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The Sky This Week

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The Sky This Week The sky this week April 20 to April 26, 2020 By Joe Grida, Technical Informaon Officer, ASSA ([email protected]) elcome to the fourth edion of The Sky this Week. It is designed to keep you looking up during these rather uncertain mes. We can’t get together for Members’ Viewing Nights, so I thought I’d write this W to give you some ideas of observing targets that you can chase on any clear night this coming week. As I said in my recent Starwatch* column in The Adverser newspaper: “Even with the restricons in place, stargazing is something that you can do easily on your own. It helps to relieve stress and will keep your sense of perspecve. It’s prey hard to walk away from a night under the stars without a jusfiable sense of awe. And also without sensing a real, albeit tenuous, connecon with the cosmos at large”. * Published on the last Friday of each month Naked eye star walk Over in the eastern late evening sky, Scorpius, the Scorpion (one of the few constellaons in our sky that actually resembles what it is supposed to represent) is difficult to miss. He will keep us company over the coming chilly winter months. Its brightest star, Antares, is a huge star of gargantuan proporons. If we replaced our Sun with it, then all the planets from Mercury through to Jupiter would all find themselves engulfed within it! Just below the tail of Scorpius, you can find the star clusters designated M6 and M7. Take the trouble to observe these with binoculars. They make a beauful sight, with many bright stars sparkling like diamonds against a background of gold dust. Over in the northern sky, Leo, the Lion is easily observed. Its brightest star is Regulus. It stands at the boom of a paern of stars that looks like a backward queson mark, outlining Leo's head and mane. Regulus is about 77 light- years from Earth, so the light you see tonight le the star during the middle of WWII. Regulus is a blue star that's much hoer and more luminous than our own Sun. Higher in the sky is the star Spica in the constellaon of Virgo. Spica is 2,554,200,000,000,000 km (270 light years) away! That is, the light from Spica began its journey to the Earth 270 years ago, whilst reflected sunlight from the Moon’s surface takes just over 1 second to reach the Earth! Further to the northeast, we find the constellaon of Virgo. One of the most famous objects in the sky; a quasar known as 3C273, is located in this constellaon. In the early 1960’s, 3C273 was known as a "radio star." Though astronomers could detect it with radio telescopes, they couldn't pinpoint its locaon well enough to see it with opcal telescopes. But that changed the day the Moon passed between the radio star and Earth, blocking its radio signals and allowing astronomers to pinpoint its locaon. With this informaon, they could find the object with opcal telescopes for the first me. Even then, they couldn't quite fathom what they saw. Photographic plates showed only an inconspicuous blue star. But when astronomers split the star's light into its individual wavelengths, they found that it was unlike any known star, galaxy, or nebula. Further study eventually revealed that it's over 3.5 billion light-years away, which means it must be incredibly bright. It's also small, which means its energy source must be incredibly powerful. Today, astronomers believe that 3C 273 and the thousands of other known quasars are monstrous black holes encircled by disks of gas. As gas spirals into the black hole, it's heated, so it glows brightly; bright enough to be seen across the vast abyss of space and me. In the north-eastern sky, the brilliance of red Arcturus, shadows the other stars in the area, whilst Sirius, the brightest star in the sky begins to perform his mighty dive into the western horizon. He will disappear from our night sky by the end of May. He won’t return unl late this year. Unmistakable, high in the southern sky, is the famous Southern Cross. Surrounding it on three sides is the constellaon of Centaurus, the centaur. Its two brightest stars, Alpha and Beta Centauri, are referred to as the Pointers, because they follow the Southern Cross around the sky and always point to it. Alpha Centauri is actually the closest star system to the Earth other than our own Sun, at a distance of just over 4 light years. The third brightest star in the Southern Cross is Gamma Crucis; you can locate it at the apex of the cross. Look at it carefully, and you’ll see that it is a red star. It is a prelude of what will happen to our Sun. Today, our Sun is a yellow star, steadily fusing hydrogen into helium at its core. Billions of years from now, though, it’ll use up its hydrogen. The Sun will expand, engulfing the planet Mercury, and possibly Venus and even Earth, and its surface layers will cool and redden. In other words, the Sun will become a red-giant star. But that won’t happen for another five billion years or so. For now, Gamma Crucis is the closest red giant to Earth, at a distance of almost 90 light-years. This bright star produces about as much energy in an hour as the Sun does in an enre week. As a red giant, Gamma Crucis is nearing the end of its life. Someday, it’ll cast its outer layers into space, exposing its hot, dense core. Eventually, over billions of years, the leover core will cool, leaving only an invisible cosmic cinder. It is strongly recommemded that you download and print an all sky chart, so that it can help you locate the stars referred to in this arcle. A link is provided at the end of this document. Twinkle, twinkle lile star….. or so the nursery rhyme goes. But, some stars are massive compared to our lile Sun. The graphic on the le shows some of the large stars that are visible in our night sky right now. You can see from the scale dia- gram that our mighty Sun is relavely puny compared to some of the behemoths in the sky. Diameters are: Sun 1,000,000 kms Sirius 2,000,000 kms Pollux 12,244,000 kms Arcturus 35,000,000 kms Rigel 110,000,000 kms Betelguese 500,000,000 kms Antares 700,000,000 kms The Solar System Mercury is found in the glow of sunrise. The best day to look for it is on Wednesday, April 22 @ 5:30am. The very thin crescent waning Moon is just to the right of it very low in the eastern sky. Even though the lile plant is so close to the Sun, spacecra that have visited the planet have found ice in the shadows of craters. See more here: hps://www.space.com/18695-water-ice-mercury-explained-infographic.html Venus (magnitude –4.7, in Taurus) is the dazzling white "Evening Star" in the west soon aer sunset. On Monday evening, April 20, you can find it 30 minutes aer sunset, about 12 degrees above the western horizon. The 2.9 day old crescent waxing Moon, will be between Aldebaran and Venus on April 26. For the next three weeks Venus will connue to shine as bright as it gets. But look for it now, as it will soon be lost in the glare of the seng Sun. In a telescope, Venus has enlarged to 33 arcseconds in diameter while waning in phase to become a thick crescent 1/3 sunlit. Venus will connue to enlarge and wane, finally becoming a dramacally thin crescent low in twilight by mid-May as it approaches conjuncon with the Sun. Mars, Jupiter, and Saturn (magnitudes, +0.5, – 2.3, and +0.6, respecvely) are lined up in the east in the morning sky. Jupiter, the brightest, is the highest. It actually rises just aer 11:00pm. Saturn glows pale yellow 5° below Jupiter. The rings are perfectly placed for viewing, so be brave and get out there. Mars, below Saturn, is moving further eastward in Capricornus, away from the other two. Uranus and Neptune are hidden behind the glare of the Sun. Mercury and the Moon on April 22 @ 5:50am ACST (Created with Stellarium soware) The Moon This is the dark week. New Moon occurs mid-week on Thursday, 23 April, so we have dark skies for all hours of darkness from dusk ll dawn. Wouldn’t it be nice to have moon-less skies all year around? I asked myself that queson some years ago, and did a lile research. Aer that, I decided that whilst it was inconvenient to have a big bright Moon in the sky, without it we probably wouldn’t be here. If you’d like to know more, view my talk tled “What if we didn’t have a Moon” that I presented to the ASSA September 2015 General Meeng. You can view the video here: hps://www.assa.org.au/members/meeng-recordings/ september-2015/ Note: you must be a member of ASSA and have a logon to be able to access this talk. Comet C/2020 F8 SWAN - a naked eye comet for May? Long me ASSA member and Past President, Michael Maazzo made his 8th discovery of a comet from images taken by the Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO).
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