14.3 The History of the Milky Way

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14.3 The History of the Milky Way

• Our Goals for Learning

• What clues to our galaxys history do halo
  stars hold?
• How did our galaxy form?

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What clues to our galaxys history do halo stars
hold?
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Halo Stars 0.02-0.2 heavy elements (O, Fe,
), only old stars
Disk Stars 2 heavy elements, stars of all
ages
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Halo Stars 0.02-0.2 heavy elements (carbon,
etc.), only old stars
Halo stars formed first, then stopped
Disk Stars 2 heavy elements, stars of all
ages
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Halo Stars 0.02-0.2 heavy elements (carbon,
etc.), only old stars
Halo stars formed first, then stopped
Disk Stars 2 heavy elements, stars of all
ages
Disk stars formed later, kept forming
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How did our galaxy form?
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Our galaxy probably formed from a giant gas cloud
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Halo stars formed first as gravity caused cloud
to contract
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Remaining gas settled into spinning disk
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Stars continuously form in disk as galaxy grows
older
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Warning This model is oversimplified
Stars continuously form in disk as galaxy grows
older
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Detailed studies Halo stars formed in clumps
that later merged
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What have we learned?

• What clues to our galaxys history do halo
  stars hold?
• The halo generally contains only old, low-mass
  stars with a much smaller proportion of heavy
  elements than stars in the disk. Thus, halo stars
  must have formed early in the galaxys history,
  before the gas settled into a disk.

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What have we learned?

• How did our galaxy form?
• The galaxy probably began as a huge blob of gas
  called a protogalactic cloud. Gravity caused the
  cloud to shrink in size, and conservation of
  angular momentum caused the gas to form the
  spinning disk of our galaxy. Stars in the halo
  formed before the gas finished collapsing into
  the disk.

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Activity 45, p. 167

• WE WILL ANSWER ALMOST EVERY SINGLE QUESTION, so
  be ready with quick answers.

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1. What properties of a black hole make it
difficult to find?

1. Large mass
2. Relatively small size
3. Absorbs light
4. 1 and 2
5. 1 and 3
6. 2 and 3
7. All the above

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2. What evidence should you seek to verify that
you had discovered a black hole?

1. Large mass
2. Relatively small size
3. Little light
4. 1 and 2
5. 1 and 3
6. 2 and 3
7. All the above

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14.4 The Mysterious Galactic Center

• Our Goals for Learning
• Question 3A pinpoint the location of the center
  of our galaxy using the two longest acceleration
  vectors shown in Figure 1.

What lies in the center of our galaxy?
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3B Approximately how close, in astronomical
units (AU), does the star come to the focus of
its orbit?

1. About 2000 AU
2. About 400 AU
3. About 100 AU
4. About 10 AU
5. About 0 AU

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Galactic center in infrared light
Galactic center in radio
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3C Is 100 AU more or less distant than Plutos
distance from the Sun?

1. More
2. Less
3. Same

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Galactic center in radio
Strange radio sources in galactic center
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3D In terms of eccentricity, is the orbit of
this star more like that of a planet in our solar
system or that of a comet?

1. More like a planet
2. More like a comet

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4A To convince an astronomer that the object at
the focus of the orbit is a BH, you need to show
the object has a very high

1. Radius
2. Volume
3. Eccentricity
4. Blueshift
5. Temperature
6. Pressure
7. Mass

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Stars at galactic center
Strange radio sources in galactic center
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4C From the equation in the activity, how many
million Suns did you find are concentrated at the
focus of the orbit?

1. 1000
2. 750
3. 250
4. 40
5. 4

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4D We have a mass 4 million suns, a size lt 100
AU, and a luminosity of 40 Suns.

1. Thats not a black hole - a black hole should
   have zero luminosity.
2. Yes it is a black hole its not the black hole
   that gives off that luminosity.

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Stars appear to be orbiting something massive but
invisible a black hole. Orbits of stars
indicate a mass of about 4 million MSun
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5A The event horizon (Schwarzschild radius) of a
BH of 4 million solar masses is

1. 8 x 103 AU
2. 8 x 10-3 AU
3. 5.33 x 103 AU
4. 5.33 AU
5. 5.33 x 10-3 AU
6. 5 AU
7. None of the above

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5D Would the black hole swallow the star if the
star got as close to the black hole as Mercury
gets to the Sun?

1. Yes
2. No

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What have we learned?

• What lies in the center of our galaxy?
• Motions of stars near the center of our galaxy
  suggest that it contains a black hole about 3 to
  4 million times as massive as the Sun. The black
  hole appears to be powering a bright source of
  radio emission known as Sgr A.