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Any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the asteroid belt) between the orbits of Mars and Jupiter; also known as a minor planet. They range in size from almost 1000 km for Ceres (the first asteroid discovered, in 1801)-down to less than 10 m for the smallest so far detected. The total mass of all asteroids is 4 × 10<sup>21</sup> kg, about one-twentieth the mass of the Moon.
When an asteroid is discovered it is given a temporary designation, consisting of the year of discovery followed by two letters; the first indicates the half-month during which the asteroid was discovered, and the second the order of discovery within that half-month. Only when an accurate orbit has been determined is it assigned a permanent number, and the discoverer then has the right to name it. By the end of 1996, orbits were known for over 7200 asteroids. Increasing numbers are being discovered in dedicated searches such as the Near-Earth Asteroid Tracking (NEAT) system on Mount Haleakala, Hawaii, and the Spacewatch Program. In all, there are thought to be at least a million asteroids, of which 90–95% are in the asteroid belt.
The orbits of most asteroids have higher eccentricities and inclinations than those of the major planets. Within the main asteroid belt, orbital eccentricities average about 0.15, and inclinations about 10°; occasionally they exceed 0.5 and 30°, respectively, more typical of the orbits of short-period comets. Indeed, some objects classified as asteroids may be defunct cometary nuclei. Rotation periods of asteroids range from a few hours to several weeks, but are typically 6–24 hours. The larger asteroids are roughly spherical, but those smaller than 150 km are commonly elongated or irregular. Radar studies of a few asteroids have revealed that some may be dumbbell-shaped or possibly double; these include
Castalia and Toutatis. The asteroid Ida has a small satellite, photographed by the Galileo space probe.
Some main-belt asteroids form groups with similar orbital characteristics (semimajor axis, orbital eccentricity, and inclination), for example the Cybele, Hilda, Hungaria, and Phocaea groups. Where the group seems to have originated from the break-up of a single parent body, it is called a
Hirayama family. A small percentage of asteroids orbit outside the main asteroid belt. Members of the Amor group cross the orbit of Mars, while Apollo and Aten group asteroids cross that of Earth; these three groups are collectively termed near-Earth asteroids. Beyond the asteroid belt, the Trojan asteroids orbit at Jupiter's distance.
Asteroids are divided into various classes according to their reflectance spectra, which reveal differences in composition. The proportion of different asteroid classes changes markedly with increasing distance from the Sun. S-class (silicaceous) asteroids predominate in the inner main belt (at less than 2.4 AU). C-class (carbonaceous) asteroids are more prevalent in the middle and outer regions of the belt, with a peak near 3 AU. The dark asteroids near the outer edge of the main belt have a
reddish tinge, and may be richer in organic components; these are the P-class asteroids. Still farther out, many of the Trojan asteroids are even redder; they are termed D-class asteroids. There is an apparent concentration of M-class (metallic) asteroids in the middle of the belt, at 2.5–3.0 AU.
Asteroids are thought to have formed through the accretion of metre-sized bodies, but were prevented from aggregating into a planet by the gravitational effect of Jupiter, which had already formed. In addition, some planetesimals left over from the formation of Jupiter may have been scattered into the asteroid belt. The largest asteroids were heated by the decay of radioactive isotopes within them. They melted and became differentiated, acquiring a metallic core, overlain by a mantle and crust. Subsequent collisions led to fragmentation, and almost every asteroid is probably a fragment of a once-larger body. In addition, most meteorites are believed to be pieces of asteroids.
Industry:Astronomy
A type of galaxy with a smooth, featureless circular or elliptical appearance, no spiral arms, and little or no interstellar gas or dust; symbol E. Elliptical galaxies range from about 10<sup>7</sup> solar masses and a few thousand light years in diameter for dwarf ellipticals to over 10<sup>12</sup> solar masses and over 100 000 l.y. in diameter for giant ellipticals. Elliptical galaxies are classified from E0 to E7 according to their apparent shape, E0 appearing circular from our viewpoint and E7 the most elliptical. The degree of ellipticity is calculated from the ratios of the major (a) and minor (b) axes, using the formula 10(a–b)/a. Dwarf ellipticals are given the designation dE, while giant and supergiant ellipticals are known as D galaxies and cD galaxies respectively; such massive examples are usually found at the centres of clusters.
The stars in elliptical galaxies are mostly old (Population II), although some ellipticals also contain intermediate-age stars that formed more recently. The light from elliptical galaxies falls off in a characteristic way from the centre to the edge, except where the galaxy has been disturbed in some way, for example by tidal forces from a passing galaxy or the addition of a faint envelope by galaxy cannibalism, as occurs in D and cD galaxies. Relative to their luminosity, elliptical galaxies have the greatest number of globular star clusters of any type of galaxy; a large elliptical may possess several thousand of them. A high proportion of the bright galaxies in rich cldusters of galaxies are ellipticals (40%), whereas the general proportion of ellipticals outside rich clusters is much lower, around 10%.
The intrinsic shape of elliptical galaxies can be spheroidal (cigaror discusshaped) or truly ellipsoidal (triaxial) with different dimensions along all three axes. Some ellipticals appear to be rotating sufficiently quickly to explain their flattened shape, but many (particularly the large ellipticals) show very little rotation. The details of the origin of elliptical galaxies remain controversial. They
could either be old systems which formed rapidly and then quickly used up or lost their interstellar gas, or they could result from the merger of spiral galaxies.
Industry:Astronomy
A sudden release of energy in the Sun's corona, lasting up to several hours or, exceptionally, more than a day. Flares emit radiation over the whole spectrum, from gamma rays to radio waves. They also throw out high-speed particles (electrons, protons, and atomic nuclei), at speeds up to about 70% of the speed of light, which reach the Earth in 15 min or so, depending on their trajectory. Only the most energetic flares are visible in white light. Flares occur in active regions with complex magnetic fields, the largest flares being in the most complex regions. Most of the energy may be released in the first few minutes, with an impulsive stage that may last only a few seconds. The total energy released can be up to 10<sup>27</sup> joules; there is no well-defined lower energy limit.
Flares are classified in two ways: by their appearance in Hα light, and by their soft X-ray emission. In Hα the term subflare is given to the smallest events, and the scale then runs from 1 to 4 with increasing area; a brightness code is added from faint (f), via normal (n), to bright (b). In soft X-rays (0.1–0.8 nm) flares are classified as C, M, or X according to increasing strength, with subdivisions from 1 to 9.
In Hα, the flare may start with the disappearance of a filament (i.e. a prominence seen from above), bright areas developing into ribbons either side of the magnetic inversion line. There is generally a rapid expansion stage called the flash phase. There are often hard X-ray and microwave radio bursts forming an early impulsive stage, with soft X-rays rising more gradually to maximum a few minutes later, followed by a decline (the decay or cooling phase). Flares derive their energy from the energy stored in magnetic fields, although the exact mechanism is not known. According to one theory, flares occur when oppositely directed magnetic field lines reconnect. Particles are accelerated at the impulsive stage to give the hard X-ray emission, while the soft X-rays are emitted by a very hot (20 million K) plasma contained within coronal loops.
Industry:Astronomy
A roughly spherical group of old stars in the halo of a galaxy. Globular clusters contain from tens of thousands to millions of stars, and have diameters of 100–300 l.y. At the center of the cluster, where most stars are concentrated, the density may be over 250 stars per cubic light year. About 140 globular clusters are known in our Galaxy, travelling on highly elongated orbits around the galactic center. They are very old, about 10<sup>10</sup> years, having formed early in the history of the Galaxy. Stars in globular clusters are members of Population II, with a low content of heavy elements (only a few per cent of the solar value), although a few show values approaching that of the Sun.
On the Hertzsprung-Russell diagram of a globular cluster, the main sequence and the giant branch are smoothly joined via a subgiant branch; also conspicuous is an asymptotic giant branch and a horizontal branch, punctuated by an instability strip containing RR Lyrae variables. Globular clusters are found around all large galaxies, but are most abundant around giant elliptical galaxies. Some galaxies, particularly those that have undergone recent mergers, contain large numbers of young globular clusters.
Industry:Astronomy
An attempt to describe the weak and strong nuclear forces and electromagnetism in a single mathematical theory. Unification of the weak force with electromagnetism has been achieved in the electroweak theory. Before about 10<sup>-12</sup> seconds after the Big Bang, by which time the Universe had cooled to about 10<sup>15</sup> K, the electromagnetic and weak interactions acted as a single physical force; in the cooler temperatures since then, they have been distinct.
Attempts to unify the electroweak force with the strong nuclear force have been only partially successful. It is thought that the temperature for their unification is of the order of 10<sup>27</sup> K, which occurs only 10<sup>-36</sup> s after the Big Bang. Particles surviving to the present day from this phase are possible candidates for non-baryonic dark matter. Unification of the GUT interaction with gravity may take place at higher energies still, but there is no satisfactory theory which unifies all four physical forces. Such a theory would be called a theory of everything (TOE).
Industry:Astronomy
(symbol L) The amount of radiation a star emits, corrected for interstellar absorption. It is expressed in watts, or in terms of the luminosity of the Sun (symbol L<sub>☉</sub>), approximately 3.9 × 10<sup>26</sup> watts. Luminosity is related to the absolute bolometric magnitude of the star, M<sub>bol</sub> by the equation
M<sub>bol</sub> - 4.72 = 2.5 log(L/L<sub>☉</sub>).
The luminosities of stars range from over 10<sup>5</sup> L<sub>☉</sub> for the brightest supergiants to less than 10<sup>-5</sup> L<sub>☉</sub> for feeble red dwarfs. For X-ray sources it is usual to define luminosity not bolometrically but in particular passbands.
Industry:Astronomy
The observation of neutrinos emitted by celestial objects. Neutrinos pass through large quantities of matter without significant absorption. For example, a neutrino produced by the nuclear processes in the Sun's core has only one chance in 10<sup>10</sup> of being absorbed in escaping from the Sun. However, observations have detected only about one-third the number of neutrinos from the Sun predicted by theory (see solar neutrino unit).
Bursts of neutrinos are predicted to be produced in supernova explosions, and such a burst was detected from Supernova 1987A. Neutrinos can be detected in several ways. One uses the neutrino's interaction with the chlorine isotope <sup>37</sup>C1, which produces radioactive argon, <sup>37</sup>Ar. Detectors have also been built which utilize the conversion of gallium to germanium by a neutrino (<sup>71</sup>Ga to <sup>71</sup>Ge). Neutrino telescopes detect the direction from which the neutrino comes, as well as its existence. These rely on the neutrino colliding with an electron inside a large tank of water. The electron is then detected via its Cerenkov radiation.
Industry:Astronomy
（符號 L）那顆星發出，輻射量更正星際吸收。表示，在瓦，或大約 3.9 × 10 <sup>26</sup> 瓦 （L <sub>☉</sub> 符號），太陽的亮度。亮度絕對的明星，M <sub>bol</sub> 感測器規模有關由方程
M <sub>bol</sub>-4.72 = 2.5 日誌 (L/L <sub>☉</sub>）。
星星範圍從 10 <sup>5</sup> L <sub>☉</sub> 為對小於 10 <sup>-5</sup> L <sub>☉</sub> 為微弱的紅矮星亮 supergiants 的 luminosities。它是通常的定義亮度不 bolometrically，但在特定的 passbands 的 x 射線源。
Industry:Astronomy
觀察天體發出的中微子。中微子通過大量的無重大吸收的物質。為例，在太陽的核心核進程所產生的中微子，只有一次機會在 10 <sup>10</sup> 全神貫注地逃離太陽 然而，意見已檢測到只有約三分之一的從由理論預測太陽中微子數量 （見太陽中微子股）。
猝發的中微子，預計超新星爆炸，在生產和從超新星 1987A 檢測到這種突發。中微子可以幾種方式來檢測。一個使用中微子的互動與氯同位素 <sup>37</sup> C1，產生放射性氬氣，<sup>37</sup> Ar.探測器也已經建立，利用到鍺鎵的轉換由中微子 (<sup>71</sup> Ga 到 <sup>71</sup> Ge)。中微子望遠鏡探測中微子來自，以及其存在性的方向。這些依靠碰撞與水的大罐內的電子中微子。電子然後通過其切倫科夫輻射檢測到。
Industry:Astronomy
Qualsiasi dei molti piccoli oggetti rocciosi o metallici nel sistema solare, per lo più situata in una zona (la cintura di asteroidi) tra le orbite di Marte e Giove; noto anche come un pianeta. Essi variano nel formato da quasi 1000 km per Ceres (il primo asteroide scoperto nel 1801)-fino a meno di 10 m per i più piccoli finora rilevati. Massa il totale di tutti gli asteroidi è 4 × 10 <sup>21</sup> kg, circa un ventesimo della massa della luna.
Quando un asteroide è scoperto è dato un'indicazione provvisoria, consistendo dell'anno della scoperta seguito da due lettere; il primo indica il metà mese durante il quale l'asteroide è stato scoperto e secondo l'ordine della scoperta all'interno di quella metà mese. Solo quando un'orbita precisa è stata determinata esso è assegnato un numero permanente, e lo scopritore ha quindi il diritto per il nome. Entro la fine del 1996, orbite erano conosciute per oltre 7200 asteroidi. Crescente numeri sono essere scoperto nelle ricerche dedicate quali il sistema Near-Earth Asteroid Tracking (NEAT) sul Monte Haleakala, Hawaii e il programma Spacewatch. In tutto, ci sono probabilmente almeno 1 milione di asteroidi, di cui 90 – 95% sono nella cintura di asteroidi.
Le orbite degli asteroidi più avere maggiore eccentricità ed inclinazioni rispetto a quelli dei pianeti maggiori. All'interno della fascia principale, media eccentricità orbitale circa 0.15 e inclinazioni di circa 10°; occasionalmente superano 0,5 a 30°, rispettivamente, più tipico delle orbite delle comete di breve periodo. Infatti, alcuni oggetti classificati come asteroidi possono essere defunte nuclei cometari. Rotazione periodi di asteroidi variano da poche ore a diverse settimane, ma in genere sono 6 – 24 ore. i più grandi asteroidi sono approssimativamente sferici, ma quelli più piccoli di 150 km sono comunemente allungata o irregolare. Studi radar di alcuni asteroidi hanno rivelato che alcuni possono essere a forma di manubrio o possibilmente doppia; tra questi
Castalia e Toutatis. L'asteroide che Ida ha un piccolo satellite, fotografato dalla sonda spaziale Galileo.
Qualche forma di asteroidi della fascia principale gruppi con simili caratteristiche orbitali (semiasse maggiore eccentricità orbitale e inclinazione), ad esempio i gruppi di Cybele, Hilda, Hungaria e Phocaea. Dove il gruppo sembra avere origine dallo scioglimento di un corpo singolo genitore, si chiama un
famiglia Hirayama. Una piccola percentuale di asteroidi orbita di fuori della fascia principale. Gruppo membri dell'Amor attraversare l'orbita di Marte, mentre Apollo e Aten asteroidi gruppo cross che della terra; questi tre gruppi sono collettivamente chiamati asteroidi near-Earth. Oltre la cintura di asteroidi, l'orbita di asteroidi troiani a distanza di Giove.
Asteroidi sono divise in varie classi secondo loro spettri di riflettanza, che rivelano le differenze nella composizione. Asteroide la proporzione delle diverse classi di cambiamenti marcatamente con l'aumento della distanza dal sole asteroidi (silicati) classe S predominano nella fascia principale interna (a meno di 2,4 AU). C-classe di asteroidi (carboniosi) sono più diffusi nelle regioni intermedia ed esterne della cintura, con un picco vicino 3 AU. Buio asteroidi vicino al bordo esterno della cinghia principale hanno un
sfumatura rossastra e può essere più ricco di componenti organici; questi sono gli asteroidi P-classe. Ancora più lontano fuori, molti gli asteroidi troiani sono anche più rossa; sono chiamati asteroidi di classe D. C'è un'apparente concentrazione di asteroidi M-class (metallico) in mezzo la cinghia, presso AU 2.5 – 3.0.
Asteroidi sono probabilmente hanno formato tramite l'accrescimento di corpi di dimensioni metri, ma è sono impediti l'aggregazione in un pianeta dall'effetto gravitazionale di Giove, che si era già formata. In più, alcuni planetesimi rimasti dalla formazione di Giove potrebbero sono stati dispersi nella cintura di asteroidi. Più grandi asteroidi sono stati riscaldati dal decadimento di isotopi radioattivi all'interno di essi. Hanno fuso e divenne differenziati, acquisendo un nucleo metallico, ricoperto da un mantello e crosta. i successivi scontri portò alla frammentazione, e quasi ogni asteroide è probabilmente un frammento di un corpo più grande una volta. In più, la maggior parte dei meteoriti sono credute per essere pezzi di asteroidi.
Industry:Astronomy
