Highlights of the Southern Sky, Summer 2017

A view to the south in this composite image via Stellarium with modifications by the author, enhancing the appearance of Antares through direct image superposition. A wide-field, digital image of the star was obtained by the author and superposed on the stellarium image to create this composite image. Visible in this image are Saturn, situated in Ophiuchus and amid the galactic center with Sagittarius to the south and Antares to the west.

Later this summer we will be witness to a number of notable events. Amid the bitter-sweet ending to the NASA/ESA Cassini mission to Saturn and Titan, with the Cassini orbiter hurtling into Saturn’s cloud tops in a spectacular blaze of glory on 15 September and the 21 August Total Solar Eclipse, we observe none other than Saturn itself to the east of Antares, the ruddy-red heart of Scorpius, the scorpion. If nothing else, take a walk outside on the evening of 15 September and look south towards Saturn and wave goodby to one of humankind’s most successful missions ever completed, a mission that was extended twice and one that will continue to bear fruit for decades to come.

Antares vs. Arcturus vs. the sun! Arcturus at 37 light years pales in luminosity to Antares at 550 light years.

Weighing in at 12.4 solar masses and far larger and more distended than Arcturus, is Antares. In previous posts, most notably this time last year, and in a subsequent post on 22 September of last year, I discussed the treasure trove that is this part of the sky. Those posts are worth recalling, if for nothing else but to add to this discussion. Also posted last year and central to this discussion are Some Thoughts about Autumn, Arcturus and the Future.

As it turns out, both of these stars are visible now, at the same time, both in an evolved state, both of comparable brightness, and one, much further away than the other. The comparison of these stars provides for a good object lesson in stellar luminosities. Luminosity, or the total radiant energy output per unit time, varies as the forth power of the temperature and the second power of the size. Since they’re both of comparable temperatures, the enormous luminosity difference must be attributable to the difference in size and so it does! Although both are comparable in visual brightness and color, Arcturus at 37 light years distance and 25 solar luminosities pales to Antares at 550 light years and 57,700 solar luminosities!

Although Arcturus, now at 25 solar radii, has evolved off the main sequence (the evolutionary location of all “normal” stars, stars such as our sun) and now classified as a “red giant”, Antares, also a “red” giant star, is classified as a “supergiant” at 883 solar radii and a supernova candidate! As a thought experiment to get a better sense of the enormous size of this star, consider the above scene with Saturn and Antares in our sky where Antares would be where the sun is, at the center of the solar system; in this hypothetical situation, Jupiter, in its orbit, would skim the star’s distended photosphere and Saturn would be the second planet in orbit around the star! Weighing in at scarcely above one solar mass, Arcturus will continue to evolve as a one solar mass star and will thus, come to quite a different end than Antares.

Any star greater than eight solar masses will evolve to become a supernova candidate, a Type II or core-collapse supernova, where the star implodes in upon itself at the end of its productive life, rebounding and, in the process, destroying itself, releasing more energy in an instant than the sun will release over its entire lifetime! Much discussion as of late concerns the red [super] giant star Betelgeuse as regards its status as a supernova candidate, but not so for Antares. Both stars are at a similar evolutionary state and both are at comparable distances from earth at less than 700 light years, a sufficiently safe distance when they eventually end their lives in spectacular fashion.

A view to the southwest in this composite image via Stellarium with modifications by the author, enhancing the appearance of Arcturus and Antares through direct image superposition. Wide-field, digital images of the stars were obtained by the author and superposed on the stellarium image to create this composite image. Note the decidedly deeper red-orange hue of Antares, an aspect attributable to its decidedly cooler effective temperature of 3,400 Kelvin compared to that of Arcturus’ effective temperature of 4,300 Kelvin.

With Sagittarius and the galactic center seen far from city lights, the southern sky is as rich as it is diverse. In a follow-up series of articles, I will continue this discussion but for now, on the next clear evening, take out your binoculars, face south and start scanning, remembering that

We, all of us, are bound by this common origin, born and composed of the same raw materials forged in the nuclear cauldrons of long-dead stars; if only for a moment, we could look up at the stars and, in humility, connect with each other, fellow travelers on this magnificent blue oasis.

Note: the featured image for this article centers on the galactic center, the heart of the Milky Way galaxy, our home in the universe and was the featured image for the 6 January, 2012 edition of NASA’s Astronomy Picture of the Day (APOD). Antares is to the right (west) of the galactic center, itself 25,000 light years distant.

All our science, measured against reality, is primitive and childlike-and yet it is the most precious thing we have

An index of all articles in this blog can be found here.


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