Publications Topics:

Books

ASP Conference Series

Monograph Publications

IAU Publications

Books of Note

Purchase through the AstroShop

Journals

Publications of the ASP (PASP)

Magazines

Mercury Magazine

Archive

Guidelines for Authors

Order Mercury Issues

Mercury Advertising Rates

Newletters

The Universe in the Classroom

ASP E-mail Newsletters

Special Features

Astronomy Beat

Contact Us

Mercury, March/April 1998 Table of Contents

Virginia Trimble
University of California, Irvine
University of Maryland

How does one recognize a discovery or announcement of worth?
Some are obvious, but others need a little time and pampering.

Fluctuations in the cosmic background radiation. Tiny temperature fluctuations in this ancient radiation field, shown as patches in this sky map created from COBE satellite data, are the imprints of small density ripples present in a very young universe.

Astronomy usually progresses like a multi-dimensional amoeba, extending pseudopods in different directions, then sometimes dragging the rest of the creature along and sometimes hastily pulling back from the touch of a contradictory observation. These advances and retreats are currently reported in the form of about 6000 journal papers per year, of which the author scanned roughly 5000 in 1997. A small subset came equipped with bells, whistles, press releases, and other ornamentation that guarantee you will already have heard about them. The possible discovery of fossils in a Martian meteorite is a recent, extreme example. Many more, including some that will surely prove to be just as important in the long run, are sent out naked into the cold, printed world to fend for themselves.

Please think of the sections that follow as the appetizer tray at a new ethnic restaurant. Some of the nibbles will be as familiar as sushi; other will be strange but tasty; and still others you will be pretty sure you never want to encounter again as long as you live. But, just as there are lovers of 100-year-old eggs, there really are lovers of rapidly oscillating peculiar A stars (and some small overlap between the classes).

Planets and Such

Depending on your age, you will almost certainly remember where you were when you first heard about the bombing of Pearl Harbor or the shooting of President Kennedy or the burning of the Challenger shuttle. Most astronomers, amateur or professional, probably also remember just where and when in October, 1995 they first heard that the star 51 Pegasi is accompanied by a planet with about the mass of Jupiter, but a much shorter orbital period.

The inventory of extra-solar-system planets has since grown to about a score, most (not all) also of Jovian mass, most closer to their parent stars than the Earth is to the Sun, and most discovered by the same method that revealed 51 Peg B, though it has never been seen directly. Planets don't really orbit stars. Rather, a star and its planet(s) orbit their mutual center of mass, and the periodic motion of the star shows up, through the Doppler effect, as small shifts in the wavelengths of its spectral lines. This was how Michel Mayor and his colleagues in Switzerland and Geoffrey Marcy and Paul Butler in California spotted 51 Peg B. While they and other, newer teams were finding additional examples, theorists were busily working out ways that planets like Jupiter, which must form quite far away from their host stars, could be forced inward by interactions with other planets, left-over proto-planetary material, or another star.

An extra-solar world. In October 1995 Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of a planet with a mass about half that of Jupiter's orbiting the solar-type star 51 Pegasi.

Even as this was happening, David Gray of Ontario, Canada cast doubts on the whole enterprise, suggesting that waves on stellar surfaces, rather than Doppler shifts due to companion planets, were causing the wavelength shifts. Happily it was a false alarm, and, after I write this, but before you read it, a pair of papers from Gray and his colleagues and from another group will have shown that periodic, small Doppler shifts are, after all, the best bet. In some ways, the most surprising aspects are (1) the enthusiasm with which many astronomers as well as science writers greeted the doubts and (2) the remarkable self-restraint of the discoverers in not assigning cute names to their planets, at least not in public.

Much also happened within our own solar system during the year. The Mars Pathfinder images (which look a lot like the Voyager ones from 20 years ago) and the evidence of water on or in Jovian satellites probably count as sushi, or even pizza. Less well publicized are, first, measurements of the ratio of deuterium (heavy hydrogen) to the ordinary kind in comets Hale-Bopp and Hyakutake. These confirm a Halley result that D/H is about twice what is found in terrestrial oceans, meaning that most of the Earth's water cannot have come from our being hit by comets (at least no those kinds of comets).

Second is a pair of measurements of amino acids in bits of the meteorite that fell at Murchison, Victoria, Australia some years ago. Proteins in terrestrial living creatures are made of a subset of 20 of the chemically possible amino acids, and always entirely out of one or two possible mirror image forms of each. Presence of non-biological compounds and precise equality in the amounts of the left- and right-handed forms are the standard signatures of complex organic compounds not made by anything alive. But some of the amino acids in Murchison turn out to have about a 7% excess of the left-handed form. The excess is not a result of terrestrial contamination, because it occurs in molecules not among the favored 20 and in association with nitrogen that has a non-terrestrial ratio of the isotopes N14 and N15. How did molecules assembled in interstellar space end up with this twist? Perhaps the influence of magnetic fields or even of the kind of decay of elementary particles described by the weak interaction, the only one of the four fundamental forces with handedness built in.

Mostly About Stars

A Cataclysmic Variable is composed of a white dwarf and a less-massive companion star. The companion is a relatively cool main-sequence or giant star large enough to fill its Roche lobe, a surface on which gas can move freely without gaining or losing energy. Gas leaving the companion falls in a stream toward the white dwarf. Because the gas has too much angular momentum (rotation), it cannot fall straight onto the white dwarf, and thus spirals into a disk from which it gradually accretes onto the star.

This is a small plate of nibbles, some to be savored because they fulfilled long-standing predictions, others because they were completely unexpected. Rotating disks ought to show spiral waves under many circumstances. Spiral galaxies are the best-known example, but the same sort of thing has long been predicted for the disks orbiting white dwarfs in dwarf novae and other cataclysmic variables. IP Pegasi is the first case seen, with its spiral structure revealed by mapping spectral lines radiated by disk gas through the orbit period of the system.

In a very happy confluence of theory and observation, a brand new class of pulsating variable star was being predicted in Canada just as the first examples were being discovered by chance in South Africa. The stars are B type subdwarfs with multiple periods between 100 and 200 seconds and amplitudes generally less than 0.01 magnitude. The pulsations arise because iron in the star's atmosphere acts as a gate, alternately blocking and releasing light; Cepheid variables pulsate for much the same reason, with hydrogen as the main driver. The prototype rejoices in the name EC 14026†2647 (most of which is its position in right ascension and declination).

We don't often see stars change their spectral types in a human lifetime. Thus, FG Sagittae, which brightened, cooled from about BO to K, and added lines of carbon, barium, and other elements to its spectrum in the century after 1890 was long seemingly unique. The standard interpretation has been that it experienced its very last flash of helium shell burning (the products are carbon and oxygen) and was about to become an R Coronea Borealis variable. These are carbon-rich stars that fade suddenly and unpredictably (which FG Sge started doing a couple of years ago) and that have hydrogen-depleted atmospheres (which FG Sge has just developed). In addition, the "galloping giant" is no longer alone. Examination of old images and spectrograms reveal that V 605 Aquilae, studied by Knut Lundmark in the 1920's was a similar sort of beast, though it is now very faint And the latest recruit is V 4334 Sagittarii, better known as Sakurai's object, for its 1994 discoverer. It, too, changed both spectral type and surface composition very rapidly, and is now hydrogen-poor and carbon-rich, and well on its way to becoming the century's third new R CrB star. I confess to a private worry that three progenitors this close together in time are really too many for a class of stars of which only about 40 are known in the part of the Milky Way we can survey.

The R CrB's themselves provide a bridge to another set of highlights. The MACHO collaboration, whose primary goal is to find gravitational lensing by small stars and substellar objects that might be part of our galactic dark matter halo, has recorded a couple of new R CrB's in the Large Magellanic Cloud, a magnitude or two fainter than the three previously known there. This provides some support for the idea that the R CrB's are not really a single population, but have a range of intrinsic luminosities and kinematics. The idea is not a new one but was rediscovered this year by the present author, who received some early-release data for a subset of the R CrB's observed by the HIPPARCOS astrometric satellite as a result of a 1982 (!) proposal.

Both HIPPARCOS and the MACHO project, with its European relatives OGLE, EROS, and DUO, are proving to be rich mines of new answers and new questions in stellar astronomy. Only two of the odder puzzles can be mentioned if there is to be room saved for extragalactic desert.

MACHO (etc.) have indeed recorded many of the gravitational lensing events they were designed to find. Most of these are in the direction of the Galactic center, thus both the star being lensed and the one bending its light are members of our bulge population. But the two years of MACHO data published in 1997 included seven events seen in the direction of the Large Magellanic Cloud. Thus the stars being lensed are probably in the LMC and the lenses in our halo. The surprise is that the details of the events imply lenses of 0.3 - 0.5 solar masses. They cannot be small stars or their collective light would be seen. They are too massive for brown dwarfs, and not massive enough for neutron stars or black holes. The only category remaining is old white dwarfs, left from very early star formation, and this alternative is not very easy to live with either. If all the lenses are old white dwarfs, the total number would, indeed, account for about half of our halo of dark matter, but the parent stars should have made more carbon and more light than we see in our own and other galaxies.

The HIPPARCOS astrometric satellite. Launched in 1989, the High Precision Parallax Collectin Satellite mission lasted until early 1993, during which time HIPPARCOS measured the parallaxes of nearly 120,000 stars.

One early result from HIPPARCOS came into the world with press flags flying, because it has some bearing on how big and how old the universe is. The classic problem has been that the oldest stars we see, in globular clusters, are, at 12-18 billion years, a bit older than is comfortably consistent with the age of the universe, if the Hubble constant is somewhere between 65 and 85 km/sec/Mpc and if there is enough matter in the universe for its expansion to be slowing down.

Among the 100,000 stars whose parallaxes and proper motions were measured by HIPPARCOS were several dozen Cepheid variables, star used to calibrate distance scales and thereby to find the value of the Hubble constant. These Cepheids turned out to be a bit more distant and thus brighter than expected, stretching out the distance scale, and allowing H0 to be a bit smaller and the universe a bit older than previously advertised. Other stars in the data base include old, low-metallicity halo stars, useful for estimating distances to the globular clusters. These also moved out a bit, making the stars we date somewhat brighter, more massive, and therefore younger. So, if the universe gets older and the stars get younger, maybe the problem has gone away. Or maybe it hasn't. Other ways of using HIPPARCOS data, including distances to nearby clusters like the Pleiades and information about stellar motions, have just the opposite effect, pulling things in closer to us and so making the universe slightly younger and the stars slightly older. This can probably all be sorted out eventually, just as one can usually bake the other half of the dish.

In the realm of the neutron stars, two things long sought have finally been found, radio emission from Geminga and rotation periods in low-mass X-ray binaries. Geminga started life as a source of gamma rays in the constellation Gemini with no counterparts at any other wavelength. Over the past decade, it finally confessed via weak optical and X-ray emission to being a magnetized, rotating neutron star, that is a pulsar, with a period of 0.237 second. But all self-respecting pulsars are periodic radio sources; that's how they were discovered. Well, Geminga finally is, in data recorded by several different groups headquartered in Russia. It is just very faint at the wavelengths where people usually look for pulsars.

Neutron stars in old, low-mass X-ray binaries are also expected to rotate rapidly, because of having accreted material from disks spinning around them. The predicted millisecond periods have, however, defied detection. The Rossi X-ray Timing Explorer has finally triumphed over them, precisely because it is so good at timing things that change quickly. The data show quasi-periodic oscillations that imply rotation periods of a few milliseconds, just what we always thought.

A Galaxy or Two

The Milky Way has been trying for so long to tell us that it has a 3 million solar mass black hole at its center that the increasing strength of evidence in favor is perhaps hardly news. On the observational front, better measurements of the velocities of stars and gas near the center, compiled by Reinhard Genzel, Andrea Ghez, and others, now say that the central mass is so compact that nothing except a black hole will fit there. And, on the theoretical side, we have been told that our Galactic center is faint at X-ray and most other wavelengths despite the black hole because the gas flowing in takes most of its energy with it, beyond the black horizon. The process is called advection.

Lots of other galaxies, some with quasar and Seyfert traits, but some otherwise normal, also host black holes of 106 to 1010 solar masses. Because you need very good angular resolution to measure brightnesses and velocities near enough to galactic centers to decide this, the field has been dominated by data from the Hubble Space Telescope. The "silent" black holes in normal galaxies must either be accreting very little gas or be advecting it down secretly. Only the noisy ones, with lots of accreting gas that radiates as it goes, show up as quasars, radio galaxies, BL Lacertae objects, Seyferts, and all the rest. A previously-empty box has just been filled, by the discovery of the first quasi-stellar object with both strong radio emission and broad absorption lines. The combination was previously thought not to occur, and much theoretical ink had been poured out trying to explain why not.

Our Galaxy is, as you know, part of a small cluster called, with the enormous creativity of nomenclature for which astronomers are world renowned, the Local Group. How many other members are there? Nobody is quite sure, because a new one is discovered every couple of years. Unfortunately, two of the last three "discoveries" turn out to be galaxies that had been catalogued by Gerard de Vaucouleurs and his colleagues 20 years ago. The modern work does, however, tell us both what sorts of stars are in these galaxies (called Antlia and Tucana for the constellations you look past to see them) and how far away they are (probably on the outskirts of the Local Group).

The largest reported galaxy redshift increased from 4.92 to 5.34 as this was being written. One or two of these do no harm, but any large number would pose severe problems for all current models of how galaxies might have formed without ruffling up the very isotropic microwave background radiation.

Evidence for a supermassive black hole in the core of the Galaxy.The difficulty in proving the existence of a supermassive black hole at the center of the Milky Way Galaxy has been in observing the distribution of matter within a few parsecs of the center. This plot shows the orbital speeds of objects in the Galaxy's innermost regions versus the objects' distances from the Galactic center. Inside about 0.01 parsecs star speeds as high as 0.5% the speed of light have been observed. These high-speed stars are orbiting a small object 2.7 million times more massive than the Sun, evidence that strongly suggests the central object is indeed a black hole.

Our Universe and Others

1997 was the year that the fat lady finally burst. The flashes of gamma rays that have been hitting the Earth's atmosphere at a rate of about one per day without showing up at any other wavelength have finally been caught with a few X-ray after-glows (mostly by the Italian satellite BeppoSAX) and even fewer optical counterparts. Three had been seen as this was written, one that faded into the underlying fuzz of a small galaxy, and one that, at peak light, had absorption lines due to intervening clouds of gas, like the ones that impose forests of absorption lines in the light of distant quasars. That particular event, on 8 May 1997, thus has to have had a redshift between 0.8 (that of the intervening cloud) and 2.1 (or Lyman alpha lines would have been seen). Other kinds of evidence say that the events are probably all a single population and that they show statistical effects of redshift and time dilation, caused by the expansion of the universe at z = 1-3. This is not to say that we know exactly how to make gamma ray bursters, but at least we know to look in the extragalactic, rather than Galactic, cookbook for the recipe. Said cookbook was probably edited by Bohdan Paczynski, who was nearly alone a dozen years ago in saying that extragalactic or cosmological models were the best bet; and the recipe has at least some ingredients contributed by Peter Meszaros and Martin Rees, who have been thinking about fireballs for gamma ray bursts and other sources for almost as long.

If you accept that GRBs show effects due to universal expansion, then you can turn some of the details into a cosmic history of when they occurred. T. Totani reports that this history resembles the pattern of star formation revealed by distant galaxies, quasar absorption lines, and other indicators. The peak rate for both was at z= 1-2 and was a factor of 4-10 larger that the present rate. Numbers of quasars as a function of redshift have a similar distribution. Neutron stars and black holes, probably in binary pairs, are the only likely source for the GRBs, and the agreement of time histories, plus some absorption of X-rays in one of the after-glows and the behavior of the one radio counterpart seen so far, collectively suggest that the sources are still close to the regions where their massive parent stars formed.

The molecules in the Murchison meteorite may be left handed, but the universe as a whole is not. The extreme isotropy of the 3K microwave background radiation as revealed by the COBE satellite puts ever tighter limits on cosmic rotation and shear, and one false alarm was quickly silenced. Two physicists from the University of Kansas reported a twist to the universe on the basis of the angles of polarization of radio emission from a number of distant galaxies and quasars. But they were using archival data, not uniform over the sky and not of high enough angular resolution to see the underlying structure of the radio sources. Thus their Physical Review Letter was quickly chased into print by a refutation based on newer, better observations of radio polarization. Two extra bits of condiment: the customary name for twists, asymmetries, and handedness of this sort is chirality. And there was already an upper limit below the claimed detection in print in an obscure journal at the time the first Letter was being refereed.

COBE is, of course, famous for having provided the first clear evidence that the microwave background is not absolutely smooth over the sky. The temperature fluctuations are parts in 100,000, and the combination of COBE numbers with ones representing smaller angles on the sky is possibly telling us something about "what came before the Big Bang." The spectrum of fluctuations is much like that predicted by inflation and different from that predicted if topological defects underlie the large scale structure of galaxies, clusters, and voids that we see (see "Digging for Clues to Galaxy Formation in Large-Scale Structures").

It is, however, never safe to turn you back for a moment. Inflation also predicts (or almost so) that the universe should have just exactly the critical density that will stop its expansion in infinite time. We have no direct way of measuring the real, total density. But several indirect methods, based on supernovae, superclusters of galaxies, and the amount of gas in and out of clusters are all, at the moment, saying that the density is 30-40% of the critical value. Partial reconciliation is possible if you are willing to accept a non-zero value for Einstein's infamous cosmological constant. But the net effect is a bit as if somebody had poured hot fudge sauce on your sashimi, and, when you objected, said, "Oh, that's all right. It will taste fine if you just put a few of these black olives on top." In any case, most of the matter we know about (because it exerts gravitational forces) is unlikely to consist of the stuff we are used to, with atomic nuclei made of protons and neutrons and electrons orbiting them.

An average year sees the first appearance of half a dozen models for the universe even stranger than this. One favorite from 1997 gave up "the arbitrary assumption of the differentiability of space time" (so that redshifts would be quantized). Another posited a metric that oscillates with a period of 160 minutes, producing frequencies seen in solar and stellar oscillations. No food-based analogy that comes to mind is suitable for a family publication like Mercury.

VIRGINIA TRIMBLE oscillates at 37 nHz between the University of California, Irvine, where she is a professor of physics, and the University of Maryland, where she is a visiting professor of astronomy. She currently serves as a member of the Board of Directors of ASP, a vice president of the American Astronomical Society, a councilor of the American Physical Society, and a number of other positions of little power but remarkable temporal absorption capacity. Her email address is vtrimble@uci.edu.