Transcript A black hole

A black hole:
The ultimate space-time warp
Ch. 5.4
A black hole is an accumulation of mass so dense
that nothing can escape its gravitational force,
not even light.
Two types of black holes exist :
Small: Heavy stars collapse under their own gravitation
after burning out, forming a supernova. If they
have enough mass (>30 solar masses), they collapse
into a black hole (after shedding 90% of their mass).
Large: A black hole with millions of solar masses lurks at
the center of many galaxies including ours.
The horizon of a black hole
When light particles (photons) are emitted from a black hole,
they perform work against gravity. This work reduces the
energy of the photons. The lower energy implies a red-shift.
There is a sphere around a black hole called the horizon,
where the photons lose all of their energy trying to escape.
This is similar to the horizon at the edge of the observable
universe, where photons from distant galaxies are red-shifted
so far that their energy goes to zero (Lect. 3, Slides 3,4).
For a simulation of a clock falling into a black hole see:
http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q15.html
Simulated view of a black hole of 10 solar masses viewed from 600 km,
just before falling in (acceleration of 400 million g). The black hole acts
as strong gravitational lens (Lect. 16, Slides 8,9).
Birth of a black hole
from the death of a
big star
Supernova in a
distant galaxy
The supernova is
about as bright as
400 billion other
stars in the galaxy.
The Crab Nebula
Leftover from a
supernova in 1054
This star was not
heavy enough to
become a black
hole. It is now a
neutron star at
the center of the
exploding gases.
Eta Carinae
A nearby star ready to
become a supernova
This star belches gases
like a volcano that is
about to explode.
How do we know about black holes ?
• We can’t see a black hole directly, because light cannot
escape from it. However, if a nearby star orbits around
the black hole we can detect the black hole by its gravity.
• The mass of a black hole is obtained from the orbit and the
velocity of the visible star (obtained from the wavelength
shift). If the mass exceeds 3 solar masses and the orbit is
too small to fit a regular star of that mass, a black hole is
the only explanation.
Examples:
Small: Cygnus X-1 (an X-ray source)
Large: Cygnus A (a galaxy with jets)
Center of our galaxy
The black hole at the center of our galaxy
The center of our own galaxy contains a black hole of
about 4 million solar masses . The mass is determined
from the speed and distance of nearby stars that orbit
the black hole like the planets orbiting Earth. A large
central mass requires high speed for a planet or a star
to stay in orbit. The star below orbits 4 million suns in
only 15 years!
Observing “dark” objects ?
• One can ask the more general question how
one can observe dark objects (black holes,
dark matter, dark energy).
• Although we cannot “see” them directly,
we can detect them by their gravitation ,
which affects nearby stars and galaxies.
Indirect way to detect black holes:
Artist’s view of a black hole drawing
matter from a nearby normal star.
Hot gas forms an “accretion disk”
around the black hole. ”Jets” are
emitted along the rotation axis.
Such features are observed for both
small and large black holes, as well
as for neutron stars.
Three images of the Crab Nebula
Need X-ray vision to see accretion disk and jets.
X-rays
(hot)
Visible
Infrared
(cool)
A pair of jets emitted from a black hole, but on a much grander scale:
This giant black hole sits at the center of a galaxy. The jets are imaged
by a radio telescope array at =6 cm. This is the brightest radio source
in the sky (Cygnus A), despite its huge distance of 0.6 billion light years.
Use many types of radiation
• A lot of the new information about the early universe
and about the most violent events in the universe has
come from satellite-based telescopes. The atmosphere
absorbs or distorts a large part of the electromagnetic
spectrum.
• The far infrared and microwave spectrum samples the
early universe. It contains most of the radiation power.
• X-ray and Gamma-ray telescopes detect the hottest,
most energetic events. Gamma-ray bursts represent the
brightest explosions in the universe since the Big Bang.
Absorption by the Earth’s atmosphere
Hubble Space
Telescope:
Visible, UV
GLAST: Gamma Rays
Spitzer: Infrared
Mauna Kea Observatories (Hawaii) : Visible, Infrared
Radio telescope array
WMAP: Cosmic microwaves
Need to detect temperature differences of 20 K at 3 K
COBE satellite 1996:
Got the first results.
Nobel prize 2006
WMAP satellite 2003:
Higher resolution.
Larger features of
the COBE picture
reproduced.
Planck satellite 2009:
Detects polarization.