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Whether people grow up to be tall or short or have big or small bones depends on the genes they get from their parents. But a star’s life is shaped by only one thing: how massive it is when it is born.
Stars with greater mass burn hotter and brighter. They also burn out faster. So stars that keep cool live longer by burning slower. The hottest stars are blue, and the coolest stars are red, while others are yellow, like our Sun. Most stars in our galaxy are red dwarfs. Here are some of the different kinds of stars.
◀ Brown Dwarfs
When is a star not a star? A star is not a star when it is too small to start its nuclear furnace but too large to be called a planet. Astronomers call these objects brown dwarfs, or failed planets. They are scattered all across the galaxy. So far, only eight planets have been found to orbit brown dwarfs, but these planets would be far too cold to support life as we know it. In this image, the brown dwarf is at the lower right. It is orbiting the bigger red dwarf in the center.
▲ Red Dwarfs
This star is Lacaille 8760. It is one of the biggest and brightest red dwarf stars known. The sky is covered with red dwarfs, but most are too dim to be seen without a telescope. It’s like looking at a field of grass and knowing it is crawling with ants, but the ants are too small to be seen. Lacaille 8760 might be big and bright, but it is just barely visible to someone with very good eyesight under a clear, black sky.
▲ Yellow Dwarfs
Our Sun is a yellow dwarf star. Its surface temperature is 11,000°F. Yellow dwarfs shine for 10 billion years before burning out. This type of star would be a perfect host for an inhabited planet. That’s because its long life provides enough time for life to develop, as it did on Earth over billions of years.
◀ Luminous Blue Variable Star
The brightest star in our galaxy is really, really far away. So far, it is hidden from view by the Milky Way’s dark dust clouds. This picture, taken in infrared light, shows the star and its nebula. The star is nicknamed the Pistol Star. It is 10 million times brighter than our Sun and 150–250 times more massive. That’s about as big as a star can get!
Binary Star System
Blue-white Sirius is a large binary star system. It is in the winter constellation Canis Major, the Great Dog. Sirius A (shown here) is the brightest star in the night sky. That’s mainly because it is nearby—only 8.6 light-years away! It has a small companion, Sirius B, a burned-out star called a white dwarf. ▶
◀ Red Giants
Late in its life, a star swells up to many times its original size. That’s because the nuclear furnace is releasing a lot more energy. This causes the star to inflate like a hot air balloon. This computer model shows large cells and bright spots on a supergiant’s surface.
