The Sky Tonight - November 2023
December brings us into the season of Birak, the first summer. The silver lining of the hot dry days is the clear night skies …Explore
The Milky Way continues to be the 200 billion strong star(s) of the show in the evening sky, stretching from east to west high above. With such a good view of the central regions, now is the perfect time to see the emu. Rather than a typical asterism made by connecting lines between bright stars to make patterns, the emu is made by looking at the dark parts of the Milky Way that are obscured by dust clouds.
Start at the Coalsack Nebula underneath the Southern Cross. This forms the head of the emu. Follow Alpha and Beta Centauri down the neck, opening out into the body of the emu in Scorpius and Sagittarius. The legs are less well defined, stretching out into Scutum and Aquila, but by the time you have that much bird the rest should be easy to fill in.
Image: Milky way with and without emu.
Once you know that the emu is there it’s quite difficult to un-see it, and unsurprisingly the emu makes appearances in the stories of many indigenous Australian cultures. Pragmatically, because of the sky incrementally changing with the Earth’s motion around the sun, the position and orientation of the emu in the night sky could be used to determine when it was the right time of year time to hunt emus for meat or whether to go for the eggs in the nest instead.
You may hear word of the Perseid meteor shower in the news. Unfortunately, this is too far north to be seen from most places in Western Australia. However, if you’re way up in the Kimberley then you might catch a few meteors very low in the northern sky just before sunrise from about 12-15 Aug.
ISS sightings from Perth
The International Space Station passes overhead multiple times a day. Most of these are too faint to see but a couple of notable sightings are:
|Date & time||Appears||Max Height||Disappears||Magnitude||Duration|
|9 Aug, 5:57 AM||17° above SW||67°||10° above NE||-3.5||5.5 min|
|15 Aug, 7:22 PM||10° above NW||68°||76° above NNW||-3.6||3 min|
Table: Times and dates to spot the ISS from Perth
Use this one weird trick to tell Webb and Hubble images apart
Invariably you will find comparisons between images from Hubble and JWST. It is worth keeping in mind that they see different parts of the spectrum, with Hubble being mostly optical and JWST infrared, so comparisons between the two must be interpreted with care.
Image: Galaxy cluster SMACS 0723 viewed through Hubble left, JWST right.
You can tell the difference between images from Hubble and JWST by counting the spikes. Notice in the images above, the four-pointed spikes sticking out from the stars in the Hubble images, compared to the eight-pointed stars in the JWST images (six large spikes and two smaller ones).
These are called ‘diffraction spikes’ and they appear on any highly focused star in a Hubble or JWST image. Diffraction is complicated, but the simple answer is the spikes are due to the shape of the primary mirror and the support struts on the secondary mirror.
Hubble has a round primary mirror and a secondary mirror supported by four struts. It is these four struts that give Hubble images the four-pointed spikes, and the circular primary mirror causes concentric rings to appear around highly focused stars (see the image of Alpha Centauri AB below). On the other hand, Webb’s hexagonal primary mirror produces the six-pointed spikes, and the support structure partially overlaps with this and so produces the two extra spikes for a total of eight.
The bonus observation is that anything that doesn’t have diffraction spikes is a galaxy.
Mars near Uranus
Moon near Antares
Moon near Saturn
Mars, Aldebaran, Betelgeuse alignment
Crescent moon near Mercury after sunset
Saturn finally gives early evening planet hunters something to look at, rising at about 7pm in the east followed by the significantly brighter Jupiter at about 11pm.
Venus is visible in the morning sky in the east, though it has to contend with an increasing proximity to sunrise, so it will get harder to see as the month goes on.
Mars is still rising at the unpleasant time of around 2am. For most of the month it aligns nicely with Aldebaran and Betelgeuse, two noticeable red stars in the east. Mars will be the brightest, furthest from the eastern horizon. Betelgeuse on the right will be slightly dimmer and Aldebaran sitting in the middle is the faintest.
Image: The Moon joins a nice red alignment on 19 Aug
Mercury is visible at about 7pm low in the western sky in the second half of the month. The best time to see it is late in the month as the New Moon on the 27 Aug makes for best viewing conditions and a nice appearance of a waxing crescent above the planet on 30 Aug.
Centaurus is a large constellation in the southern skies and an easy sight for naked eye stargazing. The constellation was formalised by Ptolemy in the 2nd century CE, though was spoken of a lot earlier, and is interpreted as a Centaur – a mythical creature that is half human, half horse – which is best not to think about too hard because it certainly poses some interesting questions.
Centaurs usually appeared in stories as violent and unpleasant characters, with the exception of Chiron, the character with whom this constellation is usually associated. Chiron was a teacher and mentor to many of the characters that appear in other legends, such as Heracles (Hercules) and Jason (of Argonaut fame).
The constellation flanks the Southern Cross on three sides, and the two brightest objects Alpha and Beta – interpreted as the Centaur’s front legs – certainly earn their moniker as the ‘pointer stars’ guiding the way to the Southern Cross.
Alpha and Beta Centauri provide an interesting refresher in the inverse square law of light intensity. Beta Centauri is a triple star system containing three very luminous B-type stars, totalling about 60 000 solar luminosities. Alpha Centauri is also a triple star system with the main contributors being two Sun-like stars for a total luminosity of about twice that of the Sun. However, at 400 light years away, Beta Centauri is 100 times more distant than Alpha Centauri, so its brightness is diminished by more than ten thousand times (100 x 100). The result is it looks fainter in our sky than Alpha Centauri despite being substantially brighter in absolute magnitude.
Of course, the third contributor to the Alpha Centauri star system is the red dwarf Proxima Centauri, the closest star to the Sun. With at least two planets, one of which is in the habitable zone, Proxima Centauri is a likely target for observations with the James Webb Space Telescope.
Centaurus is also home to Centaurus A, a giant and unusual radio galaxy in the southern sky.
Image: Centaurus A, with its prominent dust lane and lobes shown in superposed radio (blue) and X ray (orange)
Centaurus A is shrouded by an unusual screen of dust which suggests a cosmologically recent merger with another galaxy, but also makes studying the galaxy difficult. A 55 million solar mass black hole at its centre is actively interacting with galactic material and ejecting some of it from the galaxy, producing enormous lobes that shine brightly in radio and X rays. At only 10-15 million light years away the galaxy is visible through even a modest telescope.
Alpha Centauri AB
The closest star system to the Sun and third brightest in the night sky is resolved into a binary star system in a small telescope. The binary is then orbited by the closer and fainter Proxima. The two Sun-like stars in the binary are about 1.1 and 0.9 solar masses and orbit each other every 80 years, while Proxima orbits them every 550 000 years.
Image: Hubble image of Alpha Centauri AB resolved into component stars. Note the 4-pointed diffraction spikes, and the concentric rings.
Alpha Centauri AB was the target of some of the observations from the NASA sounding rockets that were launched from Australia in June and July. The researchers wanted to know whether other Sun-like stars emit similar amounts of ultraviolet light as the Sun, a question not yet answered.
The reason for this is that Venus and Mars are likely to have once had oceans similar to Earth, but it is thought that ultraviolet light from the Sun eventually stripped these planets of their water. Knowing if all Sun-like stars emit similar amounts of UV radiation can help narrow down the search for habitable exoplanets.
UV light from stars is absorbed by material in the interstellar medium, making this a difficult question to study, so only nearby Sun-like stars are good candidates for study. Enter alpha Centauri AB, two Sun-like stars literally as close as we can get.
The Bayer nomenclature for stars assigns letters from the Greek alphabet in order of brightness to stars within a constellation. Hence, Alpha and Beta Centauri are the first and second brightest stars in the constellation, respectively. At first overlooked, the fainter Omega Centauri contains a surprise when viewed through even a modest telescope and is revealed to be a globular cluster.
Image: My god! It’s full of stars!
Globular clusters are extremely compact groups of thousands to millions of stars that orbit larger galaxies as a singly body. There are about 150 globular clusters orbiting the Milky Way and, with about 10 million stars, Omega Centauri is the largest.
The leading idea for the existence of globular clusters is that they are the cores of old, smaller galaxies that the Milky Way has absorbed, stripping away loosely bound stars and gas leaving only the core cluster behind. This lends weight to the idea of ‘bottom up’ structural development of galaxy evolution, suggesting that large galaxies like the Milky Way form by the combination of smaller galaxies over billions of years. The lack of gas in globular clusters inhibits star formation, resulting in much fewer young, hot, blue stars and giving globular clusters a yellowish appearance overall.
The Incredible Engineering of the SpaceX Starship
If you blinked, you might have missed the furious pace that SpaceX is setting as they develop Starship down at Starbase in Boca Chica, Texas. Originally known affectionally as the BFR, which stands for Big … Rocket, Starship has undergone serious testing and development in the last few years and is expected to perform its first orbital test flight later this year. A few explosions along the way haven’t dampened enthusiasm.
Video: Starship test flights, with explosions
Starship is intended to be the first ever fully reusable orbital-class spacecraft, with the capacity to deliver 100 tonnes of cargo to Low Earth Orbit. For comparison, the Falcon 9 can deliver about 15 tonnes to LEO. It is a two-stage rocket, with the ‘Super Heavy Booster’ used to get off the ground, while the second stage ‘Starship’ goes into orbit. Note also that the word ‘Starship’ does double duty as the second stage as well as the fully stacked combo of ‘Superheavy Booster and Starship second stage’.
Starship incorporates lessons learnt from the Space Shuttle heat shield. One of the biggest problems with refurbishing the Space Shuttle between flights was that the heat resistant tiles came in a great variety of shapes and sizes, making repair and replacement a very slow and tedious process. Starship uses standard repeating tiles in its design for easy integration and replacement.
Image: Starship tiling vs Space Shuttle.
Perhaps most importantly, Starship is also designed to be rapidly prototyped. It’s one thing to rapid prototype a product to fit a design goal, and it’s quite another when rapid prototyping is one of the design goals. This requirement has led to the unusual choice of steel as the construction material, a material not known for its lightweight features. Spacecraft need to be as light and as strong as possible and are usually made of a combination of aluminium alloys and composite materials. However, with a hefty dry mass of ~150 tonnes, building Starship out of composite materials would be prohibitively expensive. The low cost of steel, combined with its impressive mechanical properties at extremely hot and cold temperatures make it suitable for use in Starship.
SpaceX have also designed the most efficient rocket engine to date, the Raptor 2, which achieves an astonishing 99% efficiency as it burns methane with oxygen.
Here, let this guy explain it:
Video: Elon Musk explains SpaceX’s raptor engine
Credit: Everyday Astronaut
Most rocket engines are powered by a ‘preburner’, a mini engine-inside-an-engine used to power the pumps that feed the fuel and oxidiser into the rest of the engine. In order not to melt the engine, rocket engineers flood the preburner with either extra fuel or extra oxygen, creating ‘fuel-rich’ and ‘oxygen-rich’ engine cycles.
The RS-25 engines that powered the Space Shuttle flood the preburner with hydrogen fuel in a fuel-rich cycle, while the RD180 engine that powers the Atlas V launch vehicle floods its preburner with oxygen in an oxygen rich cycle. In both cases one of the fuel or oxidiser enters the main combustion chamber in a liquid state rather than as a gas. This makes it harder for the fuel and oxidiser to mix well, reducing the efficiency of the engine.
The Raptor engine sidesteps this problem by having both a fuel-rich preburner and an oxygen-rich preburner, which between them partially burn all the fuel and all the oxygen before pumping into the main combustion chamber. This setup is called ‘full flow staged combustion’ – meaning “All the fuel and oxidiser (full flow) passes through a preburner (staged combustion) before entering the main combustion chamber”. This means both the fuel and oxidiser enter the main combustion chamber in the gaseous state, resulting in better mixing and performance. This setup also allows the engine to run at higher pressures and the preburners to run at lower temperatures, increasing the efficiency and reducing the stress on the engine allowing for longer lifespan.
Image: Full flow staged combustion schematic.
Credit: Livingjw, NASASpaceFlight.com
Using methane as a fuel is what allows for the use of a closed, fuel-rich preburner in the first place. Engines that run on kerosene, such as the Merlin engine on the Falcon 9 or the F-1 engine on the Saturn V Moon rocket, cannot reuse the partially burnt fuel from the pre-burner because it is too dirty and can damage the engine. So, it must be dumped overboard, therefore reducing efficiency. The chemical simplicity of methane allows for the pre-burner exhaust (water and CO2) to be used in the main combustion chamber without damaging it.
The choice of methane is dictated by Starship’s ultimate design goal – missions to Mars. The intention is to land Starship on Mars and then refuel by manufacturing methane and oxygen from the carbon dioxide in the atmosphere and subsurface water, allowing a return trip back to Earth. Manufacturing hydrogen fuel would be much more difficult, and of course kerosene wouldn’t be an option on Mars, so methane is the way to go.
After receiving environmental approval to launch Starship from Starbase, SpaceX will likely conduct the first orbital test flight of Starship in the near future. Ship 24 and Booster 7 are the likely candidates and were undergoing pre-flight testing early in July when an unexpected explosion damaged the booster and delayed the testing program. It is unclear whether SpaceX will persist with flight 24/7 or replace it with Booster 8. If you think 24/8 sounds less cool, you are not alone.
Image: Starship being fully stacked
Other space news
Dmitry Rogozin, the volatile head of Roscosmos, has been replaced by deputy Prime Minister Yuri Borisov
NASA and Roscosmos have settled on a ridesharing agreement to launch astronauts to the International Space Station on Russian Soyuz launch vehicles in exchange for launching cosmonauts on NASA flights.
The Rosalind Franklin Mars rover, originally intended to be launched on a Russian rocket, has had its launch date pushed back to at least 2028 after the European Space Agency suspended cooperation with Roscosmos on the project due to continued tensions over the Russian invasion of Ukraine.
Russia announced it will withdraw from the International Space Station after 2024, then hastily pushed this date back to 2028. It is unclear how this debacle will proceed.
December brings us into the season of Birak, the first summer. The silver lining of the hot dry days is the clear night skies …Explore
As we enter the second half of Kambarang the Milky Way looms low in the west at sunset, taking Scorpius lower in the sky each …Explore
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