Thursday, December 31, 2009

Galactic Building Blocks

The current view of galactic formation is that galaxies form from a "bottom-up" method. In this picture, small dwarf galaxies, full of metal poor stars, were attracted by dark matter halos in the early universe which merged into larger galaxies. Many of those metal poor stars can still be seen today in the halo of the galaxy, but it was thought that the building blocks from which the galaxies were constructed were long gone or had evolved on their own and would no longer resemble the primordial building blocks.

However, earlier this year, an extremely metal poor star with only 0.00025% of the iron in the Sun was discovered in the Sculptor dwarf galaxy. If confirmed, this would show a strong link to further support the notion that metal poor dwarf galaxies were related to the metal poor stars that still populate our halo. Confirming this was the subject of a recent paper.

For their study, the authors analyzed the newly discovered star (S1020549) with a high resolution spectrograph. From this, they confirmed that the star had very little iron present (an element generally used as an indicator of overall heavy element abundance since its absorption lines feature prominently in the spectra and are easily detectable). The extremely low ratio of iron to hydrogen makes it currently the most metal poor star known in a dwarf galaxy (the overall record holder for metal deficiency is HE 1327-2327).

The study determined an overall [Fe/H] abundance of -3.8 (see how this abundance is defined here) which is very similar to the [Fe/H] abundance of archetypical halo stars of about -4.0. Furthermore, many of the other elemental abundances that were uncovered with the detailed spectroscopy (especially those of Mg, Ca, Sc, Ti, and Cr) also fit the general abundance level of stars found in our halo.

This isn't a conclusive tie between the two and more such stars will need to be uncovered to reinforce the similarities, but since S1020549 was discovered with "a relatively modest survey" this may suggest "that future observational searches should discover more such objects in Sculptor and other dwarf galaxies."

An Exotic Source for Cosmic Rays: 'Baby' Black Holes

Cosmic rays – particles that have been accelerated to near the speed of light – stream out from our Sun all of the time, though they are positively sluggish compared to what are called Ultra-High-Energy Cosmic Rays (UHECRs). These types of cosmic rays originate from sources outside of the Solar System, and are much more energetic than those from our Sun, though also much rarer. The merger between a white dwarf and neutron star or black hole may be one source of these rays, and such mergers may occur often enough to be the most significant source of these energetic particles.

The Sloan White dwArf Radial velocity data Mining Survey (SWARMS) – which is part of the Sloan Digital Sky Survey – recently uncovered a binary system of exotic objects only 50 parsecs away from the Solar System. This system, named SDSS 1257+5428, appears to be a white dwarf star that is orbiting a neutron star or low-mass black hole. Details about the system and its initial discovery can be found in a paper by Carles Badenes, et al. here.

Co-author Todd Thompson, assistant professor in the Department of Astronomy at Ohio State University, argues in a recent letter to The Astrophysical Journal Letters that this type of system, and subsequent merger of these exotic remnants of stars, may be commonplace, and could account for the amount of UHECRs that are currently observed. The merger between the white dwarf and neutron star or black hole may also create a black hole of low mass, a so-called "baby" black hole.

Thompson wrote in an email interview:

"White dwarf/neutron star or black hole binaries are thought to be quite rare, although there is a huge range in the number per Milky Way-like galaxy in the literature. SWARMS was the first to detect such a system using the "radial velocity" technique, and the first to find such an object so nearby, only 50 parsecs away (about 170 light years). For this reason, it was very surprising, and its relative proximity is what allowed us to make the argument that these systems must be quite common compared to most previous expectations. SWARMS would have had to be very lucky to see something so rare so near by."

Thompson, et al. argue that this type of merger may be the most significant source of UHECRs in the Milky Way galaxy, and that one should merge in the galaxy about every 2,000 years. These types of mergers may be slightly less common than Type Ia supernovae, which originate in binary systems of white dwarfs.

A white dwarf merging with a neutron star would also create a low-mass black hole of about 3 times the mass of the Sun. Thompson said, "In fact, this scenario is likely since we think that neutron stars cannot exist above 2-3 times the mass of the Sun. The idea is that the WD would be disrupted and accrete onto the neutron star and then the neutron star would collapse to a black hole. In this case, we might see the signal of BH formation in gravity waves."

The gravity waves produced in such a merger would be above the detectable range by the Laser Interferometer Gravitational-Wave Observatory (LIGO), an instrument that uses lasers to detect gravity waves (of which none have been detected…yet), and even possibly a spaced base gravitational wave observatory, NASA's Laser Interferometer Space Antenna, LISA.

Common cosmic rays that come from our Sun have an energy on the scale of 10^7 to 10^10 electron-volts. Ultra-high-energy cosmic rays are a rare phenomenon, but they exceed 10^20 electron-volts. How do systems like SDSS 1257+5428 produce cosmic rays of such high energy? Thompson explained that there are two equally fascinating possibilities.

In the first, the formation of a black hole and subsequent accretion disk from the merger would generate a jet somewhat like those seen at the center of galaxies, the telltale sign of a quasar. Though these jets would be much, much smaller, the shockwaves at the front of the jet would accelerate particles to the necessary energies to create UHECRs, Thompson said.

In the second scenario, the neutron star steals matter off of the white dwarf companion, and this accretion starts it rotating rapidly. The magnetic stresses that build at the surface of the neutron star, or "magnetar", would be able to accelerate any particles that interact with the intense magnetic field to ultra-high energies.

The creation of these ultra-high-energy cosmic rays by such systems is highly theoretical, and just how common they may be in our galaxy is only an estimate. It remains unclear so soon after the discovery of SDSS 1257+5428 whether the companion object of the white dwarf is a black hole or neutron star. But the fact that SWARMS made such a discovery so early in the survey is encouraging for the discovery of further exotic binary systems.

"It is not likely that SWARMS will see 10 or 100 more such systems. If it did, the rate of such mergers would be very (implausibly) high. That said, we've been surprised many times before. However, given the total area of the sky surveyed, if our estimate of the rate of such mergers is correct, SWARMS should see only about 1 more such system, and they may see none. A similar survey in the southern sky (there is nothing at present comparable to the Sloan Digital Sky Survey, on which SWARMS is based) should turn up approximately 1 such system," Thompson said.

Observations of SDSS 1257+5428 have already been made using the Swift X-ray observatory, and some measurements have been taken in the radio spectrum. No source of gamma-rays was to be found in the location of the system using the Fermi telescope.

Thompson said, "Probably the most important forthcoming observation of the system is to get a true distance via parallax. Right now, the distance is based on the properties of the observed white dwarf. In principle, it should be relatively easy to watch the system over the next year and get a parallax distance, which will alleviate many of the uncertainties surrounding the physical properties of the white dwarf."

Source: Arxiv, email interview with Todd Thompson

Monday, December 28, 2009

Astronomy Question of the Week: How old is the Universe?

The upper photograph shows the entire globular cluster, which contains several hundred thousand stars. The two lower photographs each show an enlargement of the extract marked. The weak light spots circled in blue are white dwarfs. They are between 12 and 13 billion years old and are probably some of the oldest stars in the Universe. Credit: NASA/H.Richer/NOAO/AURA/NSF. Hi-Res JPEG (1.1 MB)

In the field of archaeology the age of finds or the time of events can sometimes be determined relatively easily, for example via the number of tree rings or the rate of decomposition of radioactive elements. However, there is unfortunately no direct and absolute indicator for the age of the Universe. Astronomers have, however, found two ways to arrive at a good estimate.

The Universe is at least as old as the oldest objects within it. What are the oldest objects whose age can be determined? Stars are promising candidates; however, several things must be taken into account. The higher a star's mass, the shorter its lifetime. In addition, many of the stars that are observable today contain chemical elements that are heavier than hydrogen and helium. These stars must have formed later in the history of the Universe’s development, because heavy elements did not exist at the very beginning. The heavy elements had to be produced in the first stars or early star generations. (See also the astronomy question from week 36: Are we made of 'stardust'?)

As a result, the oldest stars must have only a relatively low mass and contain hardly any heavy elements. Stars such as this can, for example, be found in the globular clusters that are grouped around the Milky Way. In particular, white dwarf stars, which have consumed their nuclear fuel and are slowly cooling down, can be used for age determination. (See also the astronomy question from week 27: How long will the Sun continue to shine?) Observations of the globular clusters and the cooling time of white dwarfs allow us to conclude that the age of the Milky Way is approximately 12 billion years. News Archive

The Milky Way – only a little younger than the Universe

Independent of the age of individual objects that allow a minimum age to be determined for the Universe, its age can also be determined using the Big Bang theory. To do this, we use the expansion of the Universe after the Big Bang, the same expansion that is continuing today, to calculate backwards – back in time until the zero point of the expansion. However, cosmic expansion (see also the astronomy question from week 38: How quickly is the Universe expanding?) has not always taken place evenly; this would only have been the case in a completely empty Universe. Radiation, matter (including 'dark matter') and dark energy (see the question from week 39: What is dark energy?) influence this expansion. Astronomers determine these influences using satellite observations, among other things, and ultimately they calculate that the Universe is 13.7 billion years old.


Josef Hoell
German Aerospace Center
Space Agency, Space Science
Tel.: +49 228 447-381
Fax: +49 228 447-745

Wednesday, December 23, 2009

A Quarter Century of Infrared Astronomy

This composite graphic encompasses a quarter century of infrared astronomy from space, a world away from Galileo Galilei's eight-power telescope that was the cutting edge of astronomy 400 years ago. The composite recognizes the International Year of Astronomy and celebrates the dramatic progress in our understanding of the universe derived from infrared observations. It also illustrates some of the contributions from the Infrared Processing and Analysis Center (IPAC) to this progress by way of astronomical data processing, analysis, archiving and dissemination.

Infrared astronomy, especially from space, explores up a vast portion of the spectrum beyond the red end of visible light. Through this window the universe emits a tremendous variety of information about the physical and chemical composition of various regions, about their energetic states, and about the current and historical activity of star formation. Infrared and submillimeter wavelengths still hold the most promise for studying the earliest moments in the history of the universe when diffuse gas was transformed into the first stars and galaxies. This era is thought to have occurred around the first percent of the age of the universe.

While celebrating this rich legacy of infrared astronomy, infrared astronomers are also enjoying a golden age of sorts, with an unprecedented number of missions currently in-flight: NASA's Spitzer Space Telescope is continuing its warm mission; the European Space Agency Herschel and Planck telescopes are in their prime mission phase; NASA's Wide Field Infrared Survey Explorer (WISE) just launched and is expected to start mapping the sky in early 2010. All of these missions have links to IPAC.

"Rho Ophiuchi" is a complex region of star formation centered on a large cloud of molecular gas, and located about 400 light-years from Earth. Hundreds of young stars are forming out of the central cloud. Their typical age is 300,000 years, seen in the first "moments" of the billions of years in a star's life span.

IRAS, the InfraRed Astronomical Satellite, was a joint project between the U.S., the Netherlands and the United Kingdom. It operated in Earth orbit from 25 January to 22 November 1983, surveying the infrared sky and dramatically expanding our understanding of the Universe by revealing surprising new phenomena. This false-color image renders infrared light into visible light, showing 12 m emission as blue, 25 and 60 m as green, and 100 m as red. The infrared emission originates in cosmic dust at a range of temperatures, with the colder dust appearing redder in this image. The field of view is about 18 degrees to a side. The small grey inset is amplified into the views shown in the remaining three frames.

IPAC was established twenty-five years ago on the campus of the California Institute of Technology to provide expertise and support for the processing, analysis and interpretation of data from IRAS.

ISO, the Infrared Space Observatory, was a European Space Agency project with Japanese and U.S. participation, and operated from 17 November 1995 to 16 May 1998. The image shows 7 m emission in blue and 15 m in red, revealing very small dust grains heated by stars as diffuse emission, and very young stars still in the formation stages as the reddish points, whereas older stars appear as blue dots. The field of view is the same as shown for 2MASS and Spitzer, about 3/4 of a degree on a side.

The scientific and technical expertise of the IPAC staff was one of the many heritages from IRAS, prompting NASA to designate IPAC as the U.S. science support center for ISO.

2MASS, the Two Micron All-Sky Survey, a U.S. National Aeronautics and Space Administration and National Science Foundation project, gathered data from June 1997 to February 2001, mapping the whole sky in the near infrared from the ground. The picture shows 1.3 m emission as blue, 1.6 m as green, and 2.2 m as red. The sky appears dark towards dense parts of the cloud because dust absorbs the light, whereas the reddish sources signal young stars. The field of view is about 3/4 of a degree on a side.

Its unique capabilities made IPAC the obvious choice for processing the many terabytes of data from 2MASS. The University of Massachusetts developed and operated the two observatories that acquired the data. IPAC's role included design of survey strategy, data processing, quality control, and the production of final catalogs and final data products from the primary survey and the extended mission.

The Spitzer Space Telescope is an infrared observatory built and operated by the Jet Propulsion Laboratory of the California Institute of Technology for the U.S. National Aeronautics and Space Administration. It was launched on 25 August 2003 into a Sun-centered orbit, and ran out of the liquid helium that kept it cold on 15 May 2005. Today, it still observes the cosmos at 3.6 and 4.5 m, using one out of three original instruments. The picture shows 3.6 m emission as blue, 8 m as green, and 24 m as red. Dust glows with different colors depending on its composition and heating, whereas the colors of the stars indicate their age, blue for mature stars, and red for those still surrounded by natal cloud material. The field of view is again about 3/4 of a degree on a side.

In early 1998, IPAC was designated as the science operations center for the Spitzer Space Telescope, NASA's Infrared Great Observatory. The Spitzer Science Center (SSC), is an autonomously managed entity within IPAC, which relies on the skills and knowledge of IPAC experts in supporting Spitzer.

Vampires and collisions rejuvenate stars

Image credit: NASA, ESA and Francesco Ferraro (University of Bologna)
Videos: Video 1 - Video 2

Using the NASA/ESA Hubble Space Telescope, astronomers have uncovered two distinct kinds of "rejuvenated" stars in the globular cluster Messier 30. A new study shows that both stellar collisions and a process sometimes called vampirism are behind this cosmic "face lift". The scientists also uncover evidence that both sorts of blue stragglers were produced during a critical dynamical event (known as "core collapse") that occurred in Messier 30 a few billion years ago.

Stars in globular clusters [1] are generally extremely old, with ages of 12-13 billion years. However, a small fraction of them appear to be significantly younger than the average population and, because they seem to have been left behind by the stars that followed the normal path of stellar evolution and became red giants, have been dubbed blue stragglers [2]. Blue stragglers appear to regress from "old age" back to a hotter and brighter "youth", gaining a new lease on life in the process. A team of astronomers used Hubble to study the blue straggler star content in Messier 30, which formed 13 billion years ago and was discovered in 1764 by Charles Messier. Located about 28 000 light-years away from Earth, this globular cluster — a swarm of several hundred thousand stars — is about 90 light-years across.

Although blue stragglers have been known since the early 1950s, their formation process is still an unsolved puzzle in astrophysics. "It’s like seeing a few kids in the group picture of a rest-home for retired people. It is natural to wonder why they are there," says Francesco Ferraro from the University of Bologna in Italy, lead author of the study that will be published this week in Nature [3]. Researchers have been studying these stars for many years and knew that blue stragglers are indeed old. They were thought to have arisen in a tight binary system [4]. In such a pair, the less massive star acts as a "vampire", siphoning fresh hydrogen from its more massive companion star. The new fuel supply allows the smaller star to heat up, growing bluer and hotter — behaving like a star at an earlier stage in its evolution.

The new study shows that some of the blue stragglers have instead been rejuvenated by a sort of "cosmic facelift", courtesy of cosmic collisions. These stellar encounters are nearly head-on collisions in which the stars might actually merge, mixing their nuclear fuel and re-stoking the fires of nuclear fusion. Merged stars and binary systems would both be about twice the typical mass of individual stars in the cluster.

"Our observations demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster," says team member Giacomo Beccari from ESA.
Using data from the now-retired Wide Field Planetary Camera 2 (WFPC2) aboard Hubble, astronomers found that these "straggling" stars are much more concentrated towards the centre of the cluster than the average star. "This indicates that blue stragglers are more massive than the average star in this cluster," says Ferraro. "More massive stars tend to sink deep into the cluster the way a billiard ball would sink in a bucket of honey."

The central regions of high density globular clusters are crowded neighbourhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major "core collapse" that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This event significantly increased the number of collisions among stars, and favoured the formation of one of the families of blue stragglers. On the other hand, the increase of stellar crowding due to the collapse of the core also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers. "Almost ten percent of galactic globular clusters have experienced core collapse, but this is the first time that we see the effect of the core collapse imprinted on a stellar population," says Barbara Lanzoni, University of Bologna.

"The two distinct populations of blue stragglers discovered in Messier 30 are the relics of the collapse of the core that occurred two billion years ago. In a broad context our discovery is direct evidence of the impact of star cluster dynamics on stellar evolution. We should now try to see if other globular clusters present this double population of blue stragglers," concludes Ferraro.

Notes for editors:

[1] Globular clusters are dense agglomerations of several hundred thousand stars. Present among the earliest inhabitants of our Milky Way, they formed in the vast halo of our galaxy before it flattened to form a pancake-shaped spiral disc. Star formation essentially stopped in globular clusters 13 billion years ago, so astronomers expect to find only old stars and they use globular cluster ages as a benchmark for estimating the age of the Universe.

[2] In 1953, astronomer Allan Sandage found a puzzling new population of stars that seemed to go against the rules of stellar evolution in globular clusters. Sandage detected hot young blue stars in the globular cluster Messier 3, and subsequently in other globular clusters. He dubbed them stragglers because they looked like they were trailing or left behind by other blue stars in the cluster that had long ago evolved to the red giant stage.

[3] This research was presented in a paper that appears in the 24 December 2009 issue of Nature, “Two distinct sequences of blue straggler stars in the globular cluster M30”, by F. R. Ferraro et al.

[4] In 1964 astronomers Fred Hoyle and W.H. McCrea independently suggested that blue stragglers result when two stars capture each other and form a tight binary system.



Francesco Ferraro
Astronomy Department
University of Bologna
Tel: +39-051-20-95-774

Colleen Sharkey
Hubble/ESA, Garching, Germany
Tel: +49-89-3200-6306
Cell: +49-015115373591

Keck telescopes take deeper look at planetary nurseries

This artist's concept shows the development of planets within a dust disk around a young star. The Keck Interferometer probed the temperature and density of the dust disk around MWC 419 to within a fraction of an astronomical unit from the star. Credit: David A. Hardy

MWC 419, also known as V594 Cas, is a young, blue variable star located 2,100 light years away in the constellation Cassiopeia. Credit: DSS/STScI/AURUA

MAUNA KEA, HI—Astronomers using the W. M. Keck Observatory have peered far into a young planetary system, giving an unprecedented view of dust and gas that might eventually form worlds similar to Jupiter, Venus or even Earth.

“Because the gas, dust and debris that orbit young stars provide the raw materials for planets, probing the inner regions of those stars lets us learn about how Earth-like planets form,” said astronomer Sam Ragland of Keck Observatory. He and his collaborators recently measured the properties of a young planetary system at distances closer to the star than Venus is to the Sun.

The researchers used the Keck Interferometer, which combines the light-gathering power of both 10-meter Keck telescopes to act as an 85-meter telescope, much larger than any existing or planned telescope.

“Nothing else in the world provides us with the types of measurements the Keck Interferometer does,” said Wesley Traub of Caltech’s Jet Propulsion Laboratory. “In effect, it’s a zoom lens for the Keck telescopes.”

The “zoom lens” allowed the researchers to probe MWC 419, a blue, B-type star that has several times the mass of the Sun and lies about 2,100 light-years away in the constellation Cassiopeia. With an age less than ten million years, MWC 419 ranks as a stellar kindergartener.

With the interferometer and the increased ability to observe fine detail, the team measured temperatures in the planet-forming disk to within about 50 million miles of the star. “That’s about half of Earth’s distance from the Sun, and well within the orbit of Venus,” said team member William Danchi of NASA’s Goddard Space Flight Center in Greenbelt, Md.

For comparison, astronomers using a single telescope have directly observed HR 8799, Fomalhaut and GJ 758 and their orbiting planets, which are 40 to 100 times farther away from their stars.

The interferometry results were taken in near-infrared light (3.5 to 4.1 micrometers), which is a wavelength slightly longer than red light and is invisible to the human eye. The researchers used a newly implemented infrared camera, which is the only one of its kind on Earth, to make the first “L-band” interferometric observations of MWC 419.

“This unique infrared capability adds a new dimension to the Keck Interferometer in probing the density and temperature of planet-forming regions around young stars. This wavelength region is relatively unexplored,” Ragland said. “Basically, anything we see through this camera is brand new information.”

With the data, Ragland and his collaborators measured the temperature of dust at various regions throughout MWC 419’s inner disk. Temperature differences throughout the disk may indicate that the dust has different chemical compositions and physical properties that may affect how planets form. For example, in the Solar System, conditions were just right to allow rocky worlds to form closer to the Sun, while gas giants and icy moons assembled farther our in the system. The team reported their findings in the Sept. 20 issue of the Astrophysical Journal.

The observations are an “important first step” in a larger program to collect data on young stars that span the lower-mass T Tauri stars, which are the progenitors of Sun-like stars, to their more massive counterparts, like MWC 419, explained John Monnier, an interferometry scientist at the University of Michigan who was not involved with the study.

The astronomers want to study the range of developing stars because their mass, size and luminosity might affect the composition and physical characteristics of the surrounding disk. Ragland and his collaborators are continuing to collect data on young stars and will combine their infrared observations with new data from the Keck Interferometer’s “nulling” mode, a technique which will block out the light from the central star in a young planetary system.

The Keck Interferometer is funded by NASA and developed by the Keck Observatory, the Jet Propulsion Laboratory (California Institute of Technology) and the NASA Exoplanet Science Institute (California Institute of Technology). The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit

WIYN 3.5-m telescope rounds up “Blue Stragglers”

Figure 1: NGC188 is an open star cluster in the constellation Cepheus

Astronomers have reported new observations of a remarkable binary star population in a well-known star cluster. In a press release (embargoed until Dec 23, 1PM EST), Nature features two interesting studies of a class of atypical stars known as “blue stragglers”.

One paper was written by Robert Mathieu and Aaron Geller, University of Wisconsin-Madison, who used the WIYN telescope at Kitt Peak National Observatory for their observations. Mathieu and Geller found that the blue stragglers in NGC188 (Caldwell 1), an open cluster in our galaxy, have a binary fraction of 76%, which is three times the frequency for normal stars of this type.

They conclude that possibly all of these stars originate in multiple star systems, and that several formation mechanisms could be operating. Geller, a PhD candidate at the University of Wisconsin-Madison, has been granted long term observing status for this project, which is part of the WIYN Open Cluster Study.

Kitt Peak National Observatory is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. The founding members of the WIYN Observatory partnership are the University of Wisconsin-Madison, Indiana University, Yale University, and NOAO.

Friday, December 18, 2009

LOFAR maps the radio sky at Effelsberg

The radio sky above Effelsberg on November 10, 2009, observed with the newly installed LOFAR high-band array. Fig. 1a (left): Software-calibrated image with high signal-to-noise ratio at a frequency of 120 MHz; Fig. 1b (right): Movie, showing the sky above Effelsberg at radio frequencies from 35 MHz to 190 MHz. Images: James Anderson, MPIfR.

Scientists at the Max Planck Institute for Radio Astronomy have made the first LOFAR "all-sky" images in the 110 to 190 MHz range using LOFAR high-band antennas at the LOFAR station in Effelsberg, Germany. LOFAR is the Low Frequency Array, designed and developed by ASTRON. These images are the first high-band, all-sky images made from any complete LOFAR station, and mark a significant milestone in the development of the LOFAR project.

The first LOFAR all-sky high-band image is shown in Figure 1. This all-sky image has North at the top and East at the left, just as a person would have seen the entire sky when lying on their back on a flat field near Effelsberg late in the afternoon on November 10, if their eyes were sensitive to radio waves. The two bright (yellow) spots are Cygnus A, a giant radio galaxy powered by a supermassive black hole near, the center of the image, and Cassiopeia A, a bright radio source created by a supernova explosion about 300 years ago, at the upper-left in the image. The plane of our Milky Way galaxy can also be seen passing by both Cassiopeia A and Cygnus A, and extending down to the bottom of the image. The North Polar Spur, a large cloud of radio emission within our own galaxy, can also be seen extending from the direction of the Galactic center in the South, toward the western horizon in this image.

"We made this image with a single 60 second "exposure" at 120 MHz using our high-band LOFAR field in Effelsberg", says James Anderson, project manager of the Effelsberg LOFAR station. "The ability to make all-sky images in just seconds is a tremendous advancement compared to existing radio telescopes which often require weeks or months to scan the entire sky." This opens up exciting possibilities to detect and study rapid transient phenomena in the universe.

LOFAR, the LOw Frequency ARray, is an advanced new radio telescope being built in many countries across Europe. Operating at relatively low radio frequencies from 10 to 240 MHz, LOFAR has essentially no moving parts to track objects in the sky --- instead digital electronics are used to combine signals from many small antennas to electronically steer observations on the sky. In certain electronic modes, the signals from all of the individual antennas can be combined to make images of the entire radio sky visible above the horizon.

LOFAR uses two different antenna designs, to observe in two different radio bands, the so-called low-band from 10 to 80 MHz, and the high-band from 110 to 240 MHz. All-sky images using the low-band antennas at Effelsberg were made in 2007 (see press release "LOFAR picks up speed" from December 11, 2007).

Following the observation for the first high-band, all-sky image, scientists at MPIfR made a series of all-sky images covering a wide frequency range using both the low-band and high-band antennas at Effelsberg. A movie of these all-sky images has been compiled (Figure 1b). The movie starts at a frequency of 35 MHz, and each subsequent frame is about 4 MHz higher in frequency, through 190 MHz. The resolution of the Effelsberg LOFAR telescope changes with frequency. At 35 MHz the resolution is about 10 degrees, at 110 MHz it is about 3.4 degrees, and at 190 MHz it is about 1.9 degrees. This change in resolution can be seen by the apparent size of the two bright sources Cygnus A and Cassiopeia A as the frequency changes.

Scientists at MPIfR and other institutions around Europe will use measurements such as these to study the large-sky structure of the interstellar matter of our Milky Way galaxy. The low frequencies observed by LOFAR are ideal for studying the low energy cosmic ray electrons in the Milky Way, which trace out magnetic field structures through synchrotron emission. Other large-scale features such as supernova remnants, star-formation regions, and even some other nearby galaxies will need similar measurements from individual LOFAR telescopes to provide accurate information on the large-scale emission in these objects. "We plan to search for radio transients using the all-sky imaging capabilities of the LOFAR telescopes", says Michael Kramer, director at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. "The detection of rapidly variable sources using LOFAR could lead to exciting discoveries of new types of astronomical objects, similar to the discoveries of pulsars and gamma-ray bursts in the past decades."

"The low-frequency sky is now truly open in Effelsberg and we have the capability at the observatory to observe in a wide frequency range from 10 MHz to 100 GHz", says Anton Zensus, also director at MPIfR. "Thus we can cover four orders of magnitude in the electromagnetic spectrum."


LOFAR, the LOw Frequency ARray, was designed and developed by ASTRON (Netherlands Institute for Radio Astronomy) with 36 stations centered on Exloo in the northeast of The Netherlands. It is now an international project with stations being built in Germany, France, the UK and Sweden connected to the central data processing facilities in Groningen (NL) and the ASTRON operations center in Dwingeloo (NL). The first international LOFAR station (IS-DE1) was completed on the area of the Effelsberg radio observatory next to the 100-m radio telescope of the Max-Planck-Institut fur Radioastronomie (MPIfR).

Thursday, December 17, 2009

Avatar's Moon Pandora Could Be Real

This artist's conception shows a hypothetical gas giant planet with an Earth-like moon similar to the moon Pandora in the movie Avatar. New research shows that, if we find such an "exomoon" in the habitable zone of a nearby star, the James Webb Space Telescope will be able to study its atmosphere and detect key gases like carbon dioxide, oxygen, and water. The key is to find a planet that transits its star, and then find a moon orbiting that planet more than one stellar radius away, so that the moon can be studied independently of the planet. Moreover, an alien moon orbiting the gas giant planet of a red dwarf star may be more likely to be habitable than tidally locked Earth-sized planets or super-Earths. Credit: David A. Aguilar, CfA

Cambridge, MA - In the new blockbuster Avatar, humans visit the habitable - and inhabited - alien moon called Pandora. Life-bearing moons like Pandora or the Star Wars forest moon of Endor are a staple of science fiction. With NASA's Kepler mission showing the potential to detect Earth-sized objects, habitable moons may soon become science fact. If we find them nearby, a new paper by Smithsonian astronomer Lisa Kaltenegger shows that the James Webb Space Telescope (JWST) will be able to study their atmospheres and detect key gases like carbon dioxide, oxygen, and water vapor.

"If Pandora existed, we potentially could detect it and study its atmosphere in the next decade," said Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics (CfA).

So far, planet searches have spotted hundreds of Jupiter-sized objects in a range of orbits. Gas giants, while easier to detect, could not serve as homes for life as we know it. However, scientists have speculated whether a rocky moon orbiting a gas giant could be life-friendly, if that planet orbited within the star's habitable zone (the region warm enough for liquid water to exist).

"All of the gas giant planets in our solar system have rocky and icy moons," said Kaltenegger. "That raises the possibility that alien Jupiters will also have moons. Some of those may be Earth-sized and able to hold onto an atmosphere."

Kepler looks for planets that cross in front of their host stars, which creates a mini-eclipse and dims the star by a small but detectable amount. Such a transit lasts only hours and requires exact alignment of star and planet along our line of sight. Kepler will examine thousands of stars to find a few with transiting worlds.

Once they have found an alien Jupiter, astronomers can look for orbiting moons, or exomoons. A moon's gravity would tug on the planet and either speed or slow its transit, depending on whether the moon leads or trails the planet. The resulting transit duration variations would indicate the moon's existence.

Once a moon is found, the next obvious question would be: Does it have an atmosphere? If it does, those gases will absorb a fraction of the star's light during the transit, leaving a tiny, telltale fingerprint to the atmosphere's composition.

The signal is strongest for large worlds with hot, puffy atmospheres, but an Earth-sized moon could be studied if conditions are just right. For example, the separation of moon and planet needs to be large enough that we could catch just the moon in transit, while its planet is off to one side of the star.

Kaltenegger calculated what conditions are best for examining the atmospheres of alien moons. She found that alpha Centauri A, the system featured in Avatar, would be an excellent target.

"Alpha Centauri A is a bright, nearby star very similar to our Sun, so it gives us a strong signal" Kaltenegger explained. "You would only need a handful of transits to find water, oxygen, carbon dioxide, and methane on an Earth-like moon such as Pandora."

"If the Avatar movie is right in its vision, we could characterize that moon with JWST in the near future," she added.

While alpha Centauri A offers tantalizing possibilities, small, dim, red dwarf stars are better targets in the hunt for habitable planets or moons. The habitable zone for a red dwarf is closer to the star, which increases the probability of a transit.

Astronomers have debated whether tidal locking could be a problem for red dwarfs. A planet close enough to be in the habitable zone would also be close enough for the star's gravity to slow it until one side always faces the star. (The same process keeps one side of the Moon always facing Earth.) One side of the planet then would be baked in constant sunlight, while the other side would freeze in constant darkness.

An exomoon in the habitable zone wouldn't face this dilemma. The moon would be tidally locked to its planet, not to the star, and therefore would have regular day-night cycles just like Earth. Its atmosphere would moderate temperatures, and plant life would have a source of energy moon-wide.

"Alien moons orbiting gas giant planets may be more likely to be habitable than tidally locked Earth-sized planets or super-Earths," said Kaltenegger. "We should certainly keep them in mind as we work toward the ultimate goal of finding alien life."Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Lisa Kaltenegger

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics

G292.0+1.8 & Kepler's Supernova Remnant: Supernova Explosions Stay In Shape

Credit NASA/CXC/UCSC/L. Lopez et al.

These two supernova remnants are part of a new study from NASA's Chandra X-ray Observatory that shows how the shape of the remnant is connected to the way the progenitor star exploded. In this study, a team of researchers examined the shapes of 17 supernova remnants in both the Milky Way galaxy and a neighbor galaxy, the Large Magellanic Cloud.

The results revealed that one category of supernova explosion, known as "Type Ia," generated a very symmetric, circular remnant. This type of supernova is thought to be caused by a thermonuclear explosion of a white dwarf, and is often used by astronomers as a "standard candle" for measuring cosmic distances. The image in the right panel, the so-called Kepler supernova remnant, represents this type of supernova.

On the other hand, remnants tied to the "core collapse" family of supernova
explosions were distinctly more asymmetric, which is seen in the morphology of the G292.0+1.8 remnant (left). The research team measured asymmetry in two ways: how spherical or elliptical the supernova remnant was and how much one side of the remnant mirrors its opposite side. In G292, the asymmetry is subtle but can be seen in elongated features defined by the brightest emission (colored white).

Out of the 17 supernova remnants sampled, ten were independently classified as the core-collapse variety, while the remaining seven of them were classified as Type Ia. One of these, a remnant known as SNR 0548-70.4, was a bit of an "oddball". This one was considered a Type Ia based on its chemical abundances, but has the asymmetry of a core-collapse remnant.

Fast Facts for G292.0+1.8:

Scale: 11.5 arcmin across.
Supernovas & Supernova Remnants
Coordinates: (J2000) RA 11h 24m 36.00s | Dec -59° 16' 00.00"
Constellation: Centaurus
Observation Dates: 6 observations between September - October 2006
Observation Time: 144 hours
Obs. IDs: 6677-6680, 8221, and 8447
Color Code: Energy: Red (low energy); Orange (medium-low energy); Green (medium energy); Blue (high energy)

Instrument: ACIS
References Lopez, L. et al, 2009 706 L106-L109; Park, S. et al, 2007, ApJ, 670 L121-L124
Distance Estimate: 20,000 light years

Fast Facts for Kepler's Supernova Remnant:

Scale: 5 arcmin across.
Category: Supernovas & Supernova Remnants
Coordinates: (J2000) RA 17h 30m 40.80s | Dec -21° 29' 11.00"
Constellation: Ophiuchus
Observation Dates: 6 observations between April - August 2006
Observation Time 208 hours
Obs. IDs 6714-18, 7366
Color Code: Energy: Red (low energy);Yellow/Green (medium energy); Blue (high energy)
Also Known As: SN 1604, G004.5+06.8, V 843 Ophiuchi
References: Lopez, L. et al, 2009 706 L106-L109; Park, S. et al, 2007, ApJ, 670 L121-L124
Distance Estimate: 13,000 light years

Astronomers Find World with Thick, Inhospitable Atmosphere and an Icy Heart

GJ1214b (Artist’s impression)

The star GJ1214


GJ1214b (Artist’s impression)

Zoom in on the star GJ1214 (annotated)

Zoom in on the star GJ1214

Astronomers have discovered the second super-Earth exoplanet [1] for which they have determined the mass and radius, giving vital clues about its structure. It is also the first super-Earth where an atmosphere has been found. The exoplanet, orbiting a small star only 40 light-years away from us, opens up dramatic new perspectives in the quest for habitable worlds. The planet, GJ1214b, has a mass about six times that of Earth and its interior is likely to be mostly made of water ice. Its surface appears to be fairly hot and the planet is surrounded by a thick atmosphere, which makes it inhospitable for life as we know it on Earth.

In this week’s issue of Nature, astronomers announce the discovery of a planet around the nearby, low-mass star GJ1214 [2]. It is the second time a transiting super-Earth has been detected, after the recent discovery of the planet Corot-7b [3]. A transit occurs when the planet's orbit is aligned so that we see it crossing the face of its parent star. The newly discovered planet has a mass about six times that of our terrestrial home and 2.7 times its radius, falling in size between the Earth and the ice giants of the Solar System, Uranus and Neptune.

Although the mass of GJ1214b is similar to that of Corot-7b, its radius is much larger, suggesting that the composition of the two planets must be quite different. While Corot-7b probably has a rocky core and may be covered with lava, astronomers believe that three quarters of GJ1214b is composed of water ice, the rest being made of silicon and iron.

GJ1214b orbits its star once every 38 hours at a distance of only two million kilometres — 70 times closer to its star than the Earth is to the Sun. “Being so close to its host star, the planet must have a surface temperature of about 200 degrees Celsius, too hot for water to be liquid,” says David Charbonneau, lead author of the paper reporting the discovery.

When the astronomers compared the measured radius of GJ1214b with theoretical models of planets, they found that the observed radius exceeds the models’ predictions: there is something more than the planet’s solid surface blocking the star’s light — a surrounding atmosphere, 200 km thick. “This atmosphere is much thicker than that of the Earth, so the high pressure and absence of light would rule out life as we know it,” says Charbonneau, “but these conditions are still very interesting, as they could allow for some complex chemistry to take place.”

“Because the planet is too hot to have kept an atmosphere for long, GJ1214b represents the first opportunity to study a newly formed atmosphere enshrouding a world orbiting another star,” adds team member Xavier Bonfils. “Because the planet is so close to us, it will be possible to study its atmosphere even with current facilities.”

The planet was first discovered as a transiting object within the MEarth project, which follows about 2000 low-mass stars to look for transits by exoplanets [4]. To confirm the planetary nature of GJ1214b and to obtain its mass (using the so-called Doppler method), the astronomers needed the full precision of the HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla. An instrument with unrivalled stability and great precision, HARPS is the world’s most successful hunter for small exoplanets.

“This is the second super-Earth exoplanet for which the mass and radius could be obtained, allowing us to determine the density and to infer the inner structure,” adds co-author Stephane Udry. “In both cases, data from HARPS was essential to characterise the planet.”

“The differences in composition between these two planets are relevant to the quest for habitable worlds,” concludes Charbonneau. If super-Earth planets in general are surrounded by an atmosphere similar to that of GJ1214b, they may well be inhospitable to the development of life as we know it on our own planet.

[1] A super-Earth is defined as a planet between one and ten times the mass of the Earth. An exoplanet is a planet orbiting a star other than the Sun.

[2] The star GJ1214 is five times smaller than our Sun and intrinsically three hundred times less bright.

[3] Corot-7b is the smallest and fastest-orbiting exoplanet known and has a density quite similar to the Earth's, suggesting a solid, rocky world. Discovered by the CoRoT satellite as a transiting object, its true nature was revealed by HARPS (ESO 33/09).

[4] The MEarth project uses an armada of eight small telescopes each with a diameter of 40 cm, located on top of Mount Hopkins, Arizona, USA. MEarth looks for stars that change brightness. The goal is to find a planet that crosses in front of, or transits, its star. During such a mini-eclipse, the planet blocks a small portion of the star’s light, making it dimmer. NASA’s Kepler mission also uses transits to look for Earth-sized planets orbiting Sun-like stars. However, such systems dim by only one part in ten thousand. The higher precision required to detect the drop means that such worlds can only be found from space. In contrast, a super-Earth transiting a small, red dwarf star yields a greater proportional decrease in brightness and a stronger signal that is detectable from the ground.

More information

This research was presented in a paper appearing this week in Nature (“A Super-Earth Transiting a Nearby Low-Mass Star”, by David Charbonneau et al.).

The team is composed of David Charbonneau, Zachory K. Berta, Jonathan Irwin, Christopher J. Burke, Philip Nutzman, Lars Buchhave, David W. Latham, Ruth A. Murray-Clay, Matthew J. Holman, and Emilio E. Falco (Harvard-Smithsonian Center for Astrophysics, Cambridge, USA), Christophe Lovis, Stephane Udry, Didier Queloz, Francesco Pepe, and Michel Mayor (Observatoire de l’Université de Genève, Switzerland), Xavier Bonfils, Xavier Delfosse, and Thierry Forveille (University Joseph Fourier — Grenoble 1/CNRS, LOAG, Grenoble, France), and Joshua N. Winn (Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, USA).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. 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 VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Science paper
More info: Exoplanet Media Kit


Stéphane Udry
Geneva University, Switzerland
Geneva, Switzerland
Tel: +41 22 379 2467

Xavier Bonfils
Université Joseph Fourier - Grenoble 1 / CNRS, Laboratoire d'Astrophysique de Grenoble (LAOG), France
Tel: +33 47 65 14 215

David Charbonneau
Harvard-Smithsonian Center for Astrophysics
Cambridge, USA
Tel: +1 617 496 6515

Wednesday, December 16, 2009

Hubble Finds Smallest Kuiper Belt Object Ever Seen

This is an artist's impression of a small Kuiper Belt Object (KBO) occulting a star. NASA's Hubble Space Telescope recorded this brief event and allowed astronomers to determine that the KBO was only one-half of a mile across, setting a new record for the smallest object ever seen in the Kuiper Belt. Credit: NASA, ESA, and G. Bacon (STScI)

Credit: NASA, ESA, and G. Bacon (STScI)

NASA's Hubble Space Telescope has discovered the smallest object ever seen in visible light in the Kuiper Belt, a vast ring of icy debris that is encircling the outer rim of the solar system just beyond Neptune.

The needle-in-a-haystack object found by Hubble is only 3,200 feet across and a whopping 4.2 billion miles away. The smallest Kuiper Belt Object (KBO) seen previously in reflected light is roughly 30 miles across, or 50 times larger.

This is the first observational evidence for a population of comet-sized bodies in the Kuiper Belt that are being ground down through collisions. The Kuiper Belt is therefore collisionally evolving, meaning that the region's icy content has been modified over the past 4.5 billion years.

The object detected by Hubble is so faint — at 35th magnitude — it is 100 times dimmer than what Hubble can see directly.

So then how did the space telescope uncover such a small body?

In a paper published in the December 17th issue of the journal Nature, Hilke Schlichting of the California Institute of Technology in Pasadena, Calif., and her collaborators are reporting that the telltale signature of the small vagabond was extracted from Hubble's pointing data, not by direct imaging.

Hubble has three optical instruments called Fine Guidance Sensors (FGS). The FGSs provide high-precision navigational information to the space observatory's attitude control systems by looking at select guide stars for pointing. The sensors exploit the wavelike nature of light to make precise measurement of the location of stars.

Schlichting and her co-investigators determined that the FGS instruments are so good that they can see the effects of a small object passing in front of a star. This would cause a brief occultation and diffraction signature in the FGS data as the light from the background guide star was bent around the intervening foreground KBO.

They selected 4.5 years of FGS observations for analysis. Hubble spent a total of 12,000 hours during this period looking along a strip of sky within 20 degrees of the solar system's ecliptic plane, where the majority of KBOs should dwell. The team analyzed the FGS observations of 50,000 guide stars in total.

Scouring the huge database, Schlichting and her team found a single 0.3-second-long occultation event. This was only possible because the FGS instruments sample changes in starlight 40 times a second. The duration of the occultation was short largely because of the Earth's orbital motion around the Sun.

They assumed the KBO was in a circular orbit and inclined 14 degrees to the ecliptic. The KBO's distance was estimated from the duration of the occultation, and the amount of dimming was used to calculate the size of the object. "I was very thrilled to find this in the data," says Schlichting.

Hubble observations of nearby stars show that a number of them have Kuiper Belt–like disks of icy debris encircling them. These disks are the remnants of planetary formation. The prediction is that over billions of years the debris should collide, grinding the KBO-type objects down to ever smaller pieces that were not part of the original Kuiper Belt population.

The finding is a powerful illustration of the capability of archived Hubble data to produce important new discoveries. In an effort to uncover additional small KBOs, the team plans to analyze the remaining FGS data for nearly the full duration of Hubble operations since its launch in 1990.


Ray Villard
Space Telescope Science Institute, Baltimore, Md.

Hilke Schlichting
California Institute of Technology, Pasadena, Calif.

Inside the dark heart of the Eagle

Credits: ESA and the SPIRE & PACS consortia,
P. André (CEA Saclay) for the Gould’s Belt Key Programme Consortia
HI-RES JPEG (Size: 610 kb)

Herschel has peered inside an unseen stellar nursery and revealed surprising amounts of activity. Some 700 newly-forming stars are estimated to be crowded into filaments of dust stretching through the image. The image is the first new release of ‘OSHI’, ESA’s Online Showcase of Herschel Images.

This image shows a dark cloud 1000 light-years away in the constellation Aquila, the Eagle. It covers an area 65 light-years across and is so shrouded in dust that no previous infrared satellite has been able to see into it. Now, thanks to Herschel’s superior sensitivity at the longest wavelengths of the infrared, astronomers have their first picture of the interior of this cloud.

It was taken on 24 October using two of Herschel’s instruments: the Photodetector Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE). The two bright regions are areas where large newborn stars are causing hydrogen gas to shine.

The new OSHI website that goes live today will become the library of Herschel’s best images. Stunning views of the infrared sky will be made available as the mission progresses. Each will be captioned in a way to make them accessible to media representatives, educators and the public.

Embedded within the dusty filaments in the Aquila image are 700 condensations of dust and gas that will eventually become stars. Astronomers estimate that about 100 are protostars, celestial objects in the final stages of formation. Each one just needs to ignite nuclear fusion in its core to become a true star. The other 600 objects are insufficiently developed to be considered protostars, but these too will eventually become another generation of stars.

This cloud is part of Gould’s Belt, a giant ring of stars that circles the night sky – the Solar System just happens to lie near the centre of the belt. The first to notice this unexpected alignment, in the mid-19th century, was England’s John Herschel, the son of William, after whom ESA’s Herschel telescope is named. But it was Boston-born Benjamin Gould who brought the ring to wider attention in 1874.

Gould’s Belt supplies bright stars to many constellations such as Orion, Scorpius and Crux, and conveniently provides nearby star-forming locations for astronomers to study. Observing these stellar nurseries is a key programme for Herschel, which aims to uncover the demographics of star formation and its origin, or in other words, the quantities of stars that can form and the range of masses that such newborn stars can possess. Apart from this region of Aquila, Herschel will target 14 other star-forming regions as part of the Gould’s Belt Key Programme.

Notes for editors:

The scientific rights of these Herschel observations are owned by the consortium of the Gould Belt Key Programme, led by P. André (CEA Saclay). A total of 15 nearby star-forming regions such as Aquila will be studied as part of this Programme.

Black Holes in Star Clusters stir up Time and Space

An artist's representation of the burst of gravitational waves resulting from the collision of a colliding pair of black holes. Credit: LIGO Scientific Collaboration (LSC) / NASA.

Within a decade scientists could be able to detect the merger of tens of pairs of black holes every year, according to a team of astronomers at the University of Bonn’s Argelander-Institut fuer Astronomie, who publish their findings in a paper in Monthly Notices of the Royal Astronomical Society. By modelling the behaviour of stars in clusters, the Bonn team find that they are ideal environments for black holes to coalesce. These merger events produce ripples in time and space (gravitational waves) that could be detected by instruments from as early as 2015.

Clusters of stars are found throughout our own and other galaxies and most stars are thought to have formed in them. The smallest looser ‘open clusters’ have only a few stellar members, whilst the largest tightly bound ‘globular clusters’ have as many as several million stars. The highest mass stars in clusters use up their hydrogen fuel relatively quickly (in just a few million years). The cores of these stars collapse, leading to a violent supernova explosion where the outer layers of the star are expelled into space. The explosion leaves behind a stellar remnant with gravitational field so strong that not even light can escape – a black hole.

When stars are as close together as they are in clusters, then although still rare events, the likelihood of collisions and mergers between stars of all types, including black holes, is much higher. The black holes sink to the centre of the cluster, where a core that is completely made of up of black holes forms. In the core, the black holes experience a range of interactions, sometimes forming binary pairs and sometimes being ejected from the cluster completely.

Now Dr Sambaran Banerjee, Alexander von Humboldt postdoctoral fellow, has worked with his University of Bonn colleagues Dr Holger Baumgardt and Professor Pavel Kroupa to develop the first self-consistent simulation of the movement of black holes in star clusters.

The scientists assembled their own star clusters on a high-performance supercomputer, and then calculated how they would evolve by tracing the motion of each and every star and black hole within them.

According to a key prediction of Einstein’s General Theory of Relativity, black hole binaries stir the space-time around them, generating waves that propagate away like ripples on the surface of a lake. These waves of curvature in space-time are known as gravitational waves and will temporarily distort any object they pass through. But to date no-one has succeeded in detecting them.

In the cores of stars clusters, black hole binaries are sufficiently tightly bound to be significant sources of gravitational waves. If the black holes in a binary system merge, then an even stronger pulse of gravitational waves radiates away from the system.

Based on the new results, the next generation of gravitational wave observatories like the Advanced Laser Interferometer Gravitational-wave Observatory (Advanced LIGO) could detect tens of these events each year, out to a distance of almost 5000 million light years (for comparison the well known Andromeda Galaxy is just 2.5 million light years away).

Advanced LIGO will be up and running by 2015 and if the Bonn team are right, from then on we can look forward to a new era of gravitational wave astronomy.

Sambaran comments, “Physicists have looked for gravitational waves for more than half a century. But up to now they have proved elusive. If we are right then not only will gravitational waves be found so that General Relativity passes a key test but astronomers will soon have a completely new way to study the Universe. It seems fitting that almost exactly 100 years after Einstein published his theory, scientists should be able to use this exotic phenomenon to watch some of the most exotic events in the cosmos.”


Dr Sambaran Banerjee
Alexander von Humboldt postdoctoral fellow
Argelander Institut fuer Astronomie
University of Bonn
Tel: +49 (0) 228 73 3649
Mob: +49 (0) 1577 8030745

Dr Holger Baumgardt
Argelander Institut fuer Astronomie
University of Bonn
Tel: +49 (0) 228 73 6790

Professor Pavel Kroupa
University of Bonn
Argelander Institut fuer Astronomie
University of Bonn
Tel: +49 (0) 228 73 6140
Mob: +49 (0)177 9566127


An aerial view of the Laser Interferometer Gravitational wave Observatory (LIGO at Hanford in the United States. The two interferometer arms are at right angles and each arm is about 4 km long. By 2015, the present LIGO will be upgraded to Advanced LIGO (also known as LIGO 2) which will be about 10 times more sensitive than the present instrument. See for details. Credit: LIGO Scientific Collaboration (LSC)

An artist's representation of the burst of gravitational waves resulting from the collision of a colliding pair of black holes. Credit: LIGO Scientific Collaboration (LSC) / NASA


A movie depicting the movement of black holes (black dots) and stars (represented by green asterisks) in a star cluster. The black holes, formed when the most massive stars exhaust their hydrogen fuel, are initially spread over a wide region of the cluster. Then, as they are more massive than the rest of the stars, they begin to sink and concentrate within a small region at the cluster centre. When this central black hole cluster becomes dense enough, gravitational waves are emitted due to the mergers of black holes in binary systems.

The propagation of gravitational waves from these mergers is depicted by outgoing circles and the wobbling of the whole cluster, representing space-time distortion, while the cluster stars are unmoving (since the gravitational waves, travelling at the speed of light, cross the cluster before the stars can move significantly). Note that the implied propagation speed and the wobbling are not to scale.

The time line in millions of years is given on the top axis and the passage of time is denoted by the movement of the white marker. In this model the first binary black hole forms in the cluster and begins radiating gravitational waves after about 600 million years. Credit: University of Bonn


Quicktime (for a slower Internet connection):



A preprint of the paper, which will appear in Monthly Notices of the Royal Astronomical Society, is available at


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Royal Astronomical Society Press Release
Ref: RAS PN 09/65
15th December 2009

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