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Life Cycle of a Star

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Life Cycle of a Star

Explanation 1

  • Particles in space, which are leftover remains of old stars, combine together to form a new star in the nebula. The particles are pulled together by a gravitational pull.
  • As the particles are pulled together nuclear reactions begin within the core. This star is called a protostar until it reaches 10 million degrees.
  • If the mass of particles is large enough, it is called a supergiant, and will burn for 10 million years. Larger stars burn their fuel more quickly and inefficiently.
  • When the hydrogen nuclear fuel begins to run out, it will expand into a large red giant and begin to burn the rest of its gasses.
  • After the nuclear reactions end in the core, it begins to cool and its great gravitational forces will cause it to collapse in on itself.
  • This collapse will cause a huge explosion or Supernova and blow particles out into space to form a nebula, which causes the cycle to start over again.
  • The dense body left over that collapsed in on itself, is called a Neutron Star, and has a mass of one hundred million tons for every thimbleful.
  • If the mass of the supergiant is great enough it can form what we call a black hole.
  • Since our sun is not a supergiant, but instead a medium yellow star, it will have a different fate. It will eventually use up its fuel in about 10 billion years, swell into a red giant, and then shrink down to a white dwarf. When it has completely cooled it will be a cold black dwarf.

* Scientists feel our sun is already 4.5 billion years old and expect it to be around for another 5.5 billion years.

Explanation 2

  • Stars are born from clusters of interstellar matter

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Once the contraction phase is completed, the star becomes a fixed star, an "adult" so to speak, and enters the main sequence star phase. Stars in this phase produce energy as the result of a nuclear reaction that creates one helium atom from every four hydrogen atoms. This means that the amount of hydrogen in the star gradually decreases, while the helium increases. The main sequence star phase is the longest period in a star's life, and in the case of a star with a mass similar to that of our Sun, lasts for about 10 billion years. Our Sun is thought to be around 4.6 billion years old, which means that it is probably about halfway through its main sequence star phase.

  • As the outer layers expand from the helium core, the star becomes a red giant

The helium at the centre of the star continues to increase until a helium core is formed. Nuclear reaction then begins to spread outward. As the helium core grows heavier, the core's temperature also increases, and the outer layers begin to expand until the star becomes a massive red star known as a red giant. In the case of a star that is about the size of our Sun, the gases of the outer layer are expelled, and then contract, so that the star becomes what it known as a white dwarf. However, if a star has a much greater mass than our Sun, the final stages of its giant star phase end in a supernova explosion. The giant star phase is about one tenth as long as the main sequence star phase.

  • Some stars collapse under their own weight, causing supernova explosions

When a star's mass is about three times that of our Sun, after the red giant phase, it begins to collapse under its own weight, causing a supernova explosion that scatters it through space. Its brightness at this point will be 100 billion times that of the Sun. When this happens, it looks as if a bright new star has appeared in the night sky. Supernova explosions of some exceptionally massive stars leave in their wake neutron stars, called pulsars, and black holes. Sometimes the scattered clouds of gas and particles become the material of new stars.

Explanation 3

  • What is a star?

Stars are hot bodies of glowing gas that start their life in Nebulae. They vary in size, mass and temperature, diameters ranging from 450x smaller to over 1000x larger than that of the Sun. Masses range from a twentieth to over 50 solar masses and surface temperature can range from 3,000 degrees Celsius to over 50,000 degrees Celsius.

The colour of a star is determined by its temperature, the hottest stars are blue and the coolest stars are red. The Sun has a surface temperature of 5,500 degrees Celsius, its colour appears yellow.

The energy produced by the star is by nuclear fusion in the stars core. The brightness is measured in magnitude, the brighter the star the lower the magnitude goes down. There are two ways to measuring the brightness of a star, apparent magnitude is the brightness seen from Earth, and absolute magnitude which is the brightness of a star seen from a standard distance of 10 parsecs (32.6 light years).

  • Small Stars- The Life of a Star of about one Solar Mass.
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Planetary Nebula.

Stage 9 - The remaining core (that’s 80% of the original star) is now in its final stages. The core becomes a White Dwarf the star eventually cools and dims. When it stops shining, the now dead star is called a Black Dwarf

  • Massive Stars – The Life of a Star of about 10 Solar Masses

- Massive stars have a mass 3x times that of the Sun. Some are 50x that of the Sun.

Stage 1 - Massive stars evolve in a similar way to a small star until it reaches its main sequence stage (see small stars, stages 1-4). The stars shine steadily until the hydrogen has fused to form helium (it takes billions of years in a small star, but only millions in a massive star).

Stage 2 - The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas.

Stage 3 - In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core.

Stage 4 - The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. (The actual supernova shines brighter than the entire galaxy for a short time).

Stage 5 - Sometimes the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to become a tiny, very dense Neutron Star. If the core is much greater than 3 solar masses, the core contracts to become a Black Hole.

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