An Etymological Dictionary of Astronomy and Astrophysics
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The evolutionary path of a star that has exhausted its available hydrogen content in the core, between the → main-sequence turnoff and the → helium flash.

→ red giant; → branch.

A supergiant star with spectral type K or M. Red supergiants are the largest stars in the Universe, but not necessarily the most massive. Betelgeuse and Antares are the best known examples of a red supergiant.

→ red; → supergiant.

A star with a lower absolute magnitude than a normal giant star of its → spectral type. It is a star that has exhausted the hydrogen at its center and is evolving into a → red giant. Subgiants are luminosity class IV.

→ sub-; → giant.

A star with maximum intrinsic brightness and low density. The radius of a supergiant can be as large as 1000 times that of the Sun. See also → blue supergiant; → red supergiant; → yellow supergiant.

→ super-; → giant

A highly luminous → B[e] star with a luminosity greater than 10⁴L_sun. A number of such objects exist in the → Magellanic Clouds, e.g. LMC R126, R66, SMC R4, and R50. A likely example in our Galaxy is MWC 300.

→ supergiant; → B[e] star.

A technique for deriving extragalactic distances which uses the → luminosity of the brightest → red giant branch stars in old → stellar populations as a → standard candle. For old (> 2-3 Gyr), → metal-poor ([Fe/H] < -0.7) stellar populations, this luminosity is relatively well determined, and the → absolute magnitude of these stars in the I band is roughly constant (M_I = -4.1 ± 0.1).

→ tip; → red giant; → branch; → method.

A star that appears in the upper-middle part of the → H-R diagram, to the left of the → red giants. Yellow giants are low-mass evolved stars that are burning their helium, on their path to the → planetary nebula stage. Most yellow giants behave as variable stars, usually because their outer layers pulsate. Periods of these pulsations are usually days or weeks. The Sun after leaving the red giant stage will become a pulsating yellow giant for some 100 million years.

→ yellow; → giant.

An evolved, → very massive star of spectral type F or G with a very high luminosity (~10⁵ times solar) lying near the empirical upper luminosity boundary in the → H-R diagram (→ Humphreys-Davidson limit). Yellow hypergiants have high → mass loss rates (10^-5-10^-3 solar masses per year) and are in a short, transitional evolutionary stage. Their evolutionary state is thought to correspond to post-red supergiants rapidly evolving in blueward loops in the H-R diagram. In their post-RSG blueward evolution these stars enter a temperature range (6000-9000 K), called → yellow void, with increased dynamical instability. Their link to other advanced evolutionary phases of massive stars such as → Luminous Blue Variables and → Wolf-Rayet stars is still an open issue in stellar evolution theory. The most famous yellow hypergiant is → Rho Cassiopeiae.

→ yellow; → hypergiant.

A supergiant star of type F and G whose effective temperature is between 4800 and 7500 K. Yellow supergiants are extremely rare, because they represent a very short-lived phase, typically a few tens of thousands of year, in the evolution of → massive stars.

→ yellow; → supergiant.