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 = n0

radius of shell = r

thickness of shell = dr

Each shell contributes the same amount of flux, but the universe is infinite in extent (and infinitely old, with no stellar evolution).

A “forest” of finite-sized stars

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.

Q: Is the horizon a physical object?

Long optical exposure 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 3-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).

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 at various distances.

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

Are these amongst the most distant galaxies in the universe at the present day?

William Herschel’s 1785 map of the Galaxy from star counts (The Amoeba Universe)

Fig. 4 from “On the Construction of the Heavens” by William Herschel, published in Philosophical Transactions of the Royal Society of London, vol. 75 (1785)

The “Kapteyn Universe” (1922), based on star counts and no accounting for dust obscuration

Globular Star Cluster M80

Shapley’s distribution of globular star clusters (1918)

Sun is about 15 kpc from the center, and the whole distribution is about 90 kpc in diameter

Plane of Milky Way

The Great Debate / The Shapley-Curtis Debate

• April 26, 1920 at National Academy of Sciences

• What are the distances to the spirals?

• Are the spirals composed of gas or stars?

• Why do the spirals avoid the plane of the Milky Way?

• Harlow Shapley (Mt. Wilson Observatory; “establishment”) and Heber Curtis (Lick Observatory; “youngster”)

• Curtis argued for the spirals being “island universes”

• He who had seemingly the best arguments turned out to be wrong

Distances to Spirals?

• Shapley argued that they must be close for two reasons: (1) measurements of proper motions in M101 (Adrian von Maanen) lead to ludicrous rotation speeds if M101 were as big as the Milky Way; (2) comparison of brightness of SAndromedae in M31 with Nova Persei in the Milky Way

• Curtis argued for large distances because: (1) proper motion measurements might be in error, (2) comparison of novae in M31 with novae in the Milky Way

• Difference between novae and supernovae not known observationally or theoretically at the time

• Hubble & von Maanen ultimately unable to reproduce the proper motion measurements

M101, the Pinwheel Galaxy

Stars or Gas? (Shapley)

• If they are galaxies like the Milky Way, they ought to have photometric and spectral properties like those of the Milky Way

• Based on star counts, “surface brightness” (= flux/unit area) of the MW in the solar neighborhood is much less than in the spiral nebulae

• Spirals are bluer at larger radii and absorption lines characteristic of stars hard to detect in the central regions

• Spirals were apparently quite different from what was known about the photometric properties of the MW at the time; Curtis didn’t have an adequate response

Avoidance of the Milky Way?

• Slipher had shown that the spirals were receding from the MW, and combined with the “Zone of Avoidance” led Shapley to propose that the MW exerted a peculiar (and hitherto unknown) repulsive force on the spirals

• Curtis noted that many of the spirals had dark, thick bands of obscuring material and gave 3 big “ifs”: (1) if the MW has such a band, (2) if we are located in the mid-plane of the band, and (3) if the spirals are located outside the MW, then the Zone of Avoidance is caused by the obscuring material simply blocking them from our view

NGC 4565