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:

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).

Supernova in a distant galaxy The supernova is about as bright as 400 billion other stars in the galaxy. Birth of a black hole from the death of a big star

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 Visible Infrared (hot) (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.