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Goal: To know the different types
of galaxies and to understand their
differences and similarities.
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Objectives:
1) To learn about Spirals
2) To learn about Barred Spirals
3) To learn about Elliptical Galaxies
4) To learn about Irregular Galaxies
What type of galaxy is this?
Andromeda is a spiral galaxy.
What kind of galaxy do we live in?
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a) Spiral galaxy
b) Irregular galaxy
c) Elliptical galaxy
d) Barred Spiral galaxy
(M110) What kind of galaxy?
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a) Spiral
b) Irregular
c) Elliptical
d) Barred
Spiral
Large Magellanic Clouds
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What kind of galaxy?
a) Spiral
b) Irregular
c) Elliptical
d) Barred
Spiral
Spiral Galaxies
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Spiral Galaxies have 3 components:
Disk
Bulge (the center)
Halo
Spiral Galaxies have density waves in
their discs which cause star formation.
• Star formation regions are the brightest
region for a galaxy, so that is the part we
usually see.
Spiral Galaxies cont
• Spiral Galaxies usually have about a
trillion solar masses and 100-400 billion
stars (give or take a factor of 10).
• They are usually about the size of our
galaxy (give or take a factor of 10).
• Because of this you can somewhat tell the
distance to a spiral galaxy based on its
image size (further away appears smaller).
Viewing Spiral
Galaxies inclination
• The angle the disk
makes with us
changes what we see!
• (top is M51 the
Whirlpool galaxy)
Supernova!
Elliptical Galaxies
• Elliptical galaxies are egg shaped.
• Stars orbit around the center of the galaxy
HOWEVER each star has its own orbit.
• There is no orbital
plane like in spirals.
(M87)
Center of Virgo cluster
50 million light years
D > 1 million light years
Ellipticals cont.
• The smallest AND the biggest galaxies are
elliptical galaxies!
• In the centers of clusters you tend to find Giant
Elliptical Galaxies that can be 10-100 times
more massive than our galaxy.
• Spirals tend to hang on their own – but often
have small satellite galaxies.
• The smallest galaxies are dwarf ellipticals.
• Elliptical galaxies don’t have much dust or gas
so new stars don’t form.
• So, they just have the old stars.
Jet!
• M87
probably
has a
massive
black hole
in its center
(billions of
solar
masses).
• 6500 light
years long
Most though
• Most of the galaxies in our universe are
probably Dwarf Elliptical galaxies.
• However these galaxies are so dim (with
only a few million stars and no dust you
can literally see through these galaxies)
that they are hard to find at any decent
distance.
• So, we can’t find them all.
Irregular Galaxies
• Are commonly found orbiting bigger
galaxies.
• Big galaxies eat the smaller ones.
• These are the ones being slowly torn apart
before the bigger galaxy digests them.
• Yes, galaxies do interact with one another!
• The result:
Irregular galaxies look like:
• They are very distorted.
• Often they have large star forming regions.
• Last page, blue was Small Magellanic Clouds.
The yellow regions were both globular clusters
(NGC 362 top left and the larger one is the
nearer to us 47 Tucanae – still 13000 light years
away).
So, we know everything there is to
know right?
Lets find out
What kind
of galaxy is
this?
a)
b)
c)
Spiral
Elliptical
Irregular
Why does
it look like
this?
COLLISIONS!
• Weird things happen when galaxies collide
(however stars don’t collide – the
distances between stars is way too big).
• There are a few possibilities.
Case 1: small galaxy collides with
big one
• In this case the small galaxy is ripped apart
tidally.
• In the short term it may orbit the galaxy a few
times in a highly warped state (and be an
irregular galaxy).
• No matter what, it will eventually be merged into
the larger galaxy.
• There may be some star formation in the big
galaxy as this will set out a density wave, but
most of the star formation will occur in the small
galaxy – if it has any gas in it.
Case 2: equal sized galaxies
collide
• If equal sized galaxies collide a whole
multitude of possible results can occur.
• If they collide at a very high velocity the
gasses will collide and heat up a lot.
• This heated gas will escape from the
galaxy – which means an end to star
formation in the galaxy.
Case 2 usual result
• The usual result is that the 2 galaxies
merge together.
• The tidal forces of the merge mix up the
galaxies randomly.
• While it may spark a period of star
formation the end galaxy is a giant
elliptical galaxy.
• Sometimes tidal tails are also created.
Another effect
• Since you add a lot of gas which ends up
being accreted by the center of the galaxy
(where the black holes lie) you tend to get
active galaxies.
• These are galaxies that often times have
long jets and are very bright (star
formation + bright accretion disk in the
nucleus).
Conclusion
• There are 3 main types of galaxies
although some have sub groups.
• Orientation affects our view of galaxies.
• Galaxies interact creating interesting
images!
• Big galaxies eat the small galaxies.