Star
From Aetilc
A star is a hot, roughly spherical ball of gas that shines as a result of nuclear fusion reactions in its core. Stars are one of the fundamental objects in the universe. Stars—and indeed the entire universe—are made mostly of hydrogen, the simplest and lightest element. Heavier elements are created in the cores of stars, and the final act in the lives of many stars is a massive explosion that distributes the elements it has created into the galaxy. Eventually these elements may form another star, or a planet, or life on that planet.
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[edit] Star Formation
Stars are born in the interstellar medium, the region of space between stars. Drifting through this region are vast, dark clouds of gas and dust. Certain celestial events, like the nearby explosion of a massive star at the end of its life (supernova), cause these clouds to begin to contract. After a supernova, a shock wave sweeps through the interstellar medium. When it slams into the cloud, the gas and dust is violently compressed by the shock. As the particles are squeezed together, their mutual gravitational attraction grows and a blob of gas forms, giving off energy.
As the temperature in a contracting blob of gas becomes higher, the gas exerts a pressure that counteracts the inward force of gravity. At this point, perhaps millions of years after the shock wave slammed into the dark cloud, the contraction stops. If the blob of gas has become hot enough at its center to begin thermonuclear fusion of hydrogen into helium, it has become a star. It will remain in this stable state for millions or billions of years.
An interstellar cloud does not always have to be disturbed by a shock wave to form stars, however. Sometimes a cloud may be hot and dense enough to break up and contract spontaneously under its own gravity. Large clouds can break up into numerous cloudlets this way, and this process leads to the formation of star clusters—groups of stars close to each other in space. Often, two stars will form very close to one another, orbiting around a common center of gravity. This two-star system is called a binary star. Both star clusters and binary stars are more common than single stars.
[edit] Star deaths
All stars eventually exhaust their hydrogen fuel. At this point, the gas pressure within the star goes down and the star begins to contract under its own gravity. The fate awaiting a star at this point is determined by its mass.
An average-sized star will spend the final 10 percent of its life as a red giant. In this phase of a star's evolution, the star's surface temperature drops to between 1,727 and 3,727°C and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.
Buried deep inside the star is a hot, dense core. Helium left burning at the core eventually ejects the star's atmosphere, which floats off into space as a planetary nebula (a cloud of gas and dust). The remaining glowing core is called a white dwarf. Like a dying ember in a campfire, it will gradually cool off and fade into blackness. Space is littered with such dead suns.
A star up to three times the mass of the Sun explodes in a supernova, shedding much of its mass. Any remaining matter of such a star ends up as a densely packed neutron star or pulsar, a rapidly rotating neutron star that emits varying radio waves at precise intervals.
A star more than three times the mass of the Sun will also explode in a supernova. Its remaining mass becomes so concentrated that it shrinks to an indefinitely small size and its gravity becomes completely over-powering. This single point in space where pressure and density are infinite is called a black hole.
[edit] Stellar Stages
[edit] Main Sequence Stars
When the star has accumulated enough material so that the temperature and pressure are high enough, nuclear fusion reactions, which convert hydrogen into helium, begin deep within the core of the star. The energy from the reactions makes its way to the surface of the star in about a million years, causing the star to shine. The pressure from these nuclear reactions at the star's core balances the pull of gravity, and the star is now called a main sequence star.
This name is derived from the relationship between a star's intrinsic brightness and its temperature, which was discovered independently. This relationship is displayed in a Hertzsprung-Russell diagram. A star's color depends on its surface temperature; red stars are the coolest and blue stars are the hottest. The temperature, brightness, and longevity of a star on the main sequence are determined by its mass; the least massive main sequence stars are the coolest and dimmest, and the most massive stars are the hottest and brightest. Objects less than about one-thirteenth the mass of the Sun can never sustain fusion reactions. These objects are known as brown dwarfs.
[edit] Red Giants and Red Supergiants
Counterintuitively, the more massive a star is, the more rapidly it uses up the hydrogen at its core. The most massive stars deplete their central hydrogen supply in a million years, whereas stars that are only about one-tenth the mass of the Sun remain on the main sequence for hundreds of billions of years. When hydrogen becomes depleted in the core, the core starts to collapse, and the temperature and pressure rise, so that fusion reactions can begin in a shell around the helium core. This new heat supply causes the outer layers of the star to expand and cool, and the star becomes a red giant, or a red supergiant if it is very massive.
[edit] Planetary Nebulae, White Dwarfs, and Black Dwarfs
Once stars up to a few times the mass of the Sun reach the red giant phase, the core continues to contract and temperatures and pressures in the core become high enough for helium nuclei to fuse together to form carbon. This process occurs rapidly (only a few minutes in a star like the Sun), and the star begins to shed the outer layers of its atmosphere as a diffuse cloud called a planetary nebula. Eventually, only about 20 percent of the star's initial mass remains in a very dense core called a white dwarf. White dwarfs are stable because the pressure of electrons repulsing each other balances the pull of gravity. There is no fuel left to burn, so the star slowly cools over billions of years, eventually becoming a cold, dark object known as a black dwarf.
[edit] Supernovae, Neutron Stars, and Black Holes
After a star more than about five times the mass of the Sun has become a red supergiant, its core goes through several contractions, becoming hotter and denser each time, initiating a new series of nuclear reactions that release energy and temporarily halt the collapse. Once the core has become primarily iron, however, energy can no longer be released through fusion reactions, because energy is required to fuse iron into heavier elements. The core then collapses violently in less than a tenth of a second.
The energy released from this collapse sends a shock wave through the star's outer layers, compressing the material and fusing new elements and radioactive isotopes, which are propelled into space in a spectacular explosion known as a supernova. This material seeds space with heavy elements and may collide with other clouds of gas and dust, compressing them and initiating the formation of new stars. The core that remains behind after the explosion may become either a neutron star, as the intense pressure forces electrons to combine with protons , or a black hole, if the original star was massive enough so that not even the pressure of the neutrons can overcome gravity. Black holes are stars that have literally collapsed out of existence, leaving behind only an intense gravitational pull.
[edit] Words to Know
- Binary star: Double-star system in which two stars orbit each other around a central point of gravity.
- Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity. A stellar mass blackhole's mass is usually more than 5 Solar masses, while supermassive black holes mass ranges from a few hundred thousand to a few billion times the mass of the Sun.
- Core: The central region of a star, where thermonuclear fusion reactions take place that produce the energy necessary for the star to support itself against its own gravity.
- Interstellar medium: Space between the stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.
- Nebula: Cloud of interstellar gas and dust.
- Neutron star: Extremely dense, compact, neutron-filled remains of a star following a supernova. Neutron stars are usually between 1.4 to 3 Solar masses in mass.
- Nuclear fusion: Merging of two or more hydrogen nuclei into one helium nucleus, accompanied by a tremendous release of energy.
- Pulsar: Rapidly spinning, blinking neutron star.
- Red giant: Stage in which an average-sized star spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.
- Star cluster: Groups of stars close to each other in space that appear to have roughly similar characteristics and, therefore, a common origin.
- Supernova: Explosion of a massive star at the end of is lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.
- White dwarf: Cooling, shrunken core remaining after an average-sized star ceases to burn. Their mass is usually between 0.17 to 1.4 Solar masses.
