Most of the bright galaxies in the neighbourhood of the Milky Way are spiral galaxies (although irregular galaxies are actually the most common). Spirals have the smallest range of masses and sizes. Typically, these objects can contain between 10,000,000,000 and 400,000,000,000 times the mass of our Sun, and 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 its appearance. All spiral galaxy labels begin with the prefix "S", followed by a lower case letter, either 0, a, b or c, determined by the arrangement of the spiral arms, and of the bright central region called the nucleus. The appearance of a galaxy depends on its orientation. This means that the angle at which we view galaxies can affect the classification they are given.
S0 galaxies can be very similar in superficial appearance to type E7 ellipticals, and are usually regarded as being objects intermediate between these ellipticals and type Sa spirals (covered below).
The difference between S0 galaxies and ellipticals becomes apparent when they are seen edge-on. An S0 galaxy shows signs of a disc, surrounded by a more spherical halo, with the nuclear bulge inside it. This disc is not seen in ellipticals. It is also possible to distinguish S0's from true spirals when they are seen face-on because they lack the characteristic arms seen in 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 with confidence just by looking at their appearance. There are, however, more fundamental differences that set S0 galaxies apart from the others; for instance, true spiral galaxies contain many young, population I stars, whereas S0's do not. In addition, many (although not all) S0’s contain little or no gas and dust, so resembling ellipticals, while true spirals are rich in both.
The next class of spiral we look at is the Sa. These systems have very tightly wound spiral arms, and large central nuclei. 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, the difference between this and the S0 galaxies pictured above is clear, with the spiral shape of the disc being visible (note also the dense dust lane running through the centre of the disc).
Sa's, like the remainder of the spiral class of galaxies, have both young population I, and older population II stars. The fraction of young stars increases towards the Sc class. The young stars are to be found in the spiral arms, which mark recent (and ongoing) sites of star formation. These galaxies also contain a great deal of dust and gas (some of which is heated to form luminous 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 reproduced here courtesy of the SEDS archive. This is a member of the next class of spiral, the Sb. The bright central regions contain the majority of the galaxy's young stars, whilst 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 cores that emit strongly in the radio region of the electromagnetic spectrum. Observations using instruments such as the Hubble Space Telscope provide evidence that Seyferts have a central region that is powered by material (such as stars and gas) falling into a black hole. It is now believed that all galaxies have a black hole at their centre, but in Seyferts it appears to be particularly active. The majority of spiral galaxies are of the "b" classification.
Finally we come to the Sc class of spirals, which are often referred to as the grand design. Such galaxies, like NGC 2997 (left, courtesy of AAO) have very open, "untidy" spiral arms and relatively small nuclei. These spirals have the highest proportion of gas and dust of any of the normal spiral 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. Tracing along 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.
Of course, not all spiral galaxies can be classed as purely type a, b or c. Many have features common to more than one classification. Such objects are therefore given two letter designations. For example, the galaxy shown here on the left (courtesy of the SEDS archive), is Messier 94, and is designated type Sab; the spiral arms have the appearance of a "b" galaxy, but the central region is too bright, belonging to the "a" class.
The diversity 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 emerge from a bar passing through the galactic centre, rather than coming directly from the centre. Such galaxies are given the prefix 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, leading to classifications such as SBa, SBb, and SBc, as well as intermediate 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.
In addition to bars, some galaxies exhibit rings around their central regions. One such galaxy is NGC 2523 (left, courtesy of the Digital Sky Survey). Here we can see the ring around the nucleus, and the bar that passes through the galactic centre, touching the ring on opposite sides. Spiral arms emerge from the point where the ring and bar meet.
The spiral arms are the regions 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 sites of stellar formation, where hot glowing clouds of gas and dust called nebulae form the "stellar nurseries".
If spiral arms are the areas where stars are formed, what mechanism forms the spiral arms? A quick "thought experiment" shows that it isn't simply due to the rotation of the galaxy. If this was the case, the arms would "wind up" as the galaxy rotated, becoming tighter and tighter with time. Galaxy rotation periods are far shorter than the age of the Universe (for example, our own Sun completes one orbit of the galactic centre every 240 million years), so we would expect to see the great majority of galaxies in this "wound-up" state. Yet, looking out into the Universe, we see galaxies with a range of spiral arm designs - some tight, and some loose. The galaxy’s rotation cannot be the sole cause of the shape of the arms - other factors must be involved.
This problem has puzzled astronomers for many years. The favoured theory is called density wave theory. A good example of a density wave is often observed on motorway journeys, where a traffic jam forms. We slow down and join the queue, expecting to pass an obstruction on the road. Instead we see nothing unusual, and in a (hopefully!) short time, we've moved through the queue of vehicles and begin to speed up again. In reality, these hold-ups can be caused by very minor events - for instance, a car slowing down to turn off the motorway.
Now imagine what this same scene must look like from someone in a helicopter high above the road. Briefly looking down the length of the motorway, the observer would see cars travelling at similar speeds, and with some space in between each other. But at one point, the observer notices that the cars are moving very slowly and are close together, spending some time in this group before leaving it, speeding up and continuing on. So, whilst this "bunch" of cars - the queue - exists for some time, it is always made up of a different set of cars. This is a density wave.
It is believed that the density wave in a spiral galaxy rotates slower than the material in the galactic disc, so that stars and gas are able to "overtake" the wave. In effect, the spiral pattern is not "frozen into" the stars, but instead it moves through them. As gas in the interstellar medium passes into the density wave, it becomes more dense, and (as discussed in the section on stellar evolution), this can lead to the formation of new stars. The very hottest, (and brightest) stars have short lifetimes, so they are born, live their lives, and die very close to the density wave. This is why the spiral arms are traced by the brightest stars. Between the arms there are many faint stars, but few bright ones, because the density wave passed through these areas a long time ago, and the bright, hot stars have died out. In time the wave will revisit these regions, and star formation will restart.
Author: Nigel Bannister
Updated by: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001