August 2020

Planets to look for

Jupiter and Saturn are still leading the evening planet show, now above the horizon as the Sun sets and it starts to get dark. The higher they are in the sky, the less atmosphere you have to peer through and the better the view should be, although sky conditions do vary from night to night. The Moon sits between them twice during August, the first time on the night of August 2^nd, then for a second time on August 29^th. Jupiter is always brighter than Saturn; not only is Jupiter bigger than Saturn its also twice as close!

As the two gas giants soar overhead, Mars makes its evening appearance in the northeast. It is amongst the stars of Pisces at the moment, which may help you find that constellation if you’ve been looking for it, and it will brighten noticeably as the month progresses. It is already standing out a bright looking “star” towards the north in the morning sky. Peak brightness will be at opposition (to the Sun) on October 14^th.

Venus, now a morning object, rises around 4am. It will seem to drift south along the horizon, following the Sun and the ecliptic, and moves into the stars of Gemini. The Moon with pair up with it on the mornings of the 15^th and 16^th.

Mercury is too close to the Sun to be seen during August this year. It will move into the evening sky during September where it will be easily visible.

Constellation of the month

Pavo the Peacock

When we look at the constellations around the southern pole, it’s obvious there is a flock of exotic birds clustered there, introduced to the sky by the Dutch navigators Pieter Dirkszoon Kayser and Frederick de Houtman (yes, the man the Houtman Abrolhos is named after!) There is Apus, the Bird of Paradise, Toucana the Toucan, the Phoenix in flames, and Pavo the Peacock, with his splendid tail.

Peacocks are the sacred bird of Hera, the wife of Zeus, and those fabulous tail feathers came about as a sad reward for loyalty to Hera. Hera set the monster Argus Panoptes (Pan-“all”; optes – “eyes”) to watch over Io, who Zeus had turned into a white heifer to keep her safe, but Hera was suspicious. Argus had 100 eyes with which to watch, and only two slept at any one time, so he was always awake and alert. Zeus sent Hermes to rescue Io from Argus; he played his lyre and sung and eventually all 100 eyes of Argus closed, and Hermes was able to slay him and take Io away. Distraught at such trickery, Hera put the eyes of Argus on the peacock’s tail, so he could continue to serve her.

The brightest star in Pavo is known as Peacock. It only gained this name during the 1930’s to aid with aerial navigation, but it was ratified as a proper star name earlier this year by the IAU.

To find Pavo, look south of the Teapot asterism of Sagittarius and the curving crown of Corona Australis. The star Peacock marks the bird’s eye and head, he has a foot to the west and his big, showy tail spreads around up towards Ara the Altar.

The Starfish Cluster

To the south of Sagittarius lies the constellation of Pavo the Peacock. It’s not an easy constellation to find, with only one bright star but it is worth the effort to search it out for the objects within it. It contains a number of galaxies that experienced observers might enjoy but there is also a bright globular star cluster known as the Starfish Cluster that is within easy reach of small telescopes. Binocular observers may also pick the location of this object up, but you will need a telescope to see any detail.

Also known as NGC 6752, this globular cluster lies about 15,000 light years away. Take a while to look at this cluster – careful inspection should reveal how it gained its name. Imagine a starfish with a small centre and long thin arms curving and curling around it.

A careful star-hop is required to find the Starfish cluster (NGC 6752) in Pavo the Peacock.

What is a comet?

Do you know the difference between a meteor and a comet? There is a difference – a VERY big difference. The average meteor is a small bit of rock or dust, maybe about the size of a grain of rice. A comet is a large lump of icy dirt, often around 5-10 km across, although they can be larger.

The astronomical symbol for a comet.

Comets may be travelling very fast through space – from 3,000 km per hour at the furthest point in their orbit from the Sun, all the way up to 250,000 km per hour at their closest approach to the Sun – from our vantage point down here on Earth they usually don’t seem to move across much of the sky from night to night. You could go out every night for a week and still be looking in the same direction for the comet. Compare this to a meteor which streaks across the sky in a few seconds!

Comets can last for thousands of years, too, if the nucleus doesn’t break up. As they come in closer to the Sun, they begin to warm and start to release their dust and gas. But they don’t release it evenly, from all over – they release it from pits and pockets on the nucleus and form what are known as jets. These jets of matter create the fuzzy halo that is known as the coma, or “hair,” of the comet. Spacecraft have visited several comets up close and taken images of their lumpy, pitted nuclei, So we have a fairly good idea what they are like now.

The nuclei of three well studied comets: Comet Halley (L), Comet Wild 2 (M), and Comet Churyumov-Gerasimenko (R). Credits: ESA/Giotto, NASA-NSSDC Master Catalogue, ESA/Rosetta/NAVCAM.

When enough gas and dust is being released from the comet a tail can form behind it. Comet tails always point away from the Sun, as they are being blown by the solar wind. When a comet is really active, two tails can form – a dust tail and a gas or ion tail. The gas ion tail is typically blue in colour, and streams out in a strait line, but the heavier white dust tail can curve away to the side, as the dust lags behind once it is shed. Comet tails can extend for millions of kilometres behind the nucleus at their peak.

The recent visitor Comet NEOWISE at it’s best, clearly showing the strait blue ion tail point away from the Sun and the curving dust tail falling away behind it. Credit: Stefan Zeigenbalg.

No two comets are ever the same, and even a returning comet will look different each time it comes back around the Sun. While we know of regularly returning comets (which are known as periodic comets) there will always be surprise appearances. That is part of the appeal of comets for some people – you just never know what kind of a show they are going to put on for you.