by Paul Gilster | Dec 24, 2005 | Deep Sky Astronomy & Telescopes
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 components were perpendicular to the slit. Since A and B are separated by many arcseconds, there was no contamination. (Actually, a small amount of contamination would not matter too much because the two stars oscillate at very different periods).
Thus we can separate the two stars, if needed, by noting the frequency of their oscillations. The precision of these measurements continues to amaze me. Clearly, we have it in our capability to learn a great deal more about this fascinating binary system, and about the third star — Proxima Centauri — that may or may not be gravitationally attached to it. Both Centauri A and B seem potential candidates for life-bearing planets, especially since stable orbits have now been determined in the habitable zones of each.
The above image of Alpha Centauri (marked with the arrow), Beta Centauri and the Southern Cross seems a fitting one as we move into the holidays. Stepping back from the purely scientific aspects of their study, the Alpha Centauri stars hold rich symbolic meaning as the closest known targets for interstellar exploration. In that sense, they continue to captivate those who dream of a human future in the cosmos. They are natural destinations for our first probes and, perhaps one day, our colonies.
Image Copyright Akira Fujii / David Malin Images.
by Paul Gilster | Dec 23, 2005 | Deep Sky Astronomy & Telescopes
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 both Paul Butler and Geoff Marcy, planet hunters extraordinaire, were on the team studying Centauri B. After all, planet hunters know all about tiny Doppler shifts.
The star is pulsating because gas in its outer layers is in violent enough motion to create low-frequency sound waves that bounce off inner layers of the star. Measuring the light from Centauri B once a minute for seven consecutive nights, the researchers made more than 5000 observations.
From an AAO news release:
A star’s surface can oscillate in many different patterns, or modes, simultaneously. The researchers were able to determine 37 modes of oscillation in alpha Centauri B. They also measured the mode lifetimes (how long the oscillations last), the frequencies of the modes, and their amplitudes (how far the surface of the star moves in and out).
Other stars, including our own Sun, are known to vibrate like Centauri B, but these observations are the most detailed ever made of such oscillations. From the data, we should learn much about Centauri B’s internal composition. The paper is Kjeldsen, Hans et al., “Solar-like Oscillations in α Centauri B” in the December 20 issue of the Astrophysical Journal. An abstract is available here.
Centauri Dreams‘ take: It’s extraordinary to consider that we are finding ways to study the internal dynamics of stars, learning about regions within them that are completely hidden from view. Our Sun’s pulsations occur about every five minutes and are apparently caused by the same phenomenon being studied on Centauri B. People talk about stars ‘ringing’ because sound waves become trapped in an internal region that can act like the cavity of a musical instrument. The waves are actually variations in pressure that move through the gases within the star, where temperature and density change drastically between the surface and the stellar core. Studying these ‘peals’ of sound should make our nearest neighbors just a little less mysterious.
by Paul Gilster | Dec 22, 2005 | Breakthrough Propulsion, Deep Sky Astronomy & Telescopes |
Wormholes make for great science fiction because they get us around the speed-of-light conundrum. Taking a shortcut through spacetime, they connect one part of the universe to another, though where and when you would come out if you went in a wormhole would be an interesting experiment, and not one for the faint of heart. But do we have any evidence that wormholes exist, and if they did, what could we look for that might reveal their presence?
Perhaps it’s time to revisit a fascinating 1994 paper called “Natural Wormholes as Gravitational Lenses.” The authors are a compendium of names known to anyone with an interest in the physics of interstellar flight or its depiction in science fiction: John G. Cramer (whose columns in Analog set high standards for science writing); Geoffrey A. Landis (Mars Crossing and innumerable short stories); Gregory Benford (whose bibliography of novels is too long to list); Robert Forward (the leading proponent of interstellar studies) and two other physicists whose work deserves a wider audience: Michael Morris and Matt Visser.
It was Visser (Washington University, St. Louis) who suggested a possible configuration for a wormhole that frames it with ‘struts’ of exotic material, the struts having a negative mass density that could result in an interesting object indeed, what the paper describes as ‘…a flat-space wormhole mouth framed by a single continuous loop of exotic cosmic string.’
Geoffrey Landis calls cosmic strings ‘flaws in geometry,’ but you can also think of them as flaws in the structure of spacetime itself. They’re quite useful in imagining wormholes because to preserve a primordial wormhole formed at the beginning of the universe, you need to wrap it in negative energy, and a negative mass cosmic string could do the trick. There are plenty of conditions here, but Landis put it this way in an interview I did with him back in 2003: “If one of these hypothetical negative mass cosmic strings got wrapped around a hypothetical primordial wormhole, you could have a hypothetical stable primordial wormhole, one that could still exist.”
Detecting such an object becomes a fascinating exercise in itself. We know how to look for the signature of gravitational lensing, as in imagery of a distant galaxy that has been shaped by the gravitational influence of an intervening galaxy. A wormhole should show a negative mass signature that, instead of focusing light, does the opposite. The signature of that ‘defocusing’ is characteristic. Landis again:
“If the wormhole is exactly between you and another star, it would defocus the light, so it’s dim and splays out in all directions. But when the wormhole moves and it’s nearer but not in front of the star, then you would see a spike of light. So if the wormhole moves between you and another star and then moves away, you would see two spikes of light with a dip in the middle.”
As theoretical as all this sounds, it’s actually quite useful. As the authors of the wormhole paper put it: “…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.” It makes sense, then, given the interest among astronomers in observing normal gravitational lensing with positive mass objects, to keep open the possibility of finding such a signature, which would provide our first solid evidence of wormholes.
And it also corresponds with how science works today. Much of the analysis of the voluminous data collected by our instruments is performed by computer software. And the value of a paper like this one, beyond its purely scientific interest, is that it identifies a pattern that such software should be written to recognize among all the other patterns that clue researchers in to interesting findings. It would be absurd to have solid evidence of a wormhole in data that was never analyzed in precisely the right direction.
As for that wormhole itself, an ancient one from the earliest days of the cosmos wouldn’t be of much use from a transportation perspective — you have to get to it first, after all, and it might be millions of light years away. And then there’s that problem of figuring out where it comes out on the other side. But it may be that a Kardashev Type II civilization, able to use all the energies of a star, could create artificial wormholes using negative energy, and if that is the case, the universe may have shortcuts galore through the Einstein barrier.
The paper is John Cramer, Robert L. Forward, Gregory Benford et al., “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124-27, also available on the arXiv site (and thanks to Gregory Benford for forwarding a copy). Be sure to read Robert Forward’s novel Timemaster for his fictional take on negative energy and many other ideas from the outer limits of science. In addition to being a fascinating speculative romp, it’s a rich and funny book.
by Paul Gilster | Dec 21, 2005 | Astrobiology and SETI, Exoplanetary Science |
The case for life around other stars, always a strong one, has become even more persuasive of late. First we found planet formation around HD 12039, a Sun-like star about 137 light years away, revealing a system that may look like our Solar System in its infancy. Now comes news based on findings from the Spitzer Space Telescope that astronomers have observed acetylene and hydrogen cyanide in the inner regions of the debris disk around the star IRS 46. Both gases are organic compounds considered to be precursors to DNA and RNA.
IRS 46 is located in the constellation Ophiuchus about 375 light years from Earth. Like HD 12039, it is a young star, surrounded by a disk of gas and dust that should, if our theories hold, house the raw materials of planets. Astronomers at the W.M. Keck Observatory (Mauna Kea), Leiden Observatory and the Netherlands Institute for Space Research used Spitzer’s infrared spectrometer to study 100 stars, but IRS 46 was the only one to reveal signs of an organic mix. One reason may be that its disk is oriented in such a way as to allow Spitzer to extract the best observational data, according to Fred Lahuis of Leiden Observatory.
These are intriguing findings, to say the least. Acetylene and hydrogen cyanide can work together in the presence of water to become some of the necessary ingredients for DNA and protein. Here’s what Geoffrey Blake of Caltech, a co-author of the soon to be published paper on these findings, has to say about such reactions:
“If you add hydrogen cyanide, acetylene and water together in a test tube and give them an appropriate surface on which to be concentrated and react, you’ll get a slew of organic compounds including amino acids and a DNA purine base called adenine. And now, we can detect these same molecules in the planet zone of a star hundreds of light-years away.”
Further data from the James Clerk Maxwell Telescope on Mauna Kea also points to a stellar wind pushing out against the gaseous disk, one that may have a hand in eventually clearing the debris and revealing the presence of rocky, terrestrial planets several million years from now.
Image: An artist’s conception of the dusty disk orbiting IRS 46. Credit: NASA/JPL-Caltech/T. Pyle (SSC).
We’ve found organic gases like these around other celestial objects. They’re easy to spot in the atmospheres of our own gas giants and appear both on Titan and on cometary surfaces in the Solar System. The European Space Agency’s Infrared Space Observatory has also found them around certain massive stars, but IRS 46 is the first Sun-like star around which they have been confirmed. The Keck observations suggest the organic materials may be no more than 10 AU from the star, about the distance between Saturn and the Sun, but more work needs to be done to refine this distance.
The paper on this work will appear in the January 10 issue of the Astrophysical Journal Letters.
by Paul Gilster | Dec 20, 2005 | Research Tools
Springer’s series on astronautical engineering produces expensive books, as the 1999 publication of Colin McInnes’ Solar Sailing: Technology, Dynamics and Mission Applications made clear. There is no more thorough analysis of solar sailing in print, but the title was designed for professionals and printed in small quantity, with a corresponding pricetag. I was able to snag a used copy for about $100, though Amazon now has a few for $70 or so. High quality, high expense information continues to flow, raising the question of how we can open up its pages to a wider audience.
Now Stephen Kemble’s Interplanetary Mission Analysis and Design is out from Springer at $179. Like the McInnes title, it’s a solid, detailed work. Of particular interest is a thorough discussion of gravity assist and transfer techniques, along with sections on deep space communications and navigation that update earlier references. Mission designs from nuclear to ion propulsion are presented along with specific deep space scenarios. Coupled with the second edition of Gregory Matloff’s Deep Space Probes: To the Outer Solar System and Beyond (Springer, 2005), the duo offer a comprehensive if expensive background on today’s state of the art. Any sound engineering library should have these and the McInnes volume on its shelves.
But surely the term ‘on its shelves’ points to the problem. We are not doing a sufficient job at exploiting digital text to broaden the reach of our information. The independent researcher is always bedeviled by this issue. Centauri Dreams is fortunate to have three outstanding universities within easy driving distance; library access to even the most obscure journals isn’t a problem, though finding the time for the needed library trip sometimes is. Online, some key journals can be found in full-text format, others by abstract only, and this is with full access to proprietary databases, which those without academic connections are unlikely to have. As for books, I can get to the Springer titles listed above (and usually cough up the money to keep such titles on my shelves anyway), but note how restricted the availability of such volumes is for those out of university range.
I’m now studying the excellent OpenReader concept that is producing XML standards for electronic texts. The ThoutReader software produced by OSoft is a superb platform for onscreen reading and research; OSoft also intends to produce a free e-text reader in early 2006, one that embraces the OpenReader standards. Because of their implications for cost-effective publishing and broadened information access, I will be keeping a close eye on these projects here on Centauri Dreams. We are a long way from having a true digital library, but projects like these show that good people are working on breakthroughs. You should also be aware of David Rothman’s excellent Teleread weblog; Rothman is a key player in OpenReader and a tireless advocate for a national digital library system.
