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.