How do we detect black holes? How big and hot are they?

On Mon, 31 May 1999 you wrote:

> How were black holes first detected, who was credited for the initial
> discovery?

Black holes still have not been detected directly - we see them based on
the light of gas that is about ready to fall into the black hole. There
are many astronmers who have found fairly convincing evidence that black
holes exist, but nobody is officially "credited" with having discovered a
black hole. My favorite evidence is from the Hubble Space Telescope
image of the center of the elliptical galaxy NGC 4261. The picture and
spectral information show exactly what we thought gas falling into a
black hole should look like. That picture can be viewed at:
http://oposite.stsci.edu/pubinfo/jpeg/ngc4261.jpg

> What do they look like in: radio waves,microwaves, infrared, visible light,
> ultraviolet, and x-rays?

The known black hole candidates generally look just like points of light
in all parts of the visible spectrum. This is because the area around a
black hole wehere gas gets hot enough to emit light is very small, and
impossible to see in detail from the Earth. The exceptions are in radio,
where sometimes black holes show giant jets of matter spewing away from
the black hole. Also, the hubble space telescope can sometimes see some
details in the giant disks of gas that surround black holes in other
galaxies.

> What is the size or size range of the object in meters, kilometers, or light
> years?

The size of a black hole depends on its mass. A good estimate of a black
hole's size is called the Schwarzschild radius, which is 3km for each
solar mass the black hole contains. So, if the sun were to collapse into
a black hole, it would have a radius of 3 km. The black hole at the
center of the Milky Way is around 2 million solar masses, so it would be
6 million kilometers in radius. The smallest black holes that we know
exist are around 5 or 6 solar masses, and the largest ones may be over
one billion solar masses. Some astronomers think there may be tiny black
holes left over from the early universe; these may be anywhere from a few
kilograms up to a solar mass (2 times 10 to the 30th power kilograms),
but their existence is quite controversial.

> What is the temperature range of a black hole?
Stephen Hawking showed that black holes have properties similar to what
we would call "temperature". The temperature of a black hole depends
upon its mass. Larger black holes have a lower teperature. Black holes
of a few solar masses have temperatures of around one ten-millionth of a
degree, so they are very cold!

If you'd like to know more about black holes, I'd suggest that you read
Hawking's book "A Brief History of Time." The book has several chapters
on black holes, and is written well for the non-scientist.

Thanks for writing!

Sincerely,
Kurtis Williams

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