Temperature of Stars

Artist's impression of a red giant star. Image credit: ESO

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You might be surprised to know that the color of stars depends on their temperature. The coolest stars will look red, while the hottest stars will appear blue. And what defines the temperature of a star? It all comes down to mass.

The most common stars in the Universe are the relatively tiny red dwarf stars. These stars can have as little as 7.5% the mass of the Sun, and top out at about 50%. Red dwarfs use their stores of hydrogen fuel very slowly; it’s believed that a red dwarf star with about 10% the mass of the Sun may live for 10 trillion years or more. Our own Sun will only live for about 12 billion years. Red dwarf stars have a surface temperature of less than 3,500 Kelvin, and this is why they appear red to our eyes.

Our own Sun is classified as a yellow dwarf star. It has a surface temperature of about 5,800 Kelvin. Because of this temperature, the bulk of the light we see streaming from the Sun is yellow/white. Our Sun has been in the main sequence phase of its life for 4.5 billion years, and it’s expected to last another 7 billion years or so.

The hottest stars are the blue stars. These start at temperatures of about 10,000 Kelvin, and the biggest, hottest blue supergiants can be more than 40,000 Kelvin. In fact, there’s so much energy coming off the surface of a blue star that many could actually be classified as ultraviolet stars, it’s just that our eyes can’t see that high into the spectrum.

We have written many articles about stars here on Universe Today. Here’s an article about how red dwarf stars could have habitable zones, and here’s an article about how red dwarfs can clear out their dusty disks.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

What Were the First Stars?

Artist's impression of the first stars. Image credit: NASA/WMAP

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Astronomers now know that the Big Bang occurred 13.7 billion years ago. For the first few hundred million years, the entire Universe was too hot any stars to form. But then the Universe cooled down to the point that gravity could start pulling together the raw hydrogen and helium into the first ever stars.

The basic elements on the Universe, hydrogen and helium and a few trace elements, we formed during the Big Bang. For a brief moment, the entire Universe was at the temperature and pressure that hydrogen could fuse into helium. This is why we see roughly the same ratios of hydrogen to helium, everywhere we look in the Universe: 73% hydrogen, 25% helium, and the rest are trace elements.

Astronomers think that this pure hydrogen/helium mix allowed the first stars to grow much more massive than stars can get today. It’s believed that they could have gathered together several hundred solar masses. The most massive star that can form today is thought to only be about 150 solar masses. After that point, extreme winds coming from the star prevent any additional material from falling in.

This first generation of stars, which astronomers call Population III stars, would have lived short violent lives. They probably lasted just a million years or so, and then detonated as supernovae. But in their lives, these Population III stars would have created heavier and heavier elements at their cores, and in their violent deaths, they would have created the even more exotic heavier elements, like gold and uranium. It’s possible that the first stars went through a few quick cycles, pulling in material, detonating and seeing the region with heavier elements. Eventually the first long-term stars would have gotten going, stars with the amount of heavier elements we see today.

None of the first stars have ever been observed directly. There have been a few hints through gravitational lensing; using a nearby galaxy’s gravity to focus the light from a more distant quasar. The next generation of space telescopes, like the James Webb Space Telescope might be able to push the observable Universe back to these first stars.

We have written many articles about stars here on Universe Today. Here’s an article about astronomers simulating the formation of the first stars, and here’s an article about how the first stars could have been powered by dark matter.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Source: Caltech IPAC

More Images From Aurora Flights

ACES Flight from Jan. 29, 2009. Credit: Dr. Craig Heinselman.

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On January 29, two sounding rockets simultaneously flew through the veil of an aurora to collect data from both the top and bottom edges of the arc. Dr. Scott Bounds, the principal investigator for the Auroral Current and Electrodynamics Structure (ACES) mission, provided Universe Today with images from the flight, showing the rockets flying through the aurora, near Poker Flats, Alaska. The above image shows a single-stage Black Brant V rocket that flew through the lower portion of the aurora. It reached an altitude of nearly 83 vertical miles, flying for roughly eight minutes. (See below for more images.) Other rockets have flown through aurorae previously, but this is the first time two rockets were used together. These two flights for the ACES mission will provide insight on the structural subtleties of the aurora, finding details that researchers may have missed when previous measurements were done using only a single vehicle (see our original article on the flights).

ACES rocket reaches for the top of the aurora. Credit: Dr. Craig Heinselman.
ACES rocket reaches for the top of the aurora. Credit: Dr. Craig Heinselman.

The image here shows a two-stage Black Brant IX rocket launched at 12:49 a.m. on January 29 that reached an altitude of more than 226 miles and flew for just under 10 minutes.

Dr. Bounds of the University of Iowa said the payloads of each ACES rocket performed well during flight, and the ACES team will begin to analyze all of the data collected, which should keep them busy for the next year. Bounds said this information will help refine current models of aurora structure, and provide insight on the high-frequency waves and turbulence generated by aurorae.

Thanks to Dr. Bounds for sharing these images with Universe Today, and to Dr. Craig Heinselman the photographer. Below is an image taken by Dr. Bounds of an aurora in 2002, taken where the ACES flights originate at the Poker Flat Research Range near Fairbanks, Alaska.

Aurora from 2002 in Poker Flats, Alaska.  Credit: Dr. Scott Bounds
Aurora from 2002 in Poker Flats, Alaska. Credit: Dr. Scott Bounds

Weekend SkyWatcher’s Forecast – February 6-8, 2009

Greetings, fellow SkyWatchers! For some parts of the world, baby? It’s cold outside… But those -23C temperatures haven’t kept some of us from chasing Comet C/2007 N3 Lulin and just knowing a few bright stars and having a pair of binoculars is all it takes for you to spot it, too! Or maybe you’d just like to spot the ISS? If skies are clear, why not spot the Moon and where SMART-1 did some imaging? Better yet, how about spotting a bright star to learn about – or a fantastic lunar impact crater? If you’re ready to see some “spots”, then grab your observing equipment and meet me outside…

salyutFriday, February 6, 2009 – This date marks the 1991 fiery return of the Soviet space station Salyut 7. Launched in 1982, electrical and maneuvering problems plagued the mission, but cosmonauts were able to stay on board for as long as 8 months before returning. Abandoned in 1986, equipment and supplies were transferred to the orbiting Mir. If you’d like to spot a space station like the ISS, check Heavens Above on how to get information for your area.

gassendi

Return to the Moon tonight and explore the outstanding ring of crater Gassendi on the north shore of Mare Humorum. It’s over 3.6 billion years old, and about the size of the state of Arkansas! Roughly 110 kilometers in diameter and 2,010 meters deep, this ancient crater contains a central triple mountain peak and forms one of the most ‘‘perfect circles’’ on the Moon. Lava flows have eroded Gassendi’s south wall, and its floor is covered with ridges and rilles. Look for small crater Gassendi A on the northern rim. The northwestern section of the crater wall is slightly higher, saving it from the lava that formed Mare Humorum. Look carefully at the lower southern rim to find the gap where lava spilled over the walls. It’s only 200 meters above the surface, while other areas rise as much as 2,500 meters. The fact that some parts of the rim escaped the lava is what makes this an interesting area for science!

On January 13, 2006, the Advanced Moon Imaging Experiment (AMIE) on board ESA’s SMART-1 spacecraft imaged Gassendi from about 1,220 kilometers above the surface. Previous spectroscopic studies had shown that the central peaks might have experienced their own volcanic period – earlier than the event that filled the lower floors. These ancient highland rocks may one day give us significant insight into the thermal history of the Mare Humorum impact basin and the processes that formed Gassendi. Enjoy this ancient beauty!

hugginsSaturday, February 7, 2009 – On this date in 1889, the Astronomical Society of the Pacific was born. In 1926 celebrated cosmonaut Konstantin Feoktistov, who flew Voskhod 1 and helped design Salyut and Mir, was born. Yet most noteworthy today is the 1824 birth of amateur astronomer William Huggins. By age 30, he’d built his own private observatory and through his studies made important contributions to astronomy. According to scientists Kirchoff and Bunsen, the chemical composition of minerals could be determined from their spectral signatures. The inquisitive Huggins began comparing mineral samples to the spectra of celestial objects. Although his experimental methods were crude by today’s standards, his calculations were perfect. Huggins proved the spectrum of the Orion Nebula was like that of a pure gaseous emission, while the spectrum of the Andromeda Nebula was similar to that of starlight—and this long before confirmation of its galactic nature! Huggins was also the first amateur to measure the radial velocities of stars from their spectral shifts. Although most people assume only professional scientists can make such measurements, many of today’s amateurs (unpaid, but not unskilled!) have measured spectra.

saiphTonight let’s look at a star whose radial velocity has been studied both professionally and personally – Kappa Orionis (RA 05 47 45 Dec -09 40 10). Named Saiph, it’s the often-overlooked eastern ‘‘foot’’ of Orion. According to spectral analysis, this 722 light-year distant blue supergiant is moving away from us at 21 kilometers per second.

Roughly the same type, size, and distance as Rigel, it looks far fainter. But why? Oddly enough, Saiph has an extremely high temperature, burning more than 1,500 K hotter. Near the point where helium fusion replaces hydrogen fusion, the majority of its variable light output is in the ultraviolet band. And as Huggins once said: ‘‘It is remarkable that the elements diffused through the host of stars are some of the most closely connected with the living organisms of our globe.’’

mars_meteoriteSunday, February 8, 2009 – On this date in 2001, the Sayh al Uhaymir 094 Mars Meteorite was discovered. Some space-born debris from Martian impact craters is eventually captured by Earth’s gravity. The surviving meteorites can be identified by their mineral composition, as well as from tiny gas deposits matching Viking lander samples of Mars’s atmosphere.

Tonight, aim your optics toward the Moon and study an impact crater large enough to have blasted lunar material back to Earth. Its name is Tycho… Take one glance at the lunar Southern Hemisphere, and you can’t miss the dazzling display of 85-kilometer-wide Tycho, and its brilliant splash ray pattern. Perhaps 100 million years ago a comet, an asteroid, or a large meteorite impacted the Moon, flinging debris far and wide. One of Tycho’s ejecta paths (rays) crosses the Apollo 17 landing site almost 2,000 kilometers away, where it caused a landslide, revealing deeper materials. Shining like a beacon in Tycho’s center is a mountain peak originating from below the surface crust. The crater floor is lumpy and the rim broken by the force of the impact.

tycho

Could a collision like Tycho’s create Earth-bound meteoroids? Indeed, you may have walked on one unaware! The first confirmed lunar meteorite was found in 1979 in Antarctica, but it was many years before its true identity was known. Confirmation required comparison of its chemical composition to that of Apollo lunar samples. To date, only around 40 confirmed lunar meteorites are known, but as many as one in every thousand may have originated from our nearest neighbor. Noble gas measurements show some of these materials may have left the lunar surface up to 20 million years ago, but most are around 100,000 years old. They might resemble terrestrial rocks, but ones with their chemical composition are found only on the Moon. Have a look at Tycho and imagine the power that sculpted this mighty crater!

C2007 N3 Lulin Imaged on February 5 - Joe Brimacombe
C2007 N3 Lulin Imaged on February 5 - Joe Brimacombe

For those who don’t mind getting up early, be sure to keep your eye on the mighty Comet C/2007 N3 Lulin! If you have trouble reading star charts – don’t worry. It’s a whole lot easier to find in binoculars than you might think. All you need to do is just know how to identify a few bright stars! If you live in the northern hemisphere, about an hour (even two if you have a clear skyline) go out and identify Scorpius rising to the southeast – or Virgo high in the south, if you prefer. (For other locations, simply follow the ecliptic plane.) Between the two constellations you’ll see bright optical double star Alpha Librae – Zubenelgenubi. You’ll know if you have the right star because it will appear as two close stars in your binoculars. As of the morning of February 6th, Comet Lulin appeared in the 2 o’clock position with Zubenelgenubi in the field and it’s slowly headed towards Spica – Alpha Virginis. Remember as each successive day passes to start at Zubenelgenubi (it’s about a hand span east-southeast of Spica) and move the binoculars slowly towards Spica until you spot it. It will appear as a small, faint fuzzy in 5X30 binoculars and elongated in 16X60s. Even with telescopes as small as 114mm in aperture, it’s easy to make out that signature tail! Don’t wait too long to capture it, though… Because it won’t be long until the Moon is going to interfere and make this 7th magnitude comet far more difficult to spot.

Until next week? Ask for the Moon, but keep on reaching for the stars! You could just catch a comet…

This week’s awesome images are: Salyut (historical image-NASA), Gassendi (credit-Wes Higgins), William Huggins (historical image), Kappa Orionis: Saiph (credit-Palomar Observatory, courtesy of Caltech), Mars meteorite (historical image), Tycho (credit-Roger Warner) and Comet Lulin (credit-Joe Brimacombe). Thank you for sharing!

Red Supergiant Star

VY Canis Majoris. The biggest known star.
Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen

The biggest stars in the Universe are the red supergiant stars. And we’re talking really, really big. The largest known red supergiant is thought to be VY Canis Majoris, measuring about 1800 times the size of the Sun. Imagine if the Sun extended out to the orbit of Saturn. Let’s take a look at where red supergiant stars come from.

Red supergiants are similar to red giants. They form when a star runs out of hydrogen fuel in their core, begins collapsing, and then outer shells of hydrogen around the core get hot enough to begin fusion. While a red giant might form when a star with the mass of our Sun runs out of fuel, a red supergiant occurs when a star with more than 10 solar masses begins this phase.

The five largest known supergiants in the galaxy are red supergiants: VY Canis Majoris, Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni. Each of these stars has a radius larger than 1500 times the size of the Sun. In comparison, regular red giant is only 200 to 800 times the size of the Sun.

Red supergiant stars don’t last long; typically only a few hundred thousand years, maybe up to a million. Within this period, the core of the red supergiant continues to fuse heavier and heavier elements. This process stops when iron builds up in the core of the star. Iron is the equivalent of ash when it comes to nuclear fusion. The process of fusing iron actually requires more energy than it releases.

At this point, many red supergiants will detonate as Type II supernovae.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article that talks about the three largest stars discovered.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Opportunity Rover Sidelined by Charged Particle Hit

MER's camera mast, which holds several cameras, may have been hit by a cosmic ray. Credit: NASA/JPL

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The Opportunity rover recently surpassed the five-year mark on Mars. And what did she get as a birthday present? A thorough zapping by a charged particle, perhaps a cosmic ray, which has sidelined the rover for the past several days. “Opportunity stood down for a few sols as a result of a PMA (Panoramic Mast Assembly) error,” said Scott Maxwell, one of the rover drivers for the two Mars Exploration Rovers, Opportunity and Spirit. “This turned out to be due to an SEU (Single-Event Upset), as reported by the on-board motor controller.” An SEU happens when a charged particle whizzes through a transistor on the rover and flips a bit somewhere inside. “Fortunately, the motor controllers can detect and report these events, so that the rover can safely stop,” Maxwell told Universe Today. “We have good reason to hope that Opportunity’s PMA is undamaged and that she’ll be back on the road shortly.” The PMA is the rover’s “neck and head;” it is the mast that holds the Panoramic Cameras, the Navigation Cameras and the Mini-Thermal Emission Spectrometer. It would be a critical blow to the mission to lose any or all of these instruments. The Spirit rover may also have been hit recently by a cosmic ray, causing her to “loose her memory” for a short period. The good news is that Spirit seems to be back to normal and has resumed driving again.

No images from the Pancams or Navcams have been downloaded to Earth for the past four days, since sol 1787 (today is sol 1791 for Opportunity.) Opportunity has been traversing quite quickly on her way to Endeavour Crater, about 12 kilometers (7 miles) away. That distance would match the total distance Opportunity traveled from 2004 to mid-2008. Even at the 100-meter plus pace each sol, which “Oppy” was able to do back in December, the journey could take two years.

“The terrain Opportunity is passing through is good driving terrain,” said Maxwell, “although not quite a parking lot, but nothing she can’t handle. With the help of our “Martian satellite navigation system” (that is, the beautiful high-resolution orbital images we get from MRO), we expect to continue making good time through this dune field on the way to Endeavour.”

We’ll keep you posted on Opportunity’s status.

Latest panorama from Opportunity from Sol 1770.  Credit: NASA/JP
Latest panorama from Opportunity from Sol 1770. Credit: NASA/JP

Spirit resumed driving Saturday, albeit just a short drive, after engineers performed diagnostic tests to determine the cause of “unusual behavior” by the rover last week. On Spirit’s Sol 1800, the rover did not save information into its non-volatile flash memory, so the information was lost when the rover next powered down. She also seemed to be disoriented, and was not able to locate the sun correctly.

“We may not find any data that will explain what happened on Sol 1800, but there’s no evidence that whatever happened then has recurred on subsequent sols,” said Jacob Matijevic of the rover engineering team at NASA’s Jet Propulsion Laboratory, Pasadena. One possibility is that a cosmic-ray hit could have temporarily put Spirit temporarily into a mode that disables use of the flash memory.

Spirit drove only about 30 centimeters (1 foot) Saturday, during the 1,806th Martian sol. The rover team had commanded a longer drive, but Spirit stopped short after its right-front wheel, which no longer turns, struck a partially buried rock. The rover drivers prepared commands Monday for the next drive in a slightly different direction to get around that rock.

Raw image from Spirit from her Sol 1806.  Credit: NASA/JPL
Raw image from Spirit from her Sol 1806. Credit: NASA/JPL


Spirit is just north of a low plateau called “Home Plate.” It spent 2008 on a north-facing slope on the edge of Home Plate so that its solar panels stayed tilted toward the winter sun for maximum electrical output.

Spirit drove down off Home Plate on Jan. 6, 2009. It subsequently checked whether a patch of nearby soil, called “Stapledon,” had a high concentration of silica, like a silica-rich patch of soil Spirit discovered east of Home Plate in 2007. The earlier discovery was interpreted as evidence left by a hot-spring or steam-vent environment. Examination with Spirit’s alpha particle X-ray spectrometer confirmed silica at Stapledon. This indicates that the environment that deposited the silica was not limited to the location found earlier.

Sources: JPL, email exchange with Scott Maxwell

What Is A Red Giant Star?

Betelgeuse was the first star directly imaged -- besides our own Sun, of course. Image obtained by the Hubble Space Telescope. Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA

When star like our Sun reaches the end of its life, it enters one last phase, ballooning up to many times its original size. Astronomers call these objects red giant star, and you’ll want to learn more about them, since this is the future fate for the Sun. Don’t panic, we’ve got another 7 billion years or so before the Sun becomes a red giant star.

As you probably know, stars shine because they’re converting hydrogen into helium in their cores through a process called nuclear fusion. Our own Sun has been performing fusion at its core for 4.5 billion years, and will continue to do so for another 7 billions years, at least. The helium byproduct from this fusion reaction slowly builds up in the core of a star, and they have no way to get rid of it. Eventually, billions of year down the road, a star uses up the last of its hydrogen fuel.

Once a star exhausts this fuel source, it no longer has the outward light pressure to counteract the gravity pulling in on itself. And so, the star begins to collapse. Before the star can collapse too far, though, this contraction heats up a shell of hydrogen around the core of the star to the point that it can support nuclear fusion. The higher temperatures lead to increasing reaction rates, and the star’s energy output increases by a factor of 1000 to 1000x. This new extreme light pressure pushes out the star’s outer layers beginning its life as a red giant star.

A red giant will expand outward many times its original size. Our own Sun, for example will grow so large that it engulfs the orbits of Mercury, Venus and even Earth; although, it’s not certain if Earth will actually be destroyed when this happens.

The core of the star will become so hot and dense that the leftover helium fuel will no able to star fusing into heavier elements. Stars with the mass of our Sun will stop with helium, but more massive stars will keep going, fusing carbon and even heavier elements together.

Without any more fuel to burn, these stars will expel their outer layers and then contract down to become white dwarfs.

We have written many articles about stars here on Universe Today. Here’s an article about a planet surviving when its star became a red giant. And were you wondering if the Earth will survive when the Sun becomes a red giant?

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Red Stars

Betelgeuse. Image credit: Hubble

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The color of a star comes from the temperature of its surface. The hottest stars are blue, cooler stars are white and yellow, and the coolest stars of all are red. Red stars come in one color, but many different shapes and sizes. Let’s take a look at the different kids of red stars that are out there in the Universe.

First, let’s talk about temperature. As I mentioned above, the color of a star comes from its surface temperature. A star that emits mostly red light will have a surface temperature of about 3,500 Kelvin. Just for comparison, the Sun’s surface temperature is about 6,000 Kelvin and emits yellow/white light.

Red Dwarf Stars
The first kind of red stars are red dwarfs. These are actually the most common stars in the Universe. The smallest red dwarf stars can be 7.5% the mass of our Sun, and be as large as about half the mass of the Sun. Even a star with this little mass has enough temperature and pressure at its core to carry out nuclear fusion. This is where atoms of hydrogen are fused into atoms of helium; this process releases lots and lots of heat.

Red dwarfs generate much less energy than a larger star like our Sun. In fact, a red dwarf emits 1/10,000th the energy. Even the largest red dwarf only has about 10% of the Sun’s luminosity. Red dwarfs use their hydrogen fuel slowly and so they last a very long time. It’s believed that a red dwarf star could survive for 10 trillion years.

Red Giant Stars
Stars like our Sun spend most of their lives as main sequence stars with a surface temperature that’s much hotter than a red star. But at the end of their lives, when they have used up all their hydrogen fuel, these medium-sized stars will puff out much bigger than their original size – these are red giants. When our Sun becomes a red giant, it will expand out to encompass the orbit of the Earth. After a few hundred million years, it will puff its outer layers and become a white dwarf star.

They start out as regular stars, but they grow to such an enormous size that their heat is spread out across a much larger surface area. This is why they appear very bright, but still have a red color.

Red Supergiant
The biggest stars in the Universe are the red supergiants. They’re not the most massive; Betelgeuse, for example, only has about 20 times the mass of the Sun. But the largest red supergiants can expand out to be more than 1500 times the size of the Sun. Imagine a star that engulfed the orbit of Saturn! Just like the red giants, red supergiants occur when a star has used up the hydrogen fuel in its core, and then expand out during their helium-burning phase. Red supergiants will have the mass to continue fusing elements in their cores, all the way up to iron.

Stars probably only exist as red supergiants for a few hundred thousand years; a million at most. At the end of this period, the star will have used up all the fuel it can in its core; most will detonate as Type II supernovae.

We have written many articles about stars here on Universe Today. Here’s an article about a planet surviving when its star became a red giant, and here’s a deathwatch on a red giant star.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.telescope.org/pparc/res8.html
http://en.wikipedia.org/wiki/Red_giant
http://en.wikipedia.org/wiki/Red_dwarf

HiRISE Captures Bolide Break-up and Impact on Mars

Bolide impact on Mars. Credit: NASA/JPL/University of Arizona

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Incoming! Hundreds of small objects, mostly asteroid fragments, impact Mars every year. Sometimes, like on Earth, objects break up in the Martian atmosphere. But Mars’ atmosphere is much thinner than Earth’s, meaning more stuff hits the ground on the Red Planet. If a bolide breaks apart and but doesn’t disintegrate, the result can be a cluster of craters. The image here is an example of that, with this group of recently made small impact craters. Although small Martian crater clusters are common, this example is unusual because there is a dark line between the two largest craters. The HiRISE scientists hypothesize that atmospheric breakup created two nearly equal-size objects that impacted close together in space and time so the air blasts interacted with each other to disturb the dust along this line. Wow!

The impact occurred sometime between May 2003 and September 2007. A dark spot is not present in the previous image of this location with sufficient resolution to have detected it, acquired by the visible THEMIS camera on Mars Odyssey in May 2003. Check out the THEMIS site, where you can find images by clicking on a map of Mars. This impact was first discovered as a dark spot in an image taken by the Mars Reconnaissance Orbiter’s CTX (Context) Imager acquired in March 2008, but later found to be partly visible at the very edge of a CTX image acquired in September 2007. The CTX team has been discovering many new impact events on Mars, and then they request HiRISE follow-up imaging to confirm an impact origin and to identify and measure the craters.

Here’s the full HiRISE image:

Full HiRISE Image. Credit: NASA/JPL/University of Arizona
Full HiRISE Image. Credit: NASA/JPL/University of Arizona

This area is just a few hundred meters wide.The dark markings are created by removing or disturbing the surficial dust cover, and so far new impact sites have been discovered only in dust-covered regions of Mars.

A comparable number of small objects impact Earth every year as on Mars, but most explode in the upper reaches of our atmosphere and provide us with “shooting stars.”

Source: HiRISE Site

Astronomers Find Cosmic Dust Fountain

HST image of the Red Rectangle. Photo: Van Winckel, M. Cohen, H. Bond, T. Gull, ESA, NASA

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Dust is everywhere in space, but the pervasive stuff is one thing astronomers know little about. Cosmic dust is also elusive, as it lasts only about 10,000 years, a brief period in the life of a star. “We not only do not know what the stuff is, but we do not know where it is made or how it gets into space,” said Donald York, a professor at the University of Chicago. But now York and a group of collaborators have observed a double-star system, HD 44179, that may be creating a fountain of dust. The discovery has wide-ranging implications, because dust is critical to scientific theories about how stars form.

The double star system sits within what astronomers call the Red Rectangle, a nebula full of gas and dust located approximately 2,300 light years from Earth.

One of the double stars is a post-asymptotic giant branch (post-AGB) star, a type of star astronomers regard as a likely source of dust. These stars, unlike the sun, have already burned all the hydrogen in their cores and have collapsed, burning a new fuel, helium.

During the transition between burning hydrogen and helium, which takes place over tens of thousands of years, these stars lose an outer layer of their atmosphere. Dust may form in this cooling layer, which radiation pressure coming from the star’s interior pushes out the dust away from the star, along with a fair amount of gas.

In double-star systems, a disk of material from the post-AGB star may form around the second smaller, more slowly evolving star. “When disks form in astronomy, they often form jets that blow part of the material out of the original system, distributing the material in space,” York explained.

An artist’s rendition of the possible appearance of the double star system in the Red Rectangle nebula. Credit: Steve Lane
An artist’s rendition of the possible appearance of the double star system in the Red Rectangle nebula. Credit: Steve Lane

“If a cloud of gas and dust collapses under its own gravity, it immediately gets hotter and starts to evaporate,” York said. Something, possibly dust, must immediately cool the cloud to prevent it from reheating.

The giant star sitting in the Red Rectangle is among those that are far too hot to allow dust condensation within their atmospheres. And yet a giant ring of dusty gas encircles it.

Witt’s team made approximately 15 hours of observations on the double star over a seven-year period with the 3.5-meter telescope at Apache Point Observatory in New Mexico. “Our observations have shown that it is most likely the gravitational or tidal interaction between our Red Rectangle giant star and a close sun-like companion star that causes material to leave the envelope of the giant,” said collaborator Adolph Witt, from the University of Toledo.

Some of this material ends up in a disk of accumulating dust that surrounds that smaller companion star. Gradually, over a period of approximately 500 years, the material spirals into the smaller star.

Just before this happens, the smaller star ejects a small fraction of the accumulated matter in opposite directions via two gaseous jets, called “bipolar jets.”

Other quantities of the matter pulled from the envelope of the giant end up in a disk that skirts both stars, where it cools. “The heavy elements like iron, nickel, silicon, calcium and carbon condense out into solid grains, which we see as interstellar dust, once they leave the system,” Witt explained.

Cosmic dust production has eluded telescopic detection because it only lasts for perhaps 10,000 years—a brief period in the lifetime of a star. Astronomers have observed other objects similar to the Red Rectangle in Earth’s neighborhood of the Milky Way. This suggests that the process Witt’s team has observed is quite common when viewed over the lifetime of the galaxy.

“Processes very similar to what we are observing in the Red Rectangle nebula have happened maybe hundreds of millions of times since the formation of the Milky Way,” said Witt, who teamed up with longtime friends at Chicago for the study.

The team had set out to achieve a relatively modest goal: find the Red Rectangle’s source of far-ultraviolet radiation. The Red Rectangle displays several phenomena that require far-ultraviolet radiation as a power source. “The trouble is that the very luminous central star in the Red Rectangle is not hot enough to produce the required UV radiation,” Witt said, so he and his colleagues set out to find it.

It turned out neither star in the binary system is the source of the UV radiation, but rather the hot, inner region of the disk swirling around the secondary, which reaches temperatures near 20,000 degrees. Their observations, Witt said, “have been greatly more productive than we could have imagined in our wildest dreams.”

Source: University of Chicago