ASTR 200 : Lecture 22 Structure of our Galaxy 1
The 'Milky Way' is known to all cultures on Earth (perhaps, unfortunately, except for recent city-bound dwellers) 2
Fish Eye Lens of visible hemisphere (but the Milky Way wraps around the whole sky) 3
Ancient ideas about the Galaxy The term 'galaxy' comes from the greek: kyklos galaktikos ( milky circle ) As far back as ~400 BC there were proposals that the milky way was a large number of distant stars However, others believed it some kind of gas structure. Others even suggested it was something closer than the moon (but this was disproven by arabian astronomer Alhazen who showed it had no parallax). 4
Galileo immediately, with the first telescope, proved that the Milky way resolved into an enormous number of stars 5
General form of the Galaxy The idea that the Galaxy was a collection of stars held together by gravitational forces dates only to the th 18 century. The first attempt to actually 'map' the stellar distribution was star counts by the Herchels (1785) They just counted the number of stars in different directions and assumed there were roughly constant density along that direction The problem. They didn't understand dust was present Sun 6
Fish Eye Lens of entire hemisphere We now understand that the dark areas in the plane are from dust that obscures the background stars 7
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First Hint Something was Wrong The first hint something was wrong came in the 1920s, when american astronomer Harlow Shapley was studying globular clusters. These are spherical collections of ~105-6 stars They are very compact (only tens of parsecs) Their HR diagrams indicate that they are very old (no blue MS stars and already lots of white dwarfs) 11
Distances to clusters Shapley wanted to study the spatial distribution of the globular clusters. He realized that if he measured FLUX of stars along the main sequence of the cluster, he could determine how FAR the cluster would have to be such that stars of known luminosity had that flux. Flux 12
Globulars are concentrated in one half the sky Shapley estimated distances to ~100 globular clusters Found they were centered on a point further away than expected! This was accepted to Kaptyn `Universe' prove that the Sun was far from the galactic centre. Extinction by dust in the galactic plane explains why it was so confused... 13 light years
The Importance of Dust - Dust massively limits our ability to see 'out into the Galaxy' at visible wavelengths The galaxy NGC 4565 >> 14 - Happens to be edge on - Huge amount of dust in the disc, preventing us from actually seeing far when looking 'along the disc' - Note that one can see out perpendicular to disc more easily
Our modern understand of the galaxy The 3 main components: Disk, Bulge, and Halo 15
Rough scales of the galaxy The Sun is in the disk, about 8,000 pc = 8 kpc from the center 16
The Disk Stars in the disk extend out to a radius of about 15 kpc There is not a super well-defined 'edge'... Thus, the Sun is about 'half way out' Flattened, rotating with a clear sense, contains almost all the gas and dust Perpendicular to the plane, stars drop of exponentially with a scale: Thin Disk: stars of all ages. 0.3 kpc scale Thick Disk: Only old stars. 1 kpc scale 17
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Milky Way Galaxy, face on view We know the galaxy has a set of spiral arms Solar System is on the interior edge of the Orion-Cygnus arm Our bulge is slightly 'bar' shaped 19
The Halo The halo is a diffuse, more or less spherical, envelope of old stars surrounding the galaxy 2 The density of stars falls off as roughly ρ r The halo also contains the globular clusters, some nearby small galaxies, and clouds of neutral gas Unknown outer edge... at least 50 kpc... Clear that the halo dominated by dark matter 20
Rough census of STELLAR component If you measure total LUMINOSITY, then Disk: ~ 20 billion solar luminosities Bulge: ~ 2 billion solar luminosities Halo : also ~2 billion solar luminosities Because, by number, most stars are fainter than the Sun, there are about 200 billion stars represented by these numbers. However, there is more matter that is non-luminous and not represented in stars... 21
The galactic evidence for Dark Matter For a spherical distribution of mass, we know that the acceleration towards the center is like all the mass is concentrated at the center (Newton's First Theorem) G M ( < r) a (r ) = 2 r This would provide the centripetal acceleration : ac = v2 / r If you see objects that you believe are on circular orbits, and you can measure their speed v, then you could equate these and measure: 2 v r M ( <r )= G For something like the Solar system, once you get past the surface of the Sun, M(<r) is essentially constant, implying 22 1 v r
The galactic rotation curve keeps rising out into the halo! There must be way more matter beyond the solar distance (8 kpc) than we can see in visible stars... The nature of the `dark matter' (just 'non luminous' matter) is important to understand 23