4.6 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Stellar classification | 8/8 | https://en.wikipedia.org/wiki/Stellar_classification | reference | science, encyclopedia | 2026-05-05T13:33:43.575832+00:00 | kb-cron |
Stellar remnants are objects associated with the death of stars. Included in the category are white dwarfs, and as can be seen from the radically different classification scheme for class D, stellar remnants are difficult to fit into the MK system. The Hertzsprung–Russell diagram, which the MK system is based on, is observational in nature so these remnants cannot easily be plotted on the diagram, or cannot be placed at all. Old neutron stars are relatively small and cold, and would fall on the far right side of the diagram. Planetary nebulae are dynamic and tend to quickly fade in brightness as the progenitor star transitions to the white dwarf branch. If shown, a planetary nebula would be plotted to the right of the diagram's upper right quadrant. A black hole emits no visible light of its own, and therefore would not appear on the diagram. A classification system for neutron stars using Roman numerals has been proposed: type I for less massive neutron stars with low cooling rates, type II for more massive neutron stars with higher cooling rates, and a proposed type III for more massive neutron stars (possible exotic star candidates) with higher cooling rates. The more massive a neutron star is, the higher neutrino flux it carries. These neutrinos carry away so much heat energy that after only a few years the temperature of an isolated neutron star falls from the order of billions to only around a million Kelvin. This proposed neutron star classification system is not to be confused with the earlier Secchi spectral classes and the Yerkes luminosity classes.
== Replaced spectral classes == Several spectral types, all previously used for non-standard stars in the mid-20th century, have been replaced during revisions of the stellar classification system. They may still be found in old editions of star catalogs: R and N have been subsumed into the new C class as C-R and C-N.
== Stellar classification, habitability, and the search for life ==
While humans may eventually be able to colonize any kind of stellar habitat, this section will address the probability of life arising around other stars. Stability, luminosity, and lifespan are all factors in stellar habitability. Humans know of only one star that hosts life, the G-class Sun, a star with an abundance of heavy elements and low variability in brightness. The Solar System is also unlike many stellar systems in that it only contains one star (see Habitability of binary star systems). Working from these constraints and the problems of having an empirical sample set of only one, the range of stars that are predicted to be able to support life is limited by a few factors. Of the main-sequence star types, stars more massive than 1.5 times that of the Sun (spectral types O, B, and A) age too quickly for advanced life to develop (using Earth as a guideline). On the other extreme, dwarfs of less than half the mass of the Sun (spectral type M) are likely to tidally lock planets within their habitable zone, along with other problems (see Habitability of red dwarf systems). While there are many problems facing life on red dwarfs, many astronomers continue to model these systems due to their sheer numbers and longevity. For these reasons NASA's Kepler Mission is searching for habitable planets at nearby main-sequence stars that are less massive than spectral type A but more massive than type M—making the most probable stars to host life dwarf stars of types F, G, and K.
== See also == Astrograph – Type of telescope Guest star – Ancient Chinese name for cataclysmic variable stars Spectral signature – Variation of reflectance or emittance of a material with respect to wavelengths Star count – Bookkeeping survey of stars, survey of stars Stellar dynamics – Branch of astrophysics
== Notes ==
== References ==
== Further reading == Harre, Jan-Vincent; Heller, René (2021). "Digital color codes of stars". Astronomische Nachrichten. 342 (3): 578–587. arXiv:2101.06254. Bibcode:2021AN....342..578H. doi:10.1002/asna.202113868. S2CID 231627588.
== External links ==
Libraries of stellar spectra by D. Montes, UCM Spectral Types for Hipparcos Catalogue Entries Stellar Spectral Classification Archived 31 October 2010 at the Wayback Machine by Richard O. Gray and Christopher J. Corbally Spectral models of stars by P. Coelho Merrifield, Michael; Bauer, Amanda; Häußler, Boris (2010). "Star Classification". Sixty Symbols. Brady Haran for the University of Nottingham. Stellar classification table