Welcome to You Ask Andy

  Gerald Evanitsky, age 12, of Ambridge, Pa., for his question:

How big is Betelgeuse?

Betelgeuse is a star in the dazzling constellation of Orion. North of the equator, this makes it a jewel of the winter sky. Orion, the Celestial Hunter, is far too brilliant to be overlooked. Though we may not notice all the stars which form this constellation, we are all familiar with the five brilliants which form a neat narrow diamond. The three close stars in a row are in Orion's belt. The bright star directly below the belt is Rigel. The bright star directly above is Betelgeuse, in Orion's shoulder.

To the unaided eye, Betelgeuse glows with a reddish tinge. In fact, it is classed as a red star, a red giant among the stars. Compared with Big Bet in size, our sun is but a pygmy. The diameter of our sun is 860,000 miles. This gives it a volume large enough to swallow up more than a million planets of the size of our world. The diameter of Betelgeuse is 0 million miles.

We cannot understand this vast size from mere figures. We must exercise the old brain to grasp it. A good way to do this is with a few stretching exercises. Let's begin with a small idea and grow.

The diameter of our earth, which is a straight line through the middle from side to side, is a little less than 8,000 miles. On land, this distance would take you clear across the United States and back again. To grasp the diameter of the sun, you have to stretch this distance to 100 times as great, and then some. And the diameter of Big Bet is 510 times the diameter of the sun. No wonder this star is called a giant,

Let's imagine that Betelgeuse had changed places with the sun in our Solar System. Let's place the center of the red giant where the center of our sun is now. The radius of the red giant star is 230 million miles. This means that the big star would spread out this distance in all directions.

Naturally, the size of Betelgeuse would swamp the sun and spread beyond the orbits of the inner planets. Earth is some 93 million riles from the sun, So our planet would be engulfed by the big star. Mars is roughly 141 million miles from the sun and Jupiter some 483 million miles. Hence, the rim of Big Bet would be somewhere between the orbits of Mars and Jupiter. The red giant would swallow up four of the nine planets of our Solar System.

For all its size, Betelg3use is a light‑weight star, It weighs only 15 times more than the sun. For this to be so, its gases must be very rare indeed. It is estimated, that the density of Betelgeuse is about equal to the vacuum in a thermos bottle. Its gas molecules are widely separated from each other.


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