Structure in the universe can be observed over a vast range of distances. At the smaller end are the solar systems. Not only are the earth and other planets orbiting the sun, but telescopes have now revealed other suns with their own planets, many light-years from earth. At the far end of the scale are the unimaginable supercluster complexes. These ripples of matter can stretch across space for billions of light-years.
Between these extremes lie the galaxies, circular or elliptical collections of millions of stars, all interacting gravitationally. Galaxies come in a variety of shapes and sizes, depending on the history of their formation. Scientist Hubble formulated a scheme for categorizing galaxies in 1936, and with a few extensions the system is still used today.
~ What is a galaxy? ~
It is estimated that there are between 100 and 200 billion galaxies in the universe. They vary immensely in size, from dwarf galaxies containing tens of millions of stars, up to the giants which could be a million times larger. Our galaxy, the Way, sits in the middle of this range, and our sun is one of a few hundred billion.
As well as stars, and their respective solar systems, galaxies consist of the burned out remnants of old stars, dust and gas floating in the void, and mysterious matter. Most of a galaxy is simply empty space – an almost complete vacuum between stars light-years apart. However, on the universal scale, galaxies are incredibly dense conglomerations of matter, held together by the collective gravitational pull of all the matter within the galaxy.
It is believed that most galaxies have a super-massive black hole at the centre, around which all other material orbits, and which is gradually sucking in material from its closest neighbours. Studies of numerous galaxies suggests that the structure and formation of both the galaxy and its central black hole are intrinsically linked, although the process is not yet well understood.
Galaxies vary significantly in size. The smallest, which are often found orbiting larger galaxies and hence known as satellite galaxies, can be as small as three-thousand light-years in diameter. The largest could measure 350,000 light-years from one side to the other. Despite their enormous size, galaxies are usually separated by many millions of light-years – on a cosmological scale, even galaxies are not particularly large.
~ What types of galaxies are there? ~
At the simplest level, galaxies are divided into three different types: elliptical, spiral and lenticular. These types are then further subdivided according to the precise shape of the galaxy.
Elliptical galaxies appear as oval blurs, with no distinguishing features, regardless of the direction from which they are observed. There is little rotation of the member stars around the centre of the galaxy. Elliptical galaxies are generally older than the other galaxy types. Due to their age, there is much less interstellar matter than in spiral galaxies, and therefore less star formation. The shape and behaviour of these galaxies may be a result of collisions between galaxies much earlier in their history.
In Hubble’s classification, all elliptical galaxies are classified as ‘E’ type. This is then followed by a number which describes the degree of ellipticity of the galaxy. An ‘E0’ galaxy would be spherical. There is a formula for converting this number into a ratio of the short to long diameters of the ellipse. The limit appears to be around ‘E7’, for a galaxy which is about three times wider in one direction than the other.
Spiral galaxies are the beautiful swirls that most people picture when thinking about galaxies. Unlike in elliptical galaxies, the stars in spiral galaxies rotate around the centre of the galaxy, and most of the matter is contained within a flat disc. This rotation causes the formation of sweeping arms of matter, burning brighter than the surrounding void, due to the young and newly forming stars within them. Most spiral galaxies, our own included, have two of these arms.
The spiral galaxies are further split into those with barred spirals, where a straight line of bright material sticks out of the centre of the galaxy before the spiral begins, and ordinary spirals, where the spiral curvature begins immediately from the centre of the galaxy. Spirals are classified ‘S’ and barred spirals are classified ‘SB’. These are then subdivided further by a suffix, ‘a’, ‘b’ or ‘c’, depending on how tightly wound the spirals are.
The last classified type of galaxy is the lenticular. This sits between the elliptical and spiral galaxies. It has the disc form of the spiral galaxy, but without the spiral arms. Like elliptical galaxies, new star formation is uncommon, so stars in lenticular galaxies tend to be older. Lenticular galaxies are given the classification ‘S0’.
Distinguishing between galaxies is usually a simple case of checking the shape. However, this is very much debut they also differ in colour. Because elliptical and lenticular galaxies contain older stars they tend to be redder in colour than spirals, which are bluer.
There are other types of galaxies, but by their nature cannot be easily classified. Most of these are believed to be formed by the collision of two standard galaxy types, although some may have been pulled out of shape by more massive neighbouring galaxies. Generally, these are referred to as irregular galaxies, as their shape is ill-defined compared to spirals and ellipticals. Occasionally, a small galaxy passes straight through the centre of a larger galaxy and this can result in a ring formation. The larger galaxy ends up with a tight core, surrounded by empty space, and then a ring of stellar material further out.
~ Extending the Classification ~
The Hubble system of classification is still used today. However, in the 1950s, Gerard de Vaucouleurs, proposed an extension of the system to better describe the subtle differences between galaxies of the same Hubble type. His new classification includes more detailed characteristics of bars and spirals, and also describes the ring formations which weren’t present in Hubble’s scheme.
As well as providing more subtle definition of shape, de Vaucouleurs’ classification also describes the size of the galaxy. Between them, the Hubble and de Vaucouleurs systems provide a descriptive framework which encompasses the majority of observed galaxies. There are some oddballs which don’t fit, such as rare three-armed spirals, and attempts have been made to extend the classifications to include. Some scientists view this as un-necessary and misleading, as many of these forms are likely to be intermediate between known forms.
A useful discussion of these extensions can be found at the University of Alabama’s pages.
