ETSU Observatory Open House Presentation:

BLACK HOLES

Dr. Beverly Smith
East Tennessee State University Department of Physics

BLACK HOLES

WHAT IS A BLACK HOLE?

• In normal stars, there is a balance between the inwards force of gravity and the outwards pressure of the gas.
• If there is too much mass in too little space, nothing can resist the force of gravity, and the star collapses into a single point.
• A black hole is an object in which all matter has been crushed into a single point: a singularity
• The gravitational pull in a black hole is so strong, that nothing, not even light, can escape
• Einstein's Theory of Relativity: Light is attracted by gravity.

The bending of starlight due to the Sun's gravitational field (Image from P. Marmet and C. Couture.)

• This was confirmed in 1919 during a solar eclipse
• One consequence of this bending of light by gravity is the existence of gravitational lens
• A massive object (such as a galaxy or cluster of galaxies) can bend the light of objects behind them, creating double or multiple images of the background object, or arcs and rings

A diagram illustrating gravitational lensing

A Hubble Space Telescope image showing an example of gravitational lensing of background objects by a massive cluster of galaxies

• The gravitational field of a black hole is so strong that light rays are bent back to the black hole: they can't escape.
• With no photons able to leave, all communication with the world outside the black hole is impossible
• The matter inside the black hole has effectively disappeared from the universe, leaving only its gravitational field behind to betray its presence.

THE STRUCTURE OF A BLACK HOLE

• The minimum distance you can get from a black hole and still escape its gravitational field (if you are traveling the speed of light) is called the Schwarzschild radius.
• The surface of an imaginary sphere with radius equal to the Schwarzschild radius and centered on the black hole is called the event horizon.
• The Schwarzschild radius of any object depends on its mass.
• For the Earth, it is about 1 centimeter.
• For Jupiter, it is 3 meters.
• For the Sun, it is 3 kilometers.

WHAT HAPPENS NEAR A BLACK HOLE?

• Even before reaching the event horizon, an object falling into a black hole will be pulled apart by tidal forces.

• Near a black hole, space and time are warped

An illustration of spacetime warping by a black hole (Image from Andrew Hamilton.)

Imagine a star near the event horizon of a black hole. Light going away from the black hole is redshifted; light moving towards the black hole is blueshifted. (Image from Andrew Hamilton)

• Clocks near black holes run slower than clocks in a weaker gravitational field.

• SPECULATION: a black hole may lead to other Universes: wormhole

A diagram of a wormhole (Image from Andrew Hamilton.)

HOW DOES ONE FIND A BLACK HOLE?

(from Andrew Hamilton.)

• Black holes can be found by:
• 1. Their gravitational effects on other objects.
• 2. X-ray radiation from material falling into the black hole.

• One of the best examples in the Milky Way: Cygnus X-1

An X-ray image of Cygnus X-1 from the EXOSAT satellite

• Cygnus X-1 is a very bright X-ray source
• There is a blue star near this X-ray source
• Studies of the motion of this star shows it has an invisible companion, with a mass at least 10 times that of the Sun: the black hole
• Gas from the blue star is falling into the black hole.
• Before it gets inside the event horizon, it emits X-rays.
• In the center of the Milky Way galaxy (in the constellation of Sagittarius) there is believed to be a black hole a million times more massive than than the Sun.
• In the centers of some other galaxies, there are black holes that are a billion times the mass of the Sun.
• Some of these very massive black holes squirt out beams of charged particles, creating huge plumes of radio emission that extend beyond the galaxy into intergalactic space.

Top: An optical image of the peculiar elliptical galaxy Centaurus A. The white glow is from stars, the dark band is absorption from interstellar dust.
Bottom: The same image, with the radio jets superposed. (Optical image copyright Anglo-Australian Observatory, reproduced with permission. Radio image from Jack Burns.