Astronomy 100
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| Lectures |
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A galaxy is made up of stars the way a body is made up of cells, or a beehive of bees. All the stars you can see with your naked eye are in the Milky Way Galaxy. So are all the star clusters, both globular and open . So are all the supernova remnants, the planetary nebulae, and the gaseous nebulae that are forming stars now, such as the Orion nebula.
Until Copernicus, the Earth was assumed to be the center of the Solar System; similarly, the Solar System was for millenia thought to be the center of the Milky Way Galaxy.
Two kinds of objects together helped shake that assumption, which we now know to be false:
Henrietta Leavitt, astronomer at Harvard, recognized in 1912 that Cepheid variable stars in the Large Magellanic Cloud (all at roughly same distance from Earth) obeyed a Period-Luminosity Relation: the brighter they are, they slower they blink. So you can use the period of their "blink" to determine their luminosity, and thus to infer their distance!
Harlow Shapley applied the P-L Relation to globular clusters of
stars and determined that the clusters were not centered around
the Sun, but rather around a point thousands of parsecs away, in the
direction of the constellation Sagittarius! The Sun wasn't at the
center of the Milky Way Galaxy at all!
It's easier if we look in the infrared and radio, because optical light is absorbed by the enormous clouds of dust that lie between the stars in the Galaxy, but IR and radio radiation passes right through. Combining information from all different wavelengths and all different kinds of objects -- stars of all types, atomic gas, moleclar gas dust, supernova remnants, planetary nebulae, open clusters, globular clusters, etc. -- we can map out the shape, size, and contents of the two major components of the Galaxy:
| Disk Component | Spherical Component |
| flat pancake/frisbee | "Dandelion puff" |
| Diameter = 25 kpc | Diameter = 25 kpc (halo) |
| Thickness = 1 kpc | Diameter = 4 kpc (bulge) |
| blue | red |
| gas and dust | no gas or dust |
| open star clusters | globular star clusters |
| bright young stars | old dim stars |
| star formation | no star formation |
| spiral arms | little or no structure |
| smooth rotation (whirlpool) | chaotic orbits, like swarm of bees |
| metal-rich stars = Population 1 | metal-poor stars = Population 2 |
The Sun lies in one of the spiral arms in the disk component, about 8.5 kpc (25,000 ly) from the Galactic center.
The orbital speeds of stars at different radii from the center of
the Milky Way imply the mass of the Milky Way: about 1012
MSun. But it may be even higher: rather than orbit slower
and slower at greater distances from the Galactic center, like Pluto
compared to Mercury going around the Sun, stars seem to orbit just as
fast at 10 kpc from the Galactic Center as they do at 2 kpc, and just
as fast at 15 kpc -- beyond the visible edge of the Milky Way's disk
-- as they do at 10. These rotation curves imply that there
is more than meets the eye: much or even most (90%?) of the Milky Way
is made of something OTHER than stars or gas or dust, something that
has mass and therefore gravity but that doesn't shine. This dark
matter is one of the great mysteries of contemporary
astrophysics.
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