The Expanding Universe, Black Holes, and AGN's

Mass is the measure of how much matter an object contains -- the total number of subatomic particles (electrons, protons, and neutrons) in the object. It is harder to push a car than a bicycle because the car is made of more particles, i.e. the car has more mass.

Acceleration is the measure of how the speed and direction, or velocity, of something changes. When you speed up in a car, you are accelerating. When something falls to the Earth, it moves faster and faster as it falls; it is accelerating.

A force is a push or pull. If one object exerts a force on another, the second object exerts an equal and opposite force on the first. The Earth accelerates the fruit more than the fruit accelerates the Earth because the Earth has more mass and is harder to move.

Gravity is a force that all objects with mass exert on all other objects. The force of gravity we are familiar with is the force exerted on us by the Earth. We measure this force as weight.

The force of gravity from an object gets weaker as you get farther away from the object. Astronauts in space feel very little gravity because they are far from the Earth. (When in a space station the primary reason is free fall, explained below.)

Weight and mass are not the same! The acceleration of an elevator can simulate higher and lower gravity, as it pushes you up and down. When the force of gravity changes, your weight changes, but your mass is always the same.

You've probably heard of the Milky Way Galaxy. In fact, that is where you live! A Galaxy is a huge collection of stars rotating around each other like in this picture. The Milky Way is just one of billions of galaxies in the Universe. Galaxies are incredibly massive. You would think, then, that they would attract each other. Amazingly, this is not the case.

Black Holes are very small theoretical objects which are very massive. To give an idea of how small: a black hole with the mass of the earth would be the size of a ping pong ball. The existence of black holes is predicted by Einstein's Theory of General Relativity. They are called black holes because they are so massive that not even light can escape their gravitational attraction.

Some galaxies have very bright centers. These galaxies are known as Active Galactic Nuclei(AGN). AGN's are thought to have very large black holes at their center. Very distant AGN's are called Quasars. Scientists use X-Rays to try and figure out what is really going on inside the centers of these galaxies.