Most of the bright galaxies near to the Milky Way are spiral galaxies. Spirals have the smallest range of masses and sizes. They can be between 10,000,000,000 and 400,000,000,000 times the mass of our Sun, and their diameters range from 16,300 to 163,000 light years. Our own Milky Way is close to this upper value.
Each spiral galaxy is classified according to how it looks. All spiral galaxies are labelled with the letter "S", followed by a lower case letter, either 0, a, b or c, according to the appearance of the spiral arms, and of the bright central region called the nucleus. How the galaxy appears from Earth depends on the angle from which we see it. A galaxy seen sideways on will look very different to one seen face on. This can affect how it is classified.
S0 galaxies often look like ellipticals, but the difference between the two can be seen clearly when they are viewed edge-on. An S0 galaxy has a disc, surrounded by a sphere-shaped ‘halo’, with the central bulge inside it. This disc is not seen in ellipticals. When S0’s are seen face on, we can also tell them apart from true spirals because they don’t have the arms seen in other spiral galaxies.
This image (right, courtesy of the SEDS archive) clearly shows the lens shape of the S0 / Lenticular galaxy M102. This is only visible because we see it side-on.
It is hard to classify some galaxies just by looking at their appearance. There are, however, other ways of telling them apart. For instance, true spiral galaxies contain many young stars, whereas S0's do not. S0’s also hardly contain any gas or dust, so look more like ellipticals. True spirals have lots of both.
The next class of spiral we look at is the Sa. These systems have very tightly wound spiral arms, and large central bulges. A good example of such a galaxy is Messier 65 (NGC3623), shown here courtesy of AAO. Although some of the spiral detail is lost because the galaxy is not seen face-on, there is a clear difference between this and the S0 galaxies pictured above. The spiral shape of the disc is easily visible and the dense dust lane running through the centre of the disc can also be seen.
Sa's, like the later spiral classes of galaxies, have both young and older stars. The fraction of young stars increases towards the Sc class. The young stars can be found in the spiral arms, where star formation is happening. These galaxies also contain a great deal of dust and gas, some of which is heated to form glowing nebulae. Once again, the amount of gas and dust increases, with Sa types having the least (about 2% of the total mass), and Sc's the most (around 10%).
The galaxy shown on the right is Messier 77 (NGC 1068), and is pictured here courtesy of the SEDS archive. This is a member of the next class of spiral, the Sb. The bright central regions contain most of the galaxy's young stars, while regions further from the middle hold the older objects. As well as being an Sb type, M77 is also part of a class of galaxies known as Seyfert galaxies. These have centres that emit strong radio waves. Astronomers believe that Seyferts have black holes at their centres. It is now believed that this is true for all galaxies, but in Seyferts the black hole is very active. Most spiral galaxies are of the "b" classification.
Finally we come to the Sc class of spirals, which are often called the grand design. Such galaxies, like NGC 2997 (left, courtesy of AAO) have very open, "untidy" spiral arms and small nuclei. These spirals have more gas and dust than any of the other spiral galaxy types. This image shows the yellow light of the central regions, where old stars are found, and the blue glow of the hotter, younger stars that are formed in the spiral arms. Following the path of these arms, we can also see small red patches; these are glowing clouds of gas and dust, heated by nearby stars to form nebulae, and in some of these nebulae, stars are being formed, just as they are in our own galaxy, the Milky Way.
Not all spiral galaxies can be classed as purely type a, b or c. Many are a mixture of two types. They are therefore given two letter labels. For example, the galaxy shown on the right (courtesy of the SEDS archive), is Messier 94, and is labelled type Sab; the spiral arms look like a "b" galaxy, but the central region is too bright, belonging to the "a" class.
The shape of spiral galaxies does not end with the simple a,b,c classification. There is also a sub-branch known as barred spirals, in which the arms come from a bar passing through the galactic centre, rather than coming directly from the middle. These galaxies are given the letters SB instead of the simple S, and astronomers believe that the Milky Way belongs to this class. Once again these objects are labelled according to the appearance of the arms and the central bulge, so are labelled as SBa, SBb, and SBc, as well as the mixed types such as SBbc, and so on. A particularly beautiful example of a barred spiral is Messier 83 (left, courtesy of AAO). This galaxy is one of our nearest neighbours in space, lying at a distance of around 12 million light years (remember that this means the light we see from the galaxy tonight started out on its journey to us over 12 million years ago!)
There are many more examples of barred spirals, such as the SBb object NGC1365 shown right (courtesy of AAO), in which dark dust lanes can be seen running through the arms.
As well as bars, some galaxies have rings around their central regions. One of these galaxies is NGC 2523 (left, courtesy of the Digital Sky Survey). Here we can see the ring around the middle, and the bar that passes through the galactic centre, touching the ring on opposite sides. Spiral arms begin at the point where the ring and bar meet.
The spiral arms are the areas where stars are formed. Here we find the hottest, youngest and brightest stars, and this is why we can see the arms so clearly. Along with fully formed stars, we find hot glowing clouds of gas and dust called nebulae where stars are born.
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Author: Nigel Bannister
Updated by: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001