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...