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Stars - Introduction
A star is a sphere of gas held together by its own gravity. Theclosest star to Earth is our very own Sun, so we have an example nearbythat astronomers can study in detail. The lessons we learn about theSun can be applied to other stars.
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A star's life is a constant struggle against the force of gravity. Gravity constantly works to try and cause the star to collapse. Thestar's core, however is very hot which creates pressure within the gas. This pressure counteracts the force of gravity, putting the star into what is called hydrostatic equilibrium. A star is okay as long as the star has this equilibrium between gravity pulling the star inwards and pressure pushing the star outwards.
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Diagram showing the lifecycles of Sun-like and massive stars. Click image for larger version. (Credit: NASA and the Night Sky Network)
During most a star's lifetime, the interior heat and radiation is providedby nuclear reactions in the star's core. This phase of the star's life is called the main sequence.
Before a star reaches the main sequence, the star is contracting and its core is not yet hot or dense enough to begin nuclear reactions. So, until it reaches the main sequence, hydrostatic support is provided by the heat generated from the contraction.
At some point, the star will run out of material in its core for those nuclear reactions. When the star runs out of nuclear fuel, it comes to the end of its time on the main sequence. If the star is large enough, it can go through a series of less-efficient nuclear reactions to produce internal heat. However, eventually these reactions will no longer generate sufficient heat to support the star agains its own gravity and the star will collapse.
Stellar Evolution
A star is born, lives, and dies, much like everything else in nature. Using observations of stars in all phases of their lives, astronomershave constructed a lifecycle that all stars appear to go through. Thefate and life of a star depends primarily on it's mass.
Hubble image of the Eagle Nebula, a stellar nursery. (Credit: NASA/ESA/Hubble Heritage Team)
All stars begin their lives from the collapse of material in a giantmolecular cloud. These clouds are clouds that form between the stars andconsist primarily of molecular gas and dust. Turbulence within the cloudcauses knots to form which can then collapse under it's owngravitational attraction. As the knot collapses, the material at thecenter begins to heat up. That hot core is called a protostar and willeventually become a star.
The cloud doesn't collapse into just one large star, but differentknots of material will each become it's own protostar. This is why theseclouds of material are often called stellar nuseries – they areplaces where many stars form.
As the protostar gains mass, its core gets hotter and more dense. Atsome point, it will be hot enough and dense enough for hydrogen to startfusing into helium. It needs to be 15 million Kelvin in the core forfusion to begin. When the protostar starts fusing hydrogen, it entersthe 'main sequence' phase of its life.
Stars on the main sequence are those that are fusing hydrogen intohelium in their cores. The radiation and heat from this reaction keepthe force of gravity from collapsing the star during this phase of thestar's life. This is also the longest phase of a star's life. Our sunwill spend about 10 billion years on the main sequence. However, a moremassive star uses its fuel faster, and may only be on the main sequencefor millions of years.
Eventually the core of the star runs out of hydrogen. When thathappens, the star can no longer hold up against gravity. Its innerlayers start to collapse, which squishes the core, increasing thepressure and temperature in the core of the star. While the corecollapses, the outer layers of material in the star to expand outward.The star expands to larger than it has ever been – a few hundredtimes bigger! At this point the star is called a red giant.
What happens next depends on how the mass of the star.
The Fate of Medium-Sized Stars
Hubble image of planetary nebula IC 418, also known as the Spirograph Nebula. (Credit: NASA/Hubble Heritage Team)
When a medium-sized star (up to about 7 times the mass of the Sun)reaches the red giant phase of its life, the core will have enough heatand pressure to cause helium to fuse into carbon, giving the core abrief reprieve from its collapse.
Once the helium in the core is gone, the star will shed most of itsmass, forming a cloud of material called a planetary nebula. The core ofthe star will cool and shrink, leaving behind a small, hot ball called awhite dwarf. A white dwarf doesn't collapse against gravity because ofthe pressure of electrons repelling each other in its core.
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The Fate of Massive Stars
Chandra X-ray image of supernova remnant Cassiopeia A. The colors show different wavelengths of X-rays being emitted by the matter that has been ejected from the central star. In the center is a neutron star. (Credit: NASA/CSC/SAO)
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A red giant star with more than 7 times the mass of the Sun is fatedfor a more spectacular ending.
These high-mass stars go through some of the same steps as themedium-mass stars. First, the outer layers swell out into a giant star,but even bigger, forming a red supergiant. Next, the core starts toshrink, becoming very hot and dense. Then, fusion of helium into carbonbegins in the core. When the supply of helium runs out, the core willcontract again, but since the core has more mass, it will become hot anddense enough to fuse carbon into neon. In fact, when the supply ofcarbon is used up, other fusion reactions occur, until the core isfilled with iron atoms.
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Up to this point, the fusion reactions put out energy, allowing thestar to fight gravity. However, fusing iron requires an input of energy,rather than producing excess energy. With a core full of iron, the starwill lose the fight against gravity.
The core temperature rises to over 100 billion degrees as the ironatoms are crushed together. The repulsive force between thepositively-charged nuclei overcomes the force of gravity, and the corerecoils out from the heart of the star in an explosive shock wave. Inone of the most spectacular events in the Universe, the shock propelsthe material away from the star in a tremendous explosion called asupernova. The material spews off into interstellar space.
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About 75% of the mass of the star is ejected into space in thesupernova. The fate of the left-over core depends on its mass. If theleft-over core is about 1.4 to 5 times the mass of our Sun, it willcollapse into a neutron star. If the core is larger, it will collapseinto a black hole. To turn into a neutron star, a star must start withabout 7 to 20 times the mass of the Sun before the supernova. Only starswith more than 20 times the mass of the Sun will become black holes.
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Updated: February 2014