Welcome to You Ask Andy

Susie Thomasson, age 12, of Whittier,.N.C., for her question:

How does a star change?

The changing patterns in the heavens give us our daily clocks and yearly calendars. They also help us to grasp that our world also belongs in this splendid realm of endless time and space. When we behold the beauteous heavens, we get a sense of perspective    and the New Year seems a suitable time to reach out and enjoy it.

From our small planet, the view of the stars is very limited and deceptive. They appear fixed in constellations that repeat their celestial parade every year. But we know that these apparent motions are merely our changing view, as the earth spins us around and orbits the sun. However, they really do move and also change as they go. We cannot observe these real changes because distance dwindles the enormous stars and separates them by vast oceans of space.

But astronomers have traced some of the changes that really occur in the stars themselves and also in their motions. Every star has a blazing life story that may last a billion or maybe 20 billion years. In any case, these changes take too long for us to notice. A few stars appear slightly brighter or dimmer than they did 2,000 years ago. But this is because they are nearer or farther away from us.

The average star begins to form when a huge cosmic cloud condenses into a ball of dense gases. Its massive gravity causes the enormous pressures needed to ignite it. Then the newborn star becomes a blazing nuclear furnace. Nuclear fusion fuses its hydrogen atoms into atoms of helium. Some of the material is converted into nuclear energy and radiated forth in all directions.

Through billions of years, the star consumes its fuel, trading its original hydrogen for helium and other materials. Eventually its hydrogen fuel runs low and its fiery furnace subsides. Finally it goes. out and the once blazing star blazes no more. Of course, our time is much too short to observe the aging of a star. However, astronomers can detect slight changes in their positions.

These changes occur because the 100 billion stars of our Galaxy move around like a super sized pinwheel. Our Solar System is situated some 30,000 light years from the center    and around we go with the Big 1?heel. Stars closer to the center travel faster and those in the outer lanes are slower. So our neighboring stars are not really fixed in their constellations. Some catch up and pass us while others lag behind.

The vast reaches of space are so enormous that their changing positions are barely measurable. And a thousand years is too short to detect much change in the life stories. Our sun, for example, has been burning five billion years and expects to burn maybe 12 billion more. And it takes our solar family 200 million years to complete one circle around the pinwheeling Galaxy.

 

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