We saw last week that there was a lot of DUST obscuring our view in the plane of our galaxy. But we know that dust scattering affects light more strongly when the wavelength is close to the size of teh dust particles. and the dust particles are small so scattering affects blue light more than red, so it affects infrared light even less. We can use longer wavelength light (infrared) to see directly through the plane of our galaxy and see the structure of the galaxy . 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.
We can also see straight through our galaxy at even longer wavelengths - in radio. Stars don't emit much light at radio wavelengths but Hydrogen gas does! It has a line at 21cm. So we can use IR to track stars and radio 21 cm to track gas. Because its a LINE we can use the doppler shift to measure its motion - and since it has to be orbiting then it should be moving with respect to us. But we don't see that the doppler shifted 21cm line is smooth - its LUMPY. so the Hydrogen gas distribution must be lumpy too. this is tracing spiral arms in our own galaxy. We can also see the spiral arms by tracing the distribution of OB stars (but these are very blue, so we soon loose them in the dust and can't follow the arms very far).We can see spiral arms in other galaxies more easily - especially when they are face on as there is not much gas and dust! so we can easily see their spiral arms by tracing the young and bright OB stars. And we saw that we could get distances to them by using Cepheids - easy to identify because they PULSE, and highly luminous (10,000x more luminous than the sun) so can be seen at large distances. And their intrinsic luminosity is very tightly linked to the pulsation period. so we use these, plus the inverse square law (ie assuming space is empty) to get distance. Then we can work out the diameters of the 'spiral nebulae' from their apparent size on the sky - they turned out to be similar in size to our own galaxy - 100,000 light years across the stellar disk. Again, knowing the distance, we can use the apparent brightness of the galaxy to work out its intrinsic luminosity from the inverse square law. These turned out to be approximately 1011x brighter than the luminosity of the sun. Thus if they are made up of stars like the sun then these galaxies contain approximately 1011 stars!!! The 'spiral nebulae' are then separate star systems comparable in size to our own galaxy i.e. they are galaxies in their own right. The Universe consists not just of our own galaxy, the Milky Way, but of many many other galaxies too.
These are not all spirals - galaxies come in all sorts of shapes and sizes, see an introduction to galaxies. We can classify these into several main types, ellipticals, spirals (and barred spirals), and irregular galaxies. The nearest galaxy of similar size to the Milky Way is the Andromeda Galaxy - it is 2.2 million light years away, so the light we see from it left the stars in that galaxy 2.2 million years ago!! We are looking at it, not as it is now, but as it was 2.2 million years ago - we are looking back in time! And this is the NEAREST galaxy like our own.
We can currently (just!!) see Cepheids with the Hubble Space Telescope out to ~100 million light years (more details here) But what can we use after that ? We need a something where we know its absolute luminosity (called a 'standard candle'), so we can combine this with the apparent brightness via the inverse square law to get the distance. But we want this 'standard candle' to be much brighter than the Cepheids so we can use it out to larger distances. Using the galaxies where the distances are already known via Cepheids gives that the brightest globular clusters all have much the same absolute luminosity. Alternatively, the very brightest stars in each galaxy have much the same absolute luminosity. ASSUMING that the faroff galaxies are similar to the nearby ones, then we can use these to determine the distance.
But there are more faint galaxies which seem to be at still further distances. How can we get a good distance estimate to these ? We could assume that the Galaxy was like the Milky Way, and has the same absolute luminosity - this is a bit of a wild guess as we already know that galaxies come in all sorts of sizes. Ditto if we assumed that the diameter of the faint galaxies was the same as that for the Milky Way. We need something better to go further. If we take the spectra of galaxies as well then the amount of doppler broadening of the spectral lines tells us how fast the stars orbit in the galaxy, which is proportional to its mass and so to its luminosity. These can be calibrated separately to get the absolute luminosity for spiral galaxies (Tully-Fisher relation) and for elliptical galaxies (Faber-Jackson relation). More on using these to get distances to galaxies.
But theres something far more obvious in the spectra of galaxies than the breadth of the absorption lines and thats their position! In the 1920's it was already known that most galaxies had absorption lines which were doppler redshifted. Plotting the redshift versus the distance of the galaxy (found by Cepheids or any of the other methods above) gave a linear relation - the redshift that we measure in the spectrum is directly proportional to the distance. This is Hubbles Law. We can easily take a spectrum and then use the redshift to get the distance. The current record holder for the furthest galaxy whose distance can be determined by this technique is ~15.5 thousand million light years away!
But what does this expansion mean???? - don't worry, we'll come back here and do more on this in a few lectures time!
So we've seen many ways to get distances. And a lot of them build on each other forming a distance ladder (1 parsec = 3 light years) out to measure the Universe.