Star

A star is a massive, compact body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. The most familiar and closest star to the Earth is the Sun. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Stellar astronomy is the study of stars.

Individual stars differ from each other due to their total mass, their composition, and their age. The total mass determines the course of evolution of a star, as well as its eventual fate. A Hertzsprung-Russell diagram shows the pattern of the temperature of stars against their absolute magnitude, and can be used to determine the overall age of a star and the stage in its evolution. Initially, stars are composed primarily of hydrogen, with some helium and heavier trace elements that determine the metallicity of a star. Over the course of a star's evolution, a portion of the hydrogen is converted into heavier elements through the process of nuclear fusion. Part of the matter is then recycled back into the interstellar environment, where it is used to form a new generation of more metal-rich stars.

Multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. For example, a nova occurs when a white dwarf accretes matter from a companion star.

Star formation and evolution

Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or the collision of two galaxies (as in a starburst galaxy). High-mass stars powerfully illuminate the clouds from which they formed. One example of such a star-forming nebula is the Orion Nebula.

A protostar forms at the core of a collapsing cloud of gas and dust. These pre-main sequence stars are often surrounded by a protoplanetary disk, and their energy is powered through gravitational contraction. Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. The period of gravitational contraction lasts for about 10-15 million years. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.

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. Starting at zero age main sequence, the amount of helium in a star's core will steadily accumulate. 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 Gyr ago.

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, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no black dwarfs exist yet.

As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume both Mercury and Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. In low mass stars (less than the 1.4 solar mass) the helium fusion process begins with an explosive burst of energy generation known as a helium flash. The energy resulting from this event is equivalent to the luminosity of 10⁸ Suns, but it only lasts upto a few minutes. However, this energy goes into the elimination of the electron degeneracy at the core, and is not visible from the exterior.

Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process. 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 old, very massive stars, a large core of inert iron will accumulate in the center of the star.

An average-size star (less than 1.4 solar masses after explosion) will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of electron degenerate matter not massive enough for further compression to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into brown, and then black dwarfs over a very long stretch of time. Electron degenerate matter is not plasma, even though stars are generally refered to as being spheres of plasma.

In larger stars, defined as having more than 1.4 solar masses after explosion, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay. 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.

Eventually, most of the matter in a star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars (more than 3 solar masses after explosion), a black hole. In neutron stars and black holes, the star is not in a plasma state of matter, but either neutron degenerate matter or a state of matter not currently understood within the black hole.

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.

Appearance and distribution of stars

All stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds. The disks of stars are much too small to be observed with current ground-based optical telescopes, and so Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.

It has been a long-held fact that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary stars. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.

Larger groups called star clusters also exist. Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are between 50 billion and 100 billion galaxies in the known universe.

Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the known universe ^*. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way.

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (10¹²) kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbital speed of the Space Shuttle (5 miles per second—almost 30,000 kilometers per hour), it would take about 150,000 years to get there.3.99 × 10¹³ km / (3 × 10⁴ km/hr × 24 × 365.25) = 1.5 × 10⁵ years. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much farther apart in galactic halos.

Small (dwarf) stars such as the Sun generally have essentially featureless disks with only small starspots. Larger (giant) stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb-darkening (the brightness decreases towards the edge of the stellar disk). Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.

Age and size of stars

Almost everything about a star is determined by its initial overall mass, including its destiny and fate, as well as its essential characteristics, such as lifespan, luminosity, and size. Stars range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometers. However, Betelgeuse has a much lower density than the Sun.

Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang theory and stellar evolution.) The more massive the star, the shorter its lifespan will be, primarily because the greater a star’s mass, the greater the degree of pressure on its internal core, causing the star to burn its hydrogen fuel in greater amounts per second, thus depleting the star’s fuel much more rapidly. The most massive stars burn their fuel very rapidly and last about a million years on average, while stars of minimum mass (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.

Most of our understanding of stars comes from theoretical models and simulations, although these models are based on spectral observations and measurements of the diameters of stars. The first measurement of the diameter of a star other than our Sun was made in 1921 by Albert Abraham Michelson on the Hooker telescope.

One of the most massive stars known is Eta Carinae, with 100–150 times as much mass as the Sun, and its lifespan is very short, being only several million years at most. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer.

The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.

Star classification

There are different classifications of stars 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. The main classifications can be easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon. There are many other mnemonics for star classification. A variety of rare spectral types have 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 subclassifications numbered (hottest to coldest) from 0 to 9. This system matches closely with temperature, but breaks down at the extreme hottest end; class O0 and O1 stars may not exist.

In addition, stars may be classified by their "luminosity effects", which correspond to their spatial size. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). Most stars fall into the main sequence which consists of ordinary hydrogen-burning stars. These fall along a narrow band when graphed according to their absolute magnitude and spectral type.

Our Sun is a G2V (yellow dwarf), being of intermediate temperature and ordinary size. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star), and most characteristics of other stars are usually given in solar units.

{| solar mass:

M_\bigodot = 1.9891 \times 10^{30}

kg solar luminosity:

L_\bigodot = 3.827 \times 10^{26}

watts.

Stellar structure

The interior of a stable, main sequence star is in a state of equilibrium in which the forces in any small volume exactly counterbalance each other. The balancing forces consist of inward directed gravitational force and the opposing pressure from the thermal energy of the plasma gas. For these forces to balance out, the temperature at the core of a typical star to be on the order of 10⁷ °C or higher. 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 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 a shell within the star will exactly match the incoming flux.

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, as near the core, or in areas with high opacity, as in the outer envelope.

The occurance of convection in the outer envelope of a main sequence star depends on the spectral type. Massive stars 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 convective zones will also vary over time as the star ages and the constitution of the interior is modified.

The portion of a main sequence star that is visible to an observer is called the photosphere. This is the layer at which the plasma gas 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 star spots, or regions of lower than average temerature, 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. 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.

Nuclear fusion reaction pathways

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is converted into energy, according to the mass-energy relationship E=mc².

Stars begin as a cloud of mostly hydrogen with about 23–28% helium and a few percent heavier elements. In the Sun, with a 10⁷ °K core, hydrogen fuses to form helium in the proton-proton chain:

4¹H → 2²H + 2e⁺ + 2ν_e (4.0 MeV + 1.0 MeV)

2¹H + 2²H → 2³He + 2γ (5.5 MeV)

2³He → ⁴He + 2¹H (12.9 MeV)

These reactions result in the overall reaction:

4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV)

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.

In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process:

⁴He + ⁴He + 92 keV → ^8*Be

⁴He + ^8*Be + 67 keV → ^12*C

^12*C → ¹²C + γ + 7.4 MeV

For an overall reaction of:

3⁴He → ¹²C + γ + 7.2 MeV

The final stage in the stellar nucleosynthesis process is the Silicon burning process that results in the production of the stable isotope iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.

Radiation

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind, which exists as a steady stream of electrically charged particles (such as free protons, alpha particles, and beta particles) emanating from the star’s outer layers and as a steady stream of neutrinos emanating from the star’s core.

The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element deep inside the core of a star, photons of electromagnetic energy are released from the nuclear fusion reaction, which are then converted to visible light in the star’s outer layers.

The peak frequency and color of the visible light depends on the temperature of the star’s outer layers, including its photosphere. Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans across the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

Stellar Luminosity and Magnitude

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. Luminosity is typically expressed in the SI unit watts (W), in the CGS unit ergs per second (ergs/s), or in terms of solar luminosities (L_Sun); that is, how many times more energy the star radiates than the Sun, whose luminosity is 382.7 septillion watts (L_Sun = 3.827 x 10²⁶ W).

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere.

Intrinsic or absolute magnitude is the apparent magnitude a star would have if it were observed from a distance of 10 parsecs (32.6 light-years) from Earth, and it is directly related to a star’s luminosity, measured from the standard distance of 10 parsecs.

Both the apparent and absolute magnitude scales are logarithmic: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times (2.512 to be precise). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star, which is the faintest star visible to the naked eye.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness between two stars is calculated by subtracting the magnitude number of the brighter star from the magnitude number of the fainter star, then using the difference as an exponent for the base number 2.512; that is to say (m_{f_{– m_{b_{= x) and (2.512^{x^{= variation in brightness).}}}}}}

Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not exactly equivalent for an individual star; for example, the bright star Sirius has an apparent magnitude of -1.44, but it has an absolute magnitude of +1.41.

Our Sun has an apparent magnitude of -26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky, is approximately 23 times more luminous than our Sun or L_Sun x 23 W, while Canopus, the second brightest star in the night sky, with an absolute magnitude of -5.53, is approximately 14,000 times more luminous than our Sun or L_Sun x 14,000 W. Despite Canopus being vastly more luminous than Sirius, Sirius appears brighter than Canopus to our eyes, only because it is merely 8.6 light-years away from us, while Canopus is much further away from us at 310 light-years.

In terms of apparent magnitude (m), what is the difference in brightness between Sirius and Polaris?

(m_{f_{– m_{b_{= x)}}}}

(2.512^{x^{= variation in brightness)}}

The apparent magnitude of Sirius is -1.44, and the apparent magnitude of Polaris is 1.97. Polaris is the fainter of the two stars, while Sirius is the brighter.

(m_{f_{– m_{b_{= x)}}}}

(1.97 – -1.44 = x)

(1.97 – -1.44 = 3.41)

(x = 3.41)

(2.512^{x^{= variation in brightness)}}

(2.512^{3.41^{= variation in brightness)}}

(2.512^{3.41^{= 23.124)}}

(variation in brightness = 23.124)

In terms of apparent magnitude, Sirius is 23.124 times brighter than Polaris the North Star.

Variable stars

Many stars undergo significant variations in luminosity, and these are known as variable stars.

Naming of stars

Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (for instance, the "International Star Registry") purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.

See star designations for more information on how stars are named. For a list of traditional names, see the list of stars by constellation.

Star mythology

As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead or gods and goddesses. In the Greco-Roman pantheon, some "stars", later identified as planets, represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken. (Uranus, Neptune and Pluto were also Roman gods, but none of these three planets were known to the Romans due to their low brightness. Their names were assigned by later astronomers.)

References

Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press
John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.

External links

Stars

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