Galaxies

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Galaxies

• How big is the Universe?

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Why are Cepheid variable stars useful in
determining distances?

• They all have the same distance.
• Their luminosity can be determined from their
  pulsation period.
• They all have the same luminosity.
• They all have the same radius.

3
How big is the Universe?

• Spiral nebulae were identified not long after
  development of the telescope around 1600
• In the 1600s, it was suggested that spiral
  nebula are separate galaxies so far away that the
  stars blur together, but most people thought they
  were clouds of gas
• The question wasnt resolved until 1923.

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Are there different types of objects here?
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Great debate

• Two astronomers held a great debate in 1920
• Harlow Shapley argued the Milky Way was the whole
  Universe
• Heber Curtis argued the Milky Way was just one of
  many galaxies island universes
• Held in the Smithsonian's Museum of Natural
  History the auditorium still looks the same

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Distance to the Andromeda spiral nebula

• In 1923, Edwin Hubble found Cepheid variables in
  the Andromeda nebula and showed that the nebula
  was at a great distance, much larger than the
  size of the Milky Way.

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A Cepheid is found with an oscillation period of
30 days. It is 700,000 times dimmer than another
Cepheid with a period of 30 days at a known
distance of 1000 pc. How far away is the dimmer
Cepheid?
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Flux and Luminosity

• Flux decreases as we get farther from the star
  like 1/distance2
• Mathematically, if we have two stars A and B

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Standard Candles
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Standard Candles

• Measure the distance to star A to be 1000 pc.
• Measure the flux of star A.
• Measure the flux of star B with same spectral
  type and luminosity class to be lower by a factor
  of 700000
• Find the distance to star B

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How big is the Universe?

• Greeks (up about 100 B.C.)
• Earth at Center
• Universe extends to sphere of Saturn, largest
  measured distance is from Earth to Sun at several
  million miles
• Renaissance (1500-1650)
• Sun at Center
• Universe extends to distant stars with inferred
  distance of about 100 billion miles, largest
  measured distance is from Sun to Saturn at about
  1 billion miles

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How big is the Universe?

• Parallax to stars
• First parallax measured in 1838 to star 61 Cygni
  of 0.3 arcseconds for a distance of 11 ly
  71013 miles.
• Distance to center of Milky Way
• from star counts 5000-10,000 ly (1785-1810)
• from globular clusters 50,000 ly (1915)
• Distance to Andromeda nebula
• from Cepheids 2,000,000 ly (1923)
• (really 2,500,000 ly)

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What evidence do we have that there is hidden
mass in the galaxy?

• cool clouds of hydrogen
• RR Lyrae variable stars in globular clusters
• flat rotation curve at large radii
• dusty regions in the plane

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Galaxies

• Types of galaxies
• Elliptical
• Spiral
• Irregular

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M100
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NGC 1365
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M87
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NGC 3377
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NGC 4449
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Classifying Galaxies
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Elliptical galaxies

• little interstellar gas and dust
• very little star formation
• mainly old stars (billions of years old)
• few or no young stars (millions of years old)

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Elliptical galaxies
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Often occur in clusters
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Spiral galaxies
Bulge Old stars
Disk Gas, dust, Young and old stars
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Spirals vary in prominence of bulge, tightness of
arms, presence of bar
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Irregular galaxies have asymmetric shapes and
usually lots of young stars

• They are often found near other galaxies

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In which type of galaxy are stars orbits
distributed in random directions?

• elliptical galaxies
• spiral galaxies
• barred spiral galaxies
• blue galaxies

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Our Galaxy is a member of a small cluster called
the Local Group
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MW eating neighbors
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Review Questions

• What was the definitive evidence showing that
  spiral nebulae are actually entire galaxies
  outside of the the Milky Way?
• What are the types of galaxies?
• How do the rotation patterns of stars differ in
  elliptical versus spiral galaxies?
• What is the Local Group?

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Cosmic Distances

• How to measure distances
• Primary distance indicators
• Secondary and tertiary distance indicators
• Recession of galaxies
• Expansion of the Universe

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Stellar Parallax
As the Earth moves from one side of the Sun to
the other, a nearby star will seem to change its
position relative to the distant background
stars. d 1 / p d distance to nearby star in
parsecs p parallax angle of that star in
arcseconds
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Stellar Parallax

• Most accurate parallax measurements are from the
  European Space Agencys Hipparcos mission.
• Hipparcos could measure parallax as small as
  0.001 arcseconds or distances as large as 1000
  pc.
• How to find distance to objects farther than
  1000 pc?

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Standard Candles
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Distances to galaxies

• Standard candles, such as Cepheid variables, the
  most luminous supergiants, globular clusters, H
  II regions, and supernovae in a galaxy, are used
  in estimating intergalactic distances.

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The Distance Ladder

• Each stage in the ladder overlaps the previous
  and next
• Cepheid distances are critical
• Tully-Fisher, fundamental plane apply to whole
  galaxies
• Supernova are now the best estimators at large
  distances

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Doppler effect for light
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Light from distant galaxies is redshifted
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Distances and velocities of galaxies

• If you measure the distances to a large set of
  galaxies and also measure the speed of the
  galaxies using the redshift, what do you find?

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Hubble expansion v H0d
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Expansion of the Universe
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Motion at constant velocity
distance velocity ? time velocity 0.5
cm/s time distance / velocity 3 cm/(0.5 cm/s)
6 s
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Receding galaxy
Velocity 500 km/s 0.508 Mpc/Gyr
When were galaxies in the same place? time
distance / velocity 7 Mpc/(0.508 Mpc/Gyr)
13.8 Gyr ago
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Hubble expansion v H0d
Time distance/velocity d/H0d 1/H0
1/(71 km/s/Mpc) 13.8 Gyr
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An observer at a distance of 3 billion light
years from us looking in our general direction
would see

• most of the galaxies approaching her.
• the same Hubbles law that we see.
• about equal numbers of red and blue shifted
  galaxies.
• everything rushing away from a point near the
  Milky Way galaxy.

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Expansion of the Universe

• Blow up the balloon to about a 3 inch diameter.
  Twist the neck and hold it closed so that no air
  escapes, but do NOT make a knot because you will
  need to blow it up some more. Make SIX dots on
  its surface to represent galaxies and label them
  A-F.
• Measure and record the distances from cluster A
  to each of the other 5 clusters.
• Measure and record the distances from cluster D
  to each of the other 5 clusters.
• Blow up the balloon up more, to a diameter of
  about 6 inches. Measure the distances between the
  same clusters again and record them.

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Expansion of the Universe

• Are all the other clusters moving away from
  cluster A?
• Are all the other clusters moving away from
  cluster D?
• Is there a cluster that could be considered to be
  at the center of the universe as represented by
  the surface of the balloon?

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Formation of Galaxies

• Spiral versus elliptical
• Young Universe
• Collisions and Interactions
• Starbursts
• Elliptical galaxies

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Formation of a Spiral Galaxy
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Formation of an Elliptical Galaxy
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Stellar Birthrate in Galaxies
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Formation of Galaxies

• This picture of galaxy formation is incomplete
• Mergers, collisions, and interactions between
  galaxies are very important in their formation,
  particularly in the early stages of the Universe
  (why?)

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Expansion of the Universe

• The Universe is expanding
• This means that the Universe used to be smaller
• In the early stages of the Universe
• there were more galaxies
• they were closer together
• therefore, they interacted more

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Young Universe
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Young Universe
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Young Universe
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Young Universe
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Colliding galaxies
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The Mice
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Cartwheel galaxy
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Seyferts Sextet
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Interacting galaxies
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Interacting galaxies
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Starburst galaxy M82
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M82 in X-rays
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Colliding galaxies
Movie
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Galaxy interactions

• Interactions can rip stars out of galaxies,
  producing tidal tails
• Interactions can disturb gas in and between
  galaxies, producing starbursts
• Collisions can randomize stellar orbits leading
  to the formation of elliptical galaxies

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Formation of an Elliptical Galaxy

• Movie

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Galaxy growth via interactions

• Galaxies initially form from mergers of several
  gas clouds
• Galaxies then are changed by interactions
• Galaxies grow gradually by galactic cannibalism
• Interactions disturb gas leading to starbursts
• Collisions can randomize stellar orbits leading
  to the formation of elliptical galaxies

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Review Questions

• How are elliptical versus spiral versus irregular
  galaxies formed?
• How do the star formation histories of elliptical
  versus spiral galaxies differ?
• Why do galaxy interactions tend to cause star
  formation?
• Was the population of galaxies different in the
  past?