Download Week 11 Concept Summary

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Astronomy 10 Week 11 Summary
Chapters 19 and 20
1. The Milky Way: Is our galaxy. It contains about 200 billion stars and has a mass about a
trillion times larger than the Sun’s. The band of stars we see at night is looking into the
plane of the galaxy.
(a) Disk: The disk is very thin compared to its size, like a CD (but even thinner in
proportion!). It contains a lot of gas and dust, as well as many stars both young and
old. Open clusters are also found here. Most stars in the disk have conentrations of
heavy elements similar to the Sun’s. They all orbit in the plan of the galaxy and in the
same direction, though they oscillate just above and below the disk during this time.
The sun is in the disk, about 25,000 light years from the center, or about 2/3 out to
the edge.
(b) Bulge: The bulge is at the center of the galaxy. It comtains both young and old stars,
but very little gas and dust. Stars in the bulge orbit in random directions and with
random orientations.
(c) Halo: The halo contains only older stars, almost all inside the globular clusters also
found there. There is no gas and dust, and what stars are there have very low concentrations of heavy elements. They also orbit randomly in the gallaxy.
2. Interstellar Medium: This is the gas and dust that floats freely about the galaxy. It is what
blocks visible light and only allows us to see nearby stars in the plane of the galaxy, though
radio and infrared light can get through it easily. Stars collapse and form from the ISM,
build up more heavy elements, and then recycle their enriched gas back to the ISM through
stellar winds, pletary nebulae, and supernovae.
3. Spiral Arms: The Milky Way and many other galaxies show spiral arms. These are NOT
places where stars like to be. They are areas where the gas is more dense, and hence star
formation occurs, and hence have bright, hot, blue stars that die quickly before they leave
the arms, and hence the eye sees these areas better because the bright stars are there. The
arms rotate around the galaxy slower than individual stars due. The sun has moved in and
out of spiral arms many times during its life.
4. Rotation Curves: Plotting the velocity stuff moves around the galaxy versus its distance for
just about all spiral galaxies show flat rotation curves, i.e. everything moves at about the
same speed. This is very different than the solar system, and means that the galactic mass
is distributed and not concentrated at the center. In fact, all the stars and gas we know of
are not enough mass to keep the curve flat at large distances. This tells us there is much
more matter out in the halo than what we can see: first evidense for Dark Matter.
5. Distance Ladder: How do you measure how far away things are? Various techniques build
off one another, each new one reaching farther away and depending on all that came before.
(a) Radar: By bouncing radar waves off of the Moon, Venus, and other nearby objects,
and timing how long they take to get back, we can set a scale for the size of the solar
system, accurate to a few centimeters!
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(b) Parallax: Looking at how much a star moves due to the Earth orbiting the Sun, we
get the distance geometrically. However, this requires us to already know how big 1
AU is, hence it builds on radar ranging. Parallax is only useful out to a few hundred
parsecs; only the nearest stars.
(c) Main Sequence Fitting: Given a cluster, we can plot the relative colors and brightnesses
of the stars, and fit their stars onto a known main sequence, telling us how bright they
really are, and hence the distance. But this requires us to already have a main sequence
made up from the nearest stars that we have parallax’s to, and most importantly, the
Hyades Cluster is close enough to get a parallax distance to. This method works for
any cluster you can identify individual stars in, pretty much anywhere in the Milky
Way or our satellite galaxies.
(d) Standard Candles - Variable Stars: A standard candle is any bright object that you
know intrinsically how bright it is so that you can compare that to its apparent brightness and get the distance. RR Lyrae type variable stars have short periods, and all
have about the same luminosity. Cepheid variables have longer periods, and have a
well defined relationship between their periods and luminosities. But you have to have
some in local clusters in order to discover these relationships.
(e) Hubble Law: Only useful for distant galaxies, this measures how far away they are
based on how fast they appear to be recceeding due to the expansion of the universe.
Hubble found all galaxies to be red-shifted, and found that v = H0 d where v is the
v
velosity found from the redshift (recall ∆λ
λ = c ), d is the distance, and H0 is Hubble’s
Constant. Best observations say H0 = 71 km s−1 Mpc−1 .
We’ll talk about other rungs in the distance ladder later on.
6. Other galaxies: Hubble classified other galaxies in a sequence from ellipticals to spirals
to irregulars. Most large galaxies are spirals, though most galaxies in general are dwarf
ellipticals. Galaxies group into small groups and large clusters. In galaxy clusters, there
are found many giant elliptical galaxies dominating the mass. The Local Group contains
about 30-40 galaxies, mostly small irregulars and dwarfs, with only three larger spirals: the
Andromeda Galaxy, the Milky Way, and the Triangulum Galaxy (though the last one is
significantly smaller than the first two).
(a) Ellipticals: These have very little gas and dust, and mainly only red stars, though both
young and old. The vast majority of elliptical galaxies are dwarfs found everywhere,
though the centers of large galaxy clusters contain giant ellipticals many times bigger
than spirals like our Milky Way.
(b) Spirals: Like the Milky Way, all have disks, bulges, and halos. They contain a lot of gas
and dust, and are populated by both blue and red stars, young and old. Some spirals
have bars in the middle, and it is thought that the Milky Way has one. Lenticular
galaxies are like spirals, except that they lack the actual spiral arms.
(c) Irregulars: Other. Things that don’t fit in the above two classes. Most have very much
gas and dust, and are dominated by young, blue stars, and have almost no old stars in
general. Most irregulars are also very small galaxies.