Announcements

1
Announcements

• Abstract of project due Feb 9
• 1st midterm Feb 12
• If youre interested in a group project, but are
  still in need for collaborators see me after
  class.

2
Tutoring

• Location Steward room 208
• Tutors
• Ariane Lee
• Mon, Wed, Fri 1015-1115
• Thu 130-330
• Andrea Urban
• Tue, Thu 1130-130
• Shanna Shaked
• Mon, Wed 400-500
• Tue 200-500

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Lecture 10History of Cosmology IV Determining
the Size and Age of the Universe
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Size of the Universe I

• Size of the Earth
• radius 6370 km
• Eratosthenes (200 B.C.)
• Size of the solar system
• several billion km
• rough idea Aristarchus (250 B.C.)
• detailed layout 1750

5
Size of the Universe II

• Distance to the stars
• until 1838 far away
• Bessel (1838) measured the first parallax of a
  star (61 Cygni). Result 0.3
• So how far is 61 Cygni ? Recall d D ? q
• d diameter of Earths orbit (149.7 million km)
• D distance of 61 Cygni
• q parallax (0.3)

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Distance of 61 Cygni

• So lets plug in numbers ...
• But dont forget to transform angles into radians
  !!!
• 0.3 0.3/3600 8.3?10-5 º
• into radians 8.3?10-5 º ? ?/180 1.45 ?10-6
• put into formula D 149.7 ?106 km/1.45 ?10-6
  ? 1014 km
• for comparison 1 light year (LY) 1 LY 300
  000 km/s ? 86400 s/d ? 365 d/yr 9.5 ?1012 km

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Astronomers favorite length unit
1 parsec (1pc) is the distance that produces a
parallax shift of 1 or 1 parsec (1pc) is the
distance under which the radius of the Earths
orbit around the Sun spans an angle of 1

• Distance in pc 1/parallax in
• 1 pc 3.26 LY

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Shape and Size of the Milky Way

• 1600 Galileo MW collection of stars
• 1750 Immanuel Kant, Thomas WrightMW is a disk
• 1780 Herschel counted stars in 700 fields
  around the sky MW is flattened 41, Sun is near
  the centerbut is it ?

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Size of the Milky Way

• Kapteyn (1920)
• measures distances to stars in the MW
• conclusion
• MW about 5 kpc across
• Sun near the center

• Shapley (1920)
• measured distances to globular clusters
• conclusion
• MW about 100 kpc across
• Sun 20 kpc off center

Solution ???
10
Nature of spiral nebulae ?

• Curtis
• MW is 10 kpc across
• Sun near center
• spiral nebulae were other galaxies
• high recession speed
• apparent sizes of nebulae
• did not believe van Maanens measurement
• ? Milky Way one galaxy among many others

• Shapley
• MW is 100 kpc across
• Sun off center
• spiral nebulae part of the Galaxy
• apparent brightness of nova in the Andromeda
  galaxy
• measured rotation of spirals (via proper motion)
  by van Maanen
• ? Milky Way Universe

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Solution I

• Role of dust
• obscuration Kapteyn/Curtis could only see a
  small fraction of the Milky Way disk
• dimming stars appear to be dimmer ? Shapley,
  ignoring dust, concluded that globular clusters
  are farther away than they actually are.
• ? Milky Way is 30 kpc across, Sun is 8.5 kpc off
  center.

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Solution II

• Van Maanens observation (rotation of spiral
  nebulae) turned out to be wrong.
• There is a difference between novae and
  supernovae, supernovae are much brighter?
  Andromeda is farther away than anticipated by
  Shapley
• ? spiral nebulae are galaxies like the Milky Way.
  Distance millions of parsec.

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Age of the Earth I

• Before 1670 little attention, but common
  perception that the Earth is young
• 1669 Nicolaus Steno older rocks below, younger
  rocks above. Layering of rocks ? age sequence
• 1800 Realization that Earth may be very old
• 1858 Wallace and Darwin Evolution of species ?
  Earth must be very old (hundreds of million of
  years)

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Age of the Earth II

• Problem in the 19 century, the Sun was believed
  to be only 100 million years old (it would run
  out of fuel otherwise)
• Solution nuclear fusion (Eddington-Bethe-Weizsäck
  er 1930s)
• Today radioactive dating of rocks ? Earth (and
  solar system) is 4.6 billion years old
• later in this class age of the universe 14
  billion years

16
Announcements

• Abstract of project due Feb 9
• 1st midterm Feb 12
• If youre interested in a group project, but are
  still in need for collaborators see me after
  class.