Globular clusters still developing

NGC 6121
NGC 6121, shown here in this Chandra image, is located in the Milky Way and is part of the new study that shows globular clusters might be surprisingly less mature in their development than previously thought.
NASA/CXC/Northwestern Univ/J.Fregau
April 28, 2008
Some of the oldest objects in the universe may still have a long way to go, according to a new study using NASA’s Chandra X-ray Observatory. These new results indicate that globular clusters might be surprisingly less mature in their development than previously thought.

Globular clusters are incredibly dense bunches of up to millions of stars that are found in the outskirts of galaxies, including the Milky Way. They are among the oldest known objects in the universe, with most estimates of their ages ranging from 9 to 13 billions of years old. Understanding the nature of globular clusters is very important as they are thought to contain some of the first stars to form in a galaxy.

“For many years, globular clusters have been used as wonderful natural laboratories to study the evolution and interaction of stars,” says John Fregeau of Northwestern University, who conducted the study. “So, it’s exciting to discover something that may be new and fundamental about the way they evolve.”

Conventional wisdom is that globular clusters pass through three phases of development, corresponding to adolescence, middle age, and old age. For years, it’s been thought that most globular clusters are middle-aged with a few being toward the end of the lives. However, Chandra data along with theoretical work suggest this may not be the case.

When single and double stars interact in the crowded centers of globular clusters, new double stars can form that transfer mass and give off X-rays. Since such double stars are expected to mostly be formed in the middle of a globular cluster’s life and then lost in old age, the relative number of X-ray sources gives clues about the stage of evolution the cluster is in.

NGC 6397
NGC 6397 is also part of the new study.
NASA/CXC/Northwestern Univ/J.Fregau
A new study by Fregeau of 13 globular clusters in the Milky Way shows that three of them have unusually large number of X-ray sources, or X-ray binaries, suggesting the clusters are middle-aged. Previously, these globular clusters had been classified as being in old age because they had very tight concentrations of stars in their centers, another litmus test of age used by astronomers.

The implication is that most globular clusters, including the other ten studied by Fregeau, are not in middle age, as previously thought, but are actually in adolescence.

“It’s remarkable that these objects, which are thought to be some of the oldest in the universe, may really be very immature,” said Fregeau whose paper appears in The Astrophysical Journal. “This would represent a major change in thinking about the current evolutionary status of globular clusters.”

If confirmed, this result would help reconcile other observations with recent theoretical work that suggest the tightness of the central concentration of stars in the most evolved globular clusters is consistent with them being in middle age, not old age. Other theoretical studies have suggested it can take longer than the age of the Universe for globular clusters to reach old age.

Besides improving the understanding of the basic evolution of globular clusters, this result has implications for understanding stellar interactions in dense environments. It also removes the need for exotic mechanisms &#8212 some involving black holes &#8212 that were thought to be needed to prevent the many middle-aged clusters from collapsing into old age.

“Some exotic scenarios, including some of my own, have been invoked to try to make sense of the observations and save the old theory,” says Fregeau. “If this result holds up, we don’t have to worry about the exotic scenarios any more.”

Adolescence for a globular cluster is used here to describe a phase after cluster formation when the stars near the center of the cluster collapse inwards. Middle age refers to a phase when the interactions of double stars and the formation of X-ray sources near the center of the cluster prevents it from further collapse. Finally, old age describes when binaries in the center of the cluster run out and the center of the cluster collapses inwards.

June 2008: Milky Way Galaxy on collision course

WAUKESHA, WI — Five billion years from now, the Milky Way Galaxy will collide with its neighbor, the Andromeda Galaxy. The combined galaxies — “Milkomeda” — will radically transform the arrangement of stars in Earth’s night sky.

In “Our galaxy’s date with destruction,” authors Abraham Loeb and T.J. Cox use computer simulations to tell the scientists about the coming merger and how it will change our perspective of the universe.

Abraham Loeb is a professor of astronomy at Harvard University, a visiting professor at the Weizmann Institute of Science, and the director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics.

T.J. Cox is a postdoctoral fellow at the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics.

The June 2008 Astronomy will be available on newsstands May 6.

“How we junked up outer space”
Derelict spacecraft, stray nuts and bolts, and millions of even tinier fragments litter the space around Earth. “How we junked up outer space,” by Astronomy Senior Editor Richard Talcott, discusses how these minute fragments could cause big problems. The amount of junk circling Earth totals thousands of tons. And it poses a significant threat to all functioning satellites, as well as the space shuttle and the International Space Station.

The odds of someone on Earth being injured by a falling piece of debris? About 1 in a trillion. That’s roughly a million times less than the risk of being struck by lightning in the United States.

“Do cosmic flashes reveal secrets of the infant universe?”
Gamma-ray bursts are the brightest flashes in our sky since the Big Bang. Researchers use these luminous explosions to examine the early universe. “Do cosmic flashes reveal secrets of the infant universe?,” by Steve Nadis, describes how astronomers are studying the stars and galaxies these bursts inhabit to learn about the beginning of the universe.

“All about the Veil Nebula”
Located next to the constellation Cygnus the Swan, the Veil Nebula is one of the most challenging and fascinating targets for observers. “All about the Veil Nebula,” by Raymond Shubinski, goes deep inside this celestial object and details how to find it with your scope.

Raymond Shubinski, an Astronomy contributing editor, is director of the Atomic Testing Museum in Las Vegas, an astronomy historian, and a dedicated deep-sky observer.

June night sky events visible without optical aid

  • June 7 — The waxing crescent Moon joins Mars, Saturn, and the bright star Regulus at 10p local time.
  • June 26-27 — Potential outburst of Boötid meteor shower


Also in the June 2008 Astronomy

  • “Another Earth” — Astronomy columnist Bob Berman wonders what it would be like to find a planet like our own.
  • “Master the art of wide-field imaging” — Imelda B. Joson teaches you how to become an astrophotography expert.
  • “Will you see the stars tonight?” — Learn how to create your own astronomy weather forecast.
  • “The sky this month” — Exclusive pullout star charts will guide you through June’s night sky.
  • The June issue of Astronomy also includes Astro News, Glenn Chaple’s Observing Basics, Phil Harrington’s Binocular Universe, Stephen James O’Meara’s Secret Sky, New Products, and Reader Gallery.

Astronomy‘s mission:
Astronomy promotes the science and hobby of astronomy through high-quality publications that engage, inform, entertain, and inspire. More resources from Astronomy.com:

Polarized aurora

Auroral oval over the U.S.
A bright arc of aurora cuts across the Great Lakes region and illuminates clouds in eastern Canada.
Air Force Weather Agency
April 25, 2008
An international team of scientists has detected that some of the glow of Earth’s aurora is polarized, an unexpected state for such emissions. Measurements of this newfound polarization in the Northern Lights may provide scientists with fresh insights into the composition of Earth’s upper atmosphere, the configuration of its magnetic field, and the energies of particles from the Sun, the researchers say.

If observed on other planets, the phenomenon might also give clues to the shape of the Sun’s magnetic field as it curls around other bodies in the solar system.

When a beam of light is polarized, its electromagnetic waves share a common orientation, say, aligned vertically, or at some other angle. Until now, scientists thought that light from energized atoms and molecules in planetary upper atmospheres could not be polarized. The reason is simple: In spite of the low number of particles at the altitudes concerned (above 60 miles or 100 kilometers), there are still numerous collisions between molecules and gas atoms. Those collisions depolarize the emitted light.

Fifty years ago, an Australian researcher, Robert Duncan, claimed to observe what looked like polarization of auroral light, but other scientists found that single observation unconvincing.

To revisit the question, Jean Lilensten of the Laboratory of Planetology of Grenoble, France, and his colleagues studied auroral light with a custom-made telescope during the winters of 2006-2007 and 2007-2008. They made their observations from Svalbard Island, Norway, which is in the polar region, at a latitude of 79° north.

At the north and south magnetic poles, many charged particles in the solar wind &#8212 a flow of electrically charged matter from the Sun &#8212 are captured by the planet’s field and forced to plunge into the atmosphere. The particles strike atmospheric gases, causing light emissions.

Lilensten and his colleagues observed weak polarization of a red glow that radiates at an altitude of 140 miles (220 kilometers). The glow results from electrons hitting oxygen atoms. The scientists had suspected that such light might be polarized because Earth’s magnetic field at high latitudes funnels the electrons, aligning the angles at which they penetrate the atmosphere.

The finding of auroral polarization “opens a new field in planetology,” says Lilensten, who is the lead author of the study. He and his colleagues reported their results on April 19 in Geophysical Research Letters, a publication of the American Geophysical Union (AGU).

Fluctuations in the polarization measurements can reveal the energy of the particles coming from the Sun when they enter Earth’s atmosphere, Lilensten notes. The intensity of the polarization gives clues to the composition of the upper atmosphere, particularly with regard to atomic oxygen.

Because polarization is strongest when the telescope points perpendicularly to the magnetic field lines, the measurements also provide a way to determine magnetic field configurations, Lilensten adds. That could prove especially useful as astronomers train their telescopes on other planetary atmospheres. If polarized emissions are observed there as well, the measurements may enable scientists to understand how the Sun’s magnetic field is distorted by obstacles such as the planets Venus and Mars, which lack intrinsic magnetic fields.

Preparing to launch

herschel
A picture of the Herschel telescope resting on the cryostat, taken on April 16.
ESA
April 24, 2008
The mirror of the Herschel telescope has now been assembled with the payload and service module, completing the spacecraft structure &#8212 an important milestone in the days following through to launch.

The sunshield and solar arrays were assembled with the cryostat and service module on April 11. The telescope was assembled on April 16. The spacecraft will be subjected to several mechanical tests over the next few weeks.

herschel
The mirror of the Herschel telescope, with its protective cover, ready to be lifted for installation on to the cryostat.
ESA
The telescope mirror of the Herschel infrared observatory is a 3.5-meter diameter technological marvel. It is made from 12 silicon-carbide petals brazed together to form a single structure and coated with a layer of reflective aluminum, forming a remarkably lightweight mirror.

The fully-assembled telescope, which includes the primary mirror, the secondary mirror and its support structure, is a feathery 320 kilograms; remarkably low for such a sturdy structure capable of withstanding high launch loads and functioning precisely in the harsh environment of space.

herschel
The Herschel spacecraft’s sunshield being assembled with the solar array.
ESA
This powerful telescope will allow scientists to look deep into space, at long infrared wavelengths. Herschel’s spectral coverage, which ranges from far-infrared to sub-millimeter wavelengths, will be made available for space-based observations for the first time.

Herschel will make it possible to observe and study relatively cool objects everywhere in the universe, from our own back yard to distant galaxies, teaching us much more about the birth and evolution of stars and galaxies.

Galaxies gone wild

Hubble captures galaxies
Interacting galaxies are found throughout the universe, sometimes as dramatic collisions that trigger bursts of star formation, on other occasions as stealthy mergers that result in new galaxies. This collection shows 12 of the most dynamic images of the overall 59.
NASA, ESA, Hubble Heritage Team (STScI/AURA)-ESA and A. Evans/NRAO, K. Noll, and J. Westphal
April 24, 2008
Fifty-nine new images of colliding galaxies make up the largest collection of Hubble images ever released together. As this astonishing Hubble atlas of interacting galaxies illustrates, galaxy collisions produce a remarkable variety of intricate structures.

Interacting galaxies are found throughout the universe, sometimes as dramatic collisions that trigger bursts of star formation, on other occasions as stealthy mergers that result in new galaxies. A series of 59 new images of colliding galaxies has been released from the several terabytes of archived raw images from the NASA/ESA Hubble Space Telescope to mark the 18th anniversary of the telescope’s launch. This is the largest collection of Hubble images ever released to the public simultaneously.

Galaxy mergers, which were more common in the early universe than they are today, are thought to be one of the main driving forces for cosmic evolution, turning on quasars, sparking frenetic star births and explosive stellar deaths. Even apparently isolated galaxies will show signs in their internal structure that they have experienced one or more mergers in their past. Each of the various merging galaxies in this series of images is a snapshot of a different instant in the long interaction process.

Our own Milky Way contains the debris of the many smaller galaxies it has encountered and devoured in the past, and it is currently absorbing the Sagittarius dwarf elliptical galaxy. In turn, it looks as if our Milky Way will be subsumed into its giant neighbour, the Andromeda galaxy, resulting in an elliptical galaxy, dubbed “Milkomeda”, the new home for the Earth, the Sun and the rest of the Solar System in about two billion years time. The two galaxies are currently rushing towards each other at approximately 500,000 kilometres per hour.

Cutting-edge observations and sophisticated computer models, such as those pioneered by the two Estonian brothers Alar Toomre and Juri Toomre in the 1970s, demonstrate that galaxy collisions are far more common than previously thought. Interactions are slow stately affairs, despite the typically high relative speeds of the interacting galaxies, taking hundreds of millions of years to complete. The interactions usually follow the same progression, and are driven by the tidal pull of gravity. Actual collisions between stars are rare as so much of a galaxy is simply empty space, but as the gravitational webs linking the stars in each galaxy begin to mesh, strong tidal effects disrupt and distort the old patterns leading to new structures, and finally to a new stable configuration.

The pull of the Moon that produces the twice-daily rise and fall of the Earth’s oceans illustrates the nature of tidal interactions. Tides between galaxies are much more disruptive than oceanic tides for two main reasons. Firstly, stars in galaxies, unlike the matter that makes up the Earth, are bound together only by the force of gravity. Secondly, galaxies can pass much closer to each other, relative to their size, than do the Earth and the Moon. The billions of stars in each interacting galaxy move individually, following the pull of gravity from all the other stars, so the interwoven tidal forces can produce the most intricate and varied effects as galaxies pass close to each other.

Typically the first tentative sign of an interaction will be a bridge of matter as the first gentle tugs of gravity tease out dust and gas from the approaching galaxies (IC 2810). As the outer reaches of the galaxies begin to intermingle, long streamers of gas and dust, known as tidal tails, stretch out and sweep back to wrap around the cores (NGC 6786, UCG 335, NGC 6050). These long, often spectacular, tidal tails are the signature of an interaction and can persist long after the main action is over. As the galaxy cores approach each other their gas and dust clouds are buffeted and accelerated dramatically by the conflicting pull of matter from all directions (NGC 6621, NGC 5256). These forces can result in shockwaves rippling through the interstellar clouds (ARP 148).

Gas and dust are siphoned into the active central regions, fuelling bursts of star formation that appear as characteristic blue knots of young stars (NGC 454). As the clouds of dust build they are heated so that they radiate strongly, becoming some of the brightest (luminous and ultraluminous) infrared objects (APG 220) in the sky.

These objects emit up to several thousand billion times the luminosity of our Sun. They are the most rapidly star-forming galaxies in today’s universe and are linked to the occurrence of quasars. Unlike standard spiral galaxies like the Milky Way, which radiate from stars and hot gas distributed over their entire span of perhaps 100 000 light-years, the energy in luminous and ultraluminous infrared galaxies is primarily generated within their central portion, over an extent of 1000 to 10,000 light-years. This energy emanates both from vigorous star formation processes, which can generate up to a few hundred solar masses of new stars per year (in comparison, the Milky Way generates a few solar masses of new stars per year), and from massive accreting black holes, a million to a billion times the mass of the Sun, in the central region.

Intense star formation regions and high levels of infrared and far- infrared radiation are typical of the most active central period of the interaction and are seen in many of the objects in this release. Other visible signs of an interaction are disruptions to the galaxy nuclei (NGC 3256, NGC 17). This disruption may persist long after the interaction is over, both for the case where a larger galaxy has swallowed a much smaller companion and where two more closely matched galaxies have finally separated.

Most of the 59 new Hubble images are part of a large investigation of luminous and ultraluminous infrared galaxies called the GOALS project (Great Observatories All-sky LIRG Survey). This survey combines observations from Hubble, the NASA Spitzer Space Observatory, the NASA Chandra X-Ray Observatory and NASA Galaxy Explorer. The Hubble observations are led by Professor Aaron S. Evans from the University of Virginia and the National Radio Astronomy Observatory (USA).

A number of the interacting galaxies seen here are included in the The Atlas of Peculiar Galaxies, a remarkable catalog produced by the astronomer Halton Arp in the mid-1960s that built on work by B.A. Vorontsov-Velyaminov from 1959. Arp compiled the catalogue in a pioneering attempt to solve the mystery of the bizarre shapes of galaxies observed by ground-based telescopes. Today, the peculiar structures seen by Arp and others are well understood as the result of complex gravitational interactions.

Astronomy magazine podcast: Eta Aquarids

Eta Aquarids
Tidbits from Halley’s Comet burn up in Earth’s atmosphere this month. The Eta Aquarid meteor shower peaks May 5.
Astronomy: Roen Kelly
April 24, 2008
The 2008 Eta Aquarids peak Monday, May 5. In this week’s show, Senior Editor Francis Reddy provides a background on this meteor shower.

After you listen, e-mail us here and let us know what you think.

If you would like to subscribe to our podcast, click here.

Downloadable File(s)

The birth of Milkomeda

Over the next 5 billion years, our home galaxy will merge with the Andromeda Galaxy. The galaxies will interact through several near-misses, drawing out long tidal “tails” of material that could cause our Sun to jump ship and join Andromeda before the final merger. This video sequence is based on actual computer simulations by astronomers T.J. Cox of the Harvard-Smithsonian Center for Astrophysics.

Inside blazars

blazar
A blazar is a very compact and highly variable energy source associated with a supermassive black hole. It is also characterized by a relativistic jet that is pointing in the general direction of the Earth. Blazars are among the most violent phenomena in the universe and are an important topic in extragalactic astronomy.
Boston University – Cosmovision
April 23, 2008
For the first time, astronomers have observed a blazar in action, substantiating a prevailing theory about how these luminous and energetic galactic cores work.

Two University of Michigan astronomers contributed to the research, which was led by Alan Marscher of the Institute for Astrophysical Research at Boston University. A paper on the observations is published in the April 24 issue of Nature.

Blazars, among the most energetic objects in the universe, are fueled by supermassive black holes at the core of certain giant elliptical galaxies. Periodically, they emit jets of high-energy plasma at almost the speed of light. Competing theoretical models sought to explain how this phenomenon occurs.

One model predicted that the jets were propelled by magnetic fields that were twisted by the gravity of the black hole and the materials falling into it. This is the behavior the astronomers detected.

“What we’ve observed is the mechanism by which the acceleration of relativistic particles in the emanating jets occurs. Knowing that mechanism enhances our understand of the physics that goes into the acceleration process,” says Hugh Aller, a professor in the U-M Department of Astronomy.

Relativistic particles are particles traveling close to the speed of light.

“Often, we’d observe blazars, but they didn’t do anything. It’s been difficult to catch these outbursts when they occur,” he adds.

Scientists from across the globe aimed a variety of telescopes at the blazar BL Lacertae, about 950 million light-years away from Earth. Optical, X-ray and radio telescopes monitored the galaxy at different electromagnetic wavelengths periodically for several years. U-M recorded radio light curves at the Radio Astronomy Observatory at Peach Mountain in Dexter.

“This is the first observational evidence that really fits with the picture that the theoreticians have had,” says Margo Aller, a research scientist and lecturer in the U-M Department of Astronomy. “The reason we have this evidence is a very fine sampling of a large number of instruments, including the Michigan radio telescopes.”

Scientists hope to get a closer look at blazar jets when NASA launches its Gamma-ray Large Area Space Telescope (GLAST) satellite observatory in May.

A growing problem

The numbers get so big so fast, it doesn’t take long for the eyes to glaze over. China’s successful test of an antisatellite weapon in January 2007 created more than 2,000 pieces of orbital debris at least 4 inches (10 centimeters) across — big enough that the U.S. Space Surveillance Network can track them. And the number of smaller shards likely numbers in the hundreds of thousands, if not millions.