In the news this month:
When a massive star runs out of fuel in its core it explodes violently as a supernova, releasing a vast amount of energy and briefly becoming bright enough to be visible from other galaxies. Over several weeks or months the light from such an explosion gradually fades from view as the expanding shell of debris expands, forming a supernova remnant. The nearest supernova to have been observed in the last few hundred years was spotted in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way, in 1987. Known as SN 1987A, the explosion was bright enough to be seen without a telescope. Because this event happened so close, in cosmic terms, it has been extensively studied over the twenty four years since the explosion, making this one of the best studied supernovae to date. Now, for the first time, the transition between two phases of supernova remnant evolution has been directly observed.
In the early stages of the expansion of the shell of debris, the light emitted comes mainly from the decay of certain radioactive nuclei created in the explosion itself, mainly nickel-56, nickel-57 and titanium-44. These nuclei are unstable and undergo a transformation into more stable nuclei in a process which releases energy. For a sample of any particular radioactive nucleus, the time taken for half of the nuclei in the sample to decay to a more stable form is known as the half-life. Between 1994 and 2001, observations showed that the decline in light detected from supernova 1987A matched what would be expected from the decay of titanium-44 nuclei. However, continued observations showed that the light output from the expanding remnant began to increase after 2001, with the brightness more than doubling by 2009.
In a paper published in the journal Nature on the 23rd of June, an international team led by Josefin Larsson at Stockholm University show evidence that this recent increase in brightness is due to the remnant entering the next stage of evolution, a transition which has not been directly observed before since most supernovae are observed in other galaxies and are just too far away. What their analysis suggests is that this increase in brightness is due to the ejecta (the expanding shell of debris) beginning to interact with the surrounding material. Surrounding the star before its explosion was the pre-existing interstellar material, a tenuous gas which exists between the stars, as well as the gas shed from the star itself in a stellar wind, but also a denser ring of material ejected from the star some 20,000 years before the final catastrophic explosion. Now, 24 years after the explosion was observed, astronomers are watching the outer parts of the ejecta starting to interact with this denser ring of material.
This interaction is causing the ring to brighten across the entire electromagnetic spectrum, and this increase in brightness shows that there must be a new source of energy, other than just radioactive decay. Larsson and her colleagues show that the most likely explanation for this brightening is the heating caused by X-rays which are being produced in the collision of the expanding ejecta with the 20,000 year old ring of stellar material. This is the first time the transition from supernova to supernova remnant has directly been observed, and the researchers suggest that this ongoing transition may reveal more about the structure and chemistry of the progenitor star as the collision moves further into the circumstellar ring.
Many hundreds of supernova events are detected each year, both by professional and amateur astronomers. In all known supernovae, the radiation we see comes from energy deposited in the outflowing ejected material by either radioactive decay of elements created in the explosion, heat deposited by the explosion shock in the envelope of the progenitor star, or in the interaction between the debris and slowly moving, hydrogen-rich circumstellar material ejected by the star prior to the explosion. But in the same issue of Nature, Robert Quimby and colleagues from the Palomar Transient Factory, report observations of a new class of luminous supernovae whose properties cannot be explained by any of these processes.
The Palomar Transient Factory (or PTF) is a fully-automated, wide-field survey of the sky, searching for supernovae and other, more exotic, transient events. As of today, the project has discovered 1207 supernovae since it began searching in 2009. Most of these fit with existing models of supernovae explosions, but this new class of supernova contains four newly discovered events from the PTF survey, and two previously unexplained highly luminous supernovae. These events are about ten times brighter than any known thermonuclear supernova and are bright in ultraviolet light for an unusually long period of time. The light fades at a rate which is inconsistent with the amount of radioactive elements created in the explosion, and results from the six supernova so far discovered suggest that the observed light is emitted by hydrogen-free material distributed over a large radius (10 billion kilometres) and expanding at high speeds (faster than 10,000 kilometres per second). Since most stars are made up of mostly hydrogen, this means that these stars must have lost their outer layers some time before the final explosion. Taken together, the properties of these six events do not match any existing class of supernova.
Any suggested mechanism for these events must explain the large amount of energy deposited in hydrogen-poor, rapidly expanding material. Two such explanations are proposed by the authors. The first is the explosion of an extremely massive star. Stars with initial masses of between 90 and 130 times that of the Sun are thought to undergo massive pulsations, driving off outer layers of stellar material at high speeds. Such stars evolve quickly and would soon use up their hydrogen, resulting in a core collapse supernova with a surrounding hydrogen-poor environment. The second suggestion is that the peak luminosity could be explained by the energy output from a compact spinning object such as a magnetar.
Whatever the cause, these long-lived and luminous events make them excellent targets for high redshift studies since they can be seen at great distances. As they light up their surroundings, they could be used to probe star-forming regions and primitive galaxies in the distant universe.
And finally: There were a couple of near misses this month, both on Earth and off it. On June 27th an asteroid the size of a small house came within 20,000 kilometres of the Earth. While this may sound like a long way, it is only about one 20th of the distance between the Earth and the Moon, close enough to be inside the orbit of some high-altitude communications satellites. Asteroid 2011 MD was only discovered on June 22nd, just five days before closest approach, by LINEAR, a pair of robotic telescopes in New Mexico which search the skies for such objects. Luckily for us, even if 2011 MD had entered the atmosphere it would have burned up in a spectacular fireball rather than causing the kind of destruction often seen in apocalyptic science fiction movies. Only a day later, the crew of the International Space Station had to evacuate the station due to a passing piece of debris which was only spotted 14 hours before closest approach. The unidentified object was large enough to cause serious damage to the station, currently home to six astronauts, but was spotted too late for the normal avoidance manoeuvres to be made, prompting the crew to take shelter in the attached Soyuz spacecraft as a precautionary measure. Luckily, the object passed within a few hundred metres of the station and within minutes the six-man crew were safely back at work.
Interview with Dr Giovanna Tinetti
Dr Giovanna Tinetti (University College London) researches exoplanets - planets orbiting around other stars. At the time of release there are 564 known exoplanets and Dr Tinetti begins this interview by explaining the different methods used to detect these planets. Instead of just trying to find these planets, Dr Tinetti's research is focused on characterising exoplanet atmospheres by observing them as they pass in front of, and behind, their parent star. She explains how studying the atmospheres can give us information about how the planets formed and even if there is life on them. We also talk about work done by Dr Tinetti's former PhD student, Dr David Kipping, looking at the possibility of detecting exomoons, especially using data from the Kepler mission. Finally Dr Tinetti tells us about a proposed future mission, EChO, which would be dedicated to studying exoplanet atmospheres. EChO is one of four missions currently under review by ESA and, if selected, would be launched between 2020 and 2022.
The Night Sky
Ian Morison tells us what we can see in the northern hemisphere night sky during July 2011.
The nights are getting slightly longer. One effect of this is that the night we see soon after sunset stays sort of the same throughout the late summer and autumn because as the stars move round a bit earlier by about 4 minutes per day, the sunset also gets a bit earlier so the same sort of things are visible. The brightest star in the July night sky is Arcturus in Bootes, which is the second brightest star in the northern night sky after Sirius. Up to the left of Arcturus is a little circlet of stars called Corona Borealis and over to its left, towards the bright star Vega in Lyra, is the constellation of Hercules. There are four stars at the heart of Hercules which form an asterism known as the keystone because of its shape. With binoculars or a small telescope, up the right hand side is a fuzzy object called M13, which is the best globular cluster we can see in the northern night sky. Often overlooked, just above the keystone, is a second globular cluster called M92, which can be found by scanning to the west from Vega. Below Hercules is a large constellation called Ophiuchus. Over to the east, fairly high up at 11pm in the middle of month, is Cygnus the swan with Deneb its brightest star, Lyra with Vega and Aquila with Altair - those three bright stars make up the summer triangle. Down to the left of Cygnus, across from Altair, is a nice trapezium of stars with a couple more making a tail, this is Delphinus the dolphin. All through the month, the Moon is at very low declination so doesn't rise very high above the horizon, resulting in the illusion that the Moon looks larger.
- Jupiter is a pre-dawn object but by the end of July it rises at midnight and will have an elevation of 50° at dawn. Its angular size is increasing to 40 arcseconds by the end of month. A small telescope will show the equitorial bands, look to see if the southern one has come back, and you'll also see the 4 Galilean moons as they weave their way around Jupiter.
- Saturn is still visible in the evening sky soon after twilight. It is still quite bright so a small telescope will pick it out before it is fully dark. The rings span about 35 arcseconds but the disk is only 17 arcseconds across. A telescope will see some markings on the surface and also pick up the moon Titan at magnitude +8 (binoculars in very good conditions should also pick out Titan).
- Mercury is just visible above the north-west horizon, its magnitude reduces through the month to +0.7. It reaches greatest elongation from the Sun on July 20 when it will have an elevation of 8° at sunset. Not a good month for Mercury, but have a look to see where the Sun sets on the horizon and the look in the same direction 45 minutes later.
- Mars is a pre-dawn object, but is at a low elevation. A good low eastern horizon and binoulars are probably needed to spot it. Its magnitude is +1.4 and it has an angular size of 4 arcseconds so unlikely to pick out any details. It will have an elevation of 29° at sunrise, it is moving through Taurus, and will get close to the boundary with Gemini by the end of the month.
- Venus is getting close in angle to the Sun. It might be glimpsed before dawn, low above the horizon, in the east-northeast at the beginning of the month. Its magnitude is -3.9, a telescope will see a virtually fully illuminated disk, 10 arcseconds across.
- Saturn is close to the double star called Porrima or Gamma Virginis. On a night of good seeing, a small telescope will split Porrima into two equal doubles. Find Saturn, and then go up a bit to the right to find Porrima. Saturn's rings have been getting slightly narrower in angle but now they are opening out again and continue to do so.
- The summer triangle contains two highlights this month that binoculars or a small telescope will help with. A third of the way from Altair to Vega is an asterism called Brocchi's Cluster or the Coathanger. Up to the left from the Coathanger is the star Albireo which forms the head of Cygnus the swan, and is also the base of the Northern Cross. A small telescope will show that it is a double star, one component is magnitude 3 and amber in colour, the other is magnitude 5 and blue-green.
- At dawn on July 27 there will be a thin cresent moon with Jupiter and Mars in the pre-dawn sky. Look east-southeast about 40 minutess before sunrise. Also spot the Pleiades and Hyades clusters in the same field of view up and to the right of Mars.
- Around the summer solstise is a good time to spot Noctilucent clouds. Look for them in the north in the deep twilight.
Unfortunately, the Carter Observatory have been unable to supply us with a southern night sky segment this month. We apologise for this and suggest you check out the night sky podcast from the Sydney observatory instead. Hopefully we will be back to normal in August.
Odds and Ends
The last ever space shuttle launch is scheduled for July 8. Space shuttle Atlantis will be launched on a 12 day mission (STS-135) to the International Space Station.
|Interview:||Dr Giovanna Tinetti, Jen Gupta and Mark Purver|
|Night sky:||Ian Morison|
|Presenters:||David Ault and Jen Gupta|
|Editors:||Jen Gupta, Megan Argo and Melanie Gendre|
|Intro/outro:||Dr Chris Lintott|
|Segment Voice:||Liz Guzman|
|Website:||Jen Gupta and Stuart Lowe|
|Cover art:||Image of the asteroid 2011MD on its near approach to Earth. CREDIT:: Nick Howes/Faulkes Telescope South/LCOGT|