Globular
Clusters Images (Anglo-Australian
Observatory)
Space Telescope Science
Institute
Interferometry on Stars
The Messier Page
PASADENA-The cluster of stars known as the Pleiades is one of the most recognizable objects in the night sky, and for millennia has been celebrated in literature and legend. Now, a group of astronomers has obtained a highly accurate distance to one of the stars of the Pleiades known since antiquity as Atlas. The new results will be useful in the longstanding effort to improve the cosmic distance scale, as well as to research the stellar life-cycle.
In the January 22 issue of the journal Nature, astronomers from the California Institute of Technology and the Jet Propulsion Laboratory report the best-ever distance to the double-star Atlas. The star, along with "wife" Pleione and their daughters, the "seven sisters," are the principal stars of the Pleiades that are visual to the unaided eye, although there are actually thousands of stars in the cluster. Atlas, according to the team's decade of careful interferometric measurements, is somewhere between 434 and 446 light-years from Earth.
The range of distance to the Pleiades cluster may seem somewhat imprecise, but in fact is accurate by astronomical standards. The traditional method of measuring distance is by noting the precise position of a star and then measuring its slight change in position when Earth itself has moved to the other side of the sun. This approach can also be used to find distance on Earth. If you carefully record the position of a tree an unknown distance away, move a specific distance to your side, and measure how far the tree has apparently "moved," it's possible to calculate the actual distance to the tree by using trigonometry.
However, this procedure gives only a rough estimate to the distance of even the nearest stars, due to the gigantic distances involved and the subtle changes in stellar position that must be measured.
Further, the team's new measurement settles a controversy that arose when the European satellite Hipparcos provided a distance measurement to the Pleiades so much nearer the distance than assumed that the findings contradicted theoretical models of the life cycles of stars.
This contradiction was due to the physical laws of luminosity and its relationship to distance. A 100-watt light bulb one mile away looks exactly as bright as a 25-watt light bulb half a mile away. So to figure out the wattage of a distant light bulb, we have to know how far away it is. Similarly, to figure out the "wattage" (luminosity) of observed stars, we have to measure how far away they are. Theoretical models of the internal structure and nuclear reactions of stars of known mass also predict their luminosities. So the theory and measurements can be compared.
However, the Hipparcos data provided a distance lower than that assumed from the theoretical models, thereby suggesting either that the Hipparcos distance measurements themselves were off, or else that there was something wrong with the models of the life cycles of stars. The new results show that the Hipparcos data was in error, and that the models of stellar evolution are indeed sound.
The new results come from careful observation of the orbit of Atlas and its companion--a binary relationship that wasn't conclusively demonstrated until 1974 and certainly was unknown to ancient watchers of the sky. Using data from the Mt. Wilson stellar interferometer (located next to the historic Mt. Wilson Observatory in the San Gabriel range) and the Palomar Testbed Interferometer at Caltech's Palomar Observatory in San Diego County, the team determined a precise orbit of the binary.
Interferometry is an advanced technique that allows, among other things, for the "splitting" of two bodies that are so far away that they normally appear as a single blur, even in the biggest telescopes. Knowing the orbital period and combining it with orbital mechanics allowed the team to infer the distance between the two bodies, and with this information, to calculate the distance of the binary to Earth.
"For many months I had a hard time believing our distance estimate was 10 percent larger than that published by the Hipparcos team," said the lead author, Xiao Pei Pan of JPL. "Finally, after intensive rechecking, I became confident of our result."
Coauthor Shrinivas Kulkarni, MacArthur Professor of Astronomy and Planetary Science at Caltech, said, "Our distance estimate shows that all is well in the heavens. Stellar models used by astronomers are vindicated by our value."
"Interferometry is a young technique in astronomy and our result paves the way for wonderful returns from the Keck Interferometer and the anticipated Space Interferometry Mission that is expected to be launched in 2009," said coauthor Michael Shao of JPL. Shao is also the principal scientist for the Keck Interferometer and the Space Interferometry Mission.
The Palomar Testbed Interferometer was designed and built by a team of researchers from JPL led by Shao and JPL engineer Mark Colavita. Funded by NASA, the interferometer is located at the Palomar Observatory near the historic 200-inch Hale Telescope.
The device served as an engineering testbed for the interferometer that now links the 10-meter Keck Telescopes atop Mauna Kea in Hawaii.The local celestial neighborhood just got more crowded with a discovery of a star that may be the third closest to the Sun. The star, "SO25300.5+165258," is a faint red dwarf star estimated to be about 7.8 light-years from Earth in the direction of the constellation Aries.
"Our new stellar neighbor is a pleasant surprise, since we weren't looking for it," said Dr. Bonnard Teegarden, an astrophysicist at NASA's Goddard Space Flight Center, Greenbelt, Md. Teegarden is lead author of a paper announcing the discovery to be published by the Astrophysical Journal. This work has been done in close collaboration with Dr. Steven Pravdo of NASA's Jet Propulsion Laboratory (JPL).
If its distance estimate is confirmed, the newfound star will be the Sun's third-closest stellar neighbor, slightly farther than the Alpha Centauri system, actually a group of three stars a bit more than four light-years away, and Barnard's star, about six light-years away. One light-year is almost six trillion miles, or nearly 9.5 trillion kilometers.
The new star has only about seven percent of the mass of the Sun, and it is 300,000 times fainter. The star's feeble glow is the reason why it has not been seen until now, despite being relatively close.
"We discovered this star in September 2002 while searching for white dwarf stars in an unrelated program," said Teegarden. The team was looking for white dwarf stars that move rapidly across the sky. Celestial objects with apparent rapid motion are called High Proper Motion (HPM) objects. A HPM object can be discovered in successive images of an area of sky because it noticeably shifts its position while its surroundings remain fixed. Since either a distant star moving quickly or a nearby star moving slower can exhibit the same HPM, astronomers must use other measurements to determine its distance from Earth.
During its star search, the team used the SkyMorph database for the Near Earth Asteroid Tracking (NEAT) program. NEAT is a NASA program, run by the Jet Propulsion Laboratory (JPL), Pasadena, Calif., to search for asteroids that might be on a collision course for Earth. SkyMorph was separately supported by NASA's Applied Information Systems Research Program. Like HPM stars, asteroids reveal themselves when they shift their position against background stars in successive images. Automated telescopes scan the sky, accumulating thousands of images for the NEAT program, which have been incorporated into SkyMorph, a web-accessible database, for use in other types of astronomical research.
Once the star revealed itself in the NEAT images, the team found other images of the same patch of sky to establish a rough distance estimate by a technique called trigonometric parallax. This technique is used to calculate distances to relatively close stars. As the Earth progresses in its orbit around the Sun, the position of a nearby star will appear to shift compared to background stars much farther away -- the larger the shift, the closer the star.
The team refined their initial distance estimate with another technique called photometric parallax. They used the 3.5-meter Astrophysical Research Consortium telescope at the Apache Point Observatory, Sunspot, N.M., to observe the star and separate its light into its component colors for analysis. This allowed the team to determine what kind of star it is. The analysis indicates it's similar to a red dwarf star (spectral type M6.5) that's shining by fusing hydrogen atoms in its core, like our Sun (called a main sequence star).
Once the type of star is known, its true brightness, called intrinsic luminosity, can be determined. Since all light-emitting objects appear dimmer as distance from them increases, the team compared how bright the new star appeared in their images to its intrinsic luminosity to improve their distance estimate.
Although the star resembles a M6.5 red dwarf, it actually appears three times dimmer than expected for this kind of star at the initial distance estimate of 7.8 light-years. The star could therefore really be farther than the rough trigonometric distance indicates; or, if the initial estimate holds, it could have unusual properties that make it shine less brightly than typical M6.5 red dwarfs. A more precise measurement of the new star's position to establish an improved trigonometric parallax distance is underway at the U.S. Naval Observatory. This will confirm or refute its status as one of our closest neighbors by late this year. Either way, we might get even more company soon: "Since the NEAT survey only covered a band of the sky (+/- 25 degrees in declination), it is entirely possible that other faint nearby objects remain to be discovered," said Teegarden.
The team includes B. J. Teegarden, T. McGlynn (NASA/Goddard); S. H. Pravdo, M. Hicks, S. B. Shaklan (NASA/JPL); K. Covey, O. Fraser, S. H. Hawley (U. of Wash.); and I. N. Reid (Space Telescope Science Institute).
For an image and more information, refer to:
http://www.gsfc.nasa.gov/topstory/2003/0520newstar.html
The German satellite DIVA's launch has been postponed to 2007 (Nature, 417, p. 6, 2 May 2002). The delay may wind up bringing it too close to an even better mission, GAIA, the European Space Agency's astrometry mission now scheduled for some time between 2010 and 2014 for launch.
Particle Physics and Astronomy Council, UK, Press Release, 22 April 2002
The Sun emits electron-neutrinos, elementary particles of matter that have no electric charge and very little mass, created in vast numbers by the thermonuclear reactions that fuel our parent star. Since the early 1970s, several experiments have detected neutrinos arriving on Earth, but they have found only a fraction of the number expected from detailed theories of energy production in the Sun. This meant there was either something wrong with our theories of the Sun, or our understanding of neutrinos. It turns out that our theories of how the Sun is powered look like being correct according to a team of scientists from the UK, the US and Canada whose latest results from research into solar neutrinos were announced on Saturday [20 April 2002]. What's more, these ghostly particles have 'chameleon' type capabilities, changing from one type of neutrino into another on their journey from the Sun to Earth.
The scientists used data taken entirely from the Sudbury Neutrino Observatory [SNO] in Canada which shows without doubt that the number of observed solar neutrinos is only a fraction of the total emitted from the Sun - clear evidence that they have chameleon type properties and change type en-route to Earth.
Says Project Director Art McDonald of Queen's University, Canada, "These new results show in a clear, simple and accurate way that solar neutrinos change their type. The total number of neutrinos we observe is in excellent agreement with calculations of the nuclear reactions powering the Sun. The SNO team is really excited because these measurements enable neutrino properties to be defined with much greater certainty in fundamental theories of elementary particles."
Neutrinos are known to exist in three types related to three different charged particles - the electron, and its lesser known relatives the muon and the tau. The Sun emits electron neutrinos, which are created in the thermonuclear reactions in the solar core. Previous experiments have found fewer electron neutrinos than suggested by calculations based on how the Sun burns - the famous "solar neutrino problem".
The results announced on Saturday at the Joint American Physical Society/American Astronomical Society meetings in Albuquerque, New Mexico, show that the number of electron-neutrinos detected is about 1/3 of the number expected according to calculations based on the latest sophisticated models of the solar core. The SNO detector uses the unique properties of heavy water - where the hydrogen has an extra neutron in its nucleus - to detect not only electron neutrinos through one type of reaction, but also all three known neutrino types through a different reaction. The total number of all three types of neutrino agrees well with the calculations. This shows unambiguously that electron neutrinos emitted by the Sun have changed to muon or tau neutrinos before they reach Earth.
Dr. Andre Hamer, of Los Alamos National Laboratory, said, "In order to make these measurements we had to restrict the radioactivity in the detector to minute levels and determine both neutrino signals and the detector background very accurately - to show clearly that we are observing neutrinos from the Sun. The care taken throughout this experiment to minimise radioactivity, and the careful calibration and analysis of our data, has enabled us to make these neutrino measurements with great accuracy"
In June last year results from the detection of electron neutrinos at SNO first indicated, with a certainty of 99.9%, that neutrinos change type on their way from the Sun, thus solving the long-standing problem - or so it was thought. However, these conclusions were based on comparisons of the SNO results with those from a different experiment, the Super-Kamiokande detector, located in Japan.
Prof. Dave Wark of the University of Sussex and the Rutherford Appleton Laboratory, Oxford, commented, " Whenever a scientific conclusion relies on two experiments, and on the theory connecting them, it is twice as hard to be certain that you understand what is going on. We are therefore much more certain now that we have really shown that solar neutrinos change type".
The latest results, entirely from the SNO detector, (and which have been submitted to Physical Review Letters) are 99.999% accurate, and are of great importance because of the way in which physicists think that the neutrinos - long thought to be massless particles - change types only happens if the different types have different masses.
For further information
Additional information about the conference presentations, the SNO
laboratory, the neutrino measurements being made and the participating
institutions can be found at:
www.sno.phy.queensu.ca and from
http://www.sno.phy.queensu.ca/sno/results_04_02/.
A set of high resolution lab photos can be downloaded from the SNO web site as well.
Background Information
The Sudbury Neutrino Observatory is a unique neutrino telescope, the size of a ten-storey building 2 kilometers underground in INCO's Creighton Mine near Sudbury Ontario planned, constructed and operated by a 100 member team of scientists from Canada, the United States and the United Kingdom. Through its use of heavy water, the SNO detector provides unique ways to detect neutrinos from the sun and other astrophysical objects and measure their properties. For many years, the number of solar neutrinos measured by other underground detectors had been found to be smaller than expected from theories of energy generation in the sun. This had led scientists to infer that either the understanding of the Sun was incomplete, or that the neutrinos were changing from one type to another in transit from the core of the Sun. In results presented in June 2001, SNO scientists compared the number of electron-type neutrinos reaching the SNO detector to the number of neutrinos seen by a second reaction which includes contributions from the other two types of neutrinos, making use of additional data from the Super-Kamiokande detector in Japan. The observed difference in these two numbers showed conclusively that neutrinos change their type enroute to Earth, and arrive as a mixture of electron neutrinos and the other two types. The results to be announced this month are based on the SNO detector's ability, through a third type of neutrino reaction, to measure independently the total rate of all of the three known types of neutrinos. The new data provides independent and more accurate information on the neutrino changes and on the accuracy of models of the sun.
The SNO detector consists of 1000 tonnes of ultrapure heavy water enclosed in a 12 meter diameter acrylic plastic vessel, which in turn is surrounded by ultrapure ordinary water in a giant 22 meter diameter by 34 meter high cavity. Outside the acrylic vessel is a 17 meter diameter geodesic sphere containing 9600 light sensors or photomultiplier tubes, which detect tiny flashes of light emitted as neutrinos are stopped or scattered in the heavy water. The flashes are recorded and analyzed to extract information about the neutrinos causing them. At a detection rate of about one neutrino per hour, many days of operation are required to provide sufficient data for a complete analysis. The laboratory includes electronics and computer facilities, a control room, and water purification systems for both heavy and regular water.
The construction of the SNO Laboratory began in 1990 and was completed in 1998 at a cost of $80M CDN with support from the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, the Northern Ontario Heritage Foundation, Industry, Science and Technology Canada, INCO Limited, the United States Department of Energy, and the Particle Physics and Astronomy Research Council of the UK. The heavy water is on loan from Canada's federal agency AECL with the cooperation of Ontario Power Generation, and the unique underground location is provided through the cooperation and support of INCO Limited. Measurements at the SNO Laboratory began in 1999, and the detector has been in almost continuous operation since November 1999 when, after a period of calibration and testing, its operating parameters were set in their final configurations.
In June 2001, the second phase of measurements with the SNO detector was begun, in which ultra pure sodium chloride (salt) was added to the heavy water core of the detector, to enhance signals for some of SNO's neutrino reactions and add further to the accuracy of SNO's neutrino determinations. Plans call for the observatory to continue measurements until at least the end of 2005, with a third phase of measurements set to begin in 2003.
Further background information can be found on the SNO website:
www.sno.phy.queensu.ca
University of California at Santa Cruz Press Release, March 27, 2002
Astronomers searching for globular star clusters in a nearby galaxy have discovered an entirely new class of objects, unlike anything previously described. Much larger and fainter than typical globular clusters, the new objects were first detected in Hubble Space Telescope images of the lenticular galaxy NGC 1023. They may hold clues to how galaxies of this type formed.
The discovery of these "faint extended clusters" was made by Jean Brodie, professor of astronomy and astrophysics at the University of California, Santa Cruz, and postdoctoral researcher Soeren Larsen. The astronomers described their most recent observations of the objects--which they refer to informally as "faint fuzzies"--in a paper submitted to the Astronomical Journal. This latest work, based on observations at the W. M. Keck Observatory in Hawaii, confirmed and extended their initial report of the discovery, which was published in the Astronomical Journal in December 2000.
"It is no surprise that these objects had never been seen before, because they are very faint," Brodie said. "In all the data archives from the Hubble Space Telescope, there are only four galaxies for which we have good enough observations to be able to detect them."
Of those four galaxies, the researchers found evidence of faint extended clusters in two (NGC 1023 and NGC 3384), but ruled out their existence in the other two galaxies.
Astronomers have traditionally recognized two kinds of star clusters: open clusters, which contain young stars in relatively small numbers (a few dozen to thousands), and globular clusters, which typically contain hundreds of thousands of densely packed, very old stars. Globular clusters are thought to be the oldest radiant objects in the universe and are found in all types of galaxies, usually in large numbers. "We see globular clusters in every galaxy we look at," Larsen said.
The faint extended clusters seem to be about the same age as globular clusters, but they look and act very different. Whereas globular clusters are typically 15 to 20 light-years in diameter, faint fuzzies range from 50 to 100 light-years across. They are also extremely faint, while globular clusters are fairly bright objects. Another important difference is that the faint fuzzies are associated with the disk of their host galaxy, whereas most globular clusters are associated with the halo or spheroidal component, moving in random orbits around the host galaxy.
"The association with the disk is significant because it means that they probably formed in a very different way from globular clusters," Larsen said.
Based on the Hubble images, Brodie and Larsen determined the size and brightness of these unusual objects and their distribution in the galaxy. They also noted that the light from the objects has predominantly red colors, indicating that they contain relatively old stars.
To confirm these observations, and in particular to rule out the possibility that these were background objects and not part of the galaxy itself, the researchers needed to get spectra of the faint fuzzies. In a spectrum, light from an object is separated into its component wavelengths, revealing a wealth of information about its composition and motion. In December 2001, Brodie and Larsen spent two nights gathering spectra with the LRIS spectrograph on the Keck I Telescope in Hawaii.
"The spectra have confirmed everything we had speculated about these objects based on the Hubble data," Brodie said.
The Keck spectra showed that the faint fuzzies have about the same velocity as the host galaxy. In other words, they are moving away from us (due to the expansion of the universe) at the same speed as the galaxy, meaning they are not background objects but part of the galaxy itself. Furthermore, when the researchers plotted the velocities of the clusters as a function of their positions in the galaxy, they could see that the clusters are rotating around the center of the galaxy like the disk does.
The two galaxies in which the researchers have detected faint fuzzies are both lenticular galaxies. A lenticular galaxy has a disk component similar to that of a spiral galaxy, except that there are no spiral arms and most of the stars in the disk are old. In spiral galaxies, the arms of the disk are sites of very active star formation. In many respects, lenticular galaxies are more like elliptical galaxies, which are large, football-shaped galaxies with no disk and very old stars.
"At this point, we don't know how common these clusters are or if they occur exclusively in lenticular galaxies," Brodie said. "We have some speculation as to how they may have formed, but no explanation of why they are so large," she added.
NGC 1023 appears to be interacting with a nearby dwarf companion galaxy. At Keck, the researchers took spectra of some bright spots in the dwarf companion, which turned out to be clusters of very blue, very young stars. Without Hubble images of the companion, they can't tell how large these star clusters are, but presumably they are forming as a result of the gravitational interaction of the two galaxies.
"It may be that in the past, other dwarf galaxies have interacted with NGC 1023 and been drawn into the disk, giving rise to the faint extended clusters. Over time, the stars in the clusters would redden as they aged. That doesn't explain why the clusters are so big, but it is an interesting possibility," Brodie said.
Images can be downloaded from the web at
HST Image
LRIS Image
US Naval Observatory Press Release, 7 March 2002
Astronomers from the U.S. Naval Observatory (USNO), the Naval Research Laboratory (NRL), and Lowell Observatory announced today that they have successfully combined the light from six independent telescopes to form a single, high-resolution image of a distant multiple-star system. This is the first time that this has ever been accomplished in the optical region of the electromagnetic spectrum. The Navy Prototype Optical Interferometer (NPOI) at Lowell Observatory's Anderson Mesa site near Flagstaff, Arizona observed the triple star system Eta Virginis, located about 130 light-years away from Earth.
"This development makes it possible to 'synthesize' telescopes with apertures in excess of hundreds of meters," says Dr. Kenneth Johnston, Scientific Director of the Naval Observatory. "It will lead to the direct imaging of the surfaces of stars and of star spots, analogous to the sunspots on the Sun. This technology can also be applied to space systems for remote sensing of the Earth and other objects in the solar system, as well as stars and galaxies."
Optical interferometers combine the light from several independent telescopes to form a "synthetic" telescope whose ability to make a high-resolution image is proportional to the maximum separation of the telescopes. They are the answer to the prohibitive costs and immense technical difficulties of building extremely large, monolithic single-mirror telescopes. Since the rate at which a giant telescope aperture is synthesized with an interferometer array is equal to the number of combinations between any two telescopes of the array, the combination of the six NPOI telescopes has more than quadrupled NPOI's capability to collect data over its competitors.
USNO and NRL, in collaboration with Lowell Observatory and with funding from the Office of Naval Research and the Oceanographer of the Navy, joined forces in 1991 to build the instrument. Stellar observations have been conducted with a three-station array since its "first light" in 1996.
However, due to the technical difficulty associated with linking even a small number of separate telescopes, the high-resolution capabilities of optical interferometers have only been used to date on relatively simple stellar sources. Basic questions, such as a star's apparent diameter or the existence and motions of nearby stellar companions, are easily answered for such sources. However, to increase the spatial resolution and sensitivity to stellar structure, interferometers must link more telescopes together to provide an even sampling of the synthesized aperture. Three combined telescopes provide three measurements in the synthesized aperture, but six telescopes provide 15 combinations.
To merge the six beams, the NPOI team has designed a new type of hybrid beam combiner. In addition, new hardware and control systems have been developed to uniquely encode every possible telescope combination in the recorded data so that the information necessary for the alignment and superposition of the starlight wave-fronts and the image reconstruction may be properly decoded.
The field of interferometry is a rapidly developing one, with giants like the twin Keck 10-meter telescopes having achieved "first fringes" last year, and the European Southern Observatory's VLTI planning to combine the light from four 8-meter telescopes. More modest but versatile imaging interferometers like CHARA, COAST, and IOTA have also been operating for a few years, but NPOI is the first to combine light from a full array of six telescopes.
In the near future, NPOI will be commissioning all of the remaining stations onto which any of the six telescopes can be mounted for a maximum array size of 430 meters, the largest baseline of all current imaging interferometer projects.
Stellar astrophysics will be revolutionized by the capability to directly image stars other than the Sun. Ultimately, when employed in space with the experience collected from ground-based experiments, optical interferometry may develop the capability to image Jupiter-sized planets orbiting distant stars.
"Remember the early days of radio interferometry and look at the world- wide arrays we routinely use today," says Dr. Johnston. "We've gone from simple two-element arrays to continent-sized ones with 10 or more antennas that produce extremely fine-scale images of distant quasars. We are standing on the brink of achieving similar results for visible-light sources."
An online version of this release, with images of the Eta Virginis triple-star system and the NPOI facility at Anderson Mesa, may be found at http://www.usno.navy.mil/pao/press/NPOIRel020306.shtml.
See
http://www.macho.mcmaster.ca/Demos/Cepheids/WebPL.html
for the ability to click on any star on the equivalent of Figure 20-16
of the text in order to see the star's light curve.
The Macho Project's homepage is www.macho.mcmaster.ca
The American Association of Variable Star Observers (AAVSO) has a Web site that includes information on variable stars. Their multi-media educational project Hands-On Astrophysics is included.
