Saturday, May 23, 2015

Galaxy’s snacking habits revealed

A composite image of the galaxy NGC 1512, located 38 million light years away in the direction of the constellation of Horologium, in the southern hemisphere of the sky. The image shows the regions of unusual chemical enrichment that demonstrate that NGC 1512 has absorbed other galaxies earlier in its history. Credit: Angel R. Lopez-Sanchez (AAO / MQU), & Baerbel Koribalski (CSIRO / CASS). Click  here for a full size image 

A team of Australian and Spanish astronomers have caught a greedy galaxy gobbling on its neighbours and leaving crumbs of evidence about its dietary past.

Galaxies grow by churning loose gas from their surroundings into new stars, or by swallowing neighbouring galaxies whole. However, they normally leave very few traces of their cannibalistic habits.

A study published today in Monthly Notices of the Royal Astronomical Society not only reveals a spiral galaxy devouring a nearby compact dwarf galaxy, but shows evidence of its past galactic snacks in unprecedented detail.

Australian Astronomical Observatory (AAO) and Macquarie University astrophysicist, Ángel R. López-Sánchez, and his collaborators have been studying the galaxy NGC 1512 to see if its chemical story matches its physical appearance.

The team of researchers used the unique capabilities of the 3.9-metre Anglo-Australian Telescope (AAT), near Coonabarabran, New South Wales, to measure the level of chemical enrichment in the gas across the entire face of NGC 1512.

Chemical enrichment occurs when stars churn the hydrogen and helium from the Big Bang into heavier elements through nuclear reactions at their cores.

These new elements are released back into space when the stars die, enriching the surrounding gas with chemicals like oxygen, which the team measured.

“We were expecting to find fresh gas or gas enriched at the same level as that of the galaxy being consumed, but were surprised to find the gases were actually the remnants of galaxies swallowed earlier,” Dr López-Sánchez said.

“The diffuse gas in the outer regions of NGC 1512 is not the pristine gas created in the Big Bang but is gas that has already been processed by previous generations of stars.”

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) Australia Telescope Compact Array (ATCA), a powerful 6-km diameter radio interferometer (an array of radio antennas that effectively act as a larger single instrument) located in eastern Australia, was used to detect large amounts of cold hydrogen gas that extends way beyond the stellar disk of the spiral galaxy NGC 1512.

"The dense pockets of hydrogen gas in the outer disk of NGC 1512 accurately pin-point regions of active star formation", said CSIRO's Dr Baerbel Koribalski, a member of the research collaboration.

When this finding was examined in combination with radio and ultraviolet observations the scientists concluded that the rich gas being processed into new stars did not come from the inner regions of the galaxy either. Instead, the gas was likely absorbed by the galaxy over its lifetime as NGC 1512 accreted other, smaller galaxies around it.

Dr Tobias Westmeier, from the International Centre for Radio Astronomy Research in Perth, said that while galaxy cannibalism has been known for many years, this is the first time that it has been observed in such fine detail.

“By using observations from both ground and space based telescopes we were able to piece together a detailed history for this galaxy and better understand how interactions and mergers with other galaxies have affected its evolution and the rate at which it formed stars,” he said.

The team’s successful and novel approach to investigating how galaxies grow is being used in a new program to further refine the best models of galaxy evolution.

For this work the astronomers used spectroscopic data from the AAT at Siding Spring Observatory in Australia to measure the chemical distribution around the galaxies. They identified the diffuse gas around the dual galaxy system using ATCA radio observations.

In addition, they identified regions of new star formation with data from the Galaxy Evolution Explorer (GALEX) orbiting space telescope.

“The unique combination of these data provide a very powerful tool to disentangle the nature and evolution of galaxies,” said Dr López-Sánchez.

“We will observe several more galaxies using the same proven techniques to improve our understanding of the past behaviour of galaxies in the local Universe.”

Media contacts

Dr Amanda Bauer
Australian Astronomical Observatory (AAO)
Tel: +61 2 9372 4852
Mob: +61 447 029 368

Pete Wheeler
The International Centre for Radio Astronomy Research (ICRAR)
Tel: +61 8 6488 7758
Mob: +61 0423 982 018

Science contacts

Dr Ángel R. López-Sánchez
Australian Astronomical Observatory / Macquarie University
Tel: +61 2 9372 4898

Dr Tobias Westmeier
The International Centre for Radio Astronomy Research / University of
Western Australia
Tel: +61 8 6488 4592

Dr Baerbel Koribalski
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Tel: +61 293 724 361

Further information

The new work is published in “Ionized gas in the XUV disc of the NGC1512/1510 system”,
Á. R. López-Sánchez, T. Westmeier, C. Esteban, and B. S. Koribalski, Monthly Notices of the Royal Astronomical Society, Oxford University Press, vol. 450 no. 4, pp. 3381-3409, 2015.

 Notes for editors

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter.

Friday, May 22, 2015

Hubble Observes One-of-a-Kind Star Nicknamed 'Nasty' Mass Star and Smaller Companion Create Vast Gas Disk (Artist's Illustration)

Mass Star and Smaller Companion Create Vast Gas Disk (Artist's Illustration) 
Artist's Illustration Credit: NASA, ESA, and G. Bacon (STScI)

Compass and Scale Image of WR 122 (NaSt1)
 Credit: NASA, ESA, and Z. Levay (STScI)

Astronomers using NASA's Hubble Space Telescope have uncovered surprising new clues about a hefty, rapidly aging star whose behavior has never been seen before in our Milky Way galaxy. In fact, the star is so weird that astronomers have nicknamed it "Nasty 1," a play on its catalog name of NaSt1. The star may represent a brief transitory stage in the evolution of extremely massive stars.

First discovered several decades ago, Nasty 1 was identified as a Wolf-Rayet star, a rapidly evolving star that is much more massive than our sun. The star loses its hydrogen-filled outer layers quickly, exposing its super-hot and extremely bright helium-burning core.

But Nasty 1 doesn't look like a typical Wolf-Rayet star. The astronomers using Hubble had expected to see twin lobes of gas flowing from opposite sides of the star, perhaps similar to those emanating from the massive star Eta Carinae, which is a Wolf-Rayet candidate. Instead, Hubble revealed a pancake-shaped disk of gas encircling the star. The vast disk is nearly 2 trillion miles wide, and may have formed from an unseen companion star that snacked on the outer envelope of the newly formed Wolf-Rayet. Based on current estimates, the nebula surrounding the stars is just a few thousand years old, and as close as 3,000 light-years from Earth.

"We were excited to see this disk-like structure because it may be evidence for a Wolf-Rayet star forming from a binary interaction," said study leader Jon Mauerhan of the University of California, Berkeley. "There are very few examples in the galaxy of this process in action because this phase is short-lived, perhaps lasting only a hundred thousand years, while the timescale over which a resulting disk is visible could be only ten thousand years or less."

According to the team's scenario, a massive star evolves very quickly, and as it begins to run out of hydrogen, it swells up. Its outer hydrogen envelope becomes more loosely bound and vulnerable to gravitational stripping, or a type of stellar cannibalism, by the nearby companion star. In that process, the more compact star winds up gaining mass, and the original massive star loses its hydrogen envelope, exposing its helium core to become a Wolf-Rayet star.

Another way Wolf-Rayet stars are said to form is when a massive star ejects its own hydrogen envelope in a strong stellar wind streaming with charged particles. The binary interaction model where a companion star is present is gaining traction because astronomers realize that at least 70 percent of massive stars are members of double-star systems. Direct mass loss alone also cannot account for the number of Wolf-Rayet stars relative to other less-evolved massive stars in the galaxy.

"We're finding that it is hard to form all the Wolf-Rayet stars we observe by the traditional wind mechanism, because mass loss isn't as strong as we used to think," said Nathan Smith of the University of Arizona in Tucson, who is a co-author on the new NaSt1 paper. "Mass exchange in binary systems seems to be vital to account for Wolf-Rayet stars and the supernovae they make, and catching binary stars in this short-lived phase will help us understand this process."

But the mass-transfer process in mammoth binary systems isn't always efficient. Some of the stripped matter can spill out during the dynamical gravitational tussle between the stars, creating a disk around the binary.

"That's what we think is happening in Nasty 1," Mauerhan said. "We think there is a Wolf-Rayet star buried inside the nebula, and we think the nebula is being created by this mass-transfer process. So this type of sloppy stellar cannibalism actually makes Nasty 1 a rather fitting nickname."

The star's catalog name, NaSt1, is derived from the first two letters of each of the two astronomers who discovered it in 1963, Jason Nassau and Charles Stephenson.

Viewing the Nasty 1 system hasn't been easy. The system is so heavily cloaked in gas and dust, it blocks even Hubble's view of the stars. So Mauerhan's team cannot measure the mass of each star, the distance between them, or the amount of material spilling onto the companion star.

Previous observations of Nasty 1 have provided some information on the gas in the disk. The material, for example, is travelling about 22,000 miles per hour in the outer nebula, slower than similar stars. The comparatively slow speed indicates that the star expelled its material through a less violent event than Eta Carinae's explosive outbursts, where the gas is travelling hundreds of thousands of miles per hour.

Nasty 1 may also be shedding the material sporadically. Past studies in infrared light have shown evidence for a compact pocket of hot dust very close to the central stars. Recent observations by Mauerhan and colleagues at the University of Arizona, using the Magellan telescope at Las Campanas Observatory in Chile, have resolved a larger pocket of cooler dust that may be indirectly scattering the light from the central stars. The presence of warm dust implies that it formed very recently, perhaps in spurts, as chemically enriched material from the two stellar winds collides at different points, mixes, flows away, and cools. Sporadic changes in the wind strength or the rate the companion star strips the main star's hydrogen envelope might also explain the clumpy structure and gaps seen farther out in the disk.

To measure the hypersonic winds from each star, the astronomers turned to NASA's Chandra X-ray Observatory. The observations revealed scorching hot plasma, indicating that the winds from both stars are indeed colliding, creating high-energy shocks that glow in X-rays. These results are consistent with what astronomers have observed from other Wolf-Rayet systems.

The chaotic mass-transfer activity will end when the Wolf-Rayet star runs out of material. Eventually, the gas in the disk will dissipate, providing a clear view of the binary system.

"What evolutionary path the star will take is uncertain, but it will definitely not be boring," said Mauerhan. "Nasty 1 could evolve into another Eta Carinae-type system. To make that transformation, the mass-gaining companion star could experience a giant eruption because of some instability related to the acquiring of matter from the newly formed Wolf-Rayet. Or, the Wolf-Rayet could explode as a supernova. A stellar merger is another potential outcome, depending on the orbital evolution of the system. The future could be full of all kinds of exotic possibilities depending on whether it blows up or how long the mass transfer occurs, and how long it lives after the mass transfer ceases."

The team's results will appear May 21 in the online edition of the Monthly Notices of the Royal Astronomical Society. 


Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514 /

Source: Hubble/Site

Thursday, May 21, 2015

News Center Supernova Hunting with Supercomputers

This computer simulation shows the debris of a Type Ia supernova (brown) slamming into its companion star (blue) at tens of millions of miles per hour. The interaction produces ultraviolet light that escapes as the supernova shell sweeps over the companion, a signal detected by Swift. Credits: UC Berkeley, Daniel Kasen
The graphic depicts a light curve of the newly discovered Type Ia supernova, KSN 2011b, from NASA's Kepler spacecraft. The light curve shows a star's brightness (vertical axis) as a function of time (horizontal axis) before, during and after the star exploded. The white diagram on the right represents 40 days of continuous observations by Kepler. In the red zoom box, the agua-colored region is the expected 'bump' in the data if a companion star is present during a supernova. The measurements remained constant (yellow line) concluding the cause to be the merger of two closely orbiting stars, most likely two white dwarfs. The finding provides the first direct measurements capable of informing scientists of the cause of the blast. Credits: NASA Ames/W. Stenzel

Animation showing a binary star system in which a white dwarf accretes matter from a normal companion star. Matter streaming from the red star accumulates on the white dwarf until the dwarf explodes. With its partner destroyed, the normal star careens into space. This scenario results in what astronomers refer to as a Type Ia supernova.Credits: NASA's Goddard Space Flight Center/Walt Feimer

Astronomers are going gaga over newborn supernova measurements taken by NASA’s Kepler and Swift spacecraft, poring over them in hopes of better understanding what sparks these world-shattering stellar explosions. Scientists are particularly fascinated with Type la supernovae, as they can serve as a lighthouse for measuring the vast distances across space.“Kepler’s unprecedented pre-event supernova observations and Swift’s agility in responding to supernova events have both produced important discoveries at the same time but at very different wavelengths,” says Paul Hertz, Director of Astrophysics. “Not only do we get insight into what triggers a Type Ia supernova, but these data allow us to better calibrate Type Ia supernovae as standard candles, and that has implications for our ability to eventually understand the mysteries of dark energy.”

Type Ia supernovae explode with similar brightness because the exploding object is always a white dwarf, the Earth-sized remnant of a star like the sun. A white dwarf can go supernova by merging with another white dwarf or by pulling too much matter from a nearby companion star, causing a thermonuclear reaction and blowing itself to smithereens.

In studies appearing in Nature on Thursday, Kepler and Swift have found supporting evidence for both star-pulverizing scenarios.

Researchers studying the Kepler data have caught three new and distant supernovae, and the dataset includes measurements taken before the violent explosions even happened. Known for its planet-hunting prowess and its unceasing gaze, the Kepler space telescope's exquisitely precise and frequent observations every 30 minutes have allowed astronomers to turn back the clock and dissect the initial moments of a supernova. The finding provides the first direct measurements capable of informing scientists of the cause of the blast.

"Our Kepler supernova discoveries strongly favor the white dwarf merger scenario, while the Swift study, led by Cao, proves that Type Ia supernovae can also arise from single white dwarfs," said Robert Olling, research associate at the University of Maryland and lead author of the study. "Just as many roads lead to Rome, nature may have several ways to explode white dwarf stars."

To capture the earliest moments of Type Ia explosions, the research team monitored 400 galaxies for two years using Kepler. The team discovered three events, designated KSN 2011b, KSN 2011c and KSN 2012a, with measurements taken before, during and after the explosions.

These early data provide a view into the physical processes that ignite these stellar bombs hundreds of millions of light years away. When a star goes supernova, the explosive burst of energy ejects the star's material at hypersonic velocity, emitting a shock wave in all directions. If a companion star is in the neighborhood, the disruption in the shock wave will be recorded in the data.

Scientists found no evidence of a companion star and concluded the cause to be the collision and merger of two closely orbiting stars, most likely two white dwarfs.

Knowing the distance to a galaxy in the Kepler survey was key to characterizing the Type of supernova uncovered by Olling and his colleagues. To determine the distance, the team turned to the powerful telescopes at the Gemini and the W. M. Keck Observatories atop Mauna Kea in Hawaii. These measurements were key for the researchers to conclude that the supernovae they had discovered were that of the Type Ia lighthouse variety.

“The Kepler spacecraft has delivered yet another surprise, playing an unexpected role in supernova science by providing the first well-sampled early time light curves of Type Ia supernovae," said Steve Howell, Kepler project scientist at NASA's Ames Research Center in Moffett Field, California. "Now in its new mission as K2, the spacecraft will search for more supernovae among many thousands of galaxies."

A separate group of astronomers have also found intriguing data on a different supernova. Led by California Institute of Technology (Caltech) graduate student Yi Cao, a team using Swift has detected an unprecedented flash of ultraviolet (UV) light in the first few days of a Type Ia supernova. Based on computer simulations of supernovae exploding in binary star systems, the researchers think the UV pulse was emitted when the supernova’s blast wave slammed into and engulfed a nearby companion star.

"If Swift had looked just a day or two later, we would have missed the prompt UV flash entirely," said Brad Cenko, a Swift team member at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Thanks to Swift's wavelength coverage and rapid scheduling capability, it is currently the only spacecraft that can regularly make these observations."

According to the analysis, the supernova debris slammed into and swept around its companion star, creating a region of UV emission. The peak temperature exceeded 19,000 degrees Fahrenheit (11,000 degrees Celsius) or about twice the surface temperature of the sun.

The explosion, designated iPTF14atg, was first seen on May 3, 2014, in the galaxy IC 831, located about 300 million light-years away in the constellation Coma Berenices. It was discovered through a wide-field robotic observing system known as the intermediate Palomar Transient Factory (iPTF), a multi-institute collaboration led by the Caltech Optical Observatories in California.

"We saw no evidence of this explosion in images taken the previous night, so we found iPTF14atg when it was only about one day old," Cao said. "Better yet, we confirmed it was a young Type Ia supernova, something we've worked hard designing our system to find."

The team immediately requested follow-up observations from other facilities, including ultraviolet and X-ray observations from NASA's Swift satellite. Although no X-rays were found, a fading spike of UV light was caught by Swift's Ultraviolet/Optical Telescope within a few days of the explosion, with no corresponding spike at visible wavelengths. After the flash faded, both UV and visible wavelengths rose together as the supernova brightened.

The UV pulse from iPTF14atg provides strong evidence for the presence of a companion star, but as white dwarfs crashing into each other can also produce supernovae, as demonstrated by the Kepler results, astronomers are working to determine the percentage of supernovae produced by each one.

The scientists add that a better understanding of the differences among Type Ia explosions will help astronomers improve their knowledge of dark energy, a mysterious force that appears to be accelerating cosmic expansion.

Ames manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corp. operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

Swift blasted into orbit Nov. 20, 2004. Managed by Goddard, the mission is operated in collaboration with Penn State University in University Park, Pennsylvania, the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.

Michele Johnson (Editor)
NASA’s Ames Research Center, Moffett Field, Calif.

Lynn Chandler
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Wednesday, May 20, 2015

The Dreadful Beauty of Medusa

ESO’s Very Large Telescope images the Medusa Nebula

The Medusa Nebula in the constellation of Gemini

Wide-field view of the sky around the Medusa Nebula


Zooming in on the Medusa
Zooming in on the Medusa

Close-up pan video showing the Medusa Nebula
Close-up pan video showing the Medusa Nebula

Astronomers using ESO’s Very Large Telescope in Chile have captured the most detailed image ever taken of the Medusa Nebula. As the star at the heart of this nebula made its transition into retirement, it shed its outer layers into space, forming this colourful cloud. The image foreshadows the final fate of the Sun, which will eventually also become an object of this kind.

This beautiful planetary nebula is named after a dreadful creature from Greek mythology — the Gorgon Medusa. It is also known as Sharpless 2-274 and is located in the constellation of Gemini (The Twins). The Medusa Nebula spans approximately four light-years and lies at a distance of about 1500 light-years. Despite its size it is extremely dim and hard to observe.

Medusa was a hideous creature with snakes in place of hair. These snakes are represented by the serpentine filaments of glowing gas in this nebula. The red glow from hydrogen and the fainter green emission from oxygen gas extends well beyond this frame, forming a crescent shape in the sky. The ejection of mass from stars at this stage of their evolution is often intermittent, which can result in fascinating structures within planetary nebulae.

For tens of thousands of years  the stellar cores of planetary nebulae are surrounded by these spectacularly colourful clouds of gas [1]. Over a further few thousand years the gas slowly disperses into its surroundings. This is the last phase in the transformation of stars like the Sun before ending their active lives as white dwarfs. The planetary nebula stage in the life of a star is a tiny fraction of its total life span — just as the time a child takes to blow a soap bubble and see it drift away is a brief instant compared to a full human life span.

Harsh ultraviolet radiation from the very hot star at the core of the nebula causes atoms in the outward-moving gas to lose their electrons, leaving behind ionised gas. The characteristic colours of this glowing gas can be used to identify objects. In particular, the presence of the green glow from doubly ionised oxygen ([O III]) is used as a tool for spotting planetary nebulae. By applying appropriate filters, astronomers can isolate the radiation from the glowing gas and make the dim nebulae appear more pronounced against a darker background.

When the green [O III] emission from nebulae was first observed, astronomers thought they had discovered a new element that they dubbed nebulium. They later realised that it was simply a rare wavelength of radiation [2] from an ionised form of the familiar element oxygen.

The nebula is also referred to as Abell 21 (more formally PN A66 21), after the American astronomer George O. Abell, who discovered this object in 1955. For some time scientists debated whether the cloud could be the remnant of a supernova explosion. In the 1970s, however, researchers were able to measure the movement and other properties of the material in the cloud and clearly identify it as a planetary nebula [3].

This image uses data from the FOcal Reducer and low dispersion Spectrograph (FORS) instrument attached to the VLT, which were acquired as part of the ESO Cosmic Gems programme [4].


[1] Counterintuitively, the stellar core of the Medusa Nebula is not the bright star in the centre of this image — this is a foreground star called TYC 776-1339-1. Medusa’s central star is a dimmer, bluish star lying just off-centre of the crescent shape and in the right-hand part of this image.

[2] This type of radiation is rare because it is created by a forbidden mechanism — transitions that are forbidden by quantum selection rules, but can still occur with a low probability. The designation [O III] means that the radiation is forbidden (the square brackets) emission from doubly ionised (the III part of the name) oxygen (O).

[3] The expansion velocity of the cloud was found to be about 50 kilometres/second — much lower than would be expected for a supernova remnant.

[4] The ESO Cosmic Gems programme is an outreach initiative to produce images of interesting, intriguing or visually attractive objects using ESO telescopes, for the purposes of education and public outreach. The programme makes use of telescope time that cannot be used for science observations. All data collected may also be suitable for scientific purposes, and are made available to astronomers through ESO’s science archive.

More Information

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.



Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591

Source: ESO

Tuesday, May 19, 2015

Star formation and magnetic turbulence in the Orion Molecular Cloud

Star formation and magnetic turbulence in the Orion Molecular Cloud
Copyright: ESA and the Planck Collaboration. Hi-res JPG
An annotated version of the image can be found here

With blue hues suggestive of marine paradises and a texture evoking the tranquil flow of sea waves, this image might make us daydream of sandy beaches and exotic holiday destinations. Instead, the subject of the scene is intense and powerful, because it depicts the formation of stars in the turbulent billows of gas and dust of the Orion Molecular Cloud.

The image is based on data from ESA’s Planck satellite, which scanned the sky between 2009 and 2013 to study the cosmic microwave background, the most ancient light in the Universe’s history. While doing so, Planck also detected foreground emission from material in the Milky Way, as well as from other galaxies.

Our Galaxy is pervaded by a diffuse mixture of gas and dust that occasionally becomes denser, creating giant gas clouds where stars can form. While present only in traces, dust is a crucial ingredient in these interstellar clouds. It also shines brightly at some of the wavelengths that were probed by Planck, so astronomers can use these data to learn more about the cradles of star formation.

In addition, dust grains have elongated shapes and tend to align their longest axis at right angles to the direction of the Galaxy’s magnetic field. This makes their emission partly ‘polarised’ – it vibrates in a preferred direction. Since Planck was equipped with polarisation-sensitive detectors, its scans also contain information about the direction of the magnetic field threading the Milky Way.

This image combines a visualisation of the total intensity of dust emission, shown in the colour scale, with an indication of the magnetic field’s orientation, represented by the texture. Blue hues correspond to regions with little dust, while the yellow and red areas reflect denser (and mostly hotter) clouds containing larger amounts of dust, as well as gas.

The red clumps at the centre of the image are part of the Orion Molecular Cloud Complex, one of the closest large regions of star formation, only about 1300 light-years from the Sun. The most prominent of the red clumps, to the lower left of centre, is the famous Orion Nebula, also known as M42. This is visible to the naked eye in the constellation Orion, just below the three stars forming the ‘belt’ of the mythological hunter. 

The magnetic field appears regular and organised in almost parallel lines in the upper part of the image: this is a result of the large-scale arrangement of the magnetic field along the Galactic plane, which is located above the top of this image. However, the field becomes less regular in the central and lower parts of the image, in the region of the Orion Molecular Cloud. Astronomers believe that the turbulent structure of the magnetic field observed in this and other star-forming clouds is related to the powerful processes taking place when stars are being born.

The emission from dust is computed from a combination of Planck observations at 353, 545 and 857 GHz, whereas the direction of the magnetic field is based on Planck polarisation data at 353 GHz. The image spans about 40º across.

Source: ESA

Monday, May 18, 2015

Kepler Observes Neptune Dance with Its Moons NASA Ames Research Center

Seventy days worth of solar system observations from NASA's Kepler spacecraft, taken during its reinvented "K2" mission, are highlighted in this sped-up movie. The planet Neptune appears on day 15, followed by its moon Triton, which looks small and faint. Keen-eyed observers can also spot Neptune's tiny moon Nereid at day 24. Neptune is not moving backward but appears to do so because of the changing position of the Kepler spacecraft as it orbits around the sun. Credits: NASA Ames/SETI Institute/J. Rowe

NASA's Kepler spacecraft, known for its planet-hunting prowess of other stars, is also studying solar system objects. In its new K2 mission, Neptune and two of its moons, Triton and Nereid, have been imaged. The movie illustrates 70 days of uninterrupted observation making this one of the longer continuous studies of an outer solar system object.

The movie, based on 101,580 images taken from November 2014 through January 2015 during K2's Campaign 3, reveals the perpetual clockwork of our solar system. The 70-day timespan is compressed into 34 seconds with the number of days noted in the top right corner.

Neptune appears on day 15 but does not travel alone in the video. The small faint object closely orbiting is its large moon Triton, which circles Neptune every 5.8 days. Appearing from the left at day 24, keen-eyed observers can also spot the tiny moon Nereid in its slow 360-day orbit around the planet. A few fast-moving asteroids make cameo appearances in the movie, showing up as streaks across the K2 field of view. The red dots are a few of the stars K2 examines in its search for transiting planets outside of our solar system.

Neptune's atmosphere reflects sunlight creating a bright appearance. The reflected light floods a number of pixels of the spacecraft's on board camera, producing the bright spikes extending above and below the planet. The celestial bodies in the stitched-together images are colored red to represent the wavelength response of the spacecraft's camera. In reality, Neptune is deep blue in color and its moons and the speeding asteroids are light grey while the background stars appear white from a distance.

Relative orbit speeds explain the interesting motion of Neptune and its moons beginning at day 42. Inner planets like Earth orbit more quickly than outer planets like Neptune. In the movie, Neptune’s apparent motion relative to the stationary stars is mostly due to the circular 372-day orbit of the Kepler spacecraft around the sun. If you look at distant objects and move your head back and forth, you will notice that objects close to you will also appear to move back and forth, relative to objects far away. The same concept is producing the apparent motion of Neptune.

While NASA’s Kepler spacecraft is known for its discoveries of planets around other stars, an international team of astronomers plans to use these data to track Neptune’s weather and probe the planet’s internal structure by studying subtle brightness fluctuations that can only be observed with K2.

NASA's Ames Research Center in Moffett Field, California, manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corp. operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

Editor: Michele Johnson

Saturday, May 16, 2015

Astronomers Baffled by Discovery of Rare Quasar

Image of the region of the space occupied by the rare quasar quartet. The four quasars are indicated by arrows. The quasars are embedded in a giant nebula of cool dense gas visible in the image as a blue haze. The nebula has an extent of one million light-years across, and these objects are so distant that their light has taken nearly 10 billion years to reach telescopes on Earth. This false color image is based on observations with the Keck 10m telescope on the summit of Maunakea in Hawaii.  Credit: Hennawi & Arrigoni-Battaia, MPIA

Maunakea, Hawaii – Using the W. M. Keck Observatory in Hawaii, a group of astronomers led by Joseph Hennawi of the Max Planck Institute for Astronomy have discovered the first quadruple quasar: four rare active black holes situated in close proximity to one another. The quartet resides in one of the most massive structures ever discovered in the distant universe, and is surrounded by a giant nebula of cool dense gas. Because the discovery comes with one-in-ten-million odds, perhaps cosmologists need to rethink their models of quasar evolution and the formation of the most massive cosmic structures. The results are being published in the May 15, 2015 edition of the journal Science.
Hitting the jackpot is one thing, but if you hit the jackpot four times in a row you might wonder if the odds were somehow stacked in your favor.

Quasars constitute a brief phase of galaxy evolution, powered by the in-fall of matter onto a supermassive black hole at the center of a galaxy. During this phase, they are the most luminous objects in the Universe, shining hundreds of times brighter than their host galaxies, which themselves contain hundreds of billions of stars. But these hyper-luminous episodes last only a tiny fraction of a galaxy’s lifetime, which is why astronomers need to be very lucky to catch any given galaxy in the act. As a result, quasars are exceedingly rare on the sky, and are typically separated by hundreds of millions of light years from one another. The researchers estimate that the odds of discovering a quadruple quasar by chance is one in ten million. How on Earth did they get so lucky? 

Clues come from peculiar properties of the quartet’s environment. The four quasars are surrounded by a giant nebula of cool dense hydrogen gas, which emits light because it is irradiated by the intense glare of the quasars. In addition, both the quartet and the surrounding nebula reside in a rare corner of the universe with a surprisingly large amount of matter. “There are several hundred times more galaxies in this region than you would expect to see at these distances,” said J. Xavier Prochaska, professor at the University of California Santa Cruz and the principal investigator of the Keck Observatory observations. 

Given the exceptionally large number of galaxies, this system resembles the massive agglomerations of galaxies, known as galaxy clusters, that astronomers observe in the present-day universe. But because the light from this cosmic metropolis has been travelling for 10 billion years before reaching Earth, the images show the region as it was 10 billion years ago, less than 4 billion years after the big bang. It is thus an example of a progenitor or ancestor of a present-day galaxy cluster, or proto-cluster for short. 

Piecing all of these anomalies together, the researchers tried to understand what appears to be their incredible stroke of luck. “If you discover something which, according to current scientific wisdom should be extremely improbable, you can come to one of two conclusions: either you just got very lucky, or you need to modify your theory,” Hennawi said. 

The researchers speculate that some physical process might make quasar activity much more likely in specific environments. One possibility is that quasar episodes are triggered when galaxies collide or merge, because these violent interactions efficiently funnel gas onto the central black hole. Such encounters are much more likely to occur in a dense proto-cluster filled with galaxies, just as one is more likely to encounter traffic when driving through a big city. 

“The giant emission nebula is an important piece of the puzzle since it signifies a tremendous amount of dense cool gas,” said Fabrizio Arrigoni-Battaia, a PhD student at the Max Planck Institute for Astronomy who was involved in the discovery.

Supermassive black holes can only shine as quasars if there is gas for them to swallow, and an environment that is gas rich could provide favorable conditions for fueling quasars.

On the other hand, given the current understanding of how massive structures in the universe form, the presence of the giant nebula in the proto-cluster is totally unexpected. “Our current models of cosmic structure formation based on supercomputer simulations predict that massive objects in the early universe should be filled with rarefied gas that is about ten million degrees, whereas this giant nebula requires gas thousands of times denser and colder,” said Sebastiano Cantalupo, currently at ETH Zurich, that led the imaging observations a the Keck Observatory during his previous research appointment at UCSC. “It is really amazing that this discovery was made the same night of the Slug Nebula while we were hunting for giant Lyman alpha nebulae illuminated by quasars – my first night at Keck Observatory and definitely the most exciting observing night I have ever had!” 

“Extremely rare events have the power to overturn long-standing theories” Hennawi said. 

As such, the discovery of the first quadruple quasar may force cosmologists to rethink their models of quasar evolution and the formation of the most massive structures in the universe. 

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems. 

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I. Since it has been commissioned it has seen two major upgrades to further enhance its capabilities: addition of a second, blue arm optimized for shorter wavelengths of light; and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument's high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.


Joseph F.Hennawi
Max Planck Institute for Astronomy, Heidelberg, Germany
+49 6221 528 -263

J. Xavier Prochaska
UCO Lick Observatory/University of California Santa Cruz
+1 831 459 2135

Sebastiano Cantalupo
ETH Zurich, Switzerland
+41 44 633 7057


Markus Pössel
Public Information Officer
Max Planck Institute for Astronomy, Heidelberg, Germany
+49 6221 528 -261

Steve Jefferson
Communications Officer
W. M. Keck Observatory
+1 808 881 3827

Scientists at Keck Discover the Fluffiest Galaxies

A collection of unidentified blobs was discovered toward the Coma cluster of galaxies, using the Dragonfly Telephoto Array. One of these puzzling objects, Dragonfly 44, was studied in detail using the Keck Observatory and confirmed as an ultra-diffuse galaxy. Even though it is 60,000 light years across, It is so far away that it appears as only a faint smudge.  Credit: P. van Dokkum, R. Abraham, J. Brodie. Hi-res image 

Reconstructed spectrum of light spread out from the ultra-diffuse galaxy, DragonFly44, as seen by the Keck/LRIS instrument. Dark bands occur where atoms and molecules absorb the galaxy’s starlight. These bands reveal the compositions and ages of the stars, and also the distance of the galaxy.  Credit: P. van Dokkum, A. Romanowsky, J. Brodie. Hi-res image

An ultra-diffuse galaxy, Dragonfly 17, is shown next to other types of galaxies, to scale. The Andromeda galaxy is a giant spiral like our own Milky Way, and a dwarf elliptical galaxy, NGC 205, is also shown. Ultra-diffuse galaxies have the same number of stars as dwarf ellipticals, but spread out over a much larger region.  Credit: B. Schoening, V. Harvey/REU program/NOAO/AURA/NSF, P. van Dokkum/Hubble Space Telescope. Hi-res image

Maunakea, Hawaii – An international team of researchers led by Pieter van Dokkum at Yale University have used the W. M. Keck Observatory to confirm the existence of the most diffuse class of galaxies known in the universe. These "fluffiest galaxies" are nearly as wide as our own Milky Way galaxy – about 60,000 light years – yet harbor only one percent as many stars. The findings were recently published in the Astrophysical Journal Letters.

“If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds”, said van Dokkum. “We are beginning to form some ideas about how they were born and it’s remarkable they have survived at all. They are found in a dense, violent region of space filled with dark matter and galaxies whizzing around, so we think they must be cloaked in their own invisible dark matter ‘shields’ that are protecting them from this intergalactic assault.”

The team made the latest discovery by combining results from one of the world's smallest telescopes as well as the largest telescope on Earth. The Dragonfly Telephoto Array used 14-centimeter state of the art telephoto lens cameras to produce digital images of the very faint, diffuse objects. Keck Observatory’s 10-meter Keck I telescope, with its Low Resolution Imaging Spectrograph, then separated the light of one of the objects into colors that diagnose its composition and distance.

Finding the distance was the clinching evidence. The data from Keck Observatory showed the diffuse "blobs" are very large and very far away, about 300 million light years, rather than small and close by. The blobs can now safely be called Ultra Diffuse Galaxies (UDGs).

“If there are any aliens living on a planet in an ultra-diffuse galaxy, they would have no band of light across the sky, like our own Milky Way, to tell them they were living in a galaxy. The night sky would be much emptier of stars,” said team member Aaron Romanowsky, of San Jose State University.

The UDGs were found in an area of the sky called the Coma cluster, where thousands of galaxies have been drawn together in a mutual gravitational dance. “Our fluffy objects add to the great diversity of galaxies that were previously known, from giant ellipticals that outshine the Milky Way, to ultra compact dwarfs,” said University of California, Santa Cruz Professor Jean Brodie.

“The big challenge now is to figure out where these mysterious objects came from,” said Roberto Abraham, of the University of Toronto. “Are they ‘failed galaxies’ that started off well and then ran out of gas? Were they once normal galaxies that got knocked around so much inside the Coma cluster that they puffed up? Or are they bits of galaxies that were pulled off and then got lost in space?”  The key next step in understanding UDGs is to to pin down exactly how much dark matter they have. Making this measurement will be even more challenging than the latest work.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems. 

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I. Since it has been commissioned it has seen two major upgrades to further enhance its capabilities: addition of a second, blue arm optimized for shorter wavelengths of light; and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument's high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

Friday, May 15, 2015

SGR 1745-2900: Magnetar Near Supermassive Black Hole Delivers Surprises

SGR 1745-2900
Credit: NASA/CXC/INAF/F.Coti Zelati et al 


This illustration shows how an extremely rapidly rotating neutron star, which has formed from the collapse of a very massive star, can produce incredibly powerful magnetic fields. (Illustration: NASA/CXC/M.Weiss)

In 2013, astronomers announced they had discovered a magnetar exceptionally close to the supermassive black hole at the center of the Milky Way using a suite of space-borne telescopes including NASA's Chandra X-ray Observatory.

Magnetars are dense, collapsed stars (called "neutron stars") that possess enormously powerful magnetic fields. At a distance that could be as small as 0.3 light years (or about 2 trillion miles) from the 4-million-solar mass black hole in the center of our Milky Way galaxy, the magnetar is by far the closest neutron star to a supermassive black hole ever discovered and is likely in its gravitational grip.

Since its discovery two years ago when it gave off a burst of X-rays, astronomers have been actively monitoring the magnetar, dubbed SGR 1745-2900, with Chandra and the European Space Agency's XMM-Newton. The main image of the graphic shows the region around the Milky Way's black hole in X-rays from Chandra (red, green, and blue are the low, medium, and high-energy X-rays respectively). The inset contains Chandra's close-up look at the area right around the black hole, showing a combined image obtained between 2005 and 2008 (left) when the magnetar was not detected, during a quiescent period, and an observation in 2013 (right) when it was caught as a bright point source during the X-ray outburst that led to its discovery.

A new study uses long-term monitoring observations to reveal that the amount of X-rays from SGR 1745-2900 is dropping more slowly than other previously observed magnetars, and its surface is hotter than expected.

The team first considered whether "starquakes" are able to explain this unusual behavior. When neutron stars, including magnetars, form, they can develop a tough crust on the outside of the condensed star. Occasionally, this outer crust will crack, similar to how the Earth's surface can fracture during an earthquake. Although starquakes can explain the change in brightness and cooling seen in many magnetars, the authors found that this mechanism by itself was unable to explain the slow drop in X-ray brightness and the hot crustal temperature. Fading in X-ray brightness and surface cooling occur too quickly in the starquake model.

The researchers suggest that bombardment of the surface of the magnetar by charged particles trapped in twisted bundles of magnetic fields above the surface may provide the additional heating of the magnetar's surface, and account for the slow decline in X-rays. These twisted bundles of magnetic fields can be generated when the neutron star forms.

The researchers do not think that the magnetar's unusual behavior is caused by its proximity to a supermassive black hole, as the distance is still too great for strong interactions via magnetic fields or gravity.
Astronomers will continue to study SGR 1745-2900 to glean more clues about what is happening with this magnetar as it orbits our galaxy's supermassive black hole.

These results appear in Monthly Notices of the Royal Astronomical Society in a paper led by the PhD student Francesco Coti Zelati (Universita' dell' Insubria, University of Amsterdam, INAF-OAB), within a large international collaboration including N. Rea (University of the Amsterdam, CSIC-IEEC), A. Papitto, D. Viganò (CSIC-IEEC), J. A. Pons (Universitat d'Alacant), R. Turolla (Universita' di Padova, MSSL), P. Esposito (INAF, CfA), D. Haggard (Amherst college), F. K. Baganoff (MIT), G. Ponti (MPE), G. L. Israel, S. Campana (INAF), D. F. Torres (CSIC-IEEC, ICREA), A. Tiengo (IUSS, INAF), S. Mereghetti (INAF), R. Perna (Stony Brook University), S. Zane (MSSL), R. P. Mignani (INAF, University of Zielona Gora), A. Possenti, L. Stella (INAF).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for the agency's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for SGR 1745-2900:

Scale: Main Image is 8 arcmin across (about 61 light years); Inset image is about 14 arcsec across (1.8 light years)
Category: Black Holes, Milky Way Galaxy
Coordinates (J2000): RA 17h 45m 40s | Dec -29° 00' 28.00"
Constellation: Sagittarius
Observation Date: Main Image: 43 pointings from September 21, 1999 to May 18, 2009; Inset: 25 pointings between 29 Apr 2013 and 30 Aug 2014
Observation Time: Main Image: 278 hours (11 days 14 hours);
Obs. ID: Main Image: 242, 1561, 2943, 2951-2954, 3392, 3393, 3549, 3663, 3665, 4683, 4684, 5360, 5950-5954, 6113, 6363, 6639, 6640-6646, 7554-7759, 9169-9174, 10556; Inset: 14702-14704, 14943-14946, 15040-15045, 15651, 15654, 16508, 16210-16217, 16597
Instrument: ACIS
References: Coti Zelati, F. et al, 2015, MNRAS 449, 2685; arXiv:1503.01307
Color Code: Energy: Red (2-3.3 keV), Green (3.3-4.7 keV), Blue (4.7-8 keV)
Distance Estimate: About 26,000 light years

Galactic onion

Credit: ESA/Hubble & NASA
Acknowledgement: Judy Schmidt

The glowing object in this image is an elliptical galaxy called NGC 3923. It is located over 90 million light-years away in the constellation of Hydra.

NGC 3923 is an example of a shell galaxy where the stars in its halo are arranged in layers.

Finding concentric shells of stars enclosing a galaxy is quite common and is observed in many elliptical galaxies. In fact, every tenth elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies. The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centres approach, they initially oscillate about a common centre, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed.

NGC 3923 has over twenty shells, with only a few of the outer ones visible in this image and its shells are much more subtle than those of other shell galaxies. The shells of this galaxy are also interestingly symmetrical, while other shell galaxies are more skewed.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.

Thursday, May 14, 2015

Hubble Catches a Stellar Exodus in Action

White Dwarfs Migrating from Globular Cluster 47 Tucanae's Core
Credit: NASA, ESA, and H. Richer and J. Heyl (University of British Columbia, Vancouver, Canada)
Acknowledgment: J. Mack (STScI) and G. Piotto (University of Padova, Italy)

Using NASA's Hubble Space Telescope, astronomers have captured for the first time snapshots of fledgling white dwarf stars beginning their slow-paced, 40-million-year migration from the crowded center of an ancient star cluster to the less populated suburbs.

White dwarfs are the burned-out relics of stars that rapidly lose mass, cool down, and shut off their nuclear furnaces. As these glowing carcasses age and shed weight, their orbits begin to expand outward from the star cluster's packed downtown. This migration is caused by a gravitational tussle among stars inside the cluster. Globular star clusters sort out stars according to their mass, governed by a gravitational billiard-ball game where lower mass stars rob momentum from more massive stars. The result is that heavier stars slow down and sink to the cluster’s core, while lighter stars pick up speed and move across the cluster to the edge. This process is known as "mass segregation." Until these Hubble observations, astronomers had never definitively seen the dynamical conveyor belt in action.

Astronomers used Hubble to watch the white-dwarf exodus in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in our Milky Way galaxy. The cluster resides 16,700 light-years away in the southern constellation Tucana.

"We've seen the final picture before: white dwarfs that have already sorted themselves out and are orbiting in a location outside the core that is appropriate for their mass," explained Jeremy Heyl of the University of British Columbia (UBC), Vancouver, Canada, first author on the science paper. The team's results appeared in the May 1 issue of The Astrophysical Journal.

"But in this study, which comprises about a quarter of all the young white dwarfs in the cluster, we're actually catching the stars in the process of moving outward and segregating themselves according to mass," Heyl said. "The entire process doesn't take very long, only a few hundreds of millions of years, out of the 10-billion-year age of the cluster, for the white dwarfs to reach their new home in the outer suburbs."

"This result hasn't been seen before, and it challenges some ideas about some of the details of how and when a star loses its mass near the end of its life," added team member Harvey Richer of UBC.

Using the ultraviolet-light capabilities of Hubble's sharp-eyed Wide Field Camera 3, the astronomers examined 3,000 white dwarfs, tracing two populations with diverse ages and orbits. One grouping was 6 million years old and had just begun their journey. Another was around 100 million years old and had already arrived at its new homestead far away from the center, roughly 1.5 light-years, or nearly 9 trillion miles, away.

Only Hubble can detect these stars because ultraviolet light is blocked by Earth's atmosphere and therefore doesn't reach ground-based telescopes. The astronomers estimated the white dwarfs' ages by analyzing their colors, which gives them the stars' temperatures. The hottest dwarfs shine fiercely in ultraviolet light.

The dwarfs were tossed out of the rough-and-tumble cluster center due to gravitational interactions with heftier stars orbiting the region. Stars in globular clusters sort themselves out by weight, with the heavier stars sinking to the middle. Before flaming out as white dwarfs, the migrating stars were among the most massive in the cluster, weighing roughly as much as our Sun. The more massive stars burned out long ago.

The migrating white dwarfs, however, are not in a hurry to leave. Their orbits expand outward at about 30 miles an hour, roughly the average speed of a car traveling in the city. The dead stars will continue this pace for about 40 million years, until they reach a location that is more appropriate for their mass.

Although the astronomers were not surprised to see the migration, they were puzzled to find that the youngest white dwarfs were just embarking on their journey. This discovery may be evidence that the stars shed much of their mass at a later stage in their lives than once thought.

About 100 million years before stars evolve into white dwarfs, they swell up and become red giant stars. Many astronomers thought that stars lose most of their mass during this phase by blowing it off into space. But the Hubble observations reveal that the stars actually dump 40 percent to 50 percent of their bulk just 10 million years before completely burning out as white dwarfs.

"This late start is evidence that these white dwarfs are losing a large amount of mass just before they become white dwarfs and not during the earlier red giant phase, as most astronomers had thought," said Richer. 

"That's why we are seeing stars still in the process of moving slowly away from the center of the cluster. It's only after they lose their mass that they get gravitationally pushed out of the core. If the stars had shed most of their weight earlier in their lives, we wouldn't see such a dramatic effect between the youngest white dwarfs and the older ones that are 100 million years old."

Although the white dwarfs have exhausted the hydrogen fuel that makes them shine as stars, these stellar relics are among the brightest stars in this primordial cluster because their brilliant hot cores have been exposed, which are luminous largely in ultraviolet light. "When a white dwarf forms, they've got all this stored-up heat in their cores, and the reason we can see a white dwarf is because over time they radiate their stored thermal energy slowly into space," Richer explained. "They're getting cooler and less luminous as time goes on because they have no nuclear sources of energy."

After making it through the gauntlet of gravitational interactions within the crowded 1.5-light-year-wide core, the traveling white dwarfs encounter few interactions as they migrate outward, because the density of stars decreases. "A lot of action happens when they're 30 million to 40 million years old, and continues up to around 100 million years, and then as they get older the white dwarfs still evolve but less dramatically," Heyl said.

The 47 Tucanae cluster is an ideal place to study the mass segregation of white dwarfs because it is nearby and has a significant number of centrally concentrated stars that can be resolved by Hubble's crisp vision.


Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514 /

Felicia Chou
NASA Headquarters, Washington, DC

Jeremy Heyl
University of British Columbia, Vancouver, BC, Canada

Source: HubbleSite