• SAO Astronomy News

Semester 1, 2007

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27th May, 2007: Caves Found on Mars?

Proposed cave site detected by the HiRISE imager on the Mars Reconnaissance Orbiter.

Image Credits: NASA, JPL, University of Arizona

The HiRISE (High Resolution Imaging Science Experiment) camera on the Mars Reconnaissance Orbiter has detected a very dark spot on the surface of Mars that appears to be the entrance to a cavern or cave.

The entrance to the cave measures approximately 100 m across and was found on a dusty lava plain to the northeast of Arsia Mons. Even though the HiRISE camera is extremely sensitive and can see details in almost any shadow on Mars, it was unable to see any details in the dark region suggesting that it must be very deep. It was also unable to see any walls, suggesting that the walls of the cave must be either perfectly vertical, extremely dark, or overhanging.

20th May, 2007: Hubble Finds Dark Matter Ring

Hubble Space Telescope image of the galaxy cluster Cl 0024+17 superimposed with a blue map showing the cluster's dark matter distribution.

Image Credits: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University).

Astronomers using NASA's Hubble Space Telescope have detected a ring of dark matter around the galaxy cluster Cl 0024+17. This is the first time that dark matter has been detected with a unique structure that is different from both the gas and galaxies in the cluster. The results suggest that the ring was formed 1 to 2 billion years ago as a result of a collision with another galaxy cluster.

Dark matter, as its name suggest, cannot be seen by telescopes directly. Instead, we must resort to indirect means, for example, by measuring the effect it has on normal matter and light. To trace the distribution of dark matter in Cl 0024+17, astronomers observed how the light from distant background galaxies were being distorted by the cluster itself. In an effect known as gravitational lensing, the clusters mass distorts and smears the light from background galaxies into arcs and streaks that can be detected by the Hubble Space Telescope. By mapping the distorted light, astronomers can work out the cluster's mass and how the dark matter is distributed in the cluster. Surprisingly, in the case of Cl 0024+17, the dark matter formed a ring, 2.6 million light-years across, around the cluster. This is the first time that dark matter has been detected so largely separated from the galaxies and hot gas that make up the cluster.

It is believed that the dark matter ring was formed as a result of a head-on collision with another cluster approximately 1 to 2 billion years ago. Spectroscopic observations of the cluster's 3D structure seem to provide further evidence that such an event occurred. Furthermore, computer simulations of such an event using theoretical models also produce results which agree with the measured distribution of the dark matter.

13th May, 2007: A "Fossil" Star Found in our Galaxy!

Astronomers measured the abundance of radioactive elements in HE 1523-0901 to determine its age.

Image Credits: ESO.

Astronomers have found one of the oldest stars known to date. The star, HE 1523-0901, is in the Milky Way and is estimated to be 13.2 billion years old. With such an age, it would have formed just 500 million years after the Big Bang and must have formed very early in the life of our very own galaxy.

To measure the age of the star, astronomers used a technique similar to the carbon-14 dating method used to measure the age of archaeological finds here on Earth. In this case, however, they measured the abundance of radioactive isotopes of Uranium and Thorium. Such a feat can only be performed by the largest telescopes. So the astronomers used ESO's VLT (Very Large Telescope) to observe the star for a total of 7.5 hours with an ultraviolet spectrograph - allowing the spectral lines from the radioactive elements to be observed. This is the first time that more than a single cosmic clock has been measured for a star. In all, a total of six measurements were made and the results suggested that HE 1523-0901 was 13.2 billion years old. This is quite an amazing find considering that the Universe is believed to have existed for only 13.7 billion years!

6th May, 2007: Want Planets? Stay Out of the Danger Zone!

Planetary danger zones are shown (blue spheres) in this Spitzer Space Telescope infrared image of the Rosette Nebula .

Image Credits: NASA, JPL-Caltech, Z. Balog (Univ. of Ariz./Univ. of Szeged).

Are you a cool star in the nice side of a stellar nursery? Want to settle down and form a couple of planets? Well watch out, there are stars out there that could disrupt your plans! Astronomers have used data from NASA's Spitzer Space Telescope to map out planetary "danger zones" - regions where planet formation is disrupted. The results of this mapping indicate that cool stars need to be at least 1.6 light-years away from hot O-type stars if they want any chance of forming planets.

O-type stars are hot and massive stars that emit copious amounts of ultraviolet radiation. This radiation is strong enough to heat and evaporate dust and gas in protoplanetary disks around nearby stars - thus disrupting planet formation. Astronomers used Spitzer's infrared eyes to look for protoplanetary disks around 1000 stars in the Rosette Nebula - a turbulent star-forming region approximately 5200 light-years away. Their observations revealed that 45% of stars had protoplanetary disks if they were further than 1.6 light-years from an O-type star, whereas, of the stars that were closer, only 27% had disks. The percentage of stars with disks decreased significantly when they were closer to an O-type star.

The study has also found that protoplanetary disks evaporated faster in the "danger zone". Move a star two times closer to an O-type star and its disk will evaporate twice as fast. In some cases, the disk may disappear within a hundred thousand years! All is not lost however, some theories of planet formation suggest that Jupiter-sized gas planets may form quickly, within a million years. Once formed, these larger planets could survive the onslaught of a nearby O-type star. Unfortunately smaller planets in the process of forming would not fare so well; they would be swept away forever.

30th April, 2007: A Goldilocks Planet?

Gliese 581 is the host of the most Earth-like exoplanet found to date.

Image Credits: ESO.

Astronomers have found a new exoplanet around the red dwarf star Gliese 581. Exoplanets have been found around many stars in the past, in fact Gliese 581 is already known to be the home of a 15 Earth-mass planet and an 8 Earth-mass planet. What makes this discovery special is that it is the most Earth-like exoplanet discovered to date. Furthermore, the planet appears to be situated within the habitable zone of the Gliese 581 system - that is, a region in which liquid water, and hence life, could exist.

The newly discovered planet has approximately 1.5 times the radius of the Earth and weighs in at about 5 Earth masses. Gliese 581 c, as it is formally known, is approximately 14 times closer to its star than the Earth is from the Sun, thus it has a rather speedy orbit that it completes in only 13 days. While its proximity to its host star may make it seem like an inhospitable place, Gliese 581 is much smaller and colder than the Sun. As a result, it is estimated that the mean temperature on Gliese 581 c lies between 0 and 40 degrees Celsius. At this temperature, liquid water and perhaps even life are a possible.

The new exoplanet was discovered using the HARPS (High Accuracy Radial Velocity for Planetary Searcher) instrument on the ESO 3.6-m telescope at La Silla in Chile. HARPS ability to measure velocities of stars with a very fine precision allows it to detect exoplanets as they tug their host star to and fro over an orbital period.

23th April, 2007: A New Kind of Lighthouse

Two radio images of the binary L dwarf 2MASSW J0746425+2000321 (marked by the arrow) taken by the VLA at 4.88 GHz - the source to the upper left hand side of the target is a field source. The left hand image was taken during the inter-pulse phase and in the right hand image taken during one of the pulses.

Image Credits: Hallinan et al., NRAO/AUI/NSF.

Brown dwarfs, those odd-ball objects that are too small to be stars but too large to be planets, were not thought to be capable of emitting any significant amounts of radio waves. However, a team of astronomers has recently discovered that brown dwarfs, like pulsars, can emit extremely bright pulses of radio waves.

As late as 2001 brown dwarfs were thought to have no significant emissions at radio wavelengths. However, when a group of summer students decided, against conventional wisdom, to observe a brown dwarf with the Very Large Array (VLA) radio telescope they discovered a strong flare of radio emission from the object - the finding surprised even the most seasoned of astronomers. Now a team of astronomers, also using the VLA, found three brown dwarfs that emit strong, repeated pulses of radio waves (a movie of one of these pulsations can be found here). They concluded that these pulses came from beams emitted from the magnetic poles of the brown dwarfs. This is similar to the beamed emission from pulsars, however, the characteristics of the emissions were more like that seen in planets such as Jupiter and the Earth. In brown dwarfs, as with planets, it is believed that radio waves produced by electrons interacting with the object's magnetic fields are amplified by natural masers - just like lasers can amplify light waves.

The findings suggest that brown dwarfs are a kind of missing link between planets and pulsars. They produce radio emissions of intermediate strength and are also at a much slower rate than those of pulsars i.e. a pulse roughly once every two to three hours compared to several times a second to hundreds of times a second for a pulsar. Although it is suspected that the mechanisms that produce radio waves is different to that in pulsars, it is hoped that there may be enough similarities to help better understand how pulsars work.

16th April, 2007: The Perfect Cosmic Red Square

The recently discovered Red Square Nebula around the dying star MWC 922.

Image Credits: Peter Tuthill, Sydney University Physics Dept, Palomar and W.M. Keck observatories.

The conspiracy theorists are going to have a field day with this - first we had a mysterious hexagon on Saturn, now astronomers have spotted a stunning red square nebula around a dying star. But to quickly quash the conspiracy bandwagon before it gets a foothold, it turns out that the nebula is not all that mysterious after all. The red square is in fact a bipolar nebula formed by a dying star as it spews out its innards from opposite poles in space. What we see are the resulting twin opposed conical cavities as seen exactly side-on.

The nebula was found around the hot star MWC 922 in the constellation Serpens using near-infrared imaging (1.6 micron) and adaptive optics on the 5 m Hale Telescope at Caltech's Palomar Observatory and the Keck-2 Telescope in Hawaii. The adaptive optics help reduce atmospheric effects by rapidly deforming the mirrors to correct for image distortions caused by the Earth's atmosphere. As a result, a high level of detail was attained in the final image of the nebula. What surprised astronomers was the startling degree of symmetry and the level of intricate linear form in the nebula - making it the most symmetrical object of comparable complexity ever imaged!

Twin opposed cones as see from different viewing angles.

Image Credits: Peter Tuthill, Sydney University Physics Dept, Palomar and W.M. Keck observatories.

The geometry of the nebula is consistent with a bipolar nebula seen directly side-on. This can be more easily seen in animation on the left which shows the twin opposed cones as they rotate. When we view the cones at 90 degrees to the axis we can visualise that the bright bars seen edge-on become elliptical rings encircling the polar axis of the system.

It is also interesting to note that the observed bipolar structure of the nebula is also consistent with what was revealed in the supernova SN1987A. Perhaps one day in the future the star MWC 922 may explode as a supernova and light up the outer reaches of its nebula into an incandescent display. For now, however, the red square nebula is perhaps one of the best laboratories yet discovered for trying to understand the physics that generate sharp polar ring systems such as the one around SN1987A.

9th April, 2007: A Tatooine Sunset

Astronomers have found dozens of planetary disks around binary stellar systems with separations of less than 500 AU.

Image Credits: NASA/JPL-Caltech/R. Hurt (SSC).

Wouldn't it be amazing to see a double sunset just like those portrayed in science-fiction movies - such as the sunset on Luke Skywalker's home planet Tatooine in the movie "Star Wars - A New Hope". Well, astronomers have used the Spitzer Space Telescope to find that places such as this exist around 40 percent of the close binary star systems that they surveyed.

Astronomers have known for quite some time that exceptionally wide binary systems could form planetary systems. In such systems the two stars are typically more than 1000 AU apart (one AU, or Astronomical Unit, is the average distance between the Earth and the sun). In fact, of the roughly 200 exo-planets planets discovered so far, about 50 of them were found in wide binary systems.

The recent Spitzer survey of 69 binary systems with separations of less than 500 AU found that approximately 40 percent of these systems showed evidence of planet-forming debris disks. These observations suggest that planetary systems appear to be at least as likely around binary stars as they are around single stars! However, astronomers were shocked to find that the disks were even more common (~60 percent) around the tightest binaries (separated by less that 3 AU). In these extremely close binaries the observed disks orbited both members of the binary - any planets in these disks would have a double sunset just like the one seen on the fictional planet Tatooine. It's not entirely clear why planet formation seems to favour tight binaries over single stars and wide binaries - one possibility is that they may be dustier locations.

The Spitzer survey also hints that not all binary systems are ideal for planet formation. In particular, binaries with separations between 3-50 AU appear to have far fewer disks. This tends to suggest that planets only seem to form around very tight binaries and fairly wide binaries but not in-between.

2nd April, 2007: Basic(?) geometry on Saturn.

Cassini image of a mysterious hexagon-shaped feature circling the north pole of Saturn.

Image Credits: NASA/JPL/SSI.

Perhaps Johannes Kepler was onto something when he thought that geometry somehow played an important role in the intricate workings of the Solar System. He eventually gave up on this idea to move onto other (more correct) ideas but recent pictures captured by NASA's Cassini mission would likely have caught his attention. Cassini has taken an image of an odd hexagonally-shaped feature circling the northern pole of Saturn.

At the moment it is winter at the Saturnian north pole so the feature is hidden in darkness and can't be observed with Cassini's visual cameras. Instead, it was revealed by its infrared mapping spectrometer which is able to image in both day and night conditions and can also peer deeper below the surface clouds. Interestingly, the detection of the hexagonal feature is not new - astronomers were first intrigued by its presence when NASA's Voyager 1 and 2 spacecraft imaged it over 20 years ago. At the time it was believed to be linked to Saturn's radio emissions or to auroral activity which just happens to lie directly above this feature. The new images taken by Cassini suggest that, whatever the cause, it is a long-lived feature. Furthermore, based on these images and information on the depth of the feature, scientists now no longer believe it is linked to radio emissions or auroral activity. The scientists are now not only intrigued but also perplexed!

The hexagonal feature appears to have remain fixed with Saturn's rotation rate and axis since the first images were taken by Voyager nearly three decades ago. While the actual rotation rate of the deep atmosphere and interior of Saturn are currently uncertain it is hoped that once the dynamical nature of the hexagon feature is understood this may give a clue to the true internal rotation rate of Saturn.

26th March, 2007: Weather forecast: Turbulence, strong winds and particle showers.

Hinode Solar Optical Telescope image showing the structure of the solar magnetic field rising vertically from a sunspot.

Image Credits: Hinode JAXA/NASA.

In these modern times we often take the humble sun for granted and spend much of our time looking at more distant stars. To most of us the sun appears as a never-changing bright disk. In reality, it is a turbulent and dynamic place with explosions that propagate through the different layers of its atmosphere. These release energetic particles and electromagnetic radiation outward into the solar system creating what is known as "space weather". Up until the last century we would have been oblivious to the influence of space weather but today, with our reliance on modern technology, it can disrupt communications, global navigation systems, cause power black-outs and even threaten the lives of astronauts. For this reason there is a great need to better understand the processes that affect space weather. The international spacecraft Hinode is helping researchers with this task by providing detailed observations of the sun in optical, x-ray and ultraviolet wavelengths.

NASA recently released some of the first detailed images taken by Hinode. The resolution of these images allow tiny granules of hot gas to be observed as they rise and fall in the sun's atmosphere. Already these images have shown that the sun's magnetic field is much more turbulent and dynamic than previously suspected.

The three different wavelengths observed by Hinode will be used to study different layers of the sun - such as the photosphere, the corona and the outer atmosphere of the sun that extends into the solar system. In particular, the interplay between these layers during the explosive events that affect space weather will be closely monitored. It is currently believed that explosive solar events are caused by magnetic reconnection - a process by which magnetic field lines are squeezed together and then suddenly reconfigured, releasing large amounts of energy in the process. It is hoped that this theory may be verified by using Hinode to monitor the evolution of solar structures that outline the magnetic field before, during and after such an event.

19th March, 2007: Bootes full of holes!

A 9.3 square-degree view of supermassive black holes in the Bootes Constellation as seen by NASA's Chandra X-ray observatory.

Image Credits: X-ray: NASA/CXC/CfA/R.Hickox et al.; Moon: NASA/JPL.

... they're black and they're supermassive to boot! Astronomers have used data from NASA's Chandra X-ray Observatory, the Spitzer Space Telescope and ground-based optical telescopes to complete a census of supermassive black holes in a region within the Bootes constellation. Using this multi-wavelength approach over 1300 of these giants were found but the results have raised doubts about our understanding of the environment that surrounds them.

Supermassive black holes form the heart of Active Galactic Nuclei (AGN); as material falls into them they generate huge amounts of light that can be detected at different wavelengths. Current models of AGN assume that a doughnut shaped region, or torus, of dense gas surrounds the supermassive black hole. Depending on the orientation of this torus from our viewpoint it can block the view of the black hole by varying degrees; some black holes will be fully obscured, some will be "naked" but most are expected to be partially obscured. To find all AGN, regardless of how much they are obscured, astronomers used observations at several different wavelengths to determine the nature of the emitting object. For example, observations in infrared (Spitzer Space Telescope) were used to separate the AGN from stars and galaxies. Optical and X-ray (Chandra X-ray Observatory) observations were then be used to verify that these objects were in fact AGN and to measure the amount by which they were obscured.

Overall, the survey found more than 600 obscured and 700 unobscured AGN located between 6 and 11 billion light-years from Earth in the region towards Bootes. What surprised the astronomers involved in this survey was that very few of the observed AGN were partially obscured - when it comes to AGN it seems like it is an all or nothing affair, either they are totally obscured or they're in their birthday suit! This finding goes against what is expected from current AGN models and so, at least for now, the ball is back in the theoreticians court to try and explain what's going on.

12th March, 2007: Solar powered asteroid spinner

Left: Radar image of the asteroid (54509) 2000 PH5; Centre: shape-model fit to the image; Right: 3-D model of the asteroid. Click on the image to view an animation.

Image Credits: ESO, animation created by E. Lenc.

Astronomers have observed evidence of the Yarkovsky-O'Keefe-Radzievskii-Paddack effect for the first time since it was first theorised several decades ago! I can just hear you now, jumping up and down with excitement, it's wonderful, it's fantastic, it's incredible, oh my - it's, it's, well, um, hold on - what on (or off) Earth is the Yarkovsky-O'Keefe-Radzievskii-Paddack effect? The Yarkovsky-O'Keefe-Radzievskii-Paddack effect, simply known as the YORP effect by their friends, is the recoil caused when heat is released from an asteroid or meteoroid surface as it is warmed by the Sun. This incredibly weak force can over millions of years increase or decrease the rate of spin of the object that is undergoing this effect.

To detect the YORP effect, astronomers used extensive optical and radar imaging of a small near-Earth asteroid, known as (54509) 2000 PH5, over a 4 year time-span. The relatively small size of this asteroid, it has a diameter of just 114 metres, made it would be more susceptible to the effect. Optical observations were used to measure slight brightness variations as the asteroid rotated, while at the same time radar observations were used to observe radio pulse echoes. By combining the two results, astronomers were able to construct a 3-D model of the asteroid with sufficient detail to allow an accurate comparison between theory and observation. After careful observation they determined that the asteroid was steadily decreasing its period of rotation by one millisecond each year - this rate of decrease was consistent with what was expected by YORP theory.

Over millions of years the YORP effect can cause asteroids to spin so fast that they change shape or even break apart, leading to a double system. The YORP effect also plays an important role in changing the orbits of asteroids, so an understanding of this effect is needed to identify those asteroids that may pose a potential threat to the Earth in future.

5th March, 2007: Just passing by ...

Tvashtar's Plume on Io as observed by NASA's New Horizons spacecraft.

Image Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Now that we've got Comet McNaught out of our (inner solar) system we can turn our attention to some recent successful milestones made by space travellers of a more artificial kind. The speedy New Horizons spacecraft has just made a successful flyby of Jupiter and in the process has received a helpful gravitational boost in its journey toward the dwarf planet Pluto. During the flyby New Horizons took the opportunity to put some of its science instruments to the test and captured a stunning image of an enormous dust plume, more than 240 kilometres high, erupting from the volcano Tvashtar on Jupiter's moon Io. The spacecraft also took images of Jupiter and several of its other moons. Many more images will become available over the following months as they are downloaded from the spacecraft.

Mars as observed by ESA's Rosetta spacecraft as it skims past the planet on its way to comet 67P Churyumov-Gerasimenko.

Image Credits: CIVA / Philae / ESA Rosetta.

At about the same time that New Horizons was getting a bit of a boost from Jupiter, ESA's Rosetta spacecraft was doing the same around Mars. It recently completed a flyby of the planet and also put its instruments to the test. Approximately 1000 kilometres from the surface, the spacecraft took a "self portrait" of itself with Mars in the background - the image shows part of the spacecraft and one of its solar arrays. The spacecraft is now heading back to Earth for yet another gravitational assist. In total, the spacecraft will need three gravitational assists with the Earth and one with Mars to reach its destination - comet 67P Churyumov-Gerasimenko in 2014.

On the surface of Mars, the Mars Exploration Rovers are still going strong and are now into their fourth year of exploring the Martian surface. A few weeks back Opportunity clocked up 10 kilometres on its odometer. After a long and perilous journey it is now exploring the rim of Victoria Crater before making the plunge down into the crater itself. Meanwhile, Spirit is also soldiering on, albeit with one less wheel and the odd spurious "warm" reset. While this has hindered its progress slightly controllers have managed to perfect the art of driving with only five wheels.

An image of Saturn and its rings taken by the Cassini spacecraft.

Image Credits: NASA/JPL/Space Science Institute.

Meanwhile, the Cassini spacecraft has taken a stunning image of Saturn and its ring system from a vantage point approximately 40 degrees above the ringplane. The image is in fact a mosaic composed of 36 images taken over a period of 2.5 hours. The primary aim of this imaging sequence was to capture the ring system in its entirety. The images were taken at a distance of approximately 1.23 million kilometres from Saturn. Images were also taken at an elevation of 60 degrees, the highest elevation to date.

26th February, 2007: Welcome to SAO Semester 1, 2007

A photo of Comet McNaught taken by its discoverer - Robert H. McNaught.

Image Credits: R.H. McNaught, Siding Spring Observatory.

A spectacular comet, one of the brightest in forty years, graced our skies in mid January of this year. Comet McNaught, known more poetically by its designation C/2006 P1, was discovered fairly recently by Scottish-Australian astronomer Robert H. McNaught on the 7th of August, 2006. Since then it made its way into the inner solar system where it swung past the sun at a distance of only 0.17 AU on January 12th, 2007 - an event that did not go unnoticed by NASA's Solar and Heliospheric Observatory (SOHO) where it appeared so bright that it saturated its camera. From our vantage point on Earth the comet appeared just as spectacular, reaching an estimated maximum apparent magnitude of -6. Comet McNaught is a non-periodic comet so I'm afraid to say that we will never see it again - I sure hope that you all went out to see it while you had the chance!

It has been said, in medieval times, that comets warned of impending doom - what utter rubbish, as an enlightened scientist I can find no evidence that supports this ancient belief. Instead we should stand in awe at their beauty and appreciate how fortunate we are to have an opportunity to observe one of these spectacular objects with our very own eyes.

In other news, NASA's Mars Reconnaissance Orbiter is suffering a number of problems with its instruments. The problems began late November 2006 when one of its 14 camera detecter pairs were showing increased noise. Images taken mid-January (just as comet McNaught was at its brightest) revealed signs of the same problem in five other detectors and there are concerns that the situation may worsen with time. The problems don't stop there, the Mars Climate Sounder instrument (which maps the temperature, ice clouds and dust distributions in the atmosphere) is suffering a glitch that causes position errors in the measurements it takes. The errors have become so frequent that the instrument has been temporarily stowed while the science team looks into the problem.

More bad news from Mars, NASA lost contact with the Mars Global Surveyor on November 2nd (3 months after comet McNaught was discovered) just as they were days away from celebrating its tenth year of service - it was only designed to operate for two years! It is believed that a failure prevented the spacecraft's solar panels from tracking the sun, resulting in an eventual power failure.

To add to these woes, one of the Hubble Space Telescopes (HST) main science instruments went offline on January 27th (just after comet McNaught rounded the Sun). The Advanced Camera for Surveys (ACS), which has produced many of the images that the HST is famous for, appears to have lost its power feed and has stopped functioning. Although there is little hope that the ACS will become operational again the remaining instruments on the HST are still functional. Furthermore, with the HST servicing mission targeted to fly in September of 2008 it is likely that we will see several newer and better instruments installed in the not too distant future.

Finally, a Zenit-3SL rocket carrying a Dutch communications satellite detonated on the Sea Launch platform on the 30th of January. It was the first failure of such a rocket in nearly seven years and the cause of the most recent failure is not known.

Like I said, there is absolutely no evidence that comets warn of impending doom!