IX. Stuff that's Bigger than the Solar System Physics II 3/26/09 Leeson
IX. A. Galaxies M104 Sombrero Galaxy (in Virgo)
M31
A. 1. What is a Galaxy? A system of stars, planets, dust, and gas held together by gravity. The word Galaxy comes from the Greek word for milk. Galileo thought that the Milky Way could be the aggregate light of many stars far away.
A. 2. Shapes of the galaxies Edwin P. Hubble divided the galaxies into four classes a. b. Barred spiral Spiral c. d. Elliptical Irregular
3. a. Spiral
Spiral (details) Spiral galaxies have a nucleus and have arms that spiral away from it Largish - their diameters range in size from 20,000 light years to 100,000 light years Examples - M31 and Milky Way Most consist of at least one billion stars How might this happen?
3. b. Barred Spiral
Barred Spiral (details) A barred galaxy is a type of galaxy where the spiraling arms start at the ends of a bar that goes through the center of the galaxy. How can this be?
3. c. Elliptical
Elliptical (details) Round galaxies without spiral arms Resemble the Nucleus of spiral galaxies Can be very dim... Sometimes very small and densely packed... sometimes exceptionally huge Can be seen from great distances... in other words, some are very old What happened to their spiral arms?
3. d. Irregular
Irregular (details) Not spiral or elliptical Irregular in shape (also asymmetrical) Various reasons for irregular shape: Collisions with other galaxies Gravitational effects of close galaxies Recently merged with smaller globular cluster
When Worlds Collide
Fireworks Show
The 'Monster' Centaurus A Elliptical (Ignore the irregular in the foreground) Discovered c. 1826 Powerful radio galaxy Among the first radioobjects to be linked as extragalactic Believed that Centaurus A has merged with a spiral galaxy within the last few billion years.
A. 3. How are Galaxies formed? Bottom-up theory - smaller objects (groups of stars) formed first, then merged to form galaxies. Top-down theory large clouds formed, then star formation began. Perhaps both are correct... Nevertheless, we think it all started with a big bang ~14bya
A. 4. Our Milky Way Galaxy a. Description 1) Large (Barred) Spiral galaxy 3 major arms typical of spirals, extending from the nucleus 2) Diameter: around 100,000 ly When seen from the side, it looks like a disk 3) Thickness: may be up to 30,000 ly at nucleus (near Sagittarius), and as little as 1000 ly at edges 4) Number of stars - 200 to 400 million stars 5) Age - some stars may be 13.6 byo 6) The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the solar system lies close to the galactic plane.
b. Our place in the Galaxy Recent estimates of our distance from the central bulge put our solar system at around 26,000 ly. Older estimates had us as far as 35,000 ly out. We are on the Orion Arm/Spur Diagram does not include the Large and Small Magellinic Clouds It may be a Barred Spiral. Why are we not sure? Why no Photo?
c. Milky Way Nucleus Sagittarius A* is a bright and very compact source of radio emission at the center of the Milky Way Galaxy. We now have evidence that it is a highly dense compact object, a supermassive Black Hole. It may have more than 3 million solar masses, in a volume less than 6.25 lighthours (45 AU). Milky Way galactic center, much further South than we can see. (red marks are the primary stars of Sagittarius)
d. Discovery Democritus and Galileo both thought that the Milky Way might be the light from many distant stars. Later, Galileo could see them with his telescopes. Origin of name The Milk of Hera Notice the high inclination of the ecliptic from the galactic plane. That is why all cultures knew it.
e. Pretty stuff in Our Galaxy The Trifid Nebula (HST)
Dark vs. Planetary / Diffuse / Emission
A. 6. What keeps it all together? When you get a few friends together, you call it a party. When you get enough stars together we call it a galaxy. Does this analogy work? Kinda. There's more to a party than just people. You can't put your finger on all aspects of what keeps the party going, just like you can't account for all of the mass of a galaxy. Some of it simply doesn't give off light. We call it Dark Matter and Dark Energy, simply because we can't "put our finger on it" yet.
X. B. Galaxy Clusters and more 1. Local Group... 2. Virgo Group...
B. 1. The Local Group The local group is the group of galaxies that the Milky Way belongs to. Includes about two dozen galaxies, including Andromeda and the large and small Magellanic Clouds. The Milky Way and Andromeda are the two largest galaxies in the local group.
B. 2. Virgo Group Includes 2 supergiantsized Ellipticals Dozens of spirals and irregulars the size of the Milky way Our local group may be orbiting the Virgo group. In other words, we may be part of the Virgo Super-cluster
B. 3. Galaxy Survey 2dF Galaxy Redshift Survey http://www.mso.anu.edu.au/2dfgrs/ APM galaxy survey (southern galactic cap)
X. C. The Universe 1. Big Bang Theory a. Define b. Evidence 1. Cosmic Background Radiation 2. Doppler Effect 2. Open-Closed Universe debate a. Entropy b. Omega c. Open d. Closed -