by Paul Gilster | Dec 19, 2005 | Deep Sky Astronomy & Telescopes
One of the important projects keeping astronomers busy as we wait for the next generation of both ground and space-based telescopes is mapping the local neighborhood. There is much to be learned, for example, in a star like Sirius, one of the Sun’s closest neighbors at 8.6 light years. Since 1862, we’ve known that Sirius is orbited by a white-dwarf star, and that this burnt-out remnant of an earlier star is terrifically difficult to study because of the glare of Sirius itself. Sirius B is about ten thousand times dimmer than Sirius.
Now Hubble has changed that picture, with an international team of astronomers having isolated the light of the white dwarf. This allows them to deduce its mass by examining how its gravitational field alters the wavelength of the light it emits.
And what a gravitational field it is. The measurements show that Sirius B is about 7500 miles in diameter (smaller than Earth) but its gravitational field is 350,000 times stronger. That’s enough to cause a substantial gravitational redshift, and enough to allow a mass prediction of 98 percent that of the Sun.
Image: An artist’s impression showing how the binary star system of Sirius A and its diminutive blue companion, Sirius B, might appear to an interstellar visitor. The large, bluish-white star Sirius A dominates the scene, while Sirius B is the small but very hot and blue white-dwarf star on the right. The two stars revolve around each other every 50 years. Credit: NASA, ESA and G. Bacon (STScI)
Martin Barstow (University of Leicester, UK), a leader of the observing team, has this to say about why the Sirius B work is significant:
“Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution. Our Sun will eventually become a white dwarf. White dwarfs are also the source of Type Ia supernova explosions that are used to measure cosmological distances and the expansion rate of the universe. Measurements based on Type Ia supernovae are fundamental to understanding ‘dark energy,’ a dominant repulsive force stretching the universe apart. Also, the method used to determine the white dwarf’s mass relies on one of the key predictions of Einstein’s theory of General Relativity; that light loses energy when it attempts to escape the gravity of a compact star.”
The work was reported in the October 2005 issue of the Monthly Notices of the Royal Astronomical Society. You can read a news release from the Particle Physics and Astronomy Research Council here.
by Paul Gilster | Dec 17, 2005 | Exoplanetary Science |
Last Saturday’s image of Proxima Centauri raised questions for several readers, who asked where Centauri A and B were in the photograph. The answer is that they are not in the field of view. To get a broader perspective, let’s step back a bit. In the image below, I’m using a photograph taken by Noël Cramer at the Observatoire de Genève. I’ve cropped the image to show the relative position of the primary Centauri stars and Proxima Centauri. If you look to the upper right of the image, you’ll see the tip of the red arrow that Cramer used to point to Proxima, which is otherwise indistinguishable.
Now ponder the bright ‘star’ at lower left. It is actually not one but two stars, Centauri A and B. The two are so close to each other, and so close to us, that they effectively merge into a single image, which is why we talk about ‘Alpha’ Centauri — it was once thought to be simply the single brightest star in the Centaur constellation. Now, of course, we know it is a triple system, with the two major stars both Sun-like and theoretically capable of supporting an Earth-like planet in their respective habitable zones.
Consider the distance between the bright Centauri A and B duo and Proxima in the above image. You’re looking at 10,000 AU, about the distance between the Sun and the Oort Cloud in our own Solar System.
Let’s back out to see a fuller view of the Cramer photograph. Now we see a second bright star at the top of the image. This is Beta Centauri. Despite the confusing nomenclature, Beta Centauri is not part of the Alpha Centauri triple star system. In other words, it’s not Centauri B; it’s merely the second brightest star in the Centaur. Alpha Centauri is the ‘star’ at the bottom, and if you look at about the 2 o’clock position at the very edge of the image, you’ll see just the tip of the red arrow that Cramer used to mark Proxima Centauri.
One thing that would surely galvanize the interstellar effort is the discovery of a terrestrial world around one of the Centauri stars. If such a blue and green world swam into view one day (perhaps through the efforts of the Terrestrial Planet Finder mission), reaching another star would be elevated to serious consideration by the various agencies that map out future space missions. Perhaps more important, however, would be the energizing effect on a public that has grown all too blasé about breakthroughs in the cosmos.
Image credits: Noël Cramer, Observatoire de Genève.
by Paul Gilster | Dec 16, 2005 | Exoplanetary Science |
Astronomy can be a time machine, taking us back to the era when the light we are observing left its source. Looking at a galaxy ten billion light years away thus tells us what galaxies looked like in that epoch. But the dizzying number of stars in our galaxy alone also lets us see into our own past, by showing us stellar systems much like ours once was. Such a system is that around HD 12039, a Sun-like star about 137 light years away.
By Sun-like, I mean a yellow G-type star, and that equates to surface temperatures between 5,000 and 7,000 degrees Fahrenheit. But HD 12039 is also much younger than the Sun, perhaps 30 million years old. In other words, it’s about the age that the Sun was when the Earth and Moon probably formed. Like our Sun in that era, the star has not yet settled into the main sequence, which marks mature nuclear-burning. It’s a bit brighter, a bit cooler, and a bit more massive than Sol.
The interesting news comes when we consider HD 12039’s debris disk. A team led by Dean C. Hines (Space Science Institute), using the Spitzer Space Telescope, has sampled a number of Sun-like stars, finding that between 10 and 20 percent of them have outer debris disks, much like the Kuiper Belt in our own Solar System. But HD 12039’s debris disk is narrow and warmer than the others, and at between 4 and 6 AU from the star, it’s about where Jupiter is in relation to the Sun. The narrowness of the disk may be caused by the gravitational attraction of an unseen planet.
Image: This artist’s concept depicts a distant hypothetical solar system, similar to the one recently discovered with the Spitzer Space Telescope. In this artist’s rendering, a narrow asteroid belt filled with rocks and dusty debris, orbits a star similar to our own Sun when it was approximately 30 million years old (about the time Earth formed). Within the belt a hypothetical planet also circles the star. Credit: NASA/JPL-Caltech/T. Pyle (SSC).
The implication? “At 30 million years, the material we see in this star likely has to come from ground-up rocks in a zone where terrestrial planets could form,” Hines said. The disk, in other words, may be in the process of planet formation, with rocky debris in a state of frequent collision. Hines adds, “This is one of a very rare class of objects that may give us a glimpse into what our solar system may have looked like during the formation of our terrestrial planets.”
Centauri Dreams‘ take: Is HD 12039 going through a typical scenario of planetary evolution, or are these ‘warmer’ debris disks uncommon? (I put ‘warmer’ in quotes because the actual temperature of this disk is minus 262 degrees Fahrenheit, warm only in comparison to outer debris disks like the Kuiper Belt). Early evidence seems to show that inner debris disks like this one show up around 1 to 3 percent of young Sun-like stars. We may be looking at a constraint on terrestrial planet formation, one that implies perhaps 3 out of 100 Sun-like stars are likely to form habitable planets. But these observations allow only the most preliminary of speculations.
by Paul Gilster | Dec 15, 2005 | Advanced Rocketry |
Ion drives may open up the outer Solar System, but they’re anything but high-thrust. With NASA’s Deep Space 1 mission and the later European Smart 1 moon mission, the idea was to operate for long periods of time with very little kick from the engine. The effect is cumulative, and it works. Japan’s Hayabusa asteroid probe used four ion engines designed to burn throughout its cruise to asteroid Itokawa. 20,000 hours of cumulative operation used up a scant 20 kilograms of propellant, highlighting the efficiency of these engines. NASA has run an NSTAR thruster at the Jet Propulsion Laboratory for over 30,000 continuous hours, almost five years of operation.
Now the European Space Agency has conducted successful tests of a new kind of ion drive, one designed to provide greater thrust than its predecessors. The Helicon Double Layer Thruster (HDLT) uses radio waves to ionise argon gas, creating two layers of plasma between which charged particles can be accelerated in a beam. The HDLT design was conceived by Dr. Christine Charles and team working in the Plasma Research Laboratory at Australia National University. Here’s how an ESA news release describes the process they invented, drawing on an interview with Dr. Pascal Chabert (Ecole Polytechnique, Paris):
To create the double layer, Chabert and colleagues created a hollow tube around which was wound a radio antenna. Argon gas was continuously pumped into the tube and the antenna transmitted helicoidal radio waves of 13 megahertz. This ionised the argon creating a plasma. A diverging magnetic field at the end of the tube then forced the plasma leaving the pipe to expand. This allowed two different plasmas to be formed, upstream within the tube and downstream, and so the double layer was created at their boundary. This accelerated further argon plasma from the tube into a supersonic beam, creating thrust.
How much thrust? At this point, ESA simply notes that at the same level of fuel efficiency as the main thruster on the SMART-1 mission, the new engine would produce “…many times more thrust at higher powers of up to 100 kW…”
The effect is not dissimilar to a natural phenomenon witnessed by many on Earth, as ESA explains:
“Essentially the concept exploits a natural phenomenon we see taking place in space,” says Dr Roger Walker of ESA’s Advanced Concepts Team. “When the solar wind, a ‘plasma’ of electrified gas released by the Sun, hits the magnetic field of the Earth, it creates a boundary consisting of two plasma layers. Each layer has differing electrical properties and this can accelerate some particles of the solar wind across the boundary, causing them to collide with the Earth’s atmosphere and create the aurora.”
Centauri Dreams‘ take: Now that ESA has confirmed the Australian results, the way is open to begin design studies for larger prototypes. The key to the recent ESA tests was to demonstrate that a double plasma layer could remain stable and thus allow for reliable acceleration of the charged particles for thrust. That it can is heady news and points to new sets of options for missions to the outer planets. We need spacecraft capable of delivering maximum payload without being dwarfed by fuel constraints. Coupling ion efficiency with higher thrust is a welcome and necessary step toward that goal.
by Paul Gilster | Dec 14, 2005 | Advanced Rocketry
First noted in Star Spangled Cosmos, this article from the Palm Beach Post about fusion research, focusing on recent progress and discussing the International Thermonuclear Experimental Reactor (ITER) consortium. The latter group plans to build an experimental fusion reactor in France by 2016. Speaking to Stephen Paul, a senior research physicist at Princeton University’s plasma laboratory, writer Ron Wiggins notes that fusion reactions have been sustained in laboratory settings for up to 24 seconds. How do we fund the next steps toward viability?
From the story:
The problem is that plasma — the primordial gas resulting when hydrogen atoms are heated to 100 million degrees — is difficult to contain in small reactors.
The bigger the reactor, the easier it is to control the reaction. Big reactors are expensive.
“Our reactor at Princeton is small — a man could walk into it. England has the biggest facility — about two stories.”
ITER would create such a reactor, a five-story experimental unit built to the tune of $5.5 billion, with the US paying up to $200 million per year once construction begins in 2006. Costs would be shared between the U.S., Japan, Russia, England and the European Union. Wiggins goes on to note that U.S. research is in a squeeze, with domestic research on fusion being cut to pay for ITER commitments. Paul told Wiggins that learning to manage plasma in small spaces should keep U.S. labs in the thick of the fusion hunt, and make them more rational repositories of funding than ITER, if the budget gets so tight that it comes down to a choice between the two.
Centauri Dreams‘ take: Getting practical fusion to work in a propulsion system is going to demand radical reductions in scale, just the sort of thing laboratories like Princeton are engaged in studying, though with more earthly considerations in mind. Even so, the annual U.S. budget for fusion research is less than $200 million per year. It seems reasonable to direct that funding toward the kind of containment systems that will make fusion practicable both for power production and, ultimately, advanced propulsion systems that could open up the outer planets.
