One of the longest optical exposures of single region of the sky (11.3 day exposure in multiple bands) Area is about 10% of area of full moon (0.02 sq. degrees) ~10,000 galaxies in the image = at least 20 billion galaxies in the observable universe This is a 2-d image of 4-d spacetime!!

The Universe as a Time Machine When you look up at the sky tonight, you will see light from the stars that is arriving at Earth tonight. The farther an object is from Earth, the longer it has taken the light to arrive at your eyes (and the farther back in time you are seeing when you look at the more distant object). The Big Dipper is visible in the Northern Sky every night, all night long

Dubhe Merak Phecda Megrez Alioth Mizar Alkaid The light that you see tonight from Dubhe left the star before your Grandmother was born! It also left Dubhe 45 years before the light that you see tonight from Merak left Merak. You can divide the sky into snapshots in time by looking at objects that are all at the same distance from you.

Each postage stamp is roughly the size of the Milky Way; on-going mergers in very distant (=young) galaxies

Darkness at Night (Olbers’ Paradox) Imagine you are a pre- 20th century astronomer. How many stars would you expect to see (under some simple assumptions about the universe)?

Even Kepler was worried… “If this is true, and if they are suns having the same nature as our sun, why do not these suns collectively outdistance our sun in brilliance?” From Conversation with the Starry Messenger (1610)

How much flux does each thin spherical shell contribute to the total brightness of the sky? # density of stars = n 0 radius of shell = r thickness of shell = dr You’ll work this out on HW#1 (already posted on AS413 website)

A “forest” of finite-sized stars