Fermi shows that Tycho's star shines in gamma rays

Fig: Gamma rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines in the highest-energy form of light. This portrait of the shattered star includes gamma rays (magenta), X-rays (yellow, green, and blue), infrared (red) and optical data. Gamma ray, NASA/DOE/Fermi LAT Collaboration; X-ray, NASA/CXC/SAO; Infrared, NASA/JPL-Caltech; Optical, MPIA, Calar Alto, O. Krause et al. and DSS

By NASA's Goddard Space Flight Center, Greenbelt, Maryland

Published: December 16, 2011

In early November 1572, observers on Earth witnessed the appearance of a “new star” in the constellation Cassiopeia, an event now recognized as the brightest naked-eye supernova in more than 400 years. It’s often called “Tycho’s supernova” after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. Now, years of data collected by NASA’s Fermi Gamma-Ray Space Telescope reveal that the shattered star’s remains shine in high-energy gamma rays.

The detection gives astronomers another clue in understanding the origin of cosmic rays, subatomic particles — mainly protons — that move through space at nearly the speed of light. Exactly where and how these particles attain such incredible energies has been a long-standing mystery because charged particles speeding through the galaxy are easily deflected by interstellar magnetic fields. This makes it impossible to track cosmic rays back to their sources.

“Fortunately, high-energy gamma rays are produced when cosmic rays strike interstellar gas and starlight,” said Francesco Giordano from the University of Bari and the National Institute of Nuclear Physics in Italy. “These gamma rays come to Fermi straight from their sources.”

Better understanding the origins of cosmic rays is one of Fermi’s key goals. Its Large Area Telescope (LAT) scans the entire sky every three hours, gradually building up an ever-deeper view of the gamma-ray sky. Because gamma rays are the most energetic and penetrating form of light, they serve as signposts for the particle acceleration that gives rise to cosmic rays.

“This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays,” said Stefan Funk from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) in Stanford, California.

In 1949, physicist Enrico Fermi — the satellite’s namesake — suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of interstellar gas clouds. In the decades that followed, astronomers showed that supernova remnants might be the galaxy’s best candidate sites for this process.

When a star explodes, it is transformed into a supernova remnant, a rapidly expanding shell of hot gas bounded by the blast’s shock wave. Scientists expect that magnetic fields on either side of the shock front can trap particles between them in what amounts to a subatomic Pingpong game.

“A supernova remnant’s magnetic fields are very weak relative to Earth’s, but they extend across a vast region, ultimately spanning thousands of light-years,” said Melitta Naumann-Godo from Paris Diderot University and the Atomic Energy Commission in Saclay, France. “They have a major influence on the course of charged particles.”

As they shuttle back and forth across the supernova shock, the charged particles gain energy with each traverse. Eventually, they break out of their magnetic confinement, escaping the supernova remnant and freely roaming the galaxy.

The LAT’s ongoing sky survey provides additional evidence favoring this scenario. Many younger remnants, like Tycho’s, tend to produce more high-energy gamma rays than older remnants. “The gamma-ray energies reflect the energies of the accelerated particles that produce them, and we expect more cosmic rays to be accelerated to higher energies in younger objects because the shock waves and their tangled magnetic fields are stronger,” Funk said. By contrast, older remnants with weaker shock waves cannot retain the highest-energy particles, and the LAT does not detect gamma rays with corresponding energies.

The supernova of 1572 was one of the great watersheds in the history of astronomy. The star blazed forth at a time when the starry sky was regarded as a fixed and unchanging part of the universe. Tycho’s candid account of his own discovery of the strange star gives a sense of how radical an event it was.

The supernova first appeared around November 6, but poor weather kept it from Tycho until November 11, when he noticed it during a walk before dinner. “When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star. ... They immediately replied with one voice that they saw it completely and that it was extremely bright,” he said.

The supernova remained visible for 15 months and exhibited no movement in the heavens, indicating that it was located far beyond the Sun, Moon, and planets. Modern astronomers estimate that the remnant lies between 9,000 and 11,000 light-years away.

After more than 2.5 years of scanning the sky, LAT data clearly show that an unresolved region of GeV (billion electron volt) gamma-ray emission is associated with the remnant of Tycho’s supernova. (For comparison, the energy of visible light is between about 2 and 3 electron volts.)

“We knew that Tycho’s supernova remnant could be an important find for Fermi because this object has been so extensively studied in other parts of the electromagnetic spectrum,” said Keith Bechtol from SLAC. “We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons.” he said.

The science team’s model of the emission is based on LAT observations along with higher-energy TeV (trillion electron volt) gamma rays mapped by ground-based facilities and radio and X-ray data. The researchers conclude that a process called pion production best explains the emission. First, a proton traveling close to the speed of light strikes a slower-moving proton. This interaction creates an unstable particle — a pion — with only 14 percent of the proton’s mass. In just 10 millionths of a billionth of a second, the pion decays into a pair of gamma rays.

If this interpretation is correct, then somewhere within the remnant protons are being accelerated to near the speed of light, and then interacting with slower particles to produce gamma rays, the most extreme form of light. With such unbelievable goings-on in what’s left of his “unbelievable” star, it’s easy to imagine that Tycho Brahe himself might be pleased.

Astronomers discover two planets that survived their star's expansion

Fig: Two planets that survived the red-giant expansion of their host star. Illustration by Stéphane Charpinet/Institut de Recherche en Astrophysique et Planétologie in Toulouse, France

By Iowa State University, Ames

Published: December 22, 2011

Astronomers have discovered two Earth-sized planets that survived getting caught in the red-giant expansion of their host star.

Steve Kawaler from Iowa State University helped the research team study data from the Kepler space telescope to confirm that tiny variations of light from a star were actually caused by two planets orbiting it. Stéphane Charpinet from the Institut de Recherche en Astrophysique et Planétologie in Toulouse, France, is the leader of the research team.

“This is a snapshot of what our solar system might look like after several billion more years of evolution,” Kawaler said. “This can help us learn about the future of planetary systems and of our own Sun.”

Kawaler said the researchers have studied pulsations of the planets’ host star, KIC 05807616 — an old star just past its red-giant stage — for about two years. While analyzing the data, Charpinet noticed two tiny variations repeated in 5.76- and 8.23-hour intervals.

He asked other astronomers, including Kawaler, to analyze the original Kepler data and a subsequent set of data to see if they also could see the variations.

“We saw them in the same place and the same periodicity,” Kawaler said. “So we knew they were real.”

That led to the next question: “So what are they?”

Kawaler has studied the fastest and slowest rates that stars could pulsate. Using that result, the team could conclude the variations seen by Kepler were too slow to be from the star itself. So the astronomers started testing the idea that the variations were from two planets orbiting the star.

Astronomers believe the variations from the two planets, KOI 55.01 and KOI 55.02, are caused by reflection of the star’s light on the planets and by temperature differences between the hot day-sides and cooler night-sides of the planets.

The astronomers also report the planets are 76 percent and 87 percent the size of Earth. That makes them among the smallest planets detected around a star other than our Sun.

They further report the planets are close to their host star, only 0.6 percent and 0.76 percent the distance between the Sun and Earth. That means conditions on the planets are harsh with temperatures up to 16,000° Fahrenheit (9,000° Celsius).

That’s so close that the host star’s expansion to a red giant would have engulfed the planets, possibly stripping gas giant planets similar to Jupiter down to their dense cores. The planets also could have contributed to the host star’s unusual loss of mass.

The research team said the discovery of the two planets raises many questions about their ability to survive such harsh conditions. It also raises questions about how planets can affect the evolution of their host stars.

A galaxy cluster gets sloshed

Fig: The hot gas in the galaxy cluster Abell 2052 is being sloshed back and forth. The sloshing was set in motion when a small cluster smashed into the larger central one. The large spiral structure on the outside of the image was also caused by that off-center collision. Sloshing of hot gas like this can affect how the giant elliptical galaxy and its supermassive black hole at the center grow.

X-ray: NASA/CXC/BU/L.Blanton; Optical: ESO/VLT

By Chandra X-ray Center, Cambridge, Massachusetts

Published: December 14, 2011

Like wine in a glass, vast clouds of hot gas are sloshing back and forth in Abell 2052, a galaxy cluster located about 480 million light-years from Earth. X-ray data (blue) from NASA’s Chandra X-ray Observatory shows the hot gas in this dynamic system, and optical data (gold) from the Very Large Telescope shows the galaxies. The hot X-ray-bright gas has an average temperature of about 30 million degrees.

A huge spiral structure in the hot gas — spanning almost a million light-years across — is seen around the outside of the image, surrounding a giant elliptical galaxy at the center. This spiral was created when a small cluster of galaxies smashed into a larger one that surrounds the central elliptical galaxy.

As the smaller cluster approached, the dense hot gas of the central cluster was attracted to it by gravity. After the smaller cluster passed the cluster core, the direction of motion of the cluster gas reversed, and it traveled back toward the cluster center. The cluster gas moved through the center again and “sloshed” back and forth. The sides of the glass push the wine back to the center, whereas in the cluster the gravitational force of the matter in the clusters pulls it back. The sloshing gas ended up in a spiral pattern because the collision between the two clusters was off-center.

This type of sloshing in Abell 2052 has important physical implications. First, it helps push some of the more dense, cooler gas located in the center of the cluster — where temperatures are only about 10 million degrees — farther away from the core. This helps prevent further cooling of this gas in the core and could limit the amount of new stars being formed in the central galaxy. Sloshing motions like those seen in Abell 2052 also redistribute heavy elements, like iron and oxygen, which are forged in supernova explosions. These elements are used in the future generations of stars and planets and are necessary for life, as we know it.

Chandra’s observation of Abell 2052 was particularly long, lasting more than a week. Such a deep observation was necessary to detect all of the details in this image. Even then, processing to emphasize more-subtle features was necessary to reveal the outer spiral structure.

In addition to the large-scale spiral feature, the deep Chandra observation reveals exquisite detail in the cluster center related to outbursts from the central supermassive black hole. The Chandra data show clear bubbles evacuated by material blasted away from the black hole, which are surrounded by dense, bright, cool rims. As with the sloshing, this activity helps prevent cooling of the gas in the cluster’s core, setting limits on the growth of the giant elliptical galaxy and its supermassive black hole.

Coronal mass ejections (CMEs) could "sandblast" the Moon

Fig: Coronal mass ejection as viewed by the Solar Dynamics Observatory
(June 7, 2011. NASA/SDO)

By NASA's Goddard Space Flight Center, Greenbelt, Maryland

Published: December 12, 2011

Solar storms and associated coronal mass ejections (CMEs) can significantly erode the lunar surface, according to a new set of computer simulations by NASA scientists. In addition to removing a surprisingly large amount of material from the lunar surface, this could be a major method of atmospheric loss for planets like Mars that are unprotected by a global magnetic field.

Rosemary Killen is leading the research from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, as part of the Dynamic Response of the Environment At the Moon (DREAM) team within the NASA Lunar Science Institute.

CMEs are basically an intense gust of the normal solar wind, a diffuse stream of electrically conductive gas called plasma that’s blown outward from the surface of the Sun into space. A strong CME may contain around a billion tons of plasma moving at up to 1 million mph (1.6 million km/h) in a cloud many times the size of Earth.

The Moon has just the barest wisp of an atmosphere, technically called an exosphere because it is so tenuous, which leaves it vulnerable to CME effects. The plasma from CMEs impacts the lunar surface, and atoms from the surface are ejected in a process called “sputtering.”

“We found that when this massive cloud of plasma strikes the Moon, it acts like a sandblaster and easily removes volatile material from the surface,” said William Farrell from NASA’s Goddard Space Flight Center. “The model predicts 100 to 200 tons of lunar material — the equivalent of 10 dump-truck loads — could be stripped off the lunar surface during the typical two-day passage of a CME.”

This is the first time researchers have attempted to predict the effects of a CME on the Moon. “Connecting various models together to mimic conditions during solar storms is a major goal of the DREAM project,” said Farrell.

Plasma is created when energetic events, like intense heat or radiation, remove electrons from the atoms in a gas, turning the atoms into electrically charged particles called ions. The Sun is so hot that the gas is emitted in the form of free ions and electrons called the solar wind plasma. Ejection of atoms from a surface or an atmosphere by plasma ions is called sputtering.

“Sputtering is among the top five processes that create the Moon’s exosphere under normal solar conditions, but our model predicts that during a CME, it becomes the dominant method by far, with up to 50 times the yield of the other methods,” said Killen.

CMEs are effective at removing lunar material not only because they are denser and faster than the normal solar wind, but also because they are enriched in highly charged, heavy ions, according to the team. The typical solar wind is dominated by lightweight hydrogen ions (protons). However, a heavier helium ion with more electrons removed, and hence a greater electric charge, can sputter tens of times more atoms from the lunar surface than a hydrogen ion.

The team used data from satellite observations that revealed this enrichment as input to their model. For example, helium ions make up about 4 percent of the normal solar wind, but observations reveal that during a CME they can increase to over 20 percent. When this enrichment is combined with the increased density and velocity of a CME, the highly charged, heavy ions in CMEs can sputter 50 times more material than protons in the normal solar wind.

“The computer models isolate the contributions from sputtering and other processes,” said Dana Hurley from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “Comparing model predictions through a range of solar wind conditions allows us to predict the conditions when sputtering should dominate over the other processes. Those predictions can later be compared to data during a solar storm.”

The researchers believe that NASA’s Lunar Atmosphere And Dust Environment Explorer (LADEE) — a lunar orbiter mission scheduled to launch in 2013 — will be able to test their predictions. The strong sputtering effect should kick lunar surface atoms to LADEE’s orbital altitude, about 12 to 31 miles (19 to 50 kilometers), so the spacecraft will see them increase in abundance.

“This huge CME sputtering effect will make LADEE almost like a surface mineralogy explorer, not because LADEE is on the surface, but because during solar storms surface atoms are blasted up to LADEE,” said Farrell.

The Moon is not the only heavenly body affected by the dense CME driver gas. Space scientists have long been aware that these solar storms dramatically affect Earth’s magnetic field and are responsible for intense aurorae (the northern and southern lights).

While certain areas of the martian surface are magnetized, Mars does not have a magnetic field that surrounds the entire planet. Therefore, CME gases have a direct path to sputter and erode that planet’s upper atmosphere. In late 2013, NASA will launch the Mars Atmosphere and Volatile Evolution (MAVEN) mission that will orbit the Red Planet to investigate exactly how solar activity, including CMEs, removes the atmosphere.

On exposed small bodies like asteroids, the dense, fast-streaming CME gas should create a sputtered-enhanced exosphere about the object, similar to that expected at the Moon.

Kepler's First Planet Inside Habitable (Outside Solar System)

This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. NASA/Ames/JPL-Caltech

By NASA Headquarters, Washington, D.C.

Published: December 5, 2011

NASA’s Kepler mission has confirmed its first planet in the “habitable zone,” the region where liquid water could exist on a planet’s surface. Kepler also has discovered more than 1,000 new planet candidates, nearly doubling its previously known count. Ten of these candidates are near Earth’s size and orbit in the habitable zone of their host star. Candidates require follow-up observations to verify they are actual planets.

The newly confirmed planet, Kepler-22b, is the smallest yet found to orbit in the middle of the habitable zone of a star similar to our Sun. The planet is about 2.4 times the radius of Earth. Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous, or liquid composition, but its discovery is a step closer to finding Earth-like planets.

Previous research hinted at the existence of near-Earth-sized planets in habitable zones, but clear confirmation proved elusive. Two other small planets orbiting stars smaller and cooler than our Sun recently were confirmed on the edges of the habitable zone, with orbits more closely resembling those of Venus and Mars.

“This is a major milestone on the road to finding Earth’s twin,” said Douglas Hudgins from NASA Headquarters in Washington, D.C. “Kepler’s results continue to demonstrate the importance of NASA’s science missions, which aim to answer some of the biggest questions about our place in the universe.”

Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or “transit,” the stars. Kepler requires at least three transits to verify a signal as a planet.

“Fortune smiled upon us with the detection of this planet,” said William Borucki from NASA Ames Research Center at Moffett Field, California. “The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season.”

The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planet candidates the spacecraft finds. The star field that Kepler observes in the constellations Cygnus and Lyra are only visible from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.

Kepler-22b is located 600 light-years away. While the planet is larger than Earth, its orbit of 290 days around a Sun-like star resembles that of our world. The planet’s host star belongs to the same class as our Sun, called G-type, although it is slightly smaller and cooler.

Of the 54 habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed.

The Kepler team is hosting its inaugural science conference at Ames December 5–9, announcing 1,094 new planet candidate discoveries. Since the last catalog was released in February, the number of planet candidates identified by Kepler has increased by 89 percent, and now totals 2,326. Of these, 207 are approximately Earth-sized, 680 are super-Earth-sized, 1,181 are Neptune-sized, 203 are Jupiter-sized, and 55 are larger than Jupiter.

The findings, based on observations conducted May 2009 to September 2010, show a dramatic increase in the numbers of smaller-sized planet candidates.


Kepler observed many large planets in small orbits early in its mission, which were reflected in the February data release. Having had more time to observe three transits of planets with longer orbital periods, the new data suggest that planets one to four times the size of Earth may be abundant in the galaxy.

The number of Earth-sized and super-Earth-sized candidates has increased by more than 200 and 140 percent since February, respectively.

There are 48 planet candidates in their stars’ habitable zones. While this is a decrease from the 54 reported in February, the Kepler team has applied a stricter definition of what constitutes a habitable zone in the new catalog to account for the warming effect of atmospheres, which would move the zone away from the star out to longer orbital periods.

“The tremendous growth in the number of Earth-size candidates tells us that we’re honing in on the planets that Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable,” said Natalie Batalha from San Jose State University in California. “The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods.”