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Life of Stars

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Material present in the forms of stars, constellations, galaxies, solar systems, planets, nova, quasars, neutrons stars, comets, interstellar dust, asteroids, and supernova in the unlimited space we call universe or sky, had fascinated Kepler, Galileo, Descartes, Newton and many others. Their revelations have made the universe less mysterious.

Distances between stars and planets and their masses are usually expressed in astronomical units such as light years (nine and half trillion kilometers, 9.5x1012 km), mass of the sun (1.989x1030 kg), and the mass of the earth (six heptillion kilograms, 5.976x1024 kg).

There are billions of stars out there in the sky, but only a small percentage of them are visible to the unaided eyes. The sun is one of them. The next nearest star, the Alpha or Proxima Centauri is 4.3 light years from the Sun, whereas the Sirius and the Procyon are 8.6 and 11.4 light years from the Sun respectively. Stars generally occur in pairs and multiple systems (clusters), but the sun and a few others exist singly.

The study of heavenly bodies is in the realm of astronomy. As to how materials in stars change or how the life of a star evolve has got some hint from studies of tiny nuclei and their reactions on earth. Based on our observation of nuclear reactions, we now have a general perception about the formation, live, and evolution of stars. Unlike chemical reactions, which rearrange or regroup atoms to form new molecules, nuclear reactions change the identities of the atomic nuclei. Fusion of deuterium to give helium and capture of neutron by an atomic nucleus are examples of nuclear reactions.

Life of Stars

Fusion Reaction in the Sun
4 H = 4He + 2 Positron + Energy
Positron is the antiparticle of electron
Distribution of hydrogen and interstellar dust in space may accidentally become so dense that they contract under their own gravity, causing temperature and density to rise. When the mass starts to give a red glow, a protostar is formed. When temperatures at the center of the mass increase to ten-million degrees Kelvin, hydrogen will fuse to form helium in nuclear fusion reactions. Unlike ordinary chemical reactions we are familiar with, nuclear reactions convert one chemical element into another such as from hydrogen (H) to helium (He), releasing a lot of energy, which causes the temperature to rise further. The energy causes the surface to heat up, and eventually, energy escapes from the mass as radiation (heat and light). At some point in time, the state is steady in that the amount of energy released from fusion reactions equals to the amount lost by radiation, and we call such a collection of mass a star.

The Sun consists mostly of hydrogen and helium, but its center is very dense (150 times the density of water) due to compression. At half of its radius, the density is about the same as water. Estimates indicated that two thirds of the original hydrogen in the sun has been converted to helium. Eventually, when hydrogen is depleted, the sun will evolve, and the amount of energy released will decrease. Fortunately, the sun will still be stable for some millions of years.

When hydrogen is depleted, stars evolve in a manner depending on their masses. A star with mass 1.4 times the mass of the sun becomes a red giant when its hydrogen is depleted. The surface become cooler and eventually it becomes a white dwarf. The expanding outer gaseous shell is then known as a planetary nebula. However, when small stars are close to its partner which is becoming a red giant, it will acquire material from the red giant's outer layer. Accumulation of the material eventually causes nuclear reactions, the energy from which blows off the accumulated material in a brief, but violent explosion. Such a stage is called a nova.

Life of a Star
  • Condense mass
  • Protostar
  • Star (stable situation)
  • Red Giant
  • White Dwarf & Planetary Nebula
  • Nova or Supernova
  • Neutron Star or Black Hole
Stars five or more times the mass of the Sun can continue generate energy after their hydrogen is depleted, because their centers are hotter (hundreds million K). Helium and other light elements continue fuse forming elements as heavy as iron. Fusion of heavy elements causes catastrophic collapse of the stellar core. The outer layers of a massive star explode forming a super nova, which may be a trillion times brighter than a star, and the illumination may last several months. The remaining core may become a neutron star, which has the mass of star, but the size is much reduced. Currently, it has been postulated that a really massive star may become a black hole after the super nova stage.

The Sun

Some useful links are placed here for futher exploration. Hypertext in Astronomy and Physics offers some interesting features, but this site is still under construction.

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