It's big, but what's so hot about that?: Heather Couper and Nigel Henbest welcome a bloated summer visitor - the red giant Antares

LURKING low in the south on July evenings is one of the biggest stars we know. It marks the glowing heart of Scorpius, the scorpion, which, alas, never rises higher than this at British latitudes. But seen from the tropics, the gigantic starry scorpion, complete with pincers and stinging tail, is one of the most memorable sights in the sky.

The name of Scorpius's chief star - Antares - stems from its ruddy colour, and has its roots in the Greek 'Anti-Ares': rivalling Mars. If you have clear southern and western horizons, you can check this out by comparing the rival with the real thing, which is just visible in the twilight. Mars's red coloration comes from its barren, iron-rich soils, which have been rusted by water - once common on the planet. Antares's redness is the result of the low temperature of the gases that make it up.

If you have ever seen a metal bar heated with an oxyacetylene torch, you will have got some idea how colours relate to temperature. First the bar is black. Then it starts to glow red. As more heat is applied, the bar glows amber, yellow, then white. If you could heat it even more, it would finally shine an incandescent blue. The same colour code applies to the stars, with hot stars (eg, Spica, in the constellation of Virgo, lying between Antares and the brilliant planet Jupiter) shining blue-white, middling stars (the Sun) shining yellow, and cool stars (Antares) glowing red. Hot and cool, as applied to the stars, are relative terms: Spica's surface gases are at a temperature of more than 20,000C; the Sun's surface is about 6,000C; while Antares comes in at 3,000C.

But what Antares lacks in heat, it more than makes up for in size. It is now thought to be more than 600 times the size of the Sun, which means that if it were placed in our solar system, it would stretch way beyond the orbit of Mars and into the asteroid belt. It will come as no surprise that astronomers call stars such as Antares 'red giants'. There are a couple of other red giants well-placed for spotting this month; Arcturus, the brightest star in Bootes (a mere 25 times larger than the Sun), and Rasalgethi in Hercules (400 times wider). But all these stars pale into insignificance against winter's Betelgeuse (in Orion), 750 times larger than the Sun.

How and why do stars grow so big? The answer seems to be that all stars do it: 'going red giant' is a phase in every star's life. Stars are born, grow up and die, although the associated changes take place extremely slowly from our point of view. But we can see stars in many different stages of development, which makes it possible to piece together how an individual star evolves from cradle to grave.

Stars shine because of nuclear fusion reactions: their centres are so hot and compressed that hydrogen nuclei fuse into helium nuclei. Because a helium nucleus weighs a tiny bit less than the individual hydrogen nuclei that go to make it up; the unwanted mass escapes as energy that goes to power the star. Every second, our Sun converts 4 million tonnes of its matter into starlight.

But a star's nuclear reserves are limited. After 10 billion years or so, a star like the Sun 'runs out of gas'. Bereft of a flow of energy from its core, the star's innards shrink. This in turn causes them to heat up, which makes the outer layers billow out to a hundred times their previous extent and cool to a dull red. This vast object is a red giant, but in cosmic terms it is as insubstantial as candyfloss. The star has no control over its distended atmosphere, and changes in brightness as its outer layers swell and shrink. Antares varies slightly in brightness; Betelgeuse more so, and some red giants (such as Mira in Cetus) can increase in brightness more than a hundredfold.

A red giant does not last long; the phase is a brief one in the life of a star. Gently and gradually, it puffs its bloated atmosphere into space, revealing its white-hot - but dead - core. Fresh star-

corpses like this are called 'planetary nebulae' (the puffed-off atmosphere bears some resemblance to the disc of a planet seen through a telescope), and there is a classic one in the overhead constellation of Lyra. Just between the bottom two stars of the 'lyre', moderate telescopes reveal a tiny oval patch, like a smoke ring. For one star, this is the end of the road - and it is a foretaste of how our Sun will look, 5 billion years hence.

The Planets

APART from Jupiter, the planets are not a great feature of July nights this year. Mars, fading fast, is setting at around 11.30pm BST now, and will shortly be gone from our skies. But Saturn is slowly moving in to fill the gap, and will rise at about 10pm BST by the end of the month. Jupiter is still the brightest object in the evening sky, although it is now setting before midnight. Look for it close to the crescent moon on the evening of 24 July.

Early risers (or late revellers) will find it hard not to spot brilliant Venus, which rises at just after 2am BST this month. And the sharp-eyed may just be able to pick out Mercury in the morning twilight at the very end of the month, when it rises just over an hour before the Sun.

The stars

Straggly Hercules, which look a little like the faint ghost of Orion, occupies centre stage this month. The constellation represents the Greek superhero, famous for his 12 labours. In the sky, Hercules is depicted upside-

down, with his head marked by the red giant Rasalgethi.

Despite its faintness, the constellation of Hercules contains some interesting objects - some visible to the unaided eye, or in binoculars. There's Rasalgethi itself, which changes in brightness by more than a factor of two as its atmosphere billows in and out. Many of Hercules's stars (including Rasalgethi) are double, but you need a small telescope to split them.

Hercules's most famous 'inhabitant' lies about a quarter of the way down the constellation's right-hand side. To the unaided eye (on a clear, moonless night), it appears as a fuzzy patch; binoculars (or better still, a small telescope) reveal it as a ball of stars. This faint smudge is the globular cluster M 13 - a clutch of half-a-million stars floating 25,000 light years above the main body of our Milky Way. Nearby (but only visible in binoculars) is the even more distant globular cluster M 92. These are only two of the 100-plus globulars that surround our galaxy, and were among the first objects in our cosmic neighbourhood to be born.

Diary (all times BST)

4 12.46am: Full Moon; Earth at aphelion (furthest from Sun)

11 11.50pm: Moon at last quarter

12 Uranus and Neptune at opposition (at closest positions to Earth, and opposite the Sun in the sky)

15 Mercury at inferior conjunction (in line with Sun)

19 12.25pm New moon

26 4.25am Moon at first quarter

(Maps omitted)