Posted on by Jason Major

Closely-Orbiting Stellar Companions Surrounded by “Mystery Dust”

Artist’s concept showing a dust disk around a binary system containing a white dwarf and a less-massive M (red) dwarf companion. (P. Marenfeld and NOAO/AURA/NSF)

Even though NASA’s Wide-field Infrared Survey Explorer spacecraft — aka WISE — ran out of coolant in October 2010, bringing its infrared survey mission to an end, the data that it gathered will be used by astronomers for decades to come as it holds clues to some of the most intriguing and hard-to-find objects in the Universe.

Recently astronomers using WISE data have found evidence of a particularly curious disk of dust and gas surrounding a pair of stars — one a dim red dwarf and the other the remains of a dead Sun-sized star — a white dwarf. The origin of the gas is a mystery, since based on standard models of stellar evolution it shouldn’t be there… yet there it is.

The binary system (which has the easy-to-remember name SDSS J0303+0054) consists of a white dwarf and a red dwarf separated by a distance only slightly larger than the radius of the Sun — about 700,000 km — which is incredibly close for two whole stars. The stars orbit each other quickly too: once every 3 hours.

The stars are so close that the system is referred to as a “post-common envelope” binary, because at one point the outer material of one star expanded out far enough to briefly engulf the other completely in what’s called a “common envelope.” This envelope of material brought the stars even closer together, transferring stellar material between them and ultimately speeding up the death of the white dwarf.

The system was first spotted during the Sloan Digital Sky Survey (hence the SDSS prefix) and was observed with WISE’s infrared abilities during a search for dust disks or brown dwarfs orbiting white dwarf stars. To find both a red (M) dwarf star 40-50 times the mass of Jupiter and a disk of dust orbiting the white dwarf in this system was unexpected — in fact, it’s the only known example of a system like it.

The entire mass of the dust (termed an infrared excess) is estimated to be “equivalent to the mass of an asteroid a few tens of kilometers in radius” and extends out to about the same distance as Venus’ orbit — just over 108 million kilometers, or 0.8 AU.

Why is the dust so unusual? Because, basically, it shouldn’t even be there. At that distance from the white dwarf, positioned just out of reach (but not terribly far away at all) anything that was within that zone when the original Sun-sized star swelled into its red giant phase should have spiraled inwards, getting swallowed up by the expanding stellar atmosphere.

Such is the fate that likely awaits the inner planets of our own Solar System — including Earth — when the Sun reaches the final phases of its stellar life.

So this requires that there are other sources of the dust. According to the WISE science update, “One possibility is that it is caused by multiple asteroids that orbit further away and somehow are perturbed close to the binary and collide with each other. [Another] is that the red dwarf companion releases a large amount of gas in a stellar wind that is trapped by the gravitational pull of its more massive white dwarf companion. The gas then condenses and forms the dust disk that is observed.

“Either way, this new discovery provides an interesting laboratory for the study of binary star evolution.”

See the team’s paper here, and read more on Berkeley’s WISE mission site here.

WISE launched into space on Dec. 14, 2009 on a mission to map the entire sky in infrared light with greatly improved sensitivity and resolution over its predecessors. From its polar orbit 525 kilometers (326 miles) in altitude it scanned the skies, collecting images taken at four infrared wavelengths of light. WISE took more than 2.7 million images over the course of its mission, capturing objects ranging from faraway galaxies to asteroids relatively close to Earth before exhausting the supply of coolant necessary to mask its own heat from its ultra-sensitive sensors.

Inset:  Infrared images of SDSS J0303+0054.  (NASA/JPL and  John H. Debes et. al.)

Posted on by Jason Major

STEREO Spots a CME Soaring Into Space

Press “play.” Say “wow.”

The enormous eruption of a solar prominence and resulting coronal mass ejection (CME) back on August 31 that was captured in amazing HD by NASA’s Solar Dynamics Observatory was also spotted by the Sun-flanking STEREO-B spacecraft, which observed the gigantic gout of solar material soaring away from the Sun.

This video shows the eruption as it passes across the fields of view of several of STEREO-B’s cameras over the course of 48 hours.

According to NASA’s Goddard Space Flight Center, “while CMEs are routinely seen in the Heliographic Imager (HI) telescopes, it’s very rare for prominences to stay visible for so long. The HI1 field of view ranges from 4 to 24 degrees away from the Sun. To get a sense of scale, we know the Sun is roughly 860,000 miles wide — and look how far the prominence holds together. And this CME is so bright it initially saturates the COR1 telescope.”

The bright spot in the red (COR2) field of view is the planet Venus.

Coronal mass ejections are huge bubbles of gas bounded by magnetic field lines that are ejected from the Sun over the course of several minutes — sometimes even hours. If they are directed toward Earth, the cloud of charged solar particles can interact with our magnetosphere and cause anything from increased auroral activity to radio interference to failure of sensitive electromagnetic equipment.

Particularly long filaments like the one that caused the August 31 CME have been known to collapse with explosive results when they hit the stellar surface.

The CME did not travel directly toward Earth but did connect with Earth’s magnetosphere with a glancing blow, causing bright aurorae to appear around the upper latitudes on the night of September 3.

Image: NASA/STEREO/GSFC

Posted on by Jason Major

A New Species of Type Ia Supernova?

Artist’s conception of a binary star system that produces recurrent novae, and ultimately, the supernova PTF 11kx. (Credit: Romano Corradi and the Instituto de Astrofísica de Canarias)

Although they have been used as the “standard candles” of cosmic distance measurement for decades, Type Ia supernovae can result from different kinds of star systems, according to recent observations conducted by the Palomar Transient Factory team at California’s Berkeley Lab.


Judging distances across intergalactic space from here on Earth isn’t easy. Within the Milky Way — and even nearby galaxies — the light emitted by regularly pulsating stars (called Cepheid variables) can be used to determine how far away a region in space is. Outside of our own local group of galaxies, however, individual stars can’t be resolved, and so in order to figure out how far away distant galaxies are astronomers have learned to use the light from much brighter objects: Type Ia supernovae, which can flare up with a brilliance equivalent to 5 billion Suns.

Type Ia supernovae are created from a special pairing of two stars orbiting each other: one super-dense white dwarf drawing material in from a companion until a critical mass — about 40% more massive than the Sun — is reached. The overpacked white dwarf suddenly undergoes a rapid series of thermonuclear reactions, exploding in an incredibly bright outburst of material and energy… a beacon visible across the Universe.

Because the energy and luminance of Type Ia supernovae have been found to be so consistently alike, distance can be gauged by their apparent brightness as seen from Earth. The dimmer one is when observed, the farther away its galaxy is. Based on this seemingly universal similarity it’s been thought that these supernovae must be created under very similar situations… especially since none have been directly observed — until now.

An international team of astronomers working on the Palomar Transient Factory collaborative survey have observed for the first time a Type Ia supernova-creating star pair — called a progenitor system — located in the constellation Lynx. Named PTF 11kx, the system, estimated to be some 600 million light-years away, contains a white dwarf and a red giant star, a coupling that has not been seen in previous (although indirect) observations.

“It’s a total surprise to find that thermonuclear supernovae, which all seem so similar, come from different kinds of stars,” says Andy Howell, a staff scientist at the Las Cumbres Observatory Global Telescope Network (LCOGT) and a co-author on the paper, published in the August 24 issue of Science. “How could these events look so similar, if they had different origins?”

The initial observations of PTF 11kx were made possible by a robotic telescope mounted on the 48-inch Samuel Oschin Telescope at California’s Palomar Observatory as well as a high-speed data pipeline provided by the NSF, NASA and Department of Energy. The supernova was identified on January 16, 2011 and supported by subsequent spectrography data from Lick Observatory, followed up by immediate “emergency” observations with the Keck Telescope in Hawaii.

“We basically called up a fellow UC observer and interrupted their observations in order to get time critical spectra,” said Peter Nugent, a senior scientist at the Lawrence Berkeley National Laboratory and a co-author on the paper.

The Keck observations showed the PTF 11kx post-supernova system to contain slow-moving clouds of gas and dust that couldn’t have come from the recent supernova event. Instead, the clouds — which registered high in calcium in the Lick spectrographic data — must have come from a previous nova event in which the white dwarf briefly ignited and blew off an outer layer of its atmosphere. This expanding cloud was then seen to be slowing down, likely due to the stellar wind from a companion red giant.

(What’s the difference between a nova and a supernova? Read NASA’s STEREO Spots a New Nova)

Eventually the decelerating nova cloud was impacted by the rapidly-moving outburst from the supernova, evidenced by a sudden burst in the calcium signal which had gradually diminished in the two months since the January event. This calcium burst was, in effect, the supernova hitting the nova and causing it to “light up”.

The observations of PTF 11kx show that Type Ia supernova can occur in progenitor systems where the white dwarf has undergone nova eruptions, possibly repeatedly — a scenario that many astronomers had previously thought couldn’t happen. This could even mean that PTF 11kx is an entirely new species of Type Ia supernova, and while previously unseen and rare, not unique.

Which means our cosmic “standard candles” may need to get their wicks trimmed.

“We know that Type 1a supernovae vary slightly from galaxy to galaxy, and we’ve been calibrating for that, but this PTF 11kx observation is providing the first explanation of why this happens,” Nugent said. “This discovery gives us an opportunity to refine and improve the accuracy of our cosmic measurements.”

Source: Berkeley Lab news center

Inset images: PTF 11kx observation (BJ Fulton, Las Cumbres Observatory Global Telescope Network) / The 48-inch Samuel Oschin Telescope dome at Palomar Observatory. Video: Romano Corradi and the Instituto de Astrofísica de Canarias

Posted on by Tammy Plotner

Weekly SkyWatcher’s Forecast: August 27-September 2, 2012

Greetings, fellow SkyWatchers! If you only get your telescope or binoculars out once in a Blue Moon, then get them out this week when a Blue Moon actually happens! However, if you can’t wait, then let’s explore some great lunar features, bright star clusters and great double stars. When you’re ready to learn some history, mystery and more, then just step on inside…

Monday, August 27 – Tonight the waxing Moon’s most notable features will be the vast area of craters dominating the south-central portion near and along the terminator. Now emerging is Ptolemaeus – just north-northeast of Albategnius. This large round crater is a mountain walled plain filled with lava flow. With the exception of interior crater Ptolemaeus A, binoculars will see it as very smooth. Telescopes, however, can reveal faint mottling in the surface of the crater’s interior, along with a single elongated craterlet to the northeast. Despite its apparent uniformity, close inspection has revealed as many as 195 interior craterlets within Ptolemaeus! Look for a variety of interior ridges and shallow depressions.

With the Moon low to the southwest, we have a chance to go northeast to Cepheus for a new study – NGC 7160 (Right Ascension: 21 : 53.7 (hours : minutes) Declination: +62 : 36). At magnitude 6.1, this small open cluster is easily identified in scopes and may be seen as a faint starfield in binoculars. You’ll find it about a finger-width north of Nu Cephei.

Tuesday, August 28 – In 1789 on this day, Sir William Herschel discovered Saturn’s moon Enceladus.

On the lunar surface tonight, we’ll start by following the southward descent of large crater rings Ptolemaeus, Alphonsus, and Arzachel to a smaller, bright one southwest named Thebit. We’re going to have a look at Hell…

Just west of Thebit and its prominent A crater to the northwest, you see the Straight Wall – Rupes Recta – appearing as a thin, white line. Continue south until you see large, eroded crater Deslandres. On its western shore, is a bright ring that marks the boundary of Hell. While this might seem like an unusual name for a crater, it was named for an astronomer – and clergyman!

Once you’ve been to Hell, let’s go to the heavens for NGC 7235 (Right Ascension: 22 : 12.6 – Declination: +57 : 17). Locate the star crowded area of Epsilon Cephei which will also include this 7.7 magnitude open cluster in the same low power field. Give it a try. Look for a small, rectangular assortment of 10th magnitude and fainter stars, including a beautiful ruby red, west-northwest of Epsilon.

Wednesday, August 29 – Due south of mighty Copernicus on the eastern edge of Mare Cognitum, you will see a ruined pair of flattened craters. They are Bonpland and Parry – with Frau Mauro just above them. The smallest and brightest of these ancient twins is the eastern Parry. Have a look at its south wall where a huge section is entirely lost. It was near this location that Ranger 7 ended its successful flight in 1964. Just south of Parry is another example of a well-worn Class V crater. See if you can distinguish the ruins of Guericke. Not much is left save for a slight U-shape to its battered walls. These are some of the oldest visible features on the Moon!

If you’d like to head for something very young, have a look at 6.8 magnitude open cluster NGC 6811 (Right Ascension: 19 : 37.3 – Declination: +46 : 23) in Cygnus. This mid-sized, unusually dense open cluster is found less than finger-width north-northwest of Delta – the westernmost star of the Northern Cross. Like most open clusters, the age of NGC 6811 is measured in millions, rather than billions, of years. Visible in binoculars on most nights, telescopes should show a half dozen or so broadly-spaced resolvable stars overlaying a fainter field. Be sure to return again on a moonless night, and have another look a disparate double Delta!

Thursday, August 30 – Today celebrates the Yohkoh Mission, launched in 1991. It was a joint effort of both Japan and the United States to monitor solar flares and the corona. While its initial mission was quite successful, on December 14, 2001 the signal was lost during a total eclipse. Unable to reposition the satellite back towards the Sun, the batteries discharged and Yohkoh became inoperable.

While the graceful Gassendi will try to steal the lunar show tonight, let’s have a go at Foucault instead. To find it, head north to Sinus Iridum and locate Bianchini in the Juras Mountains. Just northeast, and near the shore of south-eastern Mare Frigoris, look for a bright little ring.

Physicist Jean Foucault played an instrumental role in the creation of today’s parabolic mirrors. His “Foucault knife edge test” made it possible for opticians to test mirror curves for optical excellence during the final phases of shaping before metallization. Thanks to Foucault’s insight, we can turn our telescopes on such difficult double stars as Beta Delphini and resolve its 0.6 arc-second distant 5.0 magnitude companion. A challenge for smaller scopes is MU Cygni. This 4.5 and 6.0 magnitude pair should be resolvable in any scope that passed Foucault’s test!

Tonight let’s view a double star, Eta Lyra. Just on the edge of unaided visibility, you will find it around three finger-widths due east of Vega. This wide, disparate pair of 4.5 and 8.0 magnitude stars should be resolvable in just about any scope, but is beyond the reach of binoculars.

Friday, August 31 – Tonight we will begin entering the stream of the Andromedid meteor shower, which peaks off and on for the next couple of months. For those of you in the northern hemisphere, look for the lazy “W” of Cassiopeia to the northeast. This is the radiant – or relative point of origin – for this meteor stream. At times, this shower has been known to be spectacular, but let’s stick with an accepted fall rate of around 20 per hour. These are the offspring of Beila’s Comet, one that split apart leaving radically different streams – much like 73/P Schwassman-Wachmann did last year. These meteors have a reputation for red fireballs with spectacular trains, so watch for them in the weeks ahead.

It’s Blue Moon! That doesn’t mean the Moon is going to be colored any differently – it just means it’s the second full Moon within a month.

Think having all this Moon around is the pits? Then let’s venture to Zeta Sagittarii and have a look at Ascella – “The Armpit of the Centaur.” While you’ll find Zeta easily as the southern star in the handle of the teapot formation, what you won’t find is an easy double. With almost identical magnitudes, Ascella is one of the most difficult of all binaries. Discovered by W. C. Winlock in 1867, the components of this pair orbit each other very quickly – in just a little more than 21 years. While they are about 140 light-years away, this gravitationally bound pair waltz no further apart than our own Sun and Uranus!

Too difficult? Then have a look at Nu Sagittarii – Ain al Rami, or the “Eye of the Archer.” It’s one of the earliest known double stars and was recorded by Ptolemy. While Nu 1 and Nu 2 are actually not physically related to one another, they are an easy split in binoculars. Eastern Nu 2 is a K type spectral giant that is around 270 light-years from our solar system. But take a very close look at the western Nu 1 – while it appears almost as bright, this one is 1850 light-years away! As a bonus, power up in the telescope, because this is one very tight triple star system!

Saturday, September 1 – On this day 1859, solar physicist Richard Carrington (who originally assigned sunspot rotation numbers) observed the first solar flare ever recorded. Naturally enough, an intense aurora followed the next day. 120 years later in 1979, Pioneer 11 made history as it flew by Saturn.

While the Moon essentially appears to be full throughout the night, take the time to compare the western and eastern limbs. To the west, you will see the smooth arc no longer displays high contrast features. To the east you should see a broken edge now in sunset. Watch in the days ahead as many of your favorite craters begin to reveal themselves in a “different light.”

Tonight let’s visit Alya. One of the fainter stars to receive a proper name, Theta Serpens Caput is located around a hand span due east of Beta Ophiuchi. Thankfully, resolving this wide, matched magnitude pair is easier than finding it. If you have high power, self-stabilizing binoculars, this one could be real fun!

Sunday, September 2 – It won’t be long until the Moon lights the skies, so let’s have a look at disparate double Kappa Pegasi. It’s the westernmost star of northern Pegasus and is around a hand span due south of Sadr – the central star of the Northern Cross. At magnitude 4.3, look for a faint companion leading the orange-yellow primary across the sky. This one could be tough for small scopes – so make a challenge of it!

Now let’s have a look at Beta and Gamma Lyrae – the lower two stars in the “Harp.” Beta is actually a quick change variable dropping to less than half the brightness of Gamma every 12 days, but for a few days the two stars appear to be of near equal brightness. Beta is a very unusual eclipsing spectroscopic binary. Its unseen companion may be a “collapsar.”
Before you call it a night, head a finger-width north of Omicron Andromedae for 15 Lacertae. Just on the edge of unaided visibility, this carbon star is also a disparate double. The 5.2 magnitude variable primary will appear more red at its faintest, but its 11.0 magnitude companion is the faintest of all!

But don’t put the telescope away just yet. If you can locate the Moon, you can locate Uranus! Just take a look about 3 degrees away to lunar south to catch the slightly greenish orb of the outer planet.

Until next week, ask for the Moon… But keep on reaching for the stars!

Ptolemaeus Crater Image Credit: Damian Peach

Posted on by Nancy Atkinson

Bright Stars Don’t Like to Be Alone

Caption: New research using data from European Southern Observatory telescopes, including the Very Large Telescope, has revealed that the hottest and brightest stars, known as O stars, are often found in close pairs. Credit: ESA, NASA, H. Sana (Amsterdam University), and S.E. de Mink (STScI)

Like humans, stars seem to prefer the company of companions. A new study using the Very Large Telescope reveals that most very bright, high-mass O-type stars do not live alone. Surprisingly, almost three-quarters of these stars have a close companion star, far more than previously thought. But sometimes – also like humans – the relationship between companion stars can turn a little ugly, with one star becoming dominant and even disruptive by stealing matter from the other, or doing a hostile takeover.


An international team of astronomers have found that some stars will virtually suck the life out of another, and about one-third of the time, a pair of stars will ultimately merge to form a single star.

The stars included in this study are some of the biggest, brightest stars which have very high temperatures. They live fast and die young, and in their lives play a key role in the evolution of galaxies. by, which drive the evolution of galaxies. They are also linked to extreme phenomena such as gamma-ray bursts.

“These stars are absolute behemoths,” said Hugues Sana, from the University of Amsterdam, The Netherlands, lead author of the study. “They have 15 or more times the mass of our Sun and can be up to a million times brighter. These stars are so hot that they shine with a brilliant blue-white light and have surface temperatures over 54,000 degrees Fahrenheit (30,000 degrees C).”

The astronomers studied a sample of 71 O-type single stars and stars in pairs (binaries) in six nearby young star clusters in the Milky Way.
By analyzing the light coming from these targets in greater detail than before, the team discovered that 75 percent of all O-type stars exist inside binary systems, a higher proportion than previously thought, and the first precise determination of this number. More importantly, though, they found that the proportion of these pairs that are close enough to interact (through stellar mergers or transfer of mass by so-called vampire stars) is far higher than anyone had thought, which has profound implications for our understanding of galaxy evolution.

O-type stars make up just a fraction of a percent of the stars in the universe, but the violent phenomena associated with them mean they have a disproportionate effect on their surroundings. The winds and shocks coming from these stars can both trigger and stop star formation, their radiation powers the glow of bright nebulae, their supernovae enrich galaxies with the heavy elements crucial for life, and they are associated with gamma-ray bursts, which are among the most energetic phenomena in the universe. O-type stars are therefore implicated in many of the mechanisms that drive the evolution of galaxies.

“The life of a star is greatly affected if it exists alongside another star,” said Selma de Mink of the Space Telescope Science Institute, in Baltimore, Md., a co-author of the study. “If two stars orbit very close to each other they may eventually merge. But even if they don’t, one star will often pull matter off the surface of its neighbor.”

Mergers between stars, which the team estimates will be the ultimate fate of around 20 to 30 percent of O-type stars, are violent events. But even the comparatively gentle scenario of vampire stars, which accounts for a further 40 to 50 percent of cases, has profound effects on how these stars evolve.

Until now, astronomers mostly considered that closely orbiting massive binary stars were the exception, something that was only needed to explain exotic phenomena such as X-ray binaries, double pulsars, and black hole binaries. The new study shows that to properly interpret the universe, this simplification cannot be made: these heavyweight double stars are not just common, their lives are fundamentally different from those of single stars.

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For instance, in the case of vampire stars — where the smaller, lower-mass star is rejuvenated as it sucks the fresh hydrogen from its companion — its mass will increase substantially and it will outlive its companion, surviving much longer than a single star of the same mass. The victim star, meanwhile, is stripped of its envelope before it has a chance to become a luminous red supergiant. Instead, its hot, blue core is exposed. As a result, the stellar population of a distant galaxy may appear to be much younger than it really is: both the rejuvenated vampire stars, and the diminished victim stars become hotter, and bluer in color, mimicking the appearance of younger stars. Knowing the true proportion of interacting high-mass binary stars is therefore crucial to correctly characterize these faraway galaxies.

“The only information astronomers have on distant galaxies is from the light that reaches our telescopes. Without making assumptions about what is responsible for this light we cannot draw conclusions about the galaxy, such as how massive or how young it is. This study shows that the frequent assumption that most stars are single can lead to the wrong conclusions,” said Sana.

Understanding how big these effects are, and how much this new perspective will change our view of galactic evolution, will need further work. Modeling binary stars is complicated, so it will take time before all these considerations are included in models of galaxy formation.

The paper was published in the July 27 issue of the journal Science.

Paper by: Sana, de Mink, et al. (PDF document)

Sources: ESO, HubbleSite

Posted on by Jason Major

NASA To Launch The Finest Mirrors Ever Made

This Wednesday NASA will launch its High Resolution Coronal Imager (HI-C) mission from White Sands Missile Range in New Mexico, sending a sounding rocket above the atmosphere with some of the best mirrors ever made to capture incredibly-detailed ultraviolet images of our Sun.

HI-C will use a state-of-the-art imaging system to focus on a region near the center of the Sun about 135,000 miles (271,000 km) across. During its brief flight — only ten minutes long — HI-C will return some of the most detailed images of the Sun’s corona ever acquired, with a resolution five times that of previous telescopes… including NASA’s Solar Dynamics Observatory.

While SDO collects images in ten wavelengths, however, HI-C will focus on just one: 193 Angstroms, a wavelength of ultraviolet radiation that best reveals the structures of the Sun’s corona present in temperatures of 1.5 million kelvin. And although HI-C’s mirrors aren’t any larger than SDO’s — about 9.5 inches in diameter — they are “some of the finest ever made.” In addition, an interior “maze” between mirrors effectively increases HI-C’s focal length.

Researchers expect HI-C’s super-smooth mirrors to resolve coronal structures as small as 100 miles (160 km) across (0.1 arcsec/pixel).

“Other instruments in space can’t resolve things that small, but they do suggest – after detailed computer analysis of the amount of light in any given pixel – that structures in the sun’s atmosphere are about 100 miles across,” said Jonathan Cirtain, project scientist for HI-C at NASA’s Marshall Space Flight Center. “And we also have theories about the shapes of structures in the atmosphere, or corona, that expect that size. HI-C will be the first chance we have to see them.”

One of the main goals of HI-C will be to place significant new constraints on theories of coronal heating and structuring, by observing the small-scale processes that exist everywhere in hot magnetized coronal plasma and establishing whether or not there are additional structures below what can currently be seen.

“This instrument could push the limits on theories of coronal heating, answering questions such as why the temperature of the sun’s corona is millions of degrees higher than that of the surface,” said Marshall’s Dr. Jonathan Cirtain, heliophysicist and principle investigator on the mission.

Read more on the NASA news release here.

Top image: A Black Brant sounding rocket containing NASA’s HI-C mission will launch on July 11, 2012 to observe the sun’s corona. (NASA) Bottom image: TRACE image of the Sun at a resolution of 0.5 arcsec/pixel. HI-C will have a resolution 5 times finer.

Posted on by Jason Major

The Last Outbursts of a Dying Star

As stars approach the inevitable ends of their lives they run out of stellar fuel and begin to lose a gravitational grip on their outermost layers, which can get periodically blown far out into space in enormous gouts of gas — sometimes irregularly-shaped, sometimes in a neat sphere. The latter is the case with the star above, a red giant called U Cam in the constellation Camelopardalis imaged by the Hubble Space Telescope.

From the Hubble image description:

U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds. Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the center of the image. Its brightness, however, is enough to saturate the camera’s receptors, making the star look much bigger than it really is.

The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable, the shell of gas expelled from U Cam is almost perfectly spherical.

Image credit: ESA/NASA

Posted on by Nancy Atkinson

“Impossible” Binary Star Systems Found

Astronomers think about half of the stars in our Milky Way galaxy are, unlike our Sun, part of a binary system where two stars orbit each other. However, they’ve also thought there was a limit on how close the two stars could be without merging into one single, bigger star. But now a team of astronomers have discovered four pairs of stars in very tight orbits that were thought to be impossibly close. These newly discovered pairs orbit each other in less than 4 hours.

Over the last three decades, observations have shown a large population of stellar binaries, and none of them had an orbital period shorter than 5 hours. Most likely, the stars in these systems were formed close together and have been in orbit around each other from birth onwards.

A team of astronomers using the United Kingdom Infrared Telescope (UKIRT) in Hawaii made the first investigation of red dwarf binary systems. Red dwarfs can be up to ten times smaller and a thousand times less luminous than the Sun. Although they form the most common type of star in the Milky Way, red dwarfs do not show up in normal surveys because of their dimness in visible light.

But astronomers using UKIRT have been monitoring the brightness of hundreds of thousands of stars, including thousands of red dwarfs, in near-infrared light, using its state-of-the-art Wide-Field Camera (WFC).

“To our complete surprise, we found several red dwarf binaries with orbital periods significantly shorter than the 5 hour cut-off found for Sun-like stars, something previously thought to be impossible,” said Bas Nefs from Leiden Observatory in the Netherlands, lead author of the paper which was published in journal Monthly Notices of the Royal Astronomical Society. “It means that we have to rethink how these close-in binaries form and evolve.”

Since stars shrink in size early in their lifetime, the fact that these very tight binaries exist means that their orbits must also have shrunk as well since their birth, otherwise the stars would have been in contact early on and have merged. However, it is not at all clear how these orbits could have shrunk by so much.

One possible scenario is that cool stars in binary systems are much more active and violent than previously thought.

The astronomers said it is possible that the magnetic field lines radiating out from the cool star companions get twisted and deformed as they spiral in towards each other, generating the extra activity through stellar wind, explosive flaring and star spots. Powerful magnetic activity could apply the brakes to these spinning stars, slowing them down so that they move closer together.

“The active nature of these stars and their apparently powerful magnetic fields has profound implications for the environments around red dwarfs throughout our Galaxy, ” said team member said David Pinfield from the University of Hertfordshire.

UKIRT has a 3.8 meter diameter mirror, and is the second largest dedicated infrared telescope in the world. It sits at an altitude of 4,200 m on the top of the volcano Mauna Kea on the island of Hawaii.

Read the team’s paper.

Lead image caption: This artist’s impression shows the tightest of the new record breaking binary systems. Two active M4 type red dwarfs orbit each other every 2.5 hours, as they continue to spiral inwards. Eventually they will coalesce into a single star. Credit: J. Pinfield.

Posted on by Jason Major

The Case of the Disappearing Dust

Astronomy has always taught us that planets form from vast clouds of dust and gas orbiting young stars. It’s a gradual process of accretion that takes hundreds of thousands, perhaps even millions, of years… or does it?

During a 1983 sky survey with the Infrared Astronomical Satellite (IRAS) astronomers identified a young Sun-like star with a large cloud of dust surrounding it. The star, named TYC 8241 2652 1, is 450 light years away and what they had found around it was thought to be the beginnings of a solar system – the protoplanetary disc from which planets form.

Fast forward to 2008. Astronomers observed at the same star with a different infrared telescope, the Gemini South Observatory in Chile. What was observed looked a lot like what was previously seen in ’83.

Then, in 2009, they looked again. Curiously, the brightness of the dust cloud was only a third of what it was the year before. And in WISE observations made the very next year, it had disappeared entirely.

“It’s like the classic magician’s trick: now you see it, now you don’t. Only in this case we’re talking about enough dust to fill an inner solar system, and it really is gone.”

– Carl Melis, lead author and postdoctoral fellow at UC San Diego

Abracadabra?

“It’s as if you took a conventional picture of the planet Saturn today and then came back two years later and found that its rings had disappeared,” said study co-author and circumstellar disk expert Ben Zuckerman of UCLA.

It’s always been thought that planets take some time to form, in the order of hundreds of thousands of years. Although that may seem like forever to humans, it’s quick in cosmic time scales. But if what they’ve seen here with TYC 8241 is in fact planetary formation, well… it may happen a lot faster than anyone thought.

On the other hand, the star could have somehow blown all the dust out of the system. More research will be needed to see if that was the case.

The really interesting thing here is that astronomers have traditionally looked for these kinds of dust clouds around stars to spot planetary formation in action. But if planets form quicker than we thought, and the dust clouds are only fleeting features, then there may be a lot more solar systems out there that we can’t directly observe.

“People often calculate the percentage of stars that have a large amount of dust to get a reasonable estimate of the percentage of stars with planetary systems, but if the dust avalanche model is correct, we cannot do that anymore,” said study co-author Inseok Song, assistant professor of physics and astronomy at the University of Georgia. “Many stars without any detectable dust may have mature planetary systems that are simply undetectable.”

Read more in the news release from the University of Georgia.

Top image: Gemini Observatory/AURA artwork by Lynette Cook.

Posted on by Jason Major

WISE Spies a Hunter’s Flame

A vast star-forming cloud of gas and dust in the constellation Orion shines brightly in this image from NASA’s WISE space telescope, where infrared light is represented in visible wavelengths. It’s part of a recent data release from WISE, a trove of infrared images acquired during the telescope’s second sky scan from August to September of 2010 — just as it began to run out of its essential cryogenic coolant.

Shining brightly in infrared radiation, the Flame nebula (NGC 2024) is at the heart of the cloud.  Just below it is the reflection nebula NGC 2023, and the small, bright loop protruding from the edge of the gas and dust cloud just to its lower right is the Horsehead nebula  — whose famous equine profile appears quite different in infrared light than it does in visible.

The two bright blue stars at the upper right portion of the image are both stars in Orion’s belt. Alnitak, the brighter one closer to the Flame nebula, is a multiple star system located 736 light-years away whose stellar wind is responsible for ionizing the Flame nebula and causing it to shine in infrared. Alnilam, the dimmer star at the uppermost corner, is a blue supergiant 24 times the radius of our Sun and 275,000 times as bright, but 1,980 light-years distant.

The red arc at lower right is the bow shock of Sigma Orionis, a multiple-star system that’s hurtling through space at a speed of 5,260,000 mph (2,400 kilometers per second). As its stellar wind impacts the interstellar medium and piles up before it, an arc of infrared-bright radiation is emitted.

Sigma Orionis is also the star responsible for the glow of the Horsehead nebula.

This rich astronomical scene is an expanded view from WISE’s previously-released image of the region (at right) which used data from only three of its four infrared detectors. In contrast, all four detectors were used in the image above, making more of the nebulae’s intricate structures visible as well as providing comparative information for researchers.

“If you’re an astronomer, then you’ll probably be in hog heaven when it comes to infrared data,” said Edward (Ned) Wright of UCLA, the principal investigator of the WISE mission. “Data from the second sky scan are useful for studying stars that vary or move over time, and for improving and checking data from the first scan.”

Read more on the NASA news release here.

Top and right images: NASA/JPL-Caltech/WISE team. Horsehead nebula visible light image was taken with the 0.9-meter telescope at Kitt Peak National Observatory. Photo credit & copyright: Nigel Sharp (NOAO), KPNO, AURA, NSF. Comparison by J. Major/Universe Today.