Hubble proved that some distant clouds of light in the sky are actually distant galaxies that lie well beyond the boundaries of our own home galaxy, the Milky Way.
Light can act as a particle, called a photon, or it can travel through space as a wave.
The most energetic objects, events, and processes in the universe give off x-rays and gamma rays.
When a star dies, the material it created is distributed into space and recycled into new generations of stars, planets, and life.
Astronomers classify stars by their color, which reveals the temperature at the star’s photosphere (the visible “surface”).
Black holes are massive, dense objects with gravity so strong that nothing—not even light—can get out.
The Milky Way is moving through space at an estimated velocity of 630 kilometers per second.
The nearest stars are about 4.2 light-years from the Sun.
Dark energy acts opposite gravity and continues to speed up universal expansion today.
Ancient people used the sky as a calendar and a timekeeper.
Newton pointed out that you can calculate the force of gravity between two objects if you know their mass and their distance apart.