Transcript September 3 and 5 slides

Darkness at Night (Olbers’ Paradox)
Imagine you are a pre20th 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).
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
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)
Plane of Milky Way
Sun is about 15 kpc from the center, and the whole
distribution is about 90 kpc in diameter
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