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Universe Today — Space and astronomy news

New Desktop Image Alert: The Moon Over Earth

by JASON MAJOR on JULY 6, 2013


An Earthshine-lit Moon seen from orbit (NASA)

If you’re like me, you don’t change your computer’s desktop background nearly often enough… especially not considering all the fantastic space images that get released on an almost daily basis. But this picture, shared a couple of weeks ago by NASA’s Marshall Space Flight Center on their Flickr stream, really should inspire you to fix that. (I know it did for me!)

Captured by an Expedition 28 crew member aboard the International Space Station, this beautiful image shows a crescent-lit Moon seen through the upper layers of Earth’s atmosphere.

As it circles the globe, the ISS travels an equivalent distance to the Moon and back in about a day, making an excellent platform for viewing the Earth and its atmosphere. This photo shows the limb of Earth near the bottom transitioning into the orange-colored troposphere, the lowest and most dense portion of the Earth’s atmosphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange- and blue- colored atmosphere. Silvery-blue noctilucent clouds extend far above the Earth’s troposphere.

Expedition 28 began on May 23, 2011, with a crew consisting of Andrey Borisenko, Ron Garan, Alexander Samokutyaev, Sergei Volkov, Mike Fossum, and Satoshi Furukawa.

Universe Today — Space and astronomy news.


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Weekend Auroras Fill the Sky!

Weekend Auroras Fill the Sky!


Swirling aurora over Saskatoon, Saskatchewan. Credit and copyright: Colin Chatfield/ Chatfield Photography.

Swirling aurora over Saskatoon, Saskatchewan. Credit and copyright: Colin Chatfield/ Chatfield Photography.

I was personally so pumped to have seen the Aurora Borealis over the weekend in Central Minnesota! It was a beautiful display of a green and white glow with high, towering, bright spires. Unfortunately, I was in the car at the time, and I definitely need to upgrade my camera to be able to take images of the aurora. But lucky for us, astrophotographers from both hemispheres captured gorgeous shots of the Aurora Borealis and Aurora Australis.

According to, the Earth passed through a region of south-pointing magnetism in the solar wind on June 28, “and the encounter set off one of the finest geomagnetic storms of the current solar cycle.”

This shot from Colin Chatfield shows the awesome auroral scenes over Saskatchewan.

The northern lights on June 28/29, 2013 as seen from the Wintering Hills WInd Farm near Drumheller, Alberta. Credit and copyright: Alan Dyer/Amazing Sky Photography.

The northern lights on June 28/29, 2013 as seen from the Wintering Hills WInd Farm near Drumheller, Alberta. Credit and copyright: Alan Dyer/Amazing Sky Photography.

James Stone from Opossum Bay, Tasmania captured this video of the Aurora Australis:

To see some images from the southern hemisphere, Ian Musgrave has put together a great collection at his Astroblog site.

Weekend Auroras Fill the Sky!.


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Voyager 1 Has Outdistanced the Solar Wind

Voyager 1 Has Outdistanced the Solar Wind


Artist impression of Voyager 1, the first probe to traverse the heliosheath (NASA)

Artist impression of Voyager 1, the first probe to traverse the heliosheath (NASA)

The venerable Voyager spacecraft are truly going where no one has gone before. Voyager 1 has now reached a distant point at the edge of our solar system where it is no longer detecting the solar wind. At a distance of about 17.3 billion km (10.8 billion miles) from the Sun, Voyager 1 has crossed into an area where the velocity of the hot ionized gas, or plasma, emanating directly outward from the sun has slowed to zero. Scientists suspect the solar wind has been turned sideways by the pressure from the interstellar wind in the region between stars.

“The solar wind has turned the corner,” said Ed Stone, Voyager project scientist based at the California Institute of Technology in Pasadena, Calif. “Voyager 1 is getting close to interstellar space.”

The event is a major milestone in Voyager 1′s passage through the heliosheath, the turbulent outer shell of the sun’s sphere of influence, and the spacecraft’s upcoming departure from our solar system.

Since its launch on Sept. 5, 1977, Voyager 1’s Low-Energy Charged Particle Instrument has been used to measure the solar wind’s velocity.

When the speed of the charged particles hitting the outward face of Voyager 1 matched the spacecraft’s speed, researchers knew that the net outward speed of the solar wind was zero. This occurred in June, when Voyager 1 was about 10.6 billion miles from the sun.

However, velocities can fluctuate, so the scientists watched four more monthly readings before they were convinced the solar wind’s outward speed actually had slowed to zero. Analysis of the data shows the velocity of the solar wind has steadily slowed at a rate of about 45,000 mph each year since August 2007, when the solar wind was speeding outward at about 130,000 mph. The outward speed has remained at zero since June.

“When I realized that we were getting solid zeroes, I was amazed,” said Rob Decker, a Voyager Low-Energy Charged Particle Instrument co-investigator and senior staff scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again.”

Scientists believe Voyager 1 has not crossed the heliosheath into interstellar space. Crossing into interstellar space would mean a sudden drop in the density of hot particles and an increase in the density of cold particles. Scientists are putting the data into their models of the heliosphere’s structure and should be able to better estimate when Voyager 1 will reach interstellar space. Researchers currently estimate Voyager 1 will cross that frontier in about four years.

Our sun gives off a stream of charged particles that form a bubble known as the heliosphere around our solar system. The solar wind travels at supersonic speed until it crosses a shockwave called the termination shock. At this point, the solar wind dramatically slows down and heats up in the heliosheath.

A sister spacecraft, Voyager 2, was launched in Aug. 20, 1977 and has reached a position 8.8 billion miles from the sun. Both spacecraft have been traveling along different trajectories and at different speeds. Voyager 1 is traveling faster, at a speed of about 38,000 mph, compared to Voyager 2′s velocity of 35,000 mph. In the next few years, scientists expect Voyager 2 to encounter the same kind of phenomenon as Voyager 1.

The results were presented at the American Geophysical Union meeting in San Francisco.

Source: NASA

Voyager 1 Has Outdistanced the Solar Wind.


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No, This Image Was Not Taken from the Space Station, But it Sure Looks Like It

No, This Image Was Not Taken from the Space Station, But it Sure Looks Like It

by NANCY ATKINSON on JUNE 29, 2013

The Moon sets above the Continental Divide in Colorado from 86,000 feet. Taken June 27, 2013 on a meteorological balloon launched from Boulder, Colorado. Credit and copyright: Patrick Cullis.

The Moon sets above the Continental Divide in Colorado from 86,000 feet. Taken June 27, 2013 on a meteorological balloon launched from Boulder, Colorado. Credit and copyright: Patrick Cullis.

I love those images taken from the International Space Station that show the Moon rising or setting above Earth’s limb, and when I first saw this image posted on Universe Today’s Flickr Group page, I thought someone had randomly posted one of those images taken by an astronaut on the ISS. But then I saw it was taken by Patrick Cullis, one of our “regulars” in our featured astrophotography posts.

This very beautiful, crisp and clear image was taken from a meteorological balloon at 86,000 feet (26,200 meters) above Earth, and it was no fluke that Patrick captured the Moon setting above Earth — it was planned.

“Once I knew the weather was going to work out for a launch I really planned out what time it needed to happen for the Moon to show up in the frame,” Patrick said via Flickr. “Definitely got lucky since the camera is just swinging around randomly under the balloon.”

He calls this image “Divided Moon,” as it shows the Continental Divide in Colorado. “I-70 can be seen snaking up from the bottom center towards Georgetown (valley stretching from left to right,) Loveland Pass, and the Eisenhower Tunnel,” Patrick explained. If you click on the image above (or go here to see it on Flickr) you can see other landmarks labeled.

You can see more great shots from Patrick’s balloon and read more about it on his website.

No, This Image Was Not Taken from the Space Station, But it Sure Looks Like It.


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Zodiacal Light Over ESO’s La Silla Observatory

Zodiacal Light Over ESO’s La Silla Observatory

by JASON MAJOR on JUNE 29, 2013

Moonlight and zodiacal light lights up the skies over ESO's La Silla observatory. (Credit: Alan Fitzsimmons/ESO)

A band of zodiacal light glows in the sky over ESO’s La Silla Observatory. (Credit: Alan Fitzsimmons/ESO)

We don’t put much stock in astrology or horoscopes here at Universe Today, but there’s one thing related to the zodiac that’s all science and no superstition: zodiacal light, captured here in a gorgeous photo by astronomer Alan Fitzsimmons above ESO’s La Silla Observatory.

Created by sunlight reflected off fine particles of dust concentrated inside the plane of the Solar System, zodiacal light appears as a diffuse, hazy band of light visible in dark skies stretching away from a recently-set Sun (or before the Sun is about to rise).

The Moon is located just outside the frame of this picture, bathing the observatory in an eerie light that is reflected off the clouds below.

The La Silla Observatory is located at the outskirts of the Chilean Atacama Desert at an altitude of 2400 meters (7,900 feet). Like other observatories in this area, La Silla is located far from sources of light pollution and, like ESO’s Paranal Observatory, it has some of the darkest night skies on the Earth.

The dome in the foreground, just to the right, is the Swiss 1.2-metre Leonhard Euler Telescope named in honor of the famous Swiss mathematician Leonhard Euler (1707–83).Zodiacal Light Over ESO’s La Silla Observatory.


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A Wacky Distorted View of the Recent Solar Eclipse

A Wacky Distorted View of the Recent Solar Eclipse

by DAVID DICKINSON on MAY 13, 2013

A three image sequence of the rising annular eclipse. Credit: Geoff Sims. (@beyond_beneath)

A three image sequence of last Friday’s rising annular eclipse. Credit: Geoff Sims. (@beyond_beneath)

Just when we’d thought that we’ve seen every possible type of eclipse image, we’re happily surprised by the Universe.

If you’re like me, you watch the original Star Wars film and wonder what kind of eclipses could be seen from the surface of Tatooine.  Maybe you even wonder what things would look like if an extra sun and moon were to be thrown into the mix. How often, if ever, would such a bizarre alignment sync up?

Astrophotographer Geoff Sims provided us with just such a bizarre view this past weekend.

Geoff was one of a handful of intrepid photographers that braved the wilds of the Australian Outback to deliver us some stunning views of last week’s rising annular eclipse. We wrote of how to observe this celestial wonder late last month on Universe Today, and documented the efforts of photographers, both Earthbound and otherwise, the day of the eclipse this past Friday.

For this amazing image, Geoff positioned himself along the track of annularity in the Great Sandy Desert in Western Australia. Even the name of the site, the Plutonic Gold Mine outside of Newman, Australia couldn’t be beat!

The series is a composite of three exposures which were taken about three minutes apart. Mr. Simms relates how he accomplished this unforgettable image on his Facebook page:

 “The lower image shows a flattened and distorted Sun perched right on the horizon, just seconds before the annular eclipse began. The middle image shows the annular phase, while the upper image shows the Sun some minutes after annularity.”

Mr. Sims used a Canon Mark III DSLR camera with a 500mm lens shooting at 1/1,000thof a second exposures at a focal ratio of f/8 and an ISO setting of 100.

Amazingly, other photographers positioned very near the eclipse graze line caught sight of what are known as Bailey’s Beads as well. More commonly seen during a total solar eclipse, these are caused by sunlight streaming through ridges and valleys on the limb of the Moon. This can also cause the brilliant diamond ring effect seen during a total solar eclipse. In the case of an annular eclipse, this manifests as a ragged broken edge where the disk of the Sun meets the Moon:

Bailey's Beads captured very briefly during last week's annular eclipse. (Credit: Geoff Sims).

Bailey’s Beads captured very briefly during last week’s annular eclipse. (Credit: Geoff Sims).

An annular eclipse occurs when the Moon eclipses the Sun near apogee, or its most distant point in its orbit and is hence visually too small to cover the Sun as seen from the Earth. A similar eclipse occurred over the Pacific and the western U.S. last year onMay 20th, leading to a series of “horned sunset” photos taken across Texas and New Mexico.

But what is the most astonishing aspect of the eclipse sequence is the extreme distortion occurring across the very bottom image sitting on the horizon. When you’re looking low to the horizon, you’re viewing objects through a thicker cross-section of the atmosphere. This is what is termed as a higher air mass, and most astro-imagers avoid it entirely, preferring to catch objects with as little distortion as possible as they transit across the local meridian. This distortion can be extreme enough to result inatmospheric refraction of rising and setting objects like the Sun, Moon or planets, causing them to appear moments before or after they actually rose or set over the local horizon. In the case of the bottom image, the lower limb of the solar annulus (the technical name for what folks call the “ring of fire” seen during an annular eclipse) is actually distorted enough to appear along the rim of the local horizon!

To our knowledge, such an extremely distorted eclipse has never been documented before. One also wonders if a “green flash” could be captured by a properly positioned observer on a mountaintop or out to sea during a sunset or sunrise annular or total solar eclipse.

2013 will offer one more chance to try. On November 3rd, a hybrid solar eclipse will race across the Atlantic Ocean and central Africa. This is an eclipse that is literally an annular across a portion of its track and a total across another. The eclipse will begin at sunrise just south of Bermuda and end at sunset in eastern Africa. The maximum period of totality is 1 minute and 40 seconds off of the coast of Liberia, and the southern regions of Ethiopia offer the best shot at a sunset eclipse. Tantalizingly, the Florida Space Coast will get a rising partial eclipse only a few percent in magnitude.

Kudos to Mr. Sims for providing us with an unforgettable view of this rare cosmic spectacle. Australia won’t see another total solar eclipse until July 22nd, 2028, and another purely annular eclipse won’t occur until April 29th, 2014 across a very small section of the Antarctic.

And next week, we’ll have a very shallow penumbral eclipse on May 25th, and event is so subtle that few if any will notice it. Still, it is from such humble beginnings that great things are made, as we witness the birth of a new lunar saros… stay tuned!

 A Wacky Distorted View of the Recent Solar Eclipse.


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Hawaii set to build telescope capable of viewing the beginning of time | Science Recorder

Hawaii set to build telescope capable of viewing the beginning of time

Hawaii clears final hurdle in bid to build a massive telescope.

Hawaii set to build telescope capable of viewing the beginning of time

Photo credit: University of Hawaii Hilo

Science Recorder | Delila James | Sunday, April 14, 2013

The Hawaii Board of Land and Natural Resources has reportedly granted a permit to the University of Hawaii at Hilo for the construction of the $1.3 billion Thirty Meter Telescope (TMT). The telescope will be built atop Mauna Kea and will be one of the largest telescopes in the world, according to the university.

The TMT has been in development for over a decade, but the large amount of land needed for its construction raised concerns over the environmental and cultural impact of such a project. Now, however, the land board has rendered a final decision, saying that the university had satisfied the eight criteria necessary under Hawaiian state law to allow the venture to go forward.

The giant TMT will be an optical and infrared telescope with enough coverage area and sharpness to observe light from 13 billion years ago, track extrasolar planets, and observe planets and stars in their early formative years.

According to University of Hawaii officials, the land board’s decision “marked another important step forward for the future of astronomical discovery and economic opportunity on Hawaii Island.”

“It’s a billion-dollar project. It’s going to affect businesses, bring in a lot of grant money, researchers and astronomers,” Jerry Chang, UH-Hilo’s director of university relations, told Pacific Business News.

Challenges to the land board’s grant of the permit for the TMT came from organizations and individuals concerned over the environmental effects of the telescope on the surrounding region. In its decision, however, the board said that enormous telescope would serve as a benefit to the public, creating as many as 140 full-time jobs.

The board’s decision also mandated that certain conditions attach to the permit. The TMT will be required to pay a “substantial” amount in rent, which will be used for the management of Mauna Kea. It also must pay $1 million a year for a “community benefits package,” to be administered by the Hawaii Island New Knowledge Fund’s board of advisors. In addition, the telescope’s employees must take cultural and natural resources training and will be required to work with the Imiloa Astronomy Center and the Office of Mauna Kea to develop exhibits for visitors about the natural, cultural and archaeological resources at Mauna Kea.

According to the a statement released by the university, the TMT will now seek final approval of its construction plans by Hawaii’s Department of Land and Natural Resources (DLNR), before moving on to negotiate a sublease with the University of Hawaii. TMT intends to begin preparing the ground for construction on Mauna Kea before the end of the year. The construction start date is slated for April, 2014.

The TMT is made possible through a partnership by a number of governments and research universities. These include the University of California, the California Institute of Technology, the Association of Canadian Universities for Research in Astronomy, and the governments of the United States, China, India, Japan, and Canada.

Hawaii set to build telescope capable of viewing the beginning of time | Science Recorder.


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Vulcan and Cerberus win online poll to name Pluto’s smallest moons –

Vulcan and Cerberus win online poll to name Pluto’s smallest moons


This artists' rendering from NASA depicts how Pluto might look from the surface of one its three bigger moons. The two smallest moons may be named Vulcan and Kerberos.

This artists’ rendering from NASA depicts how Pluto might look from the surface of one it’s three bigger moons. The two smallest moons may be named Vulcan and Kerberos. (NASA/JPL and G. Bacon (STSci) / February 25, 2013)


By Karen Kaplan

February 25, 20134:30 p.m.

The people have spoken, and they would like the two smallest moons of Pluto to be named Vulcan and Cerberus.

When scientists at the SETI Institute stopped accepting new votes on the Pluto Rocks website at 9 a.m. Pacific time Monday, Vulcan was the only candidate with more than 100,000 votes. In fact, it blew away the rest of the field with 174,062 votes from people all over the world.

The biggest fan of the name has got to be William Shatner, who suggested it in a tweet on Feb. 12.

“So what do you think of the idea of naming the two moons of Pluto Vulcan and Romulus?” he asked of his 1.35 million followers. One day later, he tweeted this update: “Did you hear? They added the name Vulcan to the list of possible names for Pluto’s moons! You did it! I’m so happy.” Then he exhorted his followers to vote for Vulcan more than a dozen times with tweets such as this: “It’s a new day- at least here in Los Angeles- have you voted for Vulcan?”

On Monday morning, he shared the news: “174,062 votes and Vulcan came out on top of the voting for the naming of Pluto’s moons. Thank you to all who voted!”

Vulcan, of course, is the home planet of the Vulcans of Star Trek fame. Spock, who served along with Captain James T. Kirk (played by Shatner), has Vulcan heritage. Romulus is the home planet of the Romulans, the antagonistic beings who are related to Vulcans but have the opposite temperament.

At first, election organizer Mark Showalter, a planetary astronomer with SETI’s Carl Sagan Center for the Study of Life in the Universe, doubted that either name was worthy of serious consideration for P4 and P5, the temporary names for Pluto’s fourth and fifth moons. After all, there are rules, he explained on the election website Pluto Rocks: “By tradition, the names of Pluto’s moons come from Greek and Roman mythology, and are related to the ancient tales about Hades and the Underworld.”

Romulus was a non-starter because it has already been used to name one of the moons of the asteroid Sylvia. (The other moon is called Remus.)

Vulcan was a little more complicated. It’s the name of the Roman god of fire, so it satisfies the mythology requirement. But it’s also the name of a nonexistent planet that was once thought to orbit the Sun even closer than Mercury.

“Some of the world’s greatest astronomers spent quite a long time looking for it and they never saw it because it isn’t there,” Showalter explained in Google+ hangout. “Some people say, ‘No, we should save the name Vulcan for some place that’s big and hot.’ I guess one response to that would be, ‘Well, we found all the places [in the solar system] that are big and hot and they’ve all got names now.’”

So Vulcan was added to the ballot and it blew away the competition.

The second-place finisher with 99,432 votes was Cerberus, the Roman name for the three-headed dog that guards the entrance to the underworld. Although this was one of the official nominees from SETI, it turned out to be a little tricky too. The solar system is already home to an asteroid by that name, Showalter noted. One potential solution would be to change the spelling to Kerberos, as the creature is known in Greek mythology, he said.

Rounding out the top five were Styx (87,858 votes), Persephone (68,969 votes) and Orpheus (51,197 votes).

More than 450,000 total votes were cast (some people may have voted more than once), with about half of those coming from the United States. The ballot was available in more than a dozen languages, including Russian, Chinese, Korean, Japanese, Arabic and Farsi. “Almost every country on the planet has had at least a couple of votes come in,” Showalter said.

The final decision will be made by the nomenclature working group of the International Astronomical Union. Showalter is a member of that committee, but he pledged to recuse himself from the deliberations when the permanent names for P4 and P5 are considered a few months from now.

 Vulcan and Cerberus win online poll to name Pluto’s smallest moons –


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Russian Meteor Likely An Apollo Asteroid Chunk : Discovery News

Russian Meteor Likely An Apollo Asteroid Chunk

FEB 26, 2013 12:25 PM ET // BY IAN O’NEILL

On Feb. 15, the Urals region of Russia played host to a noisy cosmic visitor. A meteor entered the atmosphere and broke up over the city of Chelyabinsk, generating powerful shockwaves that slammed into the city, blowing out windows, causing 1,500 injuries and millions of dollars-worth of damage. Before it collided with Earth, however, the Chelyabinsk space rock was a 10,000 ton meteoroid and astronomers now think they know where it came from.

PHOTOS: Russian Meteor Strike Aftermath

Helped by the extensive coverage of eyewitness cameras, CCTV footage and a fortuitous observation made by the Meteosat-9 weather satellite, Jorge Zuluaga and Ignacio Ferrin of the University of Antioquia in Medellin, Colombia, have been able to reconstruct the most likely orbit of the space rock around the sun before the Earth got in its way. What’s more, they know what typeof space rock it was.

Using video evidence (most of which had precise timestamps), the location, speed and altitude of the fireball could be estimated. Add to that the location where a suspected meteorite fragment punched a hole into the ice of Lake Cherbakul and it’s a case of using some simple math to learn the characteristics of the object. But to trace the meteoroid’s path back out into space and assemble its orbital trajectory around the sun wasn’t so straight forward, according to the arXiv blog.

ANALYSIS: Russian Meteor: What’s With All The Dash Cams?

However, this analysis hinges on one important factor: “Assuming that the hole in the ice sheet of Lake Cherbakul was produced by a fragment of the meteoroid is also a very important hypothesis of this work. More importantly, our conclusions relies strongly onto assume that the direction of the trajectory of the fragment responsible for the breaking of the ice sheet in the Lake, is essentially the same as the direction of the parent body. It could be not the case. After the explosion and fragmentation of the meteoroid fragments could acquire different velocities and fall affecting areas far from the region where we expect to find,” the researchers write in their paper submitted to the arXiv pre-print service. So far, no meteorite has been recovered from Lake Cherbakul.

“According to our estimations, the Chelyabinski meteor started to brighten up when it was between 32 and 47 km up in the atmosphere … The velocity of the body predicted by our analysis was between 13 and 19 km/s (relative to the Earth) which encloses the preferred figure of 18 km/s assumed by other researchers,” they add.

Armed with this wealth of data farmed from various eyewitness sources, they used a piece of software called NOVAS (an acronym for “Naval Observatory Vector Astrometry Software”) developed by the U.S. Navy Observatory (USNO). This sophisticated program was able to consider the gravitational influence of the moon, plus eight other bodies in the solar system, ultimately helping Zuluaga and Ferrin track where the object was before impact.

NEWS: Huge Meteor Explodes Over Russia

Taking its orbit into account, the researchers were able to conclude that the Chelyabinsk-bound meteoroid originated from an Apollo-class asteroid. Apollo asteroids are well-known near-Earth asteroids that cross the orbit of Earth. Around 5,200 Apollo asteroids are currently known, the largest being 1866 Sisyphus — a 10 kilometer-wide monster that was discovered in 1972.

Large Apollos are identified as being a significant risk to our planet, so the Chelyabinsk meteoroid acted like an Apollo warning shot.

 Russian Meteor Likely An Apollo Asteroid Chunk : Discovery News.


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Fermi Measures Light from All the Stars That Have Ever Existed

Fermi Measures Light from All the Stars That Have Ever Existed


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This plot shows the locations of 150 blazars (green dots) used in the a new by the Fermi Gamma-Ray Telescope. Credit: NASA/DOE/Fermi LAT Collaboration

All the light that has been produced by every star that has ever existed is still out there, but “seeing” it and measuring it precisely is extremely difficult. Now, astronomers using data from NASA’s Fermi Gamma-ray Space Telescope were able to look at distant blazars to help measure the background light from all the stars that are shining now and ever were. This enabled the most accurate measurement of starlight throughout the universe, which in turn helps establish limits on the total number of stars that have ever shone.

“The optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine, and this creates a fossil radiation field we can explore using gamma rays from distant sources,” said lead scientist Marco Ajello from the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University in California and the Space Sciences Laboratory at the University of California at Berkeley.

Their results also provide a stellar density in the cosmos of about 1.4 stars per 100 billion cubic light-years, which means the average distance between stars in the universe is about 4,150 light-years.

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The total sum of starlight in the cosmos is called the extragalactic background light (EBL), and Ajello and his team investigated the EBL by studying gamma rays from 150 blazars, which are among the most energetic phenomena in the universe. They are galaxies powered by extremely energetic black holes: they have energies greater than 3 billion electron volts (GeV), or more than a billion times the energy of visible light.

The astronomers used four years of Fermi data on gamma rays with energies above 10 billion electron volts (GeV), and the Fermi Large Area Telescope (LAT) instrument is the first to detect more than 500 sources in this energy range.

To gamma rays, the EBL functions as a kind of cosmic fog, but Fermi measured the amount of gamma-ray absorption in blazar spectra produced by ultraviolet and visible starlight at three different epochs in the history of the universe.

Fermi measured the amount of gamma-ray absorption in blazar spectra produced by ultraviolet and visible starlight at three different epochs in the history of the universe. (Credit: NASA’s Goddard Space Flight Center)

“With more than a thousand detected so far, blazars are the most common sources detected by Fermi, but gamma rays at these energies are few and far between, which is why it took four years of data to make this analysis,” said team member Justin Finke, an astrophysicist at the Naval Research Laboratory in Washington.

Gamma rays produced in blazar jets travel across billions of light-years to Earth. During their journey, the gamma rays pass through an increasing fog of visible and ultraviolet light emitted by stars that formed throughout the history of the universe.

Occasionally, a gamma ray collides with starlight and transforms into a pair of particles — an electron and its antimatter counterpart, a positron. Once this occurs, the gamma ray light is lost. In effect, the process dampens the gamma ray signal in much the same way as fog dims a distant lighthouse.

From studies of nearby blazars, scientists have determined how many gamma rays should be emitted at different energies. More distant blazars show fewer gamma rays at higher energies — especially above 25 GeV — thanks to absorption by the cosmic fog.

The researchers then determined the average gamma-ray attenuation across three distance ranges: The closest group was from when the universe was 11.2 years old, a middle group of when the Universe was 8.6 billion years old, and the farthest group from when the Universe was 4.1 billion years old.

This animation tracks several gamma rays through space and time, from their emission in the jet of a distant blazar to their arrival in Fermi’s Large Area Telescope (LAT). During their journey, the number of randomly moving ultraviolet and optical photons (blue) increases as more and more stars are born in the universe. Eventually, one of the gamma rays encounters a photon of starlight and the gamma ray transforms into an electron and a positron. The remaining gamma-ray photons arrive at Fermi, interact with tungsten plates in the LAT, and produce the electrons and positrons whose paths through the detector allows astronomers to backtrack the gamma rays to their source.

From this measurement, the scientists were able to estimate the fog’s thickness.

“These results give you both an upper and lower limit on the amount of light in the Universe and the amount of stars that have formed,” said Finke during a press briefing today. “Previous estimates have only been an upper limit.”

And the upper and lower limits are very close to each other, said Volker Bromm, an astronomer at the University of Texas, Austin, who commented on the findings. “The Fermi result opens up the exciting possibility of constraining the earliest period of cosmic star formation, thus setting the stage for NASA’s James Webb Space Telescope,” he said. “In simple terms, Fermi is providing us with a shadow image of the first stars, whereas Webb will directly detect them.”

Measuring the extragalactic background light was one of the primary mission goals for Fermi, and Ajello said the findings are crucial for helping to answer a number of big questions in cosmology.

A paper describing the findings was published Thursday on Science Express.

 Fermi Measures Light from All the Stars That Have Ever Existed.


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