Hubble-Sandage Variables (Variable Stars)
We have already noted that rare, luminous red stars like Betelgeuse vary irregularly in brightness. In 1953, Edwin Hubble and Allan Sandage reported that, in the nearby galaxies M 31 and M 33, the most luminous stars of all colours were variable in brightness. In most cases, the variations were slow, small and irregular, but in a few cases, the variations were quite the opposite. HUBBLE-SANDAGE VARIABLES have absolute magnitudes in excess of —9; in fact, were it not for their large variations in brightness, they would be useful distance indicators for nearby galaxies.
Their main interest, however, is the fact that they tell us some¬thing about the evolution of the most massive, luminous stars. Astronomers have always wondered why there are no stars with masses much greater than about 50 solar masses. They have suspected, from theoretical studies, that more massive stars would be unstable against the pressure of their own radiation, but the precise results of these studies are conflicting.
To get a better understanding of the nature of Hubble-Sandage variables, we should look for them in our own Galaxy. Unfortunately, they are likely to be hidden within the thick dust clouds in the Milky Way. However, there are at least two good candidates: P Cygni and ? | Carinae. P Cygni varied from magnitude 3 to below magnitude 6 several times during the 17th century, but has remained near magnitude 5 since. Its absolute magnitude is now — 9, but at maximum was —11 – brighter than a million Suns. Its spectrum gives a clue to the nature of the light variations; it contains many sharp absorption lines, shifted to shorter wavelengths. These arise from an expanding shell of gas around the star. This suggests that the brightening episodes in the seventeenth century were caused by the expansion of the outer layers of the star and by the ejection of the shell of gas which .
An even more exotic case is ? Carinae. From 1600 to 1800, it rose in brightness until by 1840 it outshone every star except Sirius. It has now faded to magnitude 7 . if Carinae is embedded in a dense, expanding cloud of dust and gas, even denser than the one around P Cygni. The dense cloud is itself situated in the complex nebulosity shown in figure 4.19. Much of the energy from ? Carinae is absorbed by the dust cloud, and is re-emitted as infrared radiation ? | Carinae is the strongest infrared source in the sky, outside the Solar System.
There is much controversy about the nature of the star within the cloud. Is it a massive young star in the last stages of birth ? Is it a kind of slow supernova eruption, muffled by the cloud of material around it ? Or is it a Hubble-Sandage variable: a massive young star trying to evolve normally but incapable of doing so in stable fashion ? The key to this mystery lies buried within the obscuring cloud of dust, but modern astronomical techniques – particularly radio and infrared techniques – should uncover this key.