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.