The sky this week
April 27 to May 3, 2020
By Joe Grida, Technical Informa on Officer, ASSA ([email protected])
elcome to this edi on 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 to give W 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 Adver ser newspaper: “Even with the restric ons in place, stargazing is something that you can do easily on your own. It helps to relieve stress and will keep your sense of perspec ve. It’s pre y hard to walk away from a night under the stars without a jus fiable sense of awe. And also without sensing a real, albeit tenuous, connec on with the cosmos at large”.
* Published on the last Friday of each month
Naked eye star walk For centuries, humans have looked up at the night sky and played games of "join-the-dots” with the stars. Today, these pa erns form the 88 constella ons that cover the en re sky. The largest constella on is Hydra the water snake, which slithers across 90 degrees of the sky. To get a sense of how much sky that is, consider that the full Moon spans only half a degree. Whilst the head of Hydra is se ng in the west, its tail stars are s ll overhead! The smallest constella on is one of the most famous. Its correct name is "Crux", the cross, although it's be er known as the Southern Cross. At this me of the year, it’s high in the southern sky. Its stars are shown on the Australian and NZ flags. Crux is small, but it's packed with celes al wonders. For example, there's the Jewel Box Cluster, shown on this month’s starchart by its catalog number of 4755. Seen through a telescope, the stars of the Jewel Box shine diamond-white and sapphire-blue. A lone red star lies between Earth and the cluster, so it looks like a ruby against the other gems. The whole cluster is set against the gli ering band of the Milky Way. Of course, you’ll need a dark sky to see the Milky Way. The light polluted skies of Adelaide hide it. Due south of the Jewel Box is the Coal Sack Nebula, a dark cloud of interstellar dust. Our Galaxy contains many dark nebulae like the Coal Sack, vast, cold clouds that block the light from stars that appear behind them. But the Coal Sack is more obvious than most because it's set against a bright part of the Milky Way. It's several dozen light- years across, and it lies 500 light-years away. Surrounding the Southern Cross on three sides is the constella on 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. Elsewhere in the sky, the bright orange star Arcturus shines brightly halfway up the northern sky. Did you know that astronomers have calculated that we receive as much heat from this star, as we would from a candle 10 kilometres away? Not enough to keep the winter chills at bay! A myriad of other bright stars adorn the late-evening sky right now. Blue-white Regulus, the heart of Leo, the lion, stands low in the north-western sky. And all around the heavens, you'll see many other stars in shades of blue, white, yellow, orange, and red. High up, almost overhead, the blue-white star, Spica is easy to see. Spica actually consists of more than one star, but only one is bright enough to see without a telescope. Astronomers use two different measurements to rate the brightness of a star. Both measurements are called magnitudes, and the lower the number, the brighter the star. The first measurement, called apparent magnitude, rates how bright an object appears in the sky; Spica rates a "one." But apparent magnitude doesn't tell you if the object is intrinsically bright or just close by. So astronomers also rate stars and galaxies by absolute magnitude. This number tells us how bright different objects would appear if they were all lined up at the same distance. On that scale, Spica gets a minus three. The Sun would only rate a five, which means that Spica's about 1600 mes brighter. Did you think about the Universe the last me you drank a glass of water? The Big Bang in which our universe was born, the distant stars sprinkled across the night sky and that glass of water are in mately related. That's because water (H2O) contains hydrogen and oxygen. The hydrogen came from the Big Bang, while the oxygen was manufactured inside massive stars that blew up billions of years ago. The Big Bang, which took place 10 billion to 15 billion years ago, created three lightweight elements - hydrogen, helium, and lithium. When these elements formed, they were the only elements in the universe. There was nothing heavier. The lightest of these first elements was hydrogen; the same element that contributes two atoms to every molecule of water. And the hydrogen in every sip you take came from the Big Bang. The material from the Big Bang gathered into galaxies like the Milky Way. Stars in the galaxies fused some of the hydrogen into helium. The more massive stars then fused the helium into carbon, oxygen, and s ll heavier elements. These high-mass stars exploded, showering their heavy elements into the galaxy. The most abundant element was oxygen. When the Sun and Earth formed, they incorporated some of this oxygen. That's how the oxygen got into your water. So appreciate that glass of water - its hydrogen marks the birth of the universe; its oxygen, the death of stars. The Solar System
Mercury is out of sight in conjunc on with the Sun.
Venus (magnitude –4.7, in north-central Taurus) is the dazzling white "Evening Star" in the west during dusk. It's now as bright as it gets and will remain so for the next two weeks. Sets at 7:40pm on April 27, and 20 minutes earlier by week’s end.
In a telescope, Venus has enlarged to about 37 arcseconds in diameter while waning in phase to be a thick crescent about 28% sunlit (see diagram at right). It's changing faster now, on its way to becoming a drama cally thin crescent low in twilight in late May as it nears conjunc on with the Sun. Mars, Jupiter, and Saturn (magnitudes, +0.4, – 2.3, and +0.6, respec vely) shine high in the east before dawn, in the starfields of Sagi arius and Capricornus.
Jupiter, the brightest, is highest in Sagi arius,
Saturn glows pale yellow 5° below Jupiter, on the p of the goat’s (Capricornus) nose. The chart on the right showns the brightest satellites of Saturn on the morning of April 29 at 3:34am.
Mars is ever farther below Saturn, moving eastward against the stars away from the other two. On the morning of April 27th Mars is 16° from Saturn; by May 4th it retreats to 21° away.
Uranus is hidden in conjunc on with the Sun.
Neptune is barely risen in the east as dawn begins.
Jupiter, Saturn & Mars in the morning sky - 4:00am April 27, 2020 (Created with Stellarium) Poten ally Hazardous Asteroid 1998OR2 makes close approach to Earth
On the morning of April 29th it will safely zip past Earth at a distance of 6.3 million kilometres, or around 16 mes the distance to the Moon. The NASA defini on: Poten ally Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid’s poten al to make threatening close approaches to the Earth. Specifically, all asteroids with a minimum orbit intersec on distance (MOID) of 0.05 au or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. NASA has a great resource here: h ps:// cneos.jpl.nasa.gov/about/ neo_groups.html
The entry from Wikipedia:
(52768) 1998 OR2, provisional designa on 1998 OR2, is an asteroid on an eccentric orbit, classified as near-Earth object and poten ally hazardous asteroid of the Amor group with a diameter of 2 kilometres. It was discovered on 24 July 1998, by astronomers of the NEAT program at the Haleakala Observatory,
Hawaii. It is one of the brightest and Use this chart to find Asteroid 1998 OR2 during April 25th to 28th. The ck therefore largest poten ally hazardous marks are Universal Time. Sky &Telescope asteroids known to exist. With an observa on arc of 32 years, the asteroid has a well determined orbit and the trajectory is well known through the year 2197. The asteroid's orbit is only poten ally hazardous on the me scale of hundreds if not thousands of years. Get more informa on on observing the close approach in an excellent ar cle from Sky & Telescope magazine here: h ps://skyandtelescope.org/astronomy-blogs/explore-night-bob-king/chase-poten ally-hazardous- asteroid-earth-flyby/ Or if you aren’t able to observe it with your own telescope, you can watch it live online courtesy of the Virtual Telescope Project here: h ps://www.virtualtelescope.eu/2020/03/06/poten ally- hazardous-asteroid-52768-1998-or2-close-encounter-online-observa ons-28-apr-2020/ The live feed will start on 29 Apr. 2020 at 3:30am ACST. The Moon
The Moon returns to the evening sky this week. On Monday, 27 April it will be only 14% illuminted, but by week’s end that will have reached 74%, making the evening sky quite light polluted. During this week, as you follow the phase of the Moon get larger, you can see all 6 Apollo landing sites on the Moon. Start on Tuesday, April 28 with the Apollo 17 site. The map below shows the landing sites. There’s a great ar cle by Bob King from Sky & Telescope magazine that takes you to each landing site. Get the ar cle here: h ps://skyandtelescope.org/observing/how-to-see-all-six- apollo-moon-landing-sites/ NGC 3372 - Eta Carinae Nebula
As we turn our a en on to the south-eastern part of the sky, we find the Southern Cross. Just above the Southern Cross, the Milky Way is at it’s brightest. It’s here that we find a large cloud of gas and dust, known as the Eta Carinae Nebula. It is located some 8500 light years away. The nebula (La n for cloud) is a vast cloud of gas and dust, about 7300 light-years away in the constella on Carina, that's given birth to more than 60 hot, heavy stars. And one of those stars is the star that gave the nebula its name. Eta Carinae is a fascina ng object in itself. It first a racted a en on in the mid 1800s, when it flared to several hundred mes its normal brightness, becoming one of the brightest stars in the sky for a brief period of me. The instruments of the day couldn't reveal any detail around the star, so astronomers couldn't make much sense of what was happening. But modern telescopes reveal an impressive sight. The flare-up was caused by a massive erup on from the star's surface. It surrounded Eta Carinae with a turbulent cloud of gas and dust that looks like an hourglass with a tutu wrapped around the middle. Astronomers measured its mass at about a hundred mes that of the Sun. Current theories of star forma on say that's almost TOO big for the star to even exist. But recent observa ons suggest that Eta Carinae may consist of TWO stars, which are hidden inside the cocoon. One star could be about 30 mes as massive as the Sun, and the other about 80 mes as massive. The stars produce violent "winds" of gas. As the winds ram into each other, they produce X-rays. The X-rays are most intense every five and a half years, sugges ng that that's how long it takes the two stars to orbit each other. In the eyepiece, the Eta Carinae Nebula is an amazing sight. No ma er the size of the telescope, there’s plenty to see. And with the larger telescopes, 12 inches and above, you can even start to see the expanding shell of gas around the star. ASSA President, Paul Haese, recently imaged the nebula. See his spectacular image here h ps://paulhaese.net/ EtaCarinaAG12.html Have a look at the Wikipedia entry for the nebula here: h ps://en.wikipedia.org/wiki/Carina_Nebula Have a look at the Wikipedia entry for the star here: h ps://en.wikipedia.org/wiki/Eta_Carinae
This chart shows the loca on of the Carina Nebula within the constella on of Carina (The Keel). This map shows most of the stars visible to the unaided eye under good condi ons and the nebula itself is marked as a green square in a red circle at the le (labelled 3372 for NGC 3372). This nebula is very bright and can be seen well in small telescopes, and faintly without a telescope at all. Image: ESO, IAU and Sky & Telescope Comet C/2020 F8 SWAN update
The comet con nues to brighten. At the me of wri ng, Sunday 26 April, the comet’s magnitude was es mated to be 7.4. It will probably reach mag 6 by the end of the week. That makes it easily visible in binoculars, as long as mother nature cooperates and gives us some clear skies. The chart below, provided by the discoverer Michael Ma azzo, shows the comet’s path through Aquarius and Cetus. It makes a close approach to Iota Ce on the morning of 5 May. A separa on of only 16 arc-minutes. The photo on the right was taken by Damien Peach on April 21st.
The projected light curve for Comet F8 SWAN. Credit: Adapted from Seiichi Yoshida’s Weekly Informa on About Bright Comets.
Next week - May 4 to May 10, 2020
The Moon is very bright in the evening sky, so we’ll look for some more double stars.. And get a last look at Comet C/2020 F8 SWAN. 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 sec on 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 h p://skymaps.com/downloads.html
Astronomy Picture of the Day (APOD) - Each day a different image or photograph of our fascina ng universe is featured, along with a brief explana on wri en by a professional astronomer h ps://apod.nasa.gov/apod/astropix.html
Taki’s Magnitude 8.5 star atlas h p://www.astronomie.cz/data/2009/04/00-atlas-85.pdf
Stellarium - free open source planetarium for your computer. It shows a realis c sky in 3D, just like what you see with the naked eye, binoculars or a telescope h p://www.stellarium.org
Observe the planets in day me. Use your smartphone to polar align your equatorial mount h ps://skyandtelescope.org/observing/daylight-polar-alignment/
Please note: I have provided the full text of the URL in lieu of just the hyperlink throughout this document, because the conversion from a MS Publisher document to Adobe PDF doesn’t always carry over the hyperlinks correctly. Just copy the link and paste it into your favourite browser.