Stellar mass is the most important attribute of a star. It
determines a star’s luminosity, its physical size, and its
ultimate fate (Kaler, 2001; Philips 1999). The most massive
stars are the brightest, and they are known as “Supergiants”
because of their large size and their enormous luminosity
compared to most other stars (Moore, 2002). Supergiants have a
mass greater than 10 times that of the Sun, and the most massive
of them are known as Hypergiants (Kaler, 2001). These stars lie
above the Main Sequence of the Hertzsprung-Russell Diagram and
are often unstable with unpredictable variations in their
luminosities. They occur with spectral types O to M. An
observational upper limit to stellar luminosity has been found
in the Milky Way and the Local Group of galaxies, the
Humphreys-Davidson limit (Lamers, 1988). It is somehow related
to how massive a star can become. The Eddington limit is a
theoretical upper limit to the ratio of luminosity to mass for a
star. At the Eddington limit, the outward pressure of a star’s
energy from its central nuclear processes exceeds its inward
gravitational pull. At this point, a more massive star could not
hold itself together with a resultant outflow of stellar
material and mass loss (Kaler, 2001; Moore, 2002). This limit
has been variously estimated as 60-440 Solar masses (Moore,
2002).
Measured stellar masses range from 0.08 Solar masses to
observational claims of stellar masses as large as 150 Solar
masses (Weidner, 2004). It is difficult to directly measure the
mass of most large, luminous stars, because they are often not
part of a binary system. While it is possible to estimate
stellar mass based on a star’s effective temperature and
luminosity, this is not necessarily accurate for massive stars
that are no longer on the Main Sequence. Simply stated, the
upper limit for stellar mass is unknown. The most massive stars
are hydrogen burning O stars. Only a very small fraction (10-7)
of the stars in the Galaxy are more massive than 20 Solar
masses. Since massive stars are so uncommon, the upper limit for
their mass is unknown. The observed limit of 150 Solar masses
may reflect either a fundamental mass limit, or it may merely
reflect an observational limitation of the data (Weidner, 2004).
“There is a significant (factor 2!) discrepancy between the
masses predicted by stellar evolutionary models and stellar
atmosphere models…” (Massey, 1999). Some stellar formation
models suggest a limit of 100 Solar masses as a finite upper
limit for stellar mass, while other models allow for larger mass
limits through disk-accretion and escape of thermal radiation
pole-ward (Weidner, 2004). Stars with a mass greater than 100
Solar masses do not have their maximum luminosity in the optical
bands, which makes photometry of these stars difficult. Indirect
photometric and spectroscopic examination of the massive young
star cluster R136 in the Large Magellanic Cloud finds stars with
masses as great as 140-155 Solar masses (Massey, 1997; Weidner
2004). Direct measurement of massive stars is rare. However,
combined radial velocity and photometry for massive
spectroscopic binaries in the R136 cluster has found one primary
star with a directly computed mass of 57 Solar masses and
“…comparison of …masses with those derived from standard
evolutionary tracks shows excellent agreement” (Penny, 2001).
Thus, we can infer that determination of stellar mass for
massive stars using indirect photometric and spectroscopic
methods is reasonably accurate (Massey, 1999). |
