A guide to what's up in the sky for Southern Australia

Starwatch June 2024 (2nd Jun 2024)

About half-way up the northern evening sky, a bright star shines.

This is Arcturus, in the constellation Bootes, the herdsman. It's an orange-giant star, and it's unusually old as naked-eye stars go: about 10 billion years — roughly twice the age of the Sun, Earth, and solar system. Arcturus may thus be the oldest object you've ever seen.

Arcturus is the third-brightest star in Earth's night sky. It appears so bright for a couple of reasons. First, Arcturus really is a bright star; it produces more visible light than most stars. If you placed Arcturus side by side with our own star, the Sun, it would appear more than a hundred times brighter. And second, Arcturus is fairly close to us, at a distance of just 37 light-years.
Just below and to the right of Arcturus, look for the delicate curving line of faint stars that make up Corona Borealis, the Northern Crown. Further to the west, we find the stars of Leo, the lion and Hydra, the water snake setting in the west, whilst Aquila, the eagle and Capricornus, the goat rise in the east.

High up in the eastern sky, another orange-red star shines brightly. It’s called Antares, the brightest star in the constellation of Scorpius, the scorpion. Look out on the evening of June 21, and you’ll also see the Full Moon nearby. The 2 bodies are a real contrast of distances from us. The Moon is closest at 381,000 kms, whilst Antares, well we can’t measure its distance in kilometres; it would require way too many zeros. So, astronomers use the distance that light travels in a year as a unit of measurement. At 300,000 kms per second, it takes 552.8 years for the light from Antares to reach Earth.

Antares is one of the most impressive stars in the galaxy. A behemoth that is at least 15 times as massive as the Sun, hundreds of times wider, and tens of thousands of times brighter. It is big. It is bright. And it is doomed. All that flashiness comes at a price. Its lifetime is measured in millions of years versus billions of years for stars like the Sun.
When that lifetime is up, the star’s core will collapse, while its outer layers will blast into space as a supernova. The blast will push a mixture of chemical elements out into the galaxy. Over time, some of these elements, along with even heavier ones created in the fury of the supernova, are incorporated into new stars and planets. The iron that’s such a vital ingredient in our world and in our bodies was forged billions of years ago, in the heart of a star.

The largest and smallest constellations are visible in our evening sky at this time of the year. The largest constellation is Hydra the water snake, which slithers across 90 degrees of the sky. To get a sense of how much sky that is, consider that the full Moon spans only half a degree. Whilst the head of Hydra is setting in the west, its tail stars are still overhead!

Above: The amazing aurora of May 11, 2024 imaged by Joe Grida from Mt Barker SA, using an iPhone 13 set to night mode with a 10 sec exposure.

The smallest constellation is one of the most famous. Its correct name is "Crux", the cross, although it's better known as the Southern Cross. At this time of the year, it’s high in the southern sky. Its stars are shown on the Australian and NZ flags.

Crux is small, but it's packed with celestial wonders. For example, there's the Jewel Box Cluster, shown on this month’s starchart by its catalogue number of 4755. Seen through a telescope, the stars of the Jewel Box shine diamond-white and sapphire-blue. A lone red star lies between Earth and the cluster, so it looks like a ruby against the other gems. Their light left on its journey through interstellar space towards Earth 6,500 years ago. The whole cluster is set against the glittering band of the Milky Way. Of course, you’ll need a dark sky to see the Milky Way. The light polluted skies of Adelaide hide it fairly well.

High up, almost overhead, the blue-white star, Spica is easy to see. Spica consists of more than one star, but only one is bright enough to see without a telescope. Astronomers use two different measurements to rate the brightness of a star. Both measurements are called magnitudes, and the lower the number, the brighter the star.

The first measurement, called apparent magnitude, rates how bright an object appears in the sky; Spica rates a "one." But apparent magnitude doesn't tell you if the object is intrinsically bright or just close by.

So, astronomers also rate stars and galaxies by absolute magnitude. This number tells us how bright different objects would appear if they were all lined up at the same distance. On that scale, Spica gets a minus three. The Sun would only rate a five, which means that Spica is about 1600 times brighter.

June 21st gives us our Winter solstice; when our dark hours are at our longest. The solstice marks the Sun's northernmost point in the sky for the entire year. After the 21st, it'll start to head south; a motion that will continue until the summer solstice in December.
It’s not the Sun that’s moving though, the Earth is.

Our planet is tilted on its axis, so as we orbit the Sun, the axis tilts in different directions relative to the Sun. Right now, the south pole is tilted away from the Sun, so it's in total darkness, whereas the north pole is dipping toward the Sun, so it's bathed in constant sunlight.

The Moon is New on June 6th, at First Quarter on the 14th, Full on the 22nd, and at Last Quarter on the 29th of June.

Happy stargazing!