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

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Starwatch for September 2025 (3rd Sep 2025)

A few lingering stars of winter are still in view during the evening. Antares, Altair, Vega have lit up the cold winter nights for us. But there is really only one bright star that puts in its best showing during these early spring nights:

Fomalhaut, the leading light of Piscis Austrinus, the southern fish. You can find it high in the eastern sky. There are no other bright stars around it, so it’s hard to miss.

Fomalhaut itself is a young and vigorous star. It’s only a few hundred million years old, and it’s bigger and heavier than the Sun. That makes the star hotter and brighter than the Sun, and one reason that Fomalhaut shines so brightly in our night sky. The other reason is that it’s a close neighbour, just 25 light-years away.

A broad disk of dust encircles the young star. Building a planetary system is messy. It starts with small grains of rock and ice that come together to form larger and larger blocks. Some of those big blocks may eventually merge to make planets. But while some of the impacts are slow enough for the blocks to stick together, others are high-speed collisions that rip them apart, scattering debris back into space.

In 2008, astronomers using the Hubble Space Telescope directly imaged a tiny dot near the star, and at the time they thought it was a recently formed planet . It was named Fomalhaut b. It’s about three times as massive as Jupiter, the giant of our own solar system, and it’s far, far away from Fomalhaut itself. It takes over 2000 years to complete one revolution around its parent star. However, analyses in 2019 and 2023 of existing and new observations indicate that Fomalhaut b is not a planet, but rather an expanding region of debris from a massive planetesimal collision.

At the other end of the scale, we have a planet in the northern constellation of Cygnus, the Swan, orbiting its parent star at the dizzying rate of 8.5 hours per revolution. In the time it takes you to get a full night's sleep, the Earth-sized fireball of a planet 700 light-years away has already completed an entire year. Imagine going to bed and waking up a year older!

This is one of the shortest orbital periods ever detected. The planet is extremely close to its star, about 40 times closer than Mercury is to the Sun, and astronomers have estimated that its surface temperature may be as high as 2,000oC. In such a scorching environment, the top layer of the planet is likely completely melted, creating a massive, boiling ocean of lava.

The stars that make up the constellation of Cygnus have been identified with some sort of bird by most civilisations of the ancient world. Sometimes known as an eagle, a hen or a pigeon; the image of a graceful swan in full flight is the one I prefer. The star Deneb represents the tail of the swan, whilst Alberio is the beak.

In real life, Deneb is a star of enormous proportions. It is over 60,000 more luminous than our Sun, and 25 times more massive. If we placed our Sun at the same distance that Deneb is from us (1600 light years), then the Sun would be an insignificant little star barely visible in large amateur telescopes.

Alberio is a beautiful double star of blue and gold. A steadily held pair of powerful binoculars should show it as two distinct stars.

Higher in the sky we find Aquila, The Eagle. The brightest star in this constellation is Altair. At a distance from us of only 16 light years, it definitely classifies as a close stellar neighbour. But a bit of a strange character! It is about twice the diameter of our Sun, but spins on its axis in 6 and a half hours (our Sun takes 25 days). As a matter of fact the spin is so rapid, that the star probably resembles a flattened tomato!



Above: The approximate appearance of the Moon during the eclipse. Times shown in ACST.



Over in the western sky, the bright orange star Antares is dropping lower in the evening sky with each passing day. In European mythology, Antares represents the "heart" of Scorpius. The scorpion's head is below Antares, its body and tail above it. Antares is a star in the twilight of its years. It has used up much of its hydrogen fuel. But it’s in the nuclear furnaces of stars, just like Antares, that the many chemical elements that make up our Earth were born. Gold, silver, iron, and oxygen are just some of them. At the end of its life it will explode, creating even more elements. The blast will spread the atoms it created far and wide.

Once the elements are out in space, though, the chemistry is far from over. In fact, the space between stars is also a chemical factory, in the form of giant clouds of gas and dust. The elements within these clouds combine to make molecules.

Astronomers have discovered molecules of hydrogen peroxide in a cloud that surrounds the star Rho Ophiuchi, not far from Antares. Hydrogen peroxide consists of two atoms of hydrogen and two atoms of oxygen, the same two elements that make up water.

It’s here, also not far from Antares, that the bulge of the Milky Way is at its biggest. That subtle glow is the light from millions of stars in the disk of our home galaxy, 26,000 light years away. More than three decades ago, astronomers discovered that gas was swirling around a dark, massive object at the centre of our galaxy. We now know that a black hole, over 4 million times as massive as the Sun, lurks there.

The Southern Cross (Crux) is now getting lower in the southwestern sky, and it won't be too long before that constellation is lost to our view for a few months as it skims along the southern horizon. If you've got a good vantage point providing a clear view of the southern horizon, then the Southern Cross is one constellation that you can observe all year round.

Night owls will be treated to a Total Eclipse of the Moon in the early hours of September 8. The Moon begins to travel through Earth’s shadow at 12:58am ACST, with the darkening becoming very noticeable about an hour later. The Moon will be fully immersed in Earth’s shadow at 3:01am ACST and exits at 4:23am. As the Moon will travel near the centre of the shadow, we can expect a moderately dark eclipse. Maybe not quite a blood Moon, but definitely a nice shade of red. The changing appearance of the Moon during eclipse are shown below.

The Moon is Full on the 8th of September, at First Quarter on the 14th, New on the 22nd, and at First Quarter on September 30th.

Happy observing!