|
A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. Red dwarfs range in mass from a low of 0.075 solar masses () to about and have a surface temperature of less than 4,000 K. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot easily be observed. From Earth, not one is visible to the naked eye.〔("The Brightest Red Dwarf" ), by Ken Croswell (Accessed 6/7/08)〕 Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.〔(Exoplanets near red dwarfs suggest another Earth nearer ), 6 February 2013, Jason Palmer, ''BBC'', retrieved at 11 April 2013〕 Stellar models indicate that red dwarfs less than are fully convective. Hence the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding a buildup at the core. Red dwarfs therefore develop very slowly, having a constant luminosity and spectral type for, in theory, some trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs of advanced evolutionary stages exist. ==Description and characteristics== Red dwarfs are very-low-mass stars. Consequently they have relatively low temperatures in their cores and energy is generated at a slow rate through nuclear fusion of hydrogen into helium by the proton–proton (PP) chain mechanism. Hence these stars emit little light, sometimes as little as that of the Sun. Even the largest red dwarfs (for example HD 179930, HIP 12961 and Lacaille 8760) have only about 10% of the Sun's luminosity. In general, red dwarfs less than transport energy from the core to the surface by convection. Convection occurs because of opacity of the interior, which has a high density compared to the temperature. As a result, energy transfer by radiation is decreased, and instead convection is the main form of energy transport to the surface of the star. Above this mass, the red dwarfs will have a region around their core where convection does not occur. Because late-type red dwarfs are fully convective, helium does not accumulate at the core and, compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence. As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespan of these stars exceeds the expected 10 billion year lifespan of our Sun by the third or fourth power of the ratio of the solar mass to their masses; thus a red dwarf may continue burning for 10 trillion years.〔 As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines and the core starts to contract. The gravitational energy released by this size reduction is converted into heat, which is carried throughout the star by convection. According to computer simulations, the minimum mass a red dwarf must have in order to become a red giant is ; less massive objects, as they age, increase their surface temperatures and luminosities becoming blue dwarfs and finally become white dwarfs.〔 The less massive the star, the longer this evolutionary process takes; for example, it has been calculated that a red dwarf (approximately the mass of the nearby Barnard's Star) would stay on the main sequence during 2.5 trillion years that would be followed by five billion years as a blue dwarf, in which the star would have 1/3 of the Sun's luminosity ()〔 and a surface temperature of 6,500‒8,500 Kelvin. The fact that red dwarfs and other low-mass stars still remain on the main sequence when more massive stars have moved off the main sequence allows the age of star clusters to be estimated by finding the mass at which the stars turn off the main sequence. This provides a lower, stellar, age limit to the Universe and also allows formation timescales to be placed upon the structures within the Milky Way, namely the Galactic halo and Galactic disk. One mystery which has not been solved is the absence of red dwarfs with no metals. (In astronomy, a metal is any element heavier than hydrogen or helium.) The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and trace amounts of lithium. If such stars included red dwarfs, they should still be observable today, but none have yet been identified. The preferred explanation is that without heavy elements only large and not yet observed population III stars can form, and these rapidly burn out, leaving heavy elements which then allow for the formation of red dwarfs. Alternative explanations, such as the idea that zero-metal red dwarfs are dim and could be few in number, are considered much less likely because they seem to conflict with stellar evolution models. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Red dwarf」の詳細全文を読む スポンサード リンク
|