Posted on by Fraser Cain

Supernova’s Companion Star Found

Image credit: ESA

When the second brightest supernova seen in modern times, SN 1993J, blew up several years ago, it did leave a survivor. Using the Hubble Space Telescope, and several ground-based observatories, an international team of astronomers discovered a massive companion star that must have been orbiting the supernova at the time it exploded. This discovery is very important because it will allow astronomers watch what the remnant of SN 1993J does to its companion star. They might even be able to detect a neutron star or black hole forming in real time.

A joint European/University of Hawaii team of astronomers has for the first time observed a stellar ?survivor? to emerge from a double star system involving an exploded supernova.

Supernovae are some of the most significant sources of chemical elements in the Universe, and they are at the heart of our understanding of the evolution of galaxies.

Supernovae are some of the most violent events in the Universe. For many years astronomers have thought that they occur in either solitary massive stars (Type II supernovae) or in a binary system where the companion star plays an important role (Type I supernovae). However no one has been able to observe any such companion star. It has even been speculated that the companion stars might not survive the actual explosion…

The second brightest supernova discovered in modern times, SN 1993J, was found in the beautiful spiral galaxy M81 on 28 March 1993. From archival images of this galaxy taken before the explosion, a red supergiant was identified as the mother star in 1993 – only the second time astronomers have actually seen the progenitor of a supernova explosion (the first was SN 1987A, the supernova that exploded in 1987 in our neighbouring galaxy, the Large Magellanic Cloud).

Initially rather ordinary, SN 1993J began to puzzle astronomers as its ejecta seemed too rich in the chemical element helium and instead of fading normally it showed a bizarre sharp increase in brightness. The astronomers realised that a normal red supergiant alone could not have given rise to such a weird supernova. It was suggested that the red supergiant orbited a companion star that had shredded its outer layers just before the explosion.

Ten years after this cataclysmic event, a European/University of Hawaii team of astronomers has now peered deep into the glowing remnants of SN 1993J using the NASA/ESA Hubble Space Telescope?s Advanced Camera for Surveys (ACS) and the giant Keck telescope on Mauna Kea in Hawaii. They have discovered a massive star exactly at the position of the supernova that is the long sought companion to the supernova progenitor.

This is the first supernova companion star ever to be detected and it represents a triumph for the theoretical models. In addition, this observation allows a detailed investigation of the stellar physics leading to supernova explosions. It is now clear that during the last 250 years before the explosion 10 solar masses of gas were torn violently from the red supergiant by its partner. By observing the companion closely in the coming years it may even be possible to detect a neutron star or black hole emerge from the remnants of the explosion ?in real time?.

Given the paucity of observations of supernova progenitor systems this result, published in Nature on 8 January 2004, is likely to ‘be crucial to understanding how very massive stars explode and why we see such peculiar supernovae’ according to first author Justyn R. Maund from the University of Cambridge, UK.

The team is composed of Stephen J. Smartt and Justyn R. Maund (University of Cambridge, UK), Rolf. P. Kudritzki (University of Hawaii, USA), Philipp Podsiadlowski (University of Oxford, UK) and Gerry F. Gilmore (University of Cambridge, UK).

Stephen Smartt, also from the University of Cambridge, says, ?Supernova explosions are at the heart of our understanding of the evolution of galaxies and the formation of chemical elements in the Universe. It is essential that we know what type of stars produce them.?

For the last ten years astronomers have believed that they could understand the very peculiar behaviour of 1993J by invoking the existence of a binary companion star and now this picture has proved correct.

According to Rolf Kudritzki, from the University of Hawaii, ?The combination of the outstanding spatial resolution of Hubble and the huge light gathering power of the Keck 10- metre telescope in Hawaii has made this fantastic discovery possible.?

Supernovae occur when a star of more than about eight times the mass of the Sun reaches the end of its nuclear fuel reserves and can no longer produce enough energy to keep the star from collapsing under its own immense weight. The core of the star collapses, and the outer layers are ejected in a fast-moving shock wave.

This huge energy release causes the visible supernova we see. While astronomers are convinced that observations will match this theoretical model, they are in the embarrassing position that they have confidently identified only two stars that later exploded as supernovae ? the precursors of supernovae 1987A and 1993J.

There have been more than 2000 supernovae discovered in galaxies beyond the Milky Way and there appear to be about eight distinct sub-classes. However identifying which stars produce which flavours has proved incredibly difficult. This team has now embarked on a parallel project with the Hubble Space Telescope to image a large number of galaxies and then wait patiently for a supernova to explode.

Supernovae appear in spiral galaxies like M81 on average once every 100 years or so. The team, led by Stephen Smartt, hope to increase the numbers of supernova progenitors known from 2 to 20 over the next five years.

Original Source: ESA News Release

Posted on by Fraser Cain

Book Review: The Solar System


Our solar system is our neighbourhood. It is what supports our life, sets us apart from other regions of space and even protects us from extra-solar visitors. Yet our knowledge of this region is frustratingly incomplete. Not only is the physical space dauntingly large but the time scale of its existence is equally vast. Giovanni Caprara in his book “The Solar System” provides us with an up to date account of what we have observed in our solar system and demonstrates some of the physical processes taking place both now and at earlier times.

Caprara starts by presenting the most accepted methods for solar system formation and he continually returns to this when considering the formation of each of the planets. As interstellar dust and gas are the core building blocks there is a lot of room for variety. Perhaps what is more surprising is the quantity of similarities and patterns that result.

With an extensive chapter for the sun and each planet the reader can both learn specific details as well as make comparative studies. There are descriptions of these bodies’ physical characteristics such as the size, the distance from the Sun and the mass. Where appropriate Caprara discusses the atmosphere and climate. Also he presents distinctive surface features and their causes such as asteroid impacts or plate tectonics. He guesses at the body’s internal structure though except for the Earth and Moon there is no data to provide corroboration. If the planets have their own satellites he describes each individually in much the same manner. What these details readily show is that we really do live in a system with all components inherently linked to one another.

Pleasantly interposed within the text are vignettes that tell the reader how the information was obtained. Usually each vignette has an accompanying pictorial representation whether a drawing by 17’th century astronomers, plates from ground-based observatories or images from space probes. As most of the images are of recent photographs from space probes they add a current feel to the book. What becomes readily apparent is that only with advances to our observational ability will we get advances in our understanding.

There is also a purposeful tie-in with this information to our own benefit on Earth. The process of global warming is evident on Venus. Tidal effects between Io and Jupiter are equally present between the Earth and Moon. Asteroids are present in large numbers and obviously continually strike as seen when Shoemaker-Levy hit Jupiter. And, as seen with other stars, our Sun will eventually change and make life on Earth untenable. Knowing about our neighbourhood isn’t just for cursory interest it can also help with our survival

This book is a good source of information though a more robust list of references would aid those looking for greater detail. Also, there seems to be a few errors either from the original text or the translation. However none of these detract from the text. Further, as this book originates in Italy I was hoping that there would be a distinct European view. Unfortunately none was apparent.

The results of Caprara’s work is a thorough description of our solar system. The reader can easily feel they are travelling with the author as they discover each planet and satellite. They will also quickly become supporters of the scientists who work so hard to add even more to what we know about our neighbourhood. The next big space probe is the United States’ New Horizons mission to Pluto. This is to add to the limited fuzzy pictures which is all we have now. Let’s hope it succeeds.

Learn more from Amazon.com

Review by Mark Mortimer

Posted on by Fraser Cain

Lifeless Suns in the Early Universe

Image credit: Harvard CfA

New calculations by a pair of Harvard astronomers predict that the first “Sun-like” stars in the Universe were alone; devoid of planets or life. The very first generation of stars was hot and massive; they lived hard and died young. After they exploded as supernovae and seeded the Universe with heavier materials, other stars formed in stellar nurseries. The next generation of stars was probably similar in mass and composition to our own Sun, but there weren’t enough minerals to create rocky planets like the Earth. It took a succession of supernovae before there was enough heavy material that planets could form – probably 500 million to 2 billion years after the Big Bang.

To most people, the phrase “Sun-like star” calls to mind images of a friendly, warm yellow star accompanied by a retinue of planets possibly capable of nurturing life. But new calculations by Harvard astronomers Volker Bromm and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics), which were announced today at the 203rd meeting of the American Astronomical Society in Atlanta, show that the first Sun-like stars were lonely orbs moving through a universe devoid of planets or life.

“The window for life opened sometime between 500 million and 2 billion years after the Big Bang” says Loeb. “Billions of years ago, the first low-mass stars were lonely places. The reason for that youthful solitude is embedded in the history of our universe.”

In The Beginning
The very first generation of stars were not at all like our Sun. They were white-hot, massive stars that were very short-lived. Burning for only a few million years, they collapsed and exploded as brilliant supernovae. Those very first stars began the seeding process in the universe, spreading vital elements like carbon and oxygen, which served as planetary building blocks.

“Previously, with Lars Hernquist and Naoki Yoshida (also at the CfA), I have simulated those first supernova explosions to calculate their evolution and how much heavy elements (elements heavier than hydrogen or helium) they produced,” says Bromm. “Now, in this work, Avi Loeb and I have determined that a single first-generation supernova could produce enough heavy elements to enable the first Sun-like stars to form.”

Bromm and Loeb showed that many second-generation stars had sizes, masses, and hence temperatures similar to our Sun. Those properties resulted from the cooling influence of carbon and oxygen when the stars formed. Even elemental abundances as low as one-ten thousandth those found in the Sun proved sufficient to allow smaller, low-mass stars like our Sun to be born.

Yet those same low abundances prohibited rocky planets from forming around those first Sun-like stars due to a lack of raw materials. Only as further generations of stars lived, died, and enriched the interstellar medium with heavy elements did the birth of planets, and life itself, become possible.

“Life is a recent phenomenon,” Loeb states unequivocally. “We know that it took many supernova explosions to make all the heavy elements we find here on Earth and in our Sun and our bodies.”

Recent observational evidence corroborates their finding. Studies of known extrasolar planets have found a strong correlation between the presence of planets and the abundance of heavy elements (“metals”) in their stars. That is, a star with higher metallicity and more heavy elements is more likely to possess planets. Conversely, the lower a star’s metallicity, the less likely it is to have planets.

“We’re now just beginning to investigate the metallicity threshold for planet formation, so it’s hard to say when exactly the window for life opened. But clearly, we’re fortunate that the metallicity of the matter that birthed our solar system was high enough for the Earth to form,” says Bromm. “We owe our existence in a very direct way to all the stars whose life and death preceded the formation of our Sun. And this process began right after the Big Bang with the very first stars. As the universe evolved, it progressively seeded itself with all the heavy elements necessary for planets and life to form. Thus, the evolution of the universe was a step-by-step process that resulted in a stable G-2 star capable of sustaining life. A star we call the Sun.”

Original Source: Harvard CfA News Release

Posted on by Fraser Cain

Spirit Landing Site Named for Columbia Crew

Image credit: NASA/JPL

NASA administrator Sean O’Keefe announced on Tuesday that they plan to name the Spirit landing site in honour of the Columbia astronauts who lost their lives nearly a year ago. The place where Spirit landed in Gusev Crater will be called the Columbia Memorial Station. One image sent back by the rover shows a memorial plaque attached to Spirit’s high-gain antenna – the plaque is aluminum and approximately 15 cm across.

NASA Administrator Sean O’Keefe today announced plans to name the landing site of the Mars Spirit Rover in honor of the astronauts who died in the tragic accident of the Space Shuttle Columbia in February. The area in the vast flatland of the Gusev Crater where Spirit landed this weekend will be called the Columbia Memorial Station.

Since its historic landing, Spirit has been sending extraordinary images of its new surroundings on the red planet over the past few days. Among them, an image of a memorial plaque placed on the spacecraft to Columbia’s astronauts and the STS-107 mission.

The plaque is mounted on the back of Spirit’s high-gain antenna, a disc-shaped tool used for communicating directly with Earth. The plaque is aluminum and approximately six inches in diameter. The memorial plaque was attached March 28, 2003, at the Payload Hazardous Servicing Facility at NASA’s Kennedy Space Center, Fla. Chris Voorhees and Peter Illsley, Mars Exploration Rover engineers at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., designed the plaque.

“During this time of great joy for NASA, the Mars Exploration Rover team and the entire NASA family paused to remember our lost colleagues from the Columbia mission. To venture into space, into the unknown, is a calling heard by the bravest, most dedicated individuals,” said NASA Administrator Sean O’Keefe. “As team members gazed at Mars through Spirit’s eyes, the Columbia memorial appeared in images returned to Earth, a fitting tribute to their own spirit and dedication. Spirit carries the dream of exploration the brave astronauts of Columbia held in their hearts.”

Spirit successfully landed on Mars Jan. 3. It will spend the next three months exploring the barren landscape to determine if Mars was ever watery and suitable to sustain life. Spirit’s twin, Opportunity, will reach Mars on Jan. 25 to begin a similar examination of a site on the opposite side of the planet.

Original Source: NASA News Release

Posted on by Fraser Cain

SMART-1 Gets Out of the Radiation Belts

Image credit: ESA

The European Space Agency’s SMART-1 spacecraft has completed its 176th orbit around the Earth, finally reaching the outer limits of our planet’s Van Allen radiation belts. After weeks in the intense radiation, it looks like everything on SMART-1 is functioning normally. The spacecraft has fired its ion thruster for a total of 1,500 hours and only consumed 24 kg of Xenon fuel. SMART-1 is taking the slow road to the Moon, where it will map the surface and search for deposits of ice.

The spacecraft is now in its 176th orbit, in good status and with all functions performing nominally. The first mission target, namely to exit the most dangerous part of the radiation belts, has been achieved! The pericentre altitude (the closest distance of the spacecraft from the centre of the Earth) will reach the prelaunch target of 20 000 km on 7 January 2004.

Between 23 December 2003 and 2 January 2004, the thruster fired continuously for a record duration of more than 240 hours. This is likely to remain the record for some time because later this week SMART-1 will change from a continuous thrust strategy to a more orbitally efficient thrust arcing.

The total cumulated thrust so far of more than 1500 hours, consuming 24 kg of Xenon, has provided a velocity increment of about 1070 ms-1 (equivalent to 3850 km per hour). The electric propulsion engine’s performance, periodically monitored by means of the telemetry data transmitted by the spacecraft and by radio-tracking by the ground stations, continues to show a small over performance in thrust: varying from 1.1% to 1.5% over the last week.

The degradation of the electrical power produced by the solar arrays has now ceased. The power available has remained virtually constant since November 2003.

The communication, data handling, on-board software and thermal subsystems have been performing well in this period.

Original Source: ESA News Release

Posted on by Fraser Cain

Chandra Sees Colliding Galaxies

Image credit: Chandra

The Chandra X-Ray Observatory has found rich deposits of neon, magnesium and silicon in a pair of colliding galaxies called The Antennae, located 30 million light-years away. When these hot clouds eventually cool, they’ll serve as enormous nurseries for newborn stars. Astronomers are interested in this collision because it’s very similar to what will happen when the Milky Way and Andromeda galaxies collide in about 3 billion years.

NASA’s Chandra X-ray Observatory has discovered rich deposits of neon, magnesium, and silicon in a pair of colliding galaxies known as The Antennae. When the clouds in which these elements are present cool, an exceptionally high number of stars with planets should form. These results may foreshadow the fate of the Milky Way and its future collision with the Andromeda Galaxy.

“The amount of enrichment of elements in The Antennae is phenomenal,” said Giuseppina Fabbiano of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. at a press conference at a meeting of the American Astronomical Society in Atlanta, Ga. “This must be due to a very high rate of supernova explosions in these colliding galaxies.” Fabbiano is lead author of a paper on this discovery by a team of U.S. and U.K. scientists that will appear in an upcoming issue of The Astrophysical Journal Letters.

When galaxies collide, direct hits between stars are extremely rare, but collisions between huge gas clouds in the galaxies can trigger a stellar baby boom. The most massive of these stars race through their evolution in a few million years and explode as supernovas. Heavy elements manufactured inside these stars are blown away by the explosions and enrich the surrounding gas for thousands of light years.

“The amount of heavy elements supports earlier studies that indicate there was a very high rate of relatively recent supernovas, 30 times that of the Milky Way,” according to collaborator Andreas Zezas of the CfA.

The supernova violence also heats the gas to millions of degrees Celsius. This makes much of the matter in the clouds invisible to optical telescopes, but it can be observed by an X-ray telescope. Chandra data revealed for the first time regions of varying enrichment in the galaxies ? in one cloud magnesium and silicon are 16 and 24 times as abundant as in the Sun.

“These are the kinds of elements that form the ultimate building blocks for habitable planets,” said Andrew King of the University of Leicester, U.K. and a coauthor of the study. “This process occurs in all galaxies, but it is greatly enhanced by the collision. Usually we only see the new elements in diluted form as they are mixed up with the rest of the interstellar gas.”

CfA coauthor Alessandro Baldi commented that, “This is spectacular confirmation of the idea that the basis of chemistry, of planets, and ultimately of life is assembled inside stars and spread through galaxies by supernova explosions,”

As the enriched gas cools, a new generation of stars will form, and with them new planets. A number of studies indicate that clouds enriched in heavy elements are more likely to form stars with planetary systems, so in the future an unusually high number of planets may form in The Antennae.

“If life arises on a significant fraction of these planets, then in the future the Antennae will be teeming with life,” speculated Francois Schweizer, another coauthor who is from the Carnegie Observatories in Pasadena, Calif. “A vast number of Sun like stars and planetary systems will age in unison for billions of years.”

At a distance of about 60 million light years, The Antennae system is the nearest example of a collision between two large galaxies. The collision, which began a couple of hundred million years ago, has been so violent that gas and stars from the galaxies have been ejected into the two long arcs that give the system its name. The Chandra image shows spectacular loops of 3-million-degree gas spreading out south of the antennae. “These loops may be carrying out some of the elements dispersed by supernovas into intergalactic space,” said Trevor Ponman of Birmingham University, U.K.

The Antennae give a closeup view of the type of collisions that were common in the early universe and likely led to the formation of most of the stars that exist in the universe today. They may also provide a glimpse of the future of our Milky Way Galaxy, which is on a collision course with the Andromeda Galaxy. At the present rate, a crash such as the one now occurring in the Antennae could happen in about 3 billion years. Tremendous gravitational forces will disrupt both galaxies and reform them, probably as a giant elliptical galaxy with hundreds of millions of young Sun like stars, and possibly planetary systems.

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Original Source: Chandra News Release

Posted on by Fraser Cain

Stardust Heads for Home

Image credit: NASA/JPL

Now that it’s survived its dangerous journey through the tail of Comet Wild-2, NASA’s Stardust spacecraft is heading for home. On board is its precious cargo of comet fragments that will be returned to Earth in 2006 for analysis by scientists. Even though the spacecraft’s camera was only really designed for navigation, it took 72 photographs which are some of the best images of a comet ever taken. Scientists hope that the cometary particles will help answer questions about the earliest history of our solar system.

Having weathered its out-of-this-world sandblasting by cometary particles hurtling toward it at about six times the speed of a rifle bullet, NASA’s Stardust spacecraft begins its two-year, 1.14 billion kilometer (708 million mile) trek back to its planet of origin.

“On January 2, comet Wild 2 gave up its particles but it did not do so without a fight,” said Stardust Project Manager Tom Duxbury of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Our data indicates we flew through sheets of cometary particles that jostled the spacecraft and that on at least 10 occasions the first layer of our shielding was breeched. Glad we had a couple more layers of the stuff.”

Stardust entered the comet’s coma ? the vast cloud of dust and gas that surrounds a comet’s nucleus – on December 31, 2003. From that point on it kept its defensive shielding between it and what scientists hoped would be the caustic stream of particles it would fly through. And fly through cometary particles Stardust did, but not in the fashion the team envisioned while designing the mission.

“We thought we would see a uniform increase in the number of particles the closer we came to the comet’s nucleus and then a reduction,” said University of Washington scientist Dr. Don Brownlee, Stardust’s Principal Investigator. “Instead, our data indicate we flew through a veritable swarm of particles and then there would be almost nothing and then we would fly through another swarm.”

Stardust scooped up these cometary particles, impacting at 6.1 kilometers per second (3.8 miles per second), for almost instantaneous analysis from onboard instruments and stored other particles for later, in-depth analysis, here on Earth. Along with this cosmic taste testing, the spacecraft also took some remarkable images of comet Wild 2’s five-kilometer wide (3.1- mile wide) nucleus.

“Our navigation camera was designed to assist in navigation, not science,” said Stardust’s imaging team lead Ray Newburn. “But these are the best images ever taken of a comet and there is a remarkable amount of information in those 72 pictures. Not only did we image the jets of material spewing out from the comet, but for the first time in history we can actually see the location of their origin on the surface of the comet.”

At about 11:25 am Pacific Standard Time (2:25pm EST) on Jan. 2, only minutes after its closest approach with the comet, Stardust pointed its high gain antenna at Earth and began transmitting a data stream that took over 30 hours to send but will keep cometary scientists busy for years to come. About six hours later another event took place that goes a long way to literally increasing the scientists task load exponentially.

“Six hours after encounter we retracted the collector grid, with what we are all confident is an abundance of cometary particles, into the spacecraft’s sample return capsule,” added Duxbury. “The next time the sample return capsule is going to be opened is in a clean room at the Johnson Space Center in the days following Earth return in January 2006.”

Scientists expect in-depth terrestrial analysis of the samples will reveal much about comets and the earliest history of the solar system. Chemical and physical information locked within the particles could be the record of the formation of the planets and the materials from which they were made. More information on the Stardust mission is available at http://stardust.jpl.nasa.gov.

Stardust, a part of NASA’s Discovery Program of low-cost, highly focused science missions, was built by Lockheed Martin Space Systems, Denver, Colo., and is managed by JPL for NASA’s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Mars Express Fails to Communicate with Beagle 2

Image credit: ESA

The first opportunity for Mars Express to hear from Beagle 2 has come and gone, and so far, there’s just silence. The spacecraft passed over the anticipated landing area Tuesday, at 1213 UTC (7:13 am EST) and attempted to make contact with the lander. Several more attempts are planned for the coming days, and now that Mars Express has reached its operational orbit, there should be plenty of opportunities to hear from Beagle 2 if it’s still intact on the surface of Mars.

ESA?s Mars Express orbiter made its first attempt to establish contact with the Beagle 2 lander, after the two spacecraft separated on 19 December 2003.

The orbiter made its first pass over the Beagle 2 landing site today at 13:13 CET, but could not pick up any signal from the tiny lander. More attempts to contact Beagle 2 are planned in the days to come.

Beagle 2 was released on 19 December on a course towards the Red Planet by Mars Express, the mothership for the 400 million kilometre interplanetary cruise. Six days later it entered the Martian atmosphere and should have landed on the near-equatorial site of Isidis Planitia.

Since then, attempts to communicate with the lander through NASA?s Mars Odyssey orbiter and radio telescopes on Earth have been unsuccessful.

The Mars Express orbiter successfully entered Mars orbit at about the same time as Beagle 2?s landing. Then, in early January, it made a series of planned manoeuvres to change its equatorial orbit to a polar one, to prepare for its scientific mission and to make contact with Beagle 2.

Unlike Mars Odyssey and the radio telescopes, Mars Express has a communication system that was fully tested to contact Beagle 2, which gives ESA more confidence of picking up the signal in the coming days.

?We have not lost hope yet to contact Beagle 2, but we also know that it has landed on an unforgiving planet,? said David Southwood, ESA?s Director of Science.

?There are still opportunities to make contact with Beagle in the days to come, and we are giving our best efforts. Nevertheless, our spacecraft Mars Express has now reached its operational orbit and is working well; I know the science community is eagerly waiting for its first results.?

Original Source: ESA News Release

Posted on by Fraser Cain

Spirit’s First Colour Photo of Mars

Well, here it is, the picture we’ve all been waiting for – the first colour image from the surface of Mars taken by Spirit. I can’t believe the resolution this rover has. I’ve taken a big chunk of the image and processed it to be a 1280 x 1024 wallpaper. To set this image as your desktop wallpaper, click the following link, and then right-click anywhere in the image and select “Set as Wallpaper”.

Click here to download the first colour image of Mars taken by Spirit. (156 KB)

If you want to access the original, high-resolution image from NASA, click here instead. I’ll warn you, though, it’s more than 4 MB… and that’s just the medium resolution version.

Enjoy!

Fraser Cain
Publisher
Universe Today

Posted on by Fraser Cain

Biggest Stars Often Have Companions

Image credit: Hubble

New research from the Hubble Space Telescope indicates that the majority of large dying Wolf-Rayat stars have a smaller companion star orbiting nearby. This discovery will help astronomers understand how these unique stars evolve in the Universe, and could provide new a new method to estimate their size. Wolf-Rayat stars start out at least 20 times the mass of the Sun, last only a few million years, and then explode as supernovae. It’s now believed that these stars and their companions transfer mass as they orbit one another.

The majority of massive and brilliant but dying “Wolf-Rayet” stars have company – a smaller companion star orbiting nearby, according to new observations using the Hubble Space Telescope. The result will help astronomers understand how the biggest stars in the Universe evolve. It may also resolve the mystery of impossibly massive stars, and calls into question a certain kind of distance estimate that uses the apparent brightness of starlight.

Wolf-Rayet (WR) stars begin life as cosmic titans, with at least 20 times the mass of the Sun. They live fast and die hard, exploding as supernova and blasting vast amounts of heavy elements into space for use in later generations of stars and planets. “I tell people I study the stars that made a lot of the carbon in their bodies and the gold in their jewelry,” says Dr. Debra Wallace of NASA’s Goddard Space Flight Center, Greenbelt, Md. “Understanding how Wolf-Rayet stars evolve is a critical link in the chain of events that ultimately led to life.” Wallace is lead author of papers on this research to be published in the Astronomical Journal and the Astrophysical Journal.

By the time these stars are near the end of their brief lifetimes, during the “Wolf-Rayet” phase, they are fusing heavy elements in their cores in a frantic bid to prevent collapsing under their own immense mass. This generates intense heat and radiation that drives fierce, 2.2 million to 5.4 million mile-per-hour (3.6 million to 9 million km/hr) stellar winds characteristic of WR stars (Image 1). These winds blow off the outer layers of WR stars, greatly reducing their mass and compressing nearby interstellar clouds, triggering their gravitational collapse and igniting a new generation of stars.

Because cosmic distances are so great, what appears as a single star even when viewed through large telescopes (Image 2) may in fact be two or more stars orbiting each other (Images 3 and 4). In the new research, Wallace and her team used the superior resolving power of the Planetary Camera in the Wide-Field Planetary Camera 2 instrument on board Hubble to identify new potential companion stars for 23 of 61 WR stars in our galaxy. Although the apparent companion stars need to be confirmed with a light-analysis technique called spectroscopy, the team was conservative in declaring nearby stars companions.

“The portion of Wolf-Rayet stars having visually identified companion stars zoomed from 15 percent before Hubble to 59 percent with our observations, which included a quarter of the known WR stars in our galaxy,” said Wallace. “I wouldn’t be surprised if future observations reveal companions around an even greater percentage of them.”

The presence of a companion star should significantly influence how these stars evolve, according to the team. One of many possible influences is mass transfer. If the stars come close together at some point in their orbits, their gravitational interaction could cause one to transfer gas to the other, significantly altering their masses over time. Since more massive stars use up their fuel much faster than less massive stars, such a mass transfer could significantly change their lifetimes. Other influences include altering orbits, rotation rates, or mass-loss rates through the pull of their gravity, and the impact of stellar winds. “Astronomers assumed Wolf-Rayet stars were single when trying to calculate how they evolve, but we are finding most have company,” said Wallace. “It’s like thinking married life will be the same as life as a bachelor. A companion star has got to change the life of these stars somehow.”

Since what is seen as one star may in fact be two or even more, stupendous mass estimates of more than a hundred times that of the Sun for certain stars may have to be revised downward. “This actually helps clear up an apparent mystery, because astronomers believe there is a limit to how big a star can be,” said Wallace. “The more massive a star, the faster it consumes its fuel and the brighter it shines. Above about 100 solar masses, a star should essentially blow itself apart through its intense radiation.”

The result also makes a common technique for estimating distances to these stars more uncertain. To get a distance estimate to a star, one gets the spectral type of the star, an analysis of the star’s light that reveals its unique characteristics, like a fingerprint. For a given spectral type, one knows the star’s average absolute luminosity (how bright it would be if it were a certain distance – 32.6 light-years – away). By measuring its apparent luminosity (how bright it appears to be at its actual, but unknown, distance), one can then use the relationship between its apparent and absolute luminosity to determine the actual distance. If there are really two (or more) stars there that you don’t see, the WR star will appear to be brighter than it should for its spectral type and real distance, causing the distance to be misestimated.

The team includes Wallace; Dr. Douglas R. Gies of the Department of Physics and Astronomy, Georgia State University, Atlanta, Ga.; Anthony F. J. Moffat, D?partement de Physique, Universit? de Montr?al, Quebec, Canada; and Michael M. Shara, Department of Astrophysics, American Museum of Natural History, New York, N.Y. The research was funded by NASA.

Original Source: NASA News Release