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/ast1002/Lecture040203.pdf
Messier Objects
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1784 – Charles Messier
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Identified 103 objects in the
sky which were not stars
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M1 – Crab Nebular
these were fuzzy objects
he identified them so they would not be mistaken for
comets
These were actually galaxies, globular clusters
and such
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he was a comet hunter
far away
Now a very useful list of interesting objects for
amateur astronomers to look at
Looking at Distant Objects
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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
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compare to objects oriented
differently
Looking at the Dark Sky
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A picture of a
dark patch of the
sky using the
Hubble Space
Telescope
Galaxies
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Galaxies are large collections of stars
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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
Types of Galaxies
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Three main types of
galaxies
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Spiral
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rotating disk
(with arms)
Elliptical
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based on shape
oval disk of stars
more chaotic motion
Irregular
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none of the above
Active Galactic Nuclei
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Some galaxies have supermassive
black holes at the center
If material is falling into the black
hole, enormous amounts
of energy are released
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accretion disk
Can shine with a luminosity
of 1-100 million Suns!
Quasars are a type of AGN
Galaxy Collisions
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Occasionally galaxies
collide
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Passage of one galaxy
through/near another
causes major “stirring”
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don’t actually slam into
each other
due to gravity
Causes new activity
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star formation, AGNs,…
Think of walking
around before a
football game
Looking at the Milky Way
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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
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dark band through the middle of the Milky
Way
But we can study it in longer wavelengths
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e.g. radio waves
Spiral Galaxies
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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
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it will lag behind
Pieces of a Spiral Galaxy
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Disk
Arms
Bulge
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barred?
black hole
Halo
Globular
clusters
Globular Clusters
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Groups of old stars in the galactic halo
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Formed before the galaxy
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may have around 500,000 stars
may be around 15 lightyears across
or at very early stages
Orbiting the galaxy
No new star formation going on
Useful for studying the distribution of
material in the galaxy
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from gravity
Rotation Speeds of Galaxies
Rotation Speeds of Galaxies
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must have an explanation
For spiral galaxies,
stars farther out
should be moving
slower
But we see them all
moving with the
same speed!
Dark
Matter!
What is Dark Matter?
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Dark matter is material we can’t see
We have various evidence the Universe has
lots of matter we can’t see
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rotation speeds of galaxies
movement of global clusters around galaxies
Two primary theories
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MACHOs – MAssive Compact Halo Objects
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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
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new type of elementary particle
A Great Question
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The nature of dark matter is one of the
great questions of astronomy
Could have very important implications on
our understanding of the Universe
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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…
Studying Star Clusters
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Clusters of stars formed
at the same time of the
same materials
Studying them tells us
about the life of stars
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plot where stars fall on H-R
diagram
Looking at many clusters
tells us how stars leave
main sequence
Summary
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Galaxies are large collections of stars
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Spiral galaxies have structure
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three types/shapes
hundreds of billions of galaxies
Milky Way is a spiral galaxy
Globular clusters
Dark matter
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from rotational velocities of stars in galaxies