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

Previous Articles

Starwatch - March 2024 (6th Mar 2024)

What a wonderful time of the year this is to be observing the night sky. The weather is warm, the nights clear, and the Milky Way shines directly overhead!

Also overhead, are the two brightest stars in the whole sky. A little over to the north is Sirius, and a little to the south is Canopus.

It’s not just these two stars that make the summer night sky so glorious. Let's begin low in the north. Using this month's star chart, find Capella. Note that the star is a golden yellow in colour. It's the same type of star as our Sun, albeit a trifle larger. Unseen, except in the largest telescopes, Capella has 3 other stars that revolve with it around a common centre of gravity. At approximately 50 light years distant, the light from Capella now reaching us left at the time when palaeontologist, Donald Johanson, discovered a 3.2-million-year-old hominid skeleton in Ethiopia on November 24, 1974. The skeleton, later called Lucy, was the oldest known example of a human ancestor to have been found.

Higher and to the north-east, we find Castor and Pollux. They form the heads of the twins of Gemini. Pollux is about 11 times the diameter of our Sun, 35 times as bright, and almost twice as massive. And it’s that mass that determines the future of Pollux.

Whilst there’s no formula for predicting how long a person will live; a single number determines how long a star will live: its mass. Genetics, nutrition, exercise, and other factors all play a role in our longevity. But that’s not the case with the stars.

Mass is what makes all the difference in a star’s lifespan. At 2 billion years of age, Pollux is roughly half as old as the Sun. But it’s already finished its “normal” lifetime; the phase the Sun is in right now.

During that normal lifetime, a star “burns” the hydrogen in its core to make helium. When the hydrogen is gone, the core gets smaller and hotter, so it begins to burn the helium to make carbon and oxygen. At the same time, its outer layers puff up like a balloon, making the star much brighter. That also makes the star’s surface much cooler, so it glows reddish-orange, as Pollux does now. Because of its larger mass, it burns everything faster!

Over time, Pollux will get even bigger and brighter. Then it will expel its outer layers, briefly surrounding itself with a colourful bubble of gas. That bubble will quickly fade, though, leaving only the star’s small dead core; a hot ember known as a white dwarf. The same fate awaits the Sun, perhaps 4-5 billion years in the future.

You can see the 9-day old gibbous Moon just above Pollux on the evening of March 19.

You will notice that the bright moonlight has a significant impact on stargazing. Many of the fainter stars become invisible to the naked eye reducing the splendour of a dark sky. However, it can also be a beautiful addition to a night-time scene. Like the Full Moon rising over a body of water, or illuminating a rustic rural scene.

A couple of examples for you to look at are shown below.

Looking East, just before sunrise on March 9, 2024. Beautiful scene, with the slender crescent of a 28-day old Moon, Venus 3o to its left, and Mars 6o above Venus. (Illustration from Stellarium software).

Looking North-West, about 9:00pm local Adelaide time, on March 14, 2024. The crescent of a 5-day old Moon, Jupiter 7o to its left, and Uranus 4o above the Moon. (Illustration from Cartes du Ciel software).

The Moon probably formed in a “Big Whack.” A planet as big as Mars rammed into the young Earth, blasting molten rock into space. Much of that material came together to form the Moon.

Another “Big Wack” occurred on 5 February 1971, when astronaut Alan Shepard hit 2 golf balls from the Apollo 14 landing site. The golf balls travelled 22 & 36 metres. A golf pro could hit them at 298 kms/hr and travel 4.2 kms in the Moon’s low gravity.

Not only do the Moon’s features tell us about its geological history, they also tell stories of human achievement. That’s because most of the Moon’s features are named for scientists, philosophers, and explorers - the men and women who solved some of the mysteries of the universe, while uncovering new mysteries for others to ponder.

As an example, three of the Moon’s largest, freshest craters are named for three astronomers whose combined talents proved that Earth is not the centre of the universe.

Nicolaus Copernicus realized this great truth during the sixteenth century, but he couldn’t prove it mathematically. Later, Tycho Brahe charted the precise positions and motions of the stars and planets. And when Tycho died, his assistant, Johannes Kepler, used Tycho’s superb observations to work out the motions of Mars. He proved mathematically that Mars orbits the Sun, not Earth - moving Earth out of the centre of the universe and into its current spot as just another planet, orbiting an average star, in an outer suburb of our Milky Way galaxy.

The Moon is at Last Quarter on the 3rd of February, New on the 10th, at First Quarter on the 17th, and Full on the 24th of February.

Happy stargazing!