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On a clear night, it is possible to see more than 3,000 stars with the naked eye. There are billions of other stars up there, but we cannot see them.
The night sky holds clues about the life and death of stars. These clues are in fuzzy-looking clouds of glowing gas sprinkled with black dust. These clouds are called nebulae (NEB-u-lie). The word nebula (singular form of nebulae) comes from the Latin word for cloud. When seen through a low-power telescope, nebulae look like cotton balls. But high-power telescopes show that they are huge structures of many shapes and colors. Some nebulae signal the birth of stars. Others are leftovers from the death of stars.
▲ How Stars Burn
A star is a burning mass of gas held together by its own gravity. It uses the lightest element, hydrogen, for fuel. Hydrogen atoms have one proton each. Energy is released when two hydrogen atoms join. They form a heavier atom called helium, which has two protons. When the two hydrogen atoms combine, some of their mass becomes a huge burst of energy.
Stars Need Fuel
▲ To create energy, stars need fuel. A star fades when that fuel is used up. Stars that weigh more than the Sun (above) burn out quickly and even explode. Our Sun will not burn out for a long time—not for another 5 billion years.
◀ Mapping Where Stars Are
To help find their way around the sky, astronomers divide it into 88 regions, like counties in a state. The regions are connected to imaginary stick-figure pictures called the constellations. Many cultures have their own set of legendary sky figures. We use the mythological characters imagined by the ancient Greeks and ancient Egyptians. Orion, the Hunter, is one of the most familiar constellations. His shoulders, legs, and waist are easy to trace.
▲ Light-Years Away
Stars are much farther away from Earth than the planets are. Our Sun is 93 million miles away. Proxima Centauri, the closest star to our Sun, is 24 trillion miles away from it. Because numbers in the trillions include so many zeros, scientists use light-years to measure such long distances. A light-year is the distance a beam of light travels in a year, which is nearly 6 trillion miles. A beam of light takes 1.3 seconds to travel between the Moon and Earth. It takes 8.3 minutes to travel the 93 million miles between the Sun and Earth. Stars are too far away to visit with spacecraft. We use powerful telescopes to look at them instead.
Jewel Box of Stars
This is a telescope view into the dense heart of our galaxy. It shows that stars come in many colors. Most blue stars are young and hot, up to ten times hotter than our Sun. Red stars are cooler, only half the Sun’s temperature. ▶
