An Etymological Dictionary of Astronomy and Astrophysics
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Massive star formation outside → OB associations. Recent observational findings suggest that → massive star formation is a collective process. In other words, massive stars form in → cluster environments and the mass of the most massive star in a cluster is correlated with the mass of the cluster itself. Nevertheless, other observational results give grounds for supposing that massive stars do not necessarily form in clusters but that they can be formed as isolated stars or in very small groups. According to statistical studies nearly 95% of Galactic → O star population is located in clusters or OB associations. This means that a small percentage, about 5%, of high mass stars may form in isolation. Isolation is meant not traceable to an origin in an OB association. This definition therefore excludes → runaway massive stars, which are thought to result from either dynamical interaction in massive dense clusters, or via a kick from a → supernova explosion in a → binary system. Alternatively, isolated massive star has been defined as follows: An O-type star belonging to a cluster whose total mass is < 100 Msun and moreover is devoid of → B stars (Selier et al. 2011, A&A 529, A40 and references therein).

→ isolated; → massive star; → formation.

A → stellar magnetic field associated with a → massive star. Magnetic fields are detected only for seven to ten percent of all studied massive → OB stars, and the magnetic field occurrence does not depend on the → spectral type. Because these magnetic fields seem to be stable over long time-scales and their strength does not seem to correlate with known stellar properties, it is assumed that they are of fossil origin (→ fossil magnetic field) and are frozen into the → radiative envelope of the stars. The fields are those of the birth → molecular clouds, partly trapped inside the → pre-main sequence star during the cloud → collapse phase, possibly further enhanced by a → dynamo effect in the early fully convective stellar phase. Typically, the polar field strength ranges from about a hundred → Gauss up to several kiloGauss. However, some weaker fields, below 100 G, have recently been detected.
The stellar magnetic field influences many different regions of the star with various effects. In the deep interior of the star, the field influences the internal → mixing of the star and this affects the size of the → convective overshooting region, changing the lifetime of the star by decreasing the amount of fuel for nuclear burning. Magnetic stars can also confine their → stellar winds, due to their strong magnetic fields, into a → magnetosphere, which slows down the → rotational velocity of the star. This → magnetic braking is an efficient mechanism for → angular momentum transport. At the stellar surface, the magnetic fields can create and sustain areas of chemical over- or under-abundances and/or large temperature differences, which are called spots (Buysschaert et al., 2016, astro-ph/1709.02619).

→ magnetic; → massive; → star.

An international collaboration devoted to the study of the origin and physics of → magnetic fields in → massive stars. The project uses several observatories and a large number of telescopes equipped with → spectropolarimetric and → asteroseismologic instruments, including → HARPS, → HARPSpol, and → ESPaDOnS (Wade et al., 2016, MNRAS 456, 2).

→ magnetism; → massive; → star.

A star whose mass is larger than approximately 10 → solar masses. The → spectral types of massive stars range from about B3 (→ B star) to O2 (→ O star) and include → Wolf-Rayet stars as well as → Luminous Blue Variables. Massive stars are very rare; for each star of 20 solar masses there are some 100,000 stars of 1 solar mass. Despite this rarity, they play a key role in astrophysics. They are major sites of → nucleosynthesis beyond oxygen and, therefore, are mainly responsible for the → chemical evolution of galaxies. Due to their high ultraviolet flux and powerful → stellar winds, they bring about interesting phenomena in the → interstellar medium, like → H II regions, → turbulence, → shocks, → bubbles, and so on. Massive stars are progenitors of → supernovae (→ type Ia, → type Ic and → type II), → neutron stars, and → black holes. The formation processes of massive stars is still an unresolved problem. For massive stars the → accretion time scale is larger than the → Kelvin-Helmholtz time scale. This means that massive stars reach the → main sequence while → accretion is still going on.

→ massive; → star.

A star with an initial mass over about 120 solar masses. The existence of such stars is the present Universe is not confirmed. Such stars were proposed as an explanation for very bright O type stars in the Large Magellanic Cloud, but these are now known to be clusters of ordinary O stars. → very massive star; → massive star.

→ supermassive; → star.

A star of mass around 100 → solar masses. See also: → supermassive star, → massive star, → canonical upper limit.

→ very; → massive; → star.