When it comes to making precise observations, nothing can beat the VLBA. Short for Very Long Baseline Array, this system of ten radio antennae is dispersed over the Earth's surface from Mauna Kea (Hawaii) to St. Croix (Virgin Islands), using 25-meter dishes to create an interferometer 5000 miles wide. The array is controlled from an operations center in Socorro, New Mexico. All those dishes make for remarkably sharp resolution, the best of any telescope in existence. And they're needed to make the kind of observations recently performed by a team of astronomers studying the Perseus arm of the Milky Way. The nearest spiral arm to the Sun, the Perseus arm has now been shown to be much closer than previously thought, some 6400 light years as opposed to an earlier estimate of 14,000. The image below shows the location of the Sun and W3OH, a newly formed star in the Perseus arm in the region under study. Image: Mark Reid and his colleagues measured the distance to the Perseus spiral arm...

When a neutron star gives off pulses of radiation every time it rotates, it's called a pulsar. The radiation, which moves along the star's magnetic field lines, is often compared to a lighthouse beam sweeping across an ocean. Now a pulsar called Geminga has been found to leave a comet-like trail of high-energy electrons as it muscles its way through the nearby interstellar medium at about 120 kilometers per second. Geminga is close in interstellar terms, a mere 500 light years from Earth, and because it is moving across our line of sight, it offers unprecedented material for observation. The 'cometary' tail shows up on data gathered by the Chandra X-ray Observatory; the same team found twin x-ray tails stretching billions of kilometers behind the object in 2003, using data from ESA's XMM-Newton. As for Geminga itself, this incredibly dense core of an exploded star is about 20 kilometers across but contains the mass of our Sun. Although most pulsars emit radio waves, Geminga is silent...

The recent work on the oscillations of Centauri B, discussed in yesterday's entry, had me thinking deep into the afternoon as I dodged holiday traffic en route to the grocery. What Tim Bedding (University of Sydney) and Hans Kjeldsen (Aarhus University, Denmark) had done by coordinating the efforts of two major observatories was to explore the inner workings of one of the nearest stars. But Centauri A and B are a close pair (they close to within about 10 AU, roughly Saturn's distance to the Sun, at one point in their elliptical orbits, while at other times they are as distant as Pluto). Wouldn't Centauri A's light be a problem for a measurement as precise as this one? The answer is no, as Dr. Bedding was kind enough to clarify in an e-mail. Here's the gist of what he had to say: Each spectrograph has an entrance slit which sits at the focus of the telescope. The slit is narrow (less than an arcsecond) and can be rotated to any angle, so we ensured that is was rotated so that the two...

Information on Alpha Centauri comes in all too slowly for Centauri Dreams, but astronomers at the Anglo-Australian Telescope and the European Southern Observatory's Very Large Telescope in Chile have come to the rescue. They've teamed up to observe Centauri B, an orange K1 star slightly cooler and less massive than the Sun. In question was the rate at which the star's surface is pulsating, which tells us about its temperature and internal composition. The precision of these observations is remarkable. A moving stellar surface causes slight alterations in the wavelength of light it emits; the study of this Doppler shift supplies information. Centauri B's surface moves about 300 meters an hour, surely a tiny figure to determine given the 4.3 light-year distance to the target, not to mention the encroaching light of Centauri A, the star's close companion. And yet the sensitivity of the instruments in question was better than 1.5 cm/s, or less than 0.06 km per hour. It makes sense that...

One of the important projects keeping astronomers busy as we wait for the next generation of both ground and space-based telescopes is mapping the local neighborhood. There is much to be learned, for example, in a star like Sirius, one of the Sun's closest neighbors at 8.6 light years. Since 1862, we've known that Sirius is orbited by a white-dwarf star, and that this burnt-out remnant of an earlier star is terrifically difficult to study because of the glare of Sirius itself. Sirius B is about ten thousand times dimmer than Sirius. Now Hubble has changed that picture, with an international team of astronomers having isolated the light of the white dwarf. This allows them to deduce its mass by examining how its gravitational field alters the wavelength of the light it emits. And what a gravitational field it is. The measurements show that Sirius B is about 7500 miles in diameter (smaller than Earth) but its gravitational field is 350,000 times stronger. That's enough to cause a...

Gravitational lensing is tricky enough to measure, but how can we use it to track down the elusive 'dark matter' that constitutes the great bulk of the matter in the universe? Remarkably, researchers at Johns Hopkins, working with the Space Telescope Science Institute, think they have found a way. Using the Hubble telescope, they've measured how gravity from unseen dark matter creates small distortions in the shapes of galaxies as seen from Earth. Their work has focused on two galactic clusters in the southern sky roughly 7 billion light years away; each contains more than 400 galaxies. That dark matter is a mystery needs no elaboration here, as it's always been a reminder that our knowledge of the universe is limited to a small subset of the things we can see and understand. Indeed, dark matter is only part of the story. Some 70 percent of the entire universe is now thought to be 'dark energy,' an even more mysterious ingredient that plays a role in the expansion of the cosmos. With...

The Hubble Space Telescope used its Wide Field and Planetary Camera 2 to create the image below, which is actually made up of 24 separate exposures -- this is said to be the highest resolution image of the Crab Nebula ever made. Be sure to click on the image to explore it in detail. I had planned to use an intriguing Robert Forward quote for today's entry (Saturday's are usually a day for reflections and overviews), but this image was just too pretty to resist. The Crab Nebula is about six light years wide, the remains of a supernova that is reliably dated at 1054 as witnessed by both Chinese and Japanese astronomers. Recall that the distance from the Sun to the primary Centauri stars is 4.3 light years and you get a sense of scale here. The filaments you're seeing are primarily hydrogen, lit blue from within by a spinning neutron star that is the remaining core of the supernova. The neutron star emits twin beams of radiation that pulse 30 times a second due to its extreme rotational...

Hubble's recent findings about 'Einstein rings' remind us of the value of using gravitational lensing to observe distant objects. When light from a distant galaxy is bent by an intervening galaxy, the effect can be to create multiple separate images of the more distant source. But line up the two galaxies exactly and the gravitational bending causes the intriguing phenomenon called an Einstein ring, which is something like the pattern of a bull's eye around the foreground galaxy. Einstein rings are useful objects to astronomers, as witness this news release from the Harvard-Smithsonian Center for Astrophysics: "An Einstein ring is one of the most dramatic demonstrations of the general theory of relativity in the cosmos," said Adam Bolton of the Harvard-Smithsonian Center for Astrophysics (CfA). "It provides an unique opportunity to study the most massive galaxies in the universe." Interesting, too, from a mission point of view, for as Centauri Dreams continues to opine, a mission to...

What can you do with 1600 detectors spread out over 3000 kilometers surrounded by an array of 24 telescopes? If you're in Argentina's Mendoza Province, the answer is that you can witness the arrival of high-energy cosmic rays. The 'Cherenkov' detectors, each containing 3000 gallons of water, detect the passage of the particles while the telescopes examine the ultraviolet fluorescence produced by their arrival in the atmosphere. The detector array covers an area roughly the size of Rhode Island. All this is occurring at the Pierre Auger Observatory, just east of the Andes on the Argentine Pampas. Auger was the first scientist to observe the interactions between Earth's atmosphere and cosmic rays back in 1938. The observatory named for him draws on the talent of 60 institutions in 16 countries. The presentation of the first physics results from the site took place this week in the Argentine town of Malargüe. Image: The Andes Mountains form a snow-capped backdrop to the west of the...

We can measure interstellar distances, but can we really grasp them? The distance to the nearest stars is so immense that even the scientists who study such things have resorted to homely comparisons. The most charming to my mind is that of the English astronomer Sir John Herschel (1792-1871), the son of the famous William Herschel who discovered Uranus. A wizard at mathematics, Herschel became a leading expert on double stars and the measurement of stellar distances through parallax (the apparent change in position of a nearby star against background stars due to the Earth's changing orbit around the Sun). When it came to the distance to the Alpha Centauri stars, Herschel saw things in terms of ocean voyaging, thinking himself standing on shipboard dropping peas into the water. As he once wrote, ". . . to drop a pea at the end of every mile of a voyage on a limitless ocean to the nearest fixed star, would require a fleet of 10,000 ships of 600 tons burthen, each starting with a full...

The Spitzer Space Telescope once again dazzles us with its capabilities at infrared wavelengths. Now it's the detection of what may be some of the earliest objects in the universe, the hypothetical Population III stars that would have formed a mere 200 million years after the Big Bang itself. These short-lived objects were probably over a hundred times more massive than our Sun. If the scientists investigating the recent Spitzer data are right, they are looking at the redshifted ultraviolet light of these ancient stars, stretched to lower energy levels by the expansion of the universe and now detected as a diffuse glow of infrared light. Image: The top panel is an image from NASA's Spitzer Space Telescope of stars and galaxies in the constellation Draco, covering about 50 by 100 million light-years (6 to 12 arcminutes). This is an infrared image showing wavelengths of 3.6 microns, below what the human eye can detect. The bottom panel is the resulting image after all the stars,...

Imagine two neutron stars colliding, or even worse, a neutron star and a black hole. The release of energy would be catastrophic, and has apparently now led to the first detection in visible light from a short gamma-ray burst. Thus we're beginning to get a handle on the most powerful explosions in the known universe, whose identity has bedeviled astronomers for thirty years. There are actually two different kinds of gamma-ray bursts. The longer ones have been linked to the explosion of a massive star as it collapses into a black hole. It's the short-duration bursts that have proven the greater challenge. The new work, performed at La Silla (Chile) and at the European Southern Observatory's Very Large Telescope, used data from NASA's HETE-2 satellite to guide the observations, and the fading source was found. "That was the clue we were waiting for," said Garrett Jernigan, a research physicist at the Berkeley Space Sciences Laboratory. "Bursts seem to come mainly in two varieties - the...

The sharp-eyed Jon Lomberg writes with a correction to today's story on Xena and its moon Gabrielle. Specifically, my statement that adaptive optics 'bounces' the light of a laser off the atmosphere to create an artificial star used in refining the telescope's images. Lomberg rightly points out that what the laser actually does is to excite sodium atoms at a specific height. The glow from this excitation is then tracked and used to adjust for atmospheric distortion. The results, as we have seen, are nothing short of spectacular. What's ahead for adaptive optics? "A future improvement of the technique," writes Lomberg, "would use different lasers to excite other elements at other altitudes, thus giving a more detailed profile of distortion in the atmosphere resulting in more precise adjustments."

Are elliptical galaxies influenced by a halo of dark matter? The theory has been accepted until recently through observation of the gravitational effects apparently caused by such matter. But 2003 findings (Romanowsky et al., Science 301, pp. 1696-1698) turned up little evidence for dark matter in such galaxies. Now a different explanation for those observations has surfaced, one that seems to rescue the dark matter concept. That's good news, because dark matter ought to be there. From a University of California at Santa Cruz press release: "A dearth of dark matter in elliptical galaxies is especially puzzling in the context of the standard theory of galaxy formation, which assumes that ellipticals originate from mergers of disk galaxies," added Avishai Dekel, professor of physics at the Hebrew University of Jerusalem and first author of the Nature paper. "Massive dark matter halos are clearly detected in disk galaxies, so where did they disappear to during the mergers?" The dark...

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,...

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...