Purveyors of doom often look to the heavens for their protagonists. During the 1980s, we were briefly captivated by Nemesis, a supposed companion of the Sun that triggered a death-dealing rain of comets every 26 million years. During the 1990s we endured wild speculations about Nibiru, which managed somehow not to destroy Earth in 2003.

Now there's a new threat — but unlike Nemesis and Nibiru, this one's real. It's called Gliese 710 (pronounced GLEE-zuh), an obscure, 10th-magnitude orange dwarf star situated about 63 light-years away in the constellation Serpens. Astronomers first took note of this modest star about a decade ago, when Joan García-Sánchez (Jet Propulsion Laboratory) and others found, based on positional observations from the Hipparcos satellite, that in roughly 1½ million years Gliese 710 should pass about 1.3 light-years from the Sun.

That's not close enough for the gravitational pull of a 0.6-solar-mass star to unhinge the planets from their current orbits. But it would stir up some trouble in the Oort Cloud, the storehouse of perhaps a trillion comets that theoretically extends to the limit of the Sun's gravitational grip. García-Sánchez and her team estimated that 2.4 million Oort Cloud denizens might be perturbed into Earth-crossing orbits over a couple million years, increasing the risk of our planet being hit by a long-period comet by only 10%. No big whoop.

But a revised Hipparcos catalog of stars' distances and motions came out in 2007, and a fresh analysis of that and other datasets by Vadim V. Bobylev (Pulkovo Astronomical Observatory) shows that Gliese 710 is a little more problematic than first thought.

After running a million computer simulations that factored in various observational errors, Bobylev confirms that the star has an 86% chance of passing through the Oort Cloud's outer limit, assumed to be 1.6 light-years from the Sun. Moreover, the simulations yield a 1-in-10,000 chance of skirting within 0.02 light-year, or about 1,000 astronomical units. A star passing by only 50 times farther out than Pluto would not only wreak havoc within the Oort Cloud but also do a number on comets stashed in the Kuiper Belt.

Could such a close encounter really happen? It probably already has. Dynamicists point to the fact that the Kuiper Belt appears to end abruptly at a distance of about 60 a.u. Then there's the strange 4-million-year orbit of 90377 Sedna, whose solar distance ranges between 76 and 976 a.u. No mere planetary encounter could have flung Sedna out that far; the only plausible explanation is the gravitational yank from a close-passing star long ago.

On the plus side, Bobylev's analysis of 35,000 stars within 100 light-years of the Sun didn't turn up any new interlopers of consequence within the past or forthcoming 2 million years. He's added nine new stars to the list of past or future encounters of 6½ light-years or less, none of which come nearly as close as Gliese 710. Some 27 and 28 million years from now, respectively, the stars Proxima and Alpha Centauri sail past our solar system but come no closer than 3 light-years.

Every year, New York’s Hayden Planetarium hosts a debate in memory of the renowned science and science-fiction writer Isaac Asimov. The topic of the most recent debate, held on March 15, 2010, was “Moon, Mars and Beyond: Where Next for the Manned Space Program.” The debaters — or more accurately, panelists — were all extraordinarily well qualified to discuss the subject:

Kenneth Ford, expert on artificial intelligence and human cognition, and chairman of the NASA Advisory Council.

Lester Lyles, retired Air Force General, formerly in charge of diverse military space programs.

Paul Spudis of the Lunar and Planetary Institute, one of the world’s leading lunar scientists, and a major advocate for a sustained human presence on the Moon.

Steven Squyres of Cornell University, co-leader of and spokesman for the Mars Exploration Rover Project.

Robert Zubrin, aerospace engineer, popular writer, and leader of the movement to launch a quick and aggressive manned Mars program.

And last but by no means least, the moderator: Neil deGrasse Tyson, astrophysicist, space expert, television star, director of the Hayden Planetarium — and (like me) a graduate of the Bronx High School of Science.

The event proved to be wildly popular, despite very modest publicity. Hundreds of people were standing on line when I arrived, a half hour early, and hundreds more joined later. Every seat in the very large auditorium was taken, and a large overflow crowd watched the debate by simulcast. Many of the remarks — particularly those by Zubrin, but also many others — elicited prolonged cheers and applause.

Tyson opened the event, explaining that although billed as a debate, it would in fact be more like an informal discussion, a bunch of interested people sitting around a table at a bar. The reality was rather different, proceeding more like a television interview. After brief opening remarks by each participant, Tyson would ask each panelist a question, sum up the answer, and then ask a related question of the next panelist. In less competent hands, the format might have been stifling. But Tyson’s questions were so cogent and his summations so accurate that he elicited a very lively yet (on the whole) courteous interchange among the panelists.

Toward the end of the event, the format was broken by Buzz Aldrin, who had been invited to participate by telephone. Aldrin proceeded to lay out his program for manned spaceflight in intricate detail and at very great length. Tyson tried to cut Aldrin short several times, but it’s hard to dominate someone who isn’t physically present, and it’s equally hard to say no to the second man to walk on the Moon. After Aldrin had finished, the format was more open and direct, proceeding soon to questions from the audience — several of which were more statements or tirades than questions at all.

Tyson had intended the debate to focus on whether we should proceed directly to Mars (as advocated most strongly by Zubrin) or use the Moon as a trial and staging exercise, as proposed (though never funded) by the Bush administration. But in the interim between the planning and the event, the Obama administration had proposed yet another course, focusing neither on the Moon nor Mars but instead on the development of new technologies and the privatization of space. Zubrin reserved his greatest scorn for this new plan, pronouncing it to be a death sentence for human spaceflight, and stating that NASA had never accomplished anything useful except in pursuit of a lofty and specific goal, like the first human Moon landing. All the other panelists had more nuanced responses, ranging from very strong though mild-mannered support of NASA’s technology programs from Ford and Lyles to an agreement by Spudis and Squyres (and perhaps Tyson) that NASA needed to be more goal-oriented, but not necessarily fixated on Mars as Zubrin is.

I can only touch on a few highlights of the discussion; it ranged over a very wide range of subjects. Spudis is perhaps the strongest advocate for the Moon as a destination in its own right, not just a stepping-stone to Mars. Squyres brought up asteroids as a potential destination, suggesting that mining asteroids is the most promising commercial application of spaceflight beyond Earth orbit. Several people alluded to the possibility of landing on Mars’s moon Phobos, a proposal that’s taken very seriously by the human-spaceflight community, though it's never been noticed much by the wider public.

Almost everybody talked about the commercialization of space. Many people spoke, both pro and con, about space as a demonstration of national prestige and prowess. Intriguingly though perhaps not surprisingly, retired General Lyles was the strongest advocate of international cooperation, and the strongest opponent of using space as an instrument of national competition.

I was intrigued that nobody either on the panel or in the audience questioned the aggressive pursuit of human spaceflight. In my mind that’s very much an open question — which is not the same thing as saying that the answer to the question is necessarily no. But refusing to take such a fundamental question seriously, taking human spaceflight as axiomatic and not enquiring into its rationale, is a sign of weakness rather than strength.

But for better or worse, this was a panel and audience of true believers — and a very lively, fascinating, and invigorating bunch it was.

Few things are more dramatic in interplanetary exploration than receiving that first crucial "I've arrived" message from a far-flung craft or regaining radio contact with one after some risky maneuver. For more than 40 years, mission teams in the U.S. and elsewhere have gotten word on the status of their robotic emissaries using NASA's worldwide set of tracking dishes — collectively called the Deep Space Network.

Managed by the Jet Propulsion Laboratory, the DSN consists of three antenna clusters, two in the Northern Hemisphere and one in Australia, spaced more or less equally in longitude. For convenience, they're said to be located at "Goldstone" (nothing more than an abandoned mine and ghost town) in California's Mojave Desert, "Canberra" (actually situated in Australia's Tidbinbilla Valley), and "Madrid" (technically 20 miles west of the city in Robledo de Chavela, Spain).

Whatever you call them, the DSN's dozen dishes are constantly in demand, maintaining contact with nearly 50 spacecraft. But not every mission gets all the tracking time it wants — in 2008 about 25% of tracking requests never happened — and even bigger scheduling problems loom on the horizon.

An especially bad "asset contention period," as the dish jockeys call it, comes in 2015. That's when New Horizons zips past Pluto, Dawn settles into orbit around Ceres, and the European probe Rosetta reaches Comet 67P/Churyumov-Gerasimenko — and that's not counting four other deep-space missions that will need support during their launch and all the other craft needing some quality time with their scientists and engineers.

So the DSN's caretakers have just kicked off a major program to fortify their complexes with new antennas and to provide arguably its most important receiver with some much-needed TLC.

Topping the list are adding two 110-foot (34-m) high-efficiency antennas at the Australian site, which has only three dishes right now. (The U.S. and Australia have cooperated in spacecraft tracking since 1960, a partnership dramatized in the 2000 movie The Dish.) New 34-m receivers will be added in California and Spain as well.

Each station will also have one of its existing antennas upgraded to transmit in the high-frequency K_a band (which permits high data rates through a relatively small antenna).

Finally, DSN managers will take pains to extend the life of the towering 230-foot (70-m) behemoths that dominate each DSN site. NASA hopes to get another 10 to 20 years of life out of the Big Three, which have been workhorses for four decades (Goldstone's was build in 1966).

Already under way is what's being called "major surgery" involving the gigantic bearings on which Goldstone's big dish turns in azimuth and altitude. Talk about your engineering challenges! For this joint replacement, engineers must first raise the entire 9,000,000-pound structure by just under ¼ inch (5 mm). The repairs should be complete by November.

But the 70-m dishes won't last forever — moreover, they can't be upgraded to handle the K_a band. So they'll eventually be decommissioned, though not until all the new ones are operational. That's reassuring to asteroid scientist Lance Benner, who uses the big Goldstone dish (a.k.a. DSS 14) to bounce radar pulses off small asteroids passing near Earth.

"We have a new radar capability that uses 'chirped' waveforms," he explains. When asteroid 2010 AL₃₀ approached within
one third of the Moon's distance in January, the new system yielded range measurements accurate to about 12 feet (3.75 m) — "a five-fold improvement," Benner beams, "and twice as fine as the highest range resolution available at Arecibo."

I've been lucky enough to visit all three DSN sites — and I've crawled around on the 70-m giants in California and Australia. To look out across these antennas' curved surfaces is a surreal experience — one I hope to repeat in Spain someday to complete my tracking-station trifecta.

Any parent of teenagers will tell you that it's often a challenge to keep enough food in the house. It's amazing how often — and how much — they can eat.

The same could be said for the appetite of our home galaxy.

Today the Milky Way and the Andromeda Galaxy (M31) are the massive "anchors" in the Local Group of galaxies, about 30 denizens in all. Most of these are dwarf galaxies, diminutive systems such as the Magellanic Clouds with several billion stars (compared to our galaxy's 200 to 400 billion). But in the past there must have been far more dwarfs, and astronomers have assumed for some time that the Milky Way was a ravenous teenager, having devoured many if not most of the dwarfs in its immediate vicinity.

Like crime-scene investigators, observers have been looking for evidence of the Milky's Way's fits of cannibalism in the halo of matter that surrounds its main spiral; that's where the stars, gas, dark matter, and globular clusters from the consumed victims end up. Two recent publications demonstrate that cosmic detectives have chased down some promising leads.

First comes word, from Duncan Forbes and Terry Bridges of Australia's Swinburne University, that up to one quarter of the Milky Way's globular clusters formed elsewhere and migrated into its halo. The observers used the Hubble Space Telescope to assess the age and composition of dozens of globulars lying within about 65,000 light-years of our galaxy's center, then pooled their data with other observations.

The upshot, described in a paper submitted to Monthly Notices of the Royal Astronomical Society, is that roughly 25% of the Milky Way's globulars came from 6 to 8 dwarf galaxies accreted by the Milky Way over time. As Forbes notes in a Society press release, "Our work shows that there are more of these accreted dwarf galaxies in our Milky Way than was thought."

More evidence of the Milky Way's unsated appetite comes from Anna Frebel (Center for Astrophysics), Evan Kirby (Caltech), and Joshua Simon (Carnegie Observatories). In the March 4th issue of Nature, they make the case that a star in the Sculptor dwarf galaxy is chemically similar to the Milky Way's oldest stars and thus might be among the earliest stars to form after the Big Bang.

Frebel's team identified an 18th-magnitude star in the Sculptor that contains almost no "metal" — a term astrophysicists apply to any elements other than hydrogen or helium. In fact, this star has only 1/6000 the heavy-element content of the Sun.

Here's why that matters: Stars cook up heavy elements through fusion and supernovas, and every subsequent generation of stars incorporates the elemental ashes left by its predecessors. If a star lacks these "metals," it must have formed too long ago to have swept up the recycled carbon, iron, and other elements found in younger stars.

By this measure, lots of stars in the Milky Way are quite old — but such ancient suns had never been found in the feedstock of dwarfs that surround it. "If dwarf galaxies were the original components of the Milky Way," explains Simon in a press release, "then it's hard to understand why they wouldn't have similar stars." Now that one's been found, he adds, the idea that the Milky Way's halo represents the corpses of many dwarf galaxies "does indeed appear to be correct."

As my S&T colleague Tony Flanders describes in his observing blog, this is a particularly good time of year for northern skygazers to seek the night-sky glow known as the zodiacal light.

Eerie and elusive, it appears after evening twilight as a towering but feeble cone of light that, under ideal, ultradark circumstances, can be traced far along the ecliptic.

The zodiacal light arises from sunlight scattering off countless tiny flecks of dust drifting through the inner solar system. Imagine an enormous, swollen pancake of tenuous dust with the Sun at its center, and you'll have the right idea. Over the years many scientists have taken a stab at explaining this phenomenon — some more fanciful than others. My 40-year-old college astronomy textbook says it's likely due to a dusty tail that trails Earth as it orbits the Sun. (Not!)

Since the glow is brightest along the ecliptic, it's logical to assume that asteroids play a major role in its formation, and that's what theorists believed in the mid-1990s. More recently, however, they've come to realize that cometary dust must play a role, though the exact mix has been largely guesswork.

Last year a five-member team of dynamicists, led by David Nesvorný (Southwest Research Institute) decided to tackle the zodiacal light's origin from first principles. They modeled what would happen to dust released from various sources — asteroid collisions, comets arriving on random orbits from the Oort Cloud, and especially "Jupiter-family comets" (orbital periods of less than 20 years) — and kept track of what went where.

Then, like any good chef, they tinkered with the recipe until their model matched the zodiacal light's true appearance. It wasn't enough to match the visible-light glow in the pre- and post-twilight sky, which comes mostly from particles inside Earth's orbit that scatter sunlight strongly in our direction. The model also had to match the sizes and concentrations of dust lying outside Earth's orbit — a diffuse cloud of grit with a distinct infrared signature that's been recorded by a host of spacecraft.

At the outset, Nesvorný felt he could get a good match by combining dust from asteroids (to match the glow's peak along the ecliptic) and comets from the Oort Cloud (to explain its vertical breadth).

But the model provided a very different answer: virtually all the dust must be coming from short-period comets, with a little contribution from Oort Cloud comets. No more than 10% of it can be coming from the asteroid belt. Moreover, these Jupiter-family comets don't just sprinkle fairy dust along their orbits — more likely, they cough up pulses of debris by breaking up repeatedly, dozens of times, over their lifetimes.

The curves at right tell the story: asteroidal dust is a poor match to reality, whereas cometary dust gives a near-perfect fit. All told, Nesvorný and his team estimate that there must be some 20 trillion tons of dust in the zodiacal cloud (twice the mass of the Martian moon Phobos), and that 100,000 tons of the stuff falls to Earth every year.

The team's exhaustive analysis even considers what the situation must have been like billions of years ago, when the solar system teemed with comets. The answer is that the zodiacal light would have been hundreds or thousands of times brighter than it is now. Imagine trying to stargaze with all that "natural" light pollution in the sky!

My high-school students are probably tiring of my telling them how much information about a star can be divined simply from measuring its light, especially when the observations involve multiple spectral regions. Well, they're going to hear it again, when I tell them the amazing story of HM Cancri.

With a visual magnitude of 21, this star wasn't even known to astronomers until 1999, when the orbiting Rosat observatory discovered an X-ray source that brightens and dims every 5.4 minutes. To observers, such isolated X-ray emitters usually scream "interacting binary" — including a degenerate has-been of a star, a white dwarf or neutron star, that's pulling matter off a hapless companion. The exiting stream slams onto the dwarf or neutron star (or onto a disk around it) violently enough to generate X rays.

But could HM Cancri's twosome actually be whirling around each other in less time than I usually need to take out the trash? No other known binary system has a period this short (the closest contender, V407 Vulpeculae, clocks in at 9½ minutes). In the past decade theorists have offered two alternative interpretations, which require a binary system with a dense object either rotating or orbiting with a 5.4-minute period.

New observations seem to clinch the idea that HM Cancri's two stars really are locked in a 5.4-minute whirlwind dance. An international team led by Gijs Roelofs (Harvard-Smithsonian Center for Astrophysics) captured the system's spectrum last year using the Keck I telescope on Mauna Kea. The researchers had been doggedly persistent — they'd wanted to make these observations in 2005, 2006, and 2007, but bad weather scuttled their plans each time.

The Keck spectra reveal blue-light emission from hot neutral helium (presumed to come from the donor star) and from ionized helium (from the hotter, denser star accreting the matter). What makes the case ironclad is that these emissions are Doppler-shifted in opposite directions. When one source is moving away from us, the other is moving toward us, and vice versa, clinching the case for a fast binary. A single neutron star, no matter how fast it's spinning, can't match the Keck observations. The artist's concept at upper right shown the presumed situation.

Roelofs and his colleagues calculate that HM Cancri's stars are only about 3 Earth diameters apart — so close that the donor star (also a white dwarf, with about a quarter of the Sun's mass) must be swollen and distended by the pull of its heavier companion (with about half a Sun's mass). A double peak in the ionized-helium emission also implies that a fast-spinning ring of matter encircles the accreting star, either as a belt on its surface or as a disk above it.

One remaining loose end is finding an explanation for why the paired stars are gradually twirling faster and faster. Since the total mass is conserved, the mass transfer can't explain it. "The binary HM Cancri is a real challenge for our understanding of stellar and binary evolution," notes team member Gijs Nelemans (Radboud University, The Netherlands) in a press release. The team also hopes to pin down the system's distance, which is only roughly estimated to be 16,000 light-years.

No matter how these stars became so closely entwined (they're much too close to have been born that way), HM Cancri's tight whirlabout must make it one of the strongest sources of gravitational waves in the galaxy. As high-energy astrophysicist Tod Strohmayer (NASA-Goddard Space Flight Center) points out, "This object is likely radiating more energy in gravitational waves than in electromagnetic energy." That in itself could account for the pair's orbital-energy loss and period speedup. Not surprisingly, it's high on the list of targets for the European Space Agency's proposed (but as yet unfunded) Laser Interferometer Space Antenna, designed to detect sources of low-frequency gravitational waves using a triad of widely separated spacecraft.

Despite Saturday's massive earthquake in central Chile, and the considerable loss of life that ensued, there is at least some good news to report. Several Chilean friends have e-mailed me to report that the major professional and amateur observatories have escaped damage and remain operational.

There are two reasons why the major observatories escaped damage. First, they are well constructed, built to withstand earthquakes. Second, the largest observatories are situated in northern Chile and were thus many hundreds of miles away from the quake's epicenter near the city of Concepción.

This tragedy has affected me personally because I visited Chile for a week in early December. While there I toured two major amateur observatories and several professional telescopes. This trip was the subject of my Spectrum column for the April issue of S&T. I had the opportunity to meet many leading Chilean amateur astronomers, and I have remained in contact with them since my trip. After hearing of the earthquake I sent e-mails to several of them, and the responses I'm now getting indicate that the observatories are fine, and that everyone I met is safe. This has been a tremendous source of relief.

Eric Escalera, a staff astronomer at the Observatorio del Pangue, reports: "In Vicuña we are fine, because the earthquake was much farther to the south, although it is distressing to see that so many people disappeared. None of the great observatories have been damaged, so all the people you met are fine."

Daniel Luna, who lives near the north-central port city of La Serena, reports: "Here we are okay, the earthquake affected five regions of Chile, from Santiago to the south. It has been a dramatic fact, very chaotic and worrying. But fortunately we are all fine, even the observatories. Because Chile is a seismically active country, our observatories must be prepared for that. So the telescopes are designed to swing when there's an earthquake."

Despite this good news, the thoughts and prayers of the S&T staff go out to the people of Chile and Haiti. We wish them a speedy recovery from these disasters. And despite the tragic earthquake, Chile remains a wonderful place for astro-tourism, especially because the regions of greatest interest to amateur astronomers are in northern Chile, which suffered minimal or no damage from the quake.

Some of the major professional observatories include the European Southern Observatory sites at Paranal and La Silla, Gemini South Telescope (near Vicuña), Cerro Tololo Inter-American Observatory (also near Vicuña), and Las Campanas Observatory. Other major amateur observatories include Observatorio Cerro Mamalluca (near Vicuña) and Observatorio Cruz del Sur (near Combarbalá). I have yet to receive a report from this observatory, but have no reason to expect any significant damage.

Press announcements from major professional observatories:
European Southern Observatory
Cerro Tololo Inter-American Observatory
Las Campanas Observatory

To view photos from my December 2009 trip, which include major amateur and professional observatories, visit my personal Chile photo web gallery.

Right now I'm enduring winter in snow-encrusted Boston, but my head is in The Woodlands, Texas, where more than 1,200 researchers have gathered for the annual Lunar and Planetary Science Conference. The LPSC is always a 'best bet" for hearing new discoveries about the solar system, and yesterday got off with a bang as scientists discussed water ice on the Moon — not whether it's there, but how much of it might be lying around.

I think 2009 will be remembered as the year we finally convinced ourselves that water is present on the Moon. Planetary scientists have speculated about its existence there for decades, because some craters at the lunar poles never, ever see sunlight and thus might serve as "cold traps" where ice (arriving on comets, presumably) could remain stable. But last year three lunar spacecraft returned results that have made the case for lunar ice all but certain.

The most dramatic findings came after NASA "bombed" the polar crater Cabeus last October 9th with its Lunar Crater Observation and Sensing Satellite (LCROSS) — though it took mission scientists a while to certify that the impact had dredged up water from the crater's shadowed floor.

It turns out there's more to this story: Yesterday NASA-Ames scientist Anthony Colaprete announced that, in addition to water, LCROSS had unearthed a whole kettle of volatile compounds in the plume of impact debris that rose from Cabeus. So far the tally includes sulfur dioxide (SO₂), methanol (CH₃OH), and the curious organic molecule diacetylene (H₂C₄).

Last year scientists also realized that whiffs of water cling to much of the lunar surface. An instrument built by American scientists but carried on India's Chandrayaan 1 spacecraft found the spectroscopic signature of water and hydroxyl (OH) ions over much of the Moon. Carlé Pieters, the Brown University researcher whose team designed the Moon Mineralogy Mapper (M³), had fortuitously extended its near-infrared coverage enough to record 2.8 and 3.0 microns, where the OH and water molecules have telltale absorptions.

Yesterday another Chandrayaan 1 instrument made a splash at LPSC, so to speak. Paul Spudis (Lunar and Planetary Institute) announced that NASA's Mini-SAR instrument had found evidence for water ice in more more than 40 small craters near the Moon's north pole. (Any water ice is not going anywhere soon — the Diviner instrument aboard NASA's Lunar Reconnaissance Orbiter finds that the interiors of these craters are the coldest locations ever measured in the solar system.)

The craters range in size from 1 to 9 miles (2 to 15 km), and according to a NASA press release the lunar stash could total up to 600 million tons of ice, depending on its thickness within each crater.

Utilizing a compact synthetic aperture radar, Mini-SAR detects changes in the polarization of radio waves reflected off the lunar surface. Water ice is transparent to radio energy and causes the radar pulses to be scattered multiple times, creating a distinctive polarization echo. Spudis thinks the deposits inside some polar craters could be nearly pure ice.

If he's right, think of the potential for lunar tourism! Imagine a cozy chalet perched atop the rim of a lunar crater, basking in a perpetual lunar sunset and with the resplendent Earth periodically bobbing into view from behind a distant peak. Then you jump into your super-insulated space suit, slap on some sticks, and schuss straight down the inner rim until you bottom out.

But watch out for those lunar moguls — in space, no one can hear you "screaming starfish."

It started over coffee in China.

Last month, a group of visiting astronomers at the Kavli Institute for Astronomy and Astrophysics at Peking University were talking about the latest “hot” topic: the curious case of two small but high temperature objects found orbiting around a pair of distant stars.

The discovery was made by Kepler — NASA’s planet-hunting satellite launched one year ago this week. Kepler’s approach is brute force; it continuously monitors a field of over 150,000 stars looking for those that periodically dim when orbiting planets block a tiny fraction of their light.

In January, when mission scientists unveiled their first discoveries with Kepler, a couple of strange stars turned up. Each has a light curve that is the reverse of what's expected from Kepler. The signal is dimmest when a planet-size companion passes behind the star rather that in front of it. This means the companions are brighter per unit surface area (and therefore hotter) than the stars they circle.

Jason Rowe, a NASA postdoctoral fellow based at the Ames Research Center in Moffett Field California, was puzzled when he first saw these light curves. “They stuck out like sore thumbs,” he says. The objects were dubbed KOI-81 and KOI-74 (mission-speak for “Kepler Object of Interest”).

It’s hard to imagine planets that are hotter than stars, so the Kepler team considered an alternative. Might they be white dwarfs? The degenerate cores of dead stars that have blown off their outer layers, white dwarfs are squeezed by their powerful self-gravity into small, dense, and very hot objects. The trouble was KOI-81 and KOI-74 didn’t seem small enough to be white dwarfs.

Clearly, the most relevant information would be the masses of the two hot companions. Were they heavy, like white dwarfs, or lightweights, like planets?

Kepler can’t measure an orbiting companion’s mass directly. But in a paper discussing the objects Rowe and his coauthors estimate the masses by measuring how much the hot companions seems to physically deform the stars they orbit. (The companion’s gravity squishes the star slightly, turning its spherical shape into something that is slightly more like a football). Such deformations produce slight variations in the light curves, which Kepler can measure.

These results point to objects that are less massive than a typical white dwarf star. KOI-74, in particular, seems to weigh in at no more than one tenth the Sun’s mass. Could it be a planet after all — a planet heated to more than 12,000 kelvins (21,600°F)?

The search for a resolution to this paradox is what brings us back to the Kavli Institute in China. The astronomers sipping coffee there and chewing over Kepler’s findings in mid-January included Marten Van Kerkwijk and Stephen Justham along with Rene Breton, a visiting postdoc from the University of Toronto. Their discussions were prompted by an email from MIT astronomer Saul Rappaport, and remotely included Philipp Podsiadlowski of Oxford University and Zhanwen Han of Yunnan Observatory.

The group realized there might be another way to measure the masses of the hot companions using the Kepler data. This is because Kepler is tailor-made not only to spot slight changes in the brightness of a star but also to do so at very well-defined wavelengths. This means it’s perfectly set up to detect “Doppler boosting”.

Doppler boosting means an approaching light source looks brighter because more of its photons are arriving at your detector per unit of time compared to when the object is stationary. The faster the approach the bigger the boost. Conversely, a receding source looks dimmer because you’re getting fewer photons per unit time. (The effect can be diminished depending on the colour of the source, but for the Kepler stars with the hot companions, it’s the photon count that dominates.)

Van Kerkwijk obtained the raw Kepler data for the two stars with the mysterious hot companions and found that, indeed, there was Doppler boosting in the signal. In each case, the strength of the boost is directly related to how much the orbiting companion is pulling on the primary star. The more massive the companion, the stronger the gravitational pull.

The method allowed the China group to recalculate the masses of the hot companions. Their results neatly resolve the paradox. The objects are light but still fall into the white-dwarf mass range, especially white dwarfs that form in binary systems where some transfer of mass onto the neighboring star is likely. After the recalculation, KOI-74 ends up being more like 20% of the Sun’s mass. This fits with the larger than standard radius detected by Kepler, since lower-mass white dwarfs have less self-gravity and so are less compact.

In another recent paper, Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics has calculated that Kepler should detect about 1,000 transiting white dwarfs that are the products of mass-transfer binaries. If so, KOI-74 and KOI-81 are simply the first two entries in what could be quite a long list.

Finding a couple of white dwarfs is not exactly a home run for Kepler (it's supposed to be finding Earth-mass planets). On the other hand, showing that these objects are massive enough to be white dwarfs by using Doppler boosting is the equivalent of a nicely executed catch. It’s the kind of everyday science that doesn’t attract headlines, but that strengthens our understanding of the universe in a way that makes major discoveries possible down the road.

I see two encouraging signs from this little saga. The first is that Kepler is performing with exquisite precision. The effective use of Doppler boosting could not have been accomplished (at least not yet) from the ground. “The effect is very small,” says van Kerkwejk, “which means it can only be used with a space-based instrument.”

The second is that the excitement generated by this new piece of hardware is bringing out the best in the available software — namely, the collective brainpower of the astronomical community. Researchers are stimulated by the possibilities Kepler has opened up. They’re talking about it over coffee in China and in places around the globe where astronomers gather. And whenever there’s talk of this kind, innovations follow.

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Ivan Semeniuk is host of the podcast The Universe in Mind and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.

Those of you with a soft spot for Pluto probably know that February was a big month for this far-flung world.

First came the unveiling, on February 4th, of a new surface map painstakingly constructed from Hubble Space Telescope images. Then, on the 18th, we celebrated the 80th anniversary of Pluto's discovery by Clyde Tombaugh at Lowell Observatory.

To these you can add one more event of note: on February 25th, the New Horizons spacecraft reached the point in its trajectory that places it closer to Pluto than it is to the Sun. As the mission's website touts, the craft's approach to Pluto has begun.

New Horizons has been coasting ever since its launch on January 19, 2006. So it's taken just four years to reach this halfway point. But the Sun's gravity has gradually slowed the craft, enough so that it will require another 5½ years to reach the outermost planet second-largest dwarf planet.

No matter how you slice it, the trip has been a long and basically boring one for both New Horizons and its science team. Like a long overnight flight from Tokyo to New York, there hasn't been much to look at since a dash past Jupiter for a gravity boost and some target practice exactly three years ago today (another February date of note). So New Horizons has been in electronic hibernation since last August; it'll be roused for a checkup in a few months.

You can follow the spacecraft's progress numerous ways, but I recommend reading the occasional musings of Alan Stern, the mission's principal investigator.

(Watch Stern and others defend Pluto's planetary status this week on "The Pluto Files," a PBS special hosted by Neil deGrasse Tyson.)

Whenever I hear a claimed discovery that overturns conventional wisdom on some important aspect of astronomy, my skepticism meter goes on high alert. Such was the case on Wednesday, when I listened to a NASA press conference in which two astronomers based in Germany presented evidence arguing that the most popular model for Type Ia supernovae is incorrect, at least for elliptical galaxies.

Astronomers are in agreement that Type Ia supernovae (SN Ia) occur when a white dwarf (a collapsed, Earth-size remnant of a star) blows itself to smithereens, producing an extraordinarily powerful explosion that large telescopes can see from billions of light-years away. But despite years of intense study on both observational and theoretical fronts, the details remain shrouded in mystery.

The standard textbook explanation for SN Ia goes something like this: Type Ia supernovae occur in binary systems consisting of a white dwarf and a normal star. As the normal star gives off a wind, some of the gas becomes trapped in a disk around the white dwarf. The gas spirals in, collecting on the white dwarf's surface. If enough gas accumulates on the white dwarf to push its mass close to the Chandrasekhar limit (1.4 solar masses), it ignites a powerful thermonuclear explosion that blows up the entire white dwarf like a giant H-bomb.

But astronomers have also proposed an alternative theory: Some Type Ia supernovae occur in binary systems consisting of two white dwarfs. The white dwarfs slowly spiral inward and merge, pushing the merged object over 1.4 solar masses. This object explodes as a SN Ia.

In the standard picture, the "accreting white dwarf" scenario occurs much more frequently than the merger scenario, because accreting binary systems should be more common. In addition, some theories predict that when two white dwarfs merge, they collapse to form a neutron star, with no supernova explosion whatsoever. As astrophysicist Mario Livio (Space Telescope Science Institute) points out, "Whether or not mergers can indeed produce Type Ia supernovae is not clear from a theoretical standpoint."

But if most SN Ia are triggered by accreting white dwarfs, gas falling onto a white dwarf should give off copious amounts of X-rays. In the NASA press conference, Marat Gilfanov and Akos Bogdan (Max Planck Institute for Astrophysics in Garching, Germany) said they had searched five elliptical galaxies and the central part of the Andromeda Galaxy with NASA's Chandra X-ray Observatory. They found 30 to 50 times less X-ray emission in the elliptical galaxies than they expected to see if accreting white dwarfs make up the majority of SN Ia progenitors.

"Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging," says Bogdan in a Chandra press release. "This is probably not what many astronomers would expect."

To find out if this was a valid claim, I consulted a number of leading experts in the field. Although everyone seemed to agree that Gilfanov and Bogdan took a creative approach toward addressing the problem, the replies were all over the map, and it's clear that the origin of SN Ia remains very much an open question.

Andrew Howell (University of California, Santa Barbara) pointed out that astronomers have known for years that there aren't enough accreting white dwarf binaries to explain the number of observed SN Ia. Speaking of this new study, Howell says, "It's not a leap, but a new step down a long path toward understanding the progenitors of SN Ia. The evidence has been building for years that the accreting-white-dwarf scenario cannot explain all SN Ia."

Alex Filippenko (University of California, Berkeley) adds, "I'm not disagreeing with Gilfanov and Bogdan that some Type Ia supernovae come from white dwarf mergers. People have been saying this for a long time. It's just that we've had trouble finding actual binary white dwarfs that have the potential to merge in the age of the universe."

Brad Schaefer (Louisiana State University) points out that the Gilfanov/Bogdan study assumes that all accreting white dwarf binaries shine in X-rays at a relatively uniform and steady rate, but that doesn't hold true in many known systems that contain a white dwarf and a normal star. "Their conclusion is clearly and easily wrong," he says.

Sumner Starrfield (Arizona State University) notes that astronomers are finding that SN Ia are not as uniform as once thought. "It now looks like there are at least two kinds of SN Ia, and they may occur in different types of galaxies. So, they are probably only addressing one type," he says.

Several astronomers also pointed out that a group led by Rosanne Di Stefano (Harvard-Smithsonian Center for Astrophysics) has already announced similar results, and that these results apply to galaxies of all types. But Di Stefano cautions, "We must be very careful about the implications for the models," because it's possible that accreting white dwarfs reprocess the X-ray radiation they emit, meaning it wouldn't necessarily show up in the Chandra observations of elliptical galaxies.

Despite this new study, the origin of SN Ia remains an unsolved problem; it's quite possible that these cataclysmic explosions have multiple origins. And it's a mystery that has much broader significance. As Schaefer points out, "The Type Ia supernova progenitor problem is one of the biggest and longest-lasting questions in astrophysics. And just 10 years ago the problem became uber-important."

Why? Because cosmologists can calibrate the luminosities of SN Ia, they use them to study the expansion history of the universe. By studying distant SN Ia with both ground- and space-based telescopes, two teams announced in 1998 that cosmic expansion is accelerating, which hints that most of the universe's energy is in some unexplained form known as dark energy.

Since 1998 astronomers have used completely independent methods to confirm the accelerating universe, so this new result casts no doubt that there's a lot of dark energy out there. But lacking a complete understanding of SN Ia makes it more difficult for astronomers to use them for cosmology studies, which will make it harder to pin down the precise details of the universe's expansion history, which in turn could make it more difficult to unravel the profound mystery of dark energy.

The Gilfanov/Bogdan paper can be found by clicking here. Di Stefano's paper can be found here, with section 2.5 being particularly important.

When Edwin Hubble devised his now famous "tuning-fork" diagram in 1926, he sought to organize galaxies' many shapes into a sensible, well-ordered sequence.

He realized that spirals sometimes sport a central bar and sometimes do not, and that ellipticals are gigantic star balls — all bulge and no arms. In Hubble's scheme lenticulars represent a hybrid of those types, and all the peculiar-shaped leftovers became known as irregulars.

Yet I'll admit that when I see or hear the word "galaxy," I conjure up a vision of just one of these: an immense, stately spiral of stars like the one shown at lower right. Seasoned observers can rattle off the names of dozens of pinwheels from memory: the Andromeda galaxy, M51 and M101 in Ursa Major, and M74 in Pisces, to name a few. Ellipticals and irregulars rarely come to mind.

Cosmologists spend a lot of time thinking about spiral galaxies too — not to admire their beauty but to figure out how they exist at all. Their very shape indicates a star system that's been stable and largely unperturbed for billions of years. Yet the early universe was hardly a tranquil place. Crowding caused many young galaxies to collide, merge, and tear each other asunder.

So it's a minor miracle (and a topic of considerable debate) how all the spirals we see today managed to endure all that mayhem unscathed. "The formation of spirals is a problem," admits Christopher Conselice, a galaxy specialist at the University of Nottingham. "We don't know how they formed, or how they survive all those mergers."

Now that telescopes can peer to ever-grater distances and thus farther back in time, astronomers are attempting to take a census of the shapes and numbers of galaxies that existed 6 to 8 billion years ago. Using observations from the Sloan Digital Sky Survey and from the Hubble Space Telescope, François Hammer and Rodney Delgado-Serrano (Paris Observatory) led an effort to catalog 116 local galaxies and 148 distant ones, respectively. In effect, they've created two Hubble sequences: one for the present and one for the circumstances 6 billion years ago. Their results appear in two articles just published in the European journal Astronomy & Astrophysics.

Surprisingly, the assorted beasts in the galaxy zoo were very different long ago. Peculiar-shaped irregular galaxies were far more common (52% of the total) and spirals relatively scarce (31%) — just the opposite of what's observed today (10% irregulars, 72% spirals).

Using computer models to trace the role of interstellar gas and rates of star formation during mergers, Hammer and his team conclude that many of the spiral galaxies seen today must have resulted from collisions between irregular systems. They can't prove that spirals arose phoenix-like from the ashes of titanic collisions — the Hubble views don't reveal scads of merging galactic blobs. But the model suggests that spirals were "rebuilt" following particularly gas-rich mergers.

But Tanner and Delgado-Serrano aren't the only ones trying to crack the Hubble sequence. A competing paper, just published in the Monthly Notices of the Royal Astronomical Society, concludes that spirals would not have fared well in the bump-and-grind chaos of the early universe. Instead, argue Andrew Benson (Caltech) and Nick Devereux (Embry-Riddle Aeronautical University), spirals abound today because they managed to escape violent interactions. "The dense galaxy clusters we see today are actually unusual," Benson explains. "The quiet regions of the universe, then and now, are more common than we thought."

The Benson-Devereux computer model, called GALFORM, takes a set of assumed starting conditions and runs it forward through time. In a sense, Benson notes, the more we know about the early universe, the more difficult the modeling becomes. GALFORM incorporates the effects of unseen dark matter (which helps draw galaxies together quickly) and even more perplexing dark energy (which then pushes them apart). The result is a very good (but not perfect) match to the numbers and types of galaxies observed today and at times past.

One hurdle shared by all these models is that we don't yet know the true shapes of the earliest galaxies. Benson points to the celebrated Hubble Deep Field image, for example, in which "they just look like blobs of light" that sometimes appear as weird chains and other shapes that aren't seen now.

Fortunately, a massive new effort — approved just three weeks ago — will use the revamped Hubble Space Telescope to observe the early universe with unprecedented clarity. Led by Sandra Faber (University of California, Santa Cruz) and Harry Ferguson (Space Telescope Science Institute), the Cosmology Survey Multi-Cycle Treasury Program will employ Hubble's new Wide Field Camera 3 to survey some 250,000 galaxies in five regions of the sky. With luck, the resulting views should reveal crucial details about how galaxies looked at least 12 billion years ago. Don't expect results anytime soon, though. Faber and Ferguson don't yet know how much HST time they'll need — but it's at least a 100 orbits' worth!

If you're impatient (or even if you're not), then let me suggest that you join Galaxy Zoo — a "citizen-science" effort to classify ancient galaxies and scan them for supernovas.

Without much public notice, the team running the Wilkinson Microwave Anisotropy Probe (WMAP) recently released results from the satellite's "seven-year data set," updating the five-year data released in 2008.

WMAP has been mapping the sizes and strengths of the slight irregularities in the cosmic microwave background radiation filling the sky. The microwave background is the "wallpaper" on the sky behind everything else seen in the universe. The slight temperature irregularities written on it (seen on the all-sky map here) tell much about the cosmic conditions just 380,000 years after the Big Bang, when the universe first became transparent to its own radiation — and before, right back to the Big Bang itself.

The two more years of data have further beaten down the statistical uncertainties in the cosmic background map, allowing analysts to refine what it tells us about the cosmos as a whole. If the new, revised results didn't make much news, it's because they show modern cosmology to be steady on course. The better data only firm up confidence in what we already thought we knew.

Some high points:

The canonical age of the universe is now 13.75 plus or minus 0.11 billion years since the Big Bang, compared to 13.73 ± 0.12 billion years from the 5-year data set. That's now an age uncertainty of only 0.8% (all uncertainties are expressed at the "1-sigma," or 68%, confidence level). By comparison, astronomy books in your public library probably say the universe is "between 10 and 20" billion years old.

The Hubble constant, the rate at which the universe is expanding today, is 70.4 ± 1.4 km per second per megaparsec. Books in your library probably say the Hubble constant is "between 50 and 100."

Combining the new values for these and many other parameters, the 7-year WMAP data tightens up the standard model of cosmology by 50% overall, according to a NASA statement. This includes WMAP's detection of the expected polarization patterns around warm and cool spots in the microwave background.

A key prediction of inflationary-universe theory — which explains how the universe grew to be the way we see it, based on quantum events that happened in the first 10^–32 second after the Big Bang — is also firmed up. The simplest versions of inflation predict that quantum fluctuations during that early instant did not produce equally strong irregularities on all size scales, the way nature often works. Instead they should have produced slightly weaker fluctuations at larger scales. WMAP finds exactly this. The so-called "scalar spectral index" is clearly tilted, with a value not of exactly 1.00, but of 0.96 ± 0.01 by the new refinement. This is a big deal for boosting confidence that inflation really happened.

Space, as far away as we can see, is flat to a new degree of precision. The total density of normal matter, dark matter, and dark energy adds up to 1.0023 ± 0.0055 of the critical flatness density. This is consistent with a value of exactly 1 (flat space) to a precision of just half a percent!

The breakdown of the basic components of the universe is now:

Ordinary matter, 4.56 ± 0.16 percent

Nonbaryonic dark matter, 22.7 ± 1.4 percent

Dark energy, 72.8 ± 0.5 percent.

The dark energy's "equation of state," a parameter known as w, is –0.980 ± 0.053, consistent with a value of exactly –1 to a precision of about 5%. This is the value that w would have if the dark energy is an inherent, constant property of any given volume of space, regardless of how much the space may have expanded in the past. This property matches Albert Einstein's idea of a "cosmological constant," which he inserted into his equations in 1917, and pretty much rules out the idea that the dark energy is some kind of "quintessence" existing in space that thins out as space expands.

For the first time, WMAP has extracted evidence from the microwave background that directly reveals primordial helium emerging from the Big Bang — not just hydrogen. This was totally expected, but it's nice to see it confirmed. This is the first direct evidence of helium existing before the first stars.

The first stars and/or quasars turned on at a time corresponding to a redshift (z) of 10.4 ± 1.2, dating the start of the "reionization era" at 460 ± 80 million years after the Big Bang — in agreement with more direct astronomical evidence.

One reason why these improved results didn't make much news could be that Europe's newer and greater Planck probe of the microwave background is up and working. But its first results probably won't be available for about another year.

Here's a NASA press release with more information.

Here's the team's summary paper on WMAP's 7-year results. For the table of the new best values for all the basic cosmic parameters, scroll to page 39.

Cool tool for building your own universe to see if you can make one that matches the WMAP data. Suitable for kids — this makes it look simple!

Posted by Alan MacRobert, February 14, 2010

related content: Cosmology news

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I never tire of watching rocket launches, and thanks the Internet it's so easy to do. Last week I got up in the wee hours — twice — to watch the last-ever night launch of a Space Shuttle get postponed and then make its fiery ascent into the blackness.

This morning I looked on, mouthing the final countdown to myself, as NASA's Solar Dynamics Observatory got its ride into orbit atop an Atlas V booster and Centaur upper stage. Launch came (again after a day's delay, due to high winds) at 10:23 a.m. Eastern Standard Time. About 16 minutes later, the Centaur engines shut down, and SDO was already in orbit. Great stuff!

Solar scientists have high hopes for this space sentinel, which will occupy an inclined but geosynchronous orbit that will keep it in constant view of a tracking station in New Mexico. It's the key element in an extensive program, dubbed "Living With a Star" by NASA's moniker-makers, to understand the Sun as never before. "SDO is going to make a huge step forward in our understanding of the Sun and its effects on life and society," predicts Richard Fisher, who directs the space agency's heliophysics division.

Equipped with three major instruments, the spacecraft will focus on measuring the Sun's extreme-ultraviolet luminosity, its global magnetic field, and temperatures in the chromosphere and inner corona. The hope is to record all this data frequently for an entire 11-year-long solar cycle.

The Helioseismic and Magnetic Imager (HMI) will monitor the Sun's internal vibrations, which cause its surface to quiver like a struck bell at many different frequencies. The expected advances in helioseismology, as this science is known, should be revolutionary — physicists hope to determine the origin of solar variability and to characterize and understand the Sun’s interior and the various components of magnetic activity.

The Atmospheric Imaging Assembly, or AIA, will allow scientists to see the entire disc of the Sun and its inner corona in very high resolution. Each of its four telescopes will monitor two wavelength channels using 4,096-by-4,096-pixel CCDs. The hope is to learn what makes the corona so hot and how energetic outbursts propagate through it.

Meanwhile, the Extreme-ultraviolet Variability Experiment (EVE) will use a combination of detectors to record the Sun's ultraviolet spectrum every 10 seconds. This wavelength range is sensitive to changes in magnetic activity and should provide clues to understanding how variations in the Sun's luminosity are linked to its magnetic fields.

One aspect of all this intense scrutiny is that SDO will image the Sun every 0.75 second at extremely high resolution. There'll be a lot of data to analyze — 1.5 terabytes each day, equivalent to a CD's worth of data every 36 seconds. (That's why it needs to remain in continuous view from a single dedicated tracking station.)

You can learn more about SDO and its scientific promise at the mission's website.

Of all the International Year of Astronomy's events, nothing matched the raw involvement of last April's 100 Hours of Astronomy, which showcased telescopes and other facilities around the world via nonstop online video streaming.

IYA has come and gone, but its heady buzz lingers. "I'd heard from all around that we needed to do something to keep the momentum going," says Mike Simmons, who co-chaired the 100HA effort. So Simmons brainstormed with others in an attempt to outdo what was arguably the single greatest outreach event in the history of astronomy.

The answer, they're betting, is Global Astronomy Month.

During April, amateurs worldwide will be encouraged to participate in all kinds of events at small and large scales. At the local end will be the kinds of activities that fueled the worldwide interest in the night sky. "Most of the world's amateur astronomers engage mostly in public outreach and education," observes Simmons. "They've discovered the rest of the universe, and they want to share it with others." (As the founder of Astronomers Without Borders, the well-traveled Simmons certainly knows what he's talking about.)

Although 100HA announced local events, this time the hope is to establish more connections between activities and to provide a platform for showing off the results. For example, GAM's "broadcast channel" gives event organizers worldwide a place to show general-interest presentations, after-party highlights, and even live video streams.

For the Big Picture, organizers also have some Big Plans. There'll be repeats of successful activities like the 24-hour Global Star Party, along with more remote-observing events like January's highly successful "Big Dipper to South Pole" webcast — which despite little advance notice attracted more than 7,000 participants from 80+ countries.

Bold, new ideas are in the works as well. One is beautifully simple: a weeklong effort to observing the nightly changes in the Moon's position and phase using just your eyes. Another is a "Living Legend Series," featuring live interactive interviews with some of the most famous icons in amateur astronomy. (I could name names, but I'm sworn to secrecy!) You'll also see the inauguration of a major new effort in dark-sky awareness dubbed One Star at a Time.

There's much, much more in store — I can't cover it all here. (In fact, I emailed Simmons to get a couple of clarifications, and he sent me a 1,700-word response!). So check it out for yourselves, start thinking of how you can participate, and get ready to party!