Posted on by Jon Voisey

Gravitational Redshifts: Main Sequence vs. Giants

Pleiades
The Pleiades, Anglo-Australian Observatory/Royal Observatory

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One of the consequences of Einsteins theories of relativity is that everything will be affected by gravitational potentials, regardless of their mass. The effect of this is observed in experiments demonstrating the potential for gravity to bend light. But a more subtle realization is that light escaping such a gravitational well must lose energy, and since energy for light is related to wavelength, this will cause the light to increase in wavelength through a process known as gravitational redshifting.

Since the amount of redshift is dependent on just how deeply inside a gravitational well a photon is when it starts its journey, predictions have shown that photons being emitted from the photosphere of a main sequence star should be more redshifted than those coming from puffed out giants. With resolution having reached the threshold to detect this difference, a new paper has attempted to observationally detect this difference between the two.

Historically, gravitational redshifts have been detected on even more dense objects such as white dwarfs. By examining the average amount of redshifts for white dwarfs against main sequence stars in clusters such as the Hyades and Pleiades, teams have reported finding gravitational redshifts on the order of 30-40 km/s (NOTE: the redshift is expressed in units as if it were a recessional Doppler velocity, although it’s not. It’s just expressed this way for convenience). Even larger observations have been made for neutron stars.

For stars like the Sun, the expected amount of redshift (if the photon were to escape to infinity) is small, a mere 0.636 km/s. But because Earth also lies in the Sun’s gravitational well the amount of redshift if the photon were to escape from the distance of our orbit would only be 0.633 km/s leaving a distance of only ~0.003 km/s, a change swamped by other sources.

Thus, if astronomers wish to study the effects of gravitational redshift on stars of more normal density, other sources will be required. Thus, the team behind the new paper, led by Luca Pasquini from the European Southern Observatory, compared the shift among stars of the middling density of main sequence stars against that of giants. To eliminate effects of varying Doppler velocities, the team chose to study clusters, which have consistent velocities as a whole, but random internal velocities of individual stars. To negate the latter of these, they averaged the results of numerous stars of each type.

The team expected to find a discrepancy of ~0.6 km/s, yet when their results were processed, no such difference was detected. The two populations both showed the recessional velocity of the cluster, centered on 33.75 km/s. So where was the predicted shift?

To explain this, the team turned to models of stars and determined that main sequence stars had a mechanism which could potentially offset the redshift with a blueshift. Namely, convection in the atmosphere of the stars would blueshift material. The team states that low mass stars made up the bulk of the survey due to their number and such stars are thought to undergo greater amounts of convection than most other types of stars. Yet, it is still somewhat suspect that this offset could so precisely counter the gravitational redshift.

Ultimately, the team concludes that, regardless of the effect, the oddities observed here point to a limitation in the methodology. Trying to tease out such small effects with such a diverse population of stars may simply not work. As such, they recommend future investigations target only specific sub-classes for comparison in order to limit such effects.

Posted on by Nancy Atkinson

Tenuous Oxygen Atmosphere Found Around Saturn’s Moon Rhea

The 2 March 2010 Rhea flyby trajectory and simulated oxygen atmosphere distribution. Inset: Predicted oxygen density (yellow) compared to the INMS measurement (white) during the flyby. Image © Science/AAAS

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A few years ago, astronomers thought they found wispy rings around Saturn’s moon Rhea. Although the possibility of rings around this icy moon was later nixed, astronomers knew there was still something around Rhea that was causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon. Now, new observations have shown something else around Rhea that was completely unexpected: an oxygen atmosphere. In March of this year, the Cassini spacecraft made a close flyby of Rhea and recorded data showing a thin atmosphere made up of oxygen and carbon dioxide.

The source of the oxygen is not really a surprise: Rhea’s density of 1.233 times that of liquid water suggests that Rhea is three quarters ice and one quarter rock. The moon’s tenuous atmosphere is maintained by the ongoing chemical decomposition of ice water on the moon’s surface by irradiation from Saturn’s magnetosphere.

Oxygen has also recently been detected in the atmospheres of two of Jupiter’s moons, Europa and Ganymede. Since oxygen is a main component of the atmosphere surrounding Saturn’s rings, astronomers think there could be similar atmospheres around other icy moons that orbit inside Saturn’s magnetosphere.

“The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe,” said lead author Ben Teolis, a Cassini team scientist based at Southwest Research Institute in San Antonio. “Such chemistry could be a prerequisite for life. All evidence from Cassini indicates that Rhea is too cold and devoid of the liquid water necessary for life as we know it.”

Of course, there’s always the possibility of life as we don’t know it.

And, there must be some sort of organics on the moon – meaning carbon compounds. The source of the carbon dioxide in Rhea’s atmosphere is not yet known, but its presence suggests that radiolysis reactions between oxidants and organics are ongoing at the moon’s surface.

As far as any of these new findings having a relation to the ruled-out hypothesis of rings around Rhea, Teolis told Universe Today there is still much about Rhea’s environment that is yet to determined. “The electron depletion is currently unexplained,” Teolis said in an email. The sharp, symmetrical drop in electrons detected around Rhea was the initial finding behind the ring theory. “Our current thinking is that it may be related to the ionization of the atmosphere, perhaps in conjunction with electrostatic charging of Rhea’s surface, but I do not have a definitive answer at this point. The atmosphere – magnetosphere interaction is a complex problem, and will take some time to sort out. But for the first time at an icy moon, the Cassini findings give us an in situ observational window onto this interaction, understanding of which is still highly theoretical. We’re working on it.”

Rhea, as seen by Cassini. Credit: NASA

This latest data came from Cassini’s ion and neutral mass spectrometer and the Cassini plasma spectrometer during flybys on Nov. 26, 2005, Aug. 30, 2007, and March 2, 2010. The ion and neutral mass spectrometer saw peak densities of oxygen of around 50 billion molecules per cubic meter (1 billion molecules per cubic foot). It detected peak densities of carbon dioxide of around 20 billion molecules per cubic meter (about 600 million molecules per cubic foot).

The plasma spectrometer saw clear signatures of flowing streams of positive and negative ions, with masses that corresponded to ions of oxygen and carbon dioxide.

The scientists said the oxygen appears to rise to an atmosphere when Saturn’s magnetic field rotates over Rhea. Energetic particles trapped in the planet’s magnetic field pepper the moon’s water-ice surface. They cause chemical reactions that decompose the surface and release oxygen.

Releasing oxygen through surface irradiation could help generate conditions favorable for life at an icy body other than Rhea that has liquid water under the surface, Teolis said. If the oxygen and carbon dioxide from the surface could somehow get transported down to a sub-surface ocean, that would provide a much more hospitable environment for more complex compounds and life to form.

The scientists are unsure how the carbon dioxide is released. It could be the result of “dry ice” trapped from the primordial solar nebula, as is the case with comets, or it may be due to similar irradiation processes operating on the organic molecules trapped in the water ice of Rhea. The carbon dioxide could also come from carbon-rich materials deposited by tiny meteors that bombarded Rhea’s surface.

“Rhea is turning out to be much more interesting than we had imagined,” said Linda Spilker, Cassini project scientist at JPL. “The Cassini finding highlights the rich diversity of Saturn’s moons and gives us clues on how they formed and evolved.”

This research appears in the November 25, 2010 issue of Science Express.

Sources: Science, JPL, email exchange with Teolis

Posted on by Nancy Atkinson

Soyuz and 3 ISS Crewmembers Return Home

The Expedition 25 crew landed safely in Kazakhstan at 11:46 p.m. EST Thursday (Friday 10:46 a.m. Kazakhstan time). The trio — Doug Wheelock, Shannon Walker and Soyuz Commander Fyodor Yurchikhin — undocked in the Soyuz TMA-19 at 8:23 p.m. ending their 5-1/2 month stay at the International Space Station. Staying behind on the orbiting laboratory are Expedition 26 Commander Scott Kelly and Flight Engineers Alexander Kaleri and Oleg Skripochka.
Continue reading “Soyuz and 3 ISS Crewmembers Return Home”

Posted on by Jason Rhian

The Fall and Rise of ‘X’

As the X-37B ends its first mission and the X-34 program looks at a potential new start - are we at the dawn of a new age of 'X'? Photo Credit: NASA

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They are at the very edge of current U.S. technological capabilities; one is a supposedly mothballed technology test-bed, the other a super-secret space plane that is currently on orbit – but set to land soon. They are the X-planes, experimental spacecraft that are proving out concepts and capabilities whose beginnings can be traced to the dawn of the space age.

It would appear from amateur observers on the ground that the secretive U.S. Air Force X-37B space plane – will be landing soon. This prediction is based off the fact that the craft is dropping in altitude and the more basic fact that it is nearing the limit of its orbital capabilities and has to return to terra firma. According to the U.S. Air Force, the X-37B can remain on orbit for around nine months or 270 days at maximum, this means that the craft should be landing sometime in the middle of January.

The X-37B or Orbital Test Vehicle (OTV) lifted off from Cape Canaveral Air Force Station in Florida on Apr. 22, atop an Atlas V rocket. Not much is known after launch due to a media blackout imposed by the U.S. Air Force.

The Air Force remains mum about the details surrounding the landing and recovery of the X-37B. It is known that the spacecraft will land at Vandenberg Air Force Base in California.

In this image, the X-37B is being encapsulated in its fairing atop an Atlas V rocket. Photo Credit: USAF

In many ways the craft resembles the shuttle with stubby wings, landing gear and a powerful engine that allows the craft to alter its orbit (much to the dismay of many observers on the ground). When the X-37B does touch down, it will do so at a 15,000 foot-long runway that was originally built to support the shuttle program.

The X-37B is one-quarter the size of the space shuttle. It is about 30 feet long and roughly 10 feet tall, with a 15-foot wingspan. It has a payload bay much like its larger, manned cousin – but naturally whatever that payload was for this mission – it was classified. The space plane was constructed by the Boeing Phantom Works. It is operated out of Schriever Air Force Base, Colorado. Another launch of the craft may take place as early as this March.

The two X-34s were moved from their hangars at Dryden to the National Test Pilot School in California. Photo Credit: NASA

Meanwhile, as the X-37B is ready to head to the hangar, another X-craft appears to be given a new lease on life. Two of the X-34 spacecraft, built by Orbital Sciences Corporation (Orbital), were moved from their hangars at Dryden Flight Research Center to the National Test Pilot School located in the Mojave Desert in California. These technology test-bed demonstrator craft will be inspected by the NASA contractor with the idea of flying them once again.

The roughly 60 foot-long spacecraft were put into mothballs back in 2001. If their flight status is renewed they would add to the growing fleet of robotic spacecraft that the United States appears to be building.

The ‘X’ craft have a long and storied history in American aviation and space exploration. One of the most famous of the “X’ planes – was the legendary X-15. None other than the first man to walk on the moon, Neil Armstrong, flew in this program which tested out concepts that would be later employed in the space shuttle. As the X-37B prepares to end its first mission and the X-34 may be at the verge of a rebirth – could we be at the dawn of a new ‘X’-era? Only time will tell.

The X-37 can be seen to the left of this image with the X-34 at the right. Photo Credit: NASA
Posted on by Mike Simonsen

J-E-T-S, Jets, Jets, Jets!

Bipolar jet from a young stellar object (YSO). Credit: Gemini Observatory, artwork by Lynette Cook

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It seems oddly appropriate to be writing about astrophysical jets on Thanksgiving Day, when the New York football Jets will be featured on television. In the most recent issue of Science, Carlos Carrasco-Gonzalez and collaborators write about how their observations of radio emissions from young stellar objects (YSOs) shed light one of the unsolved problems in astrophysics; what are the mechanisms that form the streams of plasma known as polar jets? Although we are still early in the game, Carrasco-Gonzalez et al have moved us closer to the goal line with their discovery.

Astronomers see polar jets in many places in the Universe. The largest polar jets are those seen in active galaxies such as quasars. They are also found in gamma-ray bursters, cataclysmic variable stars, X-ray binaries and protostars in the process of becoming main sequence stars. All these objects have several features in common: a central gravitational source, such as a black hole or white dwarf, an accretion disk, diffuse matter orbiting around the central mass, and a strong magnetic field.

Relativistic jet from an AGN. Credit: Pearson Education, Inc., Upper Saddle River, New Jersey

When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. These are normally the jets produced by supermassive black holes in active galaxies. These jets emit energy in the form of radio waves produced by electrons as they spiral around magnetic fields, a process called synchrotron emission. Extremely distant active galactic nuclei (AGN) have been mapped out in great detail using radio interferometers like the Very Large Array in New Mexico. These emissions can be used to estimate the direction and intensity of AGNs magnetic fields, but other basic information, such as the velocity and amount of mass loss, are not well known.

On the other hand, astronomers know a great deal about the polar jets emitted by young stars through the emission lines in their spectra. The density, temperature and radial velocity of nearby stellar jets can be measured very well. The only thing missing from the recipe is the strength of the magnetic field. Ironically, this is the one thing that we can measure well in distant AGN. It seemed unlikely that stellar jets would produce synchrotron emissions since the temperatures in these jets are usually only a few thousand degrees. The exciting news from Carrasco-Gonzalez et al is that jets from young stars do emit synchrotron radiation, which allowed them to measure the strength and direction of the magnetic field in the massive Herbig-Haro object, HH 80-81, a protostar 10 times as massive and 17,000 times more luminous than our Sun.

Finally obtaining data related to the intensity and orientation of the magnetic field lines in YSO’s and their similarity to the characteristics of AGN suggests we may be that much closer to understanding the common origin of all astrophysical jets. Yet another thing to be thankful for on this day.

Posted on by Nancy Atkinson

Where In The Universe Challenge #126

Here’s this week’s image for the Where In The Universe Challenge, to test your visual knowledge of the cosmos. You know what to do: take a look at this image and see if you can determine where in the universe this image is from; give yourself extra points if you can name the spacecraft or instrument responsible for the image. We’ll provide the image today, but won’t reveal the answer until later. This gives you a chance to mull over the image and provide your answer/guess in the comment section. Please, no links or extensive explanations of what you think this is — give everyone the chance to guess.

And happy Thanksgiving to everyone who will be celebrating on Thursday.

UPDATE: Answer now posted below.

No photoshopping here, there really is a dark “X”-shape silhoutted against the nucleus of this galaxy, M51. Taken by the Hubble Space Telescope, the “X” is due to absorption by dust and this X really does mark the spot. It marks the exact position of a black hole which may have a mass equivalent to one-million stars like the sun.

This may be the first direct view of an immense ring of dust which fuels a massive black hole at the heart of M51, located 20 million light-years away. The darkest bar may be an edge-on dust ring which is 100 light-years in diameter. Surprisingly, astronomers found that the ring is standing almost perpendicularly to the relatively flat spiral galaxy, like a top spinning on its side with respect to the floor. Even more surprising is the discovery of a secondary ring or dust lane which is contrary to all expectations.

You can read more about this image of M51 at the HubbleSite.

Posted on by Nancy Atkinson

How Jupiter is Getting Its Belt Back

This image is a composite of three color images taken on Nov. 18, 2010, by the Gemini North telescope in Hawaii. The composite image shows a belt that had previously vanished in Jupiter's atmosphere is now reappearing. Image credit: NASA/JPL/UH/NIRI/Gemin

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Earlier this year, one of Jupiter’s stripes went missing. The Southern Equatorial Band started to get lighter and paler, and eventually disappeared. Now, follow-up images from both professional and amateur astronomers are showing some activity in the area of the SEB, and scientists now believe the vanished dark stripe is making a comeback. They say these new observations will help our understanding of the interaction between Jupiter’s winds and cloud chemistry.

“The reason Jupiter seemed to ‘lose’ this band – camouflaging itself among the surrounding white bands – is that the usual downwelling winds that are dry and keep the region clear of clouds died down,” said Glenn Orton, a research scientist at JPL. “One of the things we were looking for in the infrared was evidence that the darker material emerging to the west of the bright spot was actually the start of clearing in the cloud deck, and that is precisely what we saw.”

This image of Jupiter is a composite of three color images taken on Nov. 16, 2010, by NASA's Infrared Telescope Facility. The particles lofted by the initial outbreak are easily identified in green as high altitude particles at the upper right, with a second outbreak to the lower left. Image credit: NASA/JPL-Caltech/IRTF

This white cloud deck is made up of white ammonia ice. When the white clouds float at a higher altitude, they obscure the missing brown material, which floats at a lower altitude. Every few decades or so, the South Equatorial Belt turns completely white for perhaps one to three years, an event that has puzzled scientists for decades. This extreme change in appearance has only been seen with the South Equatorial Belt, making it unique to Jupiter and the entire solar system.

The white band wasn’t the only change on the big, gaseous planet. At the same time, Jupiter’s Great Red Spot became a darker red color. Orton said the color of the spot – a giant storm on Jupiter that is three times the size of Earth and a century or more old – will likely brighten a bit again as the South Equatorial Belt makes its comeback.

A false-color composite image of Jupiter and its South Equatorial Belt shows an unusually bright spot, or outbreak, where winds are lofting particles to high altitudes in this image made from data obtained by the W.M. Keck telescope on Nov. 11, 2010. Image credit: NASA/JPL-Caltech/W. M. Keck Observatory

The South Equatorial Belt underwent a slight brightening, known as a “fade,” just as NASA’s New Horizons spacecraft was flying by on its way to Pluto in 2007. Then there was a rapid “revival” of its usual dark color three to four months later. The last full fade and revival was a double-header event, starting with a fade in 1989, revival in 1990, then another fade and revival in 1993. Similar fades and revivals have been captured visually and photographically back to the early 20th century, and they are likely to be a long-term phenomenon in Jupiter’s atmosphere.

Scientists are particularly interested in observing this latest event because it’s the first time they’ve been able to use modern instruments to determine the details of the chemical and dynamical changes of this phenomenon. Observing this event carefully may help to refine the scientific questions to be posed by NASA’s Juno spacecraft, due to arrive at Jupiter in 2016, and a larger, proposed mission to orbit Jupiter and explore its satellite Europa after 2020.

Observations by amateur astronomers Christopher Go of Cebu City, Philippines and Anthony Wesley of Australia have helped, and scientists have used the “big guns” in Hawaii — NASA’s Infrared Telescope Facility, the W.M. Keck Observatory and the Gemini Observatory telescope.

Go imaged an outburst that piqued the interest of other astronomers. “I was fortunate to catch the outburst,” said Christopher Go, referring to the first signs that the band was coming back. “I had a meeting that evening and it went late. I caught the outburst just in time as it was rising. Had I imaged earlier, I would not have caught it,” he said.

Source: JPL

Posted on by Nancy Atkinson

Cassini Spacecraft Back in Operation

Space Probes
Cassini orbiting Saturn. Credit: NASA

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Great news: the Cassini spacecraft resumed normal operations today, Nov. 24. NASA says all the science instruments have been turned back on, the spacecraft is properly configured. So, Cassini appears to be in good health, no worse for wear following a flipped bit in the command and data subsystem computer, which prevented the spacecraft from registering and following instructions. Mission managers expect to get a full stream of data during next week’s flyby of the Saturnian moon Enceladus, and beyond. (see our preview article of that flyby).

Posted on by Nancy Atkinson

Shuttle Launch Could Be Delayed Into Next Year

Shuttle Discovery on the launchpad. Credit: Alan Walters (awaltersphoto.com) for Universe Today.

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While NASA managers have targeted space shuttle Discovery’s launch for no earlier than Dec. 17, they also said they don’t want to rush to any conclusions on the cracks found on the shuttle’s external tank. Therefore, shuttle program manager John Shannon said that if the team doesn’t completely understand the issues, they won’t launch until they do. That might mean mid-December, or it might mean they wait for the next launch window, which is in February of 2011 — or even later.

“It’s a complex problem,” said Shannon. “We really need to understand our risk. Clearly we’re not ready for the December 3 -7 window that’s coming up.”

“We are methodically looking at the data and we’ll let data the drive where we’re heading, drive when we launch,” said Bill Gerstenmaier, NASA’s Associate Administrator for Space Operations, speaking at a press briefing on Wednesday.

Engineering evaluations are ongoing of the four cracks on found on two 21-foot-long, U-shaped aluminum brackets called stringers on the shuttle’s external tank, and Shannon said they still need more analysis until they understand everything. The only previous time cracks like this have been seen are during the assembly process, or if the tank has been mishandled during assembly – cracks like this have never been seen at the launchpad before.

“We have worked hard to understand the exposure, and we want to understand everything,” Shannon said. “We’re looking at the fault tree from assembly, to how it gets foamed, to transport, to how it gets to KSC – every single part of that tank’s life is part of our fault tree analysis.”

It appears the biggest worry is not that the tank would fall apart during the stresses of launch, but that foam would be dislodged from the tank, which could impact the shuttle during launch. Foam from the ET is what damaged space shuttle Columbia, and caused it to disintegrate during reentry in 2003, killing all seven astronauts on board.

If the teams feel their analysis is complete and they have the flight rationale to fly, the earliest launch date would be Friday, Dec. 17 at 8:51:53 pm EST.

A Soyuz is scheduled to dock at the space station at 3 pm EST that day, carrying three new crew members to the ISS.

No launch dates are available in January 2011 because of constraints with the orbit of the space station and conflicts with other unmanned cargo launches. The launch window in February opens on the 27th and closes March 6. Another window, Feb. 3 -10 could be available if the Japanese cargo ship, scheduled to arrive in late January, can be moved to another port on the space station.

We’ll keep you posted.

Posted on by Mike Simonsen

Symbiotic Variable Star On the Verge of an Eruption?

Symbiotic variables are binary pairs in orbit around each other inside a common envelope. Credit: NASA

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November 23rd, astronomers from the Asiago Novae and Symbiotic Stars collaboration announced recent changes in the symbiotic variable star, AX Persei, could indicate the onset of a rare eruption of this system. The last major eruption took place between 1988 and1992. In the (northern hemisphere) spring of 2009, AX Per underwent a short outburst that was the first time since 1992 this star had experienced a bright phase. Now AX Per is on the rise again. This has tempted astronomers to speculate that another major eruption could be in the making. 

Symbiotic variable stars are binary systems whose members are a hot compact white dwarf in a wide orbit around a cool giant star. The orbital periods of symbiotic variables are between 100 and 2000 days. Unlike dwarf novae, compact binaries whose periods are measured in hours, where mass is transferred directly via an accretion disk around the white dwarf, siphoned directly from the surface of the secondary, in symbiotic variables the pair orbit each other far enough away that the mass exchanged between them comes from the strong stellar wind blowing off the red giant. Both stars reside within a shared cloud of gas and dust called a common envelope.

When astronomers look at the spectra of these systems they see a very complex picture. They see the spectra of a hot compact object superimposed on the spectra of a cool giant star tangled up with the spectrum of the common envelope. The term “symbiotic” was coined in 1941 to describe stars with this combined spectrum.

Typically, these systems will remain quiescent or undergo slow, irregular changes in brightness for years at a time. Only occasionally do they undergo large outbursts of several magnitudes. These outbursts are believed to be caused either by abrupt changes in the accretion flow of gas onto the primary, or by the onset of thermonuclear burning of the material piled up on the surface of the white dwarf. Whatever the cause, these major eruptions are rare and unpredictable.

The AAVSO light curve of AX Persei from 1970 to November 2010. In the middle is the eruption of 1988-1992. The precursor outburst is the sudden narrow brightening left of the larger eruption. To the right of the light curve you can see the 2009 brightening event. Is this a precursor to a coming major eruption? Credit: AAVSO

AX Per underwent a short-duration flare about one year before the onset of the major 1988-1992 outburst. Now astronomers are tempted to speculate. Could the 2009 short outburst be a similar precursor type event? The present rise in brightness by AX Per might be the onset of a major outburst event similar to that in 1988-1992. The watch begins now, and professional and amateur variable star observers will be keeping a close eye on AX Per in the coming months.

Ranging from 8.5 to 13th magnitude, AX Persei is visible to anyone with an 8-inch telescope, and if it erupts to maximum it will be visible in binoculars. You can monitor this interesting star and report your observations to the American Association of Variable Star Observers (AAVSO). Charts with comparison stars of known brightness can be plotted and printed using the AAVSO’s Variable Star Chart Plotter, VSP.

The AAVSO comparison star chart for AX Persei