Posted on by Fraser Cain

Spitzer Sees Hidden Black Holes

A distant galaxy (yellow) that houses a quasar. Image credit: NASA Click to enlarge
Most of the biggest black holes in the universe have been eating cosmic meals behind closed doors ? until now.

With its sharp infrared eyes, NASA’s Spitzer Space Telescope peered through walls of galactic dust to uncover what may be the long-sought missing population of hungry black holes known as quasars.

“From past studies using X-rays, we expected there were a lot of hidden quasars, but we couldn’t find them,” said Alejo Martinez-Sansigre of the University of Oxford, England. He is lead author of a paper about the research in this week’s Nature. “We had to wait for Spitzer to find an entire population of these dust-obscured objects.”

Quasars are super-massive black holes that are circled by a giant ring of gas and dust. They live at the heart of distant galaxies and can consume up to the equivalent mass of one thousand stars in a single year. As their black holes suck in material from their dusty rings, the material lights up brilliantly, making quasars the brightest objects in the universe. This bright light comes in many forms, including X-rays, visible and infrared light.

Astronomers have puzzled for years over the question of how many of these cosmic behemoths are out there. One standard method for estimating the number is to measure the cosmic X-ray background. Quasars outshine everything else in the universe in X-rays. By counting the background buzz of X-rays, it is possible to predict the approximate total number of quasars.

But this estimate has not matched previous X-ray and visible-light observations of actual quasars, which number far fewer than expected. Astronomers thought this might be because most quasars are blocked from our view by gas and dust. They proposed that some quasars are positioned in such a way that their dusty rings hide their light, while others are buried in dust-drenched galaxies.

Spitzer appears to have found both types of missing quasars by looking in infrared light. Unlike X-rays and visible light, infrared light can travel through gas and dust.

Researchers found 21 examples of these quasars in a small patch of sky. All the objects were confirmed as quasars by the National Radio Astronomy Observatory’s Very Large Array radio telescope in New Mexico and by the Particle Physics and Astronomy Research Council’s William Herschel Telescope in Spain.

“If you extrapolate our 21 quasars out to the rest of the sky, you get a whole lot of quasars,” said Dr. Mark Lacy of the Spitzer Science Center, California Institute of Technology, Pasadena, Calif., a co-author of the Nature paper. “This means that, as suspected, most super-massive black hole growth is hidden by dust.”

The discovery will allow astronomers to put together a more complete picture of how and where quasars form in our universe. Of the 21 quasars uncovered by Spitzer, 10 are believed to be inside fairly mature, giant, elliptical galaxies. The rest are thought to be encased in thick, dusty galaxies that are still forming stars.

A team of researchers based at the University of Arizona, Tucson, found similar quasars using Spitzer. Their research is described at http://uanews.org/science.

Other authors of the Nature paper include Drs. Steve Rawlings and Matt Jarvis, University of Oxford; Drs. Dario Fadda and Francine Marleau, Spitzer Science Center; Dr. Chris Simpson, University of Durham, England; and Dr. Chris Willott, National Research Council Canada, Victoria.

The Jet Propulsion Laboratory, Pasadena, Calif., a division of Caltech, manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Spitzer’s multiband imaging photometer, which observed the quasars, was built by Ball Aerospace Corporation, Boulder, Colo.; the University of Arizona; and Boeing North America, Canoga Park, Calif. Spitzer’s infrared array camera, which also observed the quasars, was built by NASA Goddard Space Flight Center, Greenbelt, Md.

A Spitzer false-colored picture of one of the newfound quasars is available at http://www.spitzer.caltech.edu/Media/index.shtml.

For information about NASA and agency programs visit http://www.nasa.gov/home/.

Original Source: NASA News Release

Posted on by Fraser Cain

Podcast: Planetary Disk That Refuses to Grow Up

With new instruments, astronomers are filling in all the pieces that help to explain how planets form out of extended disks of gas and dust around newborn stars. This process seems to happen quickly, often just a few million years is all it takes to go from dust to planets. But astronomers have found one proto-planetary disk that refuses to grow up. It’s 25 million years old, and still hasn’t made the transition to form planets. Lee Hartmann is with the Harvard-Smithsonian Center for Astrophysics, and the lead author on the paper announcing the find.
Continue reading “Podcast: Planetary Disk That Refuses to Grow Up”

Posted on by Fraser Cain

Audio: Planetary Disk That Refuses to Grow Up

Artist’s conception of the 25-million-year-old protoplanetary disk. Credit: David A, Aguilar (CfA). Click to enlarge
Listen to the interview: Planetary Disk That Refuses to Grow Up (6 MB)

Or subscribe to the Podcast: universetoday.com/audio.xml

Fraser Cain: You’ve found the oldest planetary disk. Can you give me a sense of how unusual this is?

Lee Hartmann: This is about the oldest planetary or protoplanetary disk. The oldest one we’ve found before was something like 10 million years old, so this is about 2 to 2.5 times as old as anything we’ve found before.

Fraser: Was that a big surprise to find something that old?

Hartmann: Yeah, it seems like half or more of stars have some kind of extended dusty disk with something that would make planets. At an age of about a million years or so. And then by 10 million years or so, you’re down to like 10% of all stars or maybe even less than that. So to find this thing at twice the age was really pretty remarkable. We thought that by 20 million years we’d really be down to zero for anything that still had dust around it that was very much like a planetary disk.

Fraser: What could keep the disk stable for so long?

Hartmann: It’s not really clear. The central system in this case is actually a close binary star and so it’s possible – unlike a single star in our solar system – there are two, almost equal mass stars that are orbiting around in a very close orbit and although something the size of somewhere between Mercury’s orbit and Venus’ orbit; something that size. That could be kind of churning things up because each star has its own gravity, and as they move around they could be churning up the disk and agitating the particles. What we think happens to make planets is that the dust, the little dust bunnies, kind of stick electrostatically into small little lumps and then it grows bigger and bigger. And it makes rocks, and then it makes things that are more like asteroids, and finally planets. And the planet forming stage is what really clears out all this dust. And so that process is thought to be very delicate and things kind of settle down over timescales of thousands to millions of years. It’s possible that if you’re churning it up a little bit, keeping the particle suspended then they don’t really stick together that well and don’t go through the rest of the planetary formation process like most other stars do.

Fraser: How common would something like this be? Since this is the oldest one that’s been found, do you think that there are others nearby, or is this just a total fluke?

Hartmann: It’s hard to imagine that there’s only one of these things in the galaxy, let alone the entire Universe. But, this must be a very rare occurrence as far as we can tell. We can see large clusters of stars that are 30 million years old, 50 million years old, 100 million years old, and they haven’t found anything like this in several hundreds or even thousands of stars in total. It’s probably 1 in 1000, maybe, or something like that. That’s sort of what I would guess, but it’s hard to know. We haven’t looked carefully enough at these things. We haven’t been able to until very recently. The Spitzer space telescope has just so much more sensitivity than anything else we were able to do before. It’s just made factors of hundreds of thousands of times our ability to detect faint sources like this thing is. We’re just taking the first baby steps to explore what’s out there and in our own neighbourhood. With the Spitzer telescope, they start looking at some of these other clusters, they’re confirming that twice the age of this system, less than 1 in 1000 is like that. It’s really a fairly unique system. We must have caught it in some special circumstances.

Fraser: Do you think that it could go on for millions and millions of years more. Is this still an early age for it?

Hartmann: This is something that we don’t understand very well. And one of the reasons to study these kinds of systems is that we really need a lot of help in understanding the physics of this. The physics of how planets form out of basically dust bunnies to start with. It’s just such a complicated process, and there are all kinds of things that we don’t quite understand that we really need to have more surveys of these things. I don’t really know what’s going to happen with this system. My own opinion is that it’s probably not going to go on and coagulate into planets if it hasn’t done it already. The theory suggests that there’s kind of a threshold that you have to meet. You have to have just enough stuff to make it happen, to really get over the hump of making larger bodies which can then sweep up all the smaller dust and clear out the disk. If you don’t ever get to that threshold, you might not ever make any planets. My guess is that it might just peter out, and some of the dust grains will either get blown out or spiral in slowly into the star and that’s the end of it, but we don’t really understand.

Fraser: Have planet forming disks been seen around binary systems before?

Hartmann: Yes, if I can just qualify to say that we’re assuming these disks make planets. We haven’t really had the complete smoking gun to say that these dusty disks actually make planets. I think it’s a very strong likelihood because we see all this distributed dust around very young stars and then it’s all gone. We know that we have to coagulate all the dust and get the small stuff and put it into big things to make planets. So that’s the assumption we’re making, but I just wanted to say that we haven’t actually connected the dots on that issue.

Fraser: Right, so have disks been seen around binary systems like this?

Hartmann: Yes, they have. This issue is that basically, you can’t have the disk at the same size orbit as the binary orbit. The other star will just swallow up all the dust, or evaporate it, or blow it away. On the other hand, if you have a very wide binary, if you have something where the other star is very far way, you can have a disk well inside that binary and it doesn’t know there’s another star orbiting around. We orbit around the Sun, and Jupiter is out there at several astronomical units, and that only makes small perturbations on the orbit of the Earth. Similarly, you could have a system in which the two stars are relatively close together and the disk is well outside the outlying area. And so, to that disk, it almost looks like there’s a single star. It’s not exactly like that because the two stars are orbiting around so the gravity is churning it up a little. But it’s not that far away from just having a single object. So as long as the disk is either a lot bigger than the binary, or smaller than the binary, you’re okay. If the disk is a lot bigger than the binary, though, it can be so tenuous, and so spread out that it never really coagulates effectively into planets. That’s something we would kind of predict, but that’s not something that we’re able to demonstrate observationally yet.

Fraser: Do you have some follow on observations planned for this?

Hartmann: What I think we would like to try and do is to get longer wavelength observations to see where the disk ends, because in this set of observations, we’re basically saying that there is a disk, but we don’t know how big it is. The question is, is there anything outside this system that could be perturbing the disk as well. It might even be a triple system for all we know, with a very much wider companion that is low mass and we haven’t seen. And that could really be churning it up and preventing the disk from letting planets coagulate, at least. And then the other thing that we’re trying to do, is that we’re trying to identify other systems like this which are also 20 million years old, 30 million years old. If we can find any more of these things, just to see how common they are, and whether they’re all binaries, or what’s special about them that enables them to last so long. Basically, what we’re trying to do is see the process how a disk turns into planets, but of course that takes millions of years, so you can’t follow that through – at least, I can’t follow it through. It’s like taking a snapshot of a population. You’ve got old people and young people and babies and so on. And you try and infer how the evolution goes from putting the various pieces together. And then some people are largish, or better nourished, and they have a different culture or whatever, and you try to see what different effects have on the population from that snapshot. To try and find other systems that are like this is a way of doing the experiment to see what happens if you have a much wider binary, or what happens if it’s a different mass star in the middle. We can’t really do the experiment, but if we find enough different kinds of objects like this, then nature has done the experiment in different places, and we just need to go out and look at it.

This discovery was originally announced on Universe Today on July 19, 2005.

Posted on by Fraser Cain

Astronauts Prepare for Spacewalk to Remove Gap Filler

Photographed from ISS while docked with Discovery. Image credit: NASA Click to enlarge
The Space Shuttle Discovery crew begins their ninth day in space with preparations for the third spacewalk of the mission. This extravehicular activity (EVA) was a preplanned activity for the mission, but now includes a new task — repair of two protruding gap fillers between tiles on the bottom the Shuttle.

The crew began the day waking up at 10:09 p.m. CDT to “Where My Heart Will Take Me,” the theme song from Star Trek: Enterprise. The song, composed by Dennis McCarthy, was selected for the crew as a surprise dedication from the Deputy Shuttle Program Manager Wayne Hale. The International Space Station Expedition 11 crew of Sergei Krikalev and John Phillips woke 30 minutes later.

Mission Specialists Steve Robinson and Soichi Noguchi are scheduled to begin their third spacewalk at 3:14 a.m. CDT as they exit out of the Space Shuttle airlock. The two will be assisted by Andy Thomas, serving as the intravehicular officer overseeing the spacewalk from inside, as well as Pilot Jim Kelly and Mission Specialists Wendy Lawrence and Charlie Camarda who will be supporting various robotic arm activities throughout the day.

The spacewalk is scheduled to last about 7 hours. The first task entails Kelly and Lawrence maneuvering the External Stowage Platform-2 (ESP-2), via the Station’s robotic arm, which they pulled from Discovery’s payload bay earlier today, onto the Station. As the ESP-2 reaches its final position, Robinson and Noguchi will guide the structure and secure it into place. With that task complete, Lawrence and Kelly will conduct a “walk off” maneuver of the Station robotic arm, by attaching the “free” end to the Mobile Base System and releasing the other end from the Destiny Laboratory module to where it will be needed as a platform for Robinson later in the EVA.

The two spacewalkers will move on to individual tasks, with Noguchi installing the Materials International Space Station Experiment-5 (MISSE-5), a materials experiment that will study the degradation of solar cell samples in the space environment. He’ll then remove the Rotary Joint Motor Controller from the Space Station truss before proceeding to a support position to assist Robinson in his final tasks.

Meanwhile, Kelly will work with Camarda, using the Orbiter Boom Sensor System to inspect repair demonstration tiles inside the Shuttle’s payload bay. Later, Camarda will also work with Krikalev and Phillips to continue stowing supplies and equipment inside Discovery and the Station. Discovery Commander Eileen Collins will monitor and supervise all the activities.

Robinson, now attached to the Station robotic arm, will attempt to repair two tile gap filler protrusions located on the underside of Discovery. He will first try to gently pull out the protruding material, and if need be, remove by trimming with a hacksaw.

Gap fillers are used in areas to restrict the flow of hot gas into the gaps between Thermal Protection System components. They consist of a layer of coated Nextel fabric and are normally about 0.020-inch thick. These protrusions were identified from photos taken during the rendezvous pitch maneuver conducted on flight day three, as Discovery approached the orbiting Space Station.

The crews are scheduled to go to sleep about 2:09 p.m. CDT.

Original Source: NASA News Release

Posted on by Fraser Cain

Discovery’s Leading Wing Edge is Safe

Astronaut Soichi Noguchi. Image credit: NASA Click to enlarge
Space Shuttle mission managers Tuesday cleared Discovery?s wing leading edge heat shield for re-entry as they methodically deal with concerns over the protruding tile gap fillers. The mission management team also discussed a ?puffed out? insulating blanket outside the commander?s cockpit window and has decided it poses no risk of overheating during entry. Engineers will continue to analyze whether it could pose a debris problem if it came loose during aerodynamic flight.

Discovery?s astronauts worked much of today on preparations for Wednesday’s gap filler repair spacewalk. Transfer of materials to and from the International Space Station continued with crewmembers of both spacecraft making good progress.

Spacewalkers Soichi Noguchi and Steve Robinson spent an hour this morning beginning about 2:40 a.m. CDT with Mission Specialists Andy Thomas and Wendy Lawrence, and Pilot Jim Kelly on a review of spacewalk procedures. Thomas, as the intravehicular crewmember, will coach and monitor the spacewalkers, while Lawrence and Kelly will operate the Station’s Canadarm2.

That robotic arm will carry Robinson to the repair sites on the underside of the forward part of Discovery where he will either gently pull out the protruding gap fillers with his hand or with forceps, or remove the protrusions with a hacksaw.

After the procedure review, Lawrence and Kelly spent the subsequent 45 minutes in computer training for the arm tasks, using the Dynamic Onboard Ubiquitous Graphics program, or DOUG. Meanwhile, the spacewalkers and Thomas worked on assembly of the hacksaw that would be used if other methods do not work.

About 7:40 a.m. Lawrence and Kelly, using Canadarm2, unberthed External Stowage Platform 2 from Discovery’s cargo bay. Noguchi and Robinson installed the platform’s attachment device on the mission’s first spacewalk on Saturday, and the platform itself is to be installed on the attachment device during Wednesday’s spacewalk.

After lunch on board, Noguchi, Robinson and Thomas worked on spacewalk tool configuration. Near the end of their work day, all nine crewmembers on board, including Discovery Commander Eileen Collins and Station crewmembers, Commander Sergei Krikalev and NASA Science Officer John Phillips, did a spacewalk review.

The spacewalkers began a prebreathe of pure oxygen about 10:50 a.m., a little more than an hour before hatches linking Discovery and the Station were closed so the Shuttle could be depressurized to 10.2 psi. Both the prebreathe and the depressurization were aimed at reducing the nitrogen content of the spacewalkers’ blood to reduce the possibility of nitrogen bubble formation in their bloodstreams during the spacewalk. Wednesday?s spacewalk is scheduled to begin at about 3:14 a.m. CDT.

Late in the crew day Tuesday, astronauts received a phone call from President George Bush. The President thanked the crew for taking risks for the sake of exploration and wished them well in the remainder of their mission.

Original Source: NASA News Release

Posted on by Fraser Cain

Messenger Swoops Past the Earth

Earth taken by MESSENGER on July, 30. Image credit: NASA Click to enlarge
NASA’s MESSENGER spacecraft, headed toward the first study of Mercury from orbit, swung by Earth today for a gravity assist that propelled it deeper into the inner solar system.

Mission operators at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md, said MESSENGER’s systems performed flawlessly. The spacecraft swooped around Earth, coming to a closest approach point of approximately 1,458 miles (2,347 kilometers) over central Mongolia at 3:13 p.m. EDT.

The spacecraft used the tug of Earth’s gravity to significantly change its trajectory. Its average orbit distance is nearly 18 million miles closer to the sun. The maneuver sent it toward Venus for another gravity-assist flyby next year.

Launched Aug. 3, 2004, from Cape Canaveral Air Force Station, Fla., the solar-powered spacecraft is approximately 581 million miles (930 million kilometers) into a 4.9 billion mile (7.9 billion kilometer) voyage that includes 14 more loops around the sun. MESSENGER will fly past Venus twice and Mercury three times before moving into orbit.

The Venus flybys in October 2006 and June 2007 will use the planet’s gravity to guide MESSENGER toward Mercury’s orbit. The Mercury flybys in January 2008, October 2008 and September 2009 will help MESSENGER match the planet’s speed. These events will set up the maneuver in March 2011 that starts a year-long science orbit around Mercury.

“This Earth flyby is the first of a number of critical mission milestones during MESSENGER’s circuitous journey toward Mercury orbit insertion,” said Sean C. Solomon, the mission’s principal investigator from the Carnegie Institution of Washington. “Not only did it help the spacecraft sharpen its aim toward our next maneuver, it presented a special opportunity to calibrate several of our science instruments.”

MESSENGER’s main camera snapped several approach shots of Earth and the moon during the past week. Today the camera is taking a series of color images, beginning with South America and continuing for one full Earth rotation. Science team members will string the images into a video documenting MESSENGER’s departure.

On Earth approach, the craft’s atmospheric and surface composition spectrometer made several scans of the moon in conjunction with the camera observations. In addition, the particle and magnetic field instruments spent several hours measuring Earth’s magnetosphere. The science team will download the data and images through NASA’s Deep Space Network over the next several weeks, continuing assessment of the instruments’ performance.

MESSENGER will conduct the first orbital study of Mercury, the least explored of the terrestrial planets that include Venus, Earth and Mars. During one Earth year (four Mercury years), MESSENGER will provide the first images of the entire planet. It will collect detailed information about the composition and structure of Mercury’s crust, its geologic history, nature of its atmosphere and magnetosphere, makeup of its core and polar materials.

MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, is the seventh mission in NASA’s Discovery Program of lower-cost scientifically focused exploration projects. APL designed, built and operates the spacecraft and manages the mission for NASA’s Science Mission Directorate.

For information about the spacecraft and mission on the Web, visit: http://messenger.jhuapl.edu

Original Source: NASA News Release

Posted on by Fraser Cain

Bright Splat on Rhea

Saturn’s moon Rhea. Image credit: NASA/JPL/SSI. Click to enlarge
This view of Saturn’s moon Rhea shows the tremendous bright splat that coats much of the moon’s leading hemisphere. The bright feature may be impact-related and is visible in other Cassini images of Rhea (see Diversity of Impacts). Rhea is 1,528 kilometers (949 miles) across.

North on Rhea is up in this view.

The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on June 25, 2005, at a distance of approximately 1.1 million kilometers (700,000 miles) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of less than one degree. Resolution in the original image was 7 kilometers (4 miles) per pixel. The image has been contrast-enhanced and magnified by a factor of two to aid visibility.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Mark Mortimer

Book Review: The Star Guide: Learn How to Read the Night Sky Star by Star

Astronomy is such a wonderful field for the observer. The scene regularly changes and surprises continually pop up. Sharing the wonder with anyone, anywhere else, should be a simple task of saying, ‘Look it’s right there overhead’. However, Kerrod knows better than to drop a coordinate frame into a book and say you’re on your own. First he pummels your senses with eye candy. Plenty of shots from the Hubble space telescope draw you in like a nearby black hole. Chaffing with desire, you continue flipping pages. The Arecibo, Parkes and Kitt Peak sites temp you with dreams of playing with the toys of the big guys. All the while shots of exploding galaxies, planetary nebulae and writhing dark clouds tease you all the more. Once primed with this, Kerrod blasts you along the learning curve for locating the stars.

And learning, with this book as an aid, is as practicable as it is enjoyable. The constellations arrive as the appertif. Their images spread all over maps. Blue circles, with apparently random yellow dots and white lines, place each of the 88 quixotic shapes. Blow-up layouts of the well known ones serve as sign posts to send you on to the next juicy morsel. Having got you salivating, Kerrod brings on the main course. On two page spreads, he dishes out the apparent skies for each month. To ready you for this meal, two half circles give the expected evening view. One portrays the southern exposure from the northern and the other portrays the northern exposure from the south. The ‘entree’ so to speak, maps a 30 degree wide by 100 degree long section of the sky at near reference time of near midnight. With these and additional choice pieces of eye candy , there’s no option but to jump in with utensils ablaze and assimilate all the information.

Just like with any good meal, this book wraps up with a delightfully light selection. Here, to relax the palate, are sunspot examples, the authors own pictures of a solar eclipse as well as maps for the near side of our moon. One simplified sketch for each lunar quadrant identifies all the key features. Then, just as a chef advising on future meals, Kerrod entices you with a final section together with lots more eye candy of the planets of the solar system. Not only do you get well satiated by this meal but there’s always that little bit more to keep you coming back.

Kerrod, with this book, really does a great job in bringing astronomy to the hobbyist. He concentrates on identifying stars and helping you up the learning curve of identification. There is very little on equipment, technique or style. He gives enough information to make the evening viewing fun without overtaxing anyone’s ability to comprehend. The eye candy is there as a practical lure but so is a caveat that reminds the viewer what they will expect to see. With the included planisphere (mine was for latitude 42deg North), learning the main stars and constellations, the learning curve will be more like a gentle slope.

But does it work? I gave it a try by bringing the book with me while visiting a friend at a cottage. Like moths to a lantern, they dived into the book. It didn’t take long for even the most staid to be curious and perusing the contents. Once the planisphere was discovered, we headed down to the shoreline. Sure enough, we identified a number of constellations and had a great time doing so. It does work.

With 88 constellations and an apparently infinite number of bright dots in the night sky, a learner can easily feel overwhelmed. There are many resources, including friends, clubs, web sites and books. Robin Kerrod with his book The Star Guide: Learn How to Read the Night Sky Star by Star adds an excellent reference. With the maps, spectacular photographs and simple yet helpful text, a reader won’t be overwhelmed for long.

Read more reviews, or purchase a copy online from Amazon.com.

Review by Mark Mortimer.

Posted on by Fraser Cain

Bend in the Rings

Saturn’s swirling clouds. Image credit: NASA/JPL/SSI. Click to enlarge
Believe it or not, this extreme close-up of Saturn’s swirling clouds was acquired from more than one million kilometers (621,370 miles) from the gas giant planet. The rings’ image is severely bent by atmospheric refraction as they pass behind the planet.
The dark region in the rings is the 4,800-kilometer-wide (2,980 mile) Cassini Division.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 25, 2005, at a distance of approximately 1 million kilometers (600,000 miles) from Saturn. The image scale is 6 kilometers (4 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Fraser Cain

Most Accurate Distance to NGC 300

Observed Fields in NGC 300. Image credit: ESO Click to enlarge
Cepheid pulsating stars have been used as distance indicators since the early discovery of Henrietta Leavitt almost a hundred years ago. From her photographic data regarding one of the Milky Way’s neighbour galaxies, the Small Magellanic Cloud, she found that the brightness of these stars closely correlate with their pulsation periods.

This period-luminosity relation, once calibrated, allows a precise distance determination of a galaxy once Cepheids have been discovered in it, and their periods and mean magnitudes have been measured.

While the Cepheid method doesn’t reach out far enough in the Universe to directly determine cosmological parameters like the Hubble constant, Cepheid distances to relatively nearby resolved galaxies have laid the foundation for such work in the past, as in the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Cepheids indeed constitute one of the first steps in the cosmic distance ladder.

The current main problem with the Cepheid method is that its dependence on a galaxy’s metallicity, that is, its content in elements more heavy than hydrogen and helium, has never been measured accurately so far. Another intriguing difficulty with the method is the fact that the total absorption of the Cepheid’s light on its way to Earth, and in particular the amount of absorption within the Cepheid’s host galaxy, must be precisely established to avoid significant errors in the distance determination.

To tackle this problem, Wolfgang Gieren (University of Concepcion, Chile) and his team devised a Large Programme at ESO: the Araucaria Project. Its aim is to obtain distances to relatively nearby galaxies with a precision better than 5 percent.

One of the key galaxies of the team’s Araucaria Project is the beautiful, near face-on galaxy NGC 300 in the Sculptor Group. In a wide-field imaging survey carried out at the ESO/MPG 2.2-m telescope on La Silla in 1999-2000, the team had discovered more than a hundred Cepheid variables spanning a broad range in pulsation period. Pictures of the galaxy, and some of its Cepheids from these data were released in ESO Press Photos 18a-h in 2002. Last year, the team presented the distance of NGC 300 as derived from these optical images in V- and I-bands.

The team complemented this unique dataset with new data taken with the ISAAC near-infrared camera and spectrometer on ESO’s 8.2-m VLT Antu telescope.

“There are three substantial advantages in the Cepheid distance work when images obtained through near-infrared passbands are used instead of optical data”, says Wolfgang Gieren. The most important gain is the fact that the absorption of starlight in the near-infrared, and particularly in the K-band, is dramatically reduced as compared to the effect interstellar matter has at visible wavelengths. A second advantage is that Cepheid light curves in the infrared have smaller amplitudes and are much more symmetrical than their optical counterparts, making it possible to measure a Cepheid’s mean K-band brightness just from a very few, and in principle from just one observation at known pulsation phase. In contrast, optical work requires the observation of full light curves to determine accurate mean magnitudes. The third basic advantage in the infrared is a reduced sensitivity of the period-luminosity relation to metallicity, and to blending with other stars in the crowded fields of a distant galaxy.

Taking this into account, one of the main purposes of the team’s Large Programme has been to conduct near-infrared follow-up observations of Cepheids in their project’s target galaxies which have previously been discovered in optical wide-field surveys.

Deep images in the J and K bands of three fields in NGC 300 containing 16 Cepheids were taken with VLT/ISAAC in 2003.

“The high quality of the data allowed a very accurate measurement of the mean J- and K- magnitudes of the Cepheids from just 2 observations of each star obtained at different times”, says Grzegorz Pietrzynski, another member of the team, also from Concepcion.

Using these remarkable data the period-luminosity relations were constructed. “They are the most accurate infrared PL relations ever obtained for a Cepheid sample in a galaxy beyond the Magellanic Clouds”, emphasizes Wolfgang Gieren.

The total absorption of light (“reddening”) of the Cepheids in NGC 300 was obtained by combining the values for the distance of the galaxy obtained in the various optical and near-infrared bands in which NGC 300 was observed. This led to the discovery that there is a very significant contribution to the total reddening from absorption intrinsic to NGC 300. This intrinsic absorption has an important effect on the determination of the distance but had not been taken into account previously.

The team was able to measure the distance to NGC 300 with the unprecedented total uncertainty of only about 3 percent. The astronomers found that NGC 300 is located 6.13 million light-years away.

Original Source: ESO News Release