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). Named Comet C/2020 F8 SWAN, it is presently 8th magnitude, compact, and brightening steadily as it travels through the southern constellaon of Grus. Grus is found in the dawn sky during these cold April mornings, so you’ll have to brave the cold if you want to see it. Current predicons suggest that it could become a naked eye object by early May. The map below, supplied by Michael, shows the comet’s path unl the end of May. The projected light curve for Comet F8 SWAN. Credit: Adapted from Seiichi Yoshida’s Weekly Informaon About Bright Comets. Comet SWAN (C/2020 F8) will connue to brighten and move rapidly northward in the May dawn sky reaching a peak magnitude of 3.5 between May 15–23 while travelling through the northern constellaons of Triangulum and across to Perseus. Perihelion (closest approach to the Sun occurs on May 27th at a distance of 64.3 million kilometres). So, if you want to have a look at this comet YOU MUST DO IT THIS MONTH. Aerwards it will be too far north. Michael currently lives in Swan Hill, Victoria. The Herald-Sun newspaper in Melbourne ran a story on the discovery. Want to learn more about this comet direct from the discoverer? Then make sure to login to watch and listen to Michael tell his story on Friday, April 24th @ 7:30pm on ASSA’s Youtube Channel. Full login details will be provided in an ASSA email broadcast in the days prior to the event. Keep an eye on your email inbox.

Great shot from ASSA member Trevor Green. The bright star to the right of the comet is mag 6.5 SAO 214261, Spec K0.

The comet’s path through the inner Solar System. Click the link for an interacve preliminary orbit for the comet hp://astro.vanbuitenen.nl/comet/2020F8 Deep Sky - High density star cies

A globular cluster is a spherical collecon of stars that orbits a galacc core. Globular clusters are very ghtly bound by gravity, which gives them their spherical shapes, and relavely high stellar densies toward their centres. Star clusters are parcularly important because they allow astronomers to check models of stellar evoluon and the ages of stars. They therefore share the same inial metallicity so any effect of this on stellar evoluon is effecvely the same for the members of the cluster. Our own Milky Way has around 150 globular clusters, with perhaps more awaing discovery, hidden by galacc dust. Our neighbouring spiral galaxy in the direcon of the constellaon Andromeda, M31, appears to have around 300 globular clusters. The stars in a globular cluster are so close together that they can’t be resolved individually telescopically. At the centre of a globular cluster, stars may reach a density of between 100 and 1,000 stars per cubic parsec. That’s in contrast to the density of stars near our sun, esmated at about 0.14 star per cubic parsec. If you were standing on a planet orbing a star in a globular cluster, your night sky would be extremely crowded with nearby stars! For southern hemisphere observers, the 2 most famous globulars (and the 2 brightest in the whole sky ) are Omega Centauri and 47 Tucanae. Both are visible to the naked eye and ploed on the star chart that I referred to earlier. Phil Stephen, an ASSA member and passionate observer of the night sky, also recommended that I point you to a couple of other globular clusters that are very prey to look at through a telescope because they happen to lie in the same line of sight as a bright foreground star. NGC 5286 is located in Centaurus, whilst NGC 6441 is located near the tail of Scorpius. We’ll look at NGC 5286 this week (see photo above), and reserve NGC 6441 unl Scorpius rises higher in the evening sky. NGC 5286 makes a striking sight in the telescope. The magnitude 4.6 SAO241157 adjacent to the globular also displays an orange/golden glow not too dis-similar to the globular cluster itself. They are just line of sight; there’s a huge distance that separates them. NGC 5286 is 36,000 light years away, whilst the star is a mere 251 light years from Earth. Get more informaon on globular clusters here: hps://www.space.com/29717-globular-clusters.html

NGC 5286 Minor planet occultaons - watch a star disappear!

More info: hp://www.occultaons.org.nz/planet/planoccs.htm

Occultaons occur when one celesal object passes in front of another celesal object. For example, when the Moon passes in front of a background star, light from the background star is prevented from reaching the Earth. A shadow of the Moon is cast by the star onto the Earth, and this shadow sweeps across the Earth at roughly the same speed as the Moon is moving. A special case of an occultaon is a Total Solar Eclipse, in which the Moon passes in front of the Sun, obscuring it from view. There’s great scienfic value in observing minor planet occultaons. If the occultaon shadow band is perhaps a few tens to a few hundred kilometres wide, then observers situated within the band and perpendicular to the direcon of moon of the shadow will each see the star occulted by a different part of the asteroid. If enough observaons are obtained, one can essenally "join the dots" to build up a picture of the shape of the asteroid. Even if only two observers see an event, so long as they are separated by a reasonable distance an average diameter for the asteroid can be deduced - the only direct way in which this informaon can be obtained. Determining the diameter of a minor planet is important because it can provide clues to the asteroid's density, which in turn tells us something about its bulk composion and thus its origin. Because the chances of a specific asteroidal occultaon track passing across an established observatory are not high, the contribuon that amateur astronomers with portable telescopes can make is considerably enhanced. Indeed, it is the ability of amateurs to relocate themselves and their telescopes at short noce which is frequently crucial to the success of an observaon. Observers in Australia and New Zealand have achieved several excellent successes in doing just this, most notably in deriving the diameters of minor planets (9) Mes, (44) Gyps and (94) Aurora. In the early hours of Saturday, April 25 asteroid 779 Nina (mag 12.4) will pass in front of mag 8.9 star TYC 7868- 00389-1. The shadow, 72 kms wide, races across the northern areas of Adelaide and the lower mid-north. The centreline passes just south of Stockport. See shadow path on the map below. The green line represents the centreline, whilst the blue lines represent the edges of the shadow. Being bright, the star should be easy to find. Although I say that with tongue in cheek, because it is located in the congested star fields of Scorpius. See the finder chart on the previous page. The eyepiece view below, with a 30 arc -minute field, and centred on the star to be occulted, shows the locaon of asteroid 779 Nina at 8:10pm the night before. If you can locate the star earlier in the evening, that will make observing the event at 1:27am a lot easier. If you have a tracking mount, just set it up, locate the star and leave the telescope to track the star unl about 15 minutes before the predicted me. This me, all I want you to do is to locate the star, and watch it dim/disappear for 21.5 seconds. Come up with some way of recording the me of the beginning of the occultaon and the end. And then compare it with predicons. If you are within the predicted shadow path, and you don’t see an occultaon, then it probably means that the path of the asteroid across the sky wasn’t fully known, and the path has shied either north or south of the predicon. So, if you don’t see it, then your observaon is just as valid! It is my plan to encourage more people to observe asteroid occultaons once the 20” telescope at Stockport has received its makeover shortly. Have a go and let me know! Want to learn more about the coming changes to the 20” telescope? Then make sure you login to watch and listen to me on Friday, April 24th @ 7:30pm on ASSA’s Youtube Channel. Full login details will be provided in an ASSA email broadcast in the days prior to the event. Keep an eye on your email inbox. Next week - April 27 to May 3, 2020

The Moon returns to the evening sky. We’ll locate the landing site of the Apollo 11 mission. We’ll learn more about the Eta Carinae Nebula (held over this week because of the comet!). And, it’s galaxy season! So, we’ll go galaxy hunn’ in Virgo. Please let me know if there’s anything that you would like me to cover for you. Happy observing, and stay safe. Joe Grida

Useful links:

This secon is designed to give you access to further items of interest. I’ll add or delete items as the season changes. Get your star chart for the month hp://skymaps.com/downloads.html

Astronomy Picture of the Day (APOD) - Each day a different image or photograph of our fascinang universe is featured, along with a brief explanaon wrien by a professional astronomer hps://apod.nasa.gov/apod/astropix.html

Taki’s Magnitude 8.5 star atlas hp://www.astronomie.cz/data/2009/04/00-atlas-85.pdf

Stellarium - free open source planetarium for your computer. It shows a realisc sky in 3D, just like what you see with the naked eye, binoculars or a telescope hp://www.stellarium.org

Observe the planets in dayme. Use your smartphone to polar align your equatorial mount hps://skyandtelescope.org/observing/daylight-polar-alignment/