Stars and Planets: October

ALTHOUGH SUMMER is officially over - the Autumn Equinox took place in the small hours of 23 September - the three prominent stars of the "Summer Triangle" ride high in October's skies. At its apex is pure-white Vega, brightest star in the tiny constellation of Lyra - the lyre. Look between its two end stars with a moderate telescope and you'll see an amazing sight: a small yet perfectly- formed cosmic smoke ring.

The Ring Nebula is a "planetary nebula" - not the most appropriate nomenclature, because these celestial puffs of smoke have nothing to do with planets. They were first recognised as a distinct group by William Herschel, who discovered the planet Uranus in 1781. He was struck by the similarity of their appearance to that of dim planets.

A planetary nebula is a sign that a star is on the way out. Although it's popularly supposed that stars explode when they die, the vast majority - more than 95% - go with a whimper rather than a bang. According to Sun Kwok of the University of Calgary, "Planetary nebulae can be considered a Sun-like star's last hurrah". The fireworks of supernova detonation are only for massive stars.

When a star like the Sun reaches the end of its life, it expands into a huge, distended red giant - Aldebaran, now rising in the east, is one such example. Like a cosmic blancmange, the star has very little control over its extremities, and it wobbles about. Planetary nebulae were once thought to be the abruptly-ejected atmospheres of these red giants. But new research by Kwok and his team, in particular with the Hubble Space Telescope, has shed new light on their origins.

In its last 10,000 years (equivalent to the last few hours of a human lifetime), a red giant star begins to lose mass very rapidly. Gas pours off the stars at a rate of a hundred-thousandth of a solar mass a year. By comparison, the Sun loses a hundred-trillionth of its mass per year. Some stars eject so much matter that they are completely obscured by a circumstellar shell of gas and dust (cosmic "soot").

These initial shells of ejected matter around the red giant form a "protoplanetary nebula". This is relatively cool, and only visible in the reflected light from its central star. But then things hot up - literally. The more the red giant boils away, the more its hot core is exposed - and this heats up the ejected matter like gas in a neon tube, making it glow. This "ionised" ejecta is a planetary nebula.

While protoplanetary nebulae are usually symmetrical, mature planetaries are often bipolar, comprising two distinct blobs. Sun Kwok thinks that this is caused by superfast stellar winds boiling off the hot core. His calculations show that any asymmetries in the envelope of ejecta can amplify the winds, allowing them to punch holes which lead to the characteristic double shape. Other astronomers believe that the asymmetries are generated by the presence of unseen companion stars, or even planets.

Planetary nebulae are the butterflies of the cosmos. Beautiful, insubstantial and ephemeral, they last only 10,000 years. In the end, they drift away to reveal the exposed core of the once-mighty red giant - a tiny white dwarf that slowly leaks away all its heat into space. In five billion years time, this will also be the fate of our Sun. Our distant descendants, having fled the Earth during the Sun's red giant era, will look back at the gently evaporating "ring nebula" that marks the death of our own Solar System.

WHAT'S UP: Saturn is at its closest to Earth on 23 October. A small telescope will show its famous rings and the largest moon, Titan, which orbits Saturn every 16 days. The ringed planet is the fainter of two giant worlds on show in the late evening: Jupiter shines more brilliantly to its right. Use binoculars to spot its four biggest moons. In the morning sky, Mars is climbing in the east.

We may be in for some shooting stars from an old comet, Giacobini-Zinner, on 8 October: this shower is usually weak, but may be due for a revival in 1998 as the comet itself is comparatively nearby. More reliable is the Orionid meteor shower - debris shed long ago by Halley's Comet - on 21 October.

Diary

(BST until 25 October)

5 th 9.12 pm full moon

8th Maximum of Giacobinid meteor shower

12th 12.11pm moon at last quarter

20th 11.10am new moon

21st Maximum of Orionid meteor shower

23rd Saturn at opposition

25th British Summer Time ends

28th 11.46am moon at first quarter