Galaxies

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Galaxies
April 2, 2003

1. Introduction to Galaxies
2. Types of Galaxies
3. The Milky Way
4. Dark Matter

http//www.hep.fsu.edu/tadams/courses/spr03/ast10
02/Lecture040203.pdf
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Messier Objects

• 1784 Charles Messier
• he was a comet hunter
• Identified 103 objects in the
• sky which were not stars
• these were fuzzy objects
• he identified them so they would not be mistaken
  for comets
• These were actually galaxies, globular clusters
  and such
• far away
• Now a very useful list of interesting objects for
  amateur astronomers to look at

M1 Crab Nebular
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Looking at Distant Objects

• Objects look different depending on how they are
  viewed
• We are unable to walk around an object which is
  millions of lightyears away
• So we have to try to interpret what we see
• compare to objects oriented differently

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Looking at the Dark Sky

• A picture of a dark patch of the sky using the
  Hubble Space Telescope

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Galaxies

• Galaxies are large collections of stars
• millions and billions of stars
• The Milky Way is our own galaxy
• There are hundreds of billions of galaxies in the
  Universe
• Millions to hundreds of billions of stars in each
  galaxy

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Types of Galaxies

• Three main types of galaxies
• based on shape
• Spiral
• rotating disk
• (with arms)
• Elliptical
• oval disk of stars
• more chaotic motion
• Irregular
• none of the above

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Active Galactic Nuclei

• Some galaxies have supermassive black holes at
  the center
• If material is falling into the black hole,
  enormous amounts
• of energy are released
• accretion disk
• Can shine with a luminosity
• of 1-100 million Suns!
• Quasars are a type of AGN

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Galaxy Collisions

• Occasionally galaxies collide
• dont actually slam into each other
• Passage of one galaxy through/near another causes
  major stirring
• due to gravity
• Causes new activity
• star formation, AGNs,

Think of walking around before a football game
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Looking at the Milky Way

• Viewed as a bright band of stars across the sky
• Galactic center appears in the southern part of
  the sky (from the northern hemisphere
• Much of the Milky Way is blocked by dust
• dark band through the middle of the Milky Way
• But we can study it in longer wavelengths
• e.g. radio waves

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Spiral Galaxies

• Spiral arms are
• a natural consequence of some rotations
• If gas/dust in a disk are rotating with the same
  speed, stuff further out will take longer to go
  around
• it will lag behind

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Pieces of a Spiral Galaxy

• Disk
• Arms
• Bulge
• barred?
• black hole
• Halo
• Globular clusters

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Globular Clusters

• Groups of old stars in the galactic halo
• may have around 500,000 stars
• may be around 15 lightyears across
• Formed before the galaxy
• or at very early stages
• Orbiting the galaxy
• No new star formation going on
• Useful for studying the distribution of material
  in the galaxy
• from gravity

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Rotation Speeds of Galaxies
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Rotation Speeds of Galaxies

• For spiral galaxies, stars farther out should be
  moving slower
• But we see them all moving with the same speed!

Dark Matter!

• must have an explanation

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What is Dark Matter?

• Dark matter is material we cant see
• We have various evidence the Universe has lots of
  matter we cant see
• rotation speeds of galaxies
• movement of global clusters around galaxies
• Two primary theories
• MACHOs MAssive Compact Halo Objects
• planets or brown dwarfs or low-mass black holes
• unlikely, people are looking, have seen a few,
  but not enough
• WIMPs Weakly Interacting Massive Particles
• new type of elementary particle

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A Great Question

• The nature of dark matter is one of the great
  questions of astronomy
• Could have very important implications on our
  understanding of the Universe
• something is out there which we do not understand
• could radically change how we think about many
  things
• Pay attention to this issue in the coming decades

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Studying Star Clusters

• Clusters of stars formed at the same time of the
  same materials
• Studying them tells us about the life of stars
• plot where stars fall on H-R diagram
• Looking at many clusters tells us how stars leave
  main sequence

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Summary

• Galaxies are large collections of stars
• three types/shapes
• hundreds of billions of galaxies
• Spiral galaxies have structure
• Milky Way is a spiral galaxy
• Globular clusters
• Dark matter
• from rotational velocities of stars in galaxies