Galaxies
and
The Milky Way
Attendance Quiz
Are you here today?
(a) yes
(b) no
(c) To infinity…and beyond!
Here!
Next Tuesday, 5/30,
I will be away at a meeting. There will be a
guest lecture by Dr. Jorge Moreno
Today’s Topics
• Galaxy types
• Spiral
• Elliptical
• Irregular
• Hubble’s classification scheme
• Structure of the Milky Way
Hubble Ultra Deep Field (11.3 day exposure)
•
Between September 3, 2003 and
January 16, 2004, the Hubble Space
Telescope stared at a single,
seemingly empty patch of sky in the
constellation Fornax
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This observation lasted for
1,000,000 seconds, or about
11.3 days
•
The resulting image was the deepest
ever taken by a telescope, and
covered a patch of sky only 1/100th
the size of a full moon
•
It revealed over 10,000 objects,
most of which are galaxies
Hubble Ultra Deep Field (10,000 galaxies)
The Beginning of Time
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As we look at distant galaxies, we are looking back in time, due to the finite speed of light
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Thus, when we see a galaxy 13 billion light years away, we are seeing it as it was 13 billion
years ago
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Since the expansion rate of the universe tells us the universe is about 14 billion years old,
we are seeing these galaxies very early in the history of the universe
Types of Galaxies
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Galaxies fall into 3 main categories
• Spiral Galaxies - flat white disks with yellowish bulges at their center.
These disks contain gas and dust, and show spiral arms, where SF is occuring
• Elliptical Galaxies - redder, ellipsoidal collections of stars with varying degrees of
flatness (some spherical, others football shaped)
• Irregular Galaxies - no coherent structure; may be the result of a galaxy collision or
other gravitational interaction with a nearby galaxy
Hubble’s Classification Scheme
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•
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Each galaxy is assigned a letter - E=elliptical, S=spiral, Irr=irregular
Ellipticals are given a number to describe their roundness (0=spherical)
Spirals are given a capital B if they are barred, and a letter a-c
a = tight spiral arms, large bulge, little dust in the disk
c = loose spiral arms, small bulge, lots of dust in the disk
Hubble’s Classification Scheme
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Hubble (and others) thought this classification scheme might be an
evolutionary sequence
However, the existence of all types of galaxies together at all times, as far
back in time as we look (by looking further away), makes this unlikely
(click for image)
Lecture Tutorial:
Galaxy Classification, pp. 139-142
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Work with one or more partners - not alone!
•
Get right to work - you have 15 minutes
•
Read the instructions and questions carefully.
•
Discuss the concepts and your answers with one another.
Take time to understand it now!!!!
•
Come to a consensus answer you all agree on.
•
Write clear explanations for your answers.
•
If you get stuck or are not sure of your answer, ask another
group.
•
If you get really stuck or don’t understand what the Lecture
Tutorial is asking, ask me for help.
A cool website
Go the website http://galaxyzoo.org/ and learn how
to classify galaxies
• Then create an account and try your hand (actually
eye & brain) at classifying real galaxies (try to do at
least 50-100; you may find you can’t stop!)
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Galaxy Collisions
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Galaxies form by collisions and mergers
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Irregular galaxies are thought to be
galaxies undergoing such mergers
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Shown here are a pair of galaxies
merging called the “Antennae Galaxies”
•
Giant Elliptical galaxies (10-100 times
the size of the Milky Way) form by
cannibalizing their neighbors over
billions of years
•
This movie shows that the fate of the
Milky Way is to collide and merge with
the Andromeda Galaxy
MW-Andromeda collision movie
Many views of a galaxy collision
Structure of the Milky Way
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•
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We now have a pretty clear idea of
the structure of our home galaxy
We live about halfway to the edge of
a highly flattened spiral galaxy
Our galaxy is a typical spiral galaxy
• Highly flattened disk (like a CD)
• Stars in this disk are in circular orbits
• There are both old and young stars
• These stars contain ~2% heavy
elements (heavier than He), like the
Sun
• Roughly spherical bulge in the center
• Halo of globular clusters and other
stars
• Stars in the bulge/halo move in 3-D
elliptical orbits
• All old stars with little or no heavy
elements
Not an actual photograph!
Difficulties of our position in the Milky Way
•
•
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Because we live in the plane of the galaxy, we have a very obscured picture of
things in the Milky Way
In particular, there is interstellar dust obscuring our view, particularly towards
the Galactic Center
Nonetheless, we can learn quite a lot, if we remain aware of our surroundings
Our Position in the Milky Way
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•
•
•
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In the late 1700s, William and Caroline Herschel (brother and sister)
mapped nearby stars and concluded that the Sun was near the center of a
slightly flattened distribution of stars
Later, more quantitative studies, in the early 1900s, showed that this
distribution is about ~500 x 2500 light years in size
Some astronomers were suspicious of this (remember Copernicus)
Q: What could make us think were were near the center of the Milky Way
when we aren’t?
A: Interstellar medium!
Our Position in the Milky Way
This modern image of the Milky Way clearly shows the dark patches caused by
•
In the early
Shapleyfor the pervasive dimming and obscuring of
interstellar
dust,1900s,
whichHarlow
is responsible
observed the distribution of globular
objects far from the Solar System
clusters, and found that they were
centered on a point 10s of thousands of
light years from the Sun
•
He also found that these clusters avoided
the plane of the Milky Way
•
From these facts, he concluded
The Sun is not at the center of the Milky
Way
2. The Milky Way is much bigger than had
been previously thought
1.
•
In the 1920s, Robert Trumpler, observing
open clusters, demonstrated that there is
an obscuring “fog” which makes distant
objects seem dimmer than they should like car headlights in a fog
Milky Way Quiz I
Why is it difficult to get a good picture of what our
Milky Way galaxy looks like?
a) We’re in the middle of one of its arms–no perspective
b) It is very large and the edges are far away and faint
c) Dust blocks our view when we look toward the center or
in the plane
d) No one has ever taken a photo from both the top and side
of it
e) All of the above
Milky Way Quiz II
You observe two identical stars at the same distance.
One is in the disk of the Milky Way, the other in a
direction perpendicularly out of the disk. Chances are
the disk star will be
a) brighter and be reddened by dust
b) less bright and be reddened by dust
c) brighter and be less reddened by dust
d) less bright and be less reddened by dust
Lecture Tutorial:
Milky Way Scales, pp. 135-137
•
Work with one or more partners - not alone!
•
Get right to work - you have 15 minutes
•
Read the instructions and questions carefully.
•
Discuss the concepts and your answers with one another.
Take time to understand it now!!!!
•
Come to a consensus answer you all agree on.
•
Write clear explanations for your answers.
•
If you get stuck or are not sure of your answer, ask another
group.
•
If you get really stuck or don’t understand what the Lecture
Tutorial is asking, ask me for help.
Milky Way Quiz III
Where is our Sun located in the Milky Way Galaxy?
a) at the exact center
b) near the center
c) about half way between the center and the outer edge
d) near the outer edge
Milky Way Quiz IV
Which letter represents M45 on the MW diagram (Q5)?
a) A
b) B
c) C
d) D
e) E
Milky Way Quiz V
Which letter represents M71 on the MW diagram (Q5)?
a) A
b) B
c) C
d) D
e) E
Milky Way Quiz V
Does the Andromeda Galaxy fit on the diagram?
a) yes
b) no