Cold Stars
Stars are often classified by temperature and cold stars describe a variety of stars and protostars with low stellar temperatures. Cold stars have surface temperatures around as low as 1,000K to 3,000K. In contrast, hot stars have temperatures around 40,000K. Cold stars encompass lower temperature brown dwarfs (protostars in which fusion has yet to commence), and also to compact objects such as old neutron stars, in which fusion has ceased.
Brown dwarfs are failed protostars of less than 0.084 solar masses. Below this critical mass, protostars are not massive enough to develop the high core temperatures (approximately 3 million degrees) needed to fuse hydrogen into helium. As a result, the energy emitted from these stars is limited to the heat generated through gravitational contraction. The spectrum of these stars may be identified by the prescience of lithium, which in normal stars is destroyed during the fusion process, and also by the presence of molecules such as methane, which is also destroyed at relatively low temperatures. It has been suggested that a large fraction of the dark matter in the universe may be contained in brown dwarfs. Brown dwarfs are difficult to detect, with no reliable estimate of their numbers feasible at this time. Current theories of star formation differ greatly on the number of brown dwarfs that might exist, with estimates ranging from numbers greater than M-class stars (a majority of the main-sequence stars that comprise over 90% of observed stars), to numbers that would make brown dwarfs a scarce cosmological species. Because of their extreme dimness, these stars were not discovered until the 1990s. The star Gliese 229B in the constellation of the Pleiades is thought to be a brown dwarf.
All protostars form within dense interstellar clouds. Gravitational perturbations arising from a variety of sources create regions of slightly higher gravity in which matter within the interior of the cloud begins to collapse, eventually forming a hot rotating disk within the cloud. The disk continues condensing into a protostar. A strong stellar wind begins, possibly as a result of compression of the magnetic field, which emerges along the axis of the disk's rotation. These bipolar flows, or jets, dispel the cloud, revealing the new star. The protostars which emerge are known as T Tauri stars, and exhibit a strong infrared spectrum, a result of the obscuring dust in the interstellar birth cloud. Protostars with sufficient mass continue to condense, begin hydrogen fusion and take their place among the hydrogen fusing main-sequence stars found on Hertzsprung-Russell diagrams.
At the other extreme of stellar evolution, cold stars may encompass old cold neutron stars that are the massive core remnants of stars which have undergone a series of supernovae explosions. These stars, more massive than the Sun, are composed mainly of neutrons and are on the order of only ten kilometers across. The Hubble Space telescope has successfully imaged a non-pulsar dim neutron star that will dim and cool over millions of years.
Cooler stars are redder than hotter stars. A blue-visible color index (B-V index) quantifies the spectra of stars. Using two different filters the blue (B) filter allows measurement of only narrow range blue-range wavelengths while the visible (V) filter only allows measurement of green-yellow wavelengths. The B-V index for stars ranges from cool stars at 2.0 to hot stars at 0 (or negative). Other methods for determining stellar temperature involve the use of Weinüs law. The peak of the continuous spectrum for cool stars is found in the longer (redder) wavelengths. As stellar temperatures increase, the continuous spectrum peak shifts to shorter (bluer) wavelengths. It is also possible to relate stellar temperatures to the strength of different absorption lines in a star's spectrum.
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