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 skym 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 - the direct way is trigonometic parallax but this can only be used on relatively few stars which are close enough to show a measureable effect so kapteyn couldn't use this. He used the indirect way which is spectroscopic. This 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.
But spectroscopic distance 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.
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! Its hard to get distances as they are further away than most of the stars seen easily in the galactic disk. But at the stars of the 20th century a class of stars called Cepheids had been identified. These are evolved stars, and are highly luminous so can be seen at large distances. But their key property is that are unstable and pulse, 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 (for those who are interested, see why cepheids pulse). 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. Getting distances from Cepheids gave a globular cluster distribution which was centred on a point thousands of light years away from the Sun. More on location of the sun .
Plainly there was something wrong with the galactic star distributions derived from their spectroscopic distances - dust!! The 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 dusk. But the stars in the plane are badly affected. Correct for this and we get that the globular clusters and stars are centered on a point 27,000 light years away in the direction of Sagittarius. See here for a nice review of the history of the structure of the Milky Way. Dust affects mainly blue light, so we can see more of the structure of the galaxy if we go to infrared wavelengths. The milky way then is clearly separated into a thin disk of stars, and a central bulge. The globular clusters then form a halo around it. Disk stars tend to have more absorption lines from 'metals' (carbon, nitogen, oxygen, silicon, sulphur and iron) than the stars found in the bulge and in globular clusters, indicating that the bulge stars formed first, evolved, fused the heavier elements, and then released these through planetary nebulae and Supernovae explosions. The interstellar medium clouds were then enriched in heavier elements and the next generation of stars formed with more 'metals'. This tells us that the bulge and halo of our galaxy formed first, then the disk. These two populations of stars are called population I and II - the the young ones are I and the old ones are II.
Stars, especially young stars, are not distributed smoothly in the disk of our galaxy - they clump into a spiral structure which can be seen in the distribution of O and B stars. But these are blue stars so they are sensitive to reddening - we get a much better map of the spiral pattern by going to radio wavelengths. Hydrogen is very abundant, so a nice strong atomic line from this at radio wavelengths would be ideal - and there is one at 21cm. Fantastic. The doppler shift gives relative velocity of the hydrogen with respect to the sun, and this maps out the spiral structure of the disk.
The sun must then be orbiting the centre, because of gravity. Assuming the globular clusters orbit randomly and add their velocities together. Anything left over then is the velocity of the centre of the galaxy with respect to the Sun. So reverse it and get the velocity of the sun with respect to the center of the galaxy - its 220 km/sec. 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 is 240 million years. This seems a long time. But the Sun is ~5 thousand million years old. So its gone around the center 20x!
But 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!!