Author: Fraser Cain

Image credit: Hubble

Recent evidence seems to indicate that the expansion of the Universe is actually accelerating – some kind of “dark energy” is pushing it apart. And a new redshift survey of galactic clusters seems to support this. Astronomers using data gathered by the Chandra X-Ray Observatory have determined that there is insufficient matter (both regular and dark matter) in various galactic clusters to account for their shape and position, so something else must be having an effect.

The universe appears to be permeated with an invisible force ? dark energy ? that is pushing it apart faster and faster. By conducting redshift surveys of galaxy clusters, astronomers hope to learn more about this mysterious force, and about the structure and geometry of the universe.

“Galaxy clusters consist of thousands of galaxies gravitationally bound into huge structures,” said Joseph Mohr, a professor of astronomy at the University of Illinois. “Because of the expansion of the universe, the clusters appear denser at larger redshifts, when the universe was younger and denser.”

Galaxy cluster surveys that probe the high-redshift universe can potentially provide a wealth of information about the amount and nature of both dark matter and dark energy, said Mohr, who will present the results of an ongoing study of galaxy clusters at a meeting of the American Physical Society, to be held in Albuquerque, N.M., April 20-23.

“Till now, galaxy clusters have only been used to study the dark matter component of the universe,” Mohr said. “We would measure the total mass in a galaxy cluster, and then determine the fraction of mass that was ordinary, baryonic matter.”

Those measurements have shown there is insufficient baryonic and dark matter to account for the geometry of the universe. Astronomers now believe the universe is expanding at ever-increasing speed, and is dominated by a mysterious dark energy that must be doing the pushing.

“The next step is to try to figure out some of the specifics of the dark energy, such as its equation of state,” Mohr said. “By mapping the redshift distribution of galaxy clusters, we should be able to measure the equation of state of dark energy, which would provide some important clues to what it is and how it came to be.”

Mohr is using data collected by NASA’s Chandra X-ray Observatory to study scaling relations ? such as the relationship between mass and luminosity or size ? of galaxy clusters and how they change with redshift. “These scaling relations are expected to evolve with redshift, reflecting the increasing density of the universe at earlier times,” Mohr said.

In particular, Mohr ? in collaboration with John Carlstrom at the University of Chicago and scientists at the University of California and Harvard Smithsonian Center for Astrophysics ? is studying the effect that hot electrons within galaxy clusters have on the cosmic microwave background, the afterglow of the big bang.

Galaxy clusters are filled with dark matter, galaxies and hot gas. Electrons in the gas scatter off the protons and produce X-rays. The emission of X-rays diminishes with higher redshift, because of the larger distances involved.

“There also is a tendency for the electrons to give some of their energy to the photons of the cosmic microwave background, which causes the blackbody spectrum to shift slightly,” Mohr said. “The resulting distortion ? called the Sunyaev-Zeldovich effect ? appears as a cold spot on the cosmic microwave background at certain frequencies. Because this is a distortion in the spectrum, however, it doesn’t dim with distance like X-rays.”

By comparing the X-ray emission and the Sunyaev-Zeldovich effect, Mohr can study even faint, high-redshift galaxy clusters that are currently inaccessible by other means. Such measurements, correlating galaxy cluster redshift distribution, structure and spatial distribution, should determine the equation of state of dark energy and, therefore, help define the essence of dark energy.

“Within the context of our standard structure formation scenario, galaxy surveys provide measurements of the geometry of the universe and the nature of the dark matter and dark energy,” Mohr said. “But, to properly interpret these surveys, we must first understand how the structure of galaxy clusters are changing as we look backward in time.”

Original Source: UIUC News Release

Image credit: Chandra

A new photograph taken by the Chandra X-Ray Observatory shows a close up view of the dynamics of star formation in the Tarantula Nebula (aka 30 Doradus). This region, located 160,000 light years away is one of the most active star forming regions in our local group of galaxies and provides a lot of clues to astronomers. In this region, astronomers have identified at least 11 extremely massive stars with ages of only 2 million years with many more young stars packed together so tightly individual stars can’t be resolved.

The Chandra image of the Tarantula Nebula gives scientists a close-up view of the drama of star formation and evolution. The Tarantula, also known as 30 Doradus, is in one of the most active star-forming regions in our Local Group of galaxies. Massive stars are producing intense radiation and searing winds of multimillion-degree gas that carve out gigantic super-bubbles in the surrounding gas. Other massive stars have raced through their evolution and exploded catastrophically as supernovas, leaving behind pulsars and expanding remnants that trigger the collapse of giant clouds of dust and gas to form new generations of stars.

30 Doradus is located about 180,000 light years from Earth in the Large Magellanic Cloud, a satellite galaxy of our Milky Way Galaxy. It allows astronomers to study the details of starbursts – episodes of extremely prolific star formation that play an important role in the evolution of galaxies.

At least 11 extremely massive stars with ages of about 2 million years are detected in the bright star cluster in the center of the primary image (left panel). This crowded region contains many more stars whose X-ray emission is unresolved. The brightest source in this region known as Melnick 34, a 130 solar-mass star located slightly to the lower left of center. On the lower right of this panel is the supernova remnant N157B, with its central pulsar.

Two off-axis ACIS-S chips (right panel) were used to expand the field of view. They show SNR N157C, possibly a large shell-like supernova remnant or a wind-blown bubble created by OB stars. Supernova 1987A is also visible just above and to the right of the Honeycomb Nebula at the bottom center.

In the image, lower energy X-rays appear red, medium energy green and high-energy are blue.

Original Source: Chandra News Release

Image credit: NASA

NASA astronomers believe that retired quasars may be a source of rare, high-energy cosmic rays. They’ve identified four elliptical galaxies relatively nearby that contain massive black holes. If these black holes are spinning, they could be a source of ultra high-energy cosmic rays. The source of cosmic rays is a mystery, but astronomers have calculated that they must come from objects within 200 million light years from the Earth – these “retired quasars” could be the source.

They are old but not forgotten. Nearby “retired” quasar galaxies, billions of years past their glory days as the brightest beacons in the Universe, may be the current source of rare, high-energy cosmic rays, the fastest-moving bits of matter known and whose origin has been a long-standing mystery, according to scientists at NASA and Princeton University.

The scientists have identified four elliptical galaxies that may have started this second career of cosmic-ray production, all located above the handle of the Big Dipper and visible with backyard telescopes. Each contains a central black hole of at least 100 million solar masses that, if spinning, could form a colossal battery sending atomic particles, like sparks, shooting off towards Earth at near light speed.

These findings are discussed today in a press conference at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, N.M. The team includes Dr. Diego Torres of Princeton University and Drs. Elihu Boldt, Timothy Hamilton and Michael Loewenstein of NASA’s Goddard Space Flight Center in Greenbelt, Md.

Quasar galaxies are thousands of times brighter than ordinary galaxies, fueled by a central black hole swallowing copious amounts of interstellar gas. In galaxies with so-called quasar remnants, the black hole nucleus is no longer a strong source of radiation.

“Some quasar remnants might not be so lifeless after all, keeping busy in their later years,” said Torres. “For the first time, we see the hint of a possible connection between the arrival directions of ultra-high energy cosmic rays and locations on the sky of nearby dormant galaxies hosting supermassive black holes.”

Ultra high-energy cosmic rays represent one of astrophysics’ greatest mysteries. Each cosmic ray — essentially a single sub-atomic particle such as a proton traveling just shy of light speed — packs as much energy as a major league baseball pitch, over 40 million trillion electron volts. (The rest energy of a proton is about a billion electron volts.) The particles’ source must be within 200 million light years of Earth, for cosmic rays from beyond this distance would lose energy as they traveled through the murk of the cosmic microwave radiation pervading the Universe. There is considerable uncertainty, however, over what kinds of objects within 200 million light years could generate such energetic particles.

“The very fact that these four giant elliptical galaxies are apparently inactive makes them viable candidates for generating ultra high-energy cosmic rays,” said Boldt. Drenching radiation from an active quasar would dampen cosmic-ray acceleration, sapping most of their energy, Boldt said.

The team concedes it cannot determine if the black holes in these galaxies are spinning, a basic requirement for a compact dynamo to accelerate ultra-high energy cosmic rays. Yet scientists have confirmed the existence of at least one spinning supermassive black hole, announced in October 2001. The prevailing theory is that supermassive black holes spin up as they accrete matter, absorbing orbital energy from the infalling matter.

Ultra-high-energy cosmic rays are detected by ground-based observatories, such as the Akeno Giant Air Shower Array near Yamanashi, Japan. They are extremely rare, striking the Earth’s atmosphere at a rate about one per square kilometer per decade. Construction is underway for the Auger Observatory, which will cover 3,000 square kilometers (1,160 square miles) on an elevated plain in western Argentina. A proposed NASA mission called OWL (Orbiting Wide-angle Light-collectors) would detect the highest-energy cosmic rays by looking down on the atmosphere from space.

Loewenstein joins NASA Goddard’s Laboratory for High Energy Astrophysics as a research associate with the University of Maryland, College Park. Hamilton, also a member of the Lab, is a National Research Council fellow.

Original Source: NASA News Release

Image credit: Harvard

During late April and May you’ll get an opportunity to see the five brightest planets lined up in a single evening. Look West in the early evening sky and you’ll be able to see Mercury, Venus, Mars, Jupiter, and Saturn grouped up. The grouping is fairly rare and won’t be seen again until 2040.

Comet Hale-Bopp dazzled us for weeks. The Perseid meteor shower thrilled us for one night. But the world hasn’t seen anything like the planetary traffic jam that’s going to occur the last week of April and the first two weeks in May!

Inching across the sky like bumper-to-bumper commuters on their way to work, a rare planetary alignment will allow sky observers to see every planet in our solar system in a single evening! “There will be other opportunities in the future to see the planets in different configurations,” says Philip Sadler, Director of the Science Education Department at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA, ” but it won’t be anything like this for at least another 70 years. This is truly a once-in-a-lifetime experience.”

In the past, many different configurations of planetary alignments have been seen from Earth. They occur due to the random positions of the planets in their eccentric orbits around the Sun. In the early 1980s and in May of 2000, the planets stacked up directly behind the Sun. Many people thought the combined gravitational pull might create havoc here on Earth resulting in giant earthquakes, sweeping tidal waves or erupting volcanoes. But, the collective gravitational pull was so insignificant, nothing happened. What was the reason? The other planets are simply too small or too far away in space to affect us back on Earth. To see just how insignificant the gravitational pull of the planets can be, let’s do what many good, red-blooded Americans like to do. Let’s go shopping!

Imagine if we stood in the produce section of a grocery store and held up a big yellow grapefruit representing the Sun. The planet Mercury would be the size of a small grain of salt orbiting around it 18 feet away. Venus would be somewhat larger, like a grain of sugar you get in those little brown packets at the coffee shops, 34 feet away. Earth, also a grain of sugar, would be located 50 feet away. Mars also would be the size of a grain of salt 75 feet away. As for the rest of the planets: Jupiter, a cherry-sized tomato, would be found at 240 feet; Saturn, the size of a green grape, at 420 feet; Uranus, a frozen green pea, at 300 yards; Neptune, also the size of a frozen pea, at 470 yards; and Pluto, represented by a speck of dust, would orbit our grapefruit-sized Sun at a distance of 475-600 yards. As you’ve probably guessed, not much gravitational pull is exerted on the Earth by these grocery store lightweights!

In early May, when the planets line up, they will not be arranged behind one another or the Sun. Instead, they will present a beautiful line across the sky from horizon to near zenith. For a period of a little more than three weeks, anyone looking west at sunset will be able to see the planets Mercury, Venus, Mars, Saturn and Jupiter. A few hours later at 4 A.M., armed with a large-size amateur telescope, they can continue their grand tour by observing Uranus, Neptune, and Pluto. By quickly glancing down at the ground, they will have completed their grand tour of the solar system.

Looking at the planets spread out across the sky during this alignment also demonstrates, better than any book, how our solar system formed 4 billion years ago; something astronomers just recently have begun seeing around other distant stars in space. “Our solar system condensed out of a nebular dust cloud that flattened down into a giant disk that resembled a big pizza pan,” says CfA astrophysicist David Wilner. “Utilizing instruments like the Hubble Space Telescope and data from the Infrared Astronomical Satellite, we are now witnessing the formation of new solar systems spread out into flattened discs of gas and dust. We are even detecting large lumps of material in the dust disks that may be the signatures of planets in formation. Astronomers are now assembling snapshots of our own past frozen in time billions of years ago.”

This pathway of planets, or the ecliptic as astronomers call it, is what remains after our dust cloud coalesced into planets. Tracing the path of this ancient dust ring across the sky is easy. Stand sideways facing south with your right hand extended and pointing to where the Sun recently set along the western horizon. Now, extend your arm up to point at the Moon or a bright planet overhead. Connecting these two points together, continue to sweep your arm in an arc until it reaches the opposite horizon. Bingo! You have just traced out the ecliptic. All the planets will be found along this line and nowhere else. And this is where the traffic jam will occur.

“Coincidentally,” says Sadler,” have you ever wondered why the zodiac sign were chosen? Why someone you know wasn’t born under the sign of Hercules or Orion?”

To the Greeks and Romans, the ecliptic was the Highway of the Gods or the path the planets and Moon moved across at night and the Sun traveled during the daytime. “Located directly behind this highway were the twelve special constellations the Gods passed by as they moved across the sky. They constituted the signs of the zodiac. This was the basis for astrology – religious beliefs and basic sky observations mixed together. It should not be confused with the science of astronomy that emerged centuries later,” says Sadler. Today, it is widely held by many historians and planetarium directors that a conjunction of the planets, similar to the one on May 5, accounts for the Star of Bethlehem that sent the Magi on their way to seek the Christ child. Certainly the timing was right. An almost identical triangular alignment of Saturn, Mars and Venus did take place on April 1, 2 B.C. And the planets Jupiter, Saturn and Mars also formed a triangular conjunction in 6 B.C., in the constellation Pisces, the sign of the Christians. However, renowned astronomical historian Prof. Owen Gingerich of the CfA disagrees. “The very, very short duration of a grouping of planets was not the Star of Bethlehem,” he states. “A conjugation like this would have meant nothing to the Magi. It was not part of their astrological tradition. It really wasn’t until Kepler became fascinated with the harmony of the planets in the 16th century that the idea of a planetary conjunction came about to try to attach a scientific explanation to this event. In fact, Kepler even went so far as to add an imaginary supernova to the conjunction of planets in 6 B.C. to try to make it even more spectacular to catch the Magi’s attention. ”

Will this event be religiously significant or just an astronomical oddity? Is it the most dramatic way to visualize how our solar system formed? Or, is it an exciting challenge for amateur astronomers to conduct their only whirlwind tour of the solar system in just one evening? Answering yes to any or all of the above makes the alignment of late April and early May something not to be missed. Nothing like it will occur again in our lifetime. At the very least, it presents a wonderful opportunity for friends and family to come together and share an experience beyond the daily routine. It also may be an opportunity to ponder our fragile existence on this tiny blue world racing around an ordinary yellow star with eight other planetary companions and maybe help us, just a little bit, bring our own world back into perspective.

Headquartered in Cambridge, Massachusetts, 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 seven research divisions study the origin, evolution, and ultimate fate of the universe.

Original Source: CfA News Release