by Paul Gilster | Sep 20, 2005 | Deep Sky Astronomy & Telescopes |
A strange blue light near the core of the Andromeda Galaxy promises to tell us much about black holes and the behavior of objects near them. First spotted in 1995 by the Hubble Space Telescope, the blue light was thought to emmanate from a single, massive star, or possibly an exotic source of energy that was little understood. But new spectroscopic observations show that the light is actually made up of 400 stars packed into a disk only one light year across.
Now this is a very strange finding, for these young stars — thought to be on the order of 200 million years old — are revolving around the black hole at the center of Andromeda so closely that they should be torn apart. How could gas and dust coalesce to form stars in such an environment? Mysteries, of course, are just what astronomers like to find; they often lead to enough new data to revise earlier theories and produce more complete explanations.
Image: This artist’s concept shows a view across a mysterious disk of young, blue stars encircling a supermassive black hole at the core of the neighboring Andromeda Galaxy (M31). The region around the black hole is barely visible at the center of the disk. The background stars are the typical older, redder population of stars that inhabit the cores of most galaxies. Under the black hole’s gravitational grip, the stars are traveling very fast: 2.2 million miles an hour (3.6 million kilometers an hour, or 1,000 kilometers a second). Credit: NASA, ESA and A. Schaller (for STScI).
And that black hole at Andromeda’s center seems to be confirmed. The Hubble data are precise enough to rule out other exotic alternatives. The black hole that emerges, so to speak, has a mass equivalent to 140 million Suns. The blue stars that surround it are, in turn, nestled inside an elliptical ring of older, redder stars. And the fact that the blue stars are as young as they are implies they are only the most recent manifestation of a recurring phenomenon.
“The blue stars in the disk are so short-lived that it is unlikely in the long 12-billion-year history of Andromeda that such a short-lived disk would appear now,” said Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona. “That’s why we think that the mechanism that formed this disk of stars probably formed other stellar disks in the past and will trigger them again in the future. We still don’t know, however, how such a disk could form in the first place. It still remains an enigma.”
Centauri Dreams‘ note: Forty detections of black holes have been accomplished, but in almost all cases the evidence is not conclusive. Until now, the only two unambiguous detections, according to astronomer John Kormendy of the University of Texas in Austin, were in the galaxy NGC 4258 and in our own Milky Way. Andromeda gives us three confirmed black holes, and opens up new speculation on star formation in circumstances where it was once thought impossible.
More in this European Space Agency news release. The team’s results will appear in the September 20 issue of The Astrophysical Journal.
by Paul Gilster | Sep 19, 2005 | Outer Solar System
Peering through Titan’s murky atmosphere with radar, the Cassini orbiter has sent back evidence of what may be a large shoreline on the moon, one where liquids flowed not long ago and may still be present. As shown in the image below, the shoreline divides a bright from a darker, smoother area where liquids seem to be involved. “This is the area where liquid or a wet surface has most likely been present, now or in the recent past, said Steve Wall, radar deputy team leader from NASA’s Jet Propulsion Laboratory. “Titan probably has episodic periods of rainfall or massive seepages of liquid from the ground.”
The evidence mounts up as scientists study bay-like features in the images, and networks of channels that may show where liquid hydrocarbons have flowed. Intriguingly, two types of channel formation seem to be present. The most recent Cassini pass shows long and deep channels indicating flow over large areas, while other images from previous radar passes show a dense network of channels that may be indicative of rainfall.
Image: This Synthetic Aperture Radar image of the surface of Saturn’s moon Titan was obtained by the Cassini spacecraft on Sept. 7, 2005. The boundary of the bright (rough) region and the dark (smooth) region appears to be a shoreline. The patterns in the dark area indicate that it may once have been flooded, with the liquid having at least partially receded. Credit: NASA/JPL.
From a JPL news release:
Dr. Larry Soderblom with the U.S. Geological Survey in Flagstaff, Ariz., said, “It looks as though fluid flowed in these channels, cutting deeply into the icy crust of Titan. Some of the channels extend over 100 kilometers (60 miles). Some of them may have been fed by springs, while others are more complicated networks that were likely filled by rainfall.”
Centauri Dreams‘ take: The most dramatic possibility pre-Cassini was that Titan might have actual oceans of liquid methane. Cassini has found nothing like that, but the evidence of a methane hydrological cycle on Titan is overwhelming. Another radar pass is scheduled for October 26, with 37 more flybys planned in the next few years. Cassini continues to unlock Titan’s mysteries, but it will take a Titan rover to study this bizarre world’s chemistry up close.
by Paul Gilster | Sep 17, 2005 | Outer Solar System
The largest known asteroid, 1 Ceres, is 930 kilometers (580 miles) across, and represents about 25 percent of the asteroid belt’s total mass. Recent work using the Hubble Space Telescope’s Advanced Camera for Surveys now shows that the asteroid is nearly round, leading to the belief that it has, like terrestrial planets, a differentiated interior: a dense silicate core surrounded by lighter minerals and covered by a crust of carbon-rich compounds.
But more extrordinary still is the suggestion that water ice is buried beneath the asteroid’s crust. The evidence is tantalizing — Ceres’ density is less than that of Earth’s crust, and its surface gives signs of water-bearing minerals. Wrapped in a solid ice mantle surrounding the asteroid’s core, Ceres could actually possess more water than all the fresh water found on Earth.
Image: NASA’s Hubble Space Telescope took these images of the asteroid 1 Ceres over a 2-hour and 20-minute span, the time it takes the Texas-sized object to complete one quarter of a rotation. One day on Ceres lasts 9 hours. Credit: Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park).
All these things lead astronomers studying the data to conclude that Ceres is considerably more interesting than once thought. “Ceres is an embryonic planet,” said Lucy A. McFadden of the Department of Astronomy at the University of Maryland, College Park and a member of the team that made the observations. “Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet.”
The team’s findings appear in P. C. Thomas, J. Wm. Parker, L. A. McFadden et al., “Differentiation of the asteroid Ceres as revealed by its shape,” Nature 437, 224-226 (08 Sep 2005). See this Cornell University press release for more.
by Paul Gilster | Sep 16, 2005 | Outer Solar System
Centauri Dreams has talked often about what may be the main driver for deep space exploration — the need to protect a vulnerable Earth from catastrophic asteroid or comet strikes. Ongoing work to understand how such impacts fit into the history of the Solar System is revealing much about their nature. Now a team of University of Arizona and Japanese scientists has uncovered the culprits in the massive bombardment of the early Solar System some 3.9 billion years ago: main belt asteroids like those now found between Mars and Jupiter.
And here’s what’s intriguing. The objects that have been cratering various planets since that era of the so-called Late Heavy Bombardment are a different species. Now it is the Near-Earth Asteroids (NEAs) that pose the greatest threat. UA Professor Emeritus Robert Strom and colleagues report their findings in the article, “The Origin of Planetary Impactors in the Inner Solar System,” which runs in the September 16 issue of Science. From a University of Arizona press release:
“One thing this says is that the present-day size-distribution of asteroids in the asteroid belt was established at least as far back as 4 billion years ago,” UA planetary scientist Renu Malhotra, a co-author of the Science paper, said. “Another thing it says is that the mechanism that caused the Late Heavy Bombardment was a gravitational event that swept objects out of the asteroid belt regardless of size.”
Thus we get a sharper picture of Solar System formation. Not long after they were formed, Jupiter and the gas giant planets would have swept up planetary debris all the way out to the Kuiper Belt. The process would have caused Jupiter to lose energy and move into a closer orbit to the Sun, creating strong gravitational effects on the asteroid belt and flinging many objects into the inner Solar System. A sampling of Apollo lunar materials has demonstrated that asteroids account for 80 percent of lunar impacts.
Image: An Apollo-era image of the lunar highlands. The new study by Strom, Malhotra, Kring and their Japanese colleagues indicates this terrain was bombarded mostly by asteroids – not comets – that were flung into the inner solar system when the asteroid belt was destabilized by migrating giant planets. The Earth was similarly bombarded but geological activity has erased most evidence of that bombardment. (Credit: NASA).
Comets, on the other hand, seem to have played a minor role in inner Solar System impacts, and in contrast to some views, may have contributed only as much as 10 percent of the Earth’s supply of water. And note this key finding with direct relevance to the emergence of life, citing the work of UA planetary scientist David A. Kring:
Kring also has developed a hypothesis that suggests that the impact events during the Late Heavy Bombardment generated vast subsurface hydrothermal systems that were critical to the early development of life. He estimated that the inner solar system cataclysm produced more than 20,000 craters between 10 kilometers to 1,000 kilometers in diameter on Earth.
Centauri Dreams‘ take: In terms of our own future, it’s the small, Earth-crossing asteroids nudged out of the main belt that pose the greatest threat. These hard to spot, fast-moving objects have accounted for all the cratering on inner planetary surfaces since the bombardment 3.9 billion years ago, and should continue to impel our push into the outer Solar System.
It’s fascinating to consider that after the period of the Late Heavy Bombardment, the surface of the Earth was so completely changed that no feature older than 3.9 billion years can be dated by cratering. But the UA team’s work seems to show that if oceans did exist earlier than 3.9 billion years ago (and there is geological evidence for this), those oceans would have been vaporized by the asteroid impacts. We often think of asteroid bombardment as inimical to life, but it may turn out to have actually set the stage for its emergence.
by Paul Gilster | Sep 15, 2005 | Deep Sky Astronomy & Telescopes
Astronomers have detected the most distant explosion ever observed, finding the afterglow of a gamma ray burst that marked the end of a massive star and the probable birth of a black hole. Named GRB 050904, the object’s redshift is 6.29, pegging it as roughly 13 billion light years from Earth. The universe itself is now thought to be 13.7 billion years old, so the burst comes from the era when stars and galaxies had only recently formed.
Gamma rays force astronomers to work fast. Most bursts are sudden events, lasting only about ten seconds, which is why alerts are sent out whenever NASA’s Swift satellite detects one. But while the bursts are brief (and don’t even penetrate the atmosphere), the afterglow of these mammoth explosions can linger long enough to be observed by instruments on the ground. Which is what UNC-Chapel Hill astronomer Daniel Reichart immediately set out to do.
As telescopes around the globe locked onto the afterglow, Reichart’s team was able to measure the explosion’s distance using an array of six automated telescopes in Chile called the Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT). The PROMPT array is just entering service, and Reichart’s initial work showed that the burst’s aftereffects were not visible. That was actually good news — it meant either that the object was enveloped in dust or that its light was shifted well into the infrared because of a high redshift.
Image (above): Stars shine by burning hydrogen. The process is called nuclear fusion. Hydrogen burning produces helium “ash.” As the star runs out of hydrogen (and nears the end of its life), it begins burning helium. The ashes of helium burning, such as carbon and oxygen, also get burned. The end result of this fusion is iron. Iron cannot be used for nuclear fuel. Without fuel, the star no longer has the energy to support its weight. The core collapses. If the star is massive enough, the core will collapse into a black hole. The black hole quickly forms jets; and shock waves reverberating through the star ultimately blow apart the outer shells. Gamma-ray bursts are the beacons of star death and black hole birth. Credit: Nicolle Rager Fuller/NSF.
“This is when we got really excited, because this is exactly the sort of signature that you are hoping to see if you are looking for very distant objects,” Reichart said. The dust interpretation was quickly rejected as new observations came in from PROMPT and the Southern Observatory for Astrophysical Research (SOAR) telescope on Cerro Pachón in Chile. Both photometric and spectroscopic observations confirmed the high redshift. Another unusual thing about GRB 050904 — it lasted over 200 seconds, twenty times the usual length of such a burst.
Unusually, Reichart relied on several undergraduate students in making the detection; a UNC junior, Josh Haislip, will be first author on the upcoming research paper. From a National Optical Astronomy Observatory news release:
“SOAR is a new telescope that is still in the advanced commissioning phase. The instrument that we used, OSIRIS, was just put on the telescope and made available for science observations a few weeks before this event,” Reichart said. “In fact, I had asked Josh to do some extra research on the instrument to be prepared just in case, since my more senior students were away at a gamma-ray burst workshop! He and the other students on the team did an excellent job, and are now well-prepared to work on future bursts with SOAR and PROMPT.”
The explosion was the object of intense study worldwide, from Japan’s Subaru Observatory in Hawaii to the European Southern Observatory’s Very Large Telescope. An interesting note on the explosion’s luminosity comes from this ESO press release: “…it is among the intrinsically brightest Gamma-Ray Bursts ever observed,” said Guido Chincarini from INAF-Osservatorio Astronomico di Brera and University of Milano-Bicocca (Italy) and leader of a team that studied the object with ESO’s Very Large Telescope. “Its luminosity is such that within a few minutes it must have released 300 times more energy than the Sun will release during its entire life of 10,000 million years.”
