In the show this time, we talk to Dr Victor Debattista about the the evolution of spiral galaxies, Stuart rounds up the latest news and we find out what we can see in the January night sky from Ian Morison and John Field. Meanwhile, we completely forget that it's the Jodcast's eighth birthday.
In the news this month: a black hole in a black hole's clothing.
A black hole is one of nature's most unusual objects: a small region of the universe in which a vast quantity of mass has accumulated, the combined gravitational field of which then acts to wrap space into a closed system from which nothing can escape. The formation of a black holes occurs almost exclusively at the end of a massive star's life, the soul survivor at the the epicentre of the parent star's supernova. For physicists and astronomers, black holes represent the potential for testing our understanding of the Universe, as very few other places have more extreme environments than that which is found on the horizon between our Universe and the unobservable, inner world of a black hole.
The problem with black holes is that finding them is difficult. Since nothing can escape a black hole, they appear to be invisible to any of the current methods used to observe astronomical objects, which rely on the emission of some form of light. So, instead of looking for black holes directly, we search for the effects they are having on the environment around them. For example: if the black hole has a nearby companion star, it is possible for the gas and dust in the star's outer layers to be torn off by the gravitational pull of the black hole. This forms a steady flow of material falling towards the black hole and eventually forms a swirling disc of material, like water down a plughole; but, unlike the water, the gas gets hotter and brighter as it swirls ever closer to the black hole's horizon. Eventually, the infalling stripped gas is heated so much that it glows with X-ray light. It is the emission from these discs that astronomers search for when looking for black holes, and such objects are known as ultraluminous X-ray sources.
There are many types of ultraluminous X-ray source, and the type depends upon the size of the black hole and the type of star being fed upon. Typically, though, you can split them into two broad categories. The ultraluminous X-ray source may be emitting high-energy X-rays, which are characteristic of a stellar-mass black hole - the sort of black hole left over from a supernova. Alternatively, an ultraluminous X-ray source can emit low-energy x-rays, which is thought to be an indicator of the presence of an intermediate-mass black hole.
Both types of black hole are important to study as both are the products of extreme natural processes and can inform us about the end-points of the evolution of stars. In fact, intermediate-mass black holes, if discovered, would be even more interesting, exactly because of their expected rarity and unusual methods for formation. There are, of course, a number of ultraluminous X-ray sources that we know of, and some of those are potentially good candidates for containing stellar-mass black holes. An example of an ultraluminous X-ray source with a stellar-mass black hole is IC 10 X-1, which is in the local galaxy IC 10 within the constellation of Cassiopeia. However, examples of intermediate-mass black-hole ultraluminous X-ray sources are much fewer. One such potential candidate was found in 2004 by the Chandra and XMM-Newton X-ray observatories, and is called M101 ULX-1. Since then, follow-up observations have worked to confirm that M101 ULX-1 is an ultraluminous X-ray source containing an intermediate-mass black hole.
One of the teams of astronomers making the follow-up observations of M101 ULX-1 has recently published its findings in Nature. Their plan was to deduce the mass of the black hole and its companion star by measuring the speed at which the two objects orbit each other. The orbital speeds can be estimated using the Doppler shift of the light coming from the star and from the hot disc of gas around the black hole. Unfortunately, though, the result indicates that the black hole in M101 ULX-1 is only a few tens of solar masses, meaning that it is only a stellar-mass black hole. This is problematic, as the X-ray observations appeared to show all the characteristics of an intermediate-mass black hole. So what, exactly, could make a stellar-mass black hole appear to be an intermediate-mass black hole? Fortunately, there is an answer, and it all stems from the unusual properties of the companion star and the separation distance of the two companions in M101 ULX-1.
Most stars, throughout their life-cycles, are fairly stable, meaning that the gravity pulling the star inwards is closely balanced by the outward push of the fusion processes in the star's core. However, some very large stars do not have this balance - they are brighter than they should be, and, because of this, spew vast quantities of their gas out into space in events like our own Sun's solar flares, only thousands of times larger; these stars are known as Wolf-Rayet stars. The companion star in M101 ULX-1 is one of these Wolf-Rayet stars - but why would this make M101 ULX-1 appear to be an intermediate-mass black hole? It is all related to how the black hole is feeding on the star's gas. As a comparison, look at the ultraluminous X-ray source IC 10 X-1: it, too, has a Wolf-Rayet star for a companion, but the orbital time is just one day, meaning that they are very close together - close enough for the black hole to strip the gas straight off the star, forming a smooth, continuous stream of gas to feed the black hole. In M101 ULX-1, the two companions are far apart, taking over eight days to complete an orbit. The black hole is therefore restricted to feeding on the gas found in the giant solar flares from its Wolf-Rayet companion. This unstable method of feeding the black hole appears to make it so that the gas in the disc around the M101 ULX-1 black hole does not heat up nearly as much as the disc in IC 10 X-1, or other stellar-mass black-hole ultraluminous X-ray sources, and therefore not emit such energetic streams of X-rays. Theorists had predicted that perhaps ultraluminous X-ray sources like M101 ULX-1 might be possible, but now these ideas can be confirmed. This will help future observers who are still searching for intermediate-mass black holes by providing a secondary way of checking their observations. It is also another step along the road understanding the interesting physics of black holes.
Interview with Dr Victor Debattista
Dr Victor Debattista is a Reader in Astrophysics at the University of Central Lancashire. He specialises in using computers to simulate the dynamics of disc galaxies, including our own Milky Way, with the goal of understanding how disc galaxies form and evolve over time. He discusses his simulations of the Milky Way, with particular emphasis on the phenomenon of 'stellar migration', and describes the many challenges in running these simulations. He also talks about his anticipation of the chance to compare his simulations to the results from the recently launched Gaia satellite.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during January 2014.
Pegasus and Andromeda are setting in the west in the evening, with the nearby galaxies of Andromeda and Triangulum visible under a dark sky. Orion the Hunter looms large in the south, with the red giant star Betelgeuse to his upper left and the blue giant Rigel to his lower right. Between them, the Belt of three stars is above the Sword, which contains the spectacular Orion Nebula. The Belt points down towards Sirius, or Alpha Canis Majoris, the brightest star after the Sun to observers on Earth. Following the Belt the other way leads to Taurus the Bull, wherein lie the beautiful Hyades and Pleiades Clusters. The orange-coloured star Aldebaran appears to be among the Hyades, but is actually closer to us. Above Orion are Gemini, the host of Jupiter this month, and Auriga, containing the bright yellow star Capella. Perseus and Cassiopeia are nearby, with the Perseus Double Cluster among the treasures to be found along the Milky Way that runs between them. Leo rises in the east as the evening wears on, followed by the planets Mars and Saturn in the early hours of the morning.
- Jupiter is visible throughout the night, crossing the southern sky. It reaches opposition (opposite to the Sun in the sky) on the 5th and attains a maximum elevation of 62°, making this a great month to observe it. With a magnitude of -2.7 and an angular diameter of 46-47", it can be clearly seen in Gemini. It is currently moving west (retrograde) through the constellation as the Earth overtakes it in their orbits around the Sun. A small telescope shows the Galilean moons, as well as the Great Red Spot in Jupiter's South Equatorial Belt.
- Saturn is visible before dawn, rising around 03:00 Universal Time (UT) at the beginning of the month and 01:30 by the end. It shines in Libra at magnitude +0.6, with a disc 16" across. Its rings are now inclined at 20° to the line of sight, but the planet does not rise very high in northern hemisphere skies.
- Mars rises at about midnight UT at the start of January and 23:00 at the end. It brightens from magnitude +0.9 to +0.3 over the month, while it increases from 6.9 to 8.8" in angular size. Using a telescope, markings can be discerned on its surface. Mars is moving down through Virgo, from beneath Porrima to above Spica.
- Mercury reached superior conjunction (behind the Sun in the sky) on the 29th of December, but reappears low in the west-south-west in the second half of January. It is at its greatest angular separation of 18° from the Sun (elongation) on the 31st, when it has a magnitude of -0.6 and an angular size of 7", and lies 10° above the horizon 45 minutes after sunset, alongside a thin crescent Moon.
- Venus passes between the Earth and the Sun (inferior conjunction) on the 11th. Nevertheless, it can be seen low in the south-west after sunset at the very beginning of the month, and low in the south-east before dawn at the end. It forms an extremely thin crescent on New Year's Day, being just 3% illuminated, but spans almost 1'.
- Jupiter, high up and far from the Sun in the sky, offers optimal viewing conditions this month. The Great Red Spot has also become more prominent recently.
- Venus lies just below the Moon 45 minutes after sunset on the 2nd, and both bodies show very slender crescent phases. Look out for earthshine, where sunlight reflected from Earth's clouds faintly illuminates the otherwise dark part of the Moon.
- It is a good time to observe the Andromeda and Triangulum Galaxies early or late month, when the Moon is new.
- Jupiter is only 6° from the Moon on the 14th, when the latter is 98.4% full.
- Saturn is close to a third-quarter Moon before dawn on the 25th.
- Venus is just 4° above a thin, waning crescent Moon before dawn on the 29th.
John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during January 2014.
Orion, Canis Manor and Taurus dominate the northern sky after sunset. The planet Jupiter is nearby in Gemini, moving westward (retrograde) relative to the stars. Castor and Pollux, the brightest stars in Gemini, are low in the north-east in the evening and represent the heads of the Heavenly Twins. Castor, the lower of the two, is a multiple star system with a combined magnitude of +1.6 whose two brightest components can be split using a medium-sized telescope. Pollux, a red giant star in the latter stages of its life, is brighter at magnitude +1.1. The open star cluster M35 resides near the star Eta Geminorum, close to Taurus. It can be seen with the naked eye, while binoculars or a small telescope reveal more of its population of some 500 stars. Taurus, in the north, is marked by a V-shape of three stars, representing the head and horns of the Bull. The red giant star Aldebaran is the Bull's Eye. The Pleiades star cluster, on the back of Taurus, is visible to the west of the head.
Orion is between Taurus and Gemini, a Hunter standing upside-down to Southern Hemisphere observers. Commonly known as the Pot in New Zealand, it plays host to the Orion Nebula in the middle of the Sword just below his Belt. This star-forming region looks like a fuzzy star to the unaided eye, or a bat-shaped cloud in binoculars or a small telescope. A telescope of 100 millimetres or more in aperture reveals stars within and around the nebula, including a tight group of four stars called the Trapezium, the brightest of which illuminates the surrounding cloud with ultra-violet radiation. Orion's left foot is the blue giant star Rigel, which is some 18 times more massive than our own Sun.
The Pleiades, the head of Taurus and the Belt and Sword of Orion make up a great Waka, or canoe, to some Māori along the east coast of Aoteroa (New Zealand). In this canoe, called Te Waka o Tamarereti, the mythical figure of Tamarereti sailed across the night sky and placed the stars into the heavens, leaving a wake in the form of the Milky Way. The constellations of Canis Major and Canis Minor are the hunting dogs of Orion, following him through the sky. The larger dog's collar is marked by Sirius, the brightest night-time star. The smaller dog's tail is the bright star Procyon. Canopus, the next-brightest star after Sirius, is almost overhead in the evening.
- Mars rises after midnight NZDT (New Zealand Daylight Time, 13 hours ahead of Universal Time), near to the star Spica in Virgo.
- Saturn comes up around 02:00 NZDT.
Odds and Ends
The Gaia spacecraft was launched by the European Space Agency on 19 December. This spacecraft will fly to L2 (a location where the gravitational forces from the Earth and Sun equal the centripetal force needed to keep an object in orbit around the Sun) and will operate for five years. During that time, Gaia will perform measurements of the positions, motions, brightness, and spectra of 1% of the 100 billion stars in the Milky Way, giving us a better map of our galaxy than ever before.
Astronauts performed several spacewalks on the International Space Station (ISS) this month. As well as fixing a faulty ammonia coolant pump, they installed a pair of cameras called UrtheCast, which will provide near-live images of the Earth's surface as the ISS flies over it. After this episode of the Jodcast was recorded, NASA reported that the cameras did not initially function and were in the process of being fixed.
Jodcast listener Christine Brooks sent us the following poem, entitled 'Birth':
I look above to velvet sky amid the Winter night,
Orion rising through the dark, Rigel blue and bright.
I see Hunter's sword where misty birthing stars shine clear,
And glowering Betelgeuse, dimly red, marks the dark months of the year.
In aged bloated body, the giant's embers glowing low,
Self devoured, consumed within, ashes choking now,
The time will come, the spark will fade, pressures no more to be borne,
And the giant will blaze in his final incandescent morn.
Betelgeuse awaits the day his fires dim and die,
When he will burst his iron heart in his final fiery cry,
The Red Hand of the Hunter will shed his sundered flesh,
In a divine wind suicidal, to nurse his children's creche.
The shattering of his death throes will seed all coming things,
Tin, silver and nitrogen, and gold, the gift of kings,
Oxygen, uranium, all these he will give,
And carbon darkly bright, that his children's childer might live.
The Hunter's sword in spangled sky shines with birthclouds bright,
Full circle round the story comes in gleam of new starlight.
"Fiat Lux" says the old tale, but the wonder strikes me through,
When from my garden step, at my own back door, I see the birthing of the new.
The new stars gleam like diamond dust studded in dusky swirl,
And shimmering vapours shroud the stars in glowing, glimmering pearl.
We live in a universe of marvels, all there for anyone to find,
Needing only open eyes and ears, and more, an open mind.
They say we are born of ashes. They say we go to dust.
But they never said how this came to be. It irks me and thus,
This night I leave the party, to stand amid icy blast,
The sound of Jingle Bells and Silent Night from the indoors drifting past.
I watch the skies through lucid air, and the birthing stars proclaim
The cyclic story, creation's glory and how the death of others became,
The birth of the new, the start of all. Creation's children are us.
Ashes to ashes? But what ashes! We are all born of stardust.
|Interview:||Dr Victor Debattista and Indy Leclercq|
|Night sky:||Ian Morison and John Field|
|Presenters:||George Bendo and Mark Purver|
|Editors:||Mark Purver, George Bendo and Stuart Harper|
|Segment Voice:||Iain McDonald|
|Website:||Mark Purver and Stuart Lowe|
|Cover art:||The Gaia telescope's Deployable Sunshield Assembly being tested at the Centre Spatial Guyanais in French Guiana. CREDIT: ESA/M. Pedoussaut|