Stars

Ken Tapping, 30th September, 2015

In the sky this week…

  • Saturn lies very low in the southwest in the evening. It is slowly sinking into the sunset glow before passing behind the Sun.
  • Venus rises about 3am, with Mars (much less bright), and Jupiter (nearly as bright) following close behind.
  • By 5am all three planets should be visible. 
  • The Moon will reach Last Quarter on the 4th.

A hydrogen bomb explosion can be described as a very small, short-lived star. In a hydrogen bomb some form of hydrogen is submitted to extreme compression and heating so that it undergoes nuclear fusion. This is achieved by using a nuclear fission bomb as a detonator. When the compressed fuel reaches temperatures of millions of degrees, the hydrogen atoms combine to form atoms of heavier elements, such as helium, and in the process liberate a prodigious amount of energy. The process stops when the fuel runs out or the fireball expands and cools too much for the reaction to continue. Stars are powered by what can be described as a continuous, supersized, tightly controlled hydrogen bomb explosion.

A gas cloud collapses to form a big ball of gas. The weight of the overlying material compresses the material in the core of the ball, making it hotter and hotter. If there is enough overlying material, the core reaches temperatures of 10-20 million degrees, and as in the case of the hydrogen bomb, nuclear fusion starts. Four hydrogen atoms combine to form one helium atom and liberate a lot of energy. However, in the case of a star we do not get an explosion. The weight of the overlying material prevents it.  If things get too hot the increased radiation pressure causes the new star to expand, which cools and decompresses the core, so the rate of fusion slows again. If the reaction gets too slow the star shrinks, the core gets more compressed and hotter, and the reaction speeds up. This stabilizes the star.

A star’s life consists of three main phases: youth, where the star has just formed and is settling down, maturity, which is the longest part, where the star behaves consistently, and then old age, where the star is running out of fuel, and may start to behave irregularly. However, although all stars have these phases of life, how long that life is can vary enormously. A really big star can go through all three phases and blow itself apart while a small star is still settling down. Our Sun is about halfway through its 10 billion year lifetime.

It all depends upon how much material the embryo star collects from its birth cloud. If it collects a lot before lighting up and blowing the cloud away the pressure in the core will be higher because of the larger amount of overlying material, and the temperature will be higher. Because it is being held down harder, its fusion furnace will burn larger and hotter. Double the mass of the star and it will shine roughly 15 times brighter, and will run in less than 2 billion years. A star about 100 times the mass of the Sun won’t last very long at all. Stars with masses comparable with the Sun end their lives when they start running out of fuel by sneezing off their outer layers and then cooling off. Big stars collapse and then blow apart.

The Summer Triangle, a grouping of three stars: Vega, Deneb and Altair, is high in the sky at the moment and easy to find. Vega is a blue-white star and almost overhead or westering. Deneb is less bright and lies to the left of Vega. Altair, lower in the sky, forms the last vertex of the triangle. Vega lies 25 light years away and is about 40 times more luminous than the Sun. Altair is closer, just 11 light years away and about 11 times more luminous. Although Deneb looks less bright and spectacular, it lies about 2500 light years away and is about 250,000 times more luminous than the Sun. It is about 20 times more massive than the Sun, so in astronomical terms, Deneb will not be with us for long, and will eventually explode.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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