AST201H1 Lecture Notes - Lecture 7: Orbital Speed, Edwin Hubble, Blueshift

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25 Aug 2016
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AST201 Week 7
Other Galaxies
There are three main types of galaxies:
Spiral
- Flat, white disks with yellowish bulges in the center. Disks are filled with cool gas and
dust
- Well-developed spiral arms which line a disk, older stars. Young massive stars (blue)
indicates star formation
- Look white because they contain stars of different colours and ages
- The rotation curve is flat because gas clouds orbiting far from the galactic center have
approx. the same orbital speed as gas clouds located further inward
Elliptical
- 3 dimensional, egg shaped/circular or oval, redder because most of their light is from old,
red stars
- Formed through the interaction of spirals. Low mass stars, little gas, mainly made out of
light.
- Dead and red stars, no ongoing star formation.
- Typically more massive than spiral galaxies.
- Form from collisions with spiral galaxies
- Lack a significant disk component, come in a wide range of sizes but mostly small (most
common)
- Compared to spiral galaxies they contain very little cool gas and dust, though they are
very hot and have ionized gas
Irregular:
- No specific shape, result of an interaction of larger galaxies, galactic winds
- Look white because they contain stars of different colours and ages (young and massive)
The Milky way is part of a group of galaxies called the Local Group: has three big
galaxies - the Milky Way, Andromeda and Triangulum, and dozens of small ones
Gravity keeps all the galaxies together (within groups and clusters). Milky Way and
Andromeda are on a collision course and will interact in about 4.5 billion years. Stars are
very far apart, spiral arms will move through each other. It will take multiple interactions.
Superclusters: the largest known structures in the universe, consisting of many clusters of
galaxies, groups of galaxies, and individual galaxies
Hubble
- Edwin Hubble began measuring the distances and redshifts of galaxies – spectrum of a
galaxy where absorption lines shift toward longer wavelengths (red)
- Hubble found that the more distant a galaxy was, the larger its redshift (well outside the
Local Group). This is known as Hubble’s Law: z = H0 d
- Hubble incorrectly interpreted these shifts as Doppler shifts/effect: results from the
relative motion of a source and receiver of waves (light, sound). Source is moving
relative to an observer. Motion of one object relative to another
oEx: the pitch of a sound that one person hears is the same as someone who hears it
on the other side of the street. It sounds the same no matter where we stand near a
stationary train. Train coming toward you – sound waves bunch up to shorter
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