Space Rumors

Stars finish their fully developed life when they have consumed all the hydrogen in their center. This happens when they have used up all their energy supply, which up to this point maintains the stars stability. The stars nucleus can no longer resist the rest of the stars weight, and begins to contract. Such compression makes the temperature of the nucleus rise and causes nuclear reactions in the layer surrounding the nucleus. The energy produced outside the center of the star makes the stars different layers dilate and cool. The star swells and then becomes reddish. This is what we call a red giant. A typical red giant is about one hundred times bigger than our sun.

When the sun becomes a red star, in about 5 billion years, it will expand out to a maximum radius of roughly 1 AU (150,000,000 km), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass and it will grow so big that it will reach and burn up the nearest planets, mercury, Venus, and probably Earth. Many stars in the red giant stage undergo periodic changes in brightness. This is because they expand and shrink rhythmically. They are called Pulsating Variables. When the hydrogen is exhausted, the star stops being a fully developed star and becomes an old star. The core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature. The life span of the stars depends on their mass, that is, on the amount of matter in the star. The greater the amount of matter from which the star formed, the more condensed and hot its nucleus is. The star is brighter and hotter; its surface is bluish. A blue star uses up its hydrogen in only some millions of years, a very short time compared to the sun. This is why we know that bright, blue stars like the Pleiades are young.