March 2024
The Sky Tonight - March 2024
April brings us into the season of Djeran, characterised by arrival of some much-needed cooler nights. The eastern sky is the …
ExploreFirst Quarter
June 7
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The casual observer
As we enter June, Scorpius and Sagittarius start to become increasingly prominent in the eastern sky during the early evenings. These constellations mark the central regions of the milky way, and winter is the best time of year to see the bright heart of our galaxy.
Image: Scorpius and Sagittarius backdropped by the centre of the milky way.
The close alignment of planets that we’ve been treated to the last couple of months is starting to spread out again, but it’s still very easy to see Saturn, Jupiter, Mars, Venus and Mercury in the eastern sky before sunrise.
This month’s full moon is a super moon. A super moon occurs when the full moon happens around perigee – the point on the moons orbit where it is closest to Earth, a distance of about 360 000km. The terminology ‘super moon’ is perhaps a little too inspiring, summoning to mind fantastic imaginings of a colossal moon in the sky. However, even a super moon can easily be covered by the fingernail on your outstretched pinkie, so unless you have a before and after picture you probably won’t notice the difference in size. This is not to say you shouldn’t be excited, just be aware of breathless social media posts showing highly specialised photographs of a gigantic full moon
Image: Supermoon (left) compared with a typical full moon (right)
The super moon will definitely be brighter though, as the larger apparent size reflects a proportionally larger amount of light back towards Earth, so much so you will easily be able to see shadows made by moonlight. If the full moon happens at perigee, this means two weeks later the new moon will occur at apogee when the moon is at its farthest point from Earth, about 405 000km away. This is called a micro-moon and is in a sense the opposite of a super moon.
The winter solstice happens on the 21 June. For people in the southern hemisphere this is the shortest day of the year, and for people in the northern it is the longest. People living on the tropic of cancer (23.5 degrees north) will see the Sun pass directly overhead. Functionally, this is the point in the Earth’s orbit where the Sun appears to stop drifting north in the sky and starts moving south. This is what ‘solstice’ means after all: Sol-sun, sistere-stands still.
The best way to get a feel for this is to make a note of where the Sun sets each day against the horizon. From 21 June onwards you will see the apparent position of the sunset against the horizon start to move south. The southerly motion of the Sun will continue until the occurrence of the summer solstice in December and the process reverses.
Image: Viewed from space, Earth’s northern hemisphere will reach its maximum solar coverage on this day, while the southern hemisphere will reach its least.
Image Copyright: 2017 EUMETSAT
Dates of interest
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Super Moon
June 14
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Moon near Saturn
June 19
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Winter Solstice
June 21
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Moon near Jupiter
June 22
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Moon near Mars
June 23
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Uranus occulted by Moon
June 25
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Moon near Venus and Mercury
June 27
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Micro Moon
June 29
Planets to look for
Saturn is rising earlier every day, now appearing at around 11pm each night. It begins retrograde motion on 5 June. It’s subtle, but if you compare Saturn’s position relative to the nearby star Delta Capricornus, you will see it begin to drift west relative to the stars over the coming weeks.
Jupiter continues its charge higher in the eastern sky as Earth catches up to it in its orbit while Venus is getting lower in the east, passing close to Uranus on 12 June.
Mercury is visible in the east before sunrise and the week centred on the 15 June is best time to see it. Now is hardly the best time of year to see Taurus but it makes a sneaky appearance as the backdrop to Venus and Mercury mid-month.
On June 25 the Moon will pass in front of Uranus as seen from Perth, an ‘occultation’ as it is called. The event will last about 40 minutes, beginning just after 4am local time.
Video: Moon-centered video of occultation. Apparent change in size and colour of moon is simulated effect of distortion by Earth’s atmosphere. The red points are just tracking markers on the moon.
Credit: Smith, Scitech
This is not the first time that astronomers in Perth have been in the right place to see chance encounters between Uranus and other celestial bodies. Observations made by astronomers at Perth Observatory of the planet occulting a distant star assisted in the discovery of the rings of Uranus. Read more about it in this previous episode of the sky tonight.
Constellation of the month
Corvus the Crow
Corvus is a small constellation in the southern sky about two handspans directly north of the southern cross. The asterism stands out amongst an otherwise dim area of the night sky as a distinct quadrilateral of four stars with a couple more bright stars on either side, making the constellation very easy to spot.
For this reason it has appeared in various mythologies, with stories interpreting it variously as a chariot, a tortoise, and most commonly a bird, as the prominent asterism resembles a bird with outstretched wings.
One story has the crow being sent by Apollo to fetch a cup of water. The bird got distracted and, realising it would displease Apollo to be so late, snatched up a snake as an excuse for being delayed bringing water. Apollo perceived the deception and in a fury cast the crow into the sky along with the snake and the cup, which became the constellations of Hydra and Crater.
Astronomers look to Corvus to see the Antenna galaxies. This well know target for telescopes is a pair of galaxies currently colliding.
Image: Tidal forces in the interacting galaxies draw material out between the galaxies, creating the resemblance to an insect antenna.
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgement: B. Whitmore (Space Telescope Science Institute) and James Long (ESA/Hubble)
When galaxies collide, the stars themselves don’t actually strike each other. Rather the galaxies pass through one another and gravity simultaneously tears them apart and brings them together. What does collide are the clouds of gas inside each galaxy, which then collapse and undergo furious star formation in a period called a starburst phase. This is on display in the Antenna galaxies which are alight with the brilliant blues of the star forming regions and the profound pinks ionised hydrogen gas. Galaxies in a starburst phase can form new stars at a rate dozens of times higher than regular galaxies.
Astronomers often draw similarities to the Antenna galaxies and the eventual merger of the Milky Way and Andromeda galaxy in several billion years.
Object for the small telescope
NGC 4361
NGC 4361 is a planetary nebula in the constellation of Corvus located at an intimidating distance of 3450 light years. It has a magnitude of about 10-11 and appears as a roundish blob in a telescope.
Planetary nebulas are unfortunately named because they have nothing to do with planets. Instead, they signal the last gasp of a dying star. In their final stages of activity, low mass stars throw off their outer layers of material in expanding clouds of gas, exposing their fiercely hot cores to the universe. Intense UV radiation from the hot cores then ionises the previously expelled gas, creating a beautiful nebula.
Image: NGC 4361
Credit and Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona
Fittingly, at the core of NGC 4361 is a 13th magnitude white dwarf star of about half a solar mass and boasting a surface temperature of about 126 000K, roughly 20 times hotter than the surface of the Sun. Because planetary nebula are illuminated by the core of a dead star they are fleeting in their existence, lasting only a few tens of thousands of years before the cooling core is no longer hot enough to ionise the gas. So, treasure the few millennia you have with NGC 4361 before it is gone for good.
The Black Hole at the Centre of the Milky Way
The scientific announcement of the year was made on 12 May, with the Event Horizon Telescope team releasing an image of Sgr A*, the black hole at the centre of our galaxy.
Image: Sgr A*, pronounced ‘Sagittarius A star’, the 4 million solar mass black hole at the heart of our galaxy (the ‘star’ is just the pronunciation of the asterisk, not to imply it’s a star, because it isn’t).
What we’re seeing is the light coming from the super-heated plasma that is spiraling around the black hole in the ‘accretion disk’. The dark part in the middle is called the ‘shadow’, because light from the accretion disk that passes through this area has been captured by the black hole. The black hole is sitting somewhere in the middle.
This is significant because it confirms that the big thing in the center of the Milky Way is definitely a black hole. Prior studies of the orbits of stars moving around SgrA* had already led scientists to strongly suspect it was a black hole, but nothing is as good as a picture.
This is only the second black hole ever imaged. In 2019 the same team released an image of the black hole M87*, located 55 million light years away in the galaxy M87. Here are the images side by side:
Image: M87* and SgrA*. Most remarkably, they look extremely similar.
While the similarities in the images may seem mundane, they become quite remarkable when we realise just how different these two black holes actually are.
Sgr A* is in the milky way, a spiral galaxy with about 200 billion stars and measuring about 100 000 ly across. It has active star formation and about 150 globular clusters orbiting around it. On the contrary, M87* is in M87, a giant elliptical galaxy with about 1 trillion stars. The galaxy is nearly spherical in shape and about 200 000 light years across. Nearly all of its stars are old, giving the galaxy a yellowish appearance because all the young blue stars have died. It has about 15 000 globular clusters orbiting around it.
Despite the very different environments, the black holes look the same. But it gets even more intriguing if we consider the sizes of the black holes themselves. M87* is about 1500 times heavier and larger than Sgr A*, but despite being a thousand times different in size and mass their appearance is the same. The identical nature of the two black holes suggests that the same physics (general relativity) applies on both scales.
Image: Side by side size scale comparison of M87* and Sgr A*
For comparison, imagine if we discovered a new species of kangaroo that grew to be a thousand metres tall. You would be right to think that there are some fundamentally different rules of biology going on here compared to regular kangaroos. On the contrary, two black holes that are a factor of a thousand different in size still turn out to look and behave exactly the same and are governed by the same physics.
In the physics community this idea is encapsulated in the phrase ‘Black holes have no hair’, to signify that the material that makes up a black hole loses its ‘identity’, and the only thing that matters is how heavy the black hole is and how fast it is spinning. All other quantities are irrelevant.
Image: Locations of the observatories making up the EHT
The Event Horizon Telescope is itself a remarkable achievement. Not a single telescope, but an array of radio telescopes dotted across the Americas, Europe, Antarctica and the Pacific Ocean. The individual telescopes all observe the same target and are then paired up with each other one by one and their data is combined in a process called ‘Very Long Baseline Interferometry’ to simulate a single telescope as large as the Earth. This effective size gives the EHT the resolution of about 1/100 millionth of a degree.
Hold out your pinkie at arm’s length and look at the part of the background covered by the nail. Now halve that. And halve it again. And again. And again and again and again. And halve it 19 more times until you have an almost infinitesimal field of view. That’s how small of a target the EHT can focus on and is exactly the sort of precision you need to be able to see black holes like Sgr A* and M87*.
Meanwhile in the Scitech planetarium
The dome is back!
The City West dome has been replaced on top of the planetarium and is lighting up the night sky of Perth once more. See the Facebook post here.
There seems to be some confusion about this dome and the planetarium so let’s set the record straight once and for all. The Scitech Planetarium dome belongs to Scitech and sits inside the building. The screen is made of aluminium panels and we project the Universe onto it. Surrounding the Scitech dome on all sides is the concrete building. Concrete floor, concrete walls, concrete roof.
On top of the concrete roof sits the City West dome. This is the dome that you see from the outside that lights up at night. Contrary to popular belief, this dome does not belong to Scitech, but we are happy to be identified with it – it’s the official logo of the Scitech Planetarium after all!
Rumour has it there’s another dome underneath the planetarium floor, and after that, it’s just domes all the way down!
Other space news
Do you have what it takes to successfully dock with the International Space Station?
Boeing’s Starliner was launched on 20 May as part of Orbital Flight Test 2. A full 24 hours later it docked with the ISS before returning to Earth on 25 May. The (mostly) successful test means we may soon see astronauts travelling to space on Starliner, a direct competitor to the SpaceX Crew Dragon.
Rocket Labs carried out a test of snatching an electron first stage booster out of the air, and mostly succeeded.
Another tour of the SpaceX Starbase facility? Yes please!
