by Paul Gilster | Jul 14, 2011 | Deep Sky Astronomy & Telescopes |
A pair of white dwarf stars too close together to distinguish visually may help us in the hunt for gravitational waves, while potentially explaining a whole class of rare, relatively faint supernovae. The system in question — called SDSS J065133.33+284423.3, or J0651 for short — was found during a spectroscopic survey looking for extremely low mass white dwarfs. J0651 includes one white dwarf with about a quarter of the Sun’s mass compacted down to Neptune-size, along with a companion white dwarf that is half the Sun’s mass and about the size of the Earth.
Usefully, this is a system oriented so that we can observe eclipses of each star by the other, which is how we can measure orbital parameters, masses and white dwarf radii. The General Theory of Relativity predicts that close pairs of stars produce gravitational waves that are ripples in the curvature of spacetime, and as the paper on the new work points out, the binary pulsar PSR B1913+16 has already given us indirect evidence for such waves through the gradual decay of the orbit as predicted by Einstein’s theory. J0651 now emerges as an opportunity to study gravitational waves again by measuring small changes in the stars’ orbital periods.
Image: Two white dwarfs have been discovered on the brink of a merger. In just 900,000 years, material will start to stream from one star to the other (as shown in this artist’s conception), beginning the process that may end with a spectacular supernova explosion. Watching these stars fall in will allow astronomers to test Einstein’s general theory of relativity as well as the origin of a special class of supernovae. Credit: David A. Aguilar (CfA)
Thus we have an unusually useful celestial laboratory. Because there seems to be no exchange of mass, the change in separation over time should be easy to measure. From the paper:
We…predict that J0651’s orbital period is shrinking by 2.7 × 10-4 sec per year due to gravitational wave radiation. The expected change in period adds up to a 5.5 sec change in time-of-eclipse in one year. When we measure this change we expect to provide yet another fundamental test of general relativity and the existence of gravitational waves.
It’s a test that could potentially be confirmed by the proposed ESA/NASA Laser Interferometer Space Antenna (LISA) mission, which the authors predict could detect this gravitational wave source within its first week of operation considering its peak sensitivity at frequencies corresponding to orbital periods like those found here. The white dwarfs complete an orbit in just 13 minutes and will merge quickly in astronomical time. On that issue the paper comments:
The absence of mass transfer in J0651 is perhaps surprising given how quickly it will merge. In all known binaries with periods comparable to J0651… one star fills its Roche lobe and transfers mass to its companion. Our data show that the J0651 primary has a Roche lobe radius 1.5 times its current radius. Under the assumption of energy and angular momentum loss due to gravitational wave radiation, the primary WD will reach its Roche lobe radius at an orbital period of 6.5 minutes in 0.9 Myr.
What happens when the stars merge 900,000 years from now is another issue. In some models, merging white dwarf pairs are the source of faint stellar explosions called underluminous supernovae. Such objects are 10 to 100 times less luminous than normal Type Ia supernovae. The supernova SN 2005E, for example, can be explained using parameters similar to the J0651 system, but mass transfer between the two stars could lead to a variety of different outcomes. Thus the need for the ongoing survey of low mass white dwarf systems using the MMT telescope at the Whipple Observatory on Mt. Hopkins, Arizona, from which the current paper draws its data.
The two white dwarfs are circling at a speed of some 600 kilometers per second. “If there were aliens living on a planet around this star system, they would see one of their two suns disappear every 6 minutes – a fantastic light show.” said Smithsonian astronomer and co-author Mukremin Kilic. So strong is the mutual gravitational pull of the white dwarfs here that the lower-mass star is deformed by three percent and, as this Harvard-Smithsonian Center for Astrophysics news release notes, a similar bulge on our own planet would result in tides 190 kilometers high.
The paper is Brown et al., “A 12 minute Orbital Period Detached White Dwarf Eclipsing Binary,” accepted by Astrophysical Journal Letters and available as a preprint.
by Paul Gilster | Jul 13, 2011 | Outer Solar System |
When the German astronomer Johann Gottfried Galle discovered Neptune on September 23, 1846, he found a world so distant from the Sun that its orbit takes 165 years to complete. With Neptune reaching its first complete revolution since discovery, an event that occurred yesterday, we can enjoy some celebratory Hubble imagery of the planet. I especially like the shot below, which not only shows atmospheric features but also has been tweaked to reveal some of Neptune’s moons. The planet has about thirty moons, most of them too faint to appear in these images.
Image: This illustration was composed from numerous separate Hubble Wide Field Camera 3 images. A color image composed of exposures made through three color filters shows the disk of Neptune, revealing clouds in its atmosphere. Forty-eight individual images from a single filter were brightened to reveal the very faint moons and composited with the color image. The white dots are Neptune’s inner moons moving along their orbits during Hubble’s observations. The solid green lines trace the full orbit of each moon. The spacing of the moon images follows the timing of each Hubble exposure. Triton, in the lower left corner, is the brightest of the moons seen in these images, farthest from the planet, and moves in a counter-clockwise sense in this view. Next closest to Neptune is Proteus, followed by Larissa, Galatea and Despina, all of which move clockwise in this view, opposite to Triton’s (retrograde) motion. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
The four images below were taken with Hubble’s Wide Field Camera 3 in late June, spaced at four hour intervals to offer a full view of the planet’s 16-hour rotation. You can make out high-altitude clouds of methane ice crystals in the northern and southern hemispheres. And just as we’ve recently looked at seasonal changes on Saturn, the Hubble imagery reveals similar changes on Neptune, with cloud activity shifting to the northern hemisphere. The southern hemisphere is now in early summer, while the northern hemisphere has entered winter. Neptune has an axial tilt of 29 degrees, creating seasons that last for some forty years each.
Image: In the Hubble images, absorption of red light by methane in Neptune’s atmosphere gives the planet its distinctive aqua color. The clouds are tinted pink because they are reflecting near-infrared light. A faint, dark band near the bottom of the southern hemisphere is probably caused by a decrease in the hazes in the atmosphere that scatter blue light. The band was imaged by NASA’s Voyager 2 spacecraft in 1989, and may be tied to circumpolar circulation created by high-velocity winds in that region. The temperature difference between Neptune’s strong internal heat source and its frigid cloud tops, about minus 260 degrees Fahrenheit, might trigger instabilities in the atmosphere that drive large-scale weather changes. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
The story of Neptune’s discovery is a fascinating topic in itself. Galileo actually recorded Neptune in his notebooks but didn’t follow up the observation. British astronomer William Herschel would later note that the orbit of Uranus did not behave as expected according to Newton’s laws, leading to speculations (like those of French astronomer Alexis Bouvard in 1821) that another planet was tugging at Uranus. Both Urbain Le Verrier and John Couch Adams predicted the location of the putative planet in the 1840s, but it was Le Verrier who described his prediction to Galle at the Berlin Observatory. After that, it was a matter of two nights of observation before Galle was able to identify the planet. Although the Royal Society would award Le Verrier the Copley Medal for his work, supporters of Adams defended his right to be considered a co-discoverer. Recent work has cast doubt on the idea.
Neptune makes numerous appearances in science fiction, such as Samuel Delany’s Triton (1976) and Jeffrey Carver’s Neptune Crossing (1994), in both of which much of the action takes place on Neptune’s largest moon. But earlier appearances are interesting, including Olaf Stapledon’s Last and First Men (1930), in which the planet serves as humanity’s final home. H.G. Wells likewise wrote about Neptune in ‘The Star’ (1897), a short story in which the planet is destroyed by a collision with what appears to be a rogue wandering planet from the interstellar deep. The event puts a brilliant new star in Earth’s sky, one that inexorably approaches our planet. Interestingly, the massive new object now gets a gravitational assist from Jupiter, as foreseen by a canny mathematician who forecasts the end of the human race:
The new planet and Neptune, locked in a fiery embrace, were whirling headlong, ever faster and faster towards the sun. Already every second this blazing mass flew a hundred miles, and every second its terrific velocity increased. As it flew now, indeed, it must pass a hundred million of miles wide of the earth and scarcely affect it. But near its destined path, as yet only slightly perturbed, spun the mighty planet Jupiter and his moons sweeping splendid round the sun. Every moment now the attraction between the fiery star and the greatest of the planets grew stronger. And the result of that attraction? Inevitably Jupiter would be deflected from its orbit into an elliptical path, and the burning star, swung by his attraction wide of its sunward rush, would “describe a curved path” and perhaps collide with, and certainly pass very close to, our earth. “Earthquakes, volcanic outbreaks, cyclones, sea waves, floods, and a steady rise in temperature to I know not what limit”–so prophesied the master mathematician.
‘The Star’ is still a good read (Wells’ Martians even make an appearance). You can read it online here, an enjoyable way to celebrate Neptune’s first full year since discovery.
by Paul Gilster | Jul 12, 2011 | Deep Sky Astronomy & Telescopes |
The supernova called SN 1987A is a prime object for scrutiny because it gives us the chance to see the various phases of stellar death over time. And as you might guess from the fact that it was visible to the naked eye when first detected back in 1987, it’s located relatively nearby, in the Large Magellanic Cloud. Working in the far-infrared, the European Space Agency’s Herschel space observatory has made new discoveries about SN 1987A while studying this small galaxy’s cold dust emissions.
The surprise result: SN 1987A is shrouded with enormous amounts of dust, 10,000 times more than previous estimates. The dust was at a temperature of roughly minus 256 to minus 249 degrees Celsius, making it a bit colder than Pluto (minus 240 degrees Celsius). These are the first far-infrared observations of this object, showing that the dust is emitting more than 200 times the energy of the Sun. Moreover, there is enough dust here to account for 200,000 planets the size of the Earth. Thus we learn that supernova explosions can be true dust factories.
Looking at SN 1987A with different instrumentation has proven valuable. The image below contrasts what Herschel can see versus the view from the Hubble Space Telescope. Here we’re contrasting Hubble with Herschel’s ability to see much longer wavelengths, and gaining a broad, composite picture of supernova dust formation.
Image: This layout compares two pictures of supernova remnant SN 1987A — the left image was taken by the Herschel Space Observatory, and the right is an enlarged view of the circled region at left, taken with NASA’s Hubble Space Telescope. The tiny pink ring in the Hubble image shows where a shock wave from the blast is hitting the surrounding material expelled from the star before the explosion. The cause for the outer, faint rings is unknown. When Herschel observed SN 1987A, it saw something different. Herschel sees infrared and submillimeter light, much longer in wavelength than the visible light Hubble detects. For this reason, its picture of the supernova is not as sharp, appearing only as a fuzzy dot inside the circle. But that dot represents an important discovery of vast reservoirs of cold dust around SN 1987A. Herschel can see very cold material — the colder something is, the longer the wavelengths of light it emits. Credit: ESA/NASA-JPL/UCL/STScI.
We need to learn as much as we can about cosmic dust because the heavy atoms — carbon, silicon, oxygen, iron — that it contains were not produced in the Big Bang but later. These elements are major constituents of the rocky planets and key players in life itself. And while old red giant stars in today’s universe are thought to be major dust producers as their gases flow outward from the star, they would not have been present for this purpose in the very early universe. But dust condensing from the gaseous debris of a supernova would have become available early on, accounting for the dust seen in young galaxies observed at huge distances from Earth.
“The Earth on which we stand is made almost entirely of material created inside a star,” explained the principal investigator of the survey project, Margaret Meixner of the Space Telescope Science Institute, Baltimore, Md. “Now we have a direct measurement of how supernovae enrich space with the elements that condense into the dust that is needed for stars, planets and life.”
We also see the value of probing the universe with different wavelengths, such as Herschel’s far-infrared. While we’ve had previous studies on supernovae and their ability to produce dust (NASA’s Spitzer Space Telescope, working with shorter infrared wavelengths than Herschel, found 10,000 Earth masses of fresh dust around the supernova remnant called Cassiopea A), this detection of a much larger reservoir of dust around SN 1987A confirms and extends our knowledge. The closest supernova witnessed in almost 400 years has proven a useful object indeed.
The paper is Matsuura et al., “Herschel detects a massive dust reservoir in supernova 1987A,” published in Science Express on 7 July 2011 (abstract). More in this JPL news release.
by Paul Gilster | Jul 11, 2011 | Culture and Society |
By Rob Swinney
Rob Swinney is a freelance writer, a member of the British Interplanetary Society and an active participant in the Tau Zero Foundation/BIS study group Project Icarus, a team of volunteers working on a practical design for an interstellar probe. Rob completed his Bachelors degree in Astronomy and Astrophysics at the University of Newcastle Upon Tyne and his Masters in Radio Astronomy at the University of Manchester (Jodrell Bank). Later he graduated from Cranfield University (then the Cranfield Institute of Technology) with a Masters degree in Avionics and Flight Control Systems. After a rewarding career in the Royal Air Force as an Aerosystems Engineer (Avionics) Officer he completed his Commision in 2006 having attained the rank of Squadron Leader. He is a Chartered Engineer registered with the UK’s Engineering Council and a Member of the Institution of Engineering and Technology. Rob recently attended the UK Space Conference on July 4 and 5th and here offers us a personal view of the proceedings. Space agencies worldwide are challenged by budget cuts and the need to develop a new vision as we enter the age of commercial space. Our recent discussions here highlighted the problems of the US space program in particular. How do things look in the UK and the European Space Agency?
The UK Space Agency emerged from the forerunner British National Space Centre. This week the agency hosted the UK’s Space Conference 2011 at Warwick University, England, where economic issues came to the fore. The agency has an apparently vastly increased budget of several hundred millions of pounds although this was created by merging funds from other areas and the figure is still well below the amounts spent by the other major European economies.
The conference, whose theme was ‘The New Space Economy,’ brought together members of the space community from industry, government and academia and painted a positive picture of the space sector in the UK, which is estimated to be worth £7.5 billion per year (over $10 billion), a figure the new agency hopes to help grow to over £40 billion in the next 20 years. There was definitely a ground swell of positive opinion about the skills in the UK to build small satellites and other specific areas of the sector that can offer business opportunities.
Interestingly, David Willetts, a Member of Parliament and the UK Minister of State for Universities and Science, suggested the health of the UK space sector was in such a good state because there hadn’t been a government agency involved in the past and this had been to its advantage. It is perhaps unclear how ringing an endorsement this is of the move to governmental executive agency status. But Willetts did announce a number of policies to try and improve the environment for doing ‘space’ business in the UK, one of which was to lower the mandatory third party insurance limit from £100m to £60m (for launch and orbital operations).
Even if the agency just focuses on lowering costs and red tape there is little chance of the UK market not growing to £40 billion in the timeframe allowed given the rate of growth for the last 10 years – even through the recession we are now experiencing. Perhaps more challenging is the aim to grow the market share of worldwide space products and services from 6 to 10%.
What the Agency actually does with its £230 million budget is then open to debate. The good news for the European Space Agency (ESA) is that it looks like more of that money will end up being pushed through to ESA. The bad news is that it is really not a lot of money anyway and only a fraction of the totals of other European countries such as France and Germany. Even so, the importance of the UK involvement with ESA was apparent from the first session, ‘Space Policy,’ which included the ESA Director General Jean-Jacques Dordain, who helped set the scene. Dordain stated that although the conference had started on America’s Independence Day, “Together we are better.” Independent agencies cooperating skillfully can achieve much.
Various tracks were offered in the parallel sessions and on the first day I attended the ‘Science and Exploration’ session followed the next day by ‘Student presentations’ and then ‘Access to Space.’ There was an interesting mix. Much in evidence was the prototype British eccentric, perhaps a throw-back from an earlier age of brilliant endeavour but also a marker for the new generation of young enthusiasts fronted by UKSEDS, the UK chapter of the Students for the Exploration and Development of Space. Also in evidence were the hard headed business people who play so significant a role in the success and focus of the space business in the UK.
The second day opened with a panel session on ‘Innovation – Science, Business and Technology’ and included UK industry ‘giant’ Sir Martin Sweeting OBE, the executive chairman of Surrey Satellite Technology Ltd., along with innovator Alan Bond, Managing Director of Reaction Engines Ltd. and a key player in the original Project Daedalus starship design.
Bond discussed the challenges facing an innovator, especially when working with game-changing technology. This is clearly not a UK-specific issue, but he did lament the lack of support from government, industry and the banks, while noting the many skills and commitments innovators had to excel at in addition to their expertise in their field (e.g. becoming brilliant at presenting and communicating the idea, attending conferences at their own expense, working with economists and creating business plans, negotiating patent law as well as being a technical expert).
Bond said that innovators were usually young and naïve and would find that they had to live with less and less of the financial rewards of their innovation. I’m not sure if he was thinking of himself and the Reaction Engines Skylon spaceplane and its SABRE engine but he did mention that the technology first appeared some 40 years ago and that it has been fully 29 years since the start of the development. Perhaps the most valuable lesson he gave was perseverance!
Image: Skylon is the design for an unmanned, reusable spaceplane intended to provide low-cost access to space. The SABRE engines that would drive it offer both air-breathing and conventional rocket capabilities, the intent being to reach orbit in a single stage. Skylon is currently in its proof-of-concept phase. Is this the low-cost way to LEO we once imagined the Space Shuttle would be? Credit: Reaction Engines Ltd.
It is perhaps unclear how the UK Space Agency might have changed this process as its focus is on partnering with others and possibly some small independent projects.
It doesn’t look like there are any immediate plans to support blue sky thinking as far out as interstellar flight. Project Icarus may have to continue on its voluntary way. Indeed I heard interstellar distances mentioned only once and then only as a way of illustrating relative distances — e.g. if you imagine the Earth about the size of a large grapefruit, then the distance from Earth to our Moon is about 4m (a scale of 1,000,000:1), making the distance to the nearest stars the distance of the actual Moon, some 250,000 miles away. An interstellar conference this was not, but full marks to the organisers for putting together a successful look at current policy.
by Paul Gilster | Jul 7, 2011 | Outer Solar System |
I had been intending to cover recent news about Saturn in an upcoming post anyway, but the images below sealed the deal. They’re further wonders from Cassini, pictures of a massive storm in Saturn’s northern hemisphere that encircles the planet. First detected on December 5, 2010, the storm has been on the rampage ever since at about 35 degrees north latitude. It covers approximately 4 billion square kilometers. Cassini’s radio and plasma wave science instrument has been showing a lightning flash rate 10 times that of any other storms the spacecraft has monitored since its arrival in Saturn orbit back in 2004. The flashes were so frequent at one point that Cassini could no longer resolve individual strokes, although the intensity has now eased.
Image: The huge storm churning through the atmosphere in Saturn’s northern hemisphere, seen here in a true-color view from NASA’s Cassini spacecraft. This view looks toward the sunlit side of the rings from just above the ring plane. The image, captured on Feb. 25, 2011, was taken about 12 weeks after the storm began, and the clouds by this time had formed a tail that wrapped around the planet. Some of the clouds moved south and got caught up in a current that flows to the east (to the right) relative to the storm head. This tail, which appears as slightly blue clouds south and west (left) of the storm head, can be seen encountering the storm head in this view. Credit: NASA/JPL-Caltech/SSI.
It’s interesting to realize that the shadow cast by Saturn’s rings has strong effects on the planet’s weather. This storm is 500 times the area of the largest southern hemisphere storms Cassini has seen on its mission, and scientists are suggesting that we are now seeing such powerful storms in the northern hemisphere because of the change of seasons after the planet’s 2009 equinox. At present, Saturn is entering early northern spring, making this a relatively early storm. In previous years, Earth-based astronomers as well as Hubble have observed huge storms called ‘Great White Spots’ in late northern summer, some of which were as large as this one. Unfortunately, we didn’t have Cassini or Voyager on the scene for previous storms of this magnitude.
“This storm is thrilling because it shows how shifting seasons and solar illumination can dramatically stir up the weather on Saturn,” said Georg Fischer, a radio and plasma wave science team member at the Austrian Academy of Sciences in Graz. “We have been observing storms on Saturn for almost seven years, so tracking a storm so different from the others has put us at the edge of our seats.”
We’re looking at a storm that covers an area eight times the surface area of Earth. Since its arrival, Cassini has detected 10 lightning storms on Saturn, found in an area of the southern hemisphere now known as ‘Storm Alley.’ We’re now getting a glimpse of just how lively a Saturnian spring can be as the Sun’s illumination of the hemispheres changes. The storm is generating huge amounts of radio noise from lightning. At issue is the sudden release of such energies, unlike what we see on Jupiter or, for that matter, the Earth, where numerous storms are active at any given moment. In sharp contrast, Saturn seems quiet for years and then erupts into frenzied activity.
Image: False-color images from Cassini showing clouds at different altitudes. Clouds that appear blue here are the highest and are semitransparent, or optically thin. Those that are yellow and white are optically thick clouds at high altitudes. Those shown green are intermediate clouds. Red and brown colors are clouds at low altitude unobscured by high clouds, and the deep blue color is a thin haze with no clouds below. The base of the clouds, where lightning is generated, is probably in the water cloud layer of Saturn’s atmosphere. The storm clouds are likely made out of water ice covered by crystallized ammonia. Taken about 11 hours — or one Saturn day — apart, the two mosaics in the lower half of this image product consist of 84 images each. The mosaic in the middle was taken earlier than the mosaic at the bottom. Both mosaics were captured on Feb. 26, 2011, and each of the two batches of images was taken over about 4.5 hours. Credit: NASA/JPL-Caltech/Space Science Institute.
The storm on Saturn made the cover of Nature this week. The papers involved are Sánchez-Lavega et al., “Deep winds beneath Saturn’s upper clouds from a seasonal long-lived planetary-scale storm,” Nature 475 (7 July 2011), 71-74 (abstract) and Fischer et al., “A giant thunderstorm on Saturn,” Nature 475 (7 July 2011), 75-77 (abstract).
