November 2022

Planets to look for

Jupiter and Saturn become visible in the evening as the sun sets, both of them slightly to the north. They are joined by Mars rising in the east at about 10pm.  

Mercury and Venus are currently on the other side of the Sun from Earth this month so will be largely unable to be observed. They do make a brief appearance towards the end of the month, just barely visible in the minutes after sunset, but you will definitely have to look hard to see them. 

Image: The full moon joins a trio of planets on 8 Nov 

Constellation of the month

Sagitta the Arrow 

Saggita is a tiny constellation viewable in the northwestern sky during the evenings of November. Its four brightest stars form a distinctive arrow shape and hence it receives it’s namesake from the Latin word for ‘arrow’. Be sure not to confuse it with its more illustrious but unrelated cousin Sagittarius the Archer. 

Covering only 0.2% of the sky, Saggita is the third smallest constellation, larger only than Equuleus and the Southern Cross. Nearby Aquila, with bright Altair at its helm, provides a useful reference to locate this small pattern of stars. 

The arrow of Sagitta is often interpreted as the weapon Heracles used to kill the eagle (Aquila) that Zeus sent to eat the liver of Prometheus, a punishment dished out for Prometheus teaching humans how to use fire as a tool. 

Image: Sagitta and Aquila in the north-western sky. 

Sagitta is home to a number of multiple star systems, the easiest to observe being Delta Sagitta, the star along the asterism where the fletching meets the shaft. This binary system consists of a cool red giant star orbiting a hot blue main sequence star. 

The constellation is also home to V Sagittae, a cataclysmic variable system. Cataclysmic variables consist of a ‘donor star’, usually a main sequence or red giant star, and a white dwarf ‘primary’. The intense gravity of the white dwarf strips material off the donor star into an accretion disk depositing material onto the dwarf star, which can occasionally flare up in a nova event.  

Image: Accreting star system 

Credit: NASA/JPL-Caltech 

V Sagittae is unusual in its highly asymmetric mass ratio between donor and primary, where in this case the mass of the donor star is 3.3 times the mass of the white dwarf. Other known cataclysmic variables have a donor star less massive than the primary. This means the donor star still has enough self-gravity to hold itself together, so as the white dwarf’s gravitational influence increases (as it acquires mass from the donor) rather than gently stripping away the tenuous upper atmosphere of the donor star, it strips away great deep layers of the donor. The result is an exponentially increasing rate of mass transfer and a correspondingly rapid decay in orbit, so fast in fact that it is estimated that the two stars will merge in the year 2083, mere decades away, compared to the millions of years that a merger typically takes. 

Interestingly, even though the white dwarf is a dead star, the sudden accretion of several solar masses still undergoing active fusion will give it a new lease of life, as the merged star will behave like a red giant star.  

Messier 71 – Mischievous metals 

M71 is a globular cluster in the constellation of Sagitta slightly beyond visibility to the naked eye but easily seen in a telescope. Containing around 50 000 stars and around 9-10 billion years old, the cluster is unusually young. Globular clusters are typically 10-12 billion years old or so, some of the oldest structures in the universe. 

 Image: Hubble image of M71 

Credit: ESA/Hubble and NASA 

In the life cycle of stars fusing elements in their cores and then returning more complex elements to the universe when they expire, each successive generation of stars contains a higher percentage of metals than the previous one (astronomers call anything more complex than helium a ‘metal’. Confusing but pervasive terminology). Because of their extreme age and low star formation rate, most globular clusters have very low amounts of metals. The unusually high metallicity of the stars in M71 combined with the comparatively sparse distribution of stars compared to other globular clusters led astronomers to originally conclude that M71 was a dense open cluster rather than a globular cluster. Only later did studies confirm that the stellar classifications in M71 showed the characteristic ‘horizontal branch’ of a globular cluster, allowing for it to be correctly identified. 

Image: Colour magnitude diagram for M71 with the small horizontal branch visible (straight line, upper centre) confirming its identity as a globular cluster. 

Credit: Image reproduced from Hodder et al 

Within a globular cluster, the most massive stars account for most of the brightness of the cluster, while the much more numerous yet dimmer stars account for most of the mass. For this reason, the ‘Mass to Luminosity’ ratio is an important characteristic of groups of stars and something that star population models seek to replicate. Studies of M71, along with other high metallicity globular clusters, have shown that the theoretical physics of star population models requires some improvements, as it currently gives slightly lower predictions for the brightness of high metallicity globular clusters than is measured. More science still to be done! 

Gamma Ray Burst 221009A 

On 9 Oct Earth was hit by a blast of gamma rays from across the universe. Despite the frightening premise, this actually happens all the time, with Gamma ray bursts (GRBs) detected by orbiting spacecraft about once per day. Gamma ray busts are the most powerful explosions in the universe, releasing as much energy in a few seconds as the sun will emit in ten billion years. Fortunately, since they are observed at such great distances then there is no danger to us here on the Earth, though some studies suggest that nearby GRBs could cause mass extinction events and play a major role in the existence or lack thereof of life in the universe. 

Gamma ray bursts fall into two categories. Long GRBs (> 2 seconds) are thought to occur when large stars collapse into a black hole or neutron star during a core collapse supernova event. Energy released from the quickly collapsing core punches through the outer layers of the star causing a tremendously bright and violent emission. Short GRBs (< 2 seconds) have been confirmed to be produced during the merger of two neutron stars. Regardless of the source, a highly directional beam of gamma rays is emitted and if it happens to be pointing in exactly the right direction then it will eventually reach Earth.  

Video: Gamma ray burst occurring during core collapse of a star. 

Credit: NASA/Swift/Cruz deWilde 

 Which brings us to the attractively named GRB 221009A. The nomenclature indicates that this was the first burst (A) observed on the date (YYMMDD) indicated. This burst is notable for its exceptional brightness, which is a combination of it being much closer than most previously detected bursts, and also being intrinsically bright. Its close proximity of only 2 billion light years away (that’s really close for a GRB!) allowed astronomers to detect the initial wave of gamma rays followed by more than 10 hours of activity, revealing details that may normally be missed in more distant GRBs. 

Image: X rays from the burst scattering of dust inside the milky way cause the appearance of bright rings around the burst. 

Credit: NASA/Swift/A Beardmore (University of Leicester) 

The tremendous duration of the burst puts it in the Long GRB category, leading astronomers to speculate it originated from a collapsing star forming a black hole. 

There is also some evidence that the energy of individual gamma ray photons may have peaked at 18TeV, which would make it the highest energy light ever observed from a GRB. (This is about as much energy per photon as turning 18 trillion hydrogen atoms directly into energy via E=mc^2) For comparison, the most powerful particle accelerator on Earth – the Large Hadron Collider – smashes protons into each other with an energy of about 6.5 TeV per proton.  

Such high energy radiation tends to interact with other light sources in the universe, including the Cosmic Microwave Background, and becomes attenuated over great distances. The proximity of this burst may be the reason for the detection of such high energy photons, and these observations could provide some clues to interesting physics of ultrahigh energy photons. 

Image: GRB221009 as observed by the Fermi Large Area Telescope 

Credit: NASA/DOE/Fermi LAT Collaboration 

The location of the burst remains a source of intense observation for international astronomers. Being such short-lived transient events GRBs are difficult to study, and their investigation requires rapid and coordinated efforts from astronomers globally. When a burst is detected by an orbiting telescope an automated message called a GCN (GRB Coordinates Network) is sent out to astronomers world-wide who then attempt to take observations if they are in the right place at the right time. The Zadko Telescope operated by the University of Western Australia is involved in these studies. 

Other space news 

DART collision with asteroid an overwhelming success 

We heard an asteroid hit Mars! 

Amazing drone ship footage of Falcon 9 launch and landing:

View from the droneship of Falcon 9’s launch and landing pic.twitter.com/5yYvT9bNcL

— SpaceX (@SpaceX) October 12, 2022

NASA is soon to test an inflatable heat shield 

Could SpaceX and Dragon save the Hubble Space Telescope? 

An eclipse of Jupiter as seen from Mars 

New models on how the moon formed suggest it was much faster than we thought