These summer evenings, a bright, red star dominates the sky near the southern horizon. If your local skies are too bright or murky to see the colour, get out a pair of binoculars and have a closer look. The colour stands out more clearly if you throw the binoculars a bit out of focus. The combination of the star being low in the sky, together with our turbulent atmosphere makes Antares flash with all the colours of the rainbow; however, the star's natural colour is orange-red.

That star is Antares, the brightest star in Scorpio, or Scorpius, "The Scorpion", which is one of the zodiac constellations. Antares means "Rival of Mars", because its reddish colour gives it a similar appearance to Mars, "The Red Planet".  Its other name is "Cor Scorpionis - The Scorpion's Heart". If you imagine Antares to be about where you would expect a scorpion's heart to be, it's easy to spot the fainter stars forming the tail, like an inverted question mark leading down to the horizon.

Antares lies between 500 and 600 light years away. To look as bright as it does while being at that distance, it has to be about 10,000 times brighter than the sun.  Actually, since most of Antares' energy is radiated in the infrared part of the spectrum, its energy output is actually more than 60,000 times that of the sun. The sun is turning its mass into energy at a rate of four million tonnes a second; Antares is doing it at 240 billion tonnes a second! Even though the red colour of Antares corresponds to a temperature of about 3,500 C compared with the sun's 6,000, if we replaced the sun with Antares, the high energy output would fry us and our world instantly. Actually, it's a bit more dramatic than that. Antares' diameter has been measured. It is about a billion kilometres, compared with the sun's 1.5 million. Since our Earth orbits the sun at a distance of almost 150 million kilometres, we would end up inside Antares.

The large size of Antares and its high-energy output are deceptive.  The star has only about 15 times the mass of the sun, so it must be very rarefied. It is what we call a red giant star, one of the final stages in the life of almost any star, including the sun. When a star starts to run out of fuel, and its core becomes a mass of waste products, the energy production zone moves towards the star's surface. This makes two things happen: the star brightens enormously and swells into a red giant.  The sun will start its transformation into a red giant in about four billion years, after a lifetime of about 10 billion years. If Antares' energy output is 60,000 times that of the Sun, and only 15 times the sun's mass, its life will be much shorter than the Sun's. Moreover, whereas small stars like our sun end up as white dwarf stars enjoying an almost indefinite retirement, Antares will probably blow itself up, in a huge explosion called a supernova. For a month or two that star will radiate more energy than all the billions of stars in its galaxy put together.  When that happens, that 600 light years distance will not count for much protection.

* Jupiter and Neptune are currently close together in the sky, and rise around 11 p.m.; Saturn lies in the southwest during the evening. Mars and Venus rise in the early hours. The moon was full on July 7, and last quarter on the 15th.

Ken Tapping is an astronomer with the National Research Council's Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory in Penticton.