Star
- For other uses, see Star (disambiguation).
A star is a massive 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. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Stellar astronomy is the study of stars.
A Hertzsprung-Russell diagram shows the pattern of the temperature of stars against their absolute magnitude.
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 nebula is the Orion Nebula.
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.
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. 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 (<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 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.
In larger stars, defined as >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 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 (>3 solar masses after explosion), a 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. 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.
Stars are almost never isolated; instead, the vast majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. 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 [1]. 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 kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit 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. 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 further 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).
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.
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: MSun = 1.9891×1030 kg
- solar luminosity: LSun = 3.827×1026 W.
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.
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+ + 2νe (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 + 2νe (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:
For an overall reaction of:
- 34He → 12C + γ + 7.2 MeV
Energy production
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. This is how and 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 (LSun); that is, how many times more energy the star radiates than the Sun, whose luminosity is 382.7 septillion watts (LSun = 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 (mf – mb = x) and (2.512x = 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 LSun 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 LSun 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?
(mf – mb = x)
(2.512x = 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.
(mf – mb = x)
(1.97 – -1.44 = x)
(1.97 – -1.44 = 3.41)
(x = 3.41)
(2.512x = variation in brightness)
(2.5123.41 = variation in brightness)
(2.5123.41 = 23.124)
(variation in brightness = 23.124)
In terms of apparent magnitude, Sirius is 23.124 times brighter than Polaris the North Star.
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.)
ReferencesISBN links support NWE through referral fees
- Cliff Pickover, "The Stars of Heaven", Oxford University Press (2001).
- John Gribbin and Mary Gribbin, "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press (2001).
See also
- Black hole
- Blue straggler
- Overview of star constellations
- Nursery rhyme Twinkle twinkle little star
- sidereal clock
- Star count
- Star clocks
- Stars with articles in Wikipedia
- Stellar navigation
- Stellar evolution
- Timeline of stellar astronomy
- Variable star
Related lists
- Lists of stars
- List of brightest stars (apparent & absolute magnitude)
- List of heaviest stars (by solar mass)
- List of largest stars (by diameter)
- List of mnemonics for star classification
- List of nearest bright stars
- List of nearest stars
- List of stars by constellation
- List of stars with confirmed extrasolar planets
External links
- Images of starspots on the surface of Betelgeuse
- Find out what is known about any given star by entering its name or position
- There are more single stars than previously thought
- Top stars picked in alien search
- Star Classification
- Star Worldbook
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
