Astronomy - Brief History of Ancient Astronomy

A Brief History of Ancient Astronomy



Nebra Sky Disc 1600 BC


Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, cosmological, calendrical, and astrological beliefs and practices of prehistory:

The origins of Western astronomy can be found in Mesopotamia, the "land between the rivers" Tigris and Euphrates, where the ancient kingdoms of Sumer, Assyria, and Babylonia were located.



A form of writing known as cuneiform emerged among the Sumerians around 3500–3000 BC. Our knowledge of Sumerian astronomy is indirect, via the earliest Babylonian star catalogues dating from about 1200 BC.



Classical sources frequently use the term Chaldeans for the astronomers of Mesopotamia, who were, in reality, priest-scribes specializing in astrology and other forms of divination.

The first evidence of recognition that astronomical phenomena are periodic and of the application of mathematics to their prediction is Babylonian. Tablets dating back to the Old Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year.

The Venus tablet of Ammi-saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic.

A significant increase in the quality and frequency of Babylonian observations appeared during the reign of Nabonassar (747–733 BC).

The systematic records of ominous phenomena in Babylonian astronomical diaries that began at this time allowed for the discovery of a repeating 18-year cycle of lunar eclipses, for example.

The last stages in the development of Babylonian astronomy took place during the time of the Seleucid Empire (323–60 BC).

In the 3rd century BC, astronomers began to use "goal-year texts" to predict the motions of the planets.

These texts compiled records of past observations to find repeating occurrences of ominous phenomena for each planet.

About the same time, or shortly afterwards, astronomers created mathematical models that allowed them to predict these phenomena directly, without consulting past records.

Babylonian astronomy was the basis for much of what was done in Greek and Hellenistic astronomy, in classical Indian astronomy, in Iran, in Byzantium, in Syria, in Islamic astronomy, in Central Asia, and in Western Europe.

Greek Astronomy

Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman, and Late Antiquity eras.

It is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the language of scholarship throughout the Hellenistic world following the conquests of Alexander.

The development of astronomy by the Greek and Hellenistic astronomers is considered by historians to be a major phase in the history of astronomy.

Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. Most of the constellations of the northern hemisphere derive from Greek astronomy, as are the names of many stars, asteroids, and planets.

The Ancient Greeks developed astronomy, which they treated as a branch of mathematics, to a highly sophisticated level. The first geometrical, three-dimensional models to explain the apparent motion of the planets were developed in the 4th century BC by Eudoxus of Cnidus and Callippus of Cyzicus. Their models were based on nested homocentric spheres centered upon the Earth. Their younger contemporary Heraclides Ponticus proposed that the Earth rotates around its axis.

The Antikythera Mechanism

An analogue computer from 150–100 BC designed to calculate the positions of astronomical objects.

A different approach to celestial phenomena was taken by natural philosophers such as Plato and Aristotle.

They were less concerned with developing mathematical predictive models than with developing an explanation of the reasons for the motions of the Cosmos.

Plato described the universe as a spherical body divided into circles carrying the planets and governed according to harmonic intervals by a world soul.

Aristotle proposed that the universe was made of a complex system of concentric spheres, whose circular motions combined to carry the planets around the earth.

Egyptian Astronomy

The precise orientation of the Egyptian pyramids affords a lasting demonstration of the high degree of technical skill in watching the heavens attained in the 3rd millennium BC. It has been shown the Pyramids were aligned towards the pole star.

Astronomy played a considerable part in Egyption religious matters for fixing the dates of festivals and determining the hours of the night.

The titles of several temple books are preserved recording the movements and phases of the sun, moon and stars.

The Astrologer's instruments (Horologium and palm) are a plumb line and sighting instrument. They have been identified with two inscribed objects in the Berlin Museum; a short handle from which a plumb line was hung, and a palm branch with a sight-slit in the broader end.

Astronomical ceiling decoration in its earliest form can be traced to the Tomb of Senenmut The tomb and the ceiling decorations date back to the 18th Dynasty of ancient Egypt (ca. 1473 B.C.).

The Celestial Diagram consisted of a northern and a southern panel which depicted circumpolar constellations in the form of discs; each divided into 24 sections suggesting a 24-hour time period, lunar cycles, and sacred deities of Egypt.

Egyptian Celestial Diagram

Indian Astronomy

Historical Jantar Mantar observatory in Jaipur, India

Astronomy in the Indian subcontinent dates back to the period of Indus Valley Civilization during 3rd millennium BC, when it was used to create calendars.

The oldest Indian astronomical text is the Vedanga Jyotisha, dating from the Vedic period. Which describes rules for tracking the motions of the Sun and the Moon for the purposes of ritual.

Aryabhata (476–550), propounded a computational system based on a planetary model in which the Earth was taken to be spinning on its axis and the periods of   the planets were given with respect to the Sun.

He accurately calculated many astronomical constants, such as the periods of the planets, times of the solar and lunar eclipses, and the instantaneous motion of the Moon.

Astronomy was advanced during the Shunga Empire and many star catalogues were produced during this time. The Shunga period is known as the "Golden age of astronomy in India".

It saw the development of calculations for the motions and places of various planets, their rising and setting, conjunctions, and the calculation of eclipses.

Indian astronomers by the 6th century believed that comets were celestial bodies that re-appeared periodically.

And by the 10th-century astronomer Bhattotpala listed the names and estimated periods of certain comets, but it is unfortunately not known how these figures were calculated or how accurate they were.

Chinese Astronomy

Su Song's star maps the oldest existent ones in print

Astronomy in China has a long history. Detailed records of astronomical observations were kept from about the 6th century BC, until the introduction of Western astronomy and the telescope in the 17th century. Chinese astronomers were able to precisely predict eclipses.

Much of early Chinese astronomy was for the purpose of timekeeping.

The Chinese used a lunisolar calendar, but because the cycles of the Sun and the Moon are different, astronomers often prepared new calendars and made observations for that purpose.

Astrological divination was also an important part of astronomy. Astronomers took careful note of "guest stars" which suddenly appeared among the fixed stars.

They were the first to record a supernova, in the Astrological Annals of the Houhanshu in 185 AD. Also, the supernova that created the Crab Nebula in 1054 is an example of a "guest star" observed by Chinese astronomers, although it was not recorded by their European contemporaries.

Maya Astronomy

El Caracol observatory temple Mexico

Maya astronomical codices (book) include detailed tables for calculating phases of the Moon, the recurrence of eclipses, and the appearance and disappearance of Venus as morning and evening star.

The Maya based their calendric in the carefully calculated cycles of the Pleiades, the Sun, the Moon, Venus, Jupiter, Saturn, Mars, and also they had a precise description of the eclipses.

A number of important Maya structures are believed to have been oriented toward the extreme risings and settings of Venus.

To the ancient Maya, Venus was the patron of war and many recorded battles are believed to have been timed to the motions of this planet.

Mars is also mentioned in preserved astronomical codices and early mythology.

Although the Maya calendar was not tied to the Sun, it has been proposed that the Maya calculated the solar year to somewhat greater accuracy than the Gregorian calendar.

Both astronomy and an intricate numerological scheme for the measurement of time were vitally important components of Maya religion

Islamic Astronomy

The Arabic and the Persian world under Islam had become highly cultured, and many important works of knowledge from Greek astronomy and Indian astronomy and Persian astronomy were translated into Arabic, used and stored in libraries throughout the area.

An important contribution by Islamic astronomers was their emphasis on observational astronomy. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century.

In the 10th century, Abd al-Rahman al-Sufi (Azophi) carried out observations on the stars and described their positions, magnitudes, brightness, and colour and drawings for each constellation in his Book of Fixed Stars.

He also gave the first descriptions and pictures of "A Little Cloud" now known as the Andromeda Galaxy.

In the late 10th century, a huge observatory was built near Tehran, Iran, by the astronomer Abu-Mahmud al-Khujandi who observed a series of meridian transits of the Sun, which allowed him to calculate the tilt of the Earth's axis relative to the Sun.

He noted that measurements by earlier (Indian, then Greek) astronomers had found higher values for this angle, possible evidence that the axial tilt is not constant but was in fact decreasing.

Other Muslim advances in astronomy included the collection and correction of previous astronomical data. 

The invention of numerous astronomical instruments, including the development of the universal latitude-independent astrolabe

Arabic astrolabe from 1208 AD

Muhammad Mūsā believed that the heavenly bodies and celestial spheres were subject to the same physical laws as Earth.

The first elaborate experiments related to astronomical phenomena, the introduction of exacting empirical observations and experimental techniques, and the introduction of empirical testing, which produced the first model of lunar motion which matched physical observations.  

Medieval Western European Astronomy

9th century diagram of the positions of the seven planets on 18 March 816

Western Europe entered the Middle Ages with great difficulties that affected the continent's intellectual production.

The advanced astronomical treatises of classical antiquity were written in Greek, and with the decline of knowledge of that language, only simplified summaries and practical texts were available for study.

In the 6th century Bishop Gregory of Tours noted that he had learned his astronomy from reading Martianus Capella, and went on to employ this rudimentary astronomy to describe a method by which monks could determine the time of prayer at night by watching the stars.

In the 7th century the English monk Bede of Jarrow published an influential text, On the Reckoning of Time.

By the 9th century rudimentary techniques for calculating the position of the planets were circulating in Western Europe; medieval scholars recognized their flaws, but texts describing these techniques continued to be copied, reflecting an interest in the motions of the planets and in their astrological significance.

Building on this astronomical background, in the 10th century European scholars began to travel to Spain and Sicily to seek out learning which they had heard existed in the Arabic-speaking world.

There they first encountered various practical astronomical techniques concerning the calendar and timekeeping, most notably those dealing with the astrolabe.

The renaissance came to astronomy with the work of Nicolaus Copernicus, who proposed a heliocentric system, in which the planets revolved around the Sun and not the Earth.

Nicolaus Copernicus

His De revolutionibus provided a full mathematical discussion of his system, using the geometrical techniques that had been traditional in astronomy since before the time of Ptolemy. His work was later defended, expanded upon and modified by Galileo Galilei and Johannes Kepler.


Astronomy - Saturn and its Moons


Diameter 74,000 miles

Distance from Sun 0.89 billion miles 9.54 AU

Its polar diameter is 90% of its equatorial diameter, this is due to its low density and fast rotation.

Saturn turns on its axis once every 10 hours and 34 minutes giving it the second-shortest day of any of the solar system’s planets.

Saturn orbits the Sun once every 29.4 Earth years.

At least 62 moons are known to orbit Saturn, of which 53 are officially named.

Saturn is the sixth planet from the Sun and the most distant that can be seen with the naked eye. the other four being Mercury, Venus, Mars and Jupiter.

Saturn is the second largest planet in the Solar System, after Jupiter.

Like Jupiter, Saturn is a gas giant and is composed of similar gasses including hydrogen, helium and methane.

Saturn Earth Size Comparison


Saturn was known to the ancients, including the Babylonians and Far Eastern observers.

It is named after the Roman god Saturnus who was the god of agriculture. And was known to the Greeks as Cronus.

Ancient Chinese and Japanese culture designated the planet Saturn as the "earth star" (土星).

Saturn ring system was first observed in 1610 by the astronomer Galileo Galilei. He thought of them as two moons on Saturn's sides.

It was not until Christiaan Huygens used greater telescopic magnification that this notion was refuted. Huygens discovered Saturn's moon Titan

Cassini later discovered four other moons: Iapetus, Rhea, Tethys and Dione. In 1675, Cassini discovered the gap between A and B ring now known as the Cassini Division.

Four spacecraft have visited Saturn.

Pioneer 11, Voyager 1 and 2, and the Cassini-Huygens mission have all studied the planet.

Pioneer 11 made the first flyby of Saturn in September 1979, when it passed within 20,000 km of the planet's cloud tops.

Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. It also measured the temperature of Titan.

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time.

Voyager 1 performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon.

In August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings.

On 1 July 2004, the Cassini–Huygens space probe performed the SOI (Saturn Orbit Insertion) manoeuvre and entered orbit around Saturn.

The orbiter completed two Titan flybys before releasing the Huygens probe on 25 December 2004.

Huygens descended onto the surface of Titan on 14 January 2005, sending a flood of data during the atmospheric descent and after the landing.

Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.

Saturn’s upper atmosphere is divided into bands of clouds.

The top layers are mostly ammonia ice. Below them, the clouds are largely water ice.

Below that are layers of cold hydrogen and sulphur ice mixtures.

Saturn has oval-shaped storms similar to Jupiter’s.

The region around its north pole has a hexagonal-shaped pattern of clouds.

Scientists think this may be a wave pattern in the upper clouds.

The planet also has a vortex over its south pole that resembles a hurricane-like storm.

Eventually, deep inside, the hydrogen becomes metallic. At the core lies a hot interior.

Saturn has the most extensive rings in the solar system.

The rings are made mostly of chunks of ice and small amounts of dust.

The rings stretch out more than 120,700 km from the planet, but are amazingly thin: only about 20 meters thick.



Saturns Moons






Largest Moon

Diameter 3,200 miles

Distance from Saturn 759,228 miles

Orbital Period 15.5 days

Titan was discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology.

Titan is the largest moon of Saturn. It is the only moon known to have a dense atmosphere, and the only object in space other than Earth where clear evidence of stable bodies of surface liquid has been found.

Titan is the sixth gravitationally rounded moon from Saturn. Frequently described as a planet-like moon, Titan is 50% larger than Earth's Moon, and it is 80% more massive.

It is the second-largest moon in the Solar System, after Jupiter's moon Ganymede, and is larger than the smallest planet, Mercury.

11.4 times larger in the sky of Saturn than the Moon from Earth.

Titan Earth Moon Size Comparison

Titan is primarily composed of water ice and rocky material.

Information from the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in Titan's polar regions.

The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

Surface of Titan From Huygens Probe

Two new studies from Cornell University in  New York show that the liquid lakes and seas on Titan follow a constant elevation relative to Titan’s gravitational pull.

In other words, just as Earth’s oceans lie at an average elevation that we call sea level, so do Titan’s seas.

Its lakes and seas are filled with hydrocarbons rather than liquid water, and water ice overlain by a layer of solid organic material serves as the bedrock surrounding these lakes and seas.

The new study suggests that elevation is important because Titan’s liquid bodies appear to be connected under the surface in something akin to an aquifer system at Earth.

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate including wind and rain creates surface features similar to those of Earth.

Such as dunes, rivers, lakes, seas and deltas, and is dominated by seasonal weather patterns as on Earth.


Diameter 949 miles

Distance from Saturn 327,387 miles

Orbital Period 4.5 days

The second-largest moon of Saturn and the ninth-largest moon in the Solar System.

It was discovered in 1672 by Giovanni Domenico Cassini.

Rhea has a rather typical heavily cratered surface, with the exceptions of a few large fractures (wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit)


Rhea Earth Moon Size Comparison

Rhea is an icy body with a density of about 1.236 g/cm3. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm3) and ~75% water ice (density ~0.93 g/cm3).

Although Rhea is the ninth-largest moon in The Solar System, it is only the tenth-most-massive moon.

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size.

This suggests that a major resurfacing event occurred some time during its formation.

Earlier it was assumed that Rhea had a rocky core in the center. However, measurements taken during a close flyby by the Cassini orbiter in 2005 cast this into doubt.

Now considered that Rhea has an almost homogeneous interior (with some compression of ice in the center).



Diameter 892 miles

Distance from Saturn 2,212,889 miles

Orbital Period 79.3 days

Iapetus is the third-largest natural satellite of Saturn, eleventh-largest in the Solar System.

Iapetus was discovered by Giovanni Domenico Cassini, in October 1671.

He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful.

Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.


Iapetus Earth Monn Size Comparison

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites.

Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles; also, its unique equatorial ridge is so high that it visibly distorts Iapetus's shape even when viewed from a distance.

These features often lead it to be characterized as walnut-shaped.

Equatorial Ridge


Diameter 697 miles

Distance from Saturn 2,212,889 miles

Orbital Period 2.7 days

It was discovered by Giovanni Domenico Cassini in 1684. It is named after the Titaness Dione of Greek mythology.

About two thirds of Dione's mass is water ice, and the remaining is a dense core, probably silicate rock.

Data gathered by Cassini Orbiter indicates that Dione has an internal liquid water ocean.

Dione Earth Moon Size Comparison

Gravity and shape data points to a 99 ± 23 km thick ice shell crust on top of a 65 ± 30 km internal liquid water global ocean.

Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.

Dione is very similar to Rhea. They both have similar features and varied terrain, and both have dissimilar leading and trailing hemispheres.

Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, however, contains an unusual and distinctive surface feature: a network of bright ice cliffs.

The Cassini probe flyby of December 13, 2004, produced close-up images. These revealed bright ice cliffs created by tectonic fractures showing that some of them are several hundred metres high. Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.

On April 7, 2010, instruments on board the Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions  around Dione, so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere.


Diameter 650 miles

Distance from Saturn 183,100 miles

Orbital Period 1.89 days

Discovered by Cassini in 1684 observed using a large aerial telescope he set up on the grounds of the Paris Observatory. And is named after the titan Tethys of Greek mythology.

Tethys has a low density of 0.98 g/cm3, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material.

The surface of Tethys is very bright, being the second-brightest of the moons of Saturn after Enceladus, and neutral in colour.

Tethys is heavily cratered and cut by a number of large faults/graben. The largest impact crater,  is about 400 km in diameter, whereas the largest graben, is about 100 km wide and more than 2000 km long.

The surface of Tethys is one of the most reflective (at visual wavelengths) in the Solar System. The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of a dark material.

This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers.

The surface of Tethys has a number of large-scale features distinguished by their colour and sometimes brightness. The trailing hemisphere gets increasingly red and dark as the anti-apex of motion is approached.

The leading hemisphere also reddens slightly as the apex of the motion is approached, although without any noticeable darkening.

On the leading hemisphere of Tethys spacecraft observations have found a dark bluish band spanning 20° to the south and north from the equator.

The band has an elliptical shape getting narrower as it approaches the trailing hemisphere.


Diameter 310 miles

Distance from Saturn 148,000 miles

Orbital Period 1.4 days

Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometres (310 mile) in diameter, about a tenth of that of Saturn's largest moon, Titan.

Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System.


Enceladus Earth Moon Size Comparison

Consequently, its surface temperature at noon only reaches −198 °C (−324 °F), far colder than a light-absorbing body would be.

Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.

Enceladus was discovered on August 28, 1789, by William Herschel but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, passed nearby in the early 1980s.

In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail.

Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space.

Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring.

In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mile).

These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active.


Diameter 246 miles

Distance from Saturn 115,289 miles

Orbital Period 0.94 days

Mimas was discovered by the astronomer William Herschel on 17 September 1789.

He recorded his discovery as follows: "The great light of my forty-foot [12 m] telescope was so useful that on the 17th of September, 1789, I remarked the seventh satellite, then situated at its greatest western elongation.“

It is named after Mimas, a son of Gaia in Greek mythology.

A number of features in Saturn's rings are related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, the A Ring and B Ring.

They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap.

The surface area of Mimas is slightly less than the land area of Spain. The low density of Mimas, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock.

Due to the tidal forces acting on it, Mimas is noticeably prolate; its longest axis is about 10% longer than the shortest.

Mimas's most distinctive feature is a giant impact crater 130 km (81 miles) across, named Herschel after the discoverer of Mimas. Herschel's diameter is almost a third of Mimas's own diameter; its walls are approximately 5 km (3 miles) high, parts of its floor measure 10 km (6 miles) deep, and its central peak rises 6 km (4 miles) above the crater floor.

If there were a crater of an equivalent scale on Earth (in relative size) it would be over 4,000 km (2,500 miles) in diameter, wider than Australia.

The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through Mimas's body.

The surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel.

Although Mimas is heavily cratered, the cratering is not uniform.

Most of the surface is covered with craters larger than 40 km (25 miles) in diameter, but in the south polar region, there are generally no craters larger than 20 km (12 miles) in diameter.

Astronomy - Extraterrestrial Life

Extraterrestrial Life


Definition of Life

Encyclopaedia Britannica

Life, living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction.


From a physics perspective, living beings are thermodynamic systems with an organized molecular structure that can reproduce itself and evolve as survival.

Extra-terrestrial life also called alien life, is life that occurs outside of Earth and that probably did not originate from Earth.

These hypothetical life forms may range from simple single cell organism to beings with civilizations far more advanced than humanity.

Since the mid-20th century, there has been an ongoing search for signs of extra-terrestrial life.

This encompasses a search for current and historic extra-terrestrial life, and a narrower search for extra-terrestrial intelligent life.

Depending on the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send communication signals.

Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe.

This hypothesis relies on the vast size and consistent physical laws of the observable universe.

According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, as well as well-regarded thinkers such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth.

Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.

The Fermi Paradox

The Fermi paradox or Fermi's paradox, named after physicist Enrico Fermi, is the apparent contradiction between the lack of evidence and high probability estimates for the existence of extra-terrestrial civilizations.

The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old.

Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread by meteoroids, for example between habitable planets.

In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.

According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.

Since the 1950s, scientists have proposed that "habitable zones" around stars are the most likely places to find life.

Numerous discoveries in such zones since 2007 have generated numerical estimates of Earth-like planets.

On 4th November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.

Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth.

More generally, life based on ammonia (rather than water) has been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.

About 29 chemical elements play an active positive role in living organisms on Earth.

About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.

These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.

Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms.

According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life.

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components, building blocks for life as we know it, may be formed extra terrestrially in outer space.

In October 2011, scientists reported that cosmic dust contains complex organic matter that could be created naturally, and rapidly, by stars. One of the scientists suggested that these compounds may have been related to the development of life on Earth.

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system.

The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA.

This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets.

Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites. Some claim to have identified evidence that microbial life has existed on Mars.

An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. Lack of corroborating evidence from other experiments on the same samples, suggests that a non-biological reaction is a more likely.

Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.

If there is an advanced extra-terrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans.

The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.

Astronomy - Observing The Night Sky

Observing The Night Sky

The Earth rotates once a day and orbits the Sun once each year.

The first motion causes sky objects to move from east to west, and the second causes different constellations to appear in each season’s sky.

Amateur astronomers use star hopping to go from stars and constellations they know to ones they don’t know yet. First, look for noticeable patterns on the sky’s dome.

One very easy pattern to find at this time of year is the constellation Orion the Hunter.

Orion is easy to find because it contains a very noticeable pattern of three medium-bright stars in a short straight row. These stars represent Orion’s Belt.

If you can find Orion, you can use it to star hop to Sirius, the sky’s brightest star, in the constellation CanisMajor.



The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the faulty world-view of the ancients.

They believed the Earth was motionless and at the center of creation.

The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over the Earth like a great dome.

The celestial dome with its starry decorations had to be a complete celestial sphere, early sky watchers realized, because we never see a bottom rim as the dome tilts and rotates around the Earth once a day.

Part of the celestial sphere is always setting behind the western horizon, while part is always rising in the east. At any time half of the celestial sphere is above the horizon, half below.

Whenever you want to specify a point on the surface of a sphere, you'll probably use spherical coordinates. In the case of Earth, these are named latitude and longitude.

Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere.

They are now called, respectively, declination and right ascension.


Directly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination.

If you stand on the Earth's equator, the celestial equator passes overhead.

Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.

On Earth, 0° longitude has long been defined as a line engraved on a brass plate set in the floor under a position-measuring telescope at the Old Royal Observatory in Greenwich.

In the sky, 0h ("zero hours") right ascension is defined as where the plane of the Earth's orbit around the Sun (the ecliptic) crosses the celestial equator in Pisces the March equinox . This point is called, for historical reasons, the First Point of Aries.



Right Ascension is measured in hours (h), minutes (m) and seconds (s) and is similar to longitude on Earth. As the Earth rotates, stars appear to rise in the East and set in the West just like the Sun.

Declination is measured in degrees (°), arc-minutes (') and arc-seconds ("), and is similar to latitude on Earth.

There are 60 arc-minutes in a degree and 60 arc-seconds in an arc-minute.

Declination measures how far overhead an object will rise in the sky, and is measured as 0° at the equator, +90° at the North Pole and -90° at the South Pole.


Hand at Full Arms Length

The word "constellation" seems to come from the Late Latin term cōnstellātiō, which can be translated as "set of stars", and came into use in English during the 14th century.

A constellation is a group of stars that are considered to form imaginary outlines or meaningful patterns on the celestial sphere, typically representing animals, mythological people or gods, mythological creatures. The 88 modern constellations are defined regions of the sky together covering the entire celestial sphere.

In 1928, the International Astronomical Union (IAU) ratified and recognized 88 modern constellations, defined regions of the sky together covering the entire celestial sphere. With contiguous boundaries defined by right ascension and declination.

Therefore, any given point in a celestial coordinate system lies in one of the modern constellations.

Locating Stars


A complete twenty-four-hour time cycle is marked on the rim of the overlay. A full twelve months of calendar dates are marked on the rim of the starchart. The window is marked to show the direction of the eastern and western horizons.

The disk and overlay are adjusted so that the observer's local time of day on the overlay corresponds to that day's date on the star chart disc.

The portion of the star chart visible in the window then represents (with a distortion because it is a flat surface representing a spherical volume) the distribution of stars in the sky at that moment for the planisphere's designed location.

Users hold the planisphere above their head with the eastern and western horizons correctly aligned to match the chart to actual star positions.



Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just as you would see with the naked eye, binoculars or a telescope.


Experience Earth and our solar system, the universe and the spacecraft exploring them, with immersive apps for Mac, PC and mobile devices.



WorldWideTelescope is an open source set of applications, data and cloud services, originally created by Microsoft Research but now an open source project hosted on GitHub. The.NET

American Astronomical Society

Astronomy - Exoplanets


This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.

Giordano Bruno (1584)

In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.

An exoplanet (extrasolar planet) is a planet located outside the Solar System.

The first evidence of an exoplanet was noted as early as 1917 but was not recognized as such.

The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet , “a hot Jupiter” 50 light-years away was found in a four-day orbit around the star 51 Pegasi.

                          Michael Mayor                            Didier Queloz

First Exoplanet Orbiting a Main-Sequence Star

As of 2 June 2018, there are 3,786 confirmed planets in 2,834 systems, with 629 systems having more than one planet.

There are currently about fifty known exoplanets whose diameters range from Mars-sized to several times the Earth's and which also reside within their stars' habitable zone – the orbital distance within which their surface temperatures permit liquid water. These exoplanets are currently our best candidates for hosting life.


In the first observation of its kind, the Hubble Space Telescope has found helium in the atmosphere of a Jupiter-class exoplanet 200 lightyears from Earth.

Another team of researchers, using Europe’s Very Large Telescope in Chile, has found an exoplanet, WASP-96b, with a cloud-free atmosphere, allowing them to detect sodium in levels similar to abundances on Earth.

Main Methods of Detecting Exoplanets

Direct Imaging                         


Radial Velocity                        


Direct Imaging

Any planet is an extremely faint light source compared to its parent star.

For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it.

In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out.

Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead.

it is easier to obtain images when the star system is relatively near to the Sun, and when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.

For those reasons, very few of the extrasolar planets reported as of April 2014 have been observed directly, with even fewer being resolved from their host star.

Direct imaging of exoplanets that is, actual pictures will play an increasingly larger role.


The vast majority of exoplanets have been found by searching for shadows: the incredibly tiny dip in the light from a star when a planet crosses its face. Astronomers call this crossing a “transit.”

The size of the dip in starlight reveals how big the transiting planet is.

NASA’s Kepler space telescope, launched in 2009, has found nearly 2,700 confirmed exoplanets this way.

The main advantage of the transit method is that the size of the planet can be determined from the light curve.

The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet.

By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet's atmosphere.

This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point.

The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems.

Radial Velocity

A star with a planet will move in its own small orbit in response to the planet's gravity.

This leads to variations in the speed with which the star moves toward or away from Earth.

The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. The radial-velocity method measures these variations in order to confirm the presence of the planet.

The size of the wobble reveals the “weight,” or mass, of the planet.

Until around 2012, the radial-velocity method was by far the most productive technique used by planet hunters (after 2012, the transit method from the Kepler spacecraft overtook it in number).

The radial velocity signal is distance independent, but requires high signal-to-noise ratio spectra to achieve high precision, and so is generally used only for relatively nearby stars, out to about 160 light-years from Earth, to find lower-mass planets.


Gravitational Microlensing

Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned.

Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.



If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing.

This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.

The main advantages of the gravitational microlensing method are that it can detect low-mass planets (in principle down to Mars mass); it can detect planets in wide orbits comparable to Saturn and Uranus, which have orbital periods too long for the radial velocity or transit methods; and it can detect planets around very distant stars.

A notable disadvantage of the method is that the lensing cannot be repeated, because the chance alignment never occurs again.

In addition, the only physical characteristic that can be determined by microlensing is the mass of the planet, within loose constraints.

Observations are usually performed using networks of robotic telescopes. In addition to the European Research Council-funded OGLE, the Microlensing Observations in Astrophysics group is working to perfect this approach.


The transit timing variation method considers whether transits occur with strict periodicity, or if there is a variation. When multiple transiting planets are detected, they can often be confirmed with the transit timing variation method. This is useful in planetary systems far from the Sun, where radial velocity methods cannot detect them due to the low signal-to-noise ratio.

If a planet has been detected by the transit method, then variations in the timing of the transit provide an extremely sensitive method of detecting additional non-transiting planets in the system with masses comparable to Earth’s. It is easier to detect transit-timing variations if planets have relatively close orbits.

The main drawback of the transit timing method is that usually not much can be learned about the planet itself.

Transit timing variation can help to determine the maximum mass of a planet.

The next generation of space telescopes is upon us. First up is the launch of TESS, the Transiting Exoplanet Survey Satellite in April. This extraordinary instrument will take a nearly full-sky survey of the closer, brighter stars to look for transiting planets.

Kepler, the past master of transits, will be passing the torch of discovery to TESS.

TESS, in turn, will reveal the best candidates for a closer look with the James Webb Space Telescope, currently schedule to launch in 2021.




Astronomy - Galaxies





A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.

The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way.

Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

Current cosmological models of the early Universe are based on the Big Bang theory.

About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination.

Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages".

It was from density fluctuations in this primordial matter that larger structures began to appear.

As a result, matter started to condense within cold dark matter halos.

These primordial structures would eventually become the galaxies we see today.

The detailed process by which early galaxies formed is an open question in astrophysics.

Theories can be divided into two categories: top-down and bottom-up.

In top-down correlations protogalaxies form on a large-scale then simultaneous collapse lasting about one hundred million years.

In bottom-up theories, small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.

Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and primordial galaxy yet seen.

Within a billion years of a galaxy's formation, key structures begin to appear.

Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form.

The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.

During this early epoch, galaxies undergo a major burst of star formation.

During the following two billion years, the accumulated matter settles into a galactic disc. A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life.

This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.

Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, containing more stars than all the grains of sand on planet Earth.

Most of the galaxies are approximately 3000 to 300,000 light years in diameter.

For comparison, the Milky Way has a diameter of at least 100,000 LY.

Galaxies are categorized according to their visual morphology

Main Types


Barred Spiral






Edwin Hubble Sequence



Barred Spiral Galaxy


A majority of spiral galaxies, including our own Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.

Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy.

Many barred spiral galaxies are active, possibly as a result of gas being channelled into the core along the arms.

Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy



Spiral Galaxies

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms.

The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.

Spiral galaxies make up roughly 77 percent of the galaxies that scientists have observed.


Irregular Galaxy

An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.

Collectively they are thought to make up about a quarter of all galaxies. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force.

Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds.

Elliptical Galaxy

These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter.

Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation.

Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions.

The stars contain low abundances of heavy elements because star formation ceases after the initial burst.


Lenticular Galaxy

A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. They contain large-scale discs but they do not have large-scale spiral arms.

Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation.

They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars (like elliptical galaxies).

Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe.


Peculiar Galaxy

A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar.

When two galaxies come close to each other, their mutual gravitational forces can cause them to acquire highly irregular shapes.

Many peculiar galaxies experience starbursts, or episodes of rapid star formation, due to the galaxies merging. The periods of elevated star formation and the luminosity resulting from active galactic nuclei cause peculiar galaxies to be slightly bluer in colour than other galaxies.

Future of an expanding universe

One theory is that Spiral galaxies, like the Milky Way, produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.

Elliptical galaxies are largely devoid of this gas, and so form few new stars.

The supply of star-forming material is finite, once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.

At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes.

Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.

Astronomy - Stars


A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.

The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth.

Historically, stars have been important to civilizations throughout the world.

They have been part of religious practices and used for celestial navigation and orientation.

Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable.

By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.

The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.

Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations.

However, most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth.

A star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space.

Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime.

Stella Nursery

A stellar nursery is a nebula (a large cloud of hydrogen gas in space) in which star formation is occurring (stars are formed from gas).

These nebulae are frequently illuminated by ultraviolet light which is emitted from the new-born stars. One example of a stellar nursery is the Eagle nebula.

Birth of Stars

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements.

When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process.

Life span

The most massive stars have the shortest lives. Stars that are 25 to 50 times that of the Sun live for only a few million years.

They die so quickly because they burn massive amounts of nuclear fuel.

Stars like our Sun live for about 10 billion years. Stars less massive than the Sun have even longer life spans.

Death of Stars

Stars expand as they grow old. As their core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright.

This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually collapse and explode. A star's life span and eventual fate are determined by the original mass of the star.


Apparent magnitude is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. The full moon has an apparent magnitude of -12.6; the sun's is -26.8.

We can see objects up to 6th magnitude without a telescope. Apparent magnitude is abbreviated m. This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C.

Absolute magnitude is a measure of the inherent brightness of a celestial object.

This scale is defined as the apparent magnitude a star would have if it were seen from a distance of 32.6 light-years.

The lower the number, the brighter the object. Negative numbers indicate extreme brightness.

Astronomers can determine the mass, age, chemical composition, and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively.

The total mass of a star is the main factor that determines its evolution and eventual fate.


The Hertzsprung -Russell (H-R) Diagram

The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars colour (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars' colour, temperature, luminosity, spectral type, and evolutionary stage.

Main Sequence Stars - Young Stars

Main sequence stars are the central band of stars on the Hertzsprung-Russell Diagram.

These stars' energy comes from nuclear fusion, as they convert Hydrogen to Helium. Most stars are Main Sequence Stars.

For these stars, the hotter they are, the brighter they are. The sun is a typical Main Sequence star.

Dwarf Stars

Dwarf stars are relatively small stars, up to 20 times larger than our sun and up to 20,000 times brighter. Our sun is a dwarf star.

Yellow Dwarf

Yellow dwarfs are small, main sequence stars. The Sun is a yellow dwarf.

Red Dwarf

A red dwarf is a small, cool, very faint, main sequence star whose surface temperature is under about 4,000 K. Red dwarfs are the most common type of star. Proxima Centauri is a red dwarf.

White Dwarf

A white dwarf is a small, very dense, hot star that is made mostly of carbon. These faint stars are what remains after a red giant star loses its outer layers. Their nuclear cores are depleted.

Brown Dwarf

A brown dwarf is a "star" whose mass is too small to have nuclear fusion occur at its core (the temperature and pressure at its core are insufficient for fusion). A brown dwarf is not very luminous.

Red Giant

A red giant is a relatively old star whose diameter is about 100 times bigger than it was originally and had become cooler (the surface temperature is under 6,500 K).

They are frequently orange in colour. Betelgeuse is a red giant. It is about 20 times as massive as the Sun about 14,000 times brighter than the Sun, and about 600 light-years from Earth.

Blue Giant

A blue giant is a huge, very hot, blue star. It is a post-main sequence star that burns helium.


A supergiant is the largest known type of star; some are almost as large as our entire solar system. Betelgeuse and Rigel are supergiant’s. These stars are rare. When supergiant’s die they supernova and become black holes.

Neutron Star

A neutron is the collapsed core of a large star which before collapse had a total of between 10 and 29 solar masses. Which is composed mostly of tightly-packed neutrons. It has a thin atmosphere of hydrogen.


A pulsar is a rapidly spinning neutron star that emits energy in pulses.

Binary Star

A binary star is a system of two stars that rotate around a common center of mass. About half of all stars are in a group of at least two stars.

Polaris (the pole star of the Northern Hemisphere of Earth) is part of a binary star system.

Double Star

A double star is two stars that appear close to one another in the sky. Some are true binaries (two stars that revolve around one another); others just appear together from the Earth because they are both in the same line-of-sight.

Eclipsing Binary

An eclipsing binary is two close stars that appear to be a single star varying in brightness.

The variation in brightness is due to the stars periodically obscuring or enhancing one another.

This binary star system is tilted (with respect to us) so that its orbital plane is viewed from its edge.

X-Ray Binary Star

X-ray binary stars are a special type of binary star in which one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole.

As matter is stripped from the normal star, it falls into the collapsed star, producing X-rays.

Astronomy - Early Famous Astronomers





Early Famous Astronomers


Ptolemy AD100-170

Claudius Ptolemy was a Greco-Roman mathematician, astronomer, geographer, astrologer, and poet. He lived in the city of Alexandria in the Roman province of Egypt, wrote in Greek, and held Roman citizenship.

Ptolemy's Almagest is the only surviving comprehensive ancient treatise on astronomy.

Ptolemy derived his geometrical models from selected astronomical observations by his predecessors spanning more than 800 years, though astronomers have for centuries suspected that his models' parameters were adopted independently of observations.

Ptolemy presented his astronomical models in convenient tables, which could be used to compute the future or past position of the planets.

His Planetary Hypotheses went beyond the mathematical model of the Almagest to present a physical realization of the universe as a set of nested spheres, in which he used the cycles of his planetary model to compute the dimensions of the universe.

He estimated the Sun was at an average distance of 1,210 Earth radii, while the radius of the sphere of the fixed stars was 20,000 times the radius of the Earth.


Nicolaus Copernicus 1473-1543

Nicolaus Copernicus was born on 19 February 1473 in the city of Thorn, in the province of Royal Prussia, in the Crown of the Kingdom of Poland.

His father was a merchant from Kraków and his mother was the daughter of a wealthy merchant.

He was a Renaissance-era mathematician and astronomer he obtained a doctorate in canon law and was also a physician, classics scholar, translator, governor, diplomat, and economist.

Who formulated a model of the universe that placed the Sun rather than the Earth at the center of the universe.


Tycho Brahe 1546-1601

Tycho Brahe was a Danish nobleman, astronomer, and writer known for his accurate and comprehensive astronomical and planetary observations.

He was born in the then Danish peninsula of Scania. Well known in his lifetime as an astronomer, astrologer and alchemist, he has been described as "the first competent mind in modern astronomy to feel ardently the passion for exact empirical facts.

Tycho worked to combine what he saw as the geometrical benefits of the Copernican system with the philosophical benefits of the Ptolemaic system into his own model of the universe, the Tychonic system. His system correctly saw the Moon as orbiting Earth, and the planets as orbiting the Sun, but erroneously considered the Sun to be orbiting the Earth. Furthermore, he was the last of the major naked-eye astronomers, working without telescopes for his observations.

On 11 November 1572, Tycho observed a very bright star, now numbered SN1572, which had unexpectedly appeared in the constellation Cassiopeia.

In 1573, he published a small book, De nova stella thereby coining the term nova for a "new" star (we now classify this star as a supernova and we know that it is 7,500 light-years from Earth).



Johannes Kepler 1571-1630 

Johannes Kepler was a German mathematician, astronomer, and astrologer.

Kepler is a key figure in the 17th-century scientific revolution.

He is best known for his laws of planetary motion. These works also provided one of the foundations for Isaac Newton's theory of universal gravitation.

Kepler was a mathematics teacher at a school in Graz. Later he became an assistant to the astronomer Tycho Brahe in Prague, and eventually the imperial mathematician to Emperor Rudolf II. He also taught mathematics in Linz.

Additionally, he did fundamental work in the field of optics, invented an improved version of the refracting telescope (the Keplerian telescope), and was mentioned in the telescopic discoveries of his contemporary Galileo Galilei.


Christiaan Huygens 1629-1695

Christiaan Huygens was a Dutch physicist, mathematician, astronomer and inventor, who is widely regarded as one of the greatest scientists of all time and a major figure in the scientific revolution.

In physics, Huygens made ground breaking contributions in optics and mechanics, while as an astronomer he is chiefly known for his studies of the rings of Saturn and the discovery of its moon Titan.

As an inventor, he improved the design of the telescope with the invention of the Huygenian eyepiece. His most famous invention, however, was the invention of the pendulum clock in 1656, which was a breakthrough in timekeeping and became the most accurate timekeeper for almost 300 years. Because he was the first to use mathematical formulae to describe the laws of physics, Huygens has been called the first theoretical physicist and the founder of mathematical physics.



Cassini was an astronomer at the Panzano Observatory, from 1648 to 1669. He was appointed professor of astronomy at the University of Bologna in 1650 and became, in 1671, director of the Paris Observatory.

He thoroughly adopted his new country, to the extent that he became interchangeably known as Jean-Dominique Cassini.

Cassini observed and published surface markings on Mars (earlier seen by Huygens but not published), determined the rotation periods of Mars and Jupiter, and discovered four satellites of Saturn.

In addition he discovered the Cassini Division in the rings of Saturn (1675). He shares with Robert Hooke credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.



Sir Isaac Newton 1642-1726

Sir Isaac Newton was an English mathematician, astronomer, and physicist (described in his own day as a "natural philosopher") who is widely recognised as one of the most influential scientists of all time and a key figure in the scientific revolution.

Newton was a fellow of Trinity College and the second Lucasian Professor of Mathematics at the University of Cambridge.

He was a devout, but unorthodox, Christian who privately rejected the doctrine of the Trinity.

Unusually for a member of the Cambridge faculty of the day, he refused to take holy orders in the Church of England.

His book Mathematical Principles of Natural Philosophy, first published in 1687, laid the foundations of classical mechanics.

Newton also made seminal contributions to optics, and shares credit with Gottfried Wilhelm Leibniz for developing the infinitesimal calculus.

In Principia, Newton formulated the laws of motion and universal gravitation that formed the dominant scientific viewpoint until it was superseded by the theory of relativity.

Newton used his mathematical description of gravity to prove Kepler's laws of planetary motion, account for tides, the trajectories of comets, the precession of the equinoxes and other phenomena, eradicating doubt about the Solar System's heliocentricity.

Newton built the first practical reflecting telescope and developed a sophisticated theory of colour based on the observation that a prism separates white light into the colours of the visible spectrum.

Politically and personally tied to the Whig party, Newton served two brief terms as Member of Parliament for the University of Cambridge, in 1689–90 and 1701–02.

He was knighted by Queen Anne in 1705 and spent the last three decades of his life in London, serving as Warden (1696–1700) and Master (1700–1727) of the Royal Mint, as well as president of the Royal Society (1703–1727).


Edmond Halley 1656-1742

Edmond Halley was an English astronomer, geophysicist, mathematician, meteorologist, and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720.

Halley was born in Haggerston, in east London. His father, Edmond Halley Sr., came from a Derbyshire family and was a wealthy soap-maker in London.

As a child, Halley was very interested in mathematics. He studied at St Paul's School where he developed his initial interest in astronomy, and from 1673 at The Queen's College, Oxford. While still an undergraduate, Halley published papers on the Solar System and sunspots.

From an observatory he constructed on Saint Helena, Halley recorded a transit of Mercury across the Sun.

He realised a similar transit of Venus could be used to determine the size of the Solar System. He also used his observations to expand contemporary star maps.

He aided in observationally proving Isaac Newton's laws of motion, and funded the publication of Newton's influential book.

From his September 1682 observations, he used the laws of motion to compute the periodicity of Halley's Comet in his 1705 Synopsis of the Astronomy of Comets. It was named after him upon its predicted return in 1758, which he did not live to see.


Frederick William Herschel 1738-1822

Frederick William Herschel was a German-British astronomer, composer and brother of fellow astronomer Caroline Herschel, with whom he worked.

Born in the Electorate of Hanover, Herschel followed his father into the Military Band of Hanover, before migrating to Great Britain in 1757 at the age of nineteen. His works were praised by Mozart, Haydn (who met Herschel in London) and Beethoven.

Herschel constructed his first large telescope in 1774, after which he spent nine years carrying out sky surveys to investigate double stars.

The resolving power of the Herschel telescopes revealed that the nebulae in the Messier catalogue were clusters of stars. Herschel published catalogues of nebulae in 1802 (2,500 objects) and in 1820 (5,000 objects).

In the course of an observation on 13 March 1781, he realized that one celestial body he had observed was not a star, but a planet, Uranus. This was the first planet to be discovered since antiquity and Herschel became famous overnight.

As a result of this discovery, George III appointed him Court Astronomer. He was elected as a Fellow of the Royal Society and grants were provided for the construction of new telescopes.

The largest and most famous of Herschel's telescopes was a reflecting telescope with a 49 1⁄2-inch-diameter (1.26 m) primary mirror and a 40-foot (12 m) focal length.

The 40-foot telescope was, at that time, the largest scientific instrument that had been built. It was hailed as a triumph of "human perseverance and zeal for science"

In 1785 Herschel approached King George 111 for money to cover the cost of building the 40-foot telescope. He received £4,000. Without royal patronage, the telescope could not have been created. As it was, it took five years, and went over budget.


Charles Messier 1730-1817

Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the Messier objects.

The purpose of the catalogue was to help astronomical observers, in particular comet hunters like himself, distinguish between permanent and transient visually diffuse objects in the sky.

Messier was born in the Lorraine region of France, being the tenth of twelve children. Six of his brothers and sisters died while young and in 1741, his father died.

Charles' interest in astronomy was stimulated by the appearance of the spectacular, great six-tailed comet in 1744 and by an annular solar eclipse visible from his hometown on 25 July 1748.

Messier's first documented observation was that of the Mercury transit of 6 May 1753, In 1764, he was made a fellow of the Royal Society, in 1769 he was elected a foreign member of the Royal Swedish Academy of Sciences, and on 30 June 1770, he was elected to the French Academy of Sciences.


Caroline Lucretia Herschel 1750-1848

Caroline Lucretia Herschel was a German astronomer, whose most significant contributions to astronomy were the discoveries of several comets, including the periodic comet 35P/Herschel–Rigollet, which bears her name.

She was the younger sister of astronomer William Herschel, with whom she worked throughout her career.

She was the first woman to be awarded a Gold Medal of the Royal Astronomical Society (1828), and to be named an Honorary Member of the Royal Astronomical Society 1835.

Following the death of their father, William suggested that Caroline join him in Bath England. In 1772 Caroline was first introduced to astronomy by her brother.

Caroline spent many hours polishing the mirrors of high performance telescopes so that the amount of light captured was maximized.

She also copied astronomical catalogues and other publications for William. After William accepted the office of King's Astronomer to George III, Caroline became his constant assistant.

With Edward C. Pickering, she is credited with the creation of the Harvard Classification Scheme, which was the first serious attempt to organize and classify stars based on their temperatures and spectral types.

She was nearly deaf throughout her career. She was a suffragist and a member of the National Women's Party.