Superluminous stars eject mass

Press, 27 May 1981, Page 11

Superluminous stars eject mass

A small number of stars known as SD variables include some of the brightest stars in terms of their absolute magnitudes. All these objects have ejected mass to form a nebular shell that is continuing expanding. The term SD simply means S Doradus, one of the most outstanding examples of this class. Their light varies in an irregular manner, although some also undergo eclipses by a companion star. Absolute magnitude is a measure of the brightness a star would have at the standard distance of 10 parsecs or 32.6 light years. It is thus a true indication of the real luminosity of a star whereas apparent magnitude only indicates the star’s brightness as we see it without regard to its distance. The Sun, for example, has an apparent magnitude of —26.8 but an absolute magnitude of 4 .85.

New Zealand astronomers have provided the details of the light variations of an SD type star, termed AG Carinae. These observations have been used in conjunction with those made from the international Ultraviolet Explorer satellite (1.U.E.). AG Carinae normally exhibits only slight changes in brightness but has undergone five periods of activity during the last'ss years. At such times this star becomes much brighter when it almost reaches naked eye visibility. During periods of activity it varies comparatively quickly. This star has ejected matter which has formed a nebul'a around the star that has the size, mass, and expansion velocity of a planetary nebula. The absolute magnitude of AG Carinae is —8.5 and it is 2000 parsecs distant. It is' thus a true supergiant star. The best known member of the SD variables is Eta Carinae, which, in 1843 became the second brightest star in the sky, being exceeded only by Sirius. Before this outburst Eta Carinae Siad ; varied in brightness between fourth and second magnitudes for two centuries.

After its 1843 outburst Eta Carinae faded slightly but remained a first magnitude object for 15 years. Then a steady fading set in and by 1865 it was no longer visible to the naked eye.

A much slower decline to eighth magnitude then occurred over some 40 years. After remaining essentially constant for many years it slowly commenced to brighten in 1951 reaching sixth magnitude the following year. It has oscillated around that brightness since then, but New Zealand observations have shown that its fluctuations have become more pronounced this year. In a telescope Eta Carinae appears as a condensation rather than the normally

sharply defined star. It is surrounded by an elliptical nebula with several knots in it.

The star and its nebula are embedded in one of the most beautiful regions of glowing interstellar gas in the sky. This striking nebula is called the Keyhole Nebula and the gas, glowing from the light of hot and very luminous stars is divided by rifts of dust that' cut dark lanes through the gas. It lies at a distance of about 6800 light years. Eta Carinae at maximum had an absolute magnitude of —l4.

S Doradus, the type of star of this class of supergiant and highly luminous stars, is one of the most luminous known. It lies in the Large Magellanic Cloud, the nearest galaxy to our own. It lies at a distance of about 160,000 light years. Recent observations by German astronomers show that S Doradus is 50 times more massive than the Sun. They estimate that it loses mass continually at a rate that requires 1500 years to eject into space a mass equal to that of the Sun.

This is a very large loss in a comparatively short time and can only be expected to • be sustained for a relatively short time. It is suggested that S Doradus began the main part of its Jife as a highly luminous star about 60 times as big as the Sun.

At first it lost matter slowly, but as its mass and temperature decreased the rate of loss suddenly increased. After passing through the hydrogen and helium burning stages ->this ' star would probably fade to ultimately become a WolfRayet star. Wolf-Rayet stars are of very high temperature and luminosity. They are classified according to the .main lines recorded in their spectra. Those with helium as the main feature are termed WR stars; those with carbon, WC type and those with nitrogen are WN type. They will all appear to have expanding atmospheres with material being ejected at high velocities of about 2000 km a second.

The theory that the most luminous and massive stars, such as S Doradus, evolve into Wolf-Rayet stars is attractive because it explains the absence of luminous red supergiant stars. That absence is well established by observations.

However, under present models of stellar evolution there should be large numbers of these luminous red supergiants. Additionally the new theory accounts for the high mass loss and by stipulating that the stars like S Doradus become WolfRayet objects helps explain the peculiar nature of the chemical composition of this latter class of objects. In this article we have

selected three diverse stars to show that it is possible that the most massive and luminous of all stars do not necessarily evolve in the same way.

AG Carinae may possibly become a planetary nebula; Eta Carinae appears to be a peculiar type of supernova, whilst S Doradus may become a Wolf-Rayet star. It is the unsolved problems that this class of stars pose that makes them so fascinating to astronomers. . Throughout June the planets, Jupiter and Saturn, will be well placed for evening viewing, passing high across the sky to the north of the zenith around 10 p.m. They remain a few degrees apart in Virgo. Venus begins to climb higher in the evening sky this month and can be seen low in the north-west after sunset, steadily increasing in altitude from night to night. Mercury can still be glimpsed in the evening twilight during the first week of the month.

On the evening of June 9 Venus and Mercury will be about 2 deg. apart. Venus will be very much the brighter of the two objects, but they can only be seen then in evening twilight.