Metallic Stars Yield Planets

Image credit: NASA

A survey of stars in our neighbourhood has revealed those rich in metals, such as iron and titanium, are five times more likely to have planets orbiting them. The survey of 61 stars with planets and 693 stars without, revealed a distinct difference in the ‘metalicity’ of stars. Debra Fisher from the University of California, Berkley, says, “If you look at the metal-rich stars, 20 percent have planets. That’s stunning.” (contributed by Darren Osborne)

A comparison of 754 nearby stars like our sun – some with planets and some without – shows definitively that the more iron and other metals there are in a star, the greater the chance it has a companion planet.

“Astronomers have been saying that only 5 percent of stars have planets, but that’s not a very precise assessment,” said Debra Fischer, a research astronomer at the University of California, Berkeley. “We now know that stars which are abundant in heavy metals are five times more likely to harbor orbiting planets than are stars deficient in metals. If you look at the metal-rich stars, 20 percent have planets. That’s stunning.”

“The metals are the seeds from which planets form,” added colleague Jeff Valenti, an assistant astronomer at the Space Telescope Science Institute (STScI) in Baltimore, Md.

Fischer will present details of the analysis by her and Valenti at 1:30 p.m. Australian Eastern Standard Time (AEST) on Monday, July 21, at the International Astronomical Union meeting in Sydney, Australia.

Iron and other elements heavier than helium – what astronomers lump together as “metals” – are created by fusion reactions inside stars and sown into the interstellar medium by spectacular supernova explosions. Thus, while metals were extremely rare in the early history of the Milky Way galaxy, over time, each successive generation of stars became richer in these elements, increasing the chances of forming a planet.

“Stars forming today are much more likely to have planets than early generations of stars,” Valenti said. “It’s a planetary baby boom.”

As the number of extrasolar planets has grown – about 100 stars are now known to have planets – astronomers have noticed that stars rich in metals are more likely to harbor planets. A correlation between a star’s “metalicity” – a measure of iron abundance in a star’s outer layer that is indicative of the abundance of many other elements, from nickel to silicon – had been suggested previously by astronomers Guillermo Gonzalez and Nuno Santos based on surveys of a few dozen planet-bearing stars.

The new survey of metal abundances by Fischer and Valenti is the first to cover a statistically large sample of 61 stars with planets and 693 stars without planets. Their analysis provides the numbers that prove a correlation between metal abundance and planet formation.

“People have looked already in fair detail at most of the stars with known planets, but they have basically ignored the hundreds of stars that don’t seem to have planets. These under-appreciated stars provide the context for understanding why planets form,” said Valenti, who is an expert at determining the chemical composition of stars.

The data show that stars like the sun, whose metal content is considered typical of stars in our neighborhood, have a 5 to 10 percent chance of having planets. Stars with three times more metal than the sun have a 20 percent chance of harboring planets, while those with 1/3 the metal content of the sun have about a 3 percent chance of having planets. The 29 most metal-poor stars in the sample, all with less than 1/3 the sun’s metal abundance, had no planets.

“These data suggest that there is a threshold metalicity, and thus not all stars in our galaxy have the same chance of forming planetary systems,” Fischer said. “Whether a star has planetary companions or not is a condition of its birth. Those with a larger initial allotment of metals have an advantage over those without, a trend we’re now able to see clearly with this new data.”

The two astronomers determined metal composition by analyzing 1,600 spectra from more than 1,000 stars before narrowing the analysis to 754 stars that had been observed long enough to rule a gas giant planet in or out. Some of these stars have been observed for 15 years by Fischer, Geoffrey Marcy, professor of astronomy at UC Berkeley, and colleague Paul Butler, now at the Carnegie Institution of Washington, in their systematic search for extrasolar planets around nearby stars. All 754 stars were surveyed for more than two years, enough time to determine whether a close-in, Jupiter-size planet is present or not.

Though the surfaces of stars contain many metals, the astronomers focused on five – iron, nickel, titanium, silicon and sodium. After four years of analysis, the astronomers were able to group the stars by metal composition and determine the likelihood that stars of a certain composition have planets. With iron, for example, the stars were ranked relative to the iron content of the sun, which is 0.0032%.

“This is the most unbiased survey of its kind,” Fischer emphasized. “It is unique because all of the metal abundances were determined with the same technique and we analyzed all of the stars on our project with more than two years of data.”
Fischer said the new data suggest why metal-rich stars are likely to develop planetary systems as they form. The data are consistent with the hypothesis that heavier elements stick together easier, allowing dust, rocks and eventually planetary cores to form around newly ignited stars. Since the young star and the surrounding disk of dust and gas would have the same composition, the metal composition observed from the star reflects the abundance of raw materials, including heavy metals, available in the disk to build planets. The data indicate a nearly linear relationship between amount of metals and the chance of harboring planets.

“These results tell us why some of the stars in our Milky Way galaxy have planets while others do not,” said Marcy. “The heavy metals must clump together to form rocks which themselves clump into the solid cores of planets.”

The research by Fischer and Valenti is supported by the National Aeronautics and Space Administration, the National Science Foundation, the Particle Physics and Astronomy Research Council (PPARC) in the United Kingdom, the Anglo-Australian Observatory, Sun Microsystems, the Keck Observatory and the University of California’s Lick Observatories.

Original Source: Berkeley News Release

Fewer Asteroids Threaten the Earth

Image credit: NASA

Researchers have built a computer simulation that better predicts how large asteroids will interact with the Earth’s atmosphere. They found that more asteroids blow up in the atmosphere than previously thought, reducing the risk of them hitting populated areas or causing tidal waves. Their model says that an asteroid has to be 200 metres in diameter or above before it can get through the atmosphere, and these only hit the Earth once every 170,000 years.

Researchers from Imperial College London and the Russian Academy of Sciences have built a computer simulation that predicts whether asteroids with a diameter up to one kilometre (km) will explode in the atmosphere or hit the surface.

The results indicate that asteroids with a diameter greater than 200 metres (the length of two football pitches) will hit the surface approximately once every 160,000 years – way down on previous estimates of impacts every 2,500 years.

The findings also predict that many more asteroids blow up in the atmosphere than previous estimates, which means the hazard posed by impact-generated tidal waves or tsunamis is lower than previous predictions. The researchers suggest that proposals to extend monitoring of Near Earth Objects (NEO) to include much smaller objects should be reviewed.

Dr Phil Bland of Imperial’s Department of Earth Science and Engineering and a Royal Society University Research Fellow, said:

“There is overwhelming evidence that impacts from space have caused catastrophes for life on Earth in the past, and will do so again.

“On the Moon it’s easier to track the number, frequency and size of collisions because there is no atmosphere, so everything hits the surface. On Earth the atmosphere acts like a screen and geological activity erodes many craters too.

“Massive impacts of the type thought to have wiped out the dinosaurs leave an indelible print on the Earth but we have not been able to accurately document the effect of smaller impacts. Now, we have a handle on the size of ‘rock’ we really need to worry about and how well the Earth’s atmosphere protects us.”

When small asteroids hit the atmosphere the two forces collide like two objects smashing together, which often breaks the asteroid into fragments. Until now, scientists have relied on the ‘pancake’ model of asteroid impact to calculate whether the asteroid will explode in the atmosphere. This treats the cascade of fragments as a single continuous liquid that spreads out over a larger area – to form a ‘pancake’. But a new model known as the ‘separate fragment’ (SF) model, which was developed by co-author of the study, Dr Natalya Artemieva of the Russian Academy of Science, has challenged this approach.

“While the pancake model can accurately predict the height from the Earth’s surface at which the asteroid will break up, it doesn’t give an accurate picture of how the asteroid will impact,” explains Dr Bland. “The SF model tracks the individual forces acting on each fragment as it descends through the atmosphere.”

To create a more accurate model of how asteroids interact with the atmosphere the researchers ran more than 1,000 simulations using both models. Objects made of either iron or stone, known as ‘impactors’, were used to reflect the composition of asteroids and experiments were run with varying diameters up to 1 km.

The researchers found the number of impacts for iron impactors were comparable using both models. For stone the pancake model significantly overestimated the survivability rate across the range used.

The SF simulations also allowed the researchers to define the different styles of fragmentation and impact rates for iron and stone, which correspond closely with crater records and meteorite data.

“Our data show that over most of the size range we investigated stony asteroids need to be 1,000 times bigger than the iron ones to make a similar sized crater. Much larger objects are disrupted in the atmosphere than previously thought.

“But we are not out of the woods yet,” added Dr Bland “asteroids that fragment in the atmosphere still pose a significant threat to human life.”

Dr Phil Bland is a member of the Meteorite and Impact Group that includes scientists from Imperial College London and the Natural History Museum.

Original Source: Imperial College News Release

Shuttle Program’s Flaws Will Be Revealed

Investigators working to determine the cause of the space shuttle Columbia disaster are expecting to reveal some serious problems with NASA’s program that oversees shuttle safety. The investigation team interviewed 72 NASA employees and contractors over a period of months and learned there were serious gaps in the agency when it came to safety. Some inspections had been removed to cut costs; quality assurance staff weren’t allowed to perform “spot checks”; and the agency was using hopelessly outdated testing equipment.

Mars Express Says Goodbye to the Earth and Moon

Image credit: ESA

Now well on its way to the Red Planet, the European Space Agency’s Mars Express spacecraft took a farewell image of the Earth and Moon. The photo was snapped on July 3 when the spacecraft was 8 million kilometres away from the Earth. The picture was taken as part of a series of tests the ESA is doing to make sure the Mars Express’ cameras and instruments are working properly. Once it reaches Mars in late December, the spacecraft will be able to resolve objects on the surface of Mars as small as 2 metres.

A unique view of our home planet and its natural satellite ? the Moon – is one of the first data sets coming from ESA’s Mars Express.

?It is very good news for the mission,? says ESA’s Mars Express Project Scientist, Agustin Chicarro. These and other data, such as those recording the major constituents of Earth as seen from space, are the actual proof that the instruments on board Mars Express, launched 2 June 2003, are working perfectly.

The routine check-outs of Mars Express’s instruments and of the Beagle-2 lander, performed during the last weeks, have been very successful. “As in all space missions little problems have arisen, but they have been carefully evaluated and solved. Mars Express continues on its way to Mars performing beautifully”, comments Chicarro.

The views of the Earth/Moon system were taken on 3 July 2003 by Mars Express’s High Resolution Stereo Camera (HRSC), when the spacecraft was 8 million kilometres from Earth. The image taken shows true colours; the Pacific Ocean appears in blue, and the clouds near the Equator and in mid to northern latitudes in white to light grey. The image was processed by the Instrument Team at the Institute of Planetary Research of DLR, Berlin (Germany). It was built by combining a super resolution black and white HRSC snap-shot image of the Earth and the Moon with colour information obtained by the blue, green, and red sensors of the instrument.

?The pictures and the information provided by the data prove the camera is working very well. They provide a good indication of what to expect once the spacecraft is in its orbit around Mars, at altitudes of only 250-300 kilometres: very high resolution images with brilliant true colour and in 3D,? says the Principal Investigator of the HRSC, Gerhard Neukum, of the Freie Universit?t of Berlin (Germany). This camera will be able to distinguish details of up to 2 metres on the Martian surface.

Another striking demonstration of Mars Express’s instruments high performance are the data taken by the OMEGA spectrometer. Once at Mars, this instrument will provide the best map of the molecular and mineralogical composition of the whole planet, with 5% of the planetary surface in high resolution. Minerals and other compounds such as water will be charted as never before. As the Red Planet is still too far away, the OMEGA team devised an ingenious test for their instrument: to detect the Earth?s surface components.

As expected, OMEGA made a direct and unambiguous detection of major and minor constituents of the Earth?s atmosphere, such as molecular oxygen, water and carbon dioxide, ozone and methane, among other molecules. “The sensitivity demonstrated by OMEGA on these Earth spectra should reveal really minute amounts of water in both Martian surface materials and atmosphere,” says the Principal Investigator of OMEGA, Jean Pierre Bibring , from the Institut d’Astrophysique Spatiale, Orsay, France.

The experts will carry on testing Mars Express?s instruments up till the arrival to the Red Planet, next December. The scientists agree on the fact that these instruments will enormously increase our understanding of the morphology and topography of the Martian surface, of the geological structures and processes – active now and in the past, and eventually of Mars?s geological evolution. With such tools, Mars Express is also able to address the important ?water? question, namely how much water there is today and how much there was in the past. Ultimately, this will also tell us whether Mars had environmental conditions that could favour the evolution of life.

Original Source: ESA News Release

SOHO is Back in Business

Image credit: ESA/NASA

ESA/NASA’s SOHO spacecraft is back to full capacity after a 9-day long blackout. On June 19, the pointing mechanism on the spacecraft’s high-gain antenna malfunctioned; however, controllers were able to retrieve data through its low-gain antenna using larger receiving dishes on Earth. The spacecraft was repositioned this week to let its antenna point directly at Earth. By repositioning it every three months, mission controllers don’t expect they will lose more than a fraction of data, allowing the spacecraft to continue operations for another five years.

ESA/NASA’s solar watchdog, SOHO, is back to full operation after its predicted 9-day-long high-gain antenna blackout. Engineers and scientists are now confident that they understand the situation and can work around it in the future to minimise the data losses.

Since 19 June 2003, SOHO’s high-gain antenna (HGA), which transmits high-speed data to Earth, has been fixed in position following the discovery of a malfunction in its pointing mechanism. This resulted in a loss of signal through SOHO’s usual 26-metre ground stations on 27 June 2003. However, 34-metre radio dishes continued to receive high-speed transmissions from the HGA until 1 July 2003.

Since then, astronomers have been relying primarily on a slower transmission rate signal, sent through SOHO’s backup antenna. It can be picked up whenever a 34-metre dish is available. However, this signal could not transmit all of SOHO’s data. Some data was recorded on board, however, and downloaded using high-speed transmissions through the backup antenna when time on the largest, 70-metre dishes could be spared.

SOHO itself orbits a point in space, 1.5 million kilometres closer to the Sun than the Earth, once every 6 months. To reorient the HGA for the next half of this orbit, engineers rolled the spacecraft through a half-circle on 8 July 2003. On 10 July, the 34-metre radio dish in Madrid re-established contact with SOHO’s HGA. Then on the morning of 14 July 2003, normal operations with the spacecraft resumed through its usual 26-metre ground stations, as predicted.

With the HGA now static, the blackouts, lasting between 9 and 16 days, will continue to occur every 3 months. Engineers will rotate SOHO by 180 degrees every time this occurs. This manoeuvre will minimise data losses. Stein Haugan, acting SOHO project scientist, says “It is good to welcome SOHO back to normal operations, as it proves that we have a good understanding of the situation and can confidently work around it.”

Original Source: ESA News Release

Clusters without a Home

Image credit: Hubble

Thousands of globular star clusters wander aimlessly between galaxies, in what was once thought to be ’empty space’. This is the finding of a joint US-UK project announced today at the International Astronomical Union General Assembly in Sydney. The group, lead by Dr. Michael West of the University of Hawaii, believes these clusters were ‘torn’ away from their parent galaxies and now drift as orphans. (contributed by Darren Osborne)

US and UK astronomers have discovered a population of previously unknown star clusters in what was thought to be the empty space between galaxies. The research is being presented today at the International Astronomical Union?s 25th General Assembly being held in Sydney, Australia, by Dr. Michael West of the University of Hawaii.

Most galaxies are surrounded by tens, hundreds or even thousands of ancient star clusters, which swarm around them like bees around a hive. Our own Milky Way galaxy has about 150 of these ?globular clusters?, as they are called. Globular clusters are systems of up to a million stars compacted together by gravity into dense sphere-shaped groupings. Studies of globular clusters have provided many important insights over the years into the formation of their parent galaxies.

The discovery of this new type of star cluster was made using images obtained last year with the Hubble Space Telescope and the giant 10-meter Keck Telescope on Mauna Kea, Hawaii. ?We found a large number of ?orphaned? globular clusters,? said Dr West. ?These clusters are no longer held within the gravitational grip of galaxies, and seem to be wandering freely through intergalactic space like cosmic vagabonds.?

Although the lonely existence of such star clusters had been predicted for half a century, it is only now that astronomers have finally been able to confirm their existence. Dr West?s team published preliminary findings about its discovery in April this year, and is today presenting new results at the International Astronomical Union?s 25th General Assembly, being held in Sydney, Australia.

?The new data from the Hubble Space Telescope and Keck Telescope confirm our discovery, and are providing new insights to the origin of these objects,? said Dr West.

According to West, these globular star clusters probably once resided in galaxies just like most of the normal globular clusters that we see in nearby galaxies today. However, the pull of gravity from a passing galaxy can rip stars and star clusters loose — in some cases entire galaxies can be damaged or destroyed by violent collisions or by the collective gravitational pull from their galactic neighbors.

It is thought that the partial or complete destruction of their parent galaxies spilled the globular star clusters into intergalactic space.

Finding these globular clusters hasn?t been easy. With only one exception, all of the intergalactic globular clusters the teams have detected are so far away (millions of light-years) that they just look like tiny points of light in a vast sea of blackness.

?Because they’re so far away these objects are very faint, almost a billion times fainter than the unaided human eye can see,? said Dr West. ?Detecting such faint objects pushes the limits of even what the Hubble Space Telescope can do.?

?By studying these intergalactic vagabonds in greater detail we hope to learn more about the numbers and types of galaxies that may have been destroyed so far during the life of the universe,? said Dr West. ?Some of these star clusters might also eventually be ?adopted? by other galaxies if they stray close enough to be captured by their gravity.?

The researchers are currently analyzing new Hubble Space Telescope images they recently obtained, and are planning to obtain more at the end of this year.

Original Source: University of Hawaii News Release

My Two Favorite Radio Programs

If you’re interested in science and discovery in general, I’d like to suggest two weekly, hour-long radio shows that you should tune into – through the Internet.

  • Quirks and Quarks – Every Canadian reader will know exactly what I’m talking about. This is a weekly radio show on the Canadian Broadcasting Channel hosted by Bob McDonald. They have archives available online going back almost 10 years.
  • NPR Science Friday – Every Friday, NPR’s Talk of the Nation is taken over by Ira Flatow to discuss the latest happening in science. It’s a great show.

Both are well worth your time. Check them out.

Fraser Cain
Universe Today

New Camera Catches a Near Earth Object Already

Image credit: NASA

Even though it’s still in its initial commissioning trials, NASA’s Quasar Equatorial Survey (or Quest) camera system attached to the 1.2 metre Oschin telescope on Palomar Mountain has already bagged an asteroid. The 250-metre near-Earth object (NEO) 2003 NL7 was discovered on the evening of July 8 by the Quest system, and then later confirmed by several other observatories. Once Quest is fully operational, it should be 3 to 4 times better than the older equipment it replaced.

NASA astronomers in pursuit of near-Earth asteroids have already made a discovery with the newly installed Quasar Equatorial Survey, or ‘Quest,’ camera mounted in mid-April on Palomar Mountain’s 1.2-meter (48-inch) Oschin telescope.

“The Quest camera is still undergoing commissioning trials,” said Dr. Steven Pravdo, project manager for the Near-Earth Asteroid Tracking Project at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “But that doesn’t mean we can’t do some real science in the meantime. What we found was a near-Earth asteroid, estimated to be about 250 meters (820 feet) in size.”

The detection of the near-Earth object, 2003 NL7, occurred on the evening of July 8. It has been confirmed by follow-up measurements from three other observatories and subsequently certified by the official clearinghouse of the solar system’s smaller inhabitants, the Minor Planet Center. While 2003 NL7 has been labeled a near-Earth asteroid, it is considered non-hazardous, with a 2.97-year orbit of the Sun in which its closest approach to Earth’s orbit is about 25.1 million kilometers (15.6 million miles).

The Quest camera is being developed as a multi-purpose instrument by Yale and Indiana universities with Dr. Charles Baltay, chairman of Yale’s physics department, as the principal investigator. It is designed for use in detecting and characterizing quasars, near-Earth asteroids, trans-Neptunian objects, supernovas, and a large variety of other astrophysical phenomena, by scientists from Yale, JPL and the California Institute of Technology in Pasadena. The complex camera consists of 112 electronic chips known as charged coupled devices (CCDs) arranged over the Oschin telescope’s focal plane. This gives the Quest camera 161-megapixel capability. By comparison, a good store-bought digital camera would probably be in the four-megapixel range.

“When Quest becomes operational, it will be a significant advancement for the Near-Earth Asteroid Tracking team,” said Dr. Raymond Bambery, the Near-Earth Asteroid Tracking Project’s principal investigator. “We expect the new camera to increase the efficiency of detection of near-Earth asteroids by some 3 to 4 times that of the camera it replaced. This will make a major contribution to NASA’s goal of discovering more than 90 percent of near-Earth objects that are greater that 1 kilometer (.62 mile) in diameter by 2008.”

The Near-Earth Asteroid Tracking System is managed by JPL for NASA’s Office of Space Science, Washington, D.C. JPL is a division of Caltech. More information on the Near-Earth Asteroid Tracking Program is available at .

Original Source: NASA/JPL News Release

NASA Creates an Independent Safety Centre

Image credit: NASA

NASA announced the creation of an independent Engineering and Safety Center (NESC) at the agency’s Langley Research Center. The 250-person division will provide independent assessment and safety review for various NASA projects and missions. The office was a response to the Columbia disaster, ideally to prevent the kind of safety problems that were uncovered by the investigation team. The team leader has yet to be announced.

NASA today announced plans to create an independent Engineering and Safety Center (NESC) at the agency’s Langley Research Center in Hampton, Va., to provide comprehensive examination of all NASA programs and projects. The center will provide a central location to coordinate and conduct robust engineering and safety assessment across the entire agency.

“Among the things we’ve learned during the investigation of the Columbia tragedy is the need to independently verify our engineering and safety standards. The new NASA Engineering and Safety center will have the capacity and authority to have direct operational influence on any agency mission,” said NASA Administrator Sean O’Keefe. “When it comes to safety and engineering analysis, we need to improve our ability to share technical information, practices and talent, and independently ensure we are in the best position to achieve mission success.”

The NESC is expected to draw on the talents of about 250 people throughout NASA and will report to former astronaut General Roy Bridges, Langley Center Director. Bryan O’Connor, also a former astronaut and Associate Administrator for the Office of Safety and Mission Assurance at NASA Headquarters in Washington, will have policy responsibility for the organization. O’Connor’s task will be to assure the effective use of all agency assets and expertise to derive the independent assessments.

“As we move forward with our ‘Return to Flight’ efforts, the development and implementation of the NESC will help us focus on the future of our technical and safety imperatives,” said O’Connor. “We have a responsibility to make our programs as safe and as sound as possible. This project raises our commitment to unprecedented levels.”

Planned activities of the new organization include:
# Independent engineering assessment and testing to support critical NASA projects and programs;
# Engineering and safety review and evaluation through independent analysis, hazard and risk assessment, safety audit, and participation in mishap investigations;
# A central location for independent trend analysis utilizing state-of-the-art tools and techniques;
# A structure to support engineering collaboration for problem resolution;
# Central coordination of engineering and programmatic lessons learned, technical standards, and technical discipline expertise; and
# Independent inspection and validation of activities to ensure the constant maintenance of NASA safety standards.

“We need to go further than what we expect to see in the findings of the Columbia Accident Investigation Board (CAIB),” added Dr. Michael Greenfield, Associate Deputy Administrator for Technical Programs at NASA Headquarters in Washington. Greenfield co-chairs the agency’s Return to Flight Team with Associate Administrator for Space Flight William F. Readdy. “We need to look beyond the CAIB and provide a centralized clearinghouse that provides NASA with authoritative and consolidated analysis and assessment for all of the agency’s high-risk endeavors,” Greenfield observed.

Original Source: NASA News Release

Supernovae Produce Dust More Efficiently Than Previously Thought

Image credit: Hubble

A new article published in the journal Nature helps settle a long-time mystery about some of the earliest solid particles in the Universe. By measuring supernova remnant Cassiopeia A with the very precise SCUBA telescope, astronomers were able to detect enormous quantities of cosmic dust below -257 degrees Celsius. Hot dust had been found in the past, but the colder dust was mostly invisible – until now. It appears that supernovae are extremely efficient at producing the dust that later forms planets, rocks, and people.

We have just discovered that some supernovae have bad habits – they belch out huge quantities of smoke, known as cosmic dust. This solves a long-standing mystery over the origin of cosmic dust and suggests that supernovae, which are exploding stars, were responsible for producing the first ever solid particles in the Universe.

The Prime Suspects
Supernovae are the violent explosions of stars occurring at the end of their lives. They occur around every 50 years or so in our Galaxy and there are two main types – Type Ia and II. Type II are the explosions of very massive stars with mass greater than 8 times the mass of the Sun (Msun). These stars are ‘live fast – die young’ using up their hydrogen and helium fuel in only a few million years, thousands of times faster than the Sun burns it’s fuel. When the fuel supply is exhausted the star must burn heavier and heavier elements until, finally, when it can do no more to keep itself alive the inner parts of the star collapse to form a neutron star or Black Hole, and the outer parts are flung off in the cataclysm we call a supernova. The enormous explosion sweeps up the surrounding gas into a shell which shines at X-ray, optical and radio wavelengths, and sends shock waves through the galaxy. Supernovae release more energy in a single instant than the Sun will produce in its whole life-time. If the nearest massive star, Betelgeuse in the constellation Orion, were to go supernova it would (for a short time) be brighter than the full moon.

The Cosmic Smoke-Screen
Interstellar dust consists of tiny particles of solid material floating around in the space between the stars – with sizes typically that of cigarette smoke. It is not the same as the dust we clean up in our houses, and in fact the Earth is a giant lump of cosmic dust! It is responsible for blocking about half of all the light emitted from stars and galaxies and profoundly affects our view of the Universe. This ‘dusty’ cloud has a silver lining though, as the astronomers can `see’ the dust radiating the stolen starlight using special cameras designed to work at longer wavelengths, in the Infra-Red (IR: 10 – 100 microns) and Submillimeter (sub-mm: 0.3 – 1mm) part of the electromagnetic spectrum. One such camera is called SCUBA and it is located on the James Clerk Maxwell Telescope in Hawaii. SCUBA is a UK-built instrument which detects light-waves at sub-mm wavelengths and is able to see dust right out to where the furthest stars and galaxies are found.

Dusty Beginnings
Recent observations with SCUBA have shown that a huge amount of dust exists in galaxies and quasars when the Universe was only 1/10th of its present age, long before the Earth and solar system had formed. The presence of all this dust in the distant Universe has a great impact on what astronomers are able to see with their giant optical telescopes, as it limits the amount of starlight which can escape from a distant galaxy and be seen on Earth.

That there were so many solid particles in Universe at such an early time was a great surprise to astronomers as they had believed that dust was mainly formed in cool winds from red giant stars near the end of their lives. Since it takes a long time for star to reach this stage in its evolution (the Sun will take around 9 billion years) there has simply not been enough time for so much dust to have been made in this way.

‘Dust has been swept under the cosmic carpet – for years astronomers have treated it as a nuisance because of the way it hides the light from the stars. But then we found that there is dust right at the edge of the Universe, in the earliest stars and galaxies, and we realised that we were ignorant of even its basic origin’ explained Dr Dunne.

Supernovae also make large amounts of heavy elements, such as carbon and oxygen, and throw them out into interstellar space. These are the elements which make up our bodies and, since they are also the elements which make up dust grains, supernovae have long been a prime suspect in the mystery of the origin of cosmic dust. As it takes only a few million years for the most massive stars to reach the end of lives and explode as supernovae, they could make dust quickly enough to explain what is seen in the early Universe. However, until this team’s work, only tiny amounts of dust had ever been found in supernovae – leaving astronomers with a smoking gun but no ‘smoke’

Haley Morgan, a PhD student at Cardiff said ‘If supernovae were efficient dust ‘factories’ they would each be producing more than the mass of the Sun in dust.’

‘As massive stars evolve to become supernovae in the blink of an eye by astronomical standards, they could easily explain why the early Universe appears so dusty.’ added Dr Rob Ivison of the Royal Observatory Edinburgh.

Supernova Sleuths
The team from Cardiff and Edinburgh used SCUBA to look for the emission from dust in the remains of a recent supernova. Cassiopeia A is the remnant of a supernova which happened around 320 years ago. It is located in the constellation Cassiopeia, 11,000 light years from Earth and is about 10 light years across. Cas A is the brightest radio source in the sky so it is well studied at many wavelengths from the optical to X-rays. The images below show Cas A in the X-rays, optical, infra-red and radio. The X-rays follow the really hot gas (10 million degrees Kelvin), and the other wavelengths trace material at: 10 thousand degrees (optical), hot dust at 100 K (IR) and high energy electrons (radio).

Although astronomers had been searching for dust in supernova remnants for decades, they had used instruments which could only detect dust that was quite warm, such as that in the ISO infra-red image above. SCUBA has the advantage here because it is able to see dust which is very cold and this is because it works at longer sub-mm wavelengths.

‘In the same way that you can only see an iron poker glowing when it’s been in a fire, you can only see dust with infra-red cameras when it is warmer than about 25 Kelvin, but SCUBA can see it when it’s colder too’ explained Dr Steve Eales, Reader in Astrophysics at Cardiff University.

Cold Hard Evidence
SCUBA found a large amount of dust in the Cas A remnant, 1-4 times more than the mass of the Sun ! This is over 1,000 times more than had been seen before. This means that Cas A was very efficient at creating dust from the elements available. The temperature of the dust is very low, only 18 Kelvin (-257 degrees Celsius), and this is the reason that it had never been seen before. Below are the two sub-mm images of Cas A at 850 and 450 microns taken with SCUBA. You can see that the left image looks a little like the radio one above, and this is because the high energy electrons which make the radio image also emit some of their energy at slightly shorter wavelengths – contaminating the sub mm emission at 850microns. The middle image is at 450 microns where the contamination is much lower, and so most of this emission is from cold dust. If we remove the contamination we get a different picture (right). All the dust is seen in the bottom half of the remnant and the two sub-mm images now look much more similar!
850 microns without radio contamination

‘The puzzle is how the dust can remain so cold when we know that there is gas at over a million degrees present from the X-ray radiation it gives off.’ commented Prof. Mike Edmunds, head of the School of Physics & Astronomy in Cardiff.

The dust also has different properties to the ‘everyday’ kind of dust in the Milky Way and other galaxies – it is better at ‘shining’ in the sub-mm, maybe because it is still very young and relatively pristine. If all supernovae were this efficient at making dust they would be the biggest dust ‘factories’ in the Galaxy. Smoking supernovae provide a solution to the mystery of the huge amounts of dust seen in the early Universe.

‘These observations give us a tantalising glimpse of how the first solid particles in the Universe were created’ said Haley Morgan.

Original Source: Cardiff University News Release