Disk Galaxies – Including the
Milky Way.
M51 – A typical spiral-disk galaxy
The M81 group of galaxies
M82
M81
Satellite galaxy
Satellite galaxy
M81 group
showing
hydrogen
emission at
radio
wavelengths
Composite image of M 81. Blue shows X-Ray
observations(high energy), pink infrared (dust)
M 82. It has a disk and lots of gas and dust
around.
M82 composite: Blue-Xrays, Red-Infrared,
White-optical
• M 82 is smaller than M 81 yet it is producing
stars at an enormous rate. Ten times faster
than the Milky Way is producing stars.
• Most of the erupted gas is coming from
supernova explosions. This is star formation
on steroids.
• Why do you think this little galaxy is producing
stars so rapidly? Think about the mechanisms
for producing stars.
Why is M82 producing stars so rapidly?
1. Stars form out of gas
clouds, so it must have
an extra large amount
of gas
2. In smaller galaxies the
gas clouds are closer
together, because the
galaxy is small
3. Something must have
forced more gas cloud
collisions in M82
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The interaction between M81 and M82 caused
molecular clouds to collide in M82
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For Friday, read about tidal forces.
Tides on the Earth– Chapter 3.6.1 & 3.6.2
Tides from Black Holes – Chapter 15.5.4
Galaxy interaction – Chapter 19.2
• There will be a quiz on this on Friday.
Where are we in the Milky Way?
It is important to understand the scale that we
are talking about here.
• Shrink the solar system to the diameter of a
human hair (about 0.1 millimeters)
• Alpha Centauri would be about 1 meter away
• The Milky Way, which has a diameter of
100,000 light years would have a diameter of
20 km, on this scale (about 13 miles)
• The thickness of the disk changes some but it
is about 1000 light years thick.
• On this scale that is about 200 meters. That is
about 700 feet. Or the height of a 70 story
building.
Lexington Financial Center is 410 feet tall.
• The thickness of the disk changes some but it
is about 1000 light years thick.
• On this scale that is about 200 meters. That is
about 700 feet. Or the height of a 70 story
building.
• The Sun orbits the Milky Way at a speed of
220 km/s and it completes one orbit in about
300 million years.
• On this scale, the Solar System moves the
width of a hair every year.
• In 10 years it moves one millimeter
• In 10,000 years, the amount of time that
human civilization has existed on Earth, the
Solar system moved 1 meter.
• Since the Sun formed (~4.6 billion years ago) it
has completed around 15 trips. Depending on
the size of the orbit you use.
So try to picture this..
• The solar system is the width of a hair.
• The Milky Way is about 3 times the size of New Circle
Drive.
• It has a thickness almost 2 times the height of the
largest building in Lexington.
• And the distance from us to the next closest star is
about 1 meter.
• What about are closest big neighbor, the Andromeda
Galaxy?
On our scale, the Andromeda Galaxy would be a city,
a little bigger than Lexington and 300 miles away.
• That’s about the distance to St. Louis.
• Remember, our solar system is the width of a
hair and it is moving about 1 meter every
10,000 years.
• So on this scale, Andromeda is about 450 km
away, or
450,000 meters away.
• How long would it take the Sun to reach
Andromeda?
How long would it take the Sun to reach Andromeda?
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1.
2.
3.
4.
4.5 trillion years
4.5 billion years
4.5 million years
450,000 years
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At the Sun’s speed it would take about 4.5
billion years.
• And remember, the Sun and solar system is
moving 220 km/s. That is 528,000 miles/hour!
• At that speed it would take 2 minute, 43
seconds to fly around the Earth.
• Our Milky Way galaxy and the Andromeda
galaxy are going to collide in the future. It will
take about 7 to 8 billion years to reach
Andromeda.
The nearest stars to
the Sun.
Barnard’s Star
Brightest
stars in
the sky
From the two H-R diagrams what can you conclude
about the types of stars in the Galaxy?
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1. Most stars must be low
mass because that is what
we find near us
2. Most stars must be high
mass because throughout
the galaxy we mostly see
luminous stars
3. If we combine the two H-R
diagrams there are about
the same number of high
and low mass stars in the
Galaxy
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• We see a lot of high mass or luminous evolved
stars in the sky. This is because they have
extremely large luminosity. They can be
readily seen at great distances.
• The stars in our little volume of the Galaxy are
almost completely, low mass stars.
• This means if we increased the volume that
we are using to search for stars, we would
start to bring in a few high mass stars, but the
number of low mass stars would skyrocket.
• Most stars in the Galaxy are low-mass, red
dwarfs. ~90% of all stars.
Why are there so many little red stars in the
Galaxy?
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1. Little low mass stars
live much longer than
high mass stars
2. Star forming regions
make more low mass
stars than high mass
3. It is the combination of
1&2
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• In young stars clusters, there are typically 5 –
10 really massive, luminous stars. But there
might be several hundred or more low mass
stars.
• The massive stars die rapidly. The really small
stars (less massive than the Sun) have never
died since the start of the universe.
• Both are important.