December 2010: Century

In this special 100th episode show, we talk to two Jodcasters, Dr. Megan Argo and Dr. Stuart Lowe about their day jobs. As always, Megan brings us the latest astronomical news and we hear what can be seen in the December night sky from Ian Morison and John Field.

The News

In the news this month:

• The usual image of the Milky Way is a fairly normal disk galaxy with spiral arms full of stars and gas, a bulge containing older stars and a supermassive black hole sitting right at the centre. But observations in various different parts of the electromagnetic spectrum show evidence for a giant diffuse structure extending many thousands of light years above and below the main Galactic disk. Now, maps made using 600 days of observations from the Fermi satellite show giant gamma-ray emitting bubbles emanating from the core of the Milky Way, stretching far above and below the plane of the disk.

  Such bubbles are seen in some other galaxies where they can be signatures of an active supermassive black hole at the galaxy's core, or a high rate of star formation powering a strong galactic wind. These newly discovered bubbles, reported in the Astrophysical Journal on November 10th, cover a large part of the sky reaching up to 50 degrees above the plane of the disk and across half of the sky as seen from our vantage point two thirds of the distance between the core and the outer edge of the Galactic disk.

  Previous observations and surveys of the sky have shown evidence of X-ray shell structures and a haze of microwave emission seen by WMAP and attributed to electrons moving in a magnetic field. Images of these features from other full-sky surveys show a strong similarity in the shape of these bubbles across the spectrum, suggesting a common cause for the gamma-ray bubbles seen by Fermi, the X-ray features observed by ROSAT, and the radio haze detected by WMAP.

  Centred on the galactic core, the two bubbles (one above, one below the plane of the disk) are very similar in shape, suggesting that they are caused by the same process. One possible origin is that jets from the central black hole have inflated these bubbles in the past, pushing the material outwards before switching off, leaving the bubbles behind. The gas, heated by the jets, would be less dense than the halo material above and so would rise away from the disk just as a helium balloon rises in air. One objection to this idea is that the rounded shape of the bubbles observed by Fermi are not typical of the much narrower jet-powered features seen in other galaxies. Another possibility is that the bubbles are driven by a wind, generated by a large number of young, hot stars created in a nuclear starburst event where many young, hot stars are created very quickly, and driven further by the energy released by subsequent supernova explosions, although the evidence from both the number of supernova remnants found in the galactic centre and the observed abundance of radioactive isotopes suggests that this scenario may not be so likely.

  Whatever the cause, these new features are a reminder that there is much we still don't understand about our own local universe.

• Theoretical models of the evolution of massive stars suggest that, for stars over a certain mass limit, the likely end result is a black hole, the compressed remnant of a stellar core which is so massive that not even light can escape its gravitational pull (making them quite hard to spot!). Luckily for our planet, such explosions are rare and happen at large distances from the Earth, but this makes studying them something of a challenge for astronomers.

  One relatively nearby explosion was that of supernova 1979C which occurred in the spiral galaxy M100, a mere 50 million light years from our solar system. The explosion happened some 50 million years ago, but due to the speed at which light travels, observers only saw the event occur in 1979. The light reaching us now shows the nature of the system 31 years after the event. Such a close example of this kind of event provides an ideal opportunity to study such a system, and test theoretical models of stellar evolution.

  With other supernovae that have been observed, there is an initial quick rise in brightness at the time of explosion, followed by a slower decline in brightness as the shockwave from the explosion expands into the surrounding gas. This gradual fading is observed in ordinary visible light, as well as in the radio and X-ray parts of the electromagnetic spectrum. But in the case of 1979C, rather than fading, researchers have found that the amount of X-ray emission has remained surprisingly constant over many years. Researchers at the Harvard-Smithsonian Centre for Astrophysics in the US, led by Daniel Patnaude, analysed archive data from instruments on board the ROSAT, XMM Newton, Swift and Chandra satellites taken over twelve years, and found that the X-ray emission does not match what is expected for a shock front. Their study, to be published in the journal New Astronomy, suggests that the peculiar nature of the X-ray emission may be due to material falling onto a stellar mass black hole formed at the time of the supernova explosion as the star's outer material collapsed. While a black hole with a mass roughly five times that of the Sun could power the observed X-ray emission, another possibility is that the supernova explosion instead created a very young, fast-spinning neutron star with a strong wind of high-energy particles. The authors point out that the current data cannot firmly show which model is correct, but that more sensitive observations could help. If the X-ray brightness varies on short time scales, this would be evidence for ongoing accretion where material is continuing to fall onto the black hole, powering the X-ray emission.

• Over the last twenty years, several hundred planets have been detected around other stars. Most of these are more massive than Jupiter and orbit fairly close to their parent star, since these are the easiest planets to find using current methods. Now, a team of astronomers have discovered a giant planet orbiting a star which may have come from another galaxy.

  The team, led by Johny Setiawan of the Max Planck Institute for Astronomy in Germany, discovered the planet using a high-resolution spectrograph which can detect the tiny motion of a star due to the tug of an orbiting planet. The star itself, known as HIP 13044, is located in the constellation of Fornax and has a motion across the sky which suggests that it was not formed in the Milky Way like the Sun, but came from a dwarf galaxy which was tidally disrupted and merged with our own galaxy between six and nine billion years ago. This means that the planet is likely to be the first exoplanet known to have formed outside the Milky Way.

  The parent star is also unusual in that it contains a relatively small amount of heavy elements. In astronomy, anything heavier than hydrogen or helium is considered a metal, and very few known exoplanets orbit stars with such low proportions of metals. The standard model of planetary formation says that the higher the metal content of a system, the more likely it is that planets will form. The star is also old, having already passed through the red giant phase where a star expands to hundreds of times the size of the Sun. Planets orbiting close to such a star are thought to be swallowed up by the star's outer layers during the red giant phase. This planet, named HIP 13044 b, orbits very close to its parent star, completing one orbit every 16.2 days, and one possibility is that something caused it to move closer to the star since the red giant phase, with its original orbit lying much further out. The rotation of the star is also quite fast for a star of this type, and one reason for this could be that other, closer, planets were swallowed up during the red giant phase, causing the star to spin faster.

  Although evidence of a planet located in the Andromeda galaxy was announced last year, that detection has not been confirmed, making HIP 13044 b the first definite planet with a known extra-Galactic origin. The fact that our own Sun will one day swell up to become a red giant makes this planet particularly interesting since it is one of very few known to have survived the red giant phase of its host star.

• And finally:

  Often described as the successor to Hubble, the James Webb Space Telescope will be the most sensitive and technically ambitious infra-red telescope ever launched. Rather than operating from low Earth orbit as Hubble does, the JWST, named after former NASA administrator James Webb, will sit at a point known as L2, a gravitationally stable point some 1.5 million kilometres from the Earth. While this has many operational advantages, it does mean that servicing missions will not be an option should something malfunction. Originally proposed a year before Hubble launched, the project was estimated to cost a total of around one billion US dollars when reviewed in 2001. By the time it received an official go-ahead in 2008, the addition of new and more complex instruments had pushed the projected cost to $5 billion. Now, an independent review of the project has revealed that the cost has spiralled to more then 6.5 billion US dollars. Since the extra money required to complete JWST will likely have to come from NASA's existing budget, this staggering overrun has implications for much of NASA's science programme and could well have effects far across the agency.

Interview with Dr. Megan Argo

Mark talks to Dr. Megan Argo about her research and life in astronomy. Megan discusses her interest in starburst galaxies, especially M82. She talks about her long association with Jodrell Bank and Macclesfield Astronomical Society, and her move across the world to Perth in 2008. She tells us about her work in radio interferometry at Curtin University, which is now capturing ever larger fields of view on the sky through the use of powerful computers. We discover her interest in practical radio astronomy via the meteor detector she helped to construct at Jodrell Bank, and she finishes by comparing the relative merits of Cheshire and Western Australia. Let us know whether you think she has developed an Australian accent!

Interview with Dr. Stuart Lowe

Dave interviews our very own Dr Stuart Lowe to find out about his work over the past few years. Stuart tells us about the Low Frequency Instrument on ESA's Planck satellite which he started to work with in 2006. He tells us about the testing efforts from the summer of 2006 leading through to launch in May 2009 and the completion of the first two sky surveys. He also describes Chromoscope which is a web site he created for viewing the sky at different wavelengths. Finally, he tells us about his new job working for the Las Cumbras Observatory Global Telescope.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during December 2010.

The Square of Pegasus is in the south in the early evening, with Andromeda, Perseus and Cassiopeia above it. The Perseus Double Cluster is between the latter two, visible as a glow to the unaided eye. Binoculars or a telescope reveal a wealth of stars in the two adjacent clusters. The right-hand three stars of Cassiopeia point towards our nearest large galaxy, Andromeda, also known as M31, whose light we see more than two million years after it set off. The constellation of Taurus the Bull rises in the east later in the evening. The Hyades Cluster forms the Bull’s head, up and to the right of the its eye, which is the red giant star Aldebaran. The Pleiades Cluster lies above and right of the Hyades, containing a triplet of stars near its centre, an arc of stars to its lower left and a double star system to its lower right. Orion the Hunter is below and left of Taurus. The red supergiant star Betelgeuse forms one of his shoulders, above which more stars mark his hand and club. His shield is to the right, and three stars across his middle make up his belt. His knee is the blue star Rigel. The stellar nursey of the Orion Nebula is a misty glow below the belt, and contains the Trapezium, a group of hot blue stars. The brightest star in the sky, Sirius, is in Canis Major, to the lower left of Orion. The cluster M41 is below this, containing a red star surrounded by many bluer ones.

The Planets

• Jupiter shines with a magnitude of -2.5 in the evening sky, the brightest object at that time of night apart from on those days when the Moon is up. It moves away and becomes slightly fainter during the month, its angular size dropping from 40 to 39”. Its four Galilean moons can be seen through binoculars. A telescope shows that its South Equatorial Belt, near the Great Red Spot, is now reappearing after several months of absence. The star 20 Piscium appears close by on the 29th.
• Saturn is in the pre-dawn sky at magnitude +0.9, rising earlier as the month progresses. Its rings continue to become more visible, reaching 10° from edge-on by the end of the month. The gap between two of its rings, known as the Cassini Division, is becoming observable.
• Mercury is very low around sunset, near the setting Sun, with a magnitude of -0.3. It reaches inferior conjunction, when it is directly between the Earth and Sun, on the 20th.
• Mars is hidden by the Sun.
• Venus is at its brightest in the first two weeks of the month, rising before dawn at magnitude -4.9. Its angular size decreases from 42 to 27” during the month, while the fraction of its surface which is illuminated increases from 25 to 45%, leaving it only slightly dimmed as seen from Earth.

Highlights

• The first total lunar eclipse in over three years to be visible from the UK occurs early on the 21st, the winter solstice. The penumbral shadow is first cast at 05:29 GMT/UT (Greenwich Mean Time/Universal Time). Totality begins at 07:41 GMT, allowing almost an hour before the sunrise washes out the view. Increased dust in the atmosphere may cause the Moon to appear greyer and less red than usual.
• The Geminid meteor shower peaks early on the 14th. It is best seen from 01:00-03:00 GMT, after the Moon has set, and about 45° to the left of the radiant, which is in the constellation of Gemini.
• Venus, Saturn, the waning crescent Moon and the star Spica appear close together in the sky on the 2nd. They rise in east before dawn in the constellation of Virgo.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern night sky during December 2010.

The summer Sun rises high in the south and the nights are short. Orion and Taurus are visible in the southern and northern hemispheres. The brightest star in Orion, Rigel, is one of his feet, and the name is Arabic for ‘foot’. To southern hemisphere observers, the Hunter is inverted and this foot is near the top of the constellation. Rigel is the seventh-brightest star in the sky, with an absolute luminosity 85,000 times greater than that of the Sun shining from a distance of 800 light-years. Its companion, Rigel B, has a magnitude of +6.7 and can be observed with an 8-inch telescope. Rigel B is itself a binary star system, identifiable only through the changing Doppler shift of its chemical spectrum. The Māori refer to Rigel as Puanga, and its rising marks the new year to some on the North Island of New Zealand. The red star Betelgeuse is Orion’s shoulder, and the tenth-brightest star in the sky. As a red giant, it has fused most of its available hydrogen and is now predominantly burning helium. This raises its internal temperature and causes the surface to expand and cool to a red-hot 3500K, barely half the outer temperature of our Sun. Betelgeuse is massive enough to end its life in a supernova, possibly within the next million years. A supernova remnant called the Crab Nebula lies near the fainter horn of Taurus the Bull, and can be seen with a small telescope. It originated in 1054, when its supernova was visible to the naked eye from Earth. A neutron star, the compressed core of the star which produced this supernova, spins at over 30 times per second in the heart of the Nebula. Taurus represents the god Zeus in Greek mythology, and is one of the twelve zodiacal constellations through which the planets appear to pass when viewed from Earth. In the early part of the month the Sun passes through Ophiuchus, the thirteenth constellation in the planets’ path around the ecliptic plane.

The Planets

• Mercury and Mars are low in the west after sunset.
• Jupiter is high in the north.
• Venus and Saturn are low in the morning sky, climbing higher as the month progresses.

Highlights

• The Milky Way and Southern Cross skirt the southern horizon in the early evening this month, climbing in the south-east during the night.
• A total lunar eclipse is visible throughout New Zealand on the 21st. A shadowed Moon rises in the night sky, turning red during totality.

Odds and Ends

On January 3-5 2011 the BBC will be broadcasting three live programmes from Jodrell Bank Observatory. Stargazing Live will be hosted by Professor Brian Cox and Dara O'Briain and shown on BBC2. You can contribute your astronomical photographs to Stargazing Live (and the Sky At Night) via the BBC's special Flickr group.

ESA's 6 newest astronauts graduated at the end of November and should start going into space in 2013. This is the first time that European astronauts have gone through an ESA programme instead of training in the USA or Russia.

At the time of recording the November 2010 episode, the pace shuttle Discovery was due to be launched at the beginning of December. However the launch was initially delayed due to poor weather conditions and then delayed further as cracks were found. The earliest date for the launch of Discovery is currently December 17.

Although a Spanish woman has claimed ownership of the Sun, citing an apparent loophole in the 1967 Outer Space Treaty, the Bad Astronomer points out that her claims have no legal weight.

Show Credits