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

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Starwatch - October 2024 (30th Sep 2024)

After a spectacular encounter with Pluto back in July 2015, the New Horizons spacecraft was redirected to visit a more distant object, known as 2014 MU69.

Like Pluto, this object is a member of the Kuiper Belt — a wide band of iceballs beyond the orbit of Neptune, the most-distant major planet. These objects are frozen leftovers from the birth of the planets. Astronomers say Kuiper Belt objects are so far from the sun that they have spent 4.6 billion years in the deep freeze and should give us a good picture of what conditions were like when the solar system was first forming.

Other stars have their own Kuiper Belts. One of the best examples is the bright star Fomalhaut. You can find it almost overhead during these early Spring nights. There are no other bright stars around it, so it’s pretty easy to pick out.

Fomalhaut has at least one planet — one of the few exoplanets that’s ever been photographed. But Fomalhaut also has a giant disk of dust that orbits beyond the planet. The dust appears to be supplied by colliding comets — balls of rock and ice like those in the Kuiper Belt.

A study a few years ago said there are hundreds of billions of comet-like objects in the belt around Fomalhaut. What’s more, up to a couple of thousand of them may collide every day. The smash-ups pulverize the colliding bodies, spraying out gas, dust, and rock. Over time, this material spreads out, helping to keep the disk full of fresh dust.

And coincidentally, this disk of material is about the same distance from Fomalhaut as the Kuiper Belt is from the Sun; a realm of icy bodies far from the star’s warmth.
And there’s a visitor from the outer solar system, which will be visible in the western sky after sunset from about mid-October.

Comet C/2023 A3 (Tsuchinshan-ATLAS) is a long-period comet from the Oort Cloud, a sphere around our solar system home to millions of comets, and it's in an elliptical orbit of roughly 80,000 years. It was first discovered at over seven Earth-sun distances by astronomers at China’s Purple Mountain Observatory in January 2023 and later re-found by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) search station in Sutherland, South Africa, in February 2023.

Comet A3 made its closest approach to the Sun on September 28, at about 59 million km. That's the riskiest time for a comet, which can break apart because of the sun's influence. It's what makes comets, and in particular their future brightness, incredibly difficult to predict. At the time of writing, it appears to have survived the encounter. It will pass closest to Earth on October 12, when it will pass about 71 million km from Earth.



he view looking west soon after sunset on October 17. The comet will be 11o above the western horizon, with the Full Moon having just risen in the east. Use binoculars to aid in the viewing of this celestial visitor. Generated with Sky Safari software.



Comet A3 has the potential to become the brightest comet in 13 years, since C/2011 W3 Lovejoy in December 2011.
The best time to see the comet is just after sunset, just as twilight ends. Make sure you have a clear western horizon and a pair of binoculars will certainly help. Use the styar map provided to guide you.

The Southern Cross 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.

Astronauts, looking out of the space station windows at night, see great clusters of lights; the glow of hundreds of thousands of individual lights that mark cities on earth. From the ground, we can see great clusters of lights in the heavens, too; cities of stars we call globular clusters. They're somewhat lost in the background of stars, but binoculars or telescopes reveal their true nature: spherical clumps of thousands or even millions of stars. Our Milky Way galaxy contains a couple of hundred globulars.

The 2 greatest of these are to be found in the southern sky, but only one is visible these October nights. NGC 104 (also known as 47 Tuc) is shown on the star chart high in the south, just next to the Small Magellanic Cloud. Imagine a spherical region of space with our Sun at the centre and the outer edge four light-years away - the same distance as Alpha Centauri, our closest star system. Now imagine that this sphere contained a hundred thousand stars. The night sky would look alive with stars - bright stars in every direction, an overpowering cosmic light show. That's how the sky would appear if the solar system were transported to the centre of 47 Tuc!.



Comet C/2023 A3 above the Parkes Radio Telescope, imaged by Alex Cherney just before sunrise on 27 September 2024. 15 x 3.2 sec exposures, 135mm lens, Canon 6D



As we gaze to other parts of the spring night sky, we find that it is splashed with references to water. To the northeast we find Pisces, the fishes. The stars that make up this constellation are quite faint, so stay well hidden from direct lights. The mythological legend associated with this constellation tells how Venus and Cupid escaped from a giant by throwing themselves into the Euphrates River and changing into fishes.

Further to the east, Cetus, the Whale greets us. For the naked eye observer, the most interesting object within this constellation is the star named Mira. It is the most famous of all the variable stars, changing its brightness over the period of approximately 330 days from well below naked eye visibility that a small telescope is required to spot it to a brightness even seen from bright light polluted city skies. Mira is a star of truly enormous proportions; it is estimated to be some 400 times the diameter of our Sun!

In the southeastern sky, you can make a splash by jumping into Eridanus, the river. It is one of the largest constellations in the whole sky. It can be traced right across the sky from the far southern sky right to the equator. We’ll need to wait till the summer stars shine in our evening sky before we can see the entire length of Eridanus.
Also in the southeastern sky, you’ll find a lone blue-white star shining very brightly. The star is Canopus, the second brightest star in the whole sky. I’ve always regarded the first sighting of this brilliant star low in the southeastern sky as a sure indication that the warmer evenings of spring and summer are on their way.

Canopus is quite different from the Sun. It's a supergiant; many times bigger and heavier than the Sun. It's also more than 10,000 times brighter than the Sun, so it's easily visible across more than 300 light-years of space. It's nearing the end of its life, and within the next few million years should explode as a supernova. When that happens, Canopus will briefly outshine everything else in the sky except the Sun.

Look high in the western sky soon after sunset, and you’ll be greeted by the planet Venus . In addition to its aesthetic appeal as it hovers in a golden sky, Venus is quite a stunning sight in a telescope. Because it is closer to the Sun than the Earth, it exhibits phases like the Moon. At the moment is resembles a 10-12 day Moon.

The Moon is New on October 3rd, at First Quarter on the 11th, Full on the 17th, Last Quarter on the 24th.

Happy stargazing!!