1. Area is about 10% of area of full moon (0.02 sq. degrees)
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!!
Completed in January 2004; Current record holder for depth is the Hubble eXtreme Deep Field (XDF) released in 2012 – exposure of 23 days and is a small part of the center of the Ultra Deep Field

2. 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

3. Alkaid
Mizar
Alioth
Megrez
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.
Phecda
Dubhe
You can divide the sky into snapshots in time by looking at objects that are all at the same distance from you.
Merak

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

7. 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)?

8. 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)

9. How much flux does each thin spherical shell contribute to the total brightness of the sky?
# density of stars = n0
radius of shell = r
thickness of shell = dr

10. A “forest” of finite-sized stars
To figure out how much flux you should observe, consider what the flux would be if one star alone filled your entire field of view (i.e., you’re positioned at the photosphere of a star)

11. How Not to Solve Olbers’ Paradox
Embed the stars in gas (it will heat up and glow)
Embed the stars in dust (you simply move the problem out of the visible part of the spectrum to the IR instead)
Replace the stars with galaxies and call on redshift (again, you simply move the problem out of the visible part of the spectrum)

12. Resolution of Olbers’ Paradox in Static Universe
Can’t see the most distant stars because the light has not had time to reach us (universe is not infinitely old).
The edge of the observable universe is the “horizon”. The distance to the horizon changes constantly.