Download Unit 1

Units to cover: 72,73,74, 75, 82, 84
Model of AGN
Depends on our observational
geometry. Superluminal motions are
possible.
The Origin of Helium
•
•
•
•
•
Immediately after the Big
Bang, only protons and
electrons existed
Shortly after the BB,
temperature and density was
high enough for deuterium to
form by fusion
After 100 seconds or so,
temperature cooled enough so
that deuterium could fuse into
helium nuclei
The temperature continued to
cool, and fusion stopped after a
few minutes.
Big Bang theory predicts that
around 24% of the matter in the
early universe was helium,
which matches what we see.
The Epoch of Inflation
• Modern technology allows
us to test theories back to a
time 10-33 seconds after the
Universe Birth (UB).
• Physics as we know it
ceases to function at 10-43
seconds after the UB, called
the Plank Time
• Using particle colliders,
scientists have uncovered a
number of clues about what
happened in the early
universe, after the Plank
time
• The early universe
underwent a period of very
rapid expansion
• By 10-33 seconds, the universe expanded from
the size of a proton to the size of a basketball
• This expansion is called inflation
Expansion Forever? Or Collapse?
• The fate of the universe is ultimately
controlled by its total amount of
energy
– Energy of expansion (positive)
– Gravitational energy that can slow the
expansion (negative)
– Binding energy
• If the total energy is positive or
zero, the expansion continues
forever
• If the total energy is negative, the
expansion will halt, and the
universe will contract and
eventually collapse.
Dark Energy
• Dark energy may provide the solution
to the mystery
• Dark energy remains constant
everywhere, regardless of the
universe’s expansion
• Provides an outward push to accelerate
expansion
• Dark energy must make up
around 70% of all of the
energy in the universe
• Much work remains to be
done on this frontier…
The Formation of the Milky Way
•
•
Our galaxy likely began 13 billion years
ago as a huge cloud of pure hydrogen and
helium, slowly rotating and collapsing
The first stars formed within this cloud,
burning out quickly and violently. This
added heavy elements to the cloud
•
•
Population II stars formed next, capturing
some of the heavy elements and settling
into elliptical orbits around the center of
the cloud
As the collapse continued, a disk formed,
and Population I stars formed from the
ashes of dying Pop I stars
Galactic Cannibalism
• There are a few observations that
are not explained by this model
– Some stars follow unusual orbits
in the galaxy
– Not all Pop II stars are the same
age
– Model predicts that the first stars
might not have been very massive,
and should still be around!
• Galactic cannibalism provides
some answers
– The Milky Way may be absorbing
another galaxy!
– Observations show streams of
stars coming from our galaxy’s
“victim”.
Composition of Interstellar Clouds
•
•
•
Light passing through an
interstellar cloud can hold
clues as to the cloud’s
composition
Atoms in the cloud absorb
specific frequencies of
starlight passing through,
creating absorption lines
Astronomers can analyze
these spectra to determine
what the clouds are made of.
• Spectra show that interstellar gas clouds are made
of mostly hydrogen and helium, just like the Sun
• Dust particles do not absorb light the same way
that gas atoms do, but using similar methods tells
us that the dust is made of silicates
Interstellar Reddening
•
•
As starlight passes through a dust cloud,
the dust particles scatter blue photons,
allowing red photons to pass through
easily
The star appears red (reddening) – it
looks older and dimmer (extinction) than
it really is.
The Galactic Center and Edge
• Despite the appearance of being
closely spaced, stars in the Milky
Way are very far apart
– At the Sun’s distance from the
center, stellar density is around 1
star per 10 cubic parsecs
• Density is much higher at the core
– Exceeds 100,000 stars per cubic
parsec!
• X-ray and gamma ray telescopes
reveal a supermassive black hole at the
Milky Way’s core
– Called Sag A*
– 5 million solar masses!
Sag A*
Star Formation in Spiral Arms
A History of Galactic Discovery
• In the early 20th century, the existence
of other galaxies was unknown
– The Milky way was the Universe!
– Other galaxies were called nebulae
• Light from galaxies always appears
fuzzy and diffuse, due to the vast
separation between the Sun and the
observed galaxy, as well as the
separation between the stars of that
galaxy!
– The paleness of visible light from distant
galaxies is called the surface brightness.
• Galaxies are therefore difficult to
observe, even with good telescopes.
More History…
• In the 1700’s, Charles Messier was
observing comets, and kept finding objects
that while fuzzy, were not comets
– He made a list (or catalog) of these undesired
objects, so he could avoid seeing them
– They became known as Messier Objects, a
number preceded by an M.
– M31 (the Andromeda galaxy) is one such object
• William and Caroline Herschel (1800’s)
developed a catalog of faint objects in the
heavens
– Now known as the New General Catalog
– Objects are known by a number preceded by the
letters NGC
– Objects can appear in both the Messier and
NGC catalogs!
M31
A Sky Full of Galaxies
• Technology has
advanced to the
point where we
have found as many
galaxies as there are
stars in the Milky
Way!
• Note the gap
running along the
zero latitude line
– Called the zone of
avoidance
– Puzzled
astronomers!
The Zone of Avoidance
Dust and the center of our own galaxy merely blocks our view –
there is no zone of avoidance!
Distances to other galaxies
•
•
•
•
We can use Cepheid variable stars
to measure the distance to other
galaxies
A Cepheid’s luminosity is
proportional to its period, so if we
know how rapidly it brightens and
dims, we know much energy it
emits
If we see a Cepheid in another
galaxy, we measure its period,
determine its luminosity, and
calculate its distance!
Distance between galaxies is huge!
– M100 is 17 million parsecs away.
The Sun’s position in the galaxy is
•
•
•
•
A. unknown
B. in the disk of the galaxy
C. in the spherical halo of the galaxy
D. in the galactic nucleus
Spiral Galaxies
• Spiral arms and a central bulge
• Type S
Elliptical Galaxies
•
•
•
•
No spiral arms
Ellipsoidal shape
Smooth, featureless appearance
Type E
Irregular Galaxies
• Stars and gas clouds scattered
in random patches
• No particular shape
• Type Irr
Galaxy collision and merger
The Mice
• These two interacting galaxies are tidally distorting
each other.
Which two quantities are shown to be related to one
another in Hubble Law?
•
•
•
•
A. distance and brightness
B. distance and recession velocity
C. brightness and recession velocity
D. brightness and dust content
How are galaxies spread through the Universe?
• A. They are grouped into clusters that in turn are
grouped into clusters of clusters (superclusters)
• B. Galaxies are spread more or less evenly throughout
the Universe
• C. They are grouped around our galaxy
• D. none of the above