Supergiant Stars - Properties

Properties

Supergiants have masses from 8 to 12 times the Sun (M_☉) upwards, and luminosities from about 10,000 to over a million times the Sun (L_☉). They vary greatly in radius, usually from 30 to 500, or even in excess of 1,000 solar radii (R_☉). They are massive enough to begin core helium burning gently before the core becomes degenerate, without a flash, and without the strong dredge-ups that lower-mass stars experience. They go on to successively ignite heavier elements, usually all the way to iron. Also because of their high masses they are destined to explode as supernovae.

The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts. Radiation pressure limits the largest cool supergiants to around 1,500 R_☉ and the most massive hot supergiants to around a million L_☉ (M_V around -9). Stars near and occasionally beyond these limits become unstable, pulsate, and experience rapid mass loss.

Supergiants are categorized on the basis of their spectra. Supergiants occur in every spectral class from young blue class O supergiants stars to highly evolved red class M supergiants. Because they are enlarged compared to main-sequence and giant stars of the same spectral type, they have lower surface gravities and changes can be observed in their line profiles. Supergiants are also evolved stars with higher levels of heavy elements than main-sequence stars. This is the basis of the MK luminosity system which assigns stars to luminosity classes purely from observing their spectra. In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials which can produce emission lines, P Cygni profiles, or forbidden lines. The MK system assigns stars to luminosity classes: Ib for supergiants; Ia for luminous supergiants; and 0 (zero) or Ia+ for hypergiants. In reality there is very much of a continuum rather than well defined bands for these classifications, and classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are frequently annotated to indicate spectral peculiarities, for example B2Iae or F8Iabpec.