Most Massive Stellar Black Hole Found in Binary System

A strange black hole locked in a tight orbit with a huge star in a nearby galaxy could be the most massive stellar black hole known, astronomers say.

Stellar black holes form when a weighty, dying star's core collapses, creating a region with such a strong gravitational pull that even light can't escape.

Astronomers also believe that supermassive black holes with masses equal to millions of suns exist at the centers of many galaxies, including our own Milky Way.

The newly described stellar black hole has a mass almost 16 times that of our sun, said lead study author Jerome A. Orosz, a professor of astronomy at San Diego State University.

"Before this, the most massive known stellar black hole was GRS 1915+105, with a black-hole mass estimated to be 14 plus or minus 4 solar masses," Orosz said. But the mass of GRS 1915+105 has recently come into question, he added.

Orosz's team also found that the black hole's companion star has a mass about 70 times that of the sun—making it the most massive star in a binary system containing a black hole.

"The present-day companion is no more than about three million years old," Orosz said. "The model suggests that it will die in another two to three million years, forming another black hole."

Orosz and colleagues report their puzzling new observations in this week's issue of the journal Nature.

X-Ray Vision

Since black holes can't be seen directly, astronomers look for the intense radiation released as hot matter spirals into a singularity.

Researchers can then detect the existence of a black hole in a binary system by observing a visible star in orbit around its invisible companion.

The mass of the black hole in such a system can then be deduced by measuring its gravitational tug on the star.

NASA's Einstein X-Ray Observatory Satellite first spotted the black hole and its companion described in the new study in 1981.

The binary system—dubbed M33 X-7—lies in Meisser 33, a spiral galaxy about three million light-years away.

Astronomers soon realized that the companion star passes directly in front of the black hole on its three-day orbit, eclipsing the black hole's x-ray emissions.

This arrangement allowed the team to combine data from NASAs orbiting Chandra X-Ray Observatory and the Gemini North Telescope on Mauna Kea, Hawaii, to calculate the two object's masses more accurately than usual.

Tight Orbit

Orosz and his team don't fully understand how a massive black hole formed in close orbit around a massive companion star.

But experts think the formation of close binaries that contain the remains of dead stars—such as white dwarfs, neutron stars, and black holes—might involve a process called the common-envelope phase.

During this phase, a dying star in a binary system expells its outer layers while sucking the other star closer.

This leads to either a merger of the two objects or the formation of a tight binary system.

"During the common-envelope phase, the progenitor of the M33 X-7 black hole must have lost a large amount of mass for the two objects to be so close," Tomasz Bulik, of the University of Warsaw in Poland, writes in a commentary article also appearing in Nature.

"In this system we know that quite a lot of mass was expelled, and on the other hand that the remaining mass formed the black hole," Bulik told National Geographic News.

"Thus we have an upper and lower bound on the amount of mass expelled."

According to lead study author Orosz, "the reason M33 X-7 is so hard to understand is that both the black hole and the companion star have a very high mass.

"The predicted mass loss [of the star as it died before becoming a black hole] is so great that M33 X-7 should not have been able to enter a common-envelope phase."

Researchers hope that by understanding systems like M33 X-7, they can gain better insight into the formation of tight binary systems.

"The common-envelope stage is poorly known," Bulik said, "and this system will help us understand it."

Also this week, an independent team led by Andrea H. Prestwich of the Harvard-Smithsonian Center for Astrophysics described what it says could be an even more massive stellar black hole weighing 24 to 33 times as much as the sun.

The object is also part of a binary system that includes a massive star, and it lies 2.2 million light-years away in the starburst galaxy IC 10.

The researchers, who published their findings online this week in the Astrophysical Journal Letters, say more data are needed to confirm that the object is actually a black hole.

Brightest Known Supernova Detected

Scientists have spotted the brightest supernova yet—a star explosion that at its peak was a hundred billion times brighter than the sun.

Robert Quimby, an astronomer at the California Institute of Technology, discovered the spectacle in 2005 using a small robotic telescope at McDonald Observatory in West Texas.

But only recently did he calculate the true power of the supernova, known as 2005ap.

The explosion is 300 times brighter than average and is the most luminous supernova ever identified, Quimby said.

"It's amazing to me that after decades of in-depth studies, the brightest and seemingly most obvious supernovae are still being found," he added.

2005ap lies about 4.7 billion light-years away in a dwarf galaxy in the constellation Coma Berenices, behind the famous Coma cluster of galaxies.

The new find is a Type II supernova. These explosions are thought to occur when the cores of massive stars—more than seven times as heavy as the sun—collapse under their own weight and trigger an explosion.

A study on the supernova will appear in the October 20 edition of Astrophysical Journal Letters.

Two in a Row

For Quimby, the discovery is almost a hat trick.

The new supernova is roughly twice as bright as the previous record holder, SN 2006gy—which he also found.

Quimby discovered that explosion last year while working on the Texas Supernova Search project, part of his doctoral degree program at the University of Texas.

He actually observed supernova 2005ap there before 2006gy. But only recent follow-up studies that pinpointed distance revealed 2005ap's true power.

"There's no question that [his results] have gotten everybody's attention," J. Craig Wheeler, Quimby's colleague and a supernova expert at the University of Texas at Austin, said in a press release.

On the Hunt

Supernovae are rare events, occurring roughly twice a century in a galaxy like the Milky Way.

Researchers study them for insights into the fates of stars bigger than our sun. The explosions also serve as evidence that the expansion of the universe is accelerating, because the most distant ones appear dimmer than expected.

Supernovae are also the only way elements heavier than iron are seeded throughout the universe and play a vital role in the evolution of galaxies, Quimby said.

"Plus, they're huge explosions, and that's just cool," he said.

Despite their massive size and power, however, finding supernovae amidst the glare of other cosmic objects isn't easy.

Galaxies get much brighter toward their cores, which can wash out the contrast of a new supernova, Quimby said.

The University of California's Lick Observatory Supernova Search actually studied the area containing 2006gy before him, Quimby said, but missed it because they couldn't distinguish the explosion from its galaxy's brilliant core.

Quimby said part of his supernova-hunting success is his technique. He checks dwarf galaxies as well as galaxies with active black holes at their centers, which other studies avoid, raising the possibility of discovering new types of supernovae previously overlooked.

2006gy, for example, was found in the core of a galaxy with a weak central black hole.

"I've worked too damn hard for this to be luck," he said.

Rare T. Rex Footprint Found?

Odds are 50/50 that the dinosaur that left this mark in Montana was a Tyrannosaurus rex, says the scientist who announced the find yesterday.

Whatever it was, it was big.

"We have two large theropods to choose from [in this region], Nanotyrannus and T. rex," Phil Manning (pictured) told National Geographic News via email. "So either [dinosaur] (or an unknown) could have generated the track."

Referring to some breathless headlines of the past 24 hours, he added, "You can imagine which one the media wants!

"Unless you find an animal dead in its tracks, it is nigh impossible to identify the track maker," added the University of Manchester paleontologist, whose work is partially funded by the Expeditions Council of the National Geographic Society (which also owns National Geographic News).

If confirmed, it would be only the second known T. rex print. The first was found in 1983 in New Mexico.

Manning originally saw the three-foot-wide (meter-wide), 65-odd-million-year-old track in 2006 in the dinosaur-rich Hell Creek formation, and he is preparing a report for full scientific review.

In the meantime, he wrote, "we intend to keep the site secret until suitable measures can be taken to preserve the track."

Titan Forecast: Cold, Drizzly Mornings

Methane drizzles down every morning on Titan, according to a new study of Saturn's largest moon.

The rain is probably widespread across Titan and may even close the loop in a methane cycle that closely resembles the water cycle on Earth, the researchers suggest..

Scientists involved with the Huygens probe, which landed on Titan's surface in late 2004, have long suspected such an atmospheric cycle.

"The most important part of these results is that there is a way to monitor methane condensation from ground-based telescopes," said Mate Adamkovics of the University of California at Berkeley, who led the new research.

"Monitoring how often and to what extent the drizzle occurs might be an indication of seasonal changes on Titan that is more sensitive than watching other types of clouds come and go."

The results will be published in this week's issue of the journal Science.

Exotic Moon

Titan is a mysterious and alluring ball of rock and ice surrounded by a thick, orange, methane-heavy atmosphere.

Some scientists believe that the moon is a likely place to harbor alien life. One reason is that several studies have suggested that Titan has long-lasting lakes most likely made of methane.

On Earth, methane is produced in gas form through biological reactions such as digestion. It can exist as a liquid only under very high pressures.

But at about minus 290 degrees Fahrenheit (about minus 180 degrees Celsius), the surface of Titan is so frigid that methane doesn't need pressure to remain in liquid form.

Using ground-based telescopes and computer models, Adamkovics and his colleagues found a solid methane cloud 15 to 21 miles (25 to 35 kilometers) from Titan's surface.

A constant methane rain pelts Xanadu, the moon's brightest continent, each morning, the researchers discovered.

It's the first time researchers have seen differences between day and nighttime weather on Titan, Adamkovics pointed out.

"They were unexpected, because day-to-night temperature changes are expected to be small," he said.

Still Mysterious

Martin Tomasko, an astronomer at the University of Arizona in Tucson who designed Huygens' camera system, has long suspected methane smog and condensation occurred on Titan.

He has proposed that methane rains from Titan's sky as a thick, tarry gunk. The chemical then travels through channels until it reaches reservoirs visible as dark patches on the moon's surface.

The exact nature of the dark patches remains in dispute, however.

If methane rain is indeed widespread, it could be the main method that atmospheric methane returns to the surface, the study authors write.

But Adamkovics said there's still a long way to go to understand the process.

"Many predictions are being made, but we are far from conclusions," he said, "and I would wager that more surprises are going to be discovered before we have a textbook description of Titan's atmospheric cycling."