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
- 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
- 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
- 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
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.
- 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
- 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