What is the overall structure of the universe?
Where are we in it?

• To get an idea of what the universe is made of at a large scale, we can look at a small patch of the sky in an image made with the Hubble Space Telescope. The patch has the size of a 1 cm square at a distance of 10 m from your eye.
• Here is the image. The universe is made of galaxies.
• A galaxy contains lots of stars. A typical number of stars in a fairly large galaxy is 10¹¹, a typical diameter is 100000 ly = 100 kly.
• One kind of galaxy that is rather common is the spiral galaxy. Here is M100.
• The evidence indicates that we live in one of these.
• You can see the evidence for yourself.

How obvious is this picture of the Milky Way?

• It is not immediately obvious that the Milky Way is of any great significance.
  • We see lots of stars.
  • We see this milky thing. Some sort of a cloud perhaps?
• Around 1600, Galileo Galilei (Venice, Florence) first looked at the sky with a telescope.
• One of the things he found was what we see as a milky band is really lots of stars.
  There is gas and dust too, as we will see.
• This observation shows (suggests?) that we are inside of a vast array of stars in a pancake shape.

How obvious it that the Milky Way is like other galaxies?

• This is not obvious at all.
• There are lots of ``nebulae'': cloud-like things.
  • The Nebula in Orion (M42).
  • A planetary nebula (NGC6543).
• 1770's: Charles Messier's catalogue
  • About 100 diffuse objects.
  • These objects should not be confused with comets.
  • We still use these Messier numbers. Eg. M42, M100 that we looked at.
• Immanuel Kant (1724-1804) proposed that these objects, or some of them, were ``island universes'' by analogy with our Milky Way.
  This speculation was mostly rejected.
• 1888 New General Catalogue of non-star objects collected.
  • eg. NGC 1201
  • Also Index Catalogue, eg IC 434.
• 1845 Lord Rosse (Ireland) builds giant telescope.
  • Sees spiral structure of some of the nebulae.
• Where are the ``spiral nebulae''?
  • Shapley and Curtis debated this question in 1920.
  • Shapley: They are inside our galaxy
    • Proper motion had been reported for some. (Wrong information.)
    • A nova (suddenly brightening star) in M31 appeared bright. (Too bad -- it was a supernova not an ordinary nova).
  • Curtis: They are outside our galaxy.
    • He advanced some fairly technical arguments that we will not discuss.
    • He noted a zone of avoidance: the spiral nebulae are mostly seen away from the plane of the Milky Way.
  • The question was not settled.
  • To proceed, we need to know about light years.
  • Edwin Hubble in 1924 measured the distance to M31.
    • M31: the nearest large galaxy, known the galaxy in Andromeda. (A nice sight with binoculars or a small telescope.)
    • He found it to be 0.5 Mly. (= 0.5 x 10⁶ ly).
    • Actually, it is 2.2 Mly.
    • Compare to the size of the Milky Way:
      • about 0.1 Mly in diameter
      • about 0.002 Mly = 2 kly thick. (1 kly =10³ ly).
    • This showed that M31 must be outside of our galaxy. Curtis was right.
• Evidently, measurements of distance are important. We will study how to find distances in some detail.
  Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu