by Paul Gilster | Jul 25, 2005 | Deep Sky Astronomy & Telescopes, Sail Concepts
Tracking down the history of a star is no easy matter, but a supernova called SN 1979C is providing unexpected assistance. Just as researchers can study ancient climates by examining the concentric rings inside a tree, astronomers using the European Space Agency’s XMM-Newton space observatory have found a way to study the rings around a star. SN 1979C, it turns out, produced huge stellar winds late in its life that flung particles into space over a period of millions of years. The result: a series of concentric rings lit up by x-rays when the star exploded.
“We can use the X-ray light from SN 1979C as a ‘time machine’ to study the life of a dead star long before it exploded,” says Dr Stefan Immler, leader of the team, from NASA’s Goddard Space Flight Center, USA. “All the important information that usually fades away in a couple of months is still there.”
Image (click to enlarge): XMM-Newton image of X-ray light from the galaxy M100. Credit: European Space Agency.
Immler and colleagues have found they can study the star’s stellar wind back to a time fully 16,000 years before the supernova explosion that took its life. What happened at SN 1979C is that the star, some 18 times more massive than the Sun, produced x-rays when the supernova shock heated the rings of stellar wind material to a temperature of several million degrees. The x-ray light illuminates the life of the star before it exploded, providing scientists with 25 years of data to work with not only in x-rays but in wavelengths ranging from radio waves to optical/ultraviolet.
How could a supernova continue shining so brightly in x-ray light? Normally, an object like this fades quickly — the usual pattern is for a supernova to be half as bright after ten days and to fade steadily after that in all wavelengths. But this star, while it has faded dramatically in optical light, is still the brightest x-ray object in its host galaxy (M100, in the constellation Coma Berenices).
One theory is that the stellar wind was so abundant around SN 1979C that it provided the needed material to keep the object glowing in these wavelengths. Compared to our Sun, the star’s circumstellar material is dense indeed. It covers a region 25 times larger than our Solar System, with a density of some 10,000 atoms per cubic centimeter (about 1000 times denser than Sol’s solar wind).
The paper is Immler, S., Fesen, Robert, Schuyler, D. Van Dyk et al., “Late-time X-Ray, UV and Optical Monitoring of Supernova 1979C,” available as a preprint on the arXiv site, and slated to appear in the 10 October issue of the Astrophysical Journal (ApJ 10 October 2005, v632 1).
by Paul Gilster | Jul 22, 2005 | Breakthrough Propulsion |
If you wanted to reach Alpha Centauri in 40 years, one way to do it would be to boost a spacecraft up to 10 percent of lightspeed as quickly as possible and then let it coast to destination. Or you could do something entirely different: push your payload at constant acceleration halfway to Centauri, turn it around at the halfway point, and perform a uniform deceleration that gets you to into Centauri space with zero speed. To achieve the latter — no small feat, needless to say — requires a constant acceleration of 0.0105g.
That number comes from the work of Italian physicist and mathematician Claudio Maccone, whose new paper “Radioactive Decay to Propel Relativistic Interstellar Probes Along a Rectilinear Hyperbolic Motion (Rindler Spacetime)” discusses a novel way to design an interstellar probe. Maccone’s study of constant acceleration (using what special relativity calls ‘hyperbolic motion’) shows that it could provide an ideal mission profile if we can find a way to propel a probe through processes of radioactive decay.
Hyperbolic motion has a fascinating history, first mentioned by Hermann Minkowski in 1908 in the same lecture where he introduced the idea of spacetime. Maccone notes a relatively unheralded 1930 book by Robert Esnault-Pelterie called L’Astronautique as using special relativity to examine relativistic interstellar flight, and also gives a nod to the German scientist Eugen Sänger, who suggested a design for a photon rocket in the 1950s (Sänger used electron/positron annihilation to produce gamma rays as exhaust, though how such an exhaust stream would be directed seems an insuperable difficulty).
But it was Carl Sagan whose work with hyperbolic motion fired the imagination of many of today’s scientists. In his 1963 paper “Direct Contact among Galactic Civilizations by Relativistic Spaceflight,” (Planetary and Space Science 11, pp. 485-98), Sagan drew from Robert W. Bussard the idea that an interstellar ramjet could accelerate continuously at 1 g. He then worked out the effects of relativistic time dilation as the ramjet continues to close on lightspeed.
Remarkably, with a constant acceleration of 1 g, a crew would reach the galactic center in a ship’s time of 20 years, even as 32,000 years passed on Earth. Even the Andromeda Galaxy would be within reach, 25 years away by ship’s time, while fully two million years passed on Earth. If all this sounds familiar, it may be because it stirs memories of Poul Anderson’s wonderful novel Tau Zero (New York: Doubleday, 1970), in which a runaway starship does something similar.
Image: M31, the Andromeda Galaxy. A constant acceleration of 1 g, as Carl Sagan showed, would allow a crew to reach it within a human lifetime, although millions of years would pass back on Earth. Credit: Adam Block/NOAO/AURA/NSF.
Maccone goes on to discuss the work of Jakob Ackeret (1898-1981), who in 1946 published the rocket equation for special relativity, an extension of the Newtonian rocket equation worked out originally by Konstantin Tsiolkovsky in 1903. Quoting Maccone: “…if one knows the probe’s velocity profile, then the Ackeret equation yields its mass decrease in proper time; or, the other way round, if the law by which the mass decreases in proper time is known, the velocity profile in proper time is obtained.”
For hyperbolic motion, the mass of the propellant decreases exponentially. The key point: “…this exponential decrease of the propellant mass formally has just the same equation as the radioactive decay equation. The way thus is paved to exploit the radioactive decay as a propulsion system to achieve the uniformly accelerated mission profile of the hyperbolic motion.” The relationship between the constant acceleration of hyperbolic motion, the radioactive decay constant and the exhaust speed of the decaying radioactive material can be used to select the appropriate radioactive material as a propellant to achieve the needed constant acceleration for a given target star.
Maccone’s paper “Radioactive Decay to Propel Relativistic Interstellar Probes Along a Rectilinear Hyperbolic Motion (Rindler Spacetime)” appeared in Acta Astronautica 57 (June, 2005), pp. 59-64.
by Paul Gilster | Jul 21, 2005 | Exoplanetary Science
So many of the planets discovered in the last ten years have been gas giants, circling their parent stars in extremely tight orbits. We assume there are rocky, terrestrial worlds out there in abundance, but until more advanced detection techniques are in place, how can we be sure? An important answer may be offered by BD +20 307, a Sun-like star some 300 light years from our Solar System. It’s surrounded by a warm disk of silicate dust particles that shows all the signs of being formed from the collision of rocky bodies up to planet size.
Located in the constellation Aries, the star has one more ace up its sleeve. Its dust — found in greater profusion than has ever been observed around a Sun-like star this long after its formation — exists at distances comparable to that of the Earth from the Sun. Finding such an infrared dust signature at Earth-like distances (i.e., 1 AU) has long been a goal of researchers. As revealed in the July 21 issue of the British science journal Nature, the finding seems to corroborate theories of how our own Solar System formed, and implies that rocky planets and their moons are not necessarily rare.
“The amount of warm dust near BD+20 307 is so unprecedented I wouldn’t be surprised if it was the result of a massive collision between planet-size objects, for example, a collision like the one which many scientists believe
formed Earth’s moon,” said Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA’s Astrobiology Institute, and a co-author on the paper. Zuckerman is quoted in a Gemini Observatory (Hawaii) press release; the observations behind this work were taken at the Gemini and W.M. Keck Observatories.
Image: Artist’s conception of a possible collision around BD +20 307 that might have created some of the dust observed in the recent Gemini/ Keck observations. The collisions responsible for this dust could range in size from the largest known asteroids (approximated here) to planets the size of the Earth or Mars. Credit: Gemini Observatory/Jon Lomberg.
Here’s what we know about BD+20 307: it’s slightly more massive than the Sun, and its dust disk was first detected by the Infared Astronomical Satellite in 1983. The star, otherwise unobserved since the 1983 findings, is estimated to be 300 million years old, implying that any large planets orbiting it have already formed. But large outer planets might have a formative effect on the rocky debris in the inner system, as evidently happened near our Sun.
What the new work shows is that the collisions that would account for the observed dust must have been between bodies as large as 300 kilometers across, roughly the size of the largest asteroids, and could have involved much larger objects up to planetary size. “Whatever massive collision ocurred, it managed to totally pulverize a lot of rock,” said team member Alycia Weinberger.
The amount of dust around BD+20 307 is remarkable, fully one million times greater than the dust remaining around our Sun today. And here’s something fascinating. Because of its properties, the team estimates that the collisions forming the dust could not have occurred more than about 1000 years ago. A longer period than that would give the dust enough time to fall into the star. So we’re dealing with what seems like one or a series of recent collisions within an Earth-like distance from the star, just the zone in which we believe a major collision produced our own Moon. BD+20 307 may well be evolving into a planetary system with marked similarities to our own.
Centauri Dreams‘ take: We’ve found dust around numerous Sun-like stars, but it tends to be cold and orbits far from its parent star, in regions analogous to the Kuiper Belt around the Sun. Only a few main-sequence stars show signs of warm dust disks, making the signature of planet formation in the ‘habitable zone’ areas roughly 1 AU from a star very hard to examine. BD+20 307 is therefore an outstanding find that will provide clues on what to look for as we extend the search for terrestrial worlds to thousands of nearby stars.
And listen to Inseok Song, a former UCLA research scientist who is now an astronomer with the Gemini Observatory in Hawaii, and lead author of the paper:
“Since the early ’80s,” Song said, “many astronomers have eagerly searched for an analogy to our solar system’s asteroidal belt at other stars. Our finding is a bona fide example of dust at the exo-asteroidal zone and it is chilling to see dust at the Earth-sun separation around a young solar analog — like seeing our own sun back in time.”
The paper is Inseok Song, B. Zuckerman, Alycia J. Weinberger and E. E. Becklin, “Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star,” Nature 436, pp. 363-365 (21 July 2005). A UCLA news release on this work can be found here.
by Paul Gilster | Jul 20, 2005 | Missions, Outer Solar System
With excitement building over what everyone hopes will be a January launch of the New Horizons mission to Pluto and Charon, astronomers have found yet another tool for studying the distant worlds. They’re taking advantage of a rare alignment in which Charon, Pluto’s moon, passes in front of a star. Such an event has been observed only once, some 25 years ago, and with less capable instrumentation.
We’ll know a lot more about the results of the July 10-11 occultation in September, when they’re presented at the 2005 meeting of the American Astronomical Society’s Division of Planetary Sciences meeting, to be held in Cambridge, England. There, scientists from MIT and Williams College will report on observations taken with four telescopes located at various sites in Chile. Remarkably, the team was able to muster more than 100 square meters of telescope surface facing Charon, a number that represents a ‘…noticeable fraction of the world’s total telescope area,’ according to an MIT news release.
Although the actual occultation lasted less than a minute, a close study of the data it produced may be able to tell us whether or not Charon has an atmosphere and determine a more accurate value for its radius. All this through an alignment that in terms of rarity can hardly be matched. “It’s amazing that people in our group could get in the right place at the right time to line up a tiny body 4 billion miles away,” said Jay M. Pasachoff, Williams College team leader and a professor in the Department of Astronomy. “It’s quite a reward for so many people who worked so hard to arrange and integrate the equipment and to get the observations.”
Image: Pluto and Charon as viewed by the Hubble Space Telescope. Note the darker tint of Charon, indicating differences in surface composition. Also, note what may be a surface feature at the center of the Pluto image. Credit: Space Telescope Science Institute.
The precision involved in the monitoring of this event is hugely impressive. One thing the research team had to do was determine the best places from which to observe the occultation. They chose their telescopes along a north-south line in Chile because predictions of the ‘starlight shadow’ of Charon were uncertain by several hundred kilometers. From the news release:
Since the star that was hidden is so far away, it casts a shadow of Charon that is the same size as Charon itself, about 1,200 kilometers in diameter. To see the event, the distant star, Charon, and the telescopes in Chile had to be perfectly aligned. All these telescopes were in clear weather and successfully observed the occultation.
There is a video of the occultation of the star C313.2 available via MIT, and it’s well worth a look. Meanwhile, I’m remembering a 2002 interview I did with the Jet Propulsion Laboratory’s James Lesh, who is chief technologist for JPL’s Advanced Multiple Mission Operations System, which includes the Deep Space Network. We had been speaking about how communications signals can be used for additional purposes, often with scientific benefits not originally forseen.
Lesh noted the value of looking at a radio signal from a spacecraft and observing how it behaves as the spacecraft goes behind a planet; such observations can unlock features of planetary atmospheres that would otherwise be hidden. Laser communications should also provide such benefits. “I claim there is a very similar field I would call light science,” Lesh said. “One could perhaps get accurate measurements of high altitude atmospherics. One could also observe transmission forward scattering through planetary ring systems, achieving very fine spatial selection. One person’s noise is often another persons signal.”
In much the same way, the light from a distant star becomes a scientific tool for ‘light science’ in studying the possible atmosphere and other features of a world that is 40 times as far from the Sun as our Earth. The ingenuity of such observations gives us a glimpse of the excitement ahead as we begin to decipher data from missions like New Horizons, and new Earth-based telescopes like the Giant Magellan Telescope, the first of whose mirrors is being cast this month at the University of Arizona.
by Paul Gilster | Jul 19, 2005 | Exoplanetary Science
How unlikely would it be to find a 200-year old person? That’s the comparison astronomer Lee Hartmann (Harvard-Smithsonian Center for Astrophysics) is using in talking about a dust disk around a pair of red dwarf stars. The disk looks conventional enough — as examined by the Spitzer Space Telescope, its inner edge is about 65 million miles from the binary stars, and it seems to extend outward for 650 million miles. That kind of disk should lead, according to current theory, to planetary formation within a few million years.
But the disk in question has been estimated to be 25 million years old, and it shows no evidence whatsoever of having created a planetary system. In fact, a dust disk that old shouldn’t exist at all; most newborn stars show no dust disks after just a few million years. All that material has by that time gone into the making of full-sized planets.
Image: Astronomers were surprised to discover a 25-million-year-old protoplanetary disk around a pair of red dwarf stars 350 light-years away. Gravitational stirring by the binary star system (shown in this artist’s conception) may have prevented planet formation. Credit: David A. Aguilar (CfA).
And that makes these two red dwarfs, located some 350 light years away in the constellation Taurus, a mystery. “We don’t know why this disk has lasted so long, because we don’t know what makes the planetary formation process start,” said Nuria Calvet of CfA, a co-author on the paper announcing the discovery.
Can these stars still form planets? The jury is out, and we can expect strenuous debate, along with a renewed search to discover how common such old disks are. The paper discussing the long-lived protoplanetary disk will be published in the Astrophysical Journal Letters. A Harvard-Smithsonian Center for Astrophysics news release is here.
“We can use the X-ray light from SN 1979C as a ‘time machine’ to study the life of a dead star long before it exploded,” says Dr Stefan Immler, leader of the team, from NASA’s Goddard Space Flight Center, USA. “All the important information that usually fades away in a couple of months is still there.”
