Transcript A Universe of Galaxies - Pennsylvania State University

Galaxy Classification
The Hubble Tuning-Fork Diagram
This is the traditional scheme for classifying galaxies:
Elliptical Galaxies
Elliptical galaxies are spherical in shape. They contain very
little gas and dust, no star formation, and only older stars. They
resemble globular clusters, except they are much larger.
Sa and SBa Galaxies
Sa and SBa galaxies have a large spherical distributions of stars
(called bulges) and a smooth disk with only a trace of spiral arms.
Sa (no bar)
Sa and SBa Galaxies
Sa and SBa galaxies have a large spherical distributions of stars
(called bulges) and a smooth disk with only a trace of spiral arms.
SBa (bar)
Sb and SBb Galaxies
In Sb and SBb galaxies, the bulge and disk components
are comparable in dominance. Spiral structure is visible in
the disk.
Sb
Sb and SBb Galaxies
In Sb and SBb galaxies, the bulge and disk components
are comparable in dominance. Spiral structure is visible in
the disk.
SBb
Sc and SBc Galaxies
Sc and SBc galaxies have little or no bulge and a large disk.
Sc
Sc and SBc Galaxies
Sc and SBc galaxies have little or no bulge and a large disk.
SBc
Irregular Galaxies
Irregular galaxies are do not have an ordered structure, and
instead have random, messy shapes. They have lots of dust,
gas, and young stars.
The Hubble Tuning-Fork Diagram
Active Galaxies
The light from most galaxies is just the sum of light from all of
the stars within it, so like starlight, a galaxy’s light is brightest
at optical wavelengths and fainter at shorter and longer
wavelengths.
Active Galaxies
But a small fraction of galaxies are different; they are much
brighter and produce more long- and short- wavelength
emission. They are called active galaxies.
The first active galaxies: Quasars
In 1962, a radio survey of the entire sky revealed a few peculiar
“stars” that were very bright at radio wavelengths (normal stars
do not produce much radio emission). They were named quasistellar radio sources (quasars, or QSOs):
The first active galaxies: Quasars
In 1962, a radio survey of the entire sky revealed a few peculiar
“stars” that were very bright at radio wavelengths (normal stars
do not produce much radio emission). They were named quasistellar radio sources (quasars, or QSOs):
Instead of this…
…Quasars look like single
points of light
Quasars: Fast, distant, and very bright
Quasars have enormous redshifts, indicating that they are moving
away from us at more than 90% of the speed of light. Stars in the
Milky Way cannot move that fast. The only way to achieve such a
high speed is if they are incredibly far away. They are therefore
incredibly bright – as bright as 1000 supernovae, and much brighter
than galaxies like the Milky Way.
Quasars: brightness varies rapidly
The brightness of
quasars varies rapidly,
on timescales of just
days and months.
Size of a Quasar’s Energy Source
Since many quasars vary in
brightness we have a crude
way to estimate their size.
• Imagine that there is some
mechanism near the center of
the quasar that controls the
object’s brightness. It says
“get bright”.
• That command goes forth no
faster than the speed of light.
• Within a few months, the
object gets bright.
• Since no signal can go faster
than the speed of light, the
object must be no bigger than
a few light-months across.
A Quasar’s Energy Source
What can outshine ~1000 supernovae for millions of years, and
be just slightly larger than our Solar System? Theoretically, not
much – only a very, very big black hole.
• Start with a black hole with a mass of 10,000,000,000 Mʘ
• Have a star come close enough to be tidally disrupted
• Have the material form into an accretion disk. Energy is
released via the friction in the disk. If you accrete ~ 1 Mʘ
per year, the friction you get will produce the luminosity
of a quasar.
Feeding the Monster
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If a star comes too close, the enormous gravity of the black
hole will cause tides on the star and rip it apart. Some of that
material will be trapped in orbit about the hole.
Explaining a Quasar’s Properties
• Because of the high speed of the gas, there is a lot of friction in
the disk. This is why quasars are very bright.
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• The temperature of the disk depends on the speed of the gas.
Near the event horizon, the friction produces x-rays. At larger
radii, where the gas revolves more slowly, optical and infrared
light is made. This is why quasars are bright at all wavelengths.
Black Holes and Jets
As matter accretes onto the black hole, particles can get ejected out
the poles of the system at 99.999% of the speed of light. How this
occurs is almost a complete mystery. But it’s often observed.
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Black Holes and Jets
As matter accretes onto the black hole, particles can get ejected out
the poles of the system at 99.999% of the speed of light. How this
occurs is almost a complete mystery. But it’s often observed.
Where are the Quasars Today?
The nearest quasar is 25% of the way across the universe; most
belong to an era when the universe was only 15% of its present
age. If supermassive black holes existed then, where they now?
In the
centers of
galaxies!
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How to Make an Active Galaxy
When a supermassive black hole is not accreting, it is invisible, and the
galaxy is “normal”. But when it is accreting, it is thousands of times
brighter than its surrounding galaxy, and the galaxy is “active”.
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How to Make an Active Galaxy
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How to Make an Active Galaxy
How to Make an Active Galaxy
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How to Make an Active Galaxy
The Milky Way’s Sleeping Monster
There’s even a 2,000,000 Mʘ black hole at the center of the
Milky Way. We can measure its mass by the motions of stars
which pass close to it.
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