Gravity’s Pull Gravity is the same force that holds you to Earth. Earth’s gravity tries to pull everything toward its center. So it pulls you down. Gravity holds air close to Earth. Which is
good, because otherwise Earth would lose all of its air. Gravity is good for another reason, too: It creates stars. To see what happens, let’s look at one of my favorite space clouds. Its name is Barnard 68. It looks like a black blob. It looks dark because dust is mixed with gases in the cloud. Although the gas is much thinner than the air you breathe, this cloud is so large that it would weigh twice as much as the sun. T ere is no star in Barnard 68 yet. But the
cloud might start to collapse as the cloud’s gravity pulls everything toward its center. When molecules start to move, they gain energy. T ey rush toward the center and crash into one another. When this happens, it makes the center hotter and hotter. Aſt er many thousands of years, the center gets so hot that it glows and is dense enough for a star to form.
T is new star is held together by gravity.
Later something else happens. T e star’s center gets so hot that particles called protons hit other protons. Every atom has at least one proton at its center, or nucleus. So we call these proton-proton reactions nuclear reactions. T ey produce lots of energy, and in most stars they make light and heat. No one knows exactly what will happen to
Barnard 68. But I’m guessing that if we come back in a million years, we’ll see a new star shining there. It may look a lot like our own sun did 4.6 billion years ago.
Wordwise atom: a particle of matter
constellation: a part of the sky, inlcuding all the stars there
dense: having parts packed closely together molecule: two or more atoms joined together
gravity: the force that causes one mass to attract another
Where did all the stars go? What used to be thought of as a hole in space is now known to astronomers at the dark molecular cloud, Barnard 68.
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