Lifecycle of stars

Shane Blok

Space: Stars appear to us as twinkling points of light in the night sky. They are, in fact, huge objects glowing with the heat from their inner nuclear furnace, which is so hot that it can change one element into another.

All light, heat and energy on Earth comes from the nuclear furnace of our nearest star: the Sun. The Sun is a rather ordinary star, but looking outward we can see gigantic cool stars, red giants, bright, hot supergiants and small stars made of incredibly dense material, white dwarfs.

There are also neutron stars and black holes, which are the dead remnants of massive stars. Many stars orbit around each other and, at times, are so close together that material from one is pulled off by the other.

Some pulsate so much that astronomers can see their light varying from hundreds of light years away. Overall, the twinkling stars are strange and exotic objects, which allow astronomers to see way beyond our own solar system.

Stars too live and die

Like people, stars are born; live their lives, grow old, and die. Unlike people, their lives are measured in millions or even billions of years-too long for us to see them age. If a visitor from another planet could visit the Earth, he or she would see many kinds of people of different sizes and shapes.

Our visitor might guess that the smallest creatures were newly born, and then try to work out how the different kinds of bigger people fitted into the human lifecycle. In a similar way, by studying the different types of stars, astronomers are able to piece together the entire stellar lifecycle.

Lifecycle of a star

1. Far out in space a cold, dark cloud of gas and dust starts to contract under the pull of its own gravity.

2. As the cloud shrinks and heats up, it breaks into smaller clumps each of which will form a protostar. (A protostar is a star in the process of formation in which nuclear fusion has not yet commenced)

3. Each protostar is shrouded in gas and dust, which flattens into a disc as the protostar spins.

4. Eventually, the contracting protostar bursts into life and strong jets of gas escape from either side of the disc.

5. Dust grains condense and stick together in the disc around the protostar, and may eventually form planets.

6. The young, fully formed star is now fusing hydrogen to make helium on the main sequence.

7. As its hydrogen fuel runs out, the star expands to become a red supergiant. (Stars with more than 10 times the mass of the Sun become supergiants, which are the most luminous stars in the Universe)

8. The core is now hotter. The star uses its helium to make carbon and oxygen.

9. Nuclear reactions produce heavier and heavier elements, until a core of iron builds up.

10. Eventually, the iron core collapses and the star explodes as a brilliant supernova. (Supernova is an enormous stellar explosion. Sending much of its material into space. There appear to be two types: one class attains a maximum luminosity of 100 million Suns, the other of 10 million Suns.)

11. Most of the star's matter is blown away by the supernova. The star's collapsed core may survive as a neutron star or a black hole. (A black hole is a collapsed object whose gravity is so strong that nothing - not even light can escape it.)

Star birth

A molecular cloud may contract under the pull of its own gravity and split up into smaller clumps. These clumps warm up as they continue to shrink and grow more dense. astronomers can detect radio waves and infrared radiation from the clumps before they are hot enough to emit light. Eventually they start to glow. At 10 million C, nuclear reactions start and new stars are born.

Star Death

Stars heavier than eight times the mass of the Sun swell and end their lives in a dramatic explosion, a supernova, leaving only a tiny, dense remnant-either a neutron star or black hole.

Sun's story

1. Most stars are not heavy enough to become supernovas. Stars like the Sun spend billions of years burning up their hydrogen on the main sequence before ending their lives in a quieter, less spectacular fashion.

2. When all the hydrogen is used, the Sun will swell to become a red giant, burning helium instead of hydrogen.

When the helium runs out, the Sun will puff off its outer layers to form a planetary nebula.

3. The planetary nebula will disperse, leaving the Sun's core exposed.

The core is a white dwarf a small, dense ball of cinders with no nuclear fuel left. Over billions of years, it will cool and fade away.

References

Stars and Planets

Astronomy Encyclopedia

Book of Astronomy and Space

The Book of Stars

The Encyclopedia of space