what two things balance to maintain the shape of a star
Stars
A star is a sphere of gas held together by its own gravity. The closest star to World is our very ain Dominicus, and so we have an example nearby that astronomers can study in particular. The lessons we learn about the Sun can be applied to other stars.
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A star's life is a constant struggle against the strength of gravity. Gravity constantly works to endeavor and cause the star to plummet. The star'southward core, however is very hot which creates pressure within the gas. This force per unit area counteracts the forcefulness 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 inward and pressure pushing the star outwards.
During nearly a star'south lifetime, the interior heat and radiation is provided by 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 still hot or dumbo enough to begin nuclear reactions. So, until it reaches the main sequence, hydrostatic back up is provided past the heat generated from the wrinkle.
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 chief sequence. If the star is big plenty, it can go through a series of less-efficient nuclear reactions to produce internal estrus. However, eventually these reactions volition no longer generate sufficient heat to back up the star agains its own gravity and the star will collapse.
Stellar Evolution
A star is built-in, lives, and dies, much like everything else in nature. Using observations of stars in all phases of their lives, astronomers accept constructed a lifecycle that all stars appear to go through. The fate and life of a star depends primarily on information technology's mass.
All stars begin their lives from the collapse of fabric in a giant molecular cloud. These clouds are clouds that grade between the stars and consist primarily of molecular gas and grit. Turbulence within the cloud causes knots to form which tin can then collapse under information technology'south ain gravitational allure. As the knot collapses, the material at the center begins to heat up. That hot cadre is chosen a protostar and will eventually go a star.
The cloud doesn't plummet into just one large star, only different knots of cloth will each become information technology's own protostar. This is why these clouds of material are often called stellar nuseries – they are places where many stars form.
As the protostar gains mass, its core gets hotter and more dense. At some point, it will be hot enough and dense enough for hydrogen to start fusing into helium. It needs to be fifteen million Kelvin in the core for fusion to begin. When the protostar starts fusing hydrogen, it enters the "main sequence" phase of its life.
Stars on the chief sequence are those that are fusing hydrogen into helium in their cores. The radiation and heat from this reaction keep the force of gravity from collapsing the star during this phase of the star's life. This is as well the longest stage of a star's life. Our lord's day will spend virtually x billion years on the master sequence. However, a more than massive star uses its fuel faster, and may only be on the master sequence for millions of years.
Eventually the core of the star runs out of hydrogen. When that happens, the star tin can no longer concord upward against gravity. Its inner layers offset to collapse, which squishes the core, increasing the pressure level and temperature in the core of the star. While the cadre collapses, the outer layers of material in the star to expand outward. The star expands to larger than it has always been – a few hundred times bigger! At this indicate the star is chosen a cerise behemothic.
What happens next depends on how the mass of the star.
The Fate of Medium-Sized Stars
When a medium-sized star (upwardly to about 7 times the mass of the Dominicus) reaches the cherry giant phase of its life, the cadre will take enough heat and pressure to crusade helium to fuse into carbon, giving the core a brief reprieve from its plummet.
Once the helium in the core is gone, the star will shed nigh of its mass, forming a cloud of material called a planetary nebula. The core of the star will cool and shrink, leaving behind a small-scale, hot brawl called a white dwarf. A white dwarf doesn't collapse against gravity because of the pressure of electrons repelling each other in its cadre.
The Fate of Massive Stars
A red giant star with more than 7 times the mass of the Dominicus is fated for a more spectacular ending.
These high-mass stars go through some of the same steps as the medium-mass stars. First, the outer layers swell out into a giant star, just fifty-fifty bigger, forming a red supergiant. Next, the core starts to compress, becoming very hot and dense. Then, fusion of helium into carbon begins in the cadre. When the supply of helium runs out, the cadre will contract once again, only since the core has more mass, it volition go hot and dense plenty to fuse carbon into neon. In fact, when the supply of carbon is used up, other fusion reactions occur, until the core is filled with fe atoms.
Up to this betoken, the fusion reactions put out energy, assuasive the star to fight gravity. However, fusing atomic number 26 requires an input of free energy, rather than producing excess free energy. With a core full of iron, the star will lose the fight confronting gravity.
The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive strength between the positively-charged nuclei overcomes the forcefulness of gravity, and the core recoils out from the heart of the star in an explosive daze wave. In ane of the nearly spectacular events in the Universe, the shock propels the material away from the star in a tremendous explosion called a supernova. The material spews off into interstellar infinite.
About 75% of the mass of the star is ejected into space in the supernova. The fate of the left-over core depends on its mass. If the left-over core is about 1.4 to 5 times the mass of our Sun, it volition collapse into a neutron star. If the cadre is larger, it will collapse into a black hole. To turn into a neutron star, a star must start with well-nigh seven to xx times the mass of the Sun earlier the supernova. Merely stars with more than 20 times the mass of the Sun will become black holes.
Updated: Feb 2014
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Related Topics
- The Dominicus
- The Solar Corona
- Eta Carinae, a Home-Grown Mystery
For Educators
- NCTM & NSES Standards
- Life Cycles of Stars Booklet
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Source: https://imagine.gsfc.nasa.gov/science/objects/stars1.html
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