by Paul Gilster | Aug 24, 2009 | Exoplanetary Science |
A conference like the recent on in Aosta offers plenty of opportunity to listen in on fascinating conversations, one of which had to do with what would happen if we found a brown dwarf closer to the Earth than the Centauri stars. The general consensus was that such a find would be a powerful stimulus to the public imagination and would probably result in renewed interest in getting to and exploring such a place. A boon, in short, for all our interstellar efforts, an awakening to a new set of possibilities.
But if there were a brown dwarf that close, wouldn’t we have other signs of it? One figure I heard mentioned at Aosta was three light years. Here I have to do some checking, because I don’t recall who dropped that figure or what paper he was referring to, but the upshot was that someone has argued that even a small brown dwarf closer to the Sun than three light years would leave an unmistakable signature in the orbits of our Solar System’s planets. I’ll see if I can track down the original reference (see note below). In any case, we didn’t have any astronomers in our number at Aosta to check the figure against.
A brown dwarf out there waiting to be discovered may not be unknown for long. In fact, we may well be no more than a few years away from finding it. The Wide-field Infrared Survey Explorer (WISE) mission is set for a November 1 launch from Vandenberg Air Force Base and should be able to put the matter to rest. Among the many activities of this observatory will be to find cold, dim stars. WISE should track down about a thousand brown dwarfs, among them those closest to our Solar System. That’s quite an exciting thought, as Peter Eisenhardt (JPL) has opined:
“We’ve been learning that brown dwarfs may have planets, so it’s possible we’ll find the closest planetary systems. We should also find many hundreds of brown dwarfs colder than 480 degrees Celsius (900 degrees Fahrenheit), a group that as of now has only nine known members.”
That figure of nine may be off by now, as Eisenhardt made this comment back in early June. But whatever the number, the infrared detectors on WISE are going to revise our understanding of the nearby brown dwarf population. Unlike the Spitzer Space Telescope and ESA’s Herschel Space Observatory, WISE will survey the entire sky in an effort to build target lists for current and future observatories like the James Webb Space Telescope. Bear in mind, as noted in this news release, that right now we’re still using the catalog produced by the Infrared Astronomical Satellite (IRAS), a fine set of data but drawn from a mission that flew in 1983.
Image: An infrared image of M16, the Eagle Nebula, taken by the ESA/ISO satellite. The false-color image was constructed from a 7.7 micron infrared exposure (shown as blue), and a 14.5 micron infrared exposure (shown as red). This nebula is the site of active star formation in the Milky Way Galaxy. WISE will observe the region in similar wavelengths of light to see the dust that often enshrouds star forming regions. Credit: European Space Agency.
The actual WISE mission is relatively short. The full-sky mapping will take six months (after a one-month checkout of the system), to be followed by a second, probably partial scan that will depend upon the health of the spacecraft’s hydrogen coolant, necessary to cool the infrared detectors. I don’t want to underplay WISE’s role in identifying both near-Earth objects and main belt asteroids, either, nor its ability to find more distant planetary systems in formation. This is going to be one significant mission.
Ponder for a moment where we are, a point in history where we’re about to answer some questions that have preoccupied scientists for decades. Does Alpha Centauri have planets around either of its main stars? Debra Fischer or Michel Mayor’s Geneva team, working separately, may well have an answer within a few short years (and, perhaps, even months). Are there other terrestrial worlds out there, and in what number? Both Kepler and COROT are sending us data that will help us make a preliminary call, again within just a few years. And now WISE, which may soon be able to tell us whether there is indeed a brown dwarf closer than Centauri. Has there ever been a time of astronomical discovery more packed with excitement than this one?
A Quick Administrative Note: I’m unexpectedly in a situation where I’m having to write these entries without a working Internet connection. I then have to shoot the entry up to the site the next time I’m in my office, making Net time difficult and keeping me away from many resources. If you happen to know the reference re the three light year limit on brown dwarfs, please let me know. I’m not quite sure when things are going to get back to normal, so posting may be a bit erratic here, at least for a while, and when I can post, it will probably be in the afternoon rather than the morning.
by Paul Gilster | Aug 21, 2009 | Deep Sky Astronomy & Telescopes |
Back in April a paper appeared in the Astrophysical Journal that drew into question our view of star populations. We’ve assumed since the 1950s that we could count the stars in a particular area of sky by looking at the light from the brightest and most massive stars. In making this assumption, we were tapping the initial mass function, a way of describing the mass distribution of a group of stars in terms of their initial mass.
We could, then, estimate the total number of stars based on a sample of the stars that were the easiest to see, assuming that a set number of smaller stars ought to have been created in the same region. Every star twenty or more times as massive as the Sun should be accompanied, in this thinking, by about 500 stars of solar mass or less. But Gerhardt R. Meurer (Johns Hopkins University) and team used data from the Galaxy Evolution Explorer to challenge these proportions.
The numbers, it turns out, don’t work out as consistently as we had thought. Says Meurer:
“What this paper is showing is that some of the standard assumptions that we’ve had – that the brightest stars tell you about the whole population of stars – this doesn’t seem to work, at least not in a constant way.”
Many galaxies, this work shows, fail to form large numbers of massive stars, but continue to produce large numbers of their less massive counterparts. Indeed, for every massive star there may be as many as 2000 lower-mass ones, depending on the galaxy or region of sky being considered.
Image: Images taken with the Galaxy Evolution Explorer at shorter ultraviolet wavelengths are dark blue, while longer ultraviolet wavelengths are light blue. The optical images are colored red and yellow; red light is shown in yellow, while specially filtered red light from a type of hydrogen emission called H-alpha is colored red. By combining the data, astronomers were able to learn that not all galaxies make stars of different sizes in the same quantities, as was previously assumed. In other words, the proportion of small to big stars can differ from galaxy to galaxy. Credit: NASA/JPL-Caltech/JHU.
Ultraviolet images from the Galaxy Evolution Explorer were contrasted with filtered optical images from the Cerro Tololo International Observatory in Chile to obtain this result, the latter sensitive to the largest stars, the former to stars three times or more massive than the Sun. We thus gain a new way of estimating stellar populations, and are forced to re-evaluate galaxies that may be more crowded than we thought.
The paper is Meurer et al., “Evidence for a Nonuniform Initial Mass Function in the Local Universe,” Astrophysical Journal 695 (10 April, 2009), pp. 765-780 (abstract). For more, see this feature on the JPL site.
by Paul Gilster | Aug 21, 2009 | Culture and Society |
Do you have any astronomy books you could spare? Larry Klaes has passed along word from Mimi Burbank, a friend from the History of Astonomy e-mail list who lives and works in Uganda. Living in Kasese, Mimi has been involved in educational activities for people living in a rural area with few resources. She’s trying to gather books on astronomy from childrens’ books up to adult levels. Mimi writes as follows:
The people here are very poor and there are no resources for education, and so I have been asking my friends from all around the world to send books and other things. I have received almost a hundred books for children of all ages, and the little NGO that I work with (BUFO) has achieved extremely high scores on their leaving exams at the end of the school year. They have instituted a Saturday reading hour, during which the older children who can read, read stories out of the books to the younger children who can not yet read, and they all love it. This is the beginning of a ‘reading culture’.
As part of that reading culture, Mimi has been teaching the children about the formation of the universe and our relationship to the cosmos around us. She notes that science is under-taught in this part of the world because of the lack of material and books, but is sharing what she has with the local schools. If you can help with any book donations, you can send them to Mimi at:
Mimi Burbank
c/o South Rwenzori Diocese
PO Box 142
Kasese, Uganda
East Africa
And she adds: “You don’t need to ship a huge shipment (very expensive) – for small packages the cost is minimal, and each and every gift is so very much appreciated. Thank goodness, books have a flat rate and a cheaper rate!”
Mimi’s own Web page is here, with links to the local groups she works with. Any donations would be received with profound gratitude.
by Paul Gilster | Aug 20, 2009 | Exotic Physics |
Sometimes what you don’t detect tells a scientific story just as important as what you do. In the case of LIGO (Laser Interferometer Gravitational-Wave Observatory) and the VIRGO Collaboration, we’re talking about setting limits to the amount of gravitational waves that would have been produced by the Big Bang. Those waves, predicted by Albert Einstein in 1916 and consistent with his theory of General Relativity, should be traceable and quite valuable to us, carrying as they do information about the earliest stages of the universe.
Image: Modeling gravitational wave complexity. Laser interferometers should be able to detect the gravitational waves produced by the most violent astrophysical events, such as the merging of two black holes. Credit: MPI for Gravitational Physics/W.Benger-ZIB.
The gravitational waves ought to be out there (General Relativity predicts that all accelerating objects should produce them) but they have yet to be observed directly. In fact, the so-called ‘stochastic background’ — this Caltech news release likens it to the superposition of numerous waves of varying strengths going in different directions on the surface of a pond — is what LIGO and the VIRGO interferometer are probing, looking into the universe as it was in the first minute of its existence. Lee Samuel Finn (Penn State) has a more colorful description:
“Space-time is the living stage upon which the drama of the universe plays out. The primordial stochastic gravitational waves are the warps, twists, and bends in space-time that were laid down as the universe expanded from its earliest moments to the present. The observations we report in this paper are the closest direct examination of the framework of the living, breathing universe.”
What we find in the new work, drawing on data from 2005 to 2007 and just published in Nature, is that the stochastic background of gravitational waves has yet to be discovered. But the data are precise enough that that fact is itself significant. It allows us to set upper limits on the phenomenon. Vuk Mandic (University of Minnesota) explains:
“Since we have not observed the stochastic background, some of these early-universe models that predict a relatively large stochastic background have been ruled out. We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old. We also know that if cosmic strings or superstrings exist, their properties must conform with the measurements we made — that is, their properties, such as string tension, are more constrained than before.”
The VIRGO Collaboration makes use of a three-kilometer long interferometer in Cascina, Italy and works with LIGO’s 700 scientists in jointly analyzing data gathered by various instruments, which include the GEO600 interferometer near Hannover, Germany. LIGO itself is composed of detectors in Hanford, WA and Livingston, LA, each using a laser beam split into two beams that travel down the interferometer arms.
The difference between the lengths of those arms should tell the tale, for General Relativity predicts that a passing gravitational wave should stretch one arm slightly while compressing the other. We’re talking about detecting a change of less than a thousandth the diameter of an atomic nucleus in the relative lengths of the arms. That’s enough to get us to today’s result, but Advanced LIGO, which comes online in 2014, is to be ten times more sensitive still, allowing us to probe various models of the early universe including those involving cosmic strings.
To place all this in perspective, recall that the Cosmic Microwave Background that has been so valuable in our studies of the early cosmos can take us back to about 380,000 years after the Big Bang. We’re now pushing into the investigation of early universe models that can take us back to the cosmos’ first minute. The paper, a joint work of the LIGO Scientific Collaboration and the Virgo Collaboration, is “An upper limit on the amplitude of stochastic gravitational wave background of cosmological origin,” Nature 460 (20 August 2009), pp. 990-994 (abstract).
by Paul Gilster | Aug 19, 2009 | Culture and Society |
On Returning to the Moon
Interesting to see that the recent debate in the pages of The Economist on whether or not we should return to the Moon has reference to the outer Solar System. The debate pits Gregg Maryniak (James S. McDonnell Planetarium, St. Louis) against Mike Gold (Bigelow Aerospace). Normally the Moon is off our agenda in these pages because of our focus on the outer system and beyond, but my friend Frank Taylor noticed that among Maryniak’s arguments for a return to the Moon was its utility as a staging point.
Specifically, Maryniak argues that in addition to its other uses, the Moon lets us get our ‘space legs’ by learning about shielding human crews and ‘living off the land’ in a deeply inhospitable place. All of this may well lead to lunar power stations or the collection of Helium-3 for fusion projects, a developing technology with profound implications. Writes Maryniak:
Once we have the ability to capture and transmit energy at the megawatt and gigawatt levels we will see fast solar system travel. By beaming power to future space travelers we can free them from the intrinsic limitations of the chemical energies embedded in their propellants. Having both abundant energy and materials available in free space will also enable such useful things as cleaning up orbital space debris and mitigating the threat of Earth impact from asteroids and comets. The use of lunar materials and later asteroid and comet resources will ultimately enable probes beyond our solar system.
The whole debate, won by Maryniak by a vote of 61 to 39 percent, is well worth reading, and is particularly worth considering in light of recent arguments by Buzz Aldrin and others that Mars is the preferable next step.
Alien Worlds on the TV
A quick note that the National Geographic Channel will be offering two shows of interest this weekend. Alien Earths is a look at exoplanet possibilities with astrobiological implications, including exotic places like those shown in the accompanying video that find ways to support life in the absence of a star.
The other show is Naked Science: Hawking’s Universe, a look at the many contributions this extraordinary physicist has made to our understanding of the universe. Check local listings for Sunday, August 23rd for these.
Dark Energy or Spacetime Waves?
I’ve been working my way through a preprint of a paper arguing that dark energy is not what many scientists think. Joel Smoller (University of Michigan) and Blake Temple (UC-Davis) believe that an expanding wave moving through spacetime could be the reason why distant galaxies appear to be accelerating as they move away from us. The dark energy debate centers on the idea that dark energy fuels the acceleration, but Smoller and Temple will have none of it. Quoted on Space.com, Temple notes:
“We’re saying there isn’t any acceleration. The galaxies are displaced from where they’re supposed to be because we’re in the aftermath of a wave that put those galaxies in a slightly different position.”
What’s interesting about this is that it allows us to explain the anomalous acceleration with the confines of classical general relativity, seeing the anomaly as not an acceleration at all, but what the authors call a ‘correction to the Standard Model due to the fact that we are looking outward into an expansion wave.” Here’s more (note that I’m quoting from the preprint, not the published paper):
Unlike the theory of Dark Energy, this provides a possible explanation for the anomalous acceleration of the galaxies that is not ad hoc in the sense that it is derivable exactly from physical principles and a mathematically rigorous theory of expansion waves. In particular, this explanation does not require the ad hoc assumption of a universe filled with an as yet unobserved form of energy with anti-gravitational properties in order to fit the data.
Those possible ‘anti-gravitational’ properties naturally arouse the interest of propulsion-minded people, implying exotic new forms of transportation. But only if the enigmatic dark energy actually exists to serve as a model. Clara Moskowitz’ story in Space.com notes how may tests the new theory will need to pass before it will become convincing. Thus Mario Livio (Johns Hopkins University), who says that a model like this must be able to predict properties of the universe that astronomers can measure, and adds “To only produce an apparent acceleration is in itself interesting, but not particularly meaningful.”
The paper is Smoller and Temple, “Expanding Wave Solutions of the Einstein Equations that Induce an Anomalous Acceleration into the Standard Model of Cosmology,” Proceedings of the National Academy of Sciences, published online August 17, 2009 (abstract). A preprint is available.
