The sky this week

April 6 to April 12, 2020

By Joe Grida, Technical Informaon Officer, ASSA ([email protected])

elcome to the second 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 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 walk

With such a bright Moon in the sky this week, our naked eye star walk will have to concentrate on just the brightest of stars and constellaons.

Brilliant Venus is sll the brightest object in the evening sky apart from the Moon, and very easy to see low in the north-western sky. It passed the Pleiades last week. I hope some of you got to see it. My western horizon is not very good, and it was cloudy for me as well. As the week begins, you can sll get Venus and the Pleiades in the same field of view of your binoculars. Just remember the vast gulf of space that separates these 2 objects: Venus is a mere 91 million kms from Earth (just over 4 minutes light travel me), the Pleiades are a whopping 380 light away!

In the other half of the sky, the Moon is sing at Leo’s hindquarters on Monday, 6 April. So, that part of the sky is a write-off for now. Over in the South, the Southern Cross is slightly higher in the sky at the same me as last week, and by this me at the end of next month, it will have reached its highest point in the sky. The two brightest stars in all the night sky, line up high in the sky during April. The brighter star is Sirius. It's known as the Dog Star because it's in the constellaon Canis Major, the big dog. The other star is , in the constellaon . Sirius and Canopus are quite different kinds of stars. Sirius is similar to our own Sun. It's bigger and hoer than the Sun, but it's in the same phase of life; it's a sedate, middle-aged star that's "burning" its vast reserves of hydrogen. It appears so bright mainly because it's one of our closest stellar neighbours, at a distance of less than nine light-years. Canopus, on the other hand, is quite different from the Sun. It's a supergiant; it is many mes bigger and heavier than the Sun. It's also more than 10,000 mes brighter than the Sun, so it's easily visible across more than 300 light- years of space. It's nearing the end of its life, and within the next few million years should explode as a supernova. When that happens, Canopus will briefly outshine everything else in the sky except the Sun. The Milky Way stretches from the northwest to the southeast, crossing directly overhead. In the southeast, , the Southern Cross, is very conspicuous. The two Pointers, Alpha (α) and Beta (β) Centauri can be found immediately below it. They are known as the Pointers because, as the Southern Cross and the Pointers rotate around the South Celesal Pole, they always point to the Southern Cross. The lower, and slightly brighter of the two, Alpha Centauri is the closest star to the Earth other than our Sun. Its light, travelling at 300,000 kms/sec, takes 4.25 years to complete its journey. Alpha Centauri is a spectacular sight in a telescope. It is in fact made up of two beauful yellow suns, just like ours, that rotate around a common centre of gravity over a period of 80 years. In a telescope they sparkle just like a pair of diamonds against the inkiness of space. A third, but much fainter companion to these two, lies about a degree away in the sky, and is impossible to see without a reasonably large telescope. Just a few years ago, astronomers announced the discovery of a planet revolving around Alpha Centauri B. Imagine a planet with 2 suns in its sky! Very much like the sunset on the planet Tatooine from the Star Wars movie. Beta Centauri looks only slightly fainter than Alpha Centauri, so the logical conclusion would be that it must be at about the same distance from us as Alpha Centauri. Wrong! Beta is actually a much brighter star that is over 200 mes further away. The Milky Way is relavely bright above the Southern Cross. I can see it clearly from my backyard in Mount Barker on a moon-less night. It might be a bit more difficult in suburban Adelaide. The photo below of the Southern Cross area provides a great view. Get a star chart for the month of April here: hp://skymaps.com/downloads.html

Above: This panorama of the southern Milky Way, by Will Godward, takes us from Alpha Centauri on the le to the complex to the right. The Planets

Other than Venus, which we are losing very rapidly in the western twilight sky, really all the planetary acon is in the morning sky at the moment. Fraser Farrell took the photo at right showing the conjuncon of Saturn and Mars at lower right, with brilliant Jupiter at top on the morning of April 1. This week, on the morning of April 9 (see below), you’ll find all 3 planets lined up in the star fields of Sagiarius and Capricornus. M75 is the un-named above Saturn on the chart. One of the great observing acvies you can do with your telescope is to watch the dance of Jupiter’s 4 Galilean moons as they orbit around the planet. You can see that movement over minutes, not days. On the morning of April 7 @ 5:15am aim your telescope at Jupiter and you’ll be greeted by the view through the eyepiece as shown at boom le. Keep looking at the view, for at 5:20am you’ll see the moon Europa wink out! It will have moved into Jupiter’s shadow, and will not re-appear from behind Jupiter unl 10:37am, well aer the Sun has risen. Another event involving Europa occurs at 00:23 on April 9, when its shadow begins to cross over Jupiter’s South Equatorial Belt. This might be a lile difficult to observe if you haven’t got a clear eastern horizon, as Jupiter is at only 6 degrees of elevaon.

5:20am ACST April 7, 2020

Both charts were created with Stellarium v0.19 soware. 5:00am ACST April 9, 2020 The Moon

As you can see from the Moon Phase chart, the Moon dominates all the period of darkness each day this week. It’s too bright to really see any detail on its surface with a telescope, even with a Moon filter in place. The Sun is shining directly over the Moon’s surface, therefore the shadows that provide the contrast that enables us to see details on the surface is not there. Look for the Full Moon as it rises in the east in the early evening of April 8th and you may noce that it appears bigger than normal in the sky. This is the so-called Supermoon! That’s because just the day before, it passed closest to Earth for its current orbit; a point called perigee (356,907 kms away).

On average, the Moon is almost 380,000 kilometres from Earth. But because of its ellipcal orbit around the Earth, the distance between Earth and Moon varies by almost 48,000 kilometres. When the Moon is closest, it actually covers an area of the sky about 25 percent larger than when it's farthest from us. Because the distance changes gradually from day to day, though, it's hard to see much of a difference.

If you live along the coast, though, you can see a direct effect of the changing distance: the des are higher when the Moon is closest. The des are produced by the gravitaonal pull of the Moon and Sun, so as the Moon gets closer, its influence gets stronger. And des are highest of all when perigee occurs around the me of new or full Moon, when the gravity of the Sun and Moon are pulling along the same line. The Stars

With a Full Moon in the sky for most of this week, it makes deep sky observing very difficult both in the evening or early morning. So, for this week, we will be content with just observing bright stellar objects. And what a beer way to keep ourselves busy than observing double stars with good colour contrast. H3945 in Canis Major is one of my favourites, but geng a lile low in the sky at the moment. See below for a list of colourful doubles visible at this me of the . They are all quite bright, so you should have lile trouble locang these in the moonlit sky. Except for the 2 stars in Bootes, all the others can be easily separated by small telescopes. To get the best view of the colour difference, defocus the stars slightly aer you’ve centred your telescope on them. Spreading the light, reduces the intensity of the star light, and makes the colour easier to see. I’ve chosen to highlight the 2 stars below from the list. Both are easy targets. 24 Com sits on the northern outskirts of the Virgo Cluster of (see Taki’s Star Atlas, chart 49. Get link at the end of this arcle). Iota Cancri offers a combinaon of pale orange and clear blue suns. Find it using Taki’s Star Atlas, chart 53. Please let me know of your impressions of these gorgeous doubles.

Name Const RA Dec Mag1 Mag2 Sep Comments Iota Cnc 08 46.7 +28 45 4.2 6.6 30 Yellow and blue or gold and light blue Tau 1 Hya 09 29.0 -02 46 4.6 7.2 66 White and lilac Tau Leo 11 28.0 +02 50 5.2 7.0 90 Yellow and blue 17 Com 12 28.9 +25 54 5.4 6.7 145 White and lilac Delta Crv 12 29.9 -16 31 3.1 8.6 24.2 Yellow and red-purple 24 Com 12 35.1 +18 22 5.2 6.7 20.3 Orange and emerald or gold and blue Alpha CVn 12 56.1 +38 18 2.9 5.8 19.4 Blue-white both or blue-white and green Epsilon Boo 14 45.0 +27 04 2.5 4.9 2.8 Yellow-orange and blue Xi Boo 14 51.4 +19 05 4.7 7.0 6.9 Yellow and red Iota Lib 15 12.3 -19 47 4.7 9.4 58 Pale yellow and red-purple NGC 4755 - The star cluster

NGC 4755, also known as the Jewel Box or Kappa Crucis Cluster, is one of the finest open clusters in the sky. It's located in the small constellaon of Crux and at magnitude 4.2 is bright enough to be seen with the naked eye. It contains over 100 stars, mostly blue or red, spread over 10 arc minutes of apparent sky. For comparison, a Full Moon is 30 arc minutes wide. NGC 4755 is 6,440 light-years away, therefore the light we receive today from those stars le about the same me that the ancient Egypans started building the first pyramids. Nicolas Louis de Lacaille discovered NGC 4755 on March 25, 1752. The name "the Jewel Box" was originally coined in 1830 by John Herschel, who described it as beauful mul- coloured cluster resembling “a superb piece of fancy jewellery”. To the naked eye, NGC 4755 appears as a hazy star one degree southeast of first magnitude Beta Crucis (β Cru - mag. +1.25). A pair of 7x50 or 10x50 binoculars resolves the brightest six stars surrounded by a sprinkling of fainter members. A small 80mm scope reveals a dozen more stars of various subtle colours, mostly reds or blues. Due to its compact size and high surface brightness, NGC 4755 takes magnificaon well. Remember when we talked about defocusing double stars to enhance the colour? It works really well with the Jewel Box. Especially on the 3 stars in a line at the centre of the cluster. What colours do you see? The Jewel Box Cluster is one of the youngest known open clusters, with an esmated age of approximately 14 million years., and is about 20 light years across. The brightest ones are supergiants. One of the cluster’s central stars is a red supergiant and stands in contrast to the nearby luminous, massive blue stars. These young, short-lived stars will likely explode as supernovae in the next few million years. Make sure your observe NGC 4755 © Starshadows Remote Observatory next me you’re out under the stars. It is truly a jewel of the southern sky.

Next week

The Moon leaves the early evening sky by mid-next week, so we’ll chase some deep sky targets. 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. A binocular tour of Canis Major hps://skyandtelescope.org/astronomy-news/night-sky-sights/bino-tour-canis-major-big-dog/

Get your star chart for the month hp://skymaps.com/downloads.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/