Black Holers

8/13/2019 Black Holers

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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 collapseinto 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.


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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
http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q15.htmlhttp://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q15.html


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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).


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Supernova in a

distant galaxy

The supernova is

about as bright as

400 billion otherstars in the galaxy.

Birth of a black hole

from the death of a

big star


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Eta Carinae

A nearby star ready to

become a supernova

This star belches gases

like a volcano that isabout to explode.


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How do we know about black holes ?

We cant see a black hole directly, because light cannotescape 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


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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 starto stay in orbit. The star below orbits 4 million suns in

only 15 years!


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Observing dark objects ?

One can ask the more general question howone 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.


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Indirect way to detect black holes:

Artists 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.


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Three images of the Crab Nebula

Need X-ray vision to see accretion disk and jets.

X-rays Visible Infrared

(hot) (cool)


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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.


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Absorption by the Earths atmosphere


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Hubble SpaceTelescope:Visible, UV


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GLAST: Gamma Rays Spitzer: Infrared


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Mauna Kea Observatories (Hawaii): Visible, Infrared


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Radio telescope array


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WMAP: Cosmic microwaves

Need to detect temperature differences of 20 K at 3 K


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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.

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