How galaxies are classified

Nobody knows how many galaxies there are in the Universe, but astronomers talk in the hundreds of billions, only a tiny fraction of which have been observed from Earth. Between 2000 and 2008, the Sloan Digital Sky Survey produced images of around a million galaxies, and the Hubble Space Telescope has continued to add to that number.

The Galaxy Zoo project is a continuing quest to make sense of this wealth of data on galaxies, primarily by seeking to classify galaxies into various types. By doing this, it is hoped to learn as much as we can about how galaxies form and, as we collect images of galaxies at much greater distances, we can see if there are significant differences between younger and older galaxies, bearing in mind that when we see an object ten billion light years away we are seeing it as it was ten billion years ago.

Galaxies have long been classified according to their shape, with Edwin Hubble himself being the first classifier in the 1920s, although he had a much smaller number of galaxies at his disposal than we have today. He recognised galaxies as being either elliptical, spiral or barred spiral, adding “irregular” as a fourth category.

Galaxy Zoo prefers a split between elliptical, spiral, lenticular and irregular, and these types are explained below. However, it must always be remembered that detecting the shape of a galaxy is not always easy because we see galaxies at every conceivable angle. A spiral is difficult to detect if it is edge-on as we see it.

Elliptical Galaxies

It was once thought that elliptical galaxies were relatively uncommon, but it is now known that they are the commonest type in the universe. This is because they tend to be of low luminosity and are often quite small. It is only as telescopes have improved that more of these have become apparent and it is now appreciated that ellipticals actually outnumber spirals. Elliptical galaxies are generally older than spiral galaxies, with star formation no longer taking place.

There is a huge range of elliptical galaxies in terms of size and shape and also luminosity. Indeed, some giant ellipticals are more luminous than any known spiral.

In terms of shape, ellipticals can be anything from a round ball shape to that of a cigar, with a classification between E0 to E7 being made (E0 is the roundest, E7 the most elongated). However, it is always possible to mistake a cigar-shaped galaxy seen end-on for a small ball-shaped galaxy.

Sizes range from giant to dwarf, with dwarves probably being the most common type of all galaxies. However, even a dwarf galaxy probably contains several million stars.

Another distinction is made according to whether the galaxy has a pronounced box or disk shape in the central area. Whether an elliptical galaxy is boxy or disky has to do with the movements of stars within the galaxy.

Spiral Galaxies

Spiral galaxies are among the most impressive objects in the Universe, comprising a central bulging core with high star density, and arms of stars spiralling out from the core, demonstrating that the whole galaxy is rotating like a slow Catherine wheel. The arms usually contain a mixture of old and young stars, with clusters of older stars and areas of nebulosity that indicate actual or potential star factories. Star density in the spirals is much less than towards the centre. Our own Milky Way is a spiral galaxy, with the Sun being a star on a spur of one of the spiral arms.

Spiral galaxies typically contain 100 billion stars and measure 100,000 light years in diameter. They may be associated with smaller galaxies and have globular clusters orbiting around them.

There are many variations in spiral galaxies. One important variant is the barred spiral, where the core resembles a bar rather than a disc. The arms begin at the two ends of the bar.

It is also of interest whether the galaxy is rotating in a clockwise or anti-clockwise direction, how many spiral arms there are (anything from one to four or more), and how tightly wound the arms are.

Edwin Hubble classified spiral galaxies as Sa, Sb or Sc, depending on the size of the central bulge and the tightness of the spirals. Fortunately, you do not get a large bulge together with a loose spiral or a tight spiral with a small bulge, so the classifications are generally safe. Sa indicates the tightest spiral and Sc the loosest. Hubble also classified barred spirals in the same way, from Sba to Sbc.

Lenticular Galaxies

These can be thought of as transitional between spiral and elliptical galaxies, and some astronomers maintain that spiral galaxies gradually evolve into ellipticals, becoming lenticular along the way. The theory is that star formation takes place in areas of dust and gas within the spiral arms, and when this is used up and absorbed into stars the spirals become tighter and tighter until they disappear altogether.

A lenticular galaxy therefore has a central disc, which appears as a bulge when seen from the side, and residual arms that are so close to the core that they have ceased to be spirals.

Irregular Galaxies

Any galaxy that does not fit into one of the above patterns is termed “irregular”, and this applies to about 3% of known galaxies. The Milky Way has two companion irregular galaxies that are known as the Large and Small Magellanic Clouds, these being visible with the naked eye from the Southern Hemisphere. They have no definite structure, comprising vast areas of nebulosity in which newly created stars can be detected.

However, it would be wrong to state that all irregular galaxies fit this pattern because there are many that clearly do not. For example, not all of them show signs of star formation. It is possible that some irregular galaxies are the result of collisions between galaxies, or are the scene of multiple supernova events or have come about due to black hole activity. There is much still to be learned here.

Other Ways of Typifying Galaxies

Mention is often made of objects such as quasars, Seyfert galaxies and N-type galaxies. These are galaxies that emit very strong radiation in the non-visible parts of the spectrum, such as radio, X-ray, ultraviolet or infrared. These have raised questions about the presence of black holes at the hearts of galaxies and the occurrence of extremely violent events in the depths of space.

Suffice it to say that the constant discovery of “new” galaxies presents more questions than answers, as there are clearly processes at work that astrophysics continues to try to understand. However, being able to classify galaxies is a very helpful first stage in getting to grips with these mysteries.

© John Welford