Understanding Galaxies What is a galaxy?

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AstroProjects Galaxies

Understanding Galaxies

What is a galaxy?

Galaxies are collections of millions to thousands of millions of stars held together in space by gravity. They come in different sizes and shapes. Two of the commonest luminous types are elliptical galaxies and spiral galaxies, like NGC 5322 and NGC 5364, shown below in images from the Sloan Digital Sky Survey (SDSS).

A typical spiral galaxy has a diameter of around 100,000 light years. In other words, it is so large that light takes a 100 thousand years to cross it, travelling at a speed of 300 thousand km each second! Some elliptical galaxies are much bigger than this. Others, known as dwarf galaxies are considerably smaller, yet still unimaginably large to us.

Given their incredible size, you might think that galaxies had to be the largest structures in the universe, but this is not the case, because there are clusters of galaxies, and even clusters of clusters, called superclusters, and such objects are even larger still. Approximately half of galaxies, especially ellipticals, are cluster galaxies, the remainder, existing on their own in space, being called field galaxies.

Elliptical galaxy NGC 5322

Spiral galaxy NGC 5364

Simplest in appearance are elliptical galaxies, like NGC 5322 shown above. Elliptical galaxies are generally featureless, apart from the fact that they are brightest in the centre, where the density of stars is greatest, and gradually decrease in brightness towards the outside as the density of stars decreases.

Individual stars in an elliptical galaxy orbit around the centre, held in place by the gravity of all the other stars. The orbits of different stars are oriented in random directions, rather like bees or mosquitoes swarming in a dense cloud that seems to hang in the air.

Elliptical galaxies contain very little gas and dust out of which new stars can form. All their stars were formed a long time ago, and are mostly yellowish in colour.

Spiral galaxy PGC 11919

Spiral galaxies, by contrast, are much more beautiful and interesting objects to look at.

At the centre they have a yellowish bulge containing old stars, which is rather like a small elliptical galaxy. Outside of the bulge, the stars form a thin disc, and they orbit around the centre of the galaxy in nearly circular orbits, just as planets do in the Solar System, each star being held in place by the gravity produced by all the other stars. Both the bulge and the disc are clearly seen in the SDSS image of an edge-on spiral galaxy above.

Most characteristic of spiral galaxies, though, are the beautiful blue spiral arms which form part of the disc of the galaxy. These spiral arms are where new stars are forming out of hydrogen and helium gas and dust. They contain the young, hot, very bright, blue stars, that give the spiral arms their colour.

Just inside the spiral arms, it is often possible to pick out dark 'dust lanes' and dotted around the spiral arms one can usually detect pinkish 'emission nebulae', regions of glowing hydrogen gas, known as 'HII' (pronounced 'H two') regions. Here hydrogen atoms have absorbed ultra-violet radiation from newly formed hot stars nearby, and are re-emitting the absorbed energy as visible light of a specific wavelength in the red part of the spectrum, giving rise to the overall pink colour. (It is not easy to spot the HII regions in the images included here because they are too small to show up.)

About half of all spiral galaxies have a central bar (some examples are shown later on). Some have a tiny central bulge, others have a very prominent bulge. In some the spiral arms are 'knotty' in appearance and not very tightly wound. In others, the spiral arms are less knotty in appearance and more tightly wound. Elliptical galaxies, vary too, in that some appear nearly circular whereas others appear quite elongated. There are also many 'lenticular' (lens shaped) galaxies that appear in images to be elliptical galaxies, but in fact have a central bulge and a disc, but no spiral arms. Then there are 'irregular' galaxies of various sorts that do not easily fit into any of the preceding categories, as well as very faint galaxies that we call dwarf elliptical and dwarf spheroidal galaxies.

It was Edwin Hubble who first brought some order into the wide variety of different galaxies visible through telescopes. The next section outlines the main features of Hubble's classification. The very faint dwarf galaxies are not included.

Hubble's classification of galaxies

Edwin Hubble published his classification of galaxy types in 1936 in a book called The Realm of the Nebulae.  Since then several people have suggested modifications and additions to his original scheme, but his basic idea of a 'tuning fork diagram' is still useful.  Below is a simple diagram of Hubble's classification.













barred lenticular

barred spiral

barred spiral

barred spiral







Examples of the different types of galaxy

The following pages show representative examples of different types of galaxy. The images were all obtained by the 2.5 metre telescope of the Sloan Digital Sky Survey (SDSS) in New Mexico.

The telescope software automatically combines images taken through three filters to produce the colour images you see here. The colours that result are close to, but not exactly the same as those that the human eye would see.

E0 type elliptical galaxy NGC 5198

E6 type elliptical galaxy NGC 4564

Elliptical galaxies look roughly egg-shaped and are relatively featureless.  All that can usually be detected is a decrease in surface brightness from the centre to the outside of the galaxy. 

Elliptical galaxies range in shape from spherical to quite elongated. Because of the angles that we view them at, however, most will appear less elongated to us than they really are. Their shapes as we see them range from circular (type E0), to elongated with the long axis being four times the short axis (type E6).

Sb type spiral galaxy NGC 3254

S0 type lenticular galaxy NGC 4526

Disc galaxies (i.e. spirals and lenticulars) have a bulge in the centre The bulge is very similar to a small elliptical galaxy, and is surrounded by a thin disc of stars, dust and gas.  The disc is usually fairly close to being circular in shape.

However, because galaxies are randomly oriented relative to our line of sight, we see most of them tilted, giving them a more or less elliptical shape in the sky, somewhere between face-on and edge-on.  The greater the viewing angle, the more elliptical they appear (a viewing angle of zero means face-on, while 90 degrees means edge-on).  (For the same reason an elliptical galaxy may well be a lot flatter in reality than it appears to us.)

Sa type spiral galaxy NGC 4580

Sb type spiral galaxy NGC 4185

Sc type spiral galaxy NGC 3180

Edge-on Sb type spiral galaxy NGC 4565

Most disc galaxies  (Sa, Sb, Sc above) also have spiral arms and are called spiral galaxies.  Some disc galaxies (S0, SB0) do not have any spiral arms and these are called lenticular (or 'lens shaped') galaxies.  They consist of a disc and a smaller bulge of variable size.

The S0 and SB0 diagrams above are just diagrammatic representations - in practice it is hard to tell lenticular galaxies at various viewing angles from elliptical galaxies (because a face-on lenticular would look like an E0 elliptical, while one inclined at about 60 degrees would appear like an E6, for example).

SBa type barred spiral galaxy NGC 4440

SBb type barred spiral galaxy NGC 5383

SBb type barred spiral galaxy M 95

SBb type barred spiral galaxy NGC 5430

About half of disc galaxies have well-define 'bars' near the centre, and these are called barred spirals (SBA, SBb, SBc) or barred lenticulars (SB0).

Irr type irregular galaxy NGC 4449

A very few of the brighter galaxies show no obvious symmetry  and do not fall into any of these categories.  They are usually just called irregular galaxies (Irr), (although  astronomers have identified many different types which have been given different names).

Differences between galaxy types

As already mentioned, elliptical galaxies contain very little cold gas and dust and their stars are moving in random orbits around the centre.  Spiral galaxies on the other hand contain large quantities of both dust and gas, especially those at the ends of the fork, Sc and SBc. Furthermore, the stars in the discs of such galaxies move around the centre in roughly circular orbits.

As we move along the top prong of the tuning fork from Sa to Sc, or along the bottom from SBa to SBc, the following changes generally occur:

  1. The bulge to disc ratio decreases.

  2. The openness of the spiral arms increases, i.e. the 'pitch angle' increases. (The pitch angle of a spiral arm is the angle that it would make with a circle centred on the centre of the galaxy. Typical pitch angles for Sa, Sb and Sc galaxies are 10, 15 and 20 degrees respectively.)

  3. Individual stars and pink emission nebulae (HII regions) become easier to pick out, and in many cases the galaxies become more 'knottier' in appearance, with the spiral arms being more clearly seen.

  4. The overall colour of the galaxy gets bluer as the spiral arms contain more young, bright, hot blue stars. This increased star formation reflects the increasing proportion of hydrogen gas within the disc region, out of which new stars can form. (The hydrogen cannot be seen in visible light images, but can be detected with radio telescopes.)

Seen edge on the progression in disc to bulge ratios looks like this:

Edge-on Sa/Sb spiral

NGC 7814

Edge-on Sb spiral

NGC 5777

Edge-on Sc spiral

UGC 4277


Richard Beare, 14th August, 2007 Version 1.1 Page of

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