all Abraham Avi Loeb stories

Two exiled stars are leaving our galaxy forever

70652986 — Mon, 26 Mar 2007 06:24:35 -0400

TV reality show contestants aren't the only ones under threat of exile. Astronomers using the MMT Observatory in Arizona have discovered two stars exiled from the Milky Way galaxy. Those stars are racing out of the Galaxy at speeds of more than 1 million miles per hour - so fast that they will never return.

"These stars literally are castaways," said Smithsonian astronomer Warren Brown (Harvard-Smithsonian Center for Astrophysics). "They have been thrown out of their home galaxy and set adrift in an ocean of intergalactic space."

Brown and his colleagues spotted the first stellar exile in 2005. European groups identified two more, one of which may have originated in a neighboring galaxy known as the Large Magellanic Cloud. The latest discovery brings the total number of known exiles to five.

"These stars form a new class of astronomical objects - exiled stars leaving the Galaxy," said Brown.

Astronomers suspect that about 1,000 exile stars exist within the Galaxy. By comparison, the Milky Way contains about 100,000,000,000 (100 billion) stars, making the search for exiles much more difficult than finding the proverbial "needle in a haystack." The Smithsonian team improved their odds by preselecting stars with locations and characteristics typical of known exiles. They sifted through dozens of candidates spread over an area of sky almost 8,000 times larger than the full moon to spot their quarry.

"Discovering these two new exiled stars was neither lucky nor random," said astronomer Margaret Geller (Smithsonian Astrophysical Observatory), a co-author on the paper. "We made a targeted search for them. By understanding their origin, we knew where to find them."

This research has been submitted to The Astrophysical Journal Letters for publication and is available online at http://arxiv.org/ abs/astro-ph/0601580. Authors on the paper are Brown, Geller, Scott Kenyon and Michael Kurtz (Smithsonian Astrophysical Observatory).

CfA researchers discover black holes aren't so black

50443248 — Fri, 20 Jul 2007 12:47:25 -0400

Common wisdom holds that we can never see a black hole because nothing can escape it - not even light. Fortunately, black holes aren't completely black. As gas is pulled into a black hole by its strong gravitational force, the gas heats up and radiates. That radiation can be used to illuminate the black hole and paint its profile.

Black holes aren't so black

70652986 — Mon, 26 Mar 2007 06:22:14 -0400

As gas is pulled into a black hole by its strong gravitational force, the gas heats up and radiates. That radiation can be used to illuminate the black hole and paint its profile.

Within a few years, astronomers believe they will be able to peer close to the horizon of the black hole at the center of the Milky Way. Already, they have spotted light from "hot spots" just outside the black hole. While current technology is not quite ready for the final plunge, Harvard theorists Avery Broderick and Avi Loeb already have modeled what observers will see when they look into the maw of this monster.

"It will be really remarkable when observers can see all the way to the edge of the Milky Way's central black hole - a hole 10 million miles in diameter that's more than 25,000 light-years away," said Broderick.

All it will take is a cross-continental array of submillimeter telescopes to effectively create a single telescope as large as the Earth. This process, known as interferometry, has already been used to study longer wavelength radio emissions from outer space. By studying shorter wavelength submillimeter emissions, astronomers could get a high-resolution view of the region just outside the black hole.

Ferreting out the first stars

70652986 — Mon, 26 Mar 2007 06:22:35 -0400

The first stars are so distant and formed so long ago that they are invisible to our best telescopes.

Until they explode. Hypernovas (more powerful cousins of supernovas) and their associated gamma-ray bursts offer astronomers the possibility of detecting light from the first generations of stars.

NASA's Swift satellite already has seen a gamma-ray burst (GRB) with a redshift of 6.29, meaning that the progenitor star exploded about 13 billion years ago, when the universe was less than a billion years old. Theorists Volker Bromm (University of Texas at Austin) and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) predict that one-tenth of the blasts Swift will spot during its operational lifetime will come from stars at a redshift of 5 or greater, that lived and died during the first billion years of the universe.

"Most of those GRBs will come from second generation or later stars," said Loeb. "But if we get lucky, Swift may even detect a burst from one of the very first stars that formed -- a star made of only hydrogen and helium."

Calculations suggest that such stars, which are called Population III for historical reasons, would have been behemoths weighing 50-500 times as much as the Sun. A Population III star would have gulped its nuclear fuel faster than an SUV, dying quickly and explosively.

"Our best guess right now is that the recent GRB was not from a Pop III star. However, its redshift is high enough to make it very interesting," said Bromm.

Taking a CAT scan of the early universe

70652986 — Mon, 26 Mar 2007 05:36:09 -0400

Reporting in the Nov. 11, 2004, issue of Nature, astrophysicists J. Stuart B. Wyithe (University of Melbourne) and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics) have calculated the size of cosmic structures that will be measured when astronomers effectively take CAT scan-like images of the early universe. Those measurements will show how the universe evolved over its first billion years of existence. "Until now, we've been limited to a single snapshot of the universe's childhood -- the cosmic microwave background," says Loeb. "This new technique will let us view an entire album full of the universe's baby photos. We can watch the universe grow up and mature." The heart of the tomography technique described by Wyithe and Loeb is the study of 21-centimeter-wavelength radiation from neutral hydrogen atoms. In our own galaxy, this radiation has helped astronomers to map the Milky Way's spherical halo. To map the distant young universe, astronomers must detect 21-cm radiation that has been redshifted: stretched to longer wavelengths (and lower frequencies) by the expansion of space itself. "Tomography is a complicated process, which is one reason why it hasn't been done before at very high redshifts," says Wyithe. "But it's also very promising because it's one of the few techniques that will let us study the first billion years of the universe's history."

Lifeless suns dominated early universe

70652986 — Mon, 26 Mar 2007 05:34:47 -0400

The very first generation of stars were not at all like our Sun. They were white-hot, massive stars that were very short-lived. Burning for only a few million years, they collapsed and exploded as brilliant supernovae. Those very first stars began the seeding process in the universe, spreading vital elements like carbon and oxygen, which served as planetary building blocks. This picture of the early universe comes from new calculations by Harvard astronomers Volker Bromm and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics). The researchers have shown that the first Sun-like stars were lonely orbs moving through a universe devoid of planets or life. "The window for life opened sometime between 500 million and 2 billion years after the Big Bang" says Loeb. "Billions of years ago, the first low-mass stars were lonely places. The reason for that youthful solitude is embedded in the history of our universe." "Life is a recent phenomenon," Loeb states unequivocally. "We know that it took many supernova explosions to make all the heavy elements we find here on Earth and in our Sun and our bodies." This research was published in the October 23, 2003, issue of the scientific journal Nature.

First Milky Ways found at edge of universe

70652986 — Mon, 26 Mar 2007 05:27:43 -0400

One key question that has puzzled astronomers for decades is: When did the first stars and galaxies form after the Big Bang occurred? The answer -- very quickly! Astronomers Rennan Barkana of Tel Aviv University and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics found the first direct evidence that galaxies as large as the Milky Way already had formed when the Universe was less than a billion years old. "In some ways, it's surprising that such large galaxies formed so quickly. Most galaxies in the early Universe were only one-hundredth that size," said Loeb. "But our model, combined with observations by other researchers, provides clear evidence that massive galaxies existed within a relatively short time after the Big Bang." Intriguingly, the large galaxies discovered by Barkana and Loeb are still around today. Over billions of years, they continued to consume smaller galaxies, like a cosmic software corporation absorbing many smaller companies. These galactic cannibals have grown from the seeds that existed in a billion-year-old Universe to become monstrous giant elliptical galaxies, resting in the centers of galaxy clusters. The research was reported in the Jan. 23, 2003 issue of the journal Nature.

Neither Rome nor universe built in a day

70652986 — Mon, 26 Mar 2007 05:21:57 -0400

Theoretical astrophysicists Stuart B. Wyithe and Abraham Loeb at the Harvard-Smithsonian Center for Astrophysics (CfA) have explained a paradox that has troubled scientists for years. Observations seem to show that giant black holes containing as much mass as three billion suns formed less than a billion years after the Big Bang. Collecting so much material so quickly was as unlikely as building a 20-room mansion in a day's time. Recently the researchers calculated that the light from a significant fraction of the most-distant quasars is likely being magnified by intervening matter, making the quasars' central black holes seem 10 to 100 times larger than they actually are. It's the equivalent of learning that what you thought was a 20-room mansion actually was a one-room shed, easily constructed in a day. The findings were published in the June 27, 2002, issue of the scientific journal Nature.

Looking toward the end

70652986 — Mon, 26 Mar 2007 05:18:40 -0400

Among astronomers there is almost a consensus that universal expansion will go on forever, with galaxies and clusters of galaxies moving away from each other so fast that gravity cannot ever pull them together. Since 1998, astronomers have found good evidence that this expansion is accelerating, that galaxies are separating at ever-increasing speed. But what will that mean for people here on Earth, far in the future?

Gamma rays may be left over from cosmic construction project

70652986 — Mon, 26 Mar 2007 05:09:31 -0400

The origin of the diffuse and pervasive background of gamma-ray radiation that exists over the universe has been one of the great unsolved mysteries in cosmology. Even the known population of powerful extragalactic gamma-ray sources, called "blazars," can account for no more than a quarter of the gamma-ray background radiation that is observed. According to a model proposed by Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics and Eli Waxman of Israel's Weizmann Institute, massive shock waves, triggered by gravity during the formation of large-scale structures such as galaxies, are sufficiently powerful to account for the background radiation. In an article in the May 11, 2000, edition of the journal Nature, Loeb and Waxman suggested that the gravity-induced shock waves generated a population of highly-relativistic electrons, which, in turn, scattered the equally pervasive microwave background, itself a remnant of the Big Bang, pumping up a fraction of the microwave photons to gamma-ray energies, thus producing the all-sky gamma-ray background seen in today's universe.