The human eye can see about 6,000 stars in the night sky. Photographs reveal millions more in every direction. All of these stars reside in our Milky Way galaxy. But they are just a tiny fraction of the several hundred billion stars in our galaxy alone. Counting all the stars in all the galaxies, there are perhaps a hundred billion billion stars in the observable universe.
An ordinary star is a massive sphere of luminous gas, mainly hydrogen and helium.
The stars are so far away that, even with the best telescopes, they generally appear as just points of light.
Stars are born, live out their lives, and die. Their appearance changes dramatically along the way.
Low-mass stars are the longest lived of the energy-producing objects in the universe.
Stars of intermediate mass have lifetimes that range between 50 million and 20 billion years.
High-mass stars are very luminous and short lived.
In any batch of newly formed stars, the most massive ones are the rarest and shortest lived.
Astronomers sort stars by placing them on a diagram according to their luminosity and surface temperature.
The material of stars is recycled over billions of years.
Telescopes reveal that about half the stars we see are actually double or multiple star systems.