Download 23.cosmology-student

• Our goals for learning
• How did Hubble prove galaxies lie beyond our
galaxy?
• How do we observe the life histories of galaxies?
• How did galaxies form?
• Why do galaxies differ?
How did Hubble prove that
galaxies lie beyond our galaxy?
The Puzzle of “Spiral Nebulae”
Before Hubble, some scientists argued that
“spiral nebulae” were entire galaxies like our
Milky Way, others maintained they were
smaller collections of stars within the Milky
Way
Hubble settled the debate by measuring the
distance to the Andromeda Galaxy using
Cepheid variables as standard candles
Cepheid Variables: A Pulsating Star
The star's atmosphere ionizes, temporarily trapping heat energy
The star temporarily gets larger, increaseing its luminosity
Pulsating Variable Stars
•
Pulse period is directly linked to luminosity
•
Stars that get very much brighter, take longer to do
so.
Cepheid Variables
• This means pulse rate can be used to calculate
luminosity -without distance.
• Comparing the calculated luminosity, with the
measured apparent brightness allows us to calculate
distance to the star
Cepheid Variables on Distance Ladder
Cepheid variables act as standard candles that
can measure distances to globular clusters and
nearby galaxies.
Thought Question
Which of these could not be used as a standard
candle for finding distance?
A. a G2 main sequence star
B. a massive star supernova
C. a cepheid variable with a measured period
D. a white dwarf supernova
Galaxies and Cosmology
• A galaxy’s age, its
distance, and the age of
the universe are all
closely related
• The study of galaxies is
thus intimately connected
with cosmology— the
study of the structure and
evolution of the universe
How do we observe the life
histories of galaxies?
Deep
observations
show us very
distant galaxies
as they were
much earlier in
time
(Old light from
young galaxies)
How did galaxies form?
We still can’t directly observe the earliest galaxies
Our best models for
galaxy formation
assume:
• Matter originally
filled all of space
almost uniformly
• Gravity of denser
regions pulled in
surrounding
matter
Denser regions
contracted, forming
protogalactic clouds
H and He gases in these
clouds formed the first
stars
These oldest stars form
the halo of a galaxy
Supernova explosions
from first stars kept
much of the gas from
forming stars
Leftover gas settled
into spinning disk
Conservation of
angular momentum
Stars continuously form in disk as galaxy grows older
What have we learned?
How did Hubble prove that galaxies lie far beyond
the Milky Way?
He measured the distance to the Andromeda galaxy
using Cepheid variable stars as standard candles
How do we observe the life histories of galaxies?
Deep observations of the universe are showing us the
history of galaxies because we are seeing galaxies as
they were at different ages
What have we learned?
• How did galaxies form?
– Our best models for galaxy formation assume
that gravity made galaxies out of regions of the
early universe that were slightly denser than
their surroundings
– Galaxies initially form from a huge cloud of
gas, with the halo stars forming first and the
disk stars forming later, after the gas settled
into a spinning disk (overly simplified model!)
Why do galaxies differ?
NGC 4414
M87
But why do some galaxies end up looking so different?
Conditions in Protogalactic Cloud?
Spin: Initial angular momentum of protogalactic cloud
could determine size of resulting disk
Conditions in Protogalactic Cloud?
Density: Elliptical galaxies could come from dense
protogalactic clouds that were able to cool and form
stars before gas settled into a disk
We must also consider the
effects of collisions
Galaxy-galaxy interactions
can create huge distortions.
Galaxies can merge and be
cannibalized.
The collisions we observe in
nearby galaxies, trigger
bursts of star formation
Collisions were much more likely early in time, because
galaxies were closer together
Many of the galaxies we see at great distances (and early times)
indeed look violently disturbed
The collisions we observe nearby trigger bursts of star
formation
Modeling such collisions on a computer shows that two spiral
galaxies can merge to make an elliptical
Giant elliptical
galaxies at the
centers of
clusters seem to
have consumed a
number of
smaller galaxies
• Our Milky Way probably formed
by the merger of many smaller
protogalaxies in a dark matter
“cloud.”
• Several of these are still orbiting the
Milky Way as satellite galaxies.
What have we learned?
• Why do galaxies differ?
– Some of the differences between galaxies may
arise from the conditions in their protogalactic
clouds
– Collisions can play a major role because they
can transform two spiral galaxies into an
elliptical galaxy
Quasars and other Active Galactic Nuclei
• Our goals for learning
• What are Active Galaxies and Quasars?
• What is the power source for quasars and other
active galactic nuclei?
• Do supermassive black holes really exist?
What are Active Galaxies and Quasars?
Characteristics of Active Galaxies
• Luminosity can be enormous (>1012 LSun)
• Luminosity can rapidly vary (comes from a
space smaller than solar system)
• Emit energy over a wide range of wavelengths
(contain matter with wide temperature range)
• Some drive jets of plasma at near light speed
If the center of a
galaxy is unusually
bright we call it an
active galactic
nucleus
Quasars are the
most luminous
examples
Active Nucleus in M87
Radio galaxies contain active nuclei shooting out vast jets of
plasma that emits radio waves coming from electrons moving at
near light speed.
The jets extend over hundreds of millions of light years
What is the power source for quasars
and other active galactic nuclei?
An active galactic
nucleus can shoot out
blobs of plasma
moving at nearly the
speed of light
Plasma is accelerated
around a small but
massive object, then
sling-shotted out.
Speed of ejection
suggests that a black
hole is present
Accretion of gas onto a supermassive black hole appears to be the
only way to explain all the properties of quasars
 Jets of material shoot out from the poles of the system.
 A dense ring (torus) of dust blocks the center.
 What we see (quasar or radio galaxy) depends on the
viewing angle.
 The orbital speeds of gas near the black hole
yield its mass.
 Supermassive black holes probably exist at the
centers of all galaxies.
 Normal galactic nuclei do not contain accretion
disks.
 Material in the accretion disk is an AGN’s
source of fuel. Without it, the black hole can
only be found by gravitational effects.
Orbital speed and distance of gas orbiting center of M87
indicate a black hole with mass of 3 billion MSun
What About Our Galaxy?
Radio emission from center
Swirling gas near center
X-ray flares from our galactic center
suggest that tidal forces of suspected
black hole occasionally tear apart
chunks of matter about to fall in.
Orbits of stars at center of Milky Way
stars indicate a black hole with mass
of 4 million MSun
Black Holes in Galaxies
• Many nearby galaxies – perhaps all of them – have
supermassive black holes at their centers
• These black holes seem to be dormant active galactic
nuclei
• All galaxies may have passed through a quasar-like
stage earlier in time
Galaxies and Black Holes
• Mass of a
galaxy’s
central
black hole is
closely
related to
mass of its
bulge
Galaxies and Black Holes
• Development
of central
black hole
must be
somehow
related to
galaxy
evolution
What have we learned?
• What are active galaxies and quasars?
– Active galactic nuclei are very bright objects seen in the
centers of some galaxies, quasars are the most
luminous
• What is the power source for quasars and other
active galactic nuclei?
– The only model that adequately explains the
observations holds that supermassive black holes are
the power source
What have we learned?
• Do supermassive black holes really exist?
– We can see particle jets, accretion disks and the rapid
orbit of stars all around the center of a galaxy.
– All of these indicate a tiny super-massive object is there.
The only possible object that fits the evidence is a supermassive back hole.
– Observations of stars and gas clouds orbiting at the
centers of galaxies indicate that many galaxies, and
perhaps all of them, have supermassive black holes