We've studied the birth, life and death of stars; now we're going on to study the birth, life and death of the Universe. By the Universe I mean everything. So what is everything ? We've already seen that there are lots of stars. How are these distributed in space and what else is there ?
When you look at the night sky the bright stars seem fairly evenly distributed - there are about as many in one direction as another. But as you look fainter you can see a milky band of light across the sky. Some all sky views of the Milky Way. The first telescopes resolved some of this faint band of light into stars. Photography plus bigger and better telescopes in the 19th century showed that most of the Milky Way was made of stars as in these higher resolution images of the Milky Way.
These observations make sense if the stars are distributed in a disk. Are we at the centre (this had a lot of theological and philosopical implications) ? At first it looked as if we were - the Milky Way is about as bright in all directions. As early at 1780 the Herschels (Willian and his sister, Caroline) counted stars in patches of sky. The idea was that the more stars they saw on a given area of sky then the further the disk extended in that direction. This gave a picture where the sun was more or less at the center of a disk of stars. Kapteyn in the 1920's got more more sophisticated than just looking at the 2 dimensional distribution of stars on the sky - He tried to get a 3 dimensional distribution by estimating their distance. Review lecture 4 for how we get distances to stars. He used spectroscopic parallax methods, which uses the spectrum of the star to classify its spectral type which then gives the intrinsic luminosity of the star. This can be used together with its apparent brightness and the inverse square law to get its distance. These results again indicated that the Sun being near the centre of the stellar disk.
Historically this picture of the Sun at the center of Universe consisting of a single disk of stars started to come under serious question in the early 20th century. The use of photographic film with large telescopes meant that ever fainter images of the sky could be taken. Looking over the whole sky, the photographs showed that as well as stars (which are point-like) there were fuzzy bits, so they called them nebulae (latin for cloud). The nebulae fell into 2 types, those that could be resolved into clusters of stars, and those which could not! Both these categories can also be split into two. Starclusters can either be open clusters such as The Pleiades which are of irregular shape and tend to be found only in the plane of the Milky Way, or globular clusters such as The Great globular cluster in Hercules, which are spherical and are also seen out of the main disk of stars. The 'fuzzies' split into ones which which looked truely diffuse, which are often the clouds of gas and dust from which stars are born such as the Orion Nebula, and ones which have spiral structure (more on these later!).
The globular clusters are out of the plane of the main stellar disk. They can't just be sitting there - gravity would pull them in. So they must be orbiting in which case their orbits should be centred on the centre of the galaxy. If the Sun is at the centre of our galaxy then we'd expect that the globular clusters should be distributed evenly around the sky. Shapley in 1917 discovered that they are not! The globular cluster distribution was instead centred on a point in the direction of a constellation called Sagitarius location of the sun .
The motions of the globular clusters also back up the conclusions from their spatial distribution. If we were at the center, then the globular clusters should be orbiting around the center, so there should be as many comming towards us as are going away. So if we measure the orbital velocities of MANY clusters from the doppler shift, then add them all together, they should all cancel out to zero. They don't! So the sun is not central to the globular cluster distribution or motion.
Plainly there is something wrong. Galactic star distances say we are at the center, globular clusters say we are not. which (if any!) of these two conflicting results should we believe? What can have gone wrong with either of them? One thing that could be wrong is in the galactic star distances. spectroscopic parallax are indirect measures - they rely on a chain of inference, in this case the use of the inverse square law brings in the ASSUMPTION that the space between the stars is empty. We now know that this isn't true - review lecture 9 on dust and gas in interstellar space. Distant stars have some of their light scattered out of our line of sight by the dust, and they appear dimmer and redder than they would have if the space were truly empty. This dust is concentrated in the galactic plane, so looking at stars in the galactic plane gives distances that are very distorted because of this. Globular clusters are not so affected by this as they lie mainly out of the plane of our galaxy so out of most of the dust.
So we should belive the globular clusters - but we need to know how far away they are in order to know the distance to the center of our galaxy. And this is difficult because its hard to see individual stars are they are FAINT. We need very bright stars, with very well definied intrinsic luminosity, that are easy to spot. At the start of the 20th century a class of stars called Cepheids were identified. These are evolved stars, and are highly luminous (10,000x more luminous than the sun) so can be seen at large distances. But their key property is that are unstable and pulse, giving rise to a characteristic and very easily identified lightcurve. And the period of pulsation gives a very good measure of the absolute luminosity. There is a nice javascript animation of a cepheid expanding and contracting. Again this is a spectroscopic distance measure, and again uses the apparent brightness and the inverse square law (ie assuming space is empty) to get its distance. Shapley was able to get distances to the globular clusters from Cepheids which was centred on a point 27,000 light years away from the Sun. See here for a nice review of the history of the structure of the Milky Way.
So, since we believe the globular clusters, we can use their orbital velocities. If we were at the center then this should be zero. But its not - add up lots of globular clusters and we have 220 km/s left over. So this is telling us the velocity of the sun with respect to the center of the galaxy. We know the distance (27000 light years equals 2.5x1017 km) and we know the speed, so we can work out that the orbital period. Then we can use our knowledge of gravity to translate this orbital period and distance to get the mass inside the suns orbit. Comes out to be 1011Msun. So if its all in stars like the Sun there are one hundred thousand million of them!!