Page 18 - Curriculum Visions Dynamic Book

Galaxy NGC 6822
 This is a glowing gas
cloud, called Hubble-V, with
a diametre of about 200 light-years. There is a dense knot of dozens of ultrahot stars nestled in the nebula, each glowing 100,000 times brighter than our Sun. They are youthful four-million-year- old stars, forming part of a small, irregular host galaxy called NGC 6822, one of the closest neighbors to the Milky Way. The galaxy is 1.6 million light-years away in the constellation Sagittarius.
Most stars are made of clouds of molecular material. A typical molecular cloud might contain
enough matter to make a million stars the size of our Sun. Even though it is hundreds of light-years across in size, the cloud has enough gravity to
cause some matter to begin to group together. Most molecular clouds contain clumps of material,
and it is in these cloud cores that stars develop. In a cloud a clump of material starts to spin
slowly. Eventually the clumps get big enough to develop sufficient gravity for them to begin to collapse in on
themselves. Almost invisible matter now suddenly becomes a concentrated and opaque ball of gas called a protostar.
Once star formation is under way, and the new star is still perhaps not much bigger than one of our gas giant planets, the gravity of the protostar causes more material to move toward it. More pressure raises the temperature of the core, and that makes light begin to be radiated from it. At first the star is not very
hot and is only visible as an infrared object. It cannot be seen through an optical telescope.
The protostar becomes more and more active, sending out jets of material as a stellar Wind. In time its stellar wind blocks any material that might be moving toward it, and because of this the star eventually stops growing.
As protostars heat up, they become visible for the first time and are called T-Tauri stars (for a picture see page 48). Unlike most newly born objects, these new stars are actually bigger than they will be as they mature, because gravity has not yet had time to pull them down to their final size. When that does happen, hydrogen fUsion starts up in the core, and the protostar becomes a main-seqUence star (see page 24).
Binary stars
About half of all stars are found in pairs in orbit around their common center of gravity. Sometimes one star will consume the material of the other and cause an intense increase in brightness. This will be discussed under nova and supernova (see pages 30–31).
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