Centauri Dreams
Imagining and Planning Interstellar Exploration
Dawn: New Imagery of Ceres
Mark January 26 on your calendar. It’s the day when the Dawn spacecraft will take images of Ceres that should exceed the resolution of the Hubble Space Telescope. We’re moving into that new world discovery phase that is so reminiscent of the Voyager images, which kept re-writing our textbooks on the outer Solar System. 2015 will be a good year for such, with Dawn being captured by Ceres gravity on March 6, and New Horizons slated for a July flyby of Pluto/Charon. In both cases, we will be seeing surfaces features never before observed.
What we have so far from Dawn can’t match earlier Hubble imagery, the best of which is about ten years old, but it’s about three times better than the calibration images taken by the spacecraft in early December. At this point, Dawn is making a series of images to be used for navigation purposes during the approach to the dwarf planet. We have sixteen months of close study of Ceres to look forward to as the excitement builds. “Already,” says Andreas Nathues, lead investigator for the framing camera team at the Max Planck Institute for Solar System Research, Gottingen, “the [latest] images hint at first surface structures such as craters.”
Image: An animated GIF showing bright and dark features on Ceres. The Dawn spacecraft observed Ceres for an hour on Jan. 13, 2015, from a distance of 383,000 kilometers. A little more than half of its surface was observed at a resolution of 27 pixels. Credit: NASA/JPL.
No spacecraft has ever visited a dwarf planet, another first for Dawn, which will also become the first spacecraft to have orbited two deep space destinations following its 2011-2012 sojourn at Vesta, where it produced over 30,000 images. Jian-Yang Li (Planetary Science Institute), who led the Hubble mapping of Ceres, says that even the early observations will be significant:
“Reproducing the Hubble observations is important to understanding the nature of Ceres’ surface. The recent detection of episodic water vapor near Ceres’ surface by the Herschel Space Observatory at a longitude observed by Dawn might arise from activity that could change Ceres’ surface over time.”
That discovery, relying on data taken in 2012, took advantage of the HIFI instrument on Herschel, which showed water vapor being emitted from the surface of Ceres, an early indication that Ceres has an icy surface and an atmosphere. Variations in the water signal during the dwarf planet’s nine hour rotation period helped Herschel scientists trace the water vapor to two spots on the surface. The two emitting regions are about five percent darker than the rest of Ceres, likely warmer regions that provide efficient sublimation of small reservoirs of water ice. In very short order, thanks to the Dawn spacecraft, we will be able to observe such features in dazzling detail.
For more on the Herschel work, see Küppers et al., “Localized sources of water vapour on the dwarf planet (1)?Ceres,” Nature 505 (23 January 2014), 525–527 (abstract).
Making the Case for Deep Space
I get few questions that are harder to answer than ‘what happened to the sense of adventure that we once had with Apollo?’ And there are few questions I get more often, usually accompanied by ‘how are we going to do starflight if we don’t even have the will to go back to the Moon?’ Both questions have unsettling answers, but the second question is open-ended. We can hope that the ‘sense of sag’ that Michael Michaud describes in talking about the post-Apollo period (for manned flight, at least) may itself evolve into something else, something far more hopeful.
But let’s dwell a moment on the first question. I’ve been looking over an essay Michaud wrote for Spaceflight in the mid-1970s, a decade of the Pioneers and the Voyagers, but also a decade when it became clear that our presence on the Moon with Apollo was going to be a short-lived affair. Instead, we were talking about Skylab, about docking operations between Soviet and American spacecraft, and the next big ticket item on the manned spaceflight agenda looked to be a reusable space shuttle. NASA wanted to deliver measurable returns, even when many of us were thinking ‘Wait! Weren’t we supposed to be headed on to Mars by now?’.
There were plenty of reasons for what some considered to be a more realistic assessment of Earth’s needs. The critics of Apollo argued that it was played as a card in the great geopolitical game of the Cold War, set up as a contest between two super-powers so that achieving the Moon meant the ‘race’ was over. We were dealing with enormous demands on spending, and space projects that could produce quantifiable results were the currency of the day. Instead of manned missions to the Moon, the goal was scientific observation of the Earth to improve our understanding of our ecosystem, to upgrade communications, to streamline navigation.
Nearby Space in Context
To our credit, the exploratory impulse remained, as the Pioneer and Voyager triumphs attest. But are we as a species destined to stay on the Earth or just above it in close orbit, letting our machines be our surrogates in deep space? A prolific essayist and author (Contact with Alien Civilizations (Copernicus, 2007),. Michaud is a former diplomat with global experience in US science policy. He argues here that such a limited view of manned spaceflight goes against the impulse that has long driven us toward expansion. Remember, this perspective is from the 1970s — our ‘sense of sag’ has been with us a long time:
Historically, spaceflight has had a philosophical purpose: to carry man to the stars. From early pioneers such as the Russian Tsiolkovsky and the American Goddard to contemporary writers such as Arthur C. Clarke, there was a consistent line of thinking: build the machines that will allow man to escape the confines of Earth and explore the Universe, and expand the realm of the human race. But military exploitation of the rocket and Cold War competition in space diverted our thinking, and Project Apollo ultimately left us unsatisfied because it had a short-term goal and did not lead directly to anything else. While Apollo proved that man could reach another sphere, it did not leave a step to be built upon.
Image: Saturn V at twilight and, at upper left, its target. Credit: NASA.
I agree completely with Michaud that a major problem with Apollo was the fact that it was in every sense a crash program. The US found itself, post-Sputnik, trying to catch up to a rapidly moving Soviet program that was setting new records in space almost monthly. In fact, looking back at how we pushed the envelope with missions like Apollo 8, I’m astounded that we didn’t lose more than one Apollo crew (and at that, the one we did lose died in a training accident). We put a crew on a never-before attempted journey outside Earth orbit to the Moon and back, launching Apollo 8 with a rocket that had never been used for a manned mission (Apollo 7 flew a less powerful variant of the Saturn rocket). It seemed dicey at the time, but from the perspective of half a century later, it seems like utter folly. The results were, of course, magnificent.
But while we still play Apollo 8’s Christmas Eve transmission from lunar orbit on YouTube and elsewhere, we’re trying to work out the next bold steps for the manned space program in a very tentative way. If Mars is truly our objective, it’s an objective that’s only vaguely in our plans for the 2030s, the kind of goal that all too easily recedes a decade at a time. NASA may or may not complete the Space Launch System, but are we sure what we will do with it? Elon Musk wants to die on Mars — does SpaceX and Falcon Heavy awaken any sort of Apollo-like passion?
Spaceflight and Evolution
For some of us, the old passion never died, but when it comes to the general public, we’re dealing with a case that has to be made all over again. If we take an evolutionary perspective, the purpose of human life, argues Michaud, is survival, as it is for all forms of life. How to broaden our survival options is a question of how we can upgrade our evolutionary potential. From the essay:
The tools for the improvement of our survivability are guided evolution, conflict limitation, a balanced relationship to our Earthly environment, and space flight. The first three can improve our chances anywhere that humans live, but are presently limited to the Earth’s biosphere. Only space flight can give man the option of surviving no matter what happens on Earth, of spreading the human race throughout the universe, of opening up new environments for long-term human evolution.
There’s that phrase again: ‘long-term.’ To engage the public in a program for human expansion into the cosmos is to reverse the Apollo preoccupation with crash programs and near-term wins. A rational program to achieve starflight is not likely to happen against a ticking clock. Rather, it will be an effort that sustains itself philosophically across the decades and centuries such an effort will require, crossing many human generations. It will be an exploratory and expansionist meme that remains alive because its triggers are fundamental to our nature as human beings.
Let me quote Edmund Burke on this, because the views of the 18th Century political theorist and philosopher are much to the point:
[Society] is not a partnership in things subservient only to the gross animal existence of a temporary and perishable nature. It is a partnership in all science, a partnership in all art; a partnership in every virtue, and in all perfection. As the ends of such a partnership cannot be obtained in many generations, it becomes a partnership not only between those who are living, but between those who are living, those who are dead, and those who are to be born. Each contract of each particular state is but a clause in the great primeval contract of eternal society.
Specifically, spaceflight challenges human capabilities and stimulates intellectual innovation. We are forced to develop new techniques, new materials and new forms of cooperation as we engage with a frontier more hostile than any we have known. Yes, these technologies will produce tangible results on Earth, many of which are obvious, from communications to power-generation. Mastering closed biospheres will teach us a great deal about our Earth. But we are also driven on a purely intellectual level to learn more about the cosmos, an engagement that broadens our understanding of physical law and provides context for all our science.
End of the Space Race
A goal of interstellar flight is not reached with a ‘space race’ mentality, although competition can boost interim steps — it will be interesting, for example, to see how the Western democracies react to a potential Chinese presence on the Moon. But the interstellar goal is too big to be comprehended by its individual components. It is a goal that begins in the short-term to expand our presence in the Solar System, reaping rewards like enhanced energy production and access to materials made available through exploitation of asteroids and other resources. There is in Michaud’s view a powerful encouragement to international cooperation in all this, for cutting costs if nothing else. Underlying early expansion is the reduction of existential risk, but also the possibility of diversifying the species as humans in new environments inevitably adapt.
Culturally, there is little to parallel the growth in philosophical perspective that may be gained by interacting with societies beginning to take hold in O’Neill-style colonies or on nearby planets like Mars, just as the Renaissance in Europe was bolstered by the influx of ideas and commodities from voyages of exploration. We might add a SETI imperative as well. If we do one day make contact with an extraterrestrial species, our own development off our world will likely be seen as an evolutionary step that is reproduced elsewhere in the universe. The potential growth in knowledge from such contact emphasizes space-based strategies to maximize the search.
All of these are short-term but necessary goals, each building on the other. But unlike Apollo, the species must have the broader context to work with. As Michaud writes:
Even if funding for major new projects seems unlikely now, interest must be kept alive. Industrial, labour, military, and scientific interests will continue to be important for the future of space flight, but farsighted political leaders also must mobilize public support with the excitement of exploration and discovery, of expansion and new opportunities, of the change in man’s place in the universe. They must hold out the promise of new and better lives, not just for a few astronauts, but for ordinary people.
None of this is easy to sell, but the collapse of interest post-Apollo should not be allowed to take hold as an inevitable consequence of success in space. The argument for a deep space exploration program is one that needs to be made again and again, for public sentiment changes slowly without precipitating events. But interplanetary growth must not become an end in itself. Interstellar flight, as Michaud reminds us, is open-ended. It opens before us an all but infinite frontier. That frontier is woven into our psychological and philosophical DNA as much as it is entwined with our evolutionary need for survival. We must continue to make the case.
The paper is Michaud, “After Apollo,” Spaceflight Volume 15 (October 1973), pp. 362-367.
Planets to Be Discovered in the Outer System?
Having just looked at the unusual ‘warped’ disk of HD 142527, I’m inclined to be skeptical when people make too many assumptions about where planets can form. Is our Solar System solely a matter of eight planets and a Kuiper Belt full of debris, with a vast cometary cloud encircling the whole? Or might there be other small planets well beyond the orbit of Neptune, planets much larger than dwarfs like Pluto but not so large that we have been able to detect them?
Certainly Carlos de la Fuente Marcos and Raúl de la Fuente Marcos (Complutense University of Madrid), working with Sverre J. Aarseth (University of Cambridge) think evidence exists for this proposition. The scientists are interested in how large objects can affect the trajectories of small ones, and in particular what a comet named 96P/Machholz 1 can reveal about how such interactions work. They’re focused on the Kozai mechanism, which explains how the larger object causes a quantified libration in the smaller object’s orbit, and use the Jupiter-family comet as a reference to understand the processes that may be happening in the outer Solar System.
Objects like Sedna are in extreme orbits that extend out hundreds of AU, with even their closest approach to the Sun well beyond the orbit of Pluto. And the the orbits of Sedna itself and others among the group the authors call ‘extreme trans-Neptunian objects’ (ETNOs) show enough discrepancies from what we would expect to point to the existence of larger objects affecting their orbits, just as Jupiter affects Comet 96P/Machholz 1. Carlos de la Fuente Marcos explains:
“This excess of objects with unexpected orbital parameters makes us believe that some invisible forces are altering the distribution of the orbital elements of the ETNO and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto. The exact number is uncertain, given that the data that we have is limited, but our calculations suggest that there are at least two planets, and probably more, within the confines of our solar system.”
We’ve recently discovered 2012 VP113, evidently another object orbiting far beyond Pluto, whose own orbit has been suggested as evidence for an undiscovered super-Earth with up to ten times the mass of our planet orbiting in a region perhaps 250 AU from the Sun. The work on 2012 VP113 by Chadwick Trujillo and Scott Sheppard is invoked by the new research, which argues that dwarf worlds like Sedna and other ETNOs may signal the existence of a large population of similar small ETNOs. Moreover, the gravitational perturbations of the outer system evident in their orbits point to something larger, as one of the two papers on the work notes:
…the architecture of that region is unlikely to be the result of a gravitationally unperturbed environment. If there are two planets, one at nearly 200 au and another one at approximately 250 au, their combined resonances may clear the area of objects in a fashion similar to what is observed between the orbits of Jupiter and Saturn.
Image: This is an orbit diagram for the outer Solar System. The Sun and Terrestrial planets are at the center. The orbits of the four giant planets, Jupiter, Saturn, Uranus and Neptune, are shown by purple solid circles. The Kuiper Belt, including Pluto, is shown by the dotted light blue region just beyond the giant planets. Sedna’s orbit is shown in orange while 2012 VP113?s orbit is shown in red. Both objects are currently near their closest approach to the Sun. Credit: Scott Sheppard.
So while we have no evidence for planets the size of Jupiter or Saturn moving in circular orbits in the outer system, the possibility of small planets beyond Neptune may still be in play, even if a wave of recent interest has resulted in no detections. The current authors think their results are unlikely to be the result of perturbations caused by Neptune or of observational bias, even while stressing that their work is based on the behavior of a relatively small number of objects. Can a moderate-sized planet affect objects beyond Pluto’s orbit in ways that New Horizons could detect? The authors think it’s possible, and it’s an interesting thought, particularly now that New Horizons is actually entering the first of its encounter phases for the Pluto/Charon flyby.
The papers are de la Fuente Marcos et al., “Flipping minor bodies: what comet 96P/Machholz 1 can tell us about the orbital evolution of extreme trans-Neptunian objects and the production of near-Earth objects on retrograde orbits.” Monthly Notices of the Royal Astronomical Society Vol. 446, Issue 2, pp. 1867-1873 (abstract / preprint) and de la Fuente Marcos, “Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets,” Monthly Notices of the Royal Astronomical Society Vol. 443, Issue 1, pp. L59-L63 (abstract / preprint). On 2012 VP113, see Trujillo & Sheppard, “A Sedna-like body with a perihelion of 80 astronomical units,” Nature 507 (27 March 2014), pp. 471–474 (abstract).
Naming Names in the Cosmos
How objects in the sky get named is always interesting to me. You may recall that the discovery of Uranus prompted some interesting naming activity. John Flamsteed, the English astronomer who was the first Astronomer Royal, observed the planet in 1690 and catalogued it as 34 Tauri, thinking it a star, as did French astronomer Pierre Lemonnier when he observed it in the mid-18th Century. William Herschel, seeing Uranus in 1781, thought at first that it was a comet, and reported it as such to the Royal Society.
By 1783, thanks to the work of the Russian astronomer Anders Lexell and Berlin-based Johann Elert Bode, Herschel came to agree that the new object was indeed a planet. Herschel, asked by then Astronomer Royal Nevil Maskelyne to name the new world, declared it to be Georgium Sidus, the ‘Georgian Planet,’ a name honoring King George III. The unpopular name soon met with alternative suggestions, including Herschel, Neptune and (Bode’s own idea) Uranus.
Image: Sir William Herschel (1738-1822), whose idea about naming a new planet met with scant approval. Credit: Lemuel Francis Abbott – National Portrait Gallery.
Herschel had worried that naming planets after the ‘ principal heroes and divinities’ of ancient eras would be out of place in his time, suggesting that naming them after the era they were discovered (hence, the reign of George III) would be the more satisfactory method. But of course we haven’t followed the suggestion, and now look not only for the names of ancient beings both human and divine as well as names related to specific cultures. The geography of Ceres, for example is to be named after mythology associated with agriculture and vegetation, a nod to Giuseppe Piazzi, its discoverer, who knew Ceres as the Roman goddess of agriculture.
The problem with all this is that we’re making so many discoveries that we’re taxing our ability to come up with the best nomenclature. Some 24 craters on Saturn’s moon Phoebe have been named by classical reference to the Argonauts, the intrepid adventurers who sailed with Jason to find the Golden Fleece. But the Gazetteer of Planetary Nomenclature also notes that future craters on Phoebe may have names associated with the goddess, who was, according to ancient lore, a Titan, the daughter of Uranus and Gaea. As mapping continues, features other than craters may acquire names based on Appollonius Rhodius’ 3rd C. text The Argonautica.
Titan, much in our thoughts with the 10th anniversary of the landing of the Huygens probe, gets plenty of attention from the International Astronomical Union, the U.S. Geological Survey, and NASA, all of whom have a hand in determining the names of features. Craters on the Saturnian moon take the names of gods and goddesses of wisdom, while a variety of surface features are open to names drawn from characters from Tolkien’s Middle Earth, characters from the Foundation series by Isaac Asimov and the names of planets from Frank Herbert’s Dune novels, surely a nod to science fictional interests among researchers.
And let’s not forget Xanadu, a plateau-like, highly reflective region on Titan, a name deriving ultimately from the Yuan Dynasty’s summer capital as established by Kublai Khan and immortalized in the West by Samuel Taylor Coleridge. Interesting places, these new worlds, and full of so many features that need names! When Makemake was discovered soon after Easter in 2005, it was immediately nicknamed Easterbunny, but later yielded to an IAU-sanctioned monicker based on fertility mythology on Rapa Nui, which most of us know as Easter Island.
I could go on with this entertaining subject indefinitely, even sticking within our own Solar System. The Uranian satellite Miranda, for instance, draws feature names from characters in Shakespeare’s plays, as do all the major moons of Uranus, though small satellites can draw on names from the poetry of Alexander Pope. We’ll doubtless have plenty of suggestions for features on Pluto once New Horizons gets close enough to see them. The theme there will be underworld deities. New moons like Nyx and Hydra have already received names according to this convention.
What happens when we turn to exoplanets? With so many being discovered, it’s no surprise that the International Astronomical Union has organized a global contest to name selected exoplanets. The NameExoWorlds contest is already open, with a first round that will allow nominations for ExoWorlds (by this, the IAU means the entire exoplanetary system and its host star) to be made available for the next stage of the contest, where names can be proposed.
Image: An artist’s impression of Alpha Centauri Bb. How many place names will we eventually have to come up with for places like this? Credit: ESO/L. Calçada/Nick Risinger.
The IAU, which goes about assigning scientifically recognized names to newly discovered objects, says that the NameExoWorlds contest will be the first opportunity for the public to name both exoplanets and the stars around which they orbit. To participate, clubs and non-profit organizations have to register with the IAU Directory of World Astronomy by May 15, 2015. The deadline for the first stage of the contest is February 15, 2015, when the nominating process for the first 20 ExoWorlds is to close. After that, each club or organization will be allowed to submit names, with a later worldwide public vote that will presumably take place over the Internet.
If you’d like to get involved, this IAU news release has all the details. News of the contest had me thinking about new categories for names, and I immediately thought about drawing ideas from Arthurian romances of the Middle Ages. But alas, I learn from the Gazetter of Planetary Nomenclature that this one has already been taken, on Mimas, of all places, where craters are to be named after people from Malory’s Morte d’Arthur. Malory scooped up most of the major characters in earlier English and French Arthurian tales, but maybe there are a few he missed. It’s worth a look, because as we keep discovering new worlds, names are going to be in short supply.
HD 142527: Shadows of a Tilted Disk
About a year ago we looked at a young star called HD 142527 in the constellation Lupus (see HD 142527: An Unusual Circumstellar Disk). A T Tauri star about five million years old, HD 142527 has drawn attention because it shows evidence of both an inner and an outer disk, each of which may be capable of producing planets. These are disks with a twist, as astronomers at the Millennium ALMA Disk Nucleus project at the Universidad de Chile demonstrate in a new paper that explains the three-dimensional geometry of this unusual system.
HD 142527’s two disks are striking because no other star shows a gap this large between an inner and outer disk, a gap that spans a region from 10 AU out to 120 AU. Two dark regions stand out in observations of the outer disk that break its continuity. The new study reveals these outer disk features to be caused by the shadow of the inner disk. The shape and orientation of the shadows thus become a measure of the inner disk’s orientation. Using radiative transfer methods that analyze the absorption, emission and scattering of light, the researchers find that the inner disk has to be tilted by about 70 degrees to produce the known features.
Image: Comparison between the the observed infrared image of HD142527 (left) and the result of the warped inner disk model (right) as predicted by radiative transfer calculations. Shadows correspond to the two dark regions or intensity nulls seen in the upper and bottom sections of the outer disk. Credit: Henning Avenhaus/Millenium ALMA Disk Nucleus.
We’re left with the puzzle of how this system came to be in this unusual configuration. One possibility, suggested in 2014, is a relatively recent close encounter with another star, although lead author Sebastian Marino argues in the paper that no candidate has yet been identified. Interactions between the circumstellar disks and possible proto-planets are more likely, and we have an example in the debris disk around Beta Pictoris where an inner disk was apparently influenced by a young planet whose orbit is aligned with an inclined warped disk component.
But that answer raises questions as well, notes Sebastian Perez, a co-author of the paper:
“The astounding fact is that this planet would most likely need to be in a highly inclined orbit, just like the inner parts of the disk. Which poses more interesting questions about the dynamical stability of such arrangement.”
It’s interesting to speculate on how shadowing like this might affect planet formation, given that the shadows create regions colder and denser than the rest of the outer disk. And can we generalize from the HD 142527 disks? This finding, notes the paper, “… poses a challenge to understand the dynamics of the HD 142527 system, and is an invitation to interpret scattered light images of gapped protoplanetary disks from the perspective of inner warp shadows.”
The paper is Marino et al., “Shadows cast by a warp in the HD 142527 protoplanetary disk,” Astrophysical Journal Letters Vol. 798, No. 2, L44 (abstract / preprint). A news release from the Millenium ALMA Disk Nucleus project is available.
A Stellar Correlation: Spin and Age
Figuring out how fast a star spins can be a tricky proposition. It’s fairly simple if you’re close by, of course — in our Solar System, we can observe sunspot patterns on our own star and watch as they make a full rotation, the spin becoming obvious. From such observations we learn that how fast the Sun spins depends on where you look. At the equator, the rotation period is 24.47 days, but this rotation rate decreases as you move toward the poles. Differential rotation means that some regions near the Sun’s poles can take as much as 38 days to make a rotation.
Because of these issues, astronomers have chosen an area about 26 degrees from the equator, where large numbers of sunspots tend to appear, as the point of reference, giving us a rotation of 25.38 days. You can imagine how complicated solar rotation gets once we look at other stars. We can’t resolve them to begin with, much less their ‘starspots,’ but what we can do is measure the decrease in light that starspots cause as they rotate around the star. You can see why studying a lightcurve like this resembles searching for transiting exoplanets, and in fact a mission like Kepler has to take the possibility of false positives from starspots into account.
Determining the Age of a Star
Stars between 80 and 140 percent of the Sun’s mass are the subject of new work on stellar spin by Søren Meibom (Harvard-Smithsonian Center for Astrophysics) and team. Using Kepler data, the researchers homed in on a 2.5 billion year old cluster called NGC 6819, part of a continuing investigation of stellar spin rates. Meibom’s work is part of the broader Kepler Cluster Study, for which he is the principal investigator. The work uses data from the original Kepler mission, the four years of data before the current K2 mission, to study star clusters.
The goal is to develop a precise method of obtaining the ages of stars like these, a method co-author Sydney Barnes (Leibniz Institute for Astrophysics) calls ‘gyrochronology.’ Stellar spin is the critical factor because stars slow down as they age, while other indicators, like size, temperature, and brightness, stay relatively constant. So far we’ve been able to use spin rate to determine stellar ages only with fast-spinning stars in young clusters. Starspot activity is prominent on young stars and thus easier to detect. We’re also helped by the fact that the pattern of color and brightness in a cluster can give us a good read on the overall cluster’s age.
Image: It’s easier to tell the age of a young star because they rotate more quickly and have larger starspots. Credit: David A. Aguilar (CfA).
Thus we can correlate spin rate with age in specific instances. Get beyond about 600 million years in stellar age, though, and there is a huge gap between these young stars and stars as old as our 4.6 billion year old Sun. How fast do stars of intermediate ages spin? In this video on his work, Meibom refers to the ‘four billion year gap’ created.by our inability to measure spin for older stars. Young, rapidly spinning stars with large starspot regions, where brightness changes are pronounced, show a far more marked lightcurve than older stars, where starspot activity has dropped and teasing out the spin rate takes us to the limit of our instruments’ capabilities.
The Kepler telescope’s exquisitely sensitive measurements of stellar brightness are what has allowed us to fill in the gap. Studies of the cluster NGC 6811 produced the first measurements for the spin rate of one billion year old stars in 2011. Analyzing NGC 6819 data, Meibom and team have now been able to measure spin rates for 30 2.5 billion year old stars. We can thus plot spin against age on a far more meaningful graph, allowing us to extrapolate that when the Sun was 2.5 billion years old, its spin rate was roughly 18 days, with the rate dropping to eleven days when the Sun was 1 billion years old. A stellar ‘clock’ like this can help us determine which stars have planets that have had time for complex life to evolve. Looking forward, studying planets like our own around much older stars will give us clues to what lies ahead for the Earth.
“Now we can derive precise ages for large numbers of cool field stars in our Galaxy by measuring their spin periods,” says Meibom. “This is an important new tool for astronomers studying the evolution of stars and their companions, and one that can help identify planets old enough for complex life to have evolved.”
The paper is Meibom et al., “A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster,” published online in Nature 5 January 2015 (abstract). Links to the full text, and to video of the AAS press conference where these findings were announced, are available along with much more material on stellar age here. A CfA news release is also available.
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