The URLs for the false color and grey scale images of Cygnus A (courtesy of Dr. Chris Carilli) are
http://pulsar.princeton.edu/~eddie/cyga.gif
and
http://www.nrao.edu/~gtaylor/cyga6cm.jpg respectively.
Lawrence Berkeley National Laboratory
January 8, 1998
Distant Exploding Stars Foretell Fate Of The Universe: It Will Expand Forever
BERKELEY, CA -- New studies of exploding stars in the farthest reaches of deep space indicate that the universe will expand forever, according to findings of the Supernova Cosmology Project, an international team of astrophysicists based at the Department of Energy's Lawrence Berkeley National Laboratory."Distant supernovae provide natural mile-marker which can be used to measure trends in the cosmic expansion," says Saul Perlmutter, leader of the project, who presented the team's research Jan. 8 at the annual meeting of the American Astronomical Society. "All the indications from our observations of supernovae spanning a large range of distances are that we live in a universe that will expand forever. Apparently there isn't enough mass in the universe for its gravity to slow the expansion, which started with the Big Bang, to a halt."
This result rests on analysis of 40 of the roughly 65 supernovae so far discovered by the Supernova Cosmology Project. Exploding stars known as supernovae are so intrinsically bright that their light is visible half-way across the observable universe. By the time the light of the most distant supernova explosions so far discovered by the team reached telescopes on earth, some seven billion years had passed since the stars exploded.
After such a journey the starlight is feeble, and its wavelength has been stretched by the expansion of the universe, a phenomenon known as redshift. By comparing the faint light of distant supernovae to that of bright nearby supernovae, researchers could tell how far the light had traveled.
Distances combined with redshifts of the supernovae give the rate of expansion of the universe over its history, allowing a determination of how much the expansion rate is slowing.
The deceleration measurement depends on the remarkable predictability of a kind of supernova called "Type Ia," whose explosions are triggered when a dying white dwarf star pulls too much gas off a neighboring red giant, igniting a thermonuclear explosion that rips the white dwarf apart.
"A Type Ia supernova can shine brighter than an entire galaxy, but only for a month or so before it becomes too faint for even the largest telescopes to observe at these distances," says Gerson Goldhaber, a Berkeley Lab researcher. Although not all Type Ia supernova have the same brightness, their intrinsic brightness can be determined by examining how quickly each supernova fades.
In fact, Type Ia supernovae seen in nearby galaxies are so predictable that, as Berkeley Lab's Peter Nugent explains, "the time at which the supernova explosion started can be determined just from looking at a spectrum. When we studied even the most distant of our supernovae, we found they had just the right spectrum on just the right day of the explosion. This tells us that Type Ia supernovae which exploded when the universe was half its present age behave essentially the same as they do today."
This result, reported in the January 1 issue of the science journal Nature, is crucial, since using supernovae as milestones rests on the comparison of nearby supernovae to distant ones. If the properties of supernovae were different when the universe was young it would confuse the comparison.
Because the most distant supernova explosions appear so faint from earth, last for such a short time, and occur at unpredictable intervals, the Supernova Cosmology Project team had to develop a tightly choreographed sequence of observations to be performed at telescopes around the world and, more recently, the Hubble Space Telescope.
"We are studying cosmic fireworks that fade away within weeks, so we have to move fast," says team member Greg Aldering. "While some team members are surveying distant galaxies using the largest telescope in the Andes Mountains of Chile, others in Berkeley are retrieving that data over the Internet and analyzing it to find supernovae. Once we find likely supernovae we rush out to Hawaii to confirm that they are supernovae and measure their redshifts using the Keck telescope, the world's largest.
"Meanwhile, team members at telescopes outside Tucson and on the Canary Islands are standing by to measure the supernovae as they fade away. The Hubble Space Telescope is called into action to study the most distant of our supernovae, since they are too hard to accurately measure from the ground."
Between Christmas of 1997 and the New Year, the Supernova Cosmology Project discovered even more deep-space supernovae, which they will use to check the results they reported Jan. 8. The Hubble Space Telescope is obtaining very precise measurements for four of these this week, even as the results from previous supernovae are being reported.
Speaking today at the American Astronomical Society meeting, Robert Knop of the Berkeley Lab said "This is our most successful supernova search yet -- our own brand of New Year's fireworks. We found over 15 supernovae, including the most distant ever confirmed with spectroscopic identification. These explosions occurred about seven billion years ago and the light has only just reached us this past week."
Team member Ariel Goobar of the University of Stockholm says, "Reaching out to these most distant supernovae teaches us about the 'Cosmological Constant,' which Einstein once called his greatest mistake" -- because if the newly discovered supernovae confirm the story told by the previous 40, astrophysicists may have to invoke Einstein's cosmological constant to obtain agreement with the popular inflation theory which explains how the universe developed shortly after the Big Bang. In other words, at least in the past some unknown force worked against gravity to produce the observed rate of expansion.
Such questions can only be addressed using supernovae so far away that only the Hubble Space Telescope can measure them, both those already observed and those yet to be found.
The work of the Supernova Cosmology Project has made extensive use of the Cerro Tololo Interamerican Observatory in Chile and the Wisconsin- Yale-NOAO telescope on Kitt Peak, both supported by the National Science Foundation, as well as the Keck 10-meter telescope in Hawaii, the William Hershel Telescope and Isaac Newton Telescopes on the Canary Islands, and of course NASA's Hubble Space Telescope.
The research is supported at the Lawrence Berkeley National Laboratory by the United States Department of Energy and the National Science Foundation's Center for Particle Astrophysics.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
California Institute of Technology
April 14, 1997
Caltech Astronomer Obtains Data That Could Resolve the "Age Problem"
PASADENA -- A California Institute of Technology astronomer has obtained data that could resolve the "age problem" of the universe, in which certain stars appear to be older than the universe itself.Dr. Neill Reid, using information collected by the European Space Agency's Hipparcos satellite, has determined that a key distance measure used to compute the age of certain Milky Way stars is off by 10 to 15 percent. The new data leads to the conclusion that the oldest stars are actually 11 to 13 billion years old, rather than 16 to 18 billion years old, as had been thought.
The new results will be of great interest to cosmologists, Reid says, because estimates of the age of the universe, based on tracking back the current rate of expansion, suggest that the Big Bang occurred no more than about 13 billion years ago. Therefore, astronomers will no longer be confronted with the nettling discrepancy between the ages of stars and the age of the universe.
"This gives us an alternate way of estimating the age of the universe," says Reid. "The ideal situation would be to have the same answer, independently given by stellar modeling and cosmology."
Reid's method focuses on a type of star (known as subdwarfs) found in globular clusters, which are spherical accumulations of hundreds of thousands of individual stars. These have long been known to be among the earliest objects to form in the universe, since the stars are composed mainly of the primordial elements hydrogen and helium, and because the clusters themselves are distributed throughout a sphere 100,000 light-years in diameter, rather than confined, like the sun, within the flattened pancake of the galactic disk. Astronomers can determine quantitative ages for the clusters by measuring the luminosity (the intrinsic brightness) of the brightest sunlike stars in each cluster. Those measurements require that the distances to the clusters be known accurately.
Reid looked at some 30 stars within about 200 light-years of Earth. Using the Hipparcos satellite, he was able to obtain very accurate distances to these stars by the parallax method. Parallax is a common method for determining relatively nearby objects. Just as a tree 10 feet away will seem to shift its position against the distant background when an observer closes one eye and then the other, a nearby star will shift its position slightly if the observer waits six months for Earth to reach the opposite side of its orbit. And if the distance between the two observing sites (the baseline) is known very accurately, the observer can then compute the distance to the object by treating the object and the two observing sites as a giant triangle.
Reid chose the 30 stars for special study (out of the 100,000 for which Hipparcos obtained parallax data) because they, like the globular cluster stars, are composed primarily of hydrogen and helium. Thus, these stars also can be assumed to be very old, and may indeed themselves once have been members of globulars that were torn apart as they orbited the galaxy.
Once distances have been measured, these nearby stars act as standard candles whose brightness can be compared to similar stars in the globular clusters. While this is a well-known technique, older investigations were only able to use lower-accuracy, pre-Hipparcos parallaxes for 10 of the 30 stars.
Reid's conclusion is that the clusters are about 10 to 15 percent farther from Earth than previously thought. This, in turn, means that the stars in those clusters are actually about 20 percent brighter than previously thought, because luminosity falls off as distance increases. Brighter stars have shorter lifetimes, so this means that the clusters themselves must be younger than once assumed.
British astronomers Michael Feast and Robin Catchpole recently arrived at very similar conclusions, also based on new data from Hipparcos, but using a different, and less direct, line of argument. They used new measurements of a type of variable known as Cepheids to determine a revised distance to the Large Magellanic Cloud, a galaxy orbiting the Milky Way.
Feast and Catchpole used another type of variable star, the RR Lyrae variables, to bridge between the LMC and globular clusters. The fact that these two independent methods give the same answer makes that answer more believable, says Reid.
"Most people previously believed that 14 billion years was the youngest age you could have for these stars," Reid says. "I think it's now accurate to say that the oldest you could make them is 14 billion years.
"No longer are we faced with the paradox of a universe younger than its stellar constituents," says Reid.
The work is set to appear in July in the Astrophysical Journal.
European Space Agency
Paris, France
14 February 1997
ESA's Hipparcos satellite revises the scale of the cosmos
The observable Universe may be about 10 per cent larger than astronomers have supposed, according to early results from the European Space Agency's Hipparcos mission. Investigators claim that the measuring ruler used since 1912 to gauge distances in the cosmos was wrongly marked.This ruler relies on the brightnesses of winking stars called Cepheids, but the distances of the nearest examples, which calibrate the ruler, could only be estimated. Direct measurements by Hipparcos imply that the Cepheids are more luminous and more distant than previously imagined.
The brightnesses of Cepheids seen in other galaxies are used as a guide to their distances. All of these galaxies may now be judged to lie farther away. At the same time the Hipparcos Cepheid scale drastically reduces the ages of the oldest stars, to about 11 billion years. By a tentative interpretation the Universe is perhaps 12 billion years old.
Michael Feast from the University of Cape Town, South Africa, announces his conclusion about the Cepheids at a meeting devoted to Hipparcos at the Royal Astronomical Society in London today (14 February 1997). It will provoke much comment and controversy, because the scale and age of the Universe is the touchiest issue in cosmology.
The best hope for confirming or modifying the result now rests with studies using Hipparcos data on other kinds of variable stars. An investigation of the variables called Miras, by Floor van Leeuwen of Royal Greenwich Observatory, Cambridge, and his colleagues, is described at the same London meeting. Full scientific reports on both the Cepheids and Miras have been accepted for publication in a leading journal, the Monthly Notices of the Royal Astronomical Society.
European teams of scientists and engineers conceived and launched the unique Hipparcos satellite, which operated from 1989 to 1993. Hipparcos fixed precise positions in the sky of 120,000 stars (Hipparcos Catalogue) and logged a million more with a little less accuracy (Tycho Catalogue). Since 1993 the largest computations in the history of astronomy have reconciled the observations, to achieve a hundredfold improvement in the accuracy of star positions compared with previous surveys.
Slight seasonal shifts in stellar positions as the Earth orbits the Sun, called parallaxes, give the first direct measurements of the distances of large numbers of stars. With the overall calculations completed, the harvest of scientific discoveries has begun. Among those delighted with the immediate irruption into cosmology, from this spacecraft made in Europe, is ESA's director of science, Roger Bonnet.
"When supporters of the Hipparcos project argued their case," Bonnet recalls, "they were competing with astrophysical missions with more obvious glamour. But they promised remarkable consequences for all branches of astronomy. And already we see that even the teams using the Hubble Space Telescope will benefit from a verdict from Hipparcos on the distance scale that underpins all their reckonings of the expansion of the Universe."
The pulse-rates of the stars
Cepheid stars alternately squeeze themselves and relax, like a beating heart. They wax and wane rhythmically in brightness, every few days or weeks, at a rate that depends on their luminosity. Henrietta Leavitt at the Harvard College Observatory discovered in the early years of this century that bigger and more brilliant Cepheids vary with a longer period, according to a strict rule. It allows astronomers to gauge relative distances simply by taking the pulse-rates of the Cepheids and measuring their apparent brightnesses.Nearby Cepheids are typically 1000-2000 light-years away. They are too far for even Hipparcos to obtain very exact distance measurements, but by taking twenty-six examples and comparing them, Michael Feast and his colleague Robin Catchpole of RGO Cambridge arrive at consistent statistics. These define the relationship between the period and the luminosity, needed to judge the distances of Cepheids. The zero point is for an imaginary Cepheid pulsating once a day. This would be a star 300 times more luminous than the Sun, according to the Hipparcos data. The slowest Cepheid in the sample, l Carinae, has a period of 36 days and is equivalent to 18,000 suns.
Applied to existing data on Cepheids seen in nearby galaxies, the Hipparcos result increases their distances. It pushes the Large Magellanic Cloud away, from 163,000 light-years, the previously accepted value, to 179,000 light-years with the Hipparcos Cepheid corrections, an increase of 10 per cent. Feast and Catchpole feed this result back to our own Milky Way Galaxy, and into calculations of the age of globular clusters, which harbour some of the oldest stars of the Universe.
The reckoning involves another kind of variable star, the RR Lyraes, and the Hipparcos investigators arrive at an age of 11 billion years for the oldest stars. Other estimates of the oldest stars assigned to them an age of 14.6 billion years. This seemed, absurdly, to leave them older than the Universe A team of astronomers using the Hubble Space Telescope recently declared the Universe to be only 9-12 billion years old. The Hipparcos Cepheid result increases that Hubble-inferred cosmic lifespan to 10-13 billion years.
"I hope we've cured a nonsensical contradiction that was a headache for cosmologists," Michael Feast says. "We judge the Universe to be a little bigger and therefore a little older, by about a billion years. The oldest stars seem to be much younger than supposed, by about 4 billion years. If we can settle on an age of the Universe at, say, 12 billion years then everything will fit nicely."
Feast and Catchpole have also cleared up a mystery about the nearest and most familiar Cepheid variable. This is Polaris, the Pole Star. Imperceptibly to the human eye, its brightness varies at a relatively high rate, every 3 days. That should make it, by the Cepheid rule, a feebler star than it appears to be.
Hipparcos fixes the distance of Polaris at 430 light-years, and the researchers conclude that Polaris pulsates with an overtone, at a rate 40 per cent faster than expected for a Cepheid of its size and luminosity. Several other Cepheids gauged by Hipparcos also exhibit overtones. Were these not recognized as fast pulsators they would give false impressions in the Cepheid distance scale.
The miraculous stars
Another famous variable star pulsates at more than twice the frequency that theorists would expect. This is Mira, the prototype of the class of stars investigated by Floor van Leeuwen and his colleagues, using the Hipparcos data. To an unaided eye, Omicron Ceti appears and disappears in a cycle of 11 months. In the 17th Century astronomers named it Mira, the miraculous star. Astrophysicists today interpret Mira as a senile star slightly more massive than the Sun. It has swollen into a red giant and started oscillating, as a prelude to greater instabilities that will in due course fling the outer layers of the star into space.Hipparcos fixes Mira's distance at 420 light-years. Other astronomers have gauged the apparent width of the star, as seen from the ground, so the Hipparcos team can compute the diameter of Mira as 650 million kilometres -- somewhat wider than the orbit of Mars. If the Sun were in Mira's state it would swallow up the Earth and all of the inner planets.
Astronomers knew that Mira was big, but the Hipparcos result confirms that it is too large to be oscillating in a simple fashion. Again its variation is an overtone, and the same is true of some other variable stars of the same type, known collectively as the Miras.
The sixteen Miras in the survey are mostly 300-1000 light-years away, at distances more comfortably within the grasp of Hipparcos parallaxes. Before Hipparcos, there was only one fairly good measurement of a Mira distance, for the star R Leonis. Even in that case, Hipparcos adjusts the distance from 390 to 330 light-years.
Patricia Whitelock of the South African Astronomical Observatory played a prominent part in the Mira study. In preparation for the Hipparcos data, observations of selected Miras from South Africa and Russia, with infrared instruments, assessed the extent to which they are dimmed by dust. Taking this effect into account, as well as the occurrence of overtones, the team arrives at a cosmic distance scale. As with the Cepheids, they can deduce distances by comparing the brightness of a Mira with its period of variation.
Applied to the Large Magellanic Cloud, where Miras have been detected, the Hipparcos Mira scale puts the galaxy at 166,000 or 171,000 light-years, depending on the method of calculation preferred. This result is intermediate between the commonly accepted distance to the Large Magellanic Cloud and the new result from the Hipparcos Cepheid scale.
"Frankly the Cepheids are at the limit of the useful range of Hipparcos, for distance measurements," comments Floor van Leeuwen. "And as for the Miras, ours is the very first attempt to gauge the absolute distance to another galaxy via parallax measurements on this type of star. So I think we should be grateful to Hipparcos, that our earliest answers are in the right ballpark and in fairly good agreement, without being hasty in drawing cosmological conclusions."
Only the beginning
Michael Perryman, ESA's project scientist for Hipparcos, anticipates a warm debate among astronomers. Should the Hipparcos Cepheid results be taken at face value, with all their implications for the size and age of the Universe? He remains confident that the issue will be settled by other results quarried from the Hipparcos data.Further Hipparcos studies of variable stars, including the RR Lyraes, are in progress. Also relevant to the distance scale are differing quantities of heavy elements present in stars of different ages, which can affect their luminosities. Any remaining confusion on this point will be dispelled by mainstream Hipparcos research devoted to the basic astrophysics of stars of different ages of origin, and at different stages of their life cycles.
"Until Hipparcos, the cosmic distance scale rested on well-informed guesses," Michael Perryman says. "The distances we now have, for stars of many kinds, provide for the very first time a firm foundation from which to gauge the distances of galaxies. The work has only just begun. If it should turn out that the Cepheids have given the final answer straight away, that might be surprising. But there will be no reason for astonishment when Hipparcos's direct measurements of stellar distances lead to a revised scale for the Universe."
The Hipparcos Cepheid scale is due to be debated in London today and in Seattle on 17 February, when Michael Feast will speak at the annual meeting the American Association for the Advancement of Science. It will also be one of the hot topics at ESA's Hipparcos Symposium in Venice,13-16 May.
The Venice meeting will celebrate the release of the Hipparcos and Tycho Catalogues to the world-wide astronomical community. It will also offer the first overview of results obtained by the groups who have had early access to the data, by virtue of their contributions to the Hipparcos mission. The subjects range from the Solar System and the Sun's neighbours among the stars, through special stars and the shape and behaviour of the Milky Way Galaxy, to the link between the starry sky of Hipparcos and the wide Universe of galaxies and quasars.
Further notifications about the Venice Symposium will be distributed to the press in due course. Meanwhile information about Hipparcos is accessible on the World Wide Web: http://astro.estec.esa.nl/Hipparcos/hipparcos.html.
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