Stars Mm Beschreibung
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Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.
This generation of supermassive population III stars is likely to have existed in the very early universe i. The combination of the radius and the mass of a star determines its surface gravity.
Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs.
The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines. The rotation rate of stars can be determined through spectroscopic measurement , or more exactly determined by tracking their starspots.
Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum —the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin.
A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.
The pulsar at the heart of the Crab nebula , for example, rotates 30 times per second. The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.
Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core. The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum.
The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star see classification below.
Smaller stars such as the Sun have surface temperatures of a few thousand K. The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation.
The particle radiation emitted by a star is manifested as the stellar wind,  which streams from the outer layers as electrically charged protons and alpha and beta particles.
Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core. The production of energy at the core is the reason stars shine so brightly: every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion product.
This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.
In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum , from the longest wavelengths of radio waves through infrared , visible light, ultraviolet , to the shortest of X-rays , and gamma rays.
From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
Using the stellar spectrum , astronomers can also determine the surface temperature, surface gravity , metallicity and rotational velocity of a star.
If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived.
The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances.
Gravitational microlensing has been used to measure the mass of a single star. The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time.
It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface.
The rapidly rotating star Vega , for example, has a higher energy flux power per unit area at its poles than along its equator. Patches of the star's surface with a lower temperature and luminosity than average are known as starspots.
Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots. Giant stars have much larger, more obvious starspots,  and they also exhibit strong stellar limb darkening.
That is, the brightness decreases towards the edge of the stellar disk. The apparent brightness of a star is expressed in terms of its apparent magnitude.
It is a function of the star's luminosity, its distance from Earth, the extinction effect of interstellar dust and gas, and the altering of the star's light as it passes through Earth's atmosphere.
Intrinsic or absolute magnitude is directly related to a star's luminosity, and is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs Both the apparent and absolute magnitude scales are logarithmic units : one whole number difference in magnitude is equal to a brightness variation of about 2.
On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter the star.
The brightest stars, on either scale, have negative magnitude numbers. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus.
This is because Sirius is merely 8. This star is at least 5,, times more luminous than the Sun. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered.
These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.
The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.
The classifications were since reordered by temperature, on which the modern scheme is based. Stars are given a single-letter classification according to their spectra, ranging from type O , which are very hot, to M , which are so cool that molecules may form in their atmospheres.
A variety of rare spectral types are given special classifications. The most common of these are types L and T , which classify the coldest low-mass stars and brown dwarfs.
Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures as classes O0 and O1 may not exist.
In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by their surface gravity.
Main sequence stars fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.
Additional nomenclature, in the form of lower-case letters added to the end of the spectral type to indicate peculiar features of the spectrum.
For example, an " e " can indicate the presence of emission lines; " m " represents unusually strong levels of metals, and " var " can mean variations in the spectral type.
White dwarf stars have their own class that begins with the letter D. This is followed by a numerical value that indicates the temperature.
Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.
During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star.
This category includes Cepheid and Cepheid-like stars , and long-period variables such as Mira. Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.
Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties.
This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.
Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.
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 10 7 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.
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 of the stellar interior where the flux of energy outward is dependent on radiative heat transfer, since convective heat transfer is inefficient in that zone.
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 making radiatative heat transfer inefficient as in the outer envelope.
The occurrence of convection in the outer envelope of a main sequence star depends on the star's 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. The photosphere is that portion of a star that is visible to an observer.
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 into space.
It is within the photosphere that sun spots , 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, just above the photosphere, is the thin chromosphere region, where spicules appear and stellar flares begin.
Beyond this is the corona , a volume of super-heated plasma that can extend outward to several million kilometres.
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, until it interacts with the interstellar medium.
For the Sun, the influence of its solar wind extends throughout a bubble-shaped region called the heliosphere. A variety of nuclear fusion reactions take place in the cores of stars, that depend upon their mass and composition.
When nuclei fuse, the mass of the fused product is less than the mass of the original parts. The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate.
In the Sun, with a million-kelvin core, hydrogen fuses to form helium in the proton—proton chain reaction : . The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy.
However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output.
In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5. In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle.
In massive stars, heavier elements can also be burned in a contracting core through the neon-burning process and oxygen-burning process.
The final stage in the stellar nucleosynthesis process is the silicon-burning process that results in the production of the stable isotope iron As an O-class main sequence star, it would be 8 times the solar radius and 62, times the Sun's luminosity.
From Wikipedia, the free encyclopedia. Astronomical spheroid of plasma. This article is about the astronomical object.
For other uses, see Star disambiguation. Main articles: Stellar designation , Astronomical naming conventions , and Star catalogue. Main article: Stellar evolution.
Main article: Star formation. Main article: Main sequence. Main articles: Supergiant star , Hypergiant , and Wolf—Rayet star.
Main article: Stellar age estimation. See also: Metallicity and Molecules in stars. Main articles: List of largest stars , List of least voluminous stars , and Solar radius.
Main article: Stellar kinematics. Main article: Stellar magnetic field. Main article: Stellar mass. Main article: Stellar rotation.
Main articles: Apparent magnitude and Absolute magnitude. Main article: Stellar classification. Main article: Variable star.
Main article: Stellar structure. Main article: Stellar nucleosynthesis. Stars portal Astronomy portal. Fusor astronomy Exoplanet host stars Lists of stars List of largest known stars Outline of astronomy Sidereal time Star clocks Star count Stars and planetary systems in fiction Stellar astronomy Stellar dynamics.
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Scientific American. July See Table 1. Astrophysics: Decoding the Cosmos. Bibcode : adc.. But his plan was much greater than just accumulating knowledge and skills: his desire was to share the experience back in his native country and be able to achieve something great in Myanmar.
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