Partial Eclipses of the Sun by Proba-2

ESA's Proba-2 Sun-watcher saw Australia's 29 April 2014 partial solar eclipse from orbit - footage captured by the Royal Observatory of Belgium.

Video credit: ESA

Star Cluster NGC 2024 in the Flame Nebula

Stars are often born in clusters or groups, in giant clouds of gas and dust. Astronomers have studied two star clusters using NASA's Chandra X-ray Observatory and infrared telescopes and the results show that the simplest ideas for the birth of these clusters cannot work.

This composite image shows one of the clusters, NGC 2024, which is found in the center of the so-called Flame Nebula about 1,400 light years from Earth. In this image, X-rays from Chandra are seen as purple, while infrared data from NASA's Spitzer Space Telescope are colored red, green and blue.

A study of NGC 2024 and the Orion Nebula Cluster, another region where many stars are forming, suggest that the stars on the outskirts of these clusters are older than those in the central regions. This is different from what the simplest idea of star formation predicts, where stars are born first in the center of a collapsing cloud of gas and dust when the density is large enough.

The research team developed a two-step process to make this discovery. First, they used Chandra data on the brightness of the stars in X-rays to determine their masses. Next, they found out how bright these stars were in infrared light using data from Spitzer, the 2MASS telescope and the United Kingdom Infrared Telescope. By combining this information with theoretical models, the ages of the stars throughout the two clusters could be estimated.

According to the new results, the stars at the center of NGC 2024 were about 200,000 years old while those on the outskirts were about 1.5 million years in age. In Orion, the age spread went from 1.2 million years in the middle of the cluster to nearly 2 million years for the stars toward the edges.

Explanations for the new findings can be grouped into three broad categories. The first is that star formation is continuing to occur in the inner regions. This could have happened because the gas in the outer regions of a star-forming cloud is thinner and more diffuse than in the inner regions. Over time, if the density falls below a threshold value where it can no longer collapse to form stars, star formation will cease in the outer regions, whereas stars will continue to form in the inner regions, leading to a concentration of younger stars there.

Another suggestion is that old stars have had more time to drift away from the center of the cluster, or be kicked outward by interactions with other stars. Finally, the observations could be explained if young stars are formed in massive filaments of gas that fall toward the center of the cluster.

These results will be published in two separate papers in The Astrophysical Journal and are available online (papers 1 and 2). They are part of the MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray) project led by Penn State astronomers.

Image credit: X-ray: NASA/CXC/PSU/K.Getman, E.Feigelson, M.Kuhn and the MYStIX team; Infrared: NASA/JPL-Caltech

Note: For more information, see NASA Delivers New Insight into Star Cluster Formation and Flame Nebula: NASA's Chandra Delivers New Insight into Formation of Star Clusters.

The Milky Way's Magnetic Field and Dust Polarization

The magnetic field of our Milky Way Galaxy as seen by ESA's Planck satellite. This image was compiled from the first all-sky observations of polarized light emitted by interstellar dust in the Milky Way. The magnetic field is displayed using a visualization technique called line integral convolution (LIC).

Darker regions correspond to stronger polarized emission, and the striations indicate the direction of the magnetic field projected on the plane of the sky. The dark band running horizontally across the center corresponds to the Galactic Plane. Here, the polarization reveals a regular pattern on large angular scales, which is due to the magnetic field lines being predominantly parallel to the plane of the Milky Way. The data also reveal variations of the polarization direction within nearby clouds of gas and dust. This can be seen in the tangled features above and below the plane, where the local magnetic field is particularly disorganized.

The image is a Mollweide projection of the full celestial sphere, with the plane of the Galaxy aligned with the horizontal axis of the oval. Certain areas in the image, mostly at high Galactic latitude, have been masked out. The overall intensity in these regions is low, complicating the separation of foreground and CMB components. Further data analysis will improve this by the time of the full data release in late 2014.

Image credit: ESA and the Planck Collaboration

Note: For more information, see Planck Takes Magnetic Fingerprint of Our Galaxy, Milky Way's Magnetic Fingerprint, PIA18048: Magnetic Map of Milky Way and Planck Takes Magnetic Fingerprint of Our Galaxy.

Ganymede's Possible Internal Structure

This artist's concept of Jupiter's moon Ganymede, the largest moon in the solar system, illustrates the "club sandwich" model of its interior oceans. Scientists suspect Ganymede has a massive ocean under an icy crust. In fact, Ganymede's oceans may have 25 times the volume of those on Earth. Previous models of the moon showed the moon's ocean sandwiched between a top and bottom later of ice. A new model, based on experiments in the laboratory that simulate salty seas, shows that the ocean and ice may be stacked up in multiple layers, more like a club sandwich.

Ice comes in different forms depending on pressures. "Ice I," the least dense form of ice, is what floats in your chilled beverages. As pressures increase, ice molecules become more tightly packed and thus more dense. Because Ganymede's oceans are up to 500 miles (800 kilometers) deep, they would experience more pressure than Earth's oceans. The deepest and most dense form of ice thought to exist on Ganymede is called "Ice VI."

When researchers added in salt into their models of the ocean, they found the situation changed from what was previously thought. With enough salt, liquid in Ganymede can become dense enough to sink to the very bottom of the seafloor, below Ice VI. Their models also suggest a complex stacking of ocean and ice, as illustrated in the picture.

What's more, the model shows that a strange phenomenon might occur in the uppermost liquid layer, where ice floats upward. In this scenario, cold plumes cause Ice III to form. As the ice forms, salt precipitates out. The salt then sinks down while the ice "snows" upward. Eventually, this ice would melt, resulting in a slushy layer in Ganymede's club sandwich structure.

Scientists say this structure may not be stable. It's possible the moon goes through a club sandwich phase, while at other times goes back to being more like a regular sandwich, with one ocean sitting below the familiar Ice I found on Earth and on top of different high-pressure ices.

The fact that salty water may persist at the bottom of the rocky seafloor, rather than ice, is favorable for the development of life. Researchers think life emerges through a series of chemical interactions at water-mineral interfaces, so a wet seafloor on Ganymede might be a key ingredient for life there.

Illustration credit: NASA/JPL-Caltech

Note: For more information, see Ganymede May Harbor 'Club Sandwich' of Oceans and Ice.

Uranus from Saturn

This view from NASA's Cassini spacecraft features a blue planet, but unlike the view from July 19, 2013 (PIA17172) that featured our home planet, this blue orb is Uranus, imaged by Cassini for the first time.

Uranus is a pale blue in this natural color image because its visible atmosphere contains methane gas and few aerosols or clouds. Methane on Uranus -- and its sapphire-colored sibling, Neptune -- absorbs red wavelengths of incoming sunlight, but allows blue wavelengths to escape back into space, resulting in the predominantly bluish color seen here. Cassini imaging scientists combined red, green and blue spectral filter images to create a final image that represents what human eyes might see from the vantage point of the spacecraft.

Uranus has been brightened by a factor of 4.5 to make it more easily visible. The outer portion of Saturn's A ring, seen at bottom right, has been brightened by a factor of two. The bright ring cutting across the image center is Saturn's narrow F ring.

Uranus was approximately 28.6 astronomical units from Cassini and Saturn when this view was obtained. An astronomical unit is the average distance from Earth to the sun, equal to 93,000,000 miles (150,000,000 kilometers).

This view was acquired by the Cassini narrow-angle camera at a distance of approximately 614,300 miles (988,600 kilometers) from Saturn on April 11, 2014. Image scale at Uranus is approximately 16,000 miles (25,700 kilometers) per pixel. Image scale at Saturn's rings is approximately 4 miles (6 kilometers) per pixel. In the image, the disk of Uranus is just barely resolved. The solar phase angle at Uranus, seen from Cassini, is 11.9 degrees.

Image credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see Cassini Spies the Ice-Giant Planet Uranus.

Saturn's C and B Rings in Ultraviolet Light

This colorful cosmic rainbow portrays a section of Saturn’s beautiful rings, four centuries after they were discovered by Galileo Galilei.

Saturn’s rings were first observed in 1610. Despite using his newly created telescope, Galileo was confounded by what he saw: he referred to the peculiar shapes surrounding the planet as “Saturn’s children”. Only later did Christiaan Huygens propose that the mysterious shapes were actually rings orbiting the planet. These were named in the order in which they were discovered, using the first seven letters of the alphabet: the D-ring is closest to the planet, followed by C, B, A, F, G and E.

The data for this image, which shows the portion of the C-ring closest to Saturn on the left, with the B-ring beginning just right of center, were acquired by Cassini’s Ultraviolet Imaging Spectrograph, or UVIS, as the spacecraft entered into orbit around Saturn on 30 June 2004.

UVIS, as its name suggests, carries out observations in ultraviolet wavelengths. During the Saturn orbit insertion maneuver, when Cassini flew closest to the rings, UVIS could resolve features up to 97 km across. The region shown in this image spans about 10,000 km.

The variation in the color of the rings arises from the differences in their composition. Turquoise-hued rings contain particles of nearly pure water ice, whereas reddish rings contain ice particles with more contaminants.

Saturn’s prominent and complex ensemble of rings is the best studied in the Solar System, but it is still not known how the rings formed. One suggestion is that they formed at the same time as the planet and that they are as old as the Solar System. Another idea is that they formed when icy material was pulled from another body into Saturn’s gravitational field, in which case the rings could be younger than the planet.

One thing is sure: as Cassini searches for answers it is providing amazing images of these rainbow rings.

The Cassini–Huygens mission is a cooperative project of NASA, ESA and Italy’s ASI space agency.

This image was first published at the NASA Cassini website, in 2004.

Image credit: NASA/JPL/University of Colorado

Caracalla Supernova in Galactic Cluster MACSJ1720+35

The heart of a vast cluster of galaxies called MACSJ1720+35 is shown in this image, taken in visible and near-infrared light by the NASA/ESA Hubble Space Telescope.

The galaxy cluster is so massive that its gravity distorts, brightens, and magnifies light from more distant objects behind it, an effect called gravitational lensing. In the top right an exploding star, located behind the cluster can just be made out. It is cataloged as SCP/SN-L2 and nicknamed Caracalla.

The supernova is a member of a special class of exploding star called Type Ia, prized by astronomers because it provides a consistent level of peak brightness that makes it reliable for making distance estimates.

Finding a gravitationally lensed Type Ia supernova gives astronomers a unique opportunity to check the optical "prescription" of the foreground lensing cluster. The supernova is one of three exploding stars discovered in the Cluster Lensing And Supernova survey with Hubble (CLASH), and was followed up as part of a Supernova Cosmology Project HST program. CLASH is a Hubble census that probed the distribution of dark matter in 25 galaxy clusters. Dark matter cannot be seen directly but is believed to make up most of the universe's matter.

The image of the galaxy cluster was taken between March and July 2012 by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys.

Image credit: (top) NASA, ESA, S. Perlmutter (UC Berkeley, LBNL), A. Koekemoer (STScI), M. Postman (STScI), A. Riess (STScI/JHU), J. Nordin (LBNL, UC Berkeley), D. Rubin (Florida State), and C. McCully (Rutgers University); (bottom) NASA, ESA, S. Perlmutter (UC Berkeley, LBNL), A. Koekemoer (STScI), M. Postman (STScI), A. Riess (STScI/JHU), J. Nordin (LBNL, UC Berkeley), D. Rubin (Florida State), and C. McCully (Rutgers University)

Note: For more information, see Cosmic Lens MACS J1720+35 Helps Hubble to Find a Distant Supernova.

Star Cluster LH63 in Emission Nebula LHA 120-N51

This stunning new Hubble image shows a small part of the Large Magellanic Cloud, one of the closest galaxies to our own. This collection of small baby stars, most weighing less than the Sun, form a young stellar cluster known as LH63. This cluster is still half-embedded in the cloud from which it was born, in a bright star-forming region known as the emission nebula LHA 120-N 51, or N51. This is just one of the hundreds of star-forming regions filled with young stars spread throughout the Large Magellanic Cloud.

The burning red intensity of the nebulae at the bottom of the picture illuminates wisps of gas and dark dust, each spanning many light-years. Moving up and across, bright stars become visible as sparse specks of light, giving the impression of pin-pricks in a cosmic cloak.

This patch of sky was the subject of observation by Hubble's WFPC2 camera. Looking for and at low-mass stars can help us to understand how stars behave when they are in the early stages of formation, and can give us an idea of how the Sun might have looked billions of years ago.

Image credit: NASA, ESA, and D. Gouliermis (University of Heidelberg)

NGC 3455

Shown here is a spiral galaxy known as NGC 3455, which lies some 65 million light-years away from us in the constellation of Leo (The Lion). Galaxies are classified into different types according to their structure and appearance. This classification system is known as the Hubble Sequence, named after its creator Edwin Hubble. In this sequence, NGC 3455 is known as a type SB galaxy — a barred spiral. Barred spiral galaxies account for approximately two thirds of all spirals. Galaxies of this type appear to have a bar of stars slicing through the bulge of stars at their center. The SB classification is further sub-divided by the appearance of a galaxy's pinwheeling spiral arms; SBa types have more tightly wound arms, whereas SBc types have looser ones. SBb types, such as NGC 3455, lie in between. NGC 3455 is part of a pair of galaxies — its partner, NGC 3454, lies out of frame. This cosmic duo belong to a group known as the NGC 3370 group, which is in turn one of the Leo II groups, a large collection of galaxies scattered some 30 million light-years to the right of the Virgo cluster. This new image is from Hubble's Advanced Camera for Surveys (ACS).

Image credit: ESA/Hubble & NASA; Acknowledgement: Nick Rose

Messier 61

This new Hubble picture is the sharpest ever image of the core of spiral galaxy Messier 61. Taken using the High Resolution Channel of Hubble's Advanced Camera for Surveys, the central part of the galaxy is shown in striking detail. Also known as NGC 4303, this galaxy is roughly 100,000 light-years across, comparable in size to our galaxy, the Milky Way.

Both Messier 61 and our home galaxy belong to a group of galaxies known as the Virgo Supercluster in the constellation of Virgo (The Virgin) — a group of galaxy clusters containing up to 2000 spiral and elliptical galaxies in total. Messier 61 is a type of galaxy known as a starburst galaxy. Starburst galaxies experience an incredibly high rate of star formation, hungrily using up their reservoir of gas in a very short period of time (in astronomical terms). But this is not the only activity going on within the galaxy; deep at its heart there is thought to be a supermassive black hole that is violently spewing out radiation.

Despite its inclusion in the Messier Catalog, Messier 61 was actually discovered by Italian astronomer Barnabus Oriani in 1779. Charles Messier also noticed this galaxy on the very same day as Oriani, but mistook it for a passing comet — the comet of 1779.

Image credit: ESA/Hubble & NASA

Beta Pictoris B

This artist’s view shows the planet orbiting the young star Beta Pictoris. This exoplanet is the first to have its rotation rate measured. Its eight-hour day corresponds to an equatorial rotation speed of 100,000 kilometers/hour — much faster than any planet in the Solar System.

Illustration credit: ESO L. Calçada/N. Risinger (skysurvey.org)

Note: For more information, see Length of Exoplanet Day Measured for First Time.

Lensed Galaxy S0901

The young galaxy SDSS090122.37+181432.3, also known as S0901, is seen here as the bright arc to the left of the central bright galaxy. The distorted view of S0901 is caused by gravitational lensing, resulting from one or more galaxies that lie between the observer and S0901. Although one effect of lensing is to distort the image, another effect is to magnify the light of the lensed object. This effect was used to enable scientists to study S0901 with Herschel's Heterodyne Instrument for the Far-Infrared (HIFI).

This image was obtained in May, 2010, using the Wide Field Camera 3 on the NASA/ESA Hubble Space Telescope.

Image credit: NASA/STScI; S. Allam and team; and the Master Lens Database, L. A. Moustakas, K. Stewart, et al (2014)

Note: For more information, see Well-Behaved, Young Galaxy Surprises Astronomers and Herschel Discovers Mature Galaxies in the Young Universe.

The Milky Way in Infrared

The majority of the stars in our Galaxy, the Milky Way, reside in a single huge disc, known as the Galactic Plane, spanning 100,000 light-years across. The Sun also resides in this crowded stellar hub, lying roughly halfway between its center and its outer edges.

This disc is filled with a diffuse mixture of gas and dust – the interstellar medium – that pervades space, filling the large gaps found between stars. Occasionally, these clouds of gas and dust cool, becoming denser and denser until they spark star formation, giving rise to new generations of stars.

This image is part of Hi-GAL, a survey of the Galactic Plane completed with ESA’s Herschel Space Observatory. Peering at the sky in infrared light, Herschel could detect the glow of dust particles dispersed between stars. This minor – but crucial – component of the interstellar medium allows astronomers to investigate how stars are born in the Milky Way, and how they affect their environment as they age.

Nestled in the Milky Way’s disc are pockets of gas and dust that have been heated by nearby newborn stars, causing them to glow brightly like cosmic gems. Through their higher temperatures, these regions glow at shorter infrared wavelengths and are depicted in violet and green, while the colder material in the surroundings – only a few tens of degrees above absolute zero – appears redder.

Laced amongst the stars is an intricate network of filaments sprinkled with tiny white spots: these are denser clumps of gas and dust that will likely evolve and give birth to new stars.

The image combines observations from the PACS and SPIRE instruments on Herschel. It spans about 12º on the longer side, corresponding to some 24 times the diameter of the full Moon. This is 1/30th of the entire Galactic Plane survey.

This image was first published in OSHI, the Online Showcase of Herschel Images, in 2011.

Image credit: ESA/PACS & SPIRE Consortium, S. Molinari, Hi-GAL Project

Messier 5

This sparkling jumble is Messier 5 — a globular cluster consisting of hundreds of thousands of stars bound together by their collective gravity.

But Messier 5 is no normal globular cluster. At 13 billion years old it is incredibly old, dating back to close to the beginning of the Universe, which is some 13.8 billion years of age. It is also one of the biggest clusters known, and at only 24,500 light-years away, it is no wonder that Messier 5 is a popular site for astronomers to train their telescopes on.

Messier 5 also presents a puzzle. Stars in globular clusters grow old and wise together. So Messier 5 should, by now, consist of old, low-mass red giants and other ancient stars. But it is actually teeming with young blue stars known as blue stragglers. These incongruous stars spring to life when stars collide, or rip material from one another.

Image credit: ESA/Hubble & NASA

WISE J085510.83-071442.5

This artist's conception shows the object named WISE J085510.83-071442.5, the coldest known brown dwarf. Brown dwarfs are dim star-like bodies that lack the mass to burn nuclear fuel as stars do.

WISE J085510.83-071442.5 is as cold as the North Pole (or between minus 54 and 9 degrees Fahrenheit, which is minus 48 to minus 13 degrees Celsius). The color of the brown dwarf in this image is arbitrary; it would have different colors when viewed in different wavelength ranges.

This celestial orb is also the fourth closest to our sun, at 7.2 light-years from Earth. In this illustration, the sun is the bright star directly to the right of the brown dwarf. Our sun's closest neighboring system (not pictured) is Alpha Centauri, at 4 light-years from Earth.

Image credit: NASA/JPL-Caltech/Penn State University

Note: For more information, see PIA18002: Cold and Quick: a Fast-Moving Brown Dwarf, PIA18003: Welcome to the Sun's Neighborhood, NASA's Spitzer, WISE Find Sun's Close, Cold Neighbor, and Discovered: A Cold, Close Neighbor of the Sun.

The Night Sky from Mars

The Mast Camera (Mastcam) on NASA's Curiosity Mars rover has captured the first image of an asteroid taken from the surface of Mars. The night-sky image actually includes two asteroids: Ceres and Vesta, plus one of Mars' two moons, Deimos, which may have been an asteroid before being captured into orbit around Mars. The image was taken after nightfall on the 606th Martian day, or sol, of Curiosity's work on Mars (April 20, 2014, PDT). In other camera pointings the same night, the Mastcam also imaged Mars' larger moon, Phobos, plus the planets Jupiter and Saturn.

Ceres, with a diameter of about 590 miles (950 kilometers), is the largest object in the asteroid belt, large enough to be classified as a dwarf planet. Vesta is the third-largest object in the asteroid belt, about 350 miles (563 kilometers) wide. These two bodies are the destinations of NASA's Dawn mission, which orbited Vesta in 2011 and 2012 and is on its way to begin orbiting Ceres in 2015.

This annotated image combines portions of images taken at the same pointing with two different exposure times, plus insets from other camera pointings. In the main portion of the image, Vesta, Ceres and three stars appear as short streaks due to the duration of a 12-second exposure. The background is detector noise, limiting what we can see to magnitude 6 or 7, much like normal human eyesight. The two asteroids and three stars would be visible to someone of normal eyesight standing on Mars. Specks are effects of cosmic rays striking the camera's light detector.

Three square insets at left show Phobos, Jupiter and Saturn at exposures of one-half second each. Deimos was much brighter than the visible stars and asteroids in the same part of the sky, in the main image. The circular inset covers a patch of sky the size that Earth's full moon appears to observers on Earth. At the center of that circular inset, Deimos appears at its correct location in the sky, in a one-quarter-second exposure. In the unannotated version of the 12-second-exposure image, the brightness of Deimos saturates that portion of the image, making the moon appear overly large.

Image credit: NASA/JPL-Caltech/MSSS/Texas A&M

Note: For more information, see Asteroids as Seen From Mars; A Curiosity Rover First.

Messier 101, Messier 81, Centaurus A, and Messier 51

Four images of galaxies that have been made using data from both amateur and professional telescopes.

This quartet of galaxies comes from a collaboration of professional and amateur astronomers that combines optical data from amateur telescopes with data from the archives of NASA missions. Starting in the upper left and moving clockwise, the galaxies are M101 (the "Pinwheel Galaxy"), M81, Centaurus A, and M51 (the "Whirlpool Galaxy"). In these images, X-rays from Chandra are in purple, infrared data from Spitzer are red, and the optical data are in red, green, and blue. The two astrophotographers who donated their images for these four images -- Detlef Hartmann and Rolf Olsen -- used their personal telescopes of 17.5 inches and 10 inches in diameter respectively.

Scale: Image is 24 arcmin on a side (about 1.5 million light years).

Image credit: X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech

Note: For more information, see Astro Pro-Am: Professional and Amateur Astronomers Join Forces.

X-Ray View of the COSMOS Survey

When we gaze up at the night sky, we are only seeing part of the story. Unfortunately, some of the most powerful and energetic events in the Universe are invisible to our eyes – and to even the best optical telescopes.

Luckily, these events are not lost; they appear vividly in the high-energy sky, making them visible to space-based telescopes like ESA's XMM-Newton, which observes the Universe in the X-ray part of the spectrum.

This image shows a patch of sky from the COSMOS survey, as viewed by XMM-Newton. COSMOS is a project studying how galaxies form and evolve, gathering observations using a variety of ground- and space-based telescopes. This image alone features about two thousand supermassive black holes, and over a hundred clusters of galaxies.

Small point sources dotted across the frame show supermassive black holes that are hungrily devouring matter from their surroundings. All massive galaxies host a black hole at their core, but not all of these are actively accreting, dragging in surrounding matter and releasing high-energy radiation and powerful jets in the process. As they are so energetic, one of the best ways to hunt these extreme bodies is by using X-ray telescopes.

The larger blobs in this image, mainly red and yellow, reveal another class of cosmic behemoths: galaxy clusters. Containing up to several thousand galaxies, galaxy clusters are the largest cosmic structures to be held together by gravity. The galaxies within these clusters are enveloped by hot gas, which releases a diffuse X-ray glow that can be detected by telescopes like XMM-Newton.

The image combines data collected by the EPIC instrument on board XMM-Newton at energies from 0.5 to 2 keV (shown in red), 2 to 4.5 keV (shown in green) and 4.5 to 10 keV (shown in blue). The observations were taken between 2003 and 2005, and the image spans 1.4 degrees on each side, corresponding to almost three times the diameter of the full Moon.

This image was first published in the paper “The XMM-Newton Wide-Field Survey in the COSMOS Field. I. Survey Description” by G. Hasinger et al. in 2007.

Image credit: ESA/XMM-Newton/Gunther Hasinger, Nico Cappelluti, and the XMM-COSMOS collaboration.

Unique Pair of Hidden Black Holes Discovered by XMM-Newton

A pair of supermassive black holes in orbit around one another have been spotted by XMM-Newton. This is the first time such a pair have been seen in an ordinary galaxy. They were discovered because they ripped apart a star when the space observatory happened to be looking in their direction.

Most massive galaxies in the Universe are thought to harbor at least one supermassive black hole at their center. Two supermassive black holes are the smoking gun that the galaxy has merged with another. Thus, finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present-day shapes and sizes.

To date, only a few candidates for close binary supermassive black holes have been found. All are in active galaxies where they are constantly ripping gas clouds apart, in the prelude to crushing them out of existence.

In the process of destruction, the gas is heated so much that it shines at many wavelengths, including X-rays. This gives the galaxy an unusually bright center, and leads to it being called active. The new discovery, reported by Fukun Liu, Peking University, Beijing, China, and colleagues, is important because it is the first to be found in a galaxy that is not active.

"There might be a whole population of quiescent galaxies that host binary black holes in their centers," says co-author Stefanie Komossa, Max-Planck-Institut für Radioastronomie, Bonn, Germany. But finding them is a difficult task because in quiescent galaxies, there are no gas clouds feeding the black holes, and so the cores of these galaxies are truly dark.

The only hope that the astronomers have is to be looking in the right direction at the moment one of the black holes goes to work, and rips a star to pieces. Such an occurrence is called a 'tidal disruption event'. As the star is pulled apart by the gravity of the black hole, it gives out a flare of X-rays.

In an active galaxy, the black hole is continuously fed by gas clouds. In a quiescent galaxy, the black hole is fed by tidal disruption events that occur sporadically and are impossible to predict. So, to increase the chances of catching such an event, researchers use ESA's X-ray observatory, XMM-Newton, in a novel way.

Usually, the observatory collects data from designated targets, one at a time. Once it completes an observation, it slews to the next. The trick is that during this movement, XMM-Newton keeps the instruments turned on and recording. Effectively this surveys the sky in a random pattern, producing data that can be analyzed for unknown or unexpected sources of X-rays.

On 10 June 2010, a tidal disruption event was spotted by XMM-Newton in galaxy SDSS J120136.02+300305.5. Komossa and colleagues were scanning the data for such events and scheduled follow-up observations just days later with XMM-Newton and NASA's Swift satellite.

The galaxy was still spilling X-rays into space. It looked exactly like a tidal disruption event caused by a supermassive black hole but as they tracked the slowly fading emission day after day something strange happened.

The X-rays fell below detectable levels between days 27 and 48 after the discovery. Then they re-appeared and continued to follow a more expected fading rate, as if nothing had happened.

Now, thanks to Fukun Liu, the behavior can be explained. "This is exactly what you would expect from a pair of supermassive black holes orbiting one another," says Liu.

Liu had been working on models of black hole binary systems that predicted a sudden plunge to darkness and then the recovery because the gravity of one of the black holes disrupted the flow of gas onto the other, temporarily depriving it of fuel to fire the X-ray flare. He found that two possible configurations were possible to reproduce the observations of J120136.

In the first, the primary black hole contained 10 million solar masses and was orbited by a black hole of about a million solar masses in an elliptical orbit. In the second solution, the primary black hole was about a million solar masses and in a circular orbit.

In both cases, the separation between the black holes was relatively small: 0.6 milliparsecs, or about 2 thousandths of a light year. This is about the width of our Solar System.

Being this close, the fate of this newly discovered black hole pair is sealed. They will radiate their orbital energy away, gradually spiraling together, until in about two million years time they will merge into a single black hole.

Now that astronomers have found this first candidate for a binary black hole in a quiescent galaxy, the search is inevitably on for more. XMM-Newton will continue its slew survey. This detection will also spur interest in a network of telescopes that search the whole sky for tidal disruption events.

"Once we have detected thousands of tidal disruption events, we can begin to extract reliable statistics about the rate at which galaxies merge," says Komossa.

There is another hope for the future as well. When binary black holes merge, they are predicted to release a massive burst of energy into the Universe but not mostly in X-rays. "The final merger is expected to be the strongest source of gravitational waves in the Universe," says Liu.

Gravitational waves are ripples in the space-time continuum. Astronomers around the world are currently building a new type of observatory to detect these ripples. ESA are also involved in opening this new window on the Universe. In 2015, ESA will launch LISA Pathfinder, which will test the necessary technology for building a space-based gravitational wave detector that must be placed in space. The search for elusive gravitational waves is also the theme for one of ESA's next large science missions, the L3 mission in the Cosmic Vision program.

In the meantime, XMM-Newton will continue to look out for the tidal disruption events that betray the presence of binary supermassive black holes candidates.

"The innovative use of XMM-Newton's slew observations made the detection of this binary supermassive black hole system possible," says Norbert Schartel, ESA's XMM-Newton Project Scientist. "This demonstrates the important role that long-lasting space observatories have in detecting rare events that can potentially open new areas in astronomy."


Background Information

The results described in this article are reported in "A milli-parsec supermassive black hole binary candidate in the galaxy SDSS J120136.02+300305.5", by F.K. Liu, Shuo Li, and S. Komossa, published in the May 10 issue of The Astrophysical Journal, 2014, Volume 786; doi:10.1088/0004-637X/786/2/103

Illustration credit: ESA - C. Carreau

The Grand Canyon

The Grand Canyon in northern Arizona is a favorite for astronauts shooting photos from the International Space Station, as well as one of the best-known tourist attractions in the world. The steep walls of the Colorado River canyon and its many side canyons make an intricate landscape that contrasts with the dark green, forested plateau to the north and south.

The Colorado River has done all the erosional work of carving away cubic kilometers of rock in a geologically short period of time. Visible as a darker line snaking along the bottom of the canyon, the river lies at an altitude of 715 meters (2,345 feet), thousands of meters below the North and South Rims. Temperatures are furnace-like on the river banks in the summer. But Grand Canyon Village, the classic outlook point for visitors, enjoys a milder climate at an altitude of 2,100 meters (6,890 feet).

The Grand Canyon has become a geologic icon — a place where you can almost sense the invisible tectonic forces within the Earth. The North and South Rims are part of the Kaibab Plateau, a gentle tectonic swell in the landscape. The uplift of the plateau had two pronounced effects on the landscape that show up in this image. First, in drier parts of the world, forests usually indicate higher places; higher altitudes are cooler and wetter, conditions that allow trees to grow. The other geologic lesson on view is the canyon itself. Geologists now know that a river can cut a canyon only if the Earth surface rises vertically. If such uplift is not rapid, a river can maintain its course by eroding huge quantities of rock and forming a canyon.

This astronaut photograph (ISS039-E-5258) was taken on March 25, 2014 by the Expedition 39 crew, with a Nikon D3S digital camera using a 180 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed.

Image credit: NASA