The sky this week
April 13 to April 19, 2020
By Joe Grida, Technical Informa on Officer, ASSA ([email protected])
elcome to the third 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 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 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 - Goodbye Orion, hello Scorpius
The crisp autumn evenings of April offer an ideal opportunity to explore the majesty of the southern sky. However, make sure you dress warmly, because a er a few short minutes in the night air, you’ll begin to feel the cold bite. Go find yourself a nice dark spot in the back-garden, now that the Moon is rising later each evening, and let your eyes become accustomed to the darkness. No ce how many more stars you can see, even a er a few minutes, as the pupils of your eyes expand to let as much light in as possible.
Some of the brightest stars in the whole sky can be seen at this me of the year. As red Betelgeuse and blue Rigel set with Orion, the Hunter in the west, fiery red Antares rises in the east. For me, I don't think there are any other constella ons in the sky, that herald the coming of the seasons so well as these two do. Orion in the east, signals summer is around the corner, whilst Scorpius in the east, brings the chill winds of winter with it.
Antares, is a star of gargantuan propor ons. 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 beau ful sight, with many bright stars sparkling like diamonds against a background of gold dust. Over to the north, the constella on of Leo, the lion is quite prominent. Many different cultures have seen a lion in this pa ern of stars. But others have seen a sickle, which forms the lion's head and shoulders, and a small triangle, which forms its hindquarters. The brightest star in the sickle is Regulus; the heart of the lion. The brightest star in the triangle is Denebola, also known as Beta Leonis because it's Leo's second-brightest star. It's about half as bright as Regulus. Denebola is an abbreviated form of an older Arabic name, Al Dhanab al Asad, which means "the lion's tail." Its name derives from the fact that Denebola is the easternmost star in Leo; at the tail end of the constella on.
Over in the northeast another star shines. This is Arcturus, in the constella on Bootes, the herdsman. It's an orange -giant star, and it's unusually old as naked-eye stars go: about 10 billion years — roughly twice the age of the Sun, Earth, and solar system. Arcturus may thus be the oldest object you've ever seen.
Arcturus is the third-brightest star in Earth's night sky. It appears so bright for a couple of reasons. First, Arcturus really is a bright star; it produces more visible light than most stars. If you placed Arcturus side by side with our own star, the Sun, it would appear more than a hundred mes brighter. And second, Arcturus is fairly close to us, at a distance of just 37 light-years. This nearness has another effect on how we see Arcturus. It moves across the sky faster than almost any other star. All stars are in constant mo on as they orbit the centre of our Milky Way galaxy. But they're so far away that their mo on is impercep ble on human mescales. Astronomers must use sensi ve instruments to measure this mo on. Arcturus is moving toward the constella on Virgo, which is higher in the sky above Arcturus, at about half a degree every millennium — a distance equal to the width of the full Moon. Arcturus will move out of Bootes and into Virgo in about 20,000 years.
As we turn our a en on to the south-east 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. More about Eta Carinae the star, and the nebula next week.
Get a star chart for the month of April here: h p://skymaps.com/downloads.html
Passes of the ISS visible from Adelaide for the coming 10 days.
In the first half of the week they are morning passes, and then they switch to evening passes for the second half of the week. Get more details at:
heavens-above.com The Solar System
The closest planet to the Sun, Mercury is lost in the glow of sunrise, as is the farthest planet, Neptune. Venus (magnitude –4.6, in north-central Taurus) is the dazzling white "Evening Star" low in the north-west during and a er dusk. Make sure you have a clear horizon as by 7:15pm, it’s only 5 degrees in eleva on. In a telescope, Venus has enlarged to 30 arcseconds in diameter while waning in phase to become a thick crescent, 40% sunlit. See inset below. Venus will con nue to enlarge and wane, becoming a drama cally thin crescent low in twilight in mid-May. Mars, Jupiter, and Saturn (magnitudes, +0.6, – 2.2, and +0.6, respec vely) are lined up in the east before and during early dawn, as shown at right. Each morning Mars moves a li le farther away from the other two. The waning crescent Moon joins the planetary trio on the morning of April 16th.
Uranus is lost in the glare of the se ng Sun.
(Note: both diagrams created with Stellarium v0.19 so ware).
North-western horizon @ 7:15pm, 15 April 2020 The Moon
Best me to observe the Moon this week comes in the morning. It rises at 10:07pm on April 13, but not un l 2:52am on April 19. Our lunar feature this week, Oceanus Procellarum (Ocean of Storms) reveals how extensive volcanism was in the Moon’s early history.
• The broad dark plain of Procellarum is a vast basalt field laid down over several billion years of intermi ent erup ons.
• Aristarchus crater is even visible in Earthshine. 450 million years old, 40 kms wide, 3000m deep
• Intense volcanic ac vity pushed up the elevated Aristarchis Plateau and dusted it with metres-deep layer of volcanic ash.
• Lava flows also washed through the snake-like rille of Schroter’s Valley arcing across the plateau. (160 kms long, 1000m deep)
• The dimples of the Marius Hills are a collec on of several hundred volcanic domes and cinder cones. The domes and cones are fairly low, so are best seen around sunrise & sunset when shadows display their shallow relief.
Best me to observe this area is on Saturday morning, 18 April when the low rays of sunset on the Moon create the great shadows and contrast that bring the features into great relief. Deep Sky Challenge - The Antlia Galaxy Cluster
High up in the sky these autumn evenings, you’ll find the small 3.0 minutes of arc it’s reasonably large, and rela vely easy to faint constella on of Antlia, the air pump. Never heard of it? see in a 10” telescope. The other bright galaxy, NGC 3271 lies It’s between Vela and Hydra. It was created by Lacaille in the 15 minutes ESE of NGC 3268. The 11.8 mag barred spiral 18th century. galaxy is of similar size to NGC 3268. Use the labeled DSS image below to assist you at the eyepiece. Probably best known for the iconic image of NGC 2997, the beau ful face-on spiral photographed by David Malin at the Located 6 minutes North of NGC 3268, is the barred spiral NGC AAO. 3269. At 12.3 mag it is s ll well within reach of smaller scopes. You observe these objects under dark skies. Going out in a However our target this month is the Antlia Galaxy Cluster. suburban sky is not going to auger well for you. This is the third closest to our Local Group, with only the Fornax Cluster and Virgo Cluster lying closer. The Antlia Cluster Just 2.5 minutes west of NGC 3268 is the 12.5 mag NGC 3267. contains about 234 galaxies and is dominated by two massive At 1.0 x 1.7 minutes in size, this spiral is the smallest of the ellip cal galaxies, NGC 3258 and NGC 3268. quartet.
The Northern subgroup of galaxies inside the cluster gravitate A number of fainter PGC galaxies also complete the view in around NGC 3268, while the Southern subgroup is centered larger scopes. around NGC 3258. Each of the two giant ellip cals contain It is an easy target in a dark sky. Our country members will several thousand globular clusters. have no trouble observing this group of galaxies. Of course, it Most galaxies in the Antlia Cluster are early type galaxies, and will be much more of a challenge for our city dwellers. Please dwarf ellip cal galaxies are the most common galaxy type. let me know how you go.
The Antlia Cluster is located between 32.58 and 32.71 million Perhaps this cluster is a good target for our astro imagers? light years from Earth.
We will concentrate our observing on the Northern subgroup centered on NGC 3268, an 11.8 mag ellip cal galaxy. At 2.5 x Next week - April 20 - 26, 2020
There’s no Moon in the evening sky, so it’s a great me to chase some faint fuzzies. The Eta Carinae nebula is perfectly placed for observa on. I’ll tell you where to find it, and what to look for. We’ll go looking for minor planet 3 Juno in the star fields of Virgo. And in Scorpius, we watch asteroid 779 Nina occult an 8.3 mag star. 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
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/
A farewell to Orion h ps://skyandtelescope.org/astronomy-news/a-farewell-to-orion/
Heavens-Above - get predic ons of ISS passes, starmap, visible satellites etc h ps://www.heavens-above.com