Recap the Black Holes

1
Recap the Black Holes

• Describe the conditions for a stellar black hole
  to be formed
• Describe the tidal effects and time dilation near
  a black hole
• Describe the X-ray observations of a black hole
• Describe Hawkings proposal on white holes
• Is there any experimental support for this idea ?
• Discuss the galactic black holes

2
Among Galaxies

• From stars to galaxies
• The Milky Way
• Looking into the Remote Past
• Galactic Explosions and Quasars
• Galactic Black Holes

3
From Stars to Galaxies

• Billions of stars form galaxies, which for the
  old astronomy were the nebulae (some nebulae
  are actually supernova remnants).
• For hundreds of years stars and nebulae were the
  fixed constellations while the planets were the
  moving cosmic objects.
• Determining the distance to galaxies became
  possible only after 1923 by measuring the
  distance to specific stars inside those galaxies.
  
• The observations with non-optical telescopes
  uncovered a world of galaxies far more violent
  than the world of stars.
• Galaxies can have different shapes oval, S,
  spiral, iregular.

4
The Milky Way

• The Sun is a medium star in our galaxy, a medium
  size spiral galaxy known also as the Milky Way.
  We are in the Orion arm, slightly above the
  galactic plane.
• Harlow Shapley was the first astronomer who after
  1920 used measurements on stars and on the
  interstellar dust (hydrogen spectrum) to map the
  Milky Way.
• Our galaxy has a galactic black hole of around
  100,000 solar masses surrounded by an abundance
  of red giants and mini white dwarfs. They are the
  oldest stars in the galaxy.
• The outside of the arms of the galaxy contains
  new stars and star nurseries.
• C.C.Lin at MIT demonstrated that our galaxy
  evolved from oval to spiral shape through
  gravitational and electromagnetic effects.

5
Spiral Galaxies
Spiral Galaxies from above ? This is the M100
spiral galaxy, similar to our Milky Way.
Spiral Galaxies from a side
high density of stars
galactic plane
black hole
Stars or groups of stars
6
Other Galaxies

• The nearest galaxies are known as Magellans
  Clouds they are about 200,000 light-years from
  us.
• In the Northern hemisphere the nearest observable
  galaxy is Andromeda (M31) at 3 million
  light-years from us. The light from M31 was sent
  to us when Earth was populated by humanoids.
• To measure the distance
• to a close galaxy one has to identify a star from
  the Cepheid class, which always has the same
  intrinsic brightness.
• for galaxies which are billions of light-years
  away one has to look for more powerful sources of
  radiation as individual stars cannot be
  identified. Type Ia supernovas are such powerful
  sources.
• In an expanding Universe the further a galaxy is
  from us the faster it moves. A fast moving light
  source would have the optical spectrum shifted
  towards large wave lengths (known as red or
  Doppler shift).

7
Huge Numbers of Galaxies
The number of galaxies observed today is of the
order 1011. Many of these galaxies emitted the
light that we observe today long before the Sun
or Earth existed.
1500 galaxies seen by the Hubble telescope in a
certain part of the Universe.
8
Many Shapes and Sizes

• Observations found galaxies of various shapes
  elliptical, spiral, barred-spiral and irregular.
• Sizes vary from dwarfs (about 1 million solar
  masses) to giants (up to 10 trillion solar
  masses).
• Among bright galaxies about 70 are spiral and
  20 elliptical, but most dwarfs are elliptical
  and therefore of the total only about 20-30 are
  spiral.

9
Explaining Different Types

• Two factors determine the type of a galaxy
• Its rotation around the galactic nucleus
• The rate of stars formation
• Elliptical galaxies show little rotation and high
  rate of stars formation.
• Spiral galaxies have rotation which creates a
  disc shaped cloud of gas, with lower stars
  formation rate and with a lot of gas left for
  future generations of stars.

10
Intergalactic Interactions

• The shapes of galaxies are also influenced by
  their interaction with neighbors.
• the Cartwheel galaxy (next slide) and
• the Antennae pair of galaxies.
• Close encounters between galaxies are also
  believed to initiate the formation of spiral
  structures. However, the preservation of the arms
  is due to the stars formation and the
  gravitational density waves.

11
Cartwheel Galaxy (I)
12
Cartwheel Galaxy (II)

• A rare and spectacular head-on collision between
  two galaxies appears in this NASA Hubble Space
  Telescope true-color image of the Cartwheel
  Galaxy, located 500 million light-years away in
  the constellation Sculptor.
• The striking ring-like feature is a direct result
  of a smaller intruder galaxy -- possibly one of
  two objects to the right of the ring -- that
  careened through the core of the host galaxy.
• Like a rock tossed into a lake, the collision
  sent a ripple of energy into space, plowing gas
  and dust in front of it. Expanding at 200,000
  miles per hour, this cosmic tsunami leaves in its
  wake a firestorm of new star creation.
• Hubble resolves bright blue knots that are
  gigantic clusters of newborn stars and immense
  loops and bubbles blown into space by exploding
  stars (supernovae) going off like a string of
  firecrackers.

13
The Local Group/Cluster

• The Magellans Clouds at 170,000 light-years
  and Andromeda (M31) and its companion M33 at 2.5
  million light-years are the best known
  neighbouring galaxies.
• These galaxies plus 14 others form our local
  group/cluster of galaxies.
• The nearest of all is an elliptic dwarf only
  50,000 light-years from the center of Milky Way
  (hidden from us by the galactic center)
• Inside a cluster a lot of interaction between
  galaxies moving with speeds up to 1500 km/s and
  huge amounts of dark matter keep the cluster
  together.

14
Clusters and Superclusters

• Beyond the center of the Milky Way one can see
  the Virgo group containing over a thousand
  galaxies. In center Virgo has 3 giant
  ellipticals, of which M87 has a mass more than
  30 times the mass of our galaxy.
• Virgo and our local group share the same
  supercluster. Our supercluster is relatively
  modest although it comprises a total about 5000
  galaxies.
• Superclusters typically occupy 100-250 million
  light-years and have various shapes. The most
  famous is the great wall of galaxies with
  500x200x15 million light-years.
• Between superclusters there are true empty spaces
  measuring hundreds of millions of light-years.

15
Radio Galaxies

• As astronomers started using radiotelescopes many
  galaxies appeared to be in an explosive state,
  with irregular shapes and with jet-type
  emissions.
• The first galactic explosion was seen in 1951 by
  G.Smith at Cambridge. Cygnus A was a radio source
  at about 750 million l-y. Its emission was also
  seen through optical telescopes.
• Observations using interferometry determined a
  structure in the radio emission, corresponding to
  periods of different level of activity.

16
Galactic Explosions and Quasars (I)

• Galactic explosions are often associated with
  quasars.
• Lick observatory (near San Francisco) is one of
  the leading centers for the study of quasars. Its
  telescope produced many results which demonstrate
  that quasars have very strong and irregular
  emissions.
• For instance 3C345 was characterized by a
  doubling of its luminosity every 12 weeks for a
  few months, while 3C279 had for short periods of
  time bursts of light about 16 times stronger than
  usually.

17
Galactic Explosions and Quasars (II)

• Rapid variations of brightness always imply a
  concentration of matter. The neutron stars which
  are a few kilometers wide emit more radio signals
  than a regular star.
• Quasars are a few light-weeks or light-months
  wide and emit much more radio waves than a
  regular galaxy. Most quasars are visible through
  optical telescopes too.
• Current explanations include gravitational
  collapse of stars or dense clouds of gases and
  the galactic black holes swallowing large clouds.
• In the scenario which includes a black hole of
  108 solar masses, a star approaching it would be
  broken into fragments by the tidal forces outside
  the event horizon. Those fragments would be
  accelerated producing the observed radiation.

18
Seyfert Galaxies

• One of the oldest variable radio-sources is
  3C120. It took many years for a clear explanation
  of its true origin. Now we know that it is a
  Seyfert galaxy, a galaxy with a very bright
  center. About one in 100 galaxies are of
  Seyfert-type.
• Although Seifert galaxies are much brighter than
  regular galaxies, quasars are still more than 100
  times brighter than Seifert galaxies.
• Many astronomers believe that the Seyfert
  galaxies and quasars are two stages in the
  evolution of the same galaxy. The majority of
  quasars correspond to galaxies about 8 billion
  light-years away and therefore very young
  galaxies.

19
Galactic Black Holes (I)

• The use of radio-interferometers allowed the
  creation of very detailed radio photos. They
  showed the radio source Cygnus A was a huge
  elliptical area almost 100 times larger than the
  center which was the galaxy NGC 5128. Its shape
  was suggesting not one but several explosions
  separated by long intervals of time.
• Blandford and Rees at Cambridge have developed a
  double emission model with two jets emitted by a
  rotating mass of gas. This model agreed well with
  the Cygnus A observations and also with the photo
  of Scorpius X-1, one of the known stellar black
  holes.

20
Galactic Black Holes (II)
The existence of black holes in the center of
galaxies is today a common belief among
cosmologists. It is not clear yet if they were
the actual seeds in the formation of galaxies or
formed in later stages.
Our galaxy contains a central black hole of about
105 solar masses and its activity is relatively
small.
21
Galactic Black Holes (III)

• The galactic black holes model can explain all 3
  types of active galaxies.
• Quasars and Seifert galaxies correspond to direct
  observations of the galactic center (with quasars
  being brighter because they are younger stages
  with more material around the black hole)
• Radio galaxies correspond to cases where the
  center is obscured by cosmic clouds and the
  observations focus on the two jets perpendicular
  to the galactic plane.