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In this Article I will try to introduce you to the variety of galaxies and to give an idea of Earth's place in the universe.
During the early 1900's, astronomers were unaware that there were other galaxies outside our own Milky Way Galaxy. When they saw a small fuzzy patch in the sky through their telescopes, they called it a nebula. When examined closely, some of the nebulae had a spiral shape. So astronomers at first called these "spiral nebulae". These nebulae were all believed to be part of our Galaxy, our community of stars.
Edwin Hubble studied the "spiral nebulae" and found that they were composed of stars. These nebulae were not nebulae at all, but other communities of billions of stars held together by gravity - galaxies! Suddenly, our universe was much bigger. We realised that our Galaxy was just one of many billions of galaxies in the universe.
Hubble studied galaxies for a very long time, and after seeing many, many galaxies, he realized that he could put them into groups based on their shape: spirals, ellipticals, or irregulars (fig. 1). It is important to realise that this diagram, sometimes referred to as the "tuning fork diagram of galaxies" is not an indication of the evolution of galaxies. In other words, an Sa spiral will not eventually become an Sb and so on.
We do not always see the galaxies as clearly as this since they are randomly orientated in space. Some we see 'face on' and these will look exactly like the ones on the diagram. Others we see at an angle and so the view we see will be distorted (fig. 2).
Charles Messier was keen and successful hunter of comets. During his observations of the sky in the eighteenth century, he discovered about 100 fuzzy blobs which resembled comets. He knew that they were not comets since they stayed in the same places with respect to the stars. In order that these would not be confused with comets, he gave them each a number, the famous Messier numbers. The Great Spiral galaxy in Andromeda is designated M31. For a full list, refer to a reference book such as Nortons Star Atlas.
Types of galaxy.
Elliptical galaxies come in a range of shapes and sizes. They can be circular or egg-shaped. They can be very large and bright (giants!) or rather small and dim (dwarfs!).
If you have the chance to see one through a small telescope, it will probably look just like a fuzzy smudge to you, but it is really formed of many billions of stars orbiting the centre of the galaxy just as the Sun is orbiting the centre of our galaxy. All the light of the galaxy comes from the stars. Elliptical galaxies are old. They formed all their stars a long time ago, and are no longer making new stars.
Galaxies are arranged together in groups called clusters. In clusters, elliptical galaxies are the most common type of galaxy. In our local group of galaxies, for example, there are no giant ellipticals, but many dwarf ellipticals orbiting both the Milky Way and the Andromeda galaxy. Two dwarf elliptical galaxies can bee seen on the image of the Great Spiral in Andromeda (fig. 2).
Spiral galaxies may remind you of a pinwheel. They are rotating disks of mostly hydrogen gas, dust and stars. Through a telescope or binoculars, the bright nucleus of the galaxy may be visible but the spiral arms are dimmer and difficult to see. You need to look carefully and over a period of time you may find that you can see the arms very vaguely.
Spiral galaxies are complex objects and have several components: a disk, a bulge, and a halo. The disk contains gas, dust, and young stars in its spiral arms. The dense bulge in the centre of the disk contains mostly old stars and no gas or dust. The halo is the home of a very few, scattered stars and globular clusters. The halo is also the home of dark matter in spiral galaxies.
Spirals are subdivided based on the appearance of the arms and the central region. Sa types have a large, bright central region and tightly wound arms, while Sc types have a smaller central region and loosely wound arms. Sb types are somewhere in between. Spiral galaxies can also have bar-like struc
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