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Pre-lab 12: Galaxies and the Expansion of the Universe
So far we have talked about our solar system and the local galactic neighborhood. In the next
few labs we will begin discussing the entire universe at a glance.
First, let us get an idea of how the individual stars in a typical galaxy are laid out. As we learned
from previous labs, stars themselves are very far apart compared to their size. The sun has a
diameter of 1.4 x 106 km. The distance to the nearest star is about 4 x 1013 km.
Calculate how many sun diameters make up the distance to the nearest star (it should be a
really big number!).
Our galaxy, the Milky Way, is 100,000 light years across. The Andromeda galaxy is the closest
spiral galaxy to us. It can be seen as a faint blob on a dark night in the constellation
Andromeda. It is located about 3 million light years away.
Clearly, even with units as big as a light year (1013 kilometers), expressing the distances between
galaxies becomes buried in unfathomably big numbers. So again we turn to a larger unit of
measure. For distances across the universe, astronomers often measure in Megaparsecs. A
Megaparsec is 1 million (106) Parsecs, and a Parsec is 3.26 light years. To get a better sense of
scale of the layout of our local group, calculate the following in Megaparsecs:
The diameter of the Milky Way:
The distance from here to Andromeda:
(hint: both numbers should be less than 1)
Calculate how many Milky Way diameters there are between the two galaxies. How does it
compare to the previously calculated number for the sun and its closest star? Based on this
calculation, why do you think that stars seldom collide, but galaxies collisions are common?
How big is the universe in total? For reasons we will discuss in the lab, it is a tricky question with
no easy answer. Some estimates have it as big as 32,000 Megaparsecs in diameter. Some
estimates have it even bigger. Some even think it goes on forever. However, there is a limit to
how far out we can see and interact with, due to the finite speed of light.