M dwarfs dominate the stellar population, accounting for three of every four stars, the nearest of which is Proxima Centauri, the closest destination beyond our Solar System. These cool stars span large ranges in luminosities (one ten-thousandth to 6% L ⊙ ) and temperatures (2,100–3,900 K) and have spectra dominated by absorption bands of titanium oxide (TiO) and, for the latest spectral types, vanadium oxide (VO). They have masses that span 0.075 to 0.61 M ⊙ , a factor of eight, which is comparable with a spread in masses for dwarf types mid- B through K . Unlike these more massive stars, in the age of the Universe no M dwarfs have evolved in any significant way. M dwarf systems are multiple roughly one-quarter of the time, with the closest binaries found in orbits that have been circularized via tides for orbital periods of about one week. Unlike any other type of main sequence star, there is a gap in the distribution of M dwarfs near masses of 0.35 M ⊙ that pinpoints the separation of partially and fully convective stars, yet both types of M dwarfs are often active, showing both Hα in emission and flares. Many planets are found orbiting M dwarfs, and most of them are terrestrial or neptunian in size, rather than jovian, yet much more work remains to be done to characterize the exoplanet population. Overall, the Solar Neighborhood is dominated by M dwarfs that are likely orbited by many small, as yet unseen, planets—some of which may harbor life very near to that in our Solar System. ▪ M dwarfs account for three of every four stars. ▪ M dwarf counts increase all the way to the end of the main sequence. ▪ M dwarfs are partially radiative at high masses and fully convective at low masses.

Todd J. Henry

Wei-Chun Jao

Annual Review of Astronomy and Astrophysics

自 1963 年出版的《天文学和天体物理学年度评论》涵盖了天文学和天体物理学领域的重大发展，包括：太阳；太阳系和系外行星；星星;星际介质；星系和星系；活跃的星系核；宇宙学；和仪器和技术，以及新研究领域的发展历史。

The Annual Review of Astronomy and Astrophysics has been published since 1963, covering significant developments in the fields of astronomy and astrophysics. This includes topics such as the Sun, the solar system and exoplanets, stars, interstellar medium, galaxies and their evolution, active galactic nuclei, cosmology, as well as advancements in instruments and techniques. The journal also discusses the historical development of new research areas.

《Annual Review of Astronomy and Astrophysics》自1963年出版，涵盖了天文学和天体物理学领域的重要发展，包括太阳、太阳系及系外行星、恒星、星际介质、星系及其演化、活跃星系核、宇宙学，以及相关仪器和技术的进展。此外，期刊还探讨了新研究领域的发展历程。

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物理与天体物理	1
物理与天体物理, 天文与天体物理	1

The Character of M Dwarfs

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Journal Info

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ASTRONOMY & ASTROPHYSICS	1