ETSU Observatory Open House Presentation:
East Tennessee State
University Department of Physics
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
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:
- The gravitational pull in a black hole is so strong,
that nothing, not even light,
- Einstein's Theory of Relativity:
Light is attracted by gravity.
The bending of starlight due to the Sun's gravitational
P. Marmet and C. Couture.)
- This was confirmed in 1919 during a
- One consequence of this bending of light by gravity
is the existence of gravitational
- A massive object (such as a
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
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
from the universe, leaving only its
gravitational field behind to betray
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
The Schwarzschild radius of any object
depends on its mass.
For the Earth, it is about
- For Jupiter, it is
- 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,
An illustration of spacetime
warping by a black hole
Imagine a star near the event horizon of a black hole.
Light going away
from the black hole is
redshifted; light moving
the black hole is
- Clocks near black holes run slower than clocks in a weaker gravitational field.
a black hole may lead to other Universes:
A diagram of a wormhole
HOW DOES ONE FIND A BLACK HOLE?
- Black holes can be found by:
- 1. Their
gravitational effects on other
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
- Cygnus X-1 is a very bright
- 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
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