Astronomical Society of South Australia

Night Watch

Updated 1 May 2012

If you venture out these cool autumn evenings, you'll be greeted by the constellation of Scorpius, the scorpion low in the eastern sky. He will keep us company over the coming chilly winter months. Its brightest star, Antares, is a huge star of gargantuan proportions. If we replaced our Sun with it, then all the planets from Mercury through to Jupiter would all find themselves engulfed within it! Just below the tail of Scorpius, you can find the star clusters M6 and M7. Take the trouble to observe these with binoculars. They make a beautiful sight, with many bright stars sparkling like diamonds against a background of gold dust.

During early May, the Earth crosses the orbit of Comet Halley, and this produces one of the southern hemisphere's most famous meteor showers, the Eta Aquarids, named for the star in the constellation of Aquarius, from near which the meteor shower appears to originate. If you're brave enough to venture outside at about 3.00am, look a third of the way up the eastern sky. You could count up to 60 or more "shooting stars" per hour. The best time is around May 5-6.

Comets are small, fragile, irregularly shaped bodies composed of a mixture of dust and frozen gases. Think of a comet as a "dirty snowball". They usually follow highly elongated paths around the Sun. Most become visible, even in telescopes, only when they get near enough to the Sun for the Sun's radiation to start evaporating the volatile gases, which in turn blow away small bits of the solid material, usually no bigger than dust grains. These form the well-known "tail" of a comet.

It is these dust grains that are left behind by the comet and form what astronomers call meteor streams.  If the Earth's orbit crosses through a meteor stream, these small particles enter the Earth's atmosphere at great speed, typically at over 40,000 kms per hour. The friction generated by this re-entry completely vaporises the dust, resulting in a brief bright flash, usually referred to as a shooting star. When we strike a large number of these particles, we experience a meteor shower.

Let's now turn our gaze to the permanent members of our evening skies. Unmistakable, high in the southern sky, is the famous Southern Cross.  Surrounding it on three sides is the constellation of Centaurus, the centaur. Its two brightest stars, Alpha and Beta Centauri, are referred to as the Pointers, because they follow the Southern Cross around the sky and always point to it. Alpha Centauri is actually the closest star system to the Earth other than our own Sun, at a distance of just over 4 light years.

The third brightest star in the Southern Cross is Gamma Crucis; you can locate it at the apex of the cross. In the early evening during May, it is the highest in the sky of the stars that make up the Southern Cross. Look at it carefully, and you'll see that it is a red star. It is a prelude of what will happen to our Sun.

Today, our Sun is a yellow star, steadily fusing hydrogen into helium at its core. Billions of years from now, though, it'll use up its hydrogen. The Sun will expand, engulfing the planet Mercury, and possibly Venus and even Earth, and its surface layers will cool and redden. In other words, the Sun will become a red-giant star. But that won't happen for another five billion years or so. For now, Gamma Crucis is the closest red giant to Earth, at a distance of almost 90 light-years. This bright star produces about as much energy in an hour as the Sun does in an entire week.

As a red giant, Gamma Crucis is nearing the end of its life. Someday, it'll cast its outer layers into space, exposing its hot, dense core. Eventually, over billions of years, the leftover core will cool, leaving only an invisible cosmic cinder.

Much closer to home, the planets are putting in a good show during May.

You just can't miss that magnificent brilliance low in the northwestern sky soon after sunset. Other than the Moon, Venus is the first object you will see in the evening sky during May. The best way to see it is when the sky is still tinged with those golden pastel colours of sunset, and the gentle lapping of the waves at the beach.

Venus might take centre-stage in the early part of the evening; but there's no argument about who owns the limelight later on. All it takes is one look through a telescope for Saturn to win in the popularity stakes hands down. Its beautiful set of rings are easily visible, even in a small telescope.

The other planet prominent in our evening sky is Mars, found in the northern sky, near the head of Leo, the lion. The Earth-Mars distance increases by about 40 million kilometers during May, so accordingly its brightness fades as well.

The Moon is Full on the 6th of May, at Last Quarter on the 13th, New on the 21st, and at First Quarter on the 29th.