Béntang
Béntang téh hartina raga plasma di luar angkasa nu ngahasilkeun énergi tina fusi inti. Teu kawas planét, nu cahyana mangrupa pantulan, béntang mah mencarkeun cahya téh alatan panasna anu kacida. Sacara ilmiah, béntang didefinisikeun salaku bal plasma self-gravitating dina kasatimbangan hidrostatik, nu ngahasilkeun énergina sorangan tina prosés fusi inti. Béntang 'leutik' samodél Panonpoé umumna boga permukaan anu kaciri polos jeung boga starspots anu leutik. Béntang nu leuwih badag (raksasa) boga starspot anu leuwih gedé jeung leuwih jelas. Salian ti éta permukaanana nunjukkeun stellar limb-darkening anu leuwih kuat (stellar limb-darkening téh caangna béntang beuki ka gigir kaciri beuki poek). Stellar astronomy nyaéta studi bentang-bentang.
kabéh béntang (kacuali Panonpoé) kaciri ku panon manusa jiga titik cahya di langit peuting, anu kelap-kelip lantaran efek atmosfer planet bumi. Teleskop Interferometer dibutuhkeun pikeun ngahasilkeun gambar-gambar objek-objek eta. Panonpoé kaasup béntang ogé, ngan lantaran deukeut pisan jarakna jadina kaciri jiga piringan, jeung nyadiakeun cahaya panonpoé.
Béntang-béntang di alam semesta téh nyebarna teu rata, nanging biasana mah ngarumpul jadi galaksi-galaksi. Hiji galaksi biasana dibentuk ku ratusan milyar milyar béntang. Mayoritas béntang silih iket alatan gravitasi jeung béntang séjén, ngabentuk bintang kembar. Sedengkeun kumpulan béntang-béntang anu leuwih loba disebut klaster béntang.
Astronom ngira-ngira paling saeutik aya 70 sextillion (7×1022) béntang di alam semesta [1]. Hartina aya 70 000 000 000 000 000 000 000, atawa 230 milyar kali leuwih loba batan 300 milyar di galaksi urang sorangan (galaksi Bima Sakti).
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Lolobana bentang téh umurna antara samilyar nepi ka 10 milyar taun. Sababaraha bentang umurna bisa nepi ka 13.7 milyar taun, nyaéta umur alam semesta nu ka ukur. (Tingal Big Bang theory jeung stellar evolution.) They range in size from the tiny neutron stars (which are actually déad stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diaméter about 1,000 times larger than the Sun —about 1.6 terametres. However, these have a much lower density than the Sun.
One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. 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 éra of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster néar the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The réason 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 héavier than lithium in their composition. This generation of supermassive star is long extinct, however, and currently only théoretical.
With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nucléar fusion in its core. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey aréa between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.
The energy produced by stars radiates into space as electromagnetic radiation, as a stréam of neutrinos from the star's core, and as a stréam of particles from the star's outer layers (its stellar wind). The péak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measured by its apparent magnitude.
The néarest star to the éarth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilométers, or 4.2 light years away (light from Proxima Centauri takes 4.2 yéars to réach éarth). Travelling at the orbit speed of the Space Shuttle (5 miles per second—almost 30,000 kilométers per hour), it would take about 150,000 yéars to get there. Distances like this are typical inside galactic discs, where the Sun and éarth are located. Stars can be much closer to éach other in the centres of galaxies and globular clusters, or much further apart in galactic halos.
Star formation and evolution
[édit | édit sumber]Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an éarthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or 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 réactions néar 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 yéars. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is gréater than the current age of the universe (13.6 billion yéars), 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 yéars, 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 héats up and contracts. Larger stars will also fuse héavier elements, all the way to iron, which is the end point of the process. Since iron nuclei are more tightly bound than any héavier nuclei, they cannot be fused to reléase energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be reléased 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 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 fusion to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into black dwarfs over very long stretches of time.
In larger stars, 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, a black hole.
The blown-off outer layers of dying stars include héavy elements which may be recycled during new star formation. These héavy 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.
Stellar evolution explains how stars are créated and die in gréater detail.
Star classification
[édit | édit sumber]There are different classifications of stars ranging from type W which are very large and bright, to M which is often just large enough to start ignition of the hydrogen. Some of the more common classifications are O, B, A, F, G, K, M, and can perhaps be more éasily remembéréd using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon (1863-1941). There are many other mnemonics for star classification; the most frequent addition tacks "Right Now, Sweetheart" for the red dwarf sub-types R, N and S. The new types L and T have also been recently appended to the end of the OBAFGKM sequence to classify the coldest low-mass stars and brown dwarfs, prompting such additions as "Lovingly Tonight" to the mnemonic.
éach letter has 9 subclassifications. Our Sun is a G2, which is very néar the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type. 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 we have), and most characteristics of other stars are usually given in solar units.
For example, the mass of the Sun is
- MSun = 1.9891×1030 kg
The masses of other stars can be given in terms of MSun.
Naming of stars
[édit | édit sumber]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 (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them, and many in the astronomy community view these organizations as frauds preying on péople 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.
Nuclear fusion reaction pathways
[édit | édit sumber]A variety of different nucléar fusion réactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis).
Stars begin as a cloud of mostly hydrogen with about 25% helium and héavier elements in smaller quantities. 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 réactions result in the overall réaction:
- 41H → 4He + 2e+ + 2γ + 2νe (26.7 MeV)
In more massive stars, helium is produced in a cycle of réactions 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 réaction of:
- 34He → 12C + γ + 7.2 MeV
Mitologi
[édit | édit sumber]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 déad, or gods/goddesses.
Rujukan
[édit | édit sumber]- 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.