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The Sky Tonight - July 2024
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ExploreNew Moon
February 1
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The casual observer
This month is all about the outer part of the Milky Way, because of where Earth is positioned along its orbit. The bright centre of the Milky Way is behind the Sun at the moment, meaning when you look up at night you are looking away from the centre to the outer parts of our galaxy.
This is prime real estate of the night sky, with many great constellations lined up and easy to recognise. Starting in the southeast at the Southern Cross and Centaurus, move through the Argo-Navis trio of Carina, Vela and Puppis to get to Canis Major, with Sirius shining bright to guide the way. From here observe Orion duelling with Taurus before finishing with Auriga the goat-herder in the north. All of this is backlit by the Milky Way’s outer sections.
As always, the Moon completes its monthly trundle across the sky, with the new moon conveniently occurring on the first day of the month.
There’s the Alpha Centaurids meteor shower in the first week or two of February. These meteors appear to emanate from their namesake star, Alpha Centauri, the closest star system to the Sun and one half of the pointer stars. It’s not a huge shower, but you might expect to see 5-6 meteors per hour coming from this part of the sky, in good conditions.
Dates of interest
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Alpha Centaurids meteor shower peaks.
February 8
Planets to look for
All the visible planets have disappeared from the early evening sky by now, with Jupiter finally setting in the west around sunset after a six-month trek across the night sky. If you are up exceedingly early, you will see Mars, Venus and Mercury at about 5am in the east just before sunrise. Be there or be … not round.
If you’re really determined, you may want to try your skills at locating Uranus. You will need a telescope for this. Let Charlie help you out with this handy episode of The Audio Guide to the Galaxy.
Constellation of the month
Musca the Fly
Musca is a small, faint constellation in the southern skies located immediately south of the Southern Cross. In is depicted as a fly, and as usual the asterism looks nothing like the artistic interpretation. In some representations it appears as a bee.
Being so far south, Musca was not included in modern star charts until the 17th Century after it was mapped by Danish astronomers Keyser and de Houtman. Musca is sometimes associated with the nearby constellation of Chameleon, as food for the bespoke lizard.
Musca is most notably home to a number of stars from the Scorpius-Centaurus OB Association – a loosely bound group of OB stars moving across the galaxy. Most of the brightest stars you see in Musca are likely members of this group. OB associations, as the name suggests, are groups of stars of spectral type O and B that are likely to have all formed at the same time in the same part of the galaxy, and so give useful information about star formation in the Milky Way.
O and B type stars are extremely hot and bright, powered by furious consumption of their nuclear fuel. Because they fuse so prolifically they live only a few million years so if you’re looking at an OB star group, you are looking at very young stars. If the history of earth was 24 hours long, dinosaurs appeared 1 hour and 15 minutes ago, and disappeared about an hour later. Stars in the Scorpius-Centaurus OB Association have only been around for the last 5 minutes on this timescale.
Object for the small telescope
Theta Musca – stellar companions in the friend zone
Theta Musca is an optical binary in the southern sky in the constellation of Musca. Through a decent telescope it separates into two discernible points. In this case however, there is less than meets the eye.
Optical binaries are stars that look close together in the sky, but in real life are not close together at all. Not to be confused with Visual Binaries – stars that look close together because they are close together as part of the same system – Optical Binaries just happen to be on the same line of sight as seen from Earth, but may be many light years distant from one another, a bit like holding your thumb up against the full moon at night. They look close together. They aren’t.
The brighter of the two objects, Theta Musca A is a gravitationally bound triple star system containing a Wolf Rayet star, one of the most unusual types of stars in the universe, and two companions. Theta Musca B is optically nearby but not related to the primary.
Wolf-Rayet stars are characterised by their extreme luminosities and temperatures, and the lack of hydrogen lines in their spectrum. At surface temperatures of order 100 000K, strong stellar winds from the star push off the outer hydrogen layers exposing helium and other heavier elements deeper in the star. Interestingly, because the surface temperature is so high most of their emission occurs in the UV, beyond the range of human vision, meaning that they actually tend not to look very bright to our eyes.
Until recently, Wolf-Rayet stars were all the more mysterious because of their seeming bashfulness about going supernova. Wolf-Rayet stars are massive enough to undergo core collapse supernova, but there didn’t seem to be any supernova remnants that matched the spectrum expected from a Wolf-Rayet star, leading astronomers to speculate that perhaps they simply went quiet into the night, turning directly into black holes while gently expelling their outer layers without any noticeable supernova. A recent study however, seems to have definitely linked supernova remnants to a Wolf-Rayet predecessor. Turns out they might go supernova after all. As always, more science will reveal the way.
James Webb Space Telescope: What happens now?
As everybody is aware, the James Webb Space Telescope (JWST) is the big new telescope on the block. Following a tense launch with an even more unpleasant deployment process, JWST has reached its target orbit around the Earth-Sun L2 point.
Image: JWST is fully deployed! Credit: NASA
While JWST is now in its final orbit, there is more to be done before we get science data back from it. The most crucial step is focusing the primary mirror. In order not to repeat the mistakes of the Hubble Space Telescope, whose single primary mirror was ground incorrectly to the wrong shape, JWST has 18 small adjustable mirrors that combine to form one single mirror 6.5 metres in diameter, as seen in the image above.
NASA learnt from the mistakes of Hubble, which required an expensive corrective optics installation, and designed JWST so that each hexagonal segment can be individually adjusted by seven actuators – one in each corner to change the angle of that part of the mirror, and one in the centre to adjust the curvature – to bring light into a precise focus. The actuators can move by mere nanometres, maximising the precision of the final focus. As the mirror cools to -230C the individual mirror segments will subtly contract and change shape, requiring fine tuning to get into precise focus. Complete focusing is expected to take about three months.
Image: JWST disappearing into space moments after separating from the launch vehicle. This is the last we will ever see of the telescope directly. Credit: NASA
If all goes well, JWST will begin science observations in about five months.
Meanwhile in the Scitech Planetarium
Dome Date Night is happening as part of Fringe World Festival. The show, Love, Death and Other Cosmic Accidents, explores exactly those topics in the context of relationships and the universe.
Part of the show involves visualising a giant elliptical galaxy, the likely result of the merger between the Milky Way and the Andromeda galaxy expected to happen about five billion years from now.
What name is befitting of such a galactic monster of the future? Surely a name that inspires awe, dread and reverence all at once. If you have any good suggestions for names, please let me know because the name that astronomers currently use to refer to this object does none of those things. All hail … Milkdromeda.
The software that runs the planetarium was only really designed to render a representation of the Milky Way, a disk-shaped spiral galaxy. Tricking it into producing a giant elliptical galaxy was an interesting challenge that required some creative programming.
A data file stores the coordinates of stars in the Milky Way. Exact coordinates for stars that are known, and a simulated distribution for stars that are too far away for precise astrometry. Throw in some templates for dust distribution and colour profiles and you have yourself a galaxy.
The first thing to do was to swap out the star distribution. Elliptical galaxies have a star density that concentrates towards the centre and exponentially drops off after that. So, we used a random number generator to produce a random distribution of coordinates in 3 dimensions weighted by an exponential function. Swapping out the colour and dust templates for something with less variation – elliptical galaxies tend to have less dust and older yellower stars – the result was achieved.
But there must be more to it than that, as the Planetarium struggled to render it at anything faster than a grand spanking 10 frames per second.
There are ways to sidestep this problem but that is a story for another day.
Other space news
ICRAR Scientists discover an unusual slow radio transient using the Murchison Widefield Array.
Have you seen Beyond the Milky Way at the WA Museum Boola Bardip yet? If not, get to it as it finishes up on 14 February.
The Hunga Tonga-Hunga Ha’apai eruption was visible from space and created pressure waves that oscillated around the entire planet.
NASA releases a video detailing their forecasted vision for 2022 – NASA 2022: The Future is Now.
They also announced it is ready roll out the Space Launch System from the Vehicle Assembly Building for testing at the launch pad sometime in February. Watch this space.
SpaceX continues its furious development at Starbase in Texas with Mechazilla coming to life and flexing the chopsticks, and the explosive testing of the ground support equipment.
