Rings in sky mean heavenly explosions

Press, 30 August 1980, Page 27

Rings in sky mean heavenly explosions

Astronomers of the last century found many hazy 'patches of light in the sky, which they termed nebulae. They had no idea what these objects were so they described them according to their visual appearance. Some appeared disc-like, resembling the telescopic views of the distant planets, Uranus and Neptune. These they called planetary nebulae. We have leamt that planetary nebulae are expanding shells of ratified ionized gas surrounding hot white dwarf stars. The . shell, or envelope, receives ultra-violet radiation from the central star. This it re-emits as visible light bv fluorescence. About 10 per cent of all the known planetary nebulae have, the typical disc shape. The best known example is the Ring Nebula in Lyra. Others appear irregular, while most appear bipolar as if an explosion was concentrated in two diametrically opposite directions. It is believed that most stars with a mass of from six-tenths to four times . that of the Sun become planetary nebulae at one stage of their, evolution. Before that they pass through the red giant stage. That ends when the central core of the star consists of carbon, surrounded by a helium burning shell. Some cases are known when the shell is still burning hydrogen. As the core contracts the shell is ejected .with, a velocity that is high enough to separate the shell from the core. The shell can be ejected in a single explosion or in a series of explosions, which eject successive shells of gas. The core that is left behind is hotter than the normal star so that much of its radiation is in the ultration at visual wavelengths. The core violet. This radiation heats up the ejected gas and causes it to re-emit radiabecomes a white dwarf. Those planetary nebulae that show a disc shape reveal a ring surrounding the central star that is nearly uniform. But most appea'r to be stretched out as if the gas was ejected only in these preferred directions. These are termed

bi-pQlar as it seems that the gas was ejected in the field of the stars magnetic poles. Simple reasoning suggests that not all planetary. nebulae are of the same age. This implies that those with their shells furthest from their central star are likely to be older than those that are more compact. As the shell expands the gas becomes thinner so that the overall brightness is fainter. Thus it is assumed that the biggest and faintest gas shells belong to the oldest planetary nebulae. The speed at which a gas shell recedes from the central star can be measured. The Ring Nebula, for example, has a shell that recedes at 19 km a second. Measurements also show that its present radius is one-third of a light-year. If we assume that the rate of expansion has remained constant since ejection then the age of the Ring Nebula is about 5,500 years. Such calculations are only approximate because it is unlikely that the speed of ejection has always been constant. However, such estimates appear to confirm that the oldest of these nebulae are also the biggest in diameter and also the faintest.

It is believed that the upper age to any planetary nebula is about 50,000 years By then the rings of ejected . gas have become so thin that they can no longer be seen. The central star has grown dimmer with time so that ultimately it is merely a burnt out cinder of a star. It presumably has changed from a white dwarf to a black dwarf no longer capable of radiating energy. A peculiar variable star, FG Sagittae, shows some of the processes described. This star has increased in brightness • from a magnitude of 13.6 in 1894 to 8.9. It is surrounded by a small, faint planetary nebula that is calculated to have been ejected 6000 years ago. In 1955 an inner shell was detected around FG Sagittae. Measurements show that between 1962 and 1977 the radius of this gas shell increased from 24 to 70 times the Sun’s radius.

The spectrum of the shell shows the normal spectral lines of the common elements, but in addition there are also the lines of some unusual elements such as zirconium, cerium and neodymium and others. Such elements are produced slowly to make new kinds of atoms. Different processes work inside stars +o produce elements other than hydrogen and helium. This production occurs well within the star so that it is only when the new elements are brought to the surface that they can be detected. FG Sagittae is the only star known in which new elements have been seen brought to the surface and carried away in gas ejected. The period of the variations in the brightness of FG Sagittae has steadily increased by about 4.9 days a, year. This means that the density of the star is diminishing. Also, the spectral type is changing. The observations therefore show that the star is crossing wnat is termed an instability strip where stars show pulsations in brightness. It has also been found that FG Sagittae is a binary star. All this shows that this star is one of the most important because the changes taking place can be seen. It will remain the centre of intensive study for many years. The evening sky in September is now becoming less interesting . for watchers of the bright planets. Jupiter and Saturn reach conjunction with the Sun on September 13 and 23 respectively. They are now too close to the Sun for viewing. Mars moves from Virgo into Libra, but is now so far away as to be uninteresting even with a moderate sized telescope. As a result its magnitude is now 1.5 making it inferior to many stars. Mercury can be seen low in the western evening sky during the second, half of September. Its rapid movement against the background of stars makes it easy to detect but it will be best placed in October. Venus rising well before dawn remains a brilliant object in the eastern morning sky.