Star
From Obsidian Fleet Database
A star (or "sun") is an energy-producing sphere of plasma and gas. The region around a star that is held by its gravity, including any planets, moons, comets, and asteroids, is called a star system.
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[edit] Stellar life cycle
[edit] Formation
Stars are born out of huge gaseous nebulae. Inside these nebulae, centers of higher density form, slowly accumulating more mass as the center's gravity increases, to form a protostar. Pressure in the interior of the protoster rises, in turn increasing the density and temperature until the gas turns to plasma, where the atomic nuclei and the electrons are dissociated from each other. At a sufficient temperature and pressure, nuclear fusion is initiated at the core, producing light: the star is born.
[edit] Structure
The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 107 K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.
As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.
In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.
The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity as in the outer envelope.
The occurrence of convection in the outer envelope of a main sequence star depends on the mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers. Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core. For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.
The portion of a star that is visible to an observer is called the photosphere. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.
Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.[108] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star. Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse.
From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.
[edit] Formation and evolution
Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[28] As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region.
[edit] Protostar formation
The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae (massive stellar explosions) or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.
As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. These can contain up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[29] These pre-main sequence stars are often surrounded by a protoplanetary disk. The period of gravitational contraction lasts for about 10–15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.
[edit] Main sequence
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The Sun loses 10−14 solar masses every year, or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution.[34] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to burn and the rate at which it burns that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 1010 years. Large stars burn their fuel very rapidly and are short-lived. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer. However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no such stars are expected to exist yet.
Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields and modify the strength of the stellar wind. Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)
[edit] Post-main sequence
As stars of at least 0.4 Solar masses exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. For example, in about 5 billion years, when the Sun is a red giant, it will expand out to a maximum radius of roughly 1 AU (150,000,000 km), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass.
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.[39] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.
After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.
[edit] Massive stars
During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium.
The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.
The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.
[edit] Collapse
An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf. The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.
In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.
Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole. In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
[edit] Stellar classification
Stars are assigned to different spectral classes.
Furthermore, a plot of stellar luminosities versus stellar spectral types on two axes, is called the Hertzsprung-Russell diagram.
[edit] Types of stars
[edit] Other Information
- Dr. Tolian Soran used a Trilithium weapon in 2371 to stop all fusion reactions inside the Amargosa sun, thereby collapsing the star and altering the gravitational conditions in the system.
As a sun gets older it begins to fuse heavier elements, like helium, as the lighter elements like hydrogen are depleted. This, however, releases more energy, causing the star to swell, which increases its surface area from which the energy is emitted. This phase marks the beginning of the star's end.
- In 2367, Dr. Timicin of the planet Kaelon II tried to save the dying star Kaelon by regulating its ever increasing temperature with the bombardment of Photon torpedoes. However, the experiment failed after testing the procedure with a star in an uninhabited solar system.
Because of its larger surface area, the star turns red and is then called a red giant. After the sun runs out of light elements and the number of fusion reactions decreases, its own gravity causes it to collapse and to expel its outer layers of matter, creating beautiful "planetary nebulae". The remnant of the star is called white dwarf.
Every star has to pass these stages of evolution. However, depending on their masses, some suns experience further changes.
Below 1.5 Sol masses: After 1-10 billion years any nuclear reactions inside the white dwarf finally cease and the star turns to a "black dwarf", a very small stellar corpse.
- In 2370, the USS Prometheus hosted Dr. Gideon Seyetik's succesful attempt to re-ignite the stellar corpse Epsilon 199 by using a protomatter-laden shuttlepod remotely sent into the star. In the case of a failure, the star could have exploded in a supernova instead.
Above 1.5 Sol masses: The white dwarf swells again, fusing all elements up to iron. After the last iron is depleted, the star turns into a supernova, where the outer layers of the sun explode, which, in turn, causes a massive shock wave. The remains of this explosion are a vast matter nebula and a tiny neutron star, which is so dense, that all protons and electrons are neutralized to neutrons. A special form of neutron stars are pulsars.
- In 2269, the star Beta Niobe turned to a supernova, when crewmembers of the Federation starship USS Enterprise were almost trapped on its planet Sarpeidon.
- The resulting electro-magnetic pulse from the supernova of Beta Magellan in 2364 was so feared by the computer-dependent Bynar in the nearby Beta Magellan system that they commandeered the USS Enterprise-D]] as a temporary dump for their planetary computer.
If the remnant of a supernova is more massive than 2.5 Sol masses, it collapses to a black hole.
