Galaxies

How big are galaxies?

• hundreds of billions of stars, 50,000+ light years across
• Most galaxies also exist in groups (clusters) of tens to thousands of galaxies.

What are some properties of the Milky Way?

• All of the stars in the Milky Way orbit a supermassive black hole in the center.
• Unlike the solar system, where inner planets orbit more quickly, all stars in the Milky Way orbit the center at about the same rate (flat rotation curve). This suggests the existence of dark matter propelling the outer stars.
• From the rotation speed we can determine that the mass of the galaxy is about $10^{11}$ solar masses.

How do we determine how far other galaxies are from us?

• Use Cepheid variable stars. Cepheid variable stars are critical to determining the distance of galaxies for two main reasons: first, because they are very bright, and second, because their brightness varies periodically. The fact that the change in brightness is predictable allows us to use the inverse square law to compare their full brightness to apparent brightness at one point in time, and deduce its distance from the Earth.

What are some types of galaxies?

• Spiral galaxies (like the Milky Way Galaxy) are a disk with spiral arms that contain new stars.
• Elliptical galaxies have no disk or arms, and very little gas and dust. They are almost entirely made up of old stars.
• Irregular galaxies also exist that don’t fit either of those two categories.

What is gravitational lensing?

• Light from a distant object is bent by the gravity of a galaxy or cluster in between them and us.
• If the object and interfering galaxy are perfectly aligned, an Einstein Ring can be observed, where the light from the object is multiplied thousands of times and appears smeared around a ring rather than the original shape.

How do we know that there is a supermassive black hole at the center of a galaxy?

• We can deduce the presence and measure the mass of a supermassive black hole in the center of a galaxy by noticing that stars are moving very rapidly near the center of a galaxy. If we measure the speeds of stars (or rotating disks of gas) and apply Newton’s law of universal gravitation, we can calculate the mass necessary to produce the observed motion. We often find that the mass is millions or even billions of solar masses, yet it is confined to a very small volume at the center of the galaxy; the only reasonable conclusion is that it’s a black hole.

What are quasars?

• Quasars are quasi-stellar radio sources: very distant objects that appear extremely bright at radio wavelengths.
• The reason why quasars are so luminous is because of the enormous energy produced by supermassive black holes swallowing nearby galaxies. The friction of gases falling into the black hole at extreme speeds causes a very large amount of light and heat to be released in a relatively small region of space, creating an accretion disk of super high-energy matter around the black hole.