How Big can a Star be? - Bigger than we Thought!

By Derek Allen

For many years the accepted wisdom has been that there is an upper limit to the possible size of any star, of about 120 times the mass of the Sun. This is based on the theory of how stars are formed by the gravitational collapse of gas clouds and the nuclear reactions which start as the temperature and pressure rise inside the resulting growing proto-star. The effect of the nuclear reactions, initially the conversion of hydrogen to helium, is to halt the collapse and form a stable star, as the outward pressure generated balances the inward gravitational force. In a proto-star growing from the collapse of a very massive gas cloud, the outward pressure from the increasing internal generation of energy eventually exceeds the effect of gravity and it starts to push away additional excess mass from its surface. It has been reliably calculated that this happens when such a proto-star has grown to about 120 solar masses, and so this is the maximum possible mass for a stable star.

Now enter star LBV 1806-20, about 45,000 light-years away on the far side of the Milky Way. This bright blue star has been known for several years and was identified as a 'luminous blue variable' (LBV), which are fairly rare, massive and short-lived stars which display light and colour variability. LBV 1806-20's light is largely blocked from our view by intervening gas and dust clouds, and only about ten percent of its infrared light in our direction actually reaches us. The star's apparent magnitude is 8.4. LBV 1806-20 has been further investigated more recently using the 200 inch Hale Telescope at the Palomar Observatory in California. The telescope's camera was equipped with new 'speckle imaging' technology to mitigate the interfering effect of the Earth's atmosphere on star observations, and this considerably improved the resulting image resolution. Observations from the four-metre telescope at the Cerro Tololo Observatory in Chile were also used.

LBV 1806-20 turns out to be an incredible body of unimaginable proportions. It is a hypergiant with a total luminosity that varies between 5 and 40 million times that of our Sun. Its mass is at least 150 solar masses, way above the previously thought limit, and its diameter has swelled to at least 200 times that of the Sun. Its surface temperature is estimated to vary between 18,000 and 60,000 0K. The previously most massive and luminous star known was the Pistol Star (named from the pistol-shaped nebula surrounding it), of about 100 solar masses and six million times brighter than the Sun. The possibility of LBV 1806-20 being an unresolved dense cluster of lower-mass stars has been ruled out by the high resolution of the observations, although there is a small chance that it could be a binary star system. Theoretically this would present other problems, first because the variable light from it is characteristic of a single star, and secondly it is not known how two massive bodies could have formed and coexist so close to one another.

So - is it back to the drawing board for the theory of stellar formation? Probably not, as LBV 1806-20 seems not to have been formed in the conventional way. It is situated at the edge of a small cluster of massive stars contained within a volume less than 6.5 light-years across, and which includes a number of other peculiar stars - two blue hypergiants, two carbon-rich stars, a rare magnetic neutron star known as a soft gamma-ray repeater, and a massive proto-star - a star still being born. LBV 1806-20 also lies at the core of a radio nebula which is powered by the tremendous stellar wind generated by LBV 1806-20 itself, and it is located close to a molecular cloud from which the whole cluster could have originated.

LBV 1806-20 is most likely to have been formed as a special case of shock-induced star formation. The nearby magnetic neutron star is the remnant of an earlier massive star which reached the end of its life span and exploded in an intense supernova between one and two million years ago. This also fits the age of LBV 1806-20. The shock wave from the exploding supernova could have quickly - over only 100,000 years or so - compressed nearby gas and dust from the molecular cloud which was already collapsing under gravity into a proto-star. The combined gravitational force and the extra shock force of compression would then have resulted in the growing proto-star growing far beyond the usual size limit described earlier, before the increasing outward pressure exceeded these combined opposing forces and excess mass started to be shed.

In general, the more massive a star, the quicker it burns its nuclear fuel and the shorter is its life. LBV 1806-20 is only about a million years old but is already middle aged. It is losing vast amounts of gas from its outer layers in an intense stellar wind and in enormous eruptions. The ejected gas, mostly hydrogen, is moving away from it at speeds of 50 to 500 km per second and this has formed the nebula, bright at radio wavelengths, seen around it. In only another million years or so it will have burned all its nuclear fuel and will explode as a Type-II supernova, or as a hypernova with an intense burst of gamma radiation. For comparison, stars like our Sun, which is five billion years old, live for about ten billion years. It is perhaps fortunate for us and our descendants that LBV 1806-20 is the other side of the Galaxy to us!

Reprinted with permission from Mercury 19 (3) July 2004, the Newsletter of Stirling Astronomical Society