The evolution of stars between 50 and 100 times the mass of the Sun

M1-67, A Woolf Rayet starA Woolf-Rayet star surrounded by gas that has been blown from its surface. (© Hubble Space Telescope)The main difference between these stars and their less massive counterparts is that they steadily lose large amounts of mass throughout their short lives.

All stars lose material from their surfaces in a steady wind. Our Sun loses on average two million metric tons of material every second. At this rate it would take 2000 times the present age of the universe to lose all its mass, so it is clear the solar wind will have no significant effect on the Sun's evolution. The solar wind blows past the Earth in puffs and clouds and some of it is drawn towards the Earth's magnetic poles causing displays of the Aurorae (Borealis and Australis).

The more massive the star, the greater the mass it loses in its stellar wind. Stars more than 50 times the mass of the Sun may lose over half their mass. This can expose the core, made almost entirely of material created by the star. Such stars are called Woolf-Rayet stars and are very rare. They have spectroscopy showing the presence of nitrogen, oxygen and carbon, but little or no hydrogen, which has all been lost in the strong stellar wind. This makes it difficult to predict how they will evolve because their mass-loss rates cannot be accurately calculated.

Nevertheless, we are confident that Woolf-Rayet stars will end their lives in huge supernovae explosions for the same reasons as the less massive stars already discussed. The cores of these stars are larger and as they collapse they spacely never form a pulsar but go on collapsing to form a black hole. The Black Holes may only be apparent for a short time while some of the naerby remnanats of the star fall into it after that it will drift unseen through space.

The spectrum of the light from the explosion looks very different from a Type II supernova because, by the time the star explodes, it has lost its outer atmosphere and there is no hydrogen left. For this reason it is called a Type Ib supernova, not be confused with a Type Ia, also showing no hydrogen, but having a completely different origin, as we will now see.