We spend a lot of time probing the borderlines of astronomy, wondering what the boundaries are between a large gas giant and a brown dwarf, for example. The other end of that question is also intriguing: When does a true star get small enough to be a brown dwarf? For main sequence stars don't operate the same way brown dwarfs do. Add hydrogen to a main sequence star and its radius increases. But brown dwarfs work the opposite way, with additional mass causing them to shrink. We see this beginning to happen at the high end of the brown dwarf mass range, somewhere between 60 and 90 Jupiter masses. Electron degeneracy pressure, which occurs when electrons are compressed into a very small volume, is at play here. No two electrons with the same spin can occupy the same energy state in the same volume -- this is the Pauli exclusion principle. When the lowest energy level is filled, added electrons are forced into higher energy states and travel at faster speeds, creating pressure. We see...

How many brown dwarfs should we expect in the Milky Way? I can recall estimates that there could be as many brown dwarfs as main sequence stars back when people started speculating about this, but we have to go by the data, and what we have so far tells another tale. The WISE (Wide-field Infrared Survey Explorer) mission can only come up with one brown dwarf for every six stars, leading Davy Kirkpatrick (Caltech), who is part of the WISE science team, to say "Now that we're finally seeing the solar neighborhood with keener, infrared vision, the little guys aren't as prevalent as we once thought" (see Brown Dwarfs Sparser than Expected). Image: Brown dwarfs in relation to the Sun and planets. Credit: NASA/WISE mission. This is true, at least, in the Sun's vicinity, where WISE identifies about 200 brown dwarfs, with 33 measured within 26 light years. In the latter volume, some 211 other stars can be found. If we extrapolated this to the entire galaxy, we would get about 33 billion...

I haven't yet read Stephen Baxter's new novel Proxima, but because of my admiration for his previous books, it's at the top of my reading list. Judging from the Amazon description, Proxima gets into issues that for me make red dwarfs utterly compelling. What would a habitable planet look like around such a star, tidally locked so that its sun never moved in the sky? What would it be like to move around this world, going from a warm substellar point toward twilight and then a frigid night on the dark side? Given that this M-class red dwarf is 18,000 times fainter than the Sun, you wouldn't expect it to make much of an impression in photographs. The one above (credit: European Southern Observatory) is instructive because it puts the entire Alpha Centauri system in context. At top left we have Centauri A and B, which are bright enough to merge together and appear as a single bright object. At the lower right is the arrow indicating Proxima Centauri, so faint as to be barely visible....

Because of my fascination with exotic venues for astrobiology, I’ve always enjoyed Karl Schroeder’s novels. The Canadian writer explored brown dwarf planets as future venues for human settlement in Permanence (2002), and in his new book Lockstep (soon to be published by Tor, currently being serialized in Analog), Schroeder looks at ‘rogue’ planets, worlds that move through the galaxy without a central star. Imagine crimson worlds baked by cosmic radiation, their surfaces building up, over the aeons, the rust red complex organic molecules called tholins. Or consider gas giants long ago ejected from the system that gave them birth by close encounters with other worlds. Objects like these and more are surely out there given what we know about gravitational interactions within planetary systems, and they’re probably out there in huge numbers. I’m not going to review how Lockstep uses them just yet -- in any case, I haven’t finished the book -- but we’ll return to its ingenious solution...

Brown dwarfs fascinate me because they're the newest addition to the celestial menagerie, exotic objects about which we know all too little. The evidence suggests that brown dwarfs can form planets, but so far we've found only a few. Two gravitational microlensing detections on low mass stars have been reported, one of which is a 3.2 Earth-mass object orbiting a primary with mass of 0.084 that of the Sun, putting it into the territory between brown dwarfs and stars. The MEarth project has uncovered a planet 6.6 times the mass of the Earth orbiting a 0.16 solar mass star. Now a new proposal to use the Spitzer Space Telescope to hunt for brown dwarfs planets is available on the Net, one that digs into what we've found so far, with reference to the discoveries I just mentioned: Accounting for their low probabilities, such detections indicate the presence of a large, mostly untapped, population of low mass planets around very low mass stars (see also Dressing & Charbonneau (2013))....

Apropos of yesterday's article on the discovery of Proxima Centauri, it's worth noting that Murray Leinster's story "Proxima Centauri," which ran in Astounding Stories in March of 1935, was published just seven years after H. A. Alden's parallax findings demonstrated beyond all doubt that Proxima was the closest star to the Sun, vindicating both Robert Innes and J. G. E. G. Voûte. Leinster's mile-wide starship makes the first interstellar crossing only to encounter a race of intelligent plants, the first science fiction story I know of to tackle the voyage to this star. The work surrounding Proxima Centauri was intensive, but another fast-moving star called Gamma Draconis in Draco, now known to be about 154 light years from Earth thanks to the precision measurements of the Hipparcos astrometry satellite, might have superseded it. About 70 percent more massive than the Sun, Gamma Draconis has an optical companion that may be an M-dwarf at about 1000 AU from the parent. Its bid for...

It's fascinating to realize how recent our knowledge of the nearest stars has emerged. A little less than a century has gone by since Proxima Centauri was discovered by one Robert Thorburn Ayton Innes (1861-1933), a Scot who had moved to Australia and went on to work at the Union Observatory in Johannesburg. Innes used a blink comparator to examine a photographic plate showing an area of 60 square degrees around Alpha Centauri, comparing a 1910 plate with one taken in 1915. Forty hours of painstaking study revealed a star with a proper motion similar to Alpha Centauri (4.87" per year), and about two degrees away from it. The question Innes faced was whether the new star was actually closer than Alpha Centauri, an issue that could be resolved only with better equipment. Ian Glass (South African Astronomical Observatory) tells the tale in a short paper written for the publication African Skies. Innes ordered a micrometer eyepiece that would be fitted to the observatory's 9-inch...

I keep a close eye on gravitational lensing, not only because of the inherent fascination of the subject but also because the prospect of using the Sun’s own lensing to study distant astrophysical phenomena could lead to near-term missions to 550 AU and beyond. And because I’m also intrigued by ‘standard candles,’ those markers of celestial distance so important in the history of astronomy, I was drawn to a new paper on the apparent gravitational lensing of a Type Ia supernova (SNIa). This is the kind of supernova that led to the discovery of the accelerating expansion of the universe by giving us ways to measure the distance to these objects. The point about Type Ia supernovae is that they are so much alike. We may not fully understand the mechanisms behind their explosions, but we have overwhelming evidence that these supernovae reach nearly standard peak luminosities. There is also a strong correlation between their luminosity and other observables like the shapes of their...

When I was in Huntsville for the recent interstellar conference, I noticed people walking around with black rubber wristbands that said ‘Build a Star Ship.’ Space educator Mike Mongo was handing these out to all concerned, and I soon picked one up to give my grandson. They’re an interesting form of marketing -- leave these in the right places and kids pick them up. Maybe it becomes a fad to wear them. The point is, you never know whose mind you might reach. And maybe once or twice, you give a wristband to a kid who starts dreaming about the stars, and pretty soon that leads to a course of study and then one day a career. The Roman historian Plutarch said it best: “The mind is not a vessel to be filled, but a fire to be ignited.” And when ignition occurs, it’s often a sudden, passionate event rather than a slow building of sequential ideas. It’s that fire that drives subsequent study and makes long hours amidst the databases and classrooms pay off in the form of eventual insights and...

Habitable zones are always easy enough to explain when you invoke the ‘Goldilocks’ principle, but every time I talk about these matters there’s always someone who wants to know how we can speak about places being ‘not too hot, not too cold, but just right.’ After all, we’re a sample of one, and why shouldn’t there be living creatures beneath icy ocean crusts or on worlds hotter than we could tolerate? I always point out that we have to work with what we know, that water and carbon-based life are what we’re likely to be able to detect, and that we need to fund the missions to find it. The last word on habitable zone models has for years been Kasting, Whitmire and Reynolds on “Habitable Zones around Main Sequence Stars.” Now Ravi Kopparapu (Penn State) has worked with Kasting and a team of researchers to tune-up the older model, giving us new boundaries based on more recent insights into how water and carbon dioxide absorb light. Both models work with well defined boundaries, the inner...

The American Astronomical Society's meeting in Long Beach is going to occupy us for several days, and not always with exoplanet news. Brown dwarfs, those other recent entrants into the gallery of research targets, continue to make waves as we learn more about their nature and distribution. The hope of finding a brown dwarf closer than Alpha Centauri has faded and recent work has emphasized that there may be fewer of these objects than thought -- WISE data point to one brown dwarf for every six stars. But habitable planets around brown dwarfs are not inconceivable, and in any case we are continuing to build the census of nearby objects. The latest from AAS offers up what could be considered a probe of brown dwarf 'weather.' If the idea of weather on a star seems odd, consider that the cooler brown dwarfs are far closer to gas giants than stars, unable to trigger hydrogen fusion and gradually cooling as they age. That means cloud patterns form and huge storms plow through the various...

How do we get from clouds of gas and dust in interstellar space to stars like the Sun? It takes the right triggering event, which can cause such a cloud to collapse under its own gravity, and we've generally assumed that the trigger was a supernova. Indeed, one way to check the theory is to look for the radioactive isotope iron 60 (60Fe), which is considered a marker for a supernova as it can only originate in such an event. Early Solar System materials have shown high levels of iron 60, so a supernova has been assumed to have nudged the Solar System into formation. But Haolan Tang and Nicolas Dauphas, two researchers from the University of Chicago, have produced results than draw this picture into question. Their samples of meteorites were the same materials other researchers had studied, but the Chicago team used different methods to remove impurities from the observation, producing results with, they believe, fewer errors. What they found was that levels of iron 60 were steady --...

Planets without stars may exist throughout the galaxy, and some studies suggest that there may be more so-called ‘rogue planets’ than main sequence stars. Now we get word of an object called CFBDSIR2149, associated with the stream of young stars called the AB Doradus Moving Group, a group of about 30 stars of the same age and metallicity associated with the star AB Doradus. The object turned up at infrared wavelengths in the Canada-France Brown Dwarfs Survey (CFBDS), which is looking for cool brown dwarf stars. If the object is indeed part of the AB Doradus group, then we can deduce that it is young (50-120 million years) and, according to this ESO news release, a number of its other properties can be inferred. Image (click to enlarge): This image captured by the SOFI instrument on ESO’s New Technology Telescope at the La Silla Observatory shows the free-floating planet CFBDSIR J214947.2-040308.9 in infrared light. This object, which appears as a faint blue dot at the centre of the...

An inventor named Tom Ditto has been casting a serious look at diffraction gratings as large primary collectors for telescopes, work that has been getting a bit of a buzz on the Internet. See, for example, An Old Idea Gives Telescopes a New Twist, and ponder how much the eponymous Dittoscope sounds like something out of a Tom Swift story. Nonetheless, an instrument based on a diffraction grating as its primary light-gathering source may prove useful in a variety of astronomical settings, including the ability to produce extremely high quality spectroscopic information for radial velocity exoplanet searches. Diffraction happens when a small obstacle or opening causes a wave of light to interfere with itself, creating patterns that depend on the size of the diffracting object and the size of the wave. A diffraction grating, in this case a flat surface with a regular pattern of grooves, can be used to separate different wavelengths of light, which will interfere at different angles. The...

I remember having a particularly strong 'sense of wonder' moment when reading Poul Anderson's "The Snows of Ganymede" when I was a kid. Anderson was good at this kind of thing, but really my reaction was not just to this story but to the whole notion of taking a distant astronomical object and placing people in it. A bright point in the telescope suddenly becomes a landscape and you feel your sense of scale - the sheer immensity of things - beginning to shift under your feet. These thoughts are triggered by the latest news from the Dawn mission about Vesta, and a UCLA news release commenting on the asteroid's steep topography, which often leads to landslides. Immediately I was thinking of stark drops and boulder-strewn regolith with no friendly blue/green Earth in the sky and wondering what it would be like to see Vesta in person. Thus dreams accumulate. Ray Bradbury found that attaching names to unknown places is a distinctively human enterprise, and one that when abandoned...

The idea of planets outside their normal settings is unsettling. It implies that beyond the stars all around us there may be worlds without suns, dark planets presumably pushed there by gravitational instabilities in their home systems. We've looked at such 'nomad' worlds before, noting that ice overlaying a frozen ocean might trap enough geothermal heat to create life-sustaining conditions. Estimates on how many such planets might exist vary widely, but in one recent paper Louis Strigari (Stanford University) has calculated that 105 objects larger than Pluto may exist for every main sequence star (see Island-Hopping to the Stars for more on Strigari's work on free-floating planets). Survey missions like Gaia may help us find some of these. If a wandering world between the stars is a chilling prospect, what about stars that wander between the galaxies? We have plenty of evidence for their existence. Strip away the emissions from our own Solar System and the Milky Way itself and what...

Our view of Pluto/Charon gets better and better as we prepare for the New Horizons flyby in 2015. It wasn't so long ago that we had no idea Pluto had a single moon, much less the five we have so far identified. When James Christy (USNO) discovered Charon in 1978, he was looking at photographic plates of Pluto that showed little more than a slight bulge that appeared periodically. The bulge came into and out of view as the [dwarf] planet rotated, indicating a genuine object and not a flaw in the observation, but although Charon had been found, we couldn't resolve separate disks until the Hubble Space Telescope's images of the 1990s. Image: Plates taken with the Kaj Strand Astrometric Reflector at the USNO Flagstaff Station in Arizona revealed the existence of Charon, visible as an elongation in the image. Later eclipses and occultations of Pluto and Charon confirmed the discovery. Charon has a mass of about 22 percent that of Pluto. Credit: Wikimedia Commons. The Hubble images,...

The Magellanic Clouds, visible in the southern hemisphere, are two dwarf galaxies that orbit the Milky Way, a fact that has always captivated me. We see the galaxy from the inside, but I have always wondered what it would be like to see it from the perspective of the Magellanics. The Large Magellanic Cloud (LMC) is, after all, only 160,000 light years out, while the Small Magellanic (SMC), its companion cloud, is about 200,000 light years away. Add in the recently discovered Sagittarius Dwarf Elliptical at 50,000 light years from the galactic core and you have three exotic venues from which to gain a visual perspective on the Milky Way, at least in the imagination. We’re so used to thinking that our solar and galactic neighborhoods are utterly commonplace that it may come as a surprise to learn that the configuration of spiral galaxy and satellite galaxies that we see in the Milky Way is actually quite unusual. New work on this comes from Aaron Robotham (International Center for...

by Dr. Gregory L. Matloff Gregory Matloff is a major figure in what might be called the 'interstellar movement,' the continuing effort to analyze our prospects for travel to the stars. Greg is Emeritus Associate Professor and Adjunct Associate Professor in the Department of Physics at New York City College of Technology as well as Hayden Associate at the American Museum of Natural History. Centauri Dreams readers will know him as the author (with Eugene Mallove) of The Starflight Handbook (Wiley, 1989) and also as author or co-author of recent books such as Deep Space Probes (2005), Living Off the Land in Space (2007) and Solar Sails: A Novel Approach to Interplanetary Travel (2010). My own acquaintance with Greg's work began with the seminal JBIS paper "Solar Sail Starships: The Clipper Ships of the Galaxy" (1981), and the flow of papers, monographs and books that followed have set high standards for those investigating our methods for going to the stars, and the reasons why we...