Why is Black Hole Black

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Why is Black Hole Black?

• Because even light can not escape the black hole!
• Escape Velocity
• The minimum velocity required to leave an
  objects surface and never returns
• G Newtons gravitational constant
• M mass of the object
• R radius of the object

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Escape Velocity
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Light in a Black Hole
Escape velocity increases as Mass is higher or
size is smaller
No light can escape ? Black Hole! R(black hole)
2GM/c2
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Size of a Black Hole

• Spacetime is so highly warped around a black
  hole, even light can not escape.
• Schwarzchild Radius the radius where the
  escape velocity equals the speed of light.
• R 2GM/c2 ?
• Rs 3 M (Rs in km M in M?)
• A sphere of radius Rs around the black hole is
  called the event horizon.

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Finding Black Holes

• We can see the effect that a black hole has on
  its stellar companion in an binary
• Cygnus X-1 was the first good candidate for a
  black hole - a blue supergiant that is rotating
  rapidly around an unseen companion
• Keplers 3rd Law gives a mass gt 3 M? for unseen
  companion
• it can not be a neutron star
• the only thing that massive, yet small enough to
  be invisible is a black hole
• We will discuss supermassive
• BH later in the class

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What have we learned?

• What is a black hole?
• A place where gravity has crushed matter into
  oblivion, creating a true hole in the universe
  from which nothing can ever escape.
• What property of a black hole determines its
  size?
• A black holes size depends on its mass,
  because the mass determines the size of the black
  holes event horizon, the boundary of the region
  from which not even light can escape.
• What observational evidence is there for the
  existence of black holes?
• We cannot see black holes directly, but we can
  infer their presence by their influence on their
  surroundings. The most definitive evidence is
  obtained by measuring the orbit of the companion.
  If that object is more massive than 3 MSun, it is
  probably a black hole. Cygnus X-1 is one of the
  binary systems thought to contain a black hole

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Lecture 11. Our Galaxy

• Sep 25, 2007
• Chap 19.1, 19.2

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The Milky Way Revealed
Our goals for learning

• Describe the general structure of the Milky Way
  Galaxy.
• Where is the Sun located within our Galaxy?
• What are the basic components of the Milky Way
  Galaxy?
• Is there a Black Hole in the Galactic Center

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Regions of the Milky Way Galaxy
Distance 1 parsec (pc) 3.3 light year(ly)
diameter of disk 100,000 l.y. (30,000 pc)
radius of disk 50,000 l.y. (15,000 pc)
thickness of disk 1,000 l.y. (300 pc)
number of stars 200 billion
Sun is in disk, 28,000 l.y. out from center
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Regions of the Milky Way Galaxy

• Disk
• younger generation of stars
• contains gas and dust
• location of the open clusters
• Where spiral arms are located
• Bulge
• mixture of both young and old stars
• Halo
• older generation of stars
• contains no gas or dust
• location of the globular clusters

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The matter in our Galaxy emits different kinds of
radiation. They come from matter in different
phases
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What is the Milky Way Galaxy made of?

• Stars
• 200 billion stars
• Age from gt10 billion years to just formed
• Many stars are located in star clusters
• Interstellar Medium
• Stuff between stars
• Nebulae, molecular clouds, and mostly empty space
  in between
• Where stars are born from, and where diffuse
  remnants of stars are stored
• Galactic Center with a supermassive Black Hole?
• Dark Matter
• The total mass of the Milky Way far exceeds the
  mass with stars and interstellar medium put
  together
• Hidden, invisible or missing component

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Stars Halo vs. Disk

• Stars in the disk are relatively young.
• fraction of heavy elements same as or greater
  than the Sun
• plenty of high- and low-mass stars, blue and red
• Stars in the halo are old.
• fraction of heavy elements much less than the Sun
• mostly low-mass, red stars
• Stars in the halo must have formed early in the
  Milky Way Galaxys history.
• they formed at a time when few heavy elements
  existed
• there is no ISM in the halo
• star formation stopped long ago in the halo when
  all the gas flattened into the disk

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Star Clusters

• Thousands to millions of stars grouping together
• Stars in a cluster have the same age, but
  different masses
• Most stars were born and once belong to a cluster
• Two types of star clusters
• Open cluster young, sparse, has fewer stars, in
  disk and spiral arms
• Globular cluster old dense, has many stars, in
  Galactic halo or bulge.

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• Open cluster

• Globular Cluster

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The Interstellar Medium (ISM)

• It is the stuff between the stars.
• It is mostly a vacuum (1 atom cm-3).
• It is composed of 90 gas and 10 dust.
• gas individual atoms and molecules
• dust large grains made of heavier elements
• ISM is the place where stars were born and the
  place where stars eject most of their matter when
  dying.

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The different components of the ISM
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Ionization Nebulae

• Found around high-mass stars (OB associations)
• O B stars (T gt 25,000K) make enough UV photons
  to ionize hydrogen in the nebula
• Gas re-emits Hydrogen line (red)

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Reflection Nebulae

• Light from central star is reflected and
  scattered by dust
• Blue light is scattered more easily than red
• Similar to our blue sky lit up by a yellow Sun

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The StarGasStar Cycle
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The StarGasStar CyclePillars of Creation
Eagle Nebulas Pillars of Creation
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Supernova Remnants Tombs of Massive Stars
Crab Nebula Remnant of a supernova Exploded in
1054 and recorded By ancient Chinese.
Supernova 1987A Exploded in 1987 in the Large
Magellenic Cloud Observed with HST
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Center of the Galaxyin Sagittarius
Infrared
Visual
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Center of the Galaxy
Radio
Although dark in visual light, there are bright
radio, IR, and X-ray sources at the center of the
Galaxy, known as Sgr A.
X-ray
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Gemini 8-m telescope image of the Galactic Center
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Adaptive OpticsOvercoming the Turbulence of the
Atmosphere
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Motion of Stars near the Black Hole
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Center of the Galaxy

• We measure the orbits of fast-moving stars near
  the Galactic center.
• these measurements must be made in the infrared
• in particular, this star passed within 1
  light-day of Sgr A
• using Keplers Law, we infer a mass of 2.6
  million M? for Sgr A
• What can be so small, yet be so massive? Black
  Hole!
• This is the best direct evidence for a
  SUPERMASSIVE black hole
• Radius of Black Hole 3 x Mass 8 million km

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Caught in Action matter falls into Black Hole
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Dark Matter in the Milky Way Galaxy
Our goals for learning

• How long does it take the Sun to orbit the
  galactic center?
• How do we determine Galactic mass from stellar
  orbits?
• What is the significance of a rotation curve that
  is flat at large distances from the Galactic
  center?

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Stellar Orbits in the Galaxy

• Stars in the disk all orbit the Galactic center
• in the same direction
• in the same plane (like planets do)
• almost circular orbit
• they bobble up and down
• this is due to gravitational pull from the disk
• this gives the disk its thickness

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Stellar Orbits in the Galaxy

• Stars in the bulge and halo all orbit the
  Galactic center
• in different directions
• at various inclinations to the disk
• They have highly eccentric orbits
• Their motions are similar to comets in the solar
  system

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Mass of the Galaxy

• We can use Keplers Third Law to estimate the
  mass
• Suns distance from center 28,000 l.y. 1.75 x
  109 AU
• Suns orbital period 230 million years (2.3 x
  108 yr)
• P2 4?2/GM a3 ? mass within Suns orbit is
  1011 M?
• Total mass of MW Galaxy 1012 M?
• Total number of stars in MW Galaxy ? 2 x 1011

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Orbital Velocities in the Disk
rotation curve a plot of rotational (orbital)
speed vs. distance from the center
for a merry-go-round (solid object)
for our Solar System (Keplers Laws)
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How to use rotation to measure mass?
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Orbital Velocities in the Disk
We can measure the rotation curve of our Milky
Way Galaxy
Here is what we observe

• the rotation curve of the Milky Way galaxy is
  FLAT
• It is small in the center
• It increases sharply near the center when moving
  outward
• Then it becomes flat ? a flat rotation curve at
  large radius

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Mass in the Milky Way Galaxy
Stars in the Galactic disk should orbit according
to Keplers Laws
Here is how we measure the mass

• the rotation curve of the Milky Way galaxy is
  FLAT
• The total mass of the Milky Way is very large
• There is a lot of mass at large distance from the
  Galactic Center
• But we observe few stars out there whats going
  on?

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Importance of the Flat Rotation Curve

• The flat rotation curve of our Galaxy implies
  that
• its mass in not concentrated in the center
• its mass extends far out into the halo
• Do these all make sense?
• Total mass of MW Galaxy 1012 M?
• Total number of stars in MW Galaxy ? 2 x 1011
• Most mass in the Milky Way are not in stars,
  missing!!!
• We do not see this mass
• we do not detect light from most of this mass
• so we refer to it as dark matter
• This is the first time in this class that we are
  going to see the evidence for dark matter, more
  later.

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What have we learned?

• Describe the general structure of the Milky Way
  Galaxy.
• The Milky Way Galaxy consists of a thin disk
  about 100,000 light-years in diameter with a
  central bulge. The spherical region surrounding
  the entire disk is called the halo.
• Where is the Sun located within our galaxy?
• The Sun is located in the disk, about 28,000
  light years from the galactic center.
• Can we see through our galaxys interstellar
  medium?
• The gas and dust that make up the interstellar
  medium absorb visible light, preventing us from
  seeing most of the galaxys disk in these
  wavelengths. However, some other wavelengths of
  light, notably infrared and radio, can pass
  through the gas and dust, allowing us to study
  regions of the galaxy whose visible light is
  blocked.

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What have we learned?

• How do halo stars differ from disk stars?
• The halo generally contains only old, low-mass
  stars, while the disk is home to stars of all
  ages. In addition, halo stars have a much smaller
  proportion of heavy elements than stars in the
  disk.

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What have we learned?

• How do stellar orbits in the disk differ from
  those in the halo?
• Stars in the disk all orbit the galactic center
  in about the same plane and in the same
  direction. Halo stars also orbit the center of
  the galaxy, but with orbits randomly inclined to
  the disk of the galaxy.
• How long does it take the Sun to orbit the
  galactic center?
• Each orbit takes about 230 million years.
• How do we determine galactic mass from stellar
  orbits?
• By using a stars orbital speed and distance from
  the galactic center in the orbital velocity law
  (Mr  (r ? v2)/G), we can calculate the mass of
  the galaxy that lies within the region enclosed
  by the stars orbit.

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What have we learned?

• What is the significance of a rotation curve that
  is flat at large distances from the galactic
  center?
• The Milky Ways flat rotation curve implies that
  the matter associated with our galaxy extends to
  large distances from the center. A rotation
  curve is a plot of the orbital speed of stars or
  gas clouds against distance from the center of
  the galaxy. If most of the galaxys mass were
  concentrated toward the center, orbital speed
  would decline as distance from the center
  increased, as in the solar system. Because the
  rotation curve of the Milky Way is flat, orbital
  speed in the Milky Way does not decline at great
  distances. Thus, the Milky Ways mass is not
  concentrated toward the center but instead
  extends far into the halo. Because we do not
  detect light from all this mass in the halo, we
  call it dark matter.

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What have we learned?

• What lies in the center of our galaxy?
• Motions of stars near the center of our galaxy
  suggest that it contains a black hole about 2.6
  million times more massive than the Sun. The
  black hole appears to be powering a bright source
  of radio emission known as Sgr A.