July 2023

Planets to look for

Venus and Mars are in the west after sunset. The very bright Venus will dominate the western sky, while Mars is so faint it might as well not be there. 

Mercury appears in the west in the second half of the month and is visible nearby to Venus after sunset. On Jul 28 Mercury, Venus and Mars can all be seen as a nice triplet setting in the west, backdropped by Leo the Lion with the bright star Regulus just above Mercury.  

Image: The sunset of Jul 28 

Credit: Stellarium 

Venus has overtaken Earth in its orbit and will soon be passing behind the sun and out of view, so now is the last good month to get a decent look before it becomes less impressive. 

Saturn is rising about 9pm but doesn’t really become good viewing until late evening. It is joined by Jupiter from about 2am, and both planets are best viewed before sunrise.  

Constellation of the month

Boötes the herdsman 

Boötes (pronounced Boh-Oh–Tees) is a large constellation in the northern celestial hemisphere. From Western Australia it is visible low on the northern horizon during the winter months. 

The constellation is dominated by the bright star Arcturus, the fourth brightest star system in the night sky, which forms the corner of a roughly diamond shaped pattern on stars that makes the primary shape of the constellation. 

Image: Boötes visible in the northern sky from Perth 

Credit: Stellarium 

Boötes is often interpreted as a herdsman of oxen or some similar relation to farming and agriculture. This is reinforced by its close association with the nearby constellation Ursa Major, inside of which is the Plough asterism. 

Another interpretation has Boötes as the ‘Bear Watcher’, watching over Callisto, one of Zeus’s lovers, who was turned into a bear (Ursa Major) by Zeus’s wife Hera in a jealous rage.  

Image: Boötes, Ursa Major and the Plough asterism in green. Simulated view from northern hemisphere. Ursa Major is mostly not visible from Perth, but can be seen from the north of Western Australia. 

Credit: Stellarium 

To observe Boötes one begins by spotting Arcturus then filling in the fainter stars of the diamond pattern below it. Arcturus is an orange giant star slightly heavier and older than the sun and is in the process of evolving into a red giant. It has exhausted the supply of hydrogen in its core and is now fusing hydrogen in the layers of material surrounding the core. This process, called hydrogen shell burning, causes the star to expand since the heat source is closer to the surface. The larger surface of the star radiates correspondingly more energy, which causes the star to cool and approach a redder colour. 

Boötes is home to a number of interesting multiple star systems including the binary system Delta Boötes and the quadruple star system Mu Boötes which consists of two binary pairs. Intriguingly, the binary systems in Mu Boötes appear to have different chemical compositions, suggesting they have different origins, and that the proximity of the two pairs of stars is a coincidence, implying that this Is not a gravitationally bound system. 

Boötes is also home to the Boötes Void. The Boötes Void is a roughly spherical region of the universe, approximately 600 million light-years across that contains very few galaxies. Distant galaxies are spread roughly uniformly across the universe, but there are slight over and underdensities where there are slightly more or fewer galaxies. The over densities are grouped into ‘superclusters’ and the underdensities are called ‘voids’ and the Boötes Void is one of the largest examples of such.  

Delta Boötes 

Delta Boötes is a binary system in the constellation of Boötes, forming one of the corners of the diamond shaped asterism shown in the images above. Whether the stars are gravitationally bound is uncertain, though they are both located about 122 light years away. The system consists of a bright evolving giant star, loosely similar to Arcturus, and a fainter main sequence star.   

Image: Delta Boötes is observed as a binary system in a modest telescope. 

Credit: Dritter, Delta Bootis.jpg,CC BY-SA 4.0 

Heavy Hitting Cosmology – Exploring the Dark Universe with Euclid 

If all goes well, the Euclid spacecraft will be launching on Jul 1 atop a SpaceX Falcon 9 rocket. Euclid is a visible and near-infrared space telescope designed to undertake enormous surveys of galaxies over very large time scales to better understand the evolution of the universe under the influence of dark matter and dark energy. That’s quite a sentence, so let’s unpack it. 

Image: Artist Impression of Euclid in space. 

Credit and Copyright: ESA/ATG medialab (spacecraft); NASA, ESA, CXC, C. Ma, H. Ebeling and E. Barrett (University of Hawaii/IfA), et al. and STScI (background) 

A project of the European Space Agency, after launch Euclid will enter a halo orbit around the Earth-Sun L2 point, a region of gravitational stability 1.5 million km from Earth also orbited by the James Webb Space Telescope and the Planck satellite. 

Euclid will then spend six years observing and cataloguing very large numbers of distant galaxies, looking everywhere except in the plane of the milky way and of the solar system, ultimately viewing about 1/3 of the total sky. 

Image: The survey regions of Euclid. The gaps in the upper left and right are because of the plane of the ecliptic. 

Credit: Credit: Euclid Consortium Survey Group/J.-C. Cuillandre 

By measuring galaxies that are nearby (not too far back in time) and comparing them to galaxies and clusters that are much further away (further back in time) scientists will be able to understand how galaxies and clusters have evolved over cosmological timescales.  

Moreover, as predicted by general relativity, light from distant galaxies will be warped and bent by the gravity of any foreground galaxies and dark matter. By studying how the light is warped, astronomers will be able to map the distribution of dark matter over the sky. To account for the unpredictable foreground distribution, galaxy shapes will be averaged over a large number of samples, meaning that galaxy morphology (shape) is an important part of the study. 

Image: Representation of light rays (orange) being deflected by matter before arriving at Euclid. 

Credit: Springel et al. (2005)/ESA/L.Linke 

Spectroscopy data collected by the near infrared camera will allow determination of redshift and distance to the galaxies studied, and this can then be correlated with the distribution of dark matter determined by gravitational lensing, with the ultimate goal to study the relationship between the dark matter and the evolution of the galaxies and galaxy clusters over cosmic timescales. 

Redshift data from distant galaxies will also reveal clues about the rate of expansion of the universe, a variable known to change over cosmological time scales, the explanation of which is bottled up into the poorly understood ‘dark energy’. Further clues about dark matter and dark energy can be teased out from the distribution of galaxy clusters, revealing the fingerprint of ‘baryon acoustic oscillations’ – sound waves that propagated through the universe after the big bang, their propagation heavily dependent upon the quantity and distribution of dark matter and dark energy. 

Image: Simulated distribution of millions of galaxies across the universe into a structure called the ‘cosmic web’. This filamentary structure gives clues to the baryon acoustic oscillations of the early universe. 

Credit: Euclid Consortium 

Did you make sense of all that? The Euclid project is enormous in both size and scope. Euclid’s 1.2m diameter mirror is sensitive enough to detect galaxies up to 10 billion light years away, and the telescope is expected to deliver about 100GB of data a day, ultimately cataloguing around 2 billion individual galaxies and 30 000 TB of information. The survey is expected to be the go-to database for galaxy and cosmology studies for decades to come.  

What’s in a name? 

The Euclid spacecraft is named after the ancient Greek mathematician of the same name. Around 300 BCE Euclid published a set of 13 books laying the foundation for what we now call Euclidean Geometry: the mathematics of circles, triangles, flat planes and sharp corners. You know, all the good stuff you studied in high school? Angles in a triangle add up to 180 degrees, parallel lines never meet, etc. etc. Good. Classic. Fun. 

Image: An excerpt from Euclid’s Elements 

Credit: Folger Shakespeare library 

In the 19th century, mathematicians started exploring non-Euclidean geometry, a problem eventually solved by the mathematician Bernhard Riemann, allowing complete descriptions of curved surfaces and warped shapes. In the early 20th century, Einstein’s genius was to understand that the same non-Euclidean geometry of curved surfaces could be applied to describe curvatures of space and time, and so general relativity was born. 

In the framework of general relativity, gravity literally is the shape of spacetime, it’s behaviours and quirks, from apples falling off a tree, to the cosmological redshift of photons hinting at dark energy and the lensing and shearing of light from distant galaxies by foreground dark matter, are all understood by the mathematics of curved spacetime. 

What would Euclid think in 300 BCE had he known that more than two thousand years later his name would be adorned on a spacecraft used to study the very shape of the universe by looking at the galaxies within it? 

Speaking of galaxies, winter isn’t going to last forever, so you’d better hurry on outside to look at ours. 

Other space news 

Intriguing new hypothesis about when Betelgeuse will go supernova 

What would happen if the Universal Logo were real? 

Boeing Starliner first crewed launch delayed indefinitely 

Phosphorous has been found in the water plumes ejected from Saturn’s moon Enceladus 

United Launch Alliance launched the second last ever Delta IV Heavy 

Astronomers present evidence for detection of low frequency gravitational waves from black hole binaries.  

Astronomers release a neutrino map of the milky way