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Giant star-jet astounds astronomers

Sanduleak's star

Sanduleak's star and the jet of matter shooting out from it at more than 5 million kilometres per hour. The jet is now 400 million million kilometres long.

  • Star shooting out jet of material 400 million million km long
  • Thought to occur due to interaction between two stars
  • Located in the Large Magellanic Cloud galaxy

ASTRONOMERS HAVE FOUND a star spitting matter into a “jet” that stretches for more than 400 million million kilometres across space.

That’s about ten times the distance between the Sun and its nearest neighbouring star (proxima Centauri).

It’s the biggest jet known from a star, and “challenges our current understanding,” said Dr Francesco Di Mille (Australian Astronomical Observatory and the University of Sydney), a member of the team that made the finding.

Theoretical models don’t deal with it, he said, “simply because nobody would ever have bet that such a giant stellar jet could exist”.

In a galaxy not so far away

The star making the jet is called Sanduleak’s star, having been discovered by astronomer Nicholas Sanduleak in 1977.

Sanduleak noted that the star varied in brightness, but didn’t see the jet.

That’s not surprising. The star is shrouded by dust, and it’s not even in our Galaxy—it’s in a small neighbouring galaxy called the Large Magellanic Cloud, about 160 thousand light-years away.

Finding the jet fell to Dr Di Mille’s team, led by Italian astronomer Rodolfo Angeloni (Pontificia Universidad Católica de Chile), which turned the 6.5-m Magellan Telescopes in Chile on the star.

Magellan Telescopes

Observations were made with the Magellan Telescopes in Chile.

Outburst 10,000 years old

Dust surrounding the star makes it hard to tell exactly what’s going on, but it seems that actually two stars are involved: a red giant and a white dwarf, tangoing closely.

The red giant’s hot “breath”—transferred matter—curls into a belt around the white dwarf’s belly. From time to time a jet shoots up and down from this disc of material, along the star’s axis of rotation.

Artist's impression of a system like Sanduleak's star

An artist's impression of a system like Sanduleak's star—a red giant star transferring matter onto a white dwarf star.

Astronomers have worked out that the current outburst has been going on for about ten thousand years, and that the material in the jet is travelling at more than 5 million kilometres per hour (1,500 km per second).

“Because we know the distance to this star we’ll be able to make good estimates of most of the jet’s properties,” Dr Di Mille said.

“It will be the best test-case for understanding jets from stars.”

The researchers have published their finding in The Astrophysical Journal Letters.

Adapted from information issued by AAO. Magellan Telescopes image courtesy Francisco Figueroa. Sanduleak’s star image courtesy R. Angeloni et al. Artist’s impression courtesy Dana Berry (STScI).

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Ancient supernova seen in a new light

RCW 86

This is all that remains of a supernova explosion that was seen by Chinese astronomers in the year 185 CE. The remnant gas cloud is called RCW 86, and is approximately 8,000 light-years from Earth.

A TWISTED AND TANGLED GAS CLOUD is all that remains of the oldest documented example of a supernova, called RCW 86.

Chinese stargazers witnessed the event in 185 CE, documenting a mysterious ‘guest star’ that remained in the sky for eight months.

The image combines data from four different space telescopes to create a multi-wavelength view.

X-ray images from the European Space Agency’s XMM-Newton Observatory and NASA’s Chandra X-ray Observatory are combined to form the blue and green colours in the image. The X-rays show the interstellar gas that has been heated to millions of degrees by the passage of the shock wave from the supernova.

Infrared data from NASA’s Spitzer Space Telescope, as well as NASA’s Wide-Field Infrared Survey Explorer (WISE) are shown in yellow and red, and reveal dust radiating at a temperature of several hundred degrees below zero, warm by comparison to normal dust in our Milky Way galaxy.

By studying the X-ray and infrared data together, astronomers were able to determine that the cause of the explosion witnessed nearly 2,000 years ago was a Type Ia supernova, in which an otherwise-stable white dwarf, or dead star, was pushed beyond the brink of stability when a companion star dumped material onto it.

Furthermore, scientists used the data to solve another mystery surrounding the remnant—how it got to be so big in such a short amount of time.

By blowing out a ‘wind’ prior to exploding, the white dwarf was able to clear out a huge ‘cavity,’ a region of very low-density surrounding the system. The explosion was able to expand into this cavity much faster than it otherwise would have.

RCW 86 is approximately 8,000 light-years away.

Adapted from information issued by NASA / JPL-Caltech / B. Williams (NCSU).

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‘The Dish’ finds a ‘diamond planet’

Artist's visualisation of the pulsar and its orbiting planet

An artist's visualisation of the pulsar and its orbiting planet, which astronomers think could be made partly of diamond or a diamond-like substance. The blue squiggly line represents the beams of radio waves emanating from the pulsar. The orange bubble represents the size of the Sun, showing that the the planet's orbit has about the same radius as the Sun (about 600,000 km), yet it whizzes around in just two hours!

  • Planet detected orbiting a pulsar 4,000 light-years away
  • It’s actually the remnant core of what was once a star
  • Probably made of compressed carbon—diamond!

ASTRONOMERS USING ‘THE DISH’—CSIRO’s radio telescope near Parkes, NSW—believe they’ve found a small planet made of diamond, orbiting an unusual star.

The discovery was made by an international research team, led by Professor Matthew Bailes of Swinburne University of Technology in Melbourne, Australia, and is reported today in the journal Science.

“Although bizarre, this planet is evidence that we’ve got the right understanding of how these binary systems evolve,” said Dr Michael Keith of CSIRO Astronomy and Space Science, one of the research team members.

Not fitting the pattern

The researchers, from Australia, Germany, Italy, the UK and the USA, first found an unusual star called a pulsar, now named PSR J1719-1438, using the 64-m Parkes radio telescope in eastern Australia.

Pulsars are small spinning stars about 20 km in diameter—the size of a small city—that emit a beam of radio waves. As the star spins and the radio beam sweeps repeatedly over Earth, radio telescopes detect a regular pattern of radio pulses.

The researchers followed up their discovery with the Lovell radio telescope in the UK and one of the Keck telescopes in Hawaii, and noticed that the arrival times of the pulsar’s pulses were systematically altered—in a way that must be caused by the gravitational pull of a small planet orbiting the pulsar.

The CSIRO's Parkes radio telescope

The CSIRO's Parkes radio telescope

Small, heavy and fast

The modulations of the radio pulses reveal several things about the planet.

First, it orbits the pulsar in just two hours and ten minutes, and the distance between the two objects is 600,000 km—a little less than the radius of our Sun.

Second, the companion must be small, less than 60,000 km (that’s about five times the Earth’s diameter). The planet is so close to the pulsar that, if it were any bigger, it would be ripped apart by the pulsar’s gravity.

But despite its small size, the planet has slightly more mass than Jupiter.

A stripped-down dwarf

“This high density of the planet provides a clue to its origin,” Professor Bailes said.

The team thinks that the ‘diamond planet’ is all that remains of a once-massive star, most of whose matter was siphoned off towards the pulsar.

But pulsar J1719-1438 and its companion are so close together that the companion can only be a very stripped-down ‘white dwarf’ star, one that has lost its outer layers and over 99.9 per cent of its original mass.

“This remnant is likely to be largely carbon and oxygen, because a star made of lighter elements like hydrogen and helium would be too big to fit the measured orbit,” said CSIRO’s Dr Keith.

The density means that this material is certain to be crystalline—that is, a large part of the star may be similar to a diamond.

The pulsar and its planet lie 4,000 light-years away in the constellation of Serpens (the Snake). The system is about an eighth of the way towards the Galactic Centre from the Earth.

Diamond planet Easy Q&A

What have they found?

  • They’ve spotted a system that comprises a weird kind of star, called a pulsar, and a medium-sized planet that is probably made of almost pure carbon…which is most likely in the form of diamond or a diamond-like substance.
  • The system is 4,000 light-years from Earth—that’s 40 thousand trillion kilometres away!
  • The pulsar emits radio waves in a regular pattern as it spins, like a lighthouse, which is what the CSIRO’s Parkes radio telescope picked up.
  • The planet itself cannot be seen as it is too small and too far away.

If they can’t see the planet, how do they know it’s really there?

  • Its presence is inferred by the distorting effect it has on the pulsar’s powerful radio emissions.
  • It whizzes around its star in just two hours (compared to one year for Earth around the Sun).
  • The data was analysed using an incredible supercomputer at Swinburne University in Melbourne.
  • The planet is about 5 times as wide as the Earth, but much, much heavier.

So why do they think it is made of diamond?

  • Now here’s the interesting bit, because the planet actually seems to be the dense, remnant core of a star, rather than a traditional planet.
  • Many stars, as they burn up their hydrogen fuel, end up having cores made of carbon.
  • The star changed into a planet, with only it’s core remaining.

How did it change from a star into a planet?

  • Because the pulsar has a huge gravitational pull and is a cosmic cannibal!
  • The pulsar and the other star would have been orbiting very close to each other.
  • The pulsar would have pulled all the outer gas layers off the other star—99.9 percent of its mass—eventually leaving it with just its carbon core.
  • If we could have seen it happening, it would have looked like a huge whirlpool of gas coming off the doomed star and spiralling onto the neighbouring pulsar.

What do astronomers hope to learn from these types of star systems?

  • For one thing, pulsars are the “end points”—the dying stages—in the lives of many kinds of big stars, so learning more about them tells us about the evolution and life cycle of those stars and the wider universe.
  • But pulsars also are important for understanding and testing laws of physics.
  • Astronomers can use them as “natural laboratories” for testing theories, such as Einstein’s theory of gravity.
  • That’s because you can only go so far testing some theories in the laboratory—to really put them to the test, you need to study massive objects travelling at high speed, and that’s what pulsar systems are.

Main text adapted from information issued by CSIRO. Q&A by Jonathan Nally, SpaceInfo.com.au Images courtesy Swinburne Astronomy Productions and David McClenaghan, CSIRO.

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Fertile ground for other Earths

Artist's impression of a white dwarf

Astronomers have suggested that white dwarfs—old, cool, burned out stars—could be good places to look for orbiting habitable planets. (Artist's impression)

  • White dwarfs are Sun-like stars reaching the end of their lives
  • Small and cool, they could be ideal places for habitable planets
  • Sky surveys should show if white dwarfs have such planets

PLANET HUNTERS HAVE FOUND hundreds of planets outside the Solar System in the last decade, though it is unclear whether even one might be habitable.

But it could be that the best place to look for planets that can support life is around dim, dying stars called white dwarfs.

In a new paper published online in The Astrophysical Journal Letters, Eric Agol, a University of Washington associate professor of astronomy, suggests that potentially habitable planets orbiting white dwarfs could be much easier to find—if they exist—than other exoplanets located so far.

White dwarfs, cooling stars believed to be in the final stage of life, typically have about 60 percent of the mass of the Sun, but by volume they are only about the size of Earth.

Though born hot, they eventually become cooler than the Sun and emit just a fraction of its energy, so the habitable zones for their planets are significantly closer than Earth is to the Sun.

“If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface—if it has water at all—and that’s a big factor for habitability,” Agol said.

A planet so close to its star could be seen using an Earth-based telescope as small as one metre wide (the largest telescope are now 8-10 metres), as the planet passes in front of, and dims the light from, the white dwarf, he said.

Red giant to white dwarf

White dwarfs evolve from stars like the Sun. When such a star’s core can no longer produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core.

That begins the transformation to a red giant, with a greatly expanded outer atmosphere that typically envelops—and destroys—any planets as close as Earth.

Finally the star sheds its outer atmosphere, leaving the glowing, gradually cooling, core as a white dwarf, with a surface temperature around 5,000 degrees Celsius.

Life cycle of a sun-like star

A star like our Sun goes through many different stages during its life, ending up as a white dwarf surrounding by a cloud of gas.

At that point, the star produces heat and light in the same way as a dying fireplace ember, though the star’s ember could last for three billion years.

Once the red giant sheds its outer atmosphere, more distant planets that were beyond the reach of that atmosphere could begin to migrate closer to the white dwarf, Agol said.

New planets also possibly could form from a ring of debris left behind by the star’s transformation.

In either case, a planet would have to move very close to the white dwarf to be habitable, perhaps 800,000 to 3.2 million kilometres from the star. That’s less than one percent of the distance from Earth to the Sun (150 million kilometres) and substantially closer than Mercury is to the Sun.

“From the planet, the star would appear slightly larger than our Sun, because it is so close, and slightly more orange, but it would look very, very similar to our Sun,” Agol said.

The planet also would be ‘tidally locked’, so the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet.

How to find other Earths

Candidate white dwarf in NGC 6397.

A candidate white dwarf star (marked in red) within the star cluster NGC 6397.

The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 9.5 trillion kilometres). It is believed to once have been five times more massive than the Sun, but now it has about the same mass as the Sun packed into the same volume as Earth.

Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a 1-metre telescope, he said, one star could be surveyed in 32 hours of observation.

If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth.

Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky.

“This could take a huge amount of time, even with such a network,” he said.

The same work could be accomplished by larger specialty telescopes, such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the UW is a founding partner.

If it turns out that the number of white dwarfs with potential Earth-like planets is very small—say one in 1,000—that telescope still would be able to track them down efficiently.

Finding an Earth-like planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable.

“Those are the reasons I find this project interesting,” he said. “And there’s also the question of, ‘Just how special is Earth?'”

Adapted from information issued by the University of Washington. Images courtesy ESA / Hubble Information Centre / ESO / NASA / G. Bacon (STScI) / S. Steinhöfel.

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A dying star’s farewell show

Planetary nebula NGC 6210

Planetary nebula NGC 6210 in the constellation Hercules. A planetary nebula is a complex cloud of gas given off during the dying stages of a Sun-like star's life.

The NASA/ESA Hubble Space Telescope has taken a striking high-resolution image of the curious planetary nebula NGC 6210.

Located about 6,500 light-years away, in the constellation of Hercules, NGC 6210 was discovered in 1825 by the German astronomer Friedrich Georg Wilhelm Struve. Although through a small telescope it appears only as a tiny disc, it is fairly bright as planetary nebulae go.

Despite their name, planetary nebulae have nothing to do with planets. They got their name because, through early telescopes, they looked more like planets than stars.

In fact, a planetary nebula is a complex cloud of gas produced in the dying stages of certain stars’ lives.

In this instance, NGC 6210 is the last gasp of a star slightly less massive than our Sun. Multiple shells of gas ejected by the dying star are superimposed on one another in different orientations, giving NGC 6210 its odd shape.

See a full-size, high-resolution wallpaper image here (new window).

This sharp image shows the inner region of this planetary nebula in unprecedented detail, where the central star is surrounded by a thin, bluish bubble that has a delicate filamentary structure. This bubble is superposed onto an asymmetric, reddish gas complex where holes, filaments and pillars are clearly visible.

A star’s life ends when the fuel available to its thermonuclear engine runs out. The estimated lifetime for a Sun-like star is some ten billion years. When the star is about to expire, it becomes unstable and ejects its outer layers, forming a planetary nebula and leaving behind a tiny, but very hot, remnant, known as white dwarf.

This compact object, visible at the centre of the image, cools down and fades very slowly. Stellar evolution theory predicts that our Sun will experience the same fate as NGC 6210 in about five billion years.

Adapted from information issued by ESA / Hubble / NASA.

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Super-blast amazes scientists

Astronomers using NASA’s Fermi Gamma-ray Space Telescope have detected gamma-rays from a nova for the first time, a finding that stunned observers and theorists alike.

The discovery overturns the notion that novae explosions lack the power to emit such high-energy radiation.

A nova is a sudden, short-lived brightening of an otherwise inconspicuous white dwarf star. The outburst occurs when a white dwarf in a binary system erupts in an enormous thermonuclear explosion.

“In human terms, this was an immensely powerful eruption, equivalent to about 1,000 times the energy emitted by the Sun every year,” said Elizabeth Hays, a Fermi deputy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“But compared to other cosmic events Fermi sees, it was quite modest. We’re amazed that Fermi detected it so strongly.”

Gamma rays are the most energetic form of light, and Fermi’s Large Area Telescope detected the nova for 15 days. Scientists believe the emission arose as a 1.5 million-kilometre-per-hour shock wave raced from the site of the explosion.

before and after images showing nova

Japanese amateur astronomers discovered Nova Cygni 2010 in an image taken on March 11 (Japan Standard Time). The erupting star (circled) was 10 times brighter than it was in an image taken several days earlier.

Discovered by Japanese amateurs

Japanese amateur astronomers first noticed that the star system, known as V407 Cyg, was 10 times brighter on March 11, 2010, than in an image they had taken three days earlier.

It was quickly followed up by other Japanese amateurs, and then by professional astronomers working with Fermi’s Large Area Telescope (LAT).

V407 Cyg lies 9,000 light-years away. The system is a so-called symbiotic binary containing a compact white dwarf and a red giant star about 500 times the size of the Sun.

“The red giant is so swollen that its outermost atmosphere is just leaking away into space,” said Adam Hill at Joseph Fourier University in Grenoble, France. The phenomenon is similar to the solar wind produced by the sun, but the flow is much stronger.

“Each decade, the red giant sheds enough hydrogen gas to equal the mass of Earth,” Hill added.

The white dwarf intercepts and captures some of this gas, which accumulates on its surface. As the gas piles on over tens to hundreds of years, it eventually becomes hot and dense enough to fuse into helium. This energy-producing process triggers a runaway reaction that explodes the accumulated gas.

The white dwarf itself, however, remains intact.

Fermi gamma ray image showing V407 Cyg nova

The Large Area Telescope aboard NASA's Fermi Gamma-ray Space Telescope saw no sign of a nova in 19 days of data prior to March 10 (left), but the eruption is obvious in data from the following 19 days (right).

A huge shock, in more ways than one

The blast created a hot, dense expanding shell called a shock front, composed of high-speed particles, ionised gas and magnetic fields. The shock wave expanded at 11 million kilometres per hour—or nearly 1 percent the speed of light.

The magnetic fields trapped particles within the shell and whipped them up to tremendous energies. Before they could escape, the particles had reached velocities near the speed of light. Scientists say that the gamma rays likely resulted when these accelerated particles smashed into the red giant’s wind.

“We know that the remnants of much more powerful supernova explosions can trap and accelerate particles like this, but no one suspected that the magnetic fields in novae were strong enough to do it as well,” said NRL’s Soebur Razzaque.

Supernovae remnants can last for 100,000 years and affect regions of space thousands of light-years across.

Adapted from information issued by Francis Reddy, NASA’s Goddard Space Flight Centre. Images courtesy NASA / DOE / Fermi LAT Collaboration / K. Nishiyama and F. Kabashima / H. Maehara, Kyoto Univ. / GSFC / Conceptual Image Lab.

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Exploding star could be new type

The image on the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right.

The image on the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right.

  • Possible new class of supernovae
  • Could explain abundance of calcium in our Galaxy

In the past decade, robotic telescopes have turned astronomers’ attention to scores of strange exploding stars, one-offs that may or may not point to new and unusual physics.

But supernova (SN) 2005E, discovered five years ago by the University of California, Berkeley’s Katzman Automatic Imaging Telescope (KAIT), is one of eight known “calcium-rich supernovae” that seem to stand out as horses of a different colour.

“With the sheer numbers of supernovae we’re detecting, we’re discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes,” said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy.

“But SN 2005E was a different kind of ‘bang.’ It and the other calcium-rich supernovae may be a true suborder, not just one of a kind.”

And then there were three

Filippenko is co-author of a paper in the journal Nature describing SN 2005E and arguing that it is distinct from the two main classes of supernovae.

The Type Ia supernovae are thought to be old, white dwarf stars that accumulate matter from a companion until they undergo a thermonuclear explosion that blows them apart entirely.

The Katzman Automatic Imaging Telescope

The supernova was spotted by the Katzman Automatic Imaging Telescope

Type Ib/c or Type II supernovae are thought to be hot, massive and short-lived stars that explode and leave behind black holes or neutron stars.

In the case of SN 2005E, the team of astronomers thinks the original star was a low-mass white dwarf stealing helium from a companion star until the temperature and pressure ignited a thermonuclear explosion – a massive fusion bomb – that blew off at least the outer layers of the star and perhaps blew the entire star to smithereens.

The researchers calculate that about half of the mass thrown out was calcium, which means that a couple of such supernova every 100 years would be enough to produce the high abundance of calcium detected in galaxies like our own Milky Way, and the calcium present in all life on Earth.

More supernova confusion

Interestingly, a team of researchers from Hiroshima University in Japan argue in the same issue of Nature that SN 2005E’s original, or progenitor, star was massive – between 8 and 12 solar masses – and that it underwent a core-collapse similar to a Type II supernova.

“It’s a confusing, muddy situation now,” said Filippenko. “But we hope that, by finding more examples of this subclass and of other unusual supernovae and observing them in greater detail, we will find new variations on the theme and get a better understanding of the physics that’s actually going on.”

To make things even muddier, Filippenko and former UC Berkeley post-doctoral fellow Dovi Poznanski, currently at Lawrence Berkeley National Laboratory and also co-author on the Nature paper, reported last November another supernova, SN 2002bj, that they believe explodes by a similar mechanism: ignition of a helium layer on a white dwarf.

“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf [stealing] helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.”

Adapted from information issued by University of California – Berkeley / Sloan Digital Sky Survey / Lick Observatory.

Watery, rocky planets may be common

An artist's impression of a massive asteroid belt in orbit around a star

An artist's impression of a massive asteroid belt in orbit around a star. New work shows that rubble around many white dwarf stars contaminates these stars with rocky material and water.

An international team of astronomers has discovered compelling evidence that rocky planets are commonplace in our Galaxy.

Leicester University scientist and lead researcher Dr Jay Farihi studied white dwarf stars—the compact remnants of stars that were once like our Sun—and found that many show signs of contamination by heavier elements and possibly even water, improving the prospects for extraterrestrial life.

White dwarf stars are the endpoint of stellar evolution for the vast majority (>90%) of all stars in the Milky Way, including our Sun. Because they should have essentially pure hydrogen or pure helium atmospheres, if heavier elements (in astronomy described as ‘metals’, examples including calcium, magnesium and iron) are found then these must be external pollutants.

For decades, it was believed that the interstellar medium, the tenuous gas between the stars, was the source of metals in these polluted white dwarfs.

Farihi and his team used data from the Sloan Digital Sky Survey (SDSS), a project that aims to survey the sky in infrared light, imaging more than 100 million objects and following up 1 million of these by obtaining their spectrum (splitting the light into its colours).

By examining the positions, motions and spectra of the white dwarfs identified in the SDSS, Farihi and his team show that this is no longer a viable theory. Instead, rocky planetary debris is almost certainly the culprit in most or all cases.

The new work indicates that at least 3% and perhaps as much as 20% of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky asteroid debris with a total mass equivalent to that of a single 140-km-diameter asteroid.

This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build terrestrial planetary systems. Astronomers are thus playing the role of celestial archaeologists by studying the ‘ruins’ of rocky planets and or their building blocks.

Adapted from information issued by RAS / NASA-JPL / Caltech / T. Pyle (SSC).

Smallest known star duo confirmed

Artist's impression of the binary star system known as HM Cancri

About 1,600 light-years away, in a binary star system known as HM Cancri, two dense white dwarf stars orbit each other once every 5.4 minutes, based on data from the Keck Observatory. This artist's rendition shows the dance of these dead stars and the resulting gravitational waves (which would actually be invisible).

Astronomers have identified the smallest known binary star system to date. Called HM Cancri, its consists of two dead stars that revolve around each other in 5.4 minutes, by far the shortest known orbital period of any pair of stars.

The team, led by Gijs Roelofs of the Harvard-Smithsonian Center of Astrophysics, used the 10-meter Keck I telescope in Hawaii and its Low Resolution Imaging Spectrograph to study the velocity changes in the spectral lines in the light coming from HM Cancri.

They saw that as the stars orbited each other, the system’s spectral lines shifted periodically from blue to red and back, in accordance with the Doppler effect. With that velocity information, the astronomers were able to confirm the binary’s 5.4-minute period.

“When the first data from the Keck telescope arrived, and our quick analysis showed the periodic shift of the spectral lines, we knew that we had succeeded. More than ten years after its discovery, we finally had deciphered the nature of HM Cancri,” said Arne Rau of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the observations at Keck.

Astronomers proposed several years ago that HM Cancri was an interacting binary consisting of two dead stars and that the 5.4 minute period observed was indeed the orbital period.

The team had been trying to make precise velocity measurements to confirm the period since 2005.

X-ray evidence

HM Cancri was discovered in 1999 as a weak X-ray source in data from the German ROSAT satellite. It comprises two white dwarfs, burnt-out cinders of stars that were once similar to the Sun and contain a highly condensed form of helium, carbon and oxygen. In 2001, the X-ray, and also optical, data suggested that the two stars orbited each other in 5.4 minutes.

Another artist's conception of HM Cancri.

Another artist's conception of HM Cancri. One star is feeding the other.

But the information suggested that the binary system was roughly eight times the diameter of the Earth—equivalent to a quarter of the distance between the Earth and the Moon—or smaller. Astronomers were reluctant to accept this physical description without additional evidence. But at a distance of 16,000 light years from Earth, the binary system shines only one millionth as bright as the faintest stars visible to the naked eye, making it very hard to study. To determine with certainty the period of such a system, astronomers needed to use world’s largest telescopes to collect the additional evidence.

“This type of observation is really at the limit of what is currently possible. Not only does one need the biggest telescopes in the world, but they also have to be equipped with the best instruments available,” said team member Paul Groot of the Radboud University Nijmegen in the Netherlands.

As a result of the successful observations with Keck, astronomers now have a new cosmic laboratory to study the evolution of stars as well as general relativity.

“We know the system must have come from two normal stars that somehow spiralled together in two earlier episodes of mass transfer, but the physics of this process is very poorly understood,” said Gijs Nelemans of the Radboud University who was also part of the team.

He added that the system must be one of the most copious emitters of gravitational waves. “We hope to detect these distortions of space-time directly with the future LISA satellite. HM Cancri will now be a cornerstone system for the mission,” he said.

Adapted from information issued by Keck Observatory / NASA / Tod Strohmayer (GSFC) / Dana Berry (Chandra X-Ray Observatory) / Rob Hynes and Paul Groot, Radboud University.