Twitter Facebook Flickr YouTube

August 2016: Sagittarius

August 2016

In the show this time, we talk to Dr. Betsy Mills about her ALMA observations of Sgr A*, Megan Argo rounds up the latest news, and we find out what we can see in the August night sky from Ian Morison and Haritina Mogosanu.

The News

This month in the news: planets found orbiting surprisingly young stars and the origin of the Moon's Imbrium sculpture

Understanding how our solar system came to be the way we see it today is not simple. A surprisingly large fraction of the exoplanet systems discovered so far consist of massive planets orbiting very close to their host stars, known as "hot Jupiters. Very few systems are known to have large planets as far from their star as Jupiter and Saturn are from our own Sun. Part of the reason for this could be selection bias: massive planets in orbits close to their parent stars are the easiest to detect. Models of star formation suggest that forming giant planets close to a star is unlikely, and that the large number of such planetary systems discovered is evidence that planets migrate over the lifetime of the star, forming in the colder, outer regions of the stellar nebula, and moving inwards over time. The question remains whether these large planets migrate inwards in the early stages, while the star and its associated planets are still embedded within the protoplanetary disk, or later once multiple planets have formed. Now, two new studies, both published in Nature during July, show evidence for early planetary migration.

The first study, led by Jean-Francois Donati in Toulouse, shows a hot Jupiter orbiting a still-forming 2-million-year-old solar-mass star. Very few exoplanets have, so far, been detected around this type of star, so discovering one can tell us a lot about the early stages of evolution within a planetary system. Using sensitive instruments on telescopes in both France and Hawaii and tomographic techniques inspired by medical imaging, the team confirmed that the star (known as V830 Tau) is spinning on its axis every 2.741 days, ten times faster than our own Sun, and found a longer periodic signal indicating the presence of an orbiting planet. By carefully analysing the data the team were able to rule out other explanations for the periodic signal and conclude that it is caused by a large gas giant planet with a mass three quarters that of Jupiter orbiting V830 Tau every 4.93 days at a distance of just over eight and a half million kilometres. The near-circular nature of the orbit of this planet strongly favours the disk-migration model of massive planets, since migration by planetary interactions produces much more eccentric, non-circular planetary orbits.

The second paper a fully-formed gas giant planet in a close orbit around a star less than 10 million years old. While the previous example was discovered by looking at the spectrum of the host star, this planet, with a size 50% larger than Neptune, was found through the transit technique, where the planetary orbit aligns with our line of sight, blocking out some of the light from the star each time it orbits. The star, known as K2-33, was detected and found to be a candidate planet host by the Kepler space telescope. A team of astronomers, led by Trevor David at the California Institute of Technology, report the independent detection and confirmation of this fully-formed Neptune-class planet orbiting its host star every 5.4 days at a distance of only eight stellar radii. This star and its associated planet are located in a star-forming region known as the Upper Scorpius OB Association, where only about 20% of low-mass stars host proto-planetary disks suggesting that planet formation is at an advanced stage. The host star itself has a mass of less than a third that of the Sun and an age of between five and ten million years. In this case, the existence of a Neptune-sized planet in close orbit around a star of this age shows that large planets can be found close to their parent stars very soon after the pre-stellar nebula has dispersed, and the authors conclude that, although it is unclear exactly where in the proto-planetary disk the planet formed, the only likely explanations permitted by the observational results are that the planet formed at its current location or that it formed further out and migrated within the gas disk before it dispersed.

The face of the Moon is today covered in the scars of many impacts from a bombardment of small rocky bodies when the solar system was much younger. Smaller impacts formed craters like Copernicus, Aristarchus and Tycho, some with large radial debris fields that stretch over significant regions of the Moon's surface. Larger impacts created the lunar mare or "seas", massive basins often filled in later by lava flows, creating a flat, darker floor. One such larger impact feature is the Mare Imbrium basin on the northern part of the Moon's surface. However Imbrium has several features that do not radiate from the centre of the Imbrium impact site, instead suggesting a more complex origin. Made up of rimmed grooves, lineations and non-circular craters, this set of features is known as the Imbrium Sculpture. Two researchers have studied this region, and determined a likely explanation for this set of features.

Peter Schultz of Brown University, and David Crawford of Sandia National Laboratories, both in the USA, combined laboratory experiments with numerical simulations to understand the creation of the features observed in the Imbrium Sculpture and the nature and trajectory of the impactor. What they found was that the grooves and lineations in the Sculpture provide important clues about the excavation process. Firstly, they found that the initial impact and excavation of material occurred up-range of the basin centre, with debris ejected down-range at high speeds and low angles. Their experiments also show that chunks of rock from the impactor sheared off and travelled far down-range at speeds close to that of the original impactor, creating groove-like features far to the south of the rim of the Imbrium basin. The researchers estimate that the impactor was approximately 250 km in diameter, a proto-planet roughly half the size of the asteroid Vesta, impacting the lunar surface at an angle of 30 degrees and a velocity of 25 km/s.

The Imbrium basin in among the largest of the impact basins created during the Late Heavy Bombardment phase of the solar system's early history, between 4.1 and 3.7 billion years ago. These new calculations have implications for estimates of the mass in the asteroid belt prior to the depletion event caused by the migration of Jupiter and Saturn and suggest that far more young asteroidal material may have intersected the lunar orbit than was previously thought.

And finally, after an exciting couple of years, the Rosetta spacecraft stopped listening for signals from the Philae lander at the end of July. The pair reached the comet 67P/Churyumov-Gerasimenko together before the Philae lander separated and attempted a nail-biting landing on the surface of the comet on November 12th 2014. Despite achieving the first-ever comet landing, Philae's final position was not ideal, resulting in less power from its solar panels than needed to power its on-board instruments. Now, since no signals have been received from Philae since the 9th of July 2015 and since scientists want to conserve power on the orbiting Rosetta spacecraft, ESA have switched off the Electrical Support System Processor Unit, the interface used for communications with Philae on the surface. Rosetta is also now being prepared for the end of its mission, when it will crash into the comet on September 30th, taking increasingly detailed images of the comet's surface as it makes its final approach.

Interview with Dr. Betsy Mills

Dr. Betsy Mills talks to Minnie about her work with ALMA observations of Sagittarius A*, the region containing the supermassive black hole at the centre of the Milky Way Galaxy. Dr. Mills has been investigating how molecular gas flows into the centre of the galaxy and has discovered a broad range of different molecules in the environment around the black hole. Additionally, Dr. Mills talks about her work in promoting equality and diversity in astronomy.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during August 2016.

Highlights of the month

August - Find the globular cluster in Hercules and spot the "Double-double" in Lyra

There are two very nice objects to spot with binoculars high in the sky after dark this month. Two thirds of the way up the right hand side of the 4 stars that make up the "keystone" in the constellation Hercules is M13, the best globular cluster visible in the northern sky.

Just to the left of the bright star Vega in Lyra is the multiple star system Epsilon Lyrae often called the double-double. With binoculars a binary star is seen but, when observed with a telescope, each of these two stars is revealed to be a double star - hence the name.

August - A good month to observe Neptune with a small telescope.

Neptune comes into opposition - when it is nearest the Earth - on the 2nd of September, so will be well placed both this month and next. Its magnitude is +7.9, so Neptune, with a disk just 3.7 arcseconds across, is easily spotted in binoculars lying in the constellation Aquarius, as shown on the chart. It rises to an elevation of ~27 degrees when due south. Given a telescope of 8 inches or greater aperture and a dark tranparent night it should even be possible to spot its moon Triton.

August 1st after sunset: Jupiter, Mercury, Regulus and Venus form a line in the Western Sky.

Given a clear sky and a very low western horizon you may be able to spot a line of Jupiter, Mercury and Venus along with Regulus in Leo. Binoculars may well be needed, but please do not use them until the Sun has set.

August 5th after sunset: Jupiter and a thin waxing crescent Moon

As Jupiter slowly sinks into the Sun's glare, given clear skies and a low western horizon it should be possible to spot Jupiter up and to the left of a thin waxing crescent Moon.

August 23rd - after sunset: Saturn and Mars lie above Antares in Scorpius

Looking South-Southwest after sunset and given a low horizon in this direction you should be able to spot Saturn (+0.4) lying above Mars (-0.3) (both in Ophiuchus) close to Antares in Scorpius.

August 27th - after sunset: Venus and Jupiter less than half a degree apart.

Looking west after sunset and given a very low horizon in this direction, binoculars may help you spot Jupiter and Venus less than half a degree apart. But please do not use binocluars until after the Sun has set.

August 11th and 25th: The Straight Wall

The Straight Wall is best observed either 1 or 2 days after First Quarter (evening of the 11th August best) or a day or so before Third Quarter (evening of the 25th August best). To honest, it is not really a wall but a gentle scarp - as Sir Patrick Moore has said, "Neither is it a wall nor is it straight."

Observe the International Space Station
Use the link below to find when the space station will be visible in the next few days. In general, the space station can be seen either in the hour or so before dawn or the hour or so after sunset - this is because it is dark and yet the Sun is not too far below the horizon so that it can light up the space station. As the orbit only just gets up the the latitude of the UK it will usually be seen to the south, and is only visible for a minute or so at each sighting. Note that as it is in low-earth orbit the sighting details vary quite considerably across the UK. The NASA website linked to below gives details for several cities in the UK and across the world.

Find details of sighting possibilities from your location from the location index. See where the space station is now using the tracking page.

The Planets

Southern Hemisphere

Haritina Mogosanu from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during August 2016.

In the Southern Hemisphere, winter is the time when the galactic center goes right all the up at Zenith. All you have to do from here in Wellington, New Zealand is to lift up your gaze and it will be right there. From there, if there is a really dark night, you can see the Milky Way Kiwi, which I spoke about in last month's jodcast. It is a dark patch at the center of our galaxy, the Milky Way, that looks just like a Kiwi bird, one of the native birds of New Zealand, and a national icon for us. It is truly remarkable how the kiwibird in the sky resembles to the one on the earth. But since you need a really dark sky, I would suggest that the best thing you can do to try spot the Milky Way Kiwi is to start looking for it in long exposed pictures of the night sky. The only place in New Zealand where I could see it with the naked eye was at Lake Tekapo's Earth and Sky. Whereas the Milky Way Kiwi is in the Scorpius Sagittarius region, holding the center of the galaxy on its head like a crown, the other famous dark patch is the Coalsack, near the Southern Cross. The coalsack is also known as the flounder, which is the Maori name for it. In deed, again if you find a truly dark sky, you will see the resemblance. However, talking about naming objects in the sky, the coalsack is also very appropriate as the dark patch, made of interstellar dust matter holds inside it a jewelbox, or the Kappa Crucis Cluster, NGC 4755. This is an open cluster in the constellation Crux, originally discovered byNicolas Louis de Lacaille during 1751-1752 and named the Jewell box by Sir John Herschel. He described its telescopic appearance as "...a superb piece of fancy jewellery". Jewel box is easily visible to the naked eye as a hazy star about 1.0 degrees southeast of the first-magnitude star Beta Crucis.

In August, after sunset, the Southern Cross, Crux, and the two pointers, Alpha and Beta Centauri, are slowly starting their descent on the south celestial carousel, our special part of the sky that is not visible from Northern latitudes above 30 degrees. They are midway down the southwest sky. The pointers point down and rightward to Crux the Southern Cross. Alpha Centauri is the third brightest star and the closest naked eye star-neighbour, 4.3 light years* away. Beta Centauri, like most of the stars in Crux, is a blue-giant star hundreds of light years away and thousands of times brighter than the sun.

There is a part of the sky here in Wellington that seems to rotate around the South Celestial Pole, the extension of the South Pole in the sky, located here at 41 degrees above the horizon. This coincides with Wellington's latitude, 41 degrees South. You can almost make the South Celestial Circle from the two pointers, Alpha and Beta Centauri, the Southern Cross, the Diamond Cross, the False Cross, then lower down on the south-western horizon Canopus, the second brightest star in the sky, and Achernar, which is easily found if you exend a line from the southern cross all the way to the southeastern horizon. The South Celestial Circle makes an entire rotation in 23 hours, 56 minutes and just about 4 seconds, which is a sidereal day. This is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars" rather than the Sun. The south celestial circle is home to the Large and Small Clouds of Magellan LMC and SMC that look like two misty patches of light low in the south, easily seen by eye on a dark moonless night. They are galaxies like our Milky Way but much smaller. The LMC is about 160000 light years away; the SMC about 200000 light years away. My two favourite objects inside the south celestial circle are the globular cluster 47 Tucanae (rival of Omega Centauri) and the Tarantula Nebula, a spectacular cloud which is one of the most active starburst region known in the Local Group of galaxies. To give you an comparison, the Tarantula Nebula is about 160 000 light years away from us. Its luminosity is so great that if it were as close to Earth as the Orion Nebula, the Tarantula Nebula would cast shadows. The South Celestial Circle never ceases to amaze me and I watch it as it rotates with the seasons and througout the night when I can. For me, the southern part of the sky, is the best and I am extremely happy that i had the chance to see it, let alone now teach about it.

Back to the ecliptic, which is visible from everywhere where people can see the Sun and the Moon, as it is defined as the path of Sun but also as the plane of our Solar System, all five naked-eye planets are visible in the early evening sky. Mercury, Venus and Jupiter are low in the west and shuffle around through the month. Mars and Saturn are north of overhead near Antares.

Venus, the brilliant silver 'evening star', sets in the west an hour after the sun at the beginning of the month, extending to nearly two hours by the end. Jupiter, higher in the west and golden-coloured, sets steadily earlier through the month: at 9 pm at the start of August and before 8 pm at the end. Mercury makes its best evening sky appearance of the year. It is roughly midway between Jupiter and Venus at the beginning of the month. Around the 20th Mercury will be left of Jupiter but much fainter. It begins to sink back into the twilight at the end of the month. On August 27-28, Jupiter and Venus will be close together, easily included in the same view in a telescope. At the beginning of August, Mars, Saturn and Antares make an isosceles triangle north of the zenith. Mars is the brightest of the three and the same colour as Antares. Saturn is cream-coloured. Saturn stays put against the background stars. Mars moves steadily eastward. On the 25th Mars will be two degrees, four full-moon diameters, from Antares making a striking pairing of orange stars.

Bright stars are widely scattered. Vega on the north skyline is balanced by Canopus low in the south. Orange Arcturus is in the northwest. The Southern Cross, Crux, and the Pointers are midway down the southwest sky. The Milky Way spans the sky from northeast to southwest. Canopus, the second brightest star, is near the south skyline at dusk. It swings upward into the southeast sky through the morning hours. On the opposite horizon is Vega, one of the brightest northern stars. It is due north in mid-evening and sets around midnight. Arcturus, Hokulea, the Zenith star of Hawaii is in the northwest at dusk. The fourth brightest star, Arcturus is currently the brightest in the northern hemisphere. It is 120 times the sun's brightness and 37 light years away. When low in the sky Arcturus twinkles red and green as the air splits up its orange light. It sets in the northwest around 10 pm. Antares marks the heart of the Scorpion. The Scorpion's tail hooks around the zenith like a back-to-front question mark. Antares and the tail make the 'fish-hook of Maui' in Maori star lore. The fishing hook drags at this time of hte year the Milky Way down from the heavens. Antares is a red giant star 600 light years away and 19000 times brighter than the sun. Below or right of the Scorpion's tail is 'the teapot' made by the brightest stars of Sagittarius. It is upside down in our southern hemisphere view.

The Milky Way is brightest and broadest overhead in Scorpius and Sagittarius. In a dark sky, it can be traced down past the Pointers and Crux into the southwest. To the northeast it passes Altair, meeting the skyline right of Vega. The Milky Way is our edgewise view of the galaxy. The thick hub of the galaxy is in Sagittarius 30000 light years away. The actual centre is hidden by dust clouds in space. The nearer dust clouds appear as gaps and slots in the Milky Way. Binoculars show many clusters of stars and some glowing gas clouds in the Milky Way.

According to Karen W. Pierce who made an excellent list of binocular objects that you can find on the site, in Ophiuchus, you can find M9, M10, M12, M14, M19, and M6, which provide examples of different concentrations of stars. Also visible is IC 4665, a big but often overlooked open cluster located near Beta Ophiuchi. On a dark night, it is visible to the naked eye as a hazy splotch nearly I degree across.

Sagittarius contains more Messier objects than any other constellation. The best way to identify them is to take them one by one. The main stars of Sagittarius form the famous "Teapot" asterism, which here in Wellington looks upside down. It is said that for the Norhtenlings, the brightest part of the Milky Way seems to emerge from the Teapot's spout like a puff of steam. In Sagittarius, M22, the Great Sagittarius star cluster is a very large globular — the best of the constellation's many globulars. At magnitude 5.1 it is an easy binocular object. M23 is another one of the many clusters in Sagittarius. M23 presents over 100 stars in an area about the size of the Moon. The Lagoon Nebula, M8, is visible with the naked eye in dark nights just north of the richest part of the Sagittarius Milky Way. The Trifid Nebula, M20, is found only 1 1/2 degrees northwest of the Lagoon Nebula. Ideal conditions and sharp eyes might detect M21, which is located just 1/2 degree northeast of M20, although it is rather faint by binocular standards. Omega Nebula, also called M17 or the Swan, the Horseshoe or the Checkmark, can be seen clearly in binoculars. In Scorpius, Antares, or Rehua in Maori, is the Heart of the Scorpion. A red giant star about 10000 times more luminous than the Sun is a good binocular object. M4 is a globular cluster that in binculars looks like a fuzzy patch. M6, the Butterfly Cluster, is a large open cluster of about 50 stars resembles a butterfly. M7 is a large, bright open cluster that lies southeast of M6 but needs to be seen through binoculars to be fully appreciated. NGC 6231 is a bright open cluster that lies in a rich region of the Milky Way. It is best surveyed in binoculars or at very low power in a telescope. In this same area of the scorpion's tail are several other binocular-visible objects but I will let you discover these, as that region of the sky comes about and remember that you don't need fancy telescopes to enjoy the night sky but a pair of good bionculars a good sky atlas and lots of hot chocolate. It's winter time here in the Souther Hemisphere and the nights are crisp but cold. Keep warm and look up.

Odds and Ends

Young stars are often surrounded by discs of gas and dust known as protoplanetary discs. As the name would suggest, these are the structures that planets form from. For stars like the sun, temperatures up to a radius of 3 AU (or three times the distance between the Earth and Sun) are great enough to keep water in its gas phase. Beyond this, water can be found as ice. Water can't exist as a liquid in the near vacuum of disc due to the extremely low pressure. The boundary between the gas and ice phases is known as the snow line. The water snow line isn't something we've been able to see before, as it is generally too close to the star. However, something unusual is going on with a particular star, V883 Orionis, which allows us to see this. The star has recently dramatically increased in brightness, as a large amount of material of its protoplanetary disc has fallen onto its surface. This star is only 30% more massive than the Sun, but because of this recent outburst, it has increased to a luminosity (or total power output) 400 times greater than the Sun. This has also increased the temperature of the star, pushing the snow line out to 40 AU, similar to the distance that Pluto orbits our Sun. More information is available in the ALMA press release.

Our listener survey has now closed. Thank you to everyone who has taken the time to fill this in. We'll be announcing the winner of the prize in the next episode.

You may remember that our April episode contained a peculiarity in the form of a series of numbers being read out, preceded by a few bars of an unknown melody. This was a recording made and inserted into the show by George and was modelled on a Numbers Station. Numbers stations are shortwave radio stations which broadcast sequences of numbers, morse code, strange melodies and intermittent beeps. They are thought to be used by countrys' intelligence services to communicate information to officers who are abroad. The best known numbers stations include UVB-76 which broadcasts from somewhere in Russia and consists of an unsettling, repeated tone, that is occasionally but very rarely interrupted by a voice that reads out a list of names. The Lincolnshire Poacher, so called because uses bars from a well known English folk song of the same name, consists of sequences of five numbers being read out by an electronic female voice. George's example contains a hidden message and he won't tell any of us what it means or how to decode it. So we're putting it to you. If you can correctly decrypt what George's numbers station is saying then he promises to send some ALMA freebies to the first person who tells us. You'll find the show here and the numbers occur at 28:20. And if you're interested, you can learn more about number stations here and here.

The computer game Pokemon Go has become quite popular. One astronomer, who goes by the name lilvipa on reddit, has suggested using the game to get people interested in astronomy. His strategy is to set up a real telescope and a virtual Lure at the location of a PokeStop. The Lure will draw both Pokemon and people playing Pokemon Go. He then introduces himself and then shows astronomical objects through the telescope. A discussion of his techniques is given on the reddit thread. However, remember that cell phone signals can interfere with the operation of many professional observatories, especially radio telescopes like the ones at Jodrell Bank.

Show Credits

News:Megan Argo
Interview:Dr. Betsy Mills and Minie Mao
Night sky:Ian Morison and Haritina Mogosanu
Presenters:James Bamber, Max Potter, and Benjamin Shaw
Editors:Adam Avison, Megan Argo, George Bendo, Haritina Mogosanu, and Nialh McCallum
Segment Voice:Iain McDonald
Website:George Bendo, Saarah Nakhuda, and Stuart Lowe
Producer:George Bendo
Cover art:Sagittarius as imaged in far-infrared light by the Herschel Space Observatory CREDIT: George J. Bendo

Download Options

Subscribe (It's free)