Astronomy 103 Tuesday, Thursday 11:00 am 12:15 pm Tom Burbine tburbinemtholyoke.edu

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Astronomy 103Tuesday, Thursday1100 am 1215
pmTom Burbinetburbine_at_mtholyoke.edu
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Exam 3

• Average was 75.3
• Grades ranged from a 28 to a 93
• Retest after class on Thursday Nov. 29 (or
  earlier if needed)

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HW 15

• Go to http//galaxyzoo.org/
• Register
• Proceed to the trial
• Pass the trial
• Hand in the page that says you passed

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What is a Galaxy?

• A Galaxy is a huge collection of stars that are
  bound together by gravity

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Number of Galaxies

• There are thought to be over 100 billion galaxies
  in the observable universe

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We live

• We live in the Milky Way Galaxy
• Thought to be 13.6 0.8 billion years old

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Why is it called Milky Way?

• It looked like a flowing ribbon of Milk to the
  Greeks

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Milky Way

• Contains between 200 and 400 billion stars
• Diameter is 100 thousand light years across

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• What two elements are the two largest
  constituents of the Milky Way Galaxy?
• A) Helium and Carbon
• B) Helium and Oxygen
• C) Hydrogen and Iron
• D) Carbon and Iron
• E) Hydrogen and Helium

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• What two elements are the two largest
  constituents of the Milky Way Galaxy?
• A) Helium and Carbon
• B) Helium and Oxygen
• C) Hydrogen and Iron
• D) Carbon and Iron
• E) Hydrogen and Helium

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Our Galaxy

• There is a disk part
• And Spherical Part Bulge and Halo

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Disk

• The disk includes all of the material (gas, dust,
  stars, and star clusters) which are confined to
  the plane of rotation of the galaxy
• Contains lots of gas and dust and is therefore
  the site of active star formation
• Lots of blue stars (O and B stars)

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Why are O and B stars evidence of active star
formation?

• A) Because they tend to be massive
• B) Because they tend to be older
• C) Because they tend to be younger
• D) Because they tend to be less massive
• E) Because they tend to be cooler

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Why are O and B stars evidence of active star
formation?

• A) Because they tend to be massive
• B) Because they tend to be older
• C) Because they tend to be younger
• D) Because they tend to be less massive
• E) Because they tend to be cooler

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Spiral Arms

• 6 spiral arms

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Spiral Arms

• Cant be fixed patterns of stars
• Stars nearer the center would complete an orbit
  much quicker than stars farther out
• Would cause the arms to wrap up into a tight coil
• We do not see that among spiral galaxies

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Spiral Arms

• Spiral Arms are actively forming new stars
• In spiral arms, see clusters of young, bright,
  blue stars
• In spiral arms, we also see lots of gas and dust
• Between the arms, you see old, red stars

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Bulge

• The bulge is at the center of the galaxy, is a
  flattened spheroid
• This is a high density region where red stars
  predominate, which are very old and about 10
  billion years old
• There is growing evidence for a very massive
  black hole at its center

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Rotation curve
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Rotation curve
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Things you need to remember

• Higher the Force of Gravity
• The faster the velocity of an object rotating
  around a larger object

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Balancing Forces

• Like a spinning hammer thrower
• Faster you spin the hammer
• Stronger the Force needed to keep it from flying
  off

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Rotation Curve

• Rotation curve of galaxy does not fall off with
  distance from center
• The mass of our galaxy is not concentrated in
  center
• More evenly distributed

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Whats the problem

• The stars in the disk do not have enough mass to
  produce this rotation curve
• The unseen mass is thought to be primarily in the
  halo

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Dark Matter

• Most (90) of the Milky Way Galaxy is thought to
  be made of Dark Matter
• Dark Matter is matter where its force of gravity
  can be felt but you cant actually see it

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Possible Dark Matter Candidates

• neutrinos with mass
• undetected brown dwarfs
• white dwarf stars
• black holes
• exotic subatomic particles

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Halo

• The halo is a diffuse spherical region, which
  surrounds the disk.
• It has a low density of old stars mainly in
  globular clusters (these consist of between
  10,000 - 1,000,000 stars).
• The halo is believed to be composed mainly of
  dark matter

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Globular Cluster

• Cluster of a million or more stars in a area of
  60-150 light years
• Tend to be found in Halos of Galaxies
• Tend to have very old stars

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M80
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Questions