The easiest way to actually measure the temperature of a planet is, in
fact, just look at it! Objects which are hot give off energy in the
form of infrared waves. For example, when you hold your hands over a
fire to warm them, your hands are feeling the infrared waves. These
infrared waves are actually another form of light which are given off
different depending on the temperature of the object.
Here is an exmaple of a picture of a man and a match taken with a camera which only sees these infrared waves.
You can tell that the match is hot because it looks very bright as
does the man's body. But his tie, glasses, and the backgroud appear
blue to show that they are cool.
Astronomers use this same procedure for planets to tell what
their temperatures are. They use an infrared telescope to get an image
of the planet and compare the colors of the plant to a scale. This also
allows astronomers to find the temperatures of galaxy's, stars, and any
other body in the universe.
Now, for the center of the galaxy. Like planets in the solar
system, every body in the galaxy is currently rotating around a fixed
center point. If look at the galaxy as a bigger verious of the solar
system, then it would appear that there must be some large mass at the
center of the galaxy. Like the sun's gravity pulling the plants into
their orbits, this large mass pulls the solar systems in the Milky Way
into their own orbits. But the Milky Way is HUGE! It takes light
100,000 years to cross it! So the mass at the center of the galaxy must
be incredible. In fact, it is believe that at the center of the galaxy
is an object so massive, that nothing, not even light can escape from
it. This object is called a black hole. It is believed that at the
center of the galaxy is a black hole which has a mass about 1,000,000
or more times that of the sun. These large objects give off huge jets
of particles and waves which we can view on earth.
(republished on 07/19/06)