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A year of discovery: astronomy highlights of 2000.

Picking the Highlights

In my Sky & Telescope reviews for 1998 and 1999, the section topics were the author's prejudiced choices. This time an effort has been made to take a sort of vote within the community, or a least a drawing of straws. If you ask a hundred people to pick a random number between 1 and 10, 3 and 7 will be greatly overrepresented (from an experiment by Caltech graduate students in response to a remark by Chris Anderson that "the random number is 3"). So the topics below are those of research papers that happen to have been recorded on line 7 of each page of my notebook whose page number ends in 3, from 93 working back through the year. Would I have cheated if this had produced 10 papers on peculiar A stars? Yeah, probably. But in fact the algorithm yielded 10 different topics, all, I think, important as a part of the total picture of what 21st century astronomy is going to be about.

Headliners

You've already read about these, in S&T and elsewhere, so the next few paragraphs are just my takes on the items, somewhat influenced by talks and discussions at the General Assembly of the International Astronomical Union, held in Manchester, England, in August.

Numbers that describe the universe. I think the real headline here is not something that begins, "newer, better." Rather, it is the fact that the numbers have been nearly stable, at least since the previous IAU gathering in Kyoto, Japan, in 1997. Thus, all forms of matter together make up to about 30 percent of the density needed to stop the cosmic expansion. Normal (baryonic) matter accounts for only a few percent of the total, but this is still more than we see in stars and gas. Dark energy or a cosmological constant adds roughly the 70 percent needed to make space very close to flat (and, by implication, to make the present expansion accelerate). And the Hubble constant has leveled off somewhere around 70 kilometers per second per megaparsec, with the Hubble Space Telescope Key Project team reporting a semifinal value of 74 and a variety of other methods averaging out to about 65.

Planets of other stars. The key word is "many"--more than 50 at last count. As searches continue, the smallest detectable masses are dropping (from "Jupiter" to "Saturn" in the last year) and the longest orbit periods are rising (to 5 or 6 years). Five to 7 percent of nearby, Sun-like stars have one or more companions, most of which are close to Jupiter in mass and closer to their stars than Earth is to its Sun. Whether this increases or decreases the odds for the other 90 to 95 percent of stars to have terrestrial planets is still up for theoretical grabs and will be for some time. But a histogram of numbers of companions versus mass does show a real gap between these "hot Jupiters" and ...

L and T dwarfs. Again, the very few have become many in these new spectral types, which overlap the divide between hydrogen-burning stars and deuterium-burning brown dwarfs. Two surveys, 2MASS and SDSS (pronounced "two-mass" and "Sloan digital") have provided most of the candidates. Recent surprises: (a) L-type spectra (with strong features due to the alkali metals lithium, sodium, and so forth) switch over to T-type spectra (with features due mostly to methane) over a very narrow range of temperatures near 1,300 Kelvin, accounting for the absence of transitional types and representing the sharpest change between adjacent spectral types anywhere along the sequence OBAFGKMLT. And (b), among the cooler Ls, a sodium feature removes so much yellow light that a suitably tuned detector might record the complementary color--you may never see a purple cow, but purple stars are possible.

And if you are wondering what ever became of types R, N, and S, they were never thought to be cooler than K and M. Instead, they have different surface compositions. The R and N types are now more often called carbon stars, while the S type includes CH subgiants and Ba II stars.

Future large-scale facilities. It's hard not to love (at least if you are a Harry Potter fan) something called OWL, the Overwhelmingly Large Telescope. This 100-meter gleam in European eyes is just one of several dozen telescopes, missions, and detectors that some national or international group of astronomers is building or wants to build. They cover the full range of photon wavelengths, from radio (the Suffa Radio Telescope, under construction in Samarkand), to millimeter and submillimeter (LMSA, with a prototype under development in Japan for possible siting in Chile), infrared (SIRTF, the mostly American Space Infrared Telescope Facility), ultraviolet (Spectrum-UV, spearheaded by Russia with European collaborators), X-rays (a proposed launch of a second Japanese-American ASTRO-E, the first having failed in January 2000), and on to gamma rays (INTEGRAL, under construction with European and some American input). Of course, the above merely samples the ambitious projects.

There are special-purpose projects to examine the Sun, the cosmic microwave background, and members of the solar system, and to measure even more precise stellar positions and motions than were possible with Hipparcos. And photons are no longer the only game in town, with new detectors for astrophysical neutrinos, very-high-energy cosmic rays, and gravitational radiation all on the horizon or a bit beyond it. Of course, with "large-scale" defined as more than $50 million on the ground and $500 million in space, we cannot have them all. So far, even the worldwide astronomical community does not have a good way of prioritizing its needs and desires, let alone a way of incorporating informed public opinions. But truly we are thinking about the issue--naturally, by appointing a committee (actually an IAU Working Group on Future Large Scale Facilities).

To B or Not To B?

Many astronomical items would not be here at all without magnetic fields--solar flares, the radiation of pulsars, and cosmic rays. At other times and places, they can be neglected up to very high levels of sophistication--planet formation, the structure of stellar interiors, and nuclear reactions in the early universe. And in between lie the cases about which, some 35 years ago, Lodewijk Woltjer said, "The larger our ignorance, the stronger the magnetic field," where existing calculations don't quite do the job, and where theorists suspect that magnetism might be the missing factor. An old example was the problem of getting rid of angular momentum so that stars could form from interstellar clouds. A magnetic field helps collimate rotating gas streams that flow out along the poles and carry away only a bit of mass but lots of angular momentum.

Well, then, why not just include the so-called B field in your calculations? First, of course, the calculations become more difficult, sometimes temporarily impossible, if you already have as many physical quantities to look after as state-of-the-art computing can accommodate. Second, what sort of field should you include: a tidy stream, a dipole, or a turbulent mess? constant strength or a gradient? supported by existing currents or a fossil field? Of course you could try all possible combinations, but this exercise will take a good long while, and your answer will be described as "model dependent" by the referee. This is not quite the same as "untrustworthy," but it comes close.

An important problem in this intermediate territory is the formation of large, dense, cool clouds of interstellar gas. These are important because they are where stars form, but even rather modest amounts of starbirth disrupt them in [10.sup.8] years or less, and they must be constantly reformed. Obviously gravity is going to be important. A long-popular picture is a sort of standoff between gravity and an orderly field just outside a galactic plane. The clouds gradually contract, weighing down the field lines, until the situation becomes unstable, the field lines pop out, and a denser cloud settles down into the galactic disk.

The clouds made this way are, however, much more elongated along the galactic spiral arms than the clouds we see. Clearly the next obvious step in complexity is to include not just the known, orderly fields that lie along spiral arms, but also the stochastic (messy, turbulent) field that, we know from observations, has about the same strength. Three cheers!--the instability then makes more nearly spherical clouds, like the ones we see. What next? Well, the first round of new calculations was analytical, so incorporation of the additional physics into numerical simulations of cloud formation will inevitably follow.

Index paper: E. N. Parker and J. R. Jokipii, June 10th Astrophysical Journal.

The Planetesimal Hypothesis

Doesn't that have a wonderfully old-fashioned sound? Yet, once upon a time, a century ago, the idea that you make big rocky things (planets) by having a bunch of little things (planetesimals) stick together was a revolutionary suggestion, put forward by Forest Ray Moulton and Thomas C. Chamberlin of the University of Chicago. It still dominates the way we think about the early history of our own solar system and about the disks of dust and gas that orbit many other stars, mostly young ones. These are generally dignified by the name "protoplanetary disks" on the assumption that the hypothesis will operate. It is probably even right, and viable alternatives are anyhow thin on the ground (oh, all right, thin on the sky). But the problem of planet formation is not entirely a solved one.

It is easy to imagine getting started. Small particles are very sticky (even my dust bunnies have dust bunnies). Later on, larger chunks will pull together and cling gravitationally. This shows in the spacings of the planet orbits, which are partially conditioned by the competition between gravity within a region pulling nearby lumps together and solar gravity tides tending to pull adjacent lumps apart.

But what about the intermediate growth stage? It is hard to imagine either surface stickiness or gravity bonding SUVs that meet in even a very gentle collision. None of the "line 7" papers reported a major breakthrough in this territory. There were, however, two minor crackthroughs, resulting from an interplay of theory, observation, and (less common for astronomical issues) laboratory data.

First, it seems that gas molecules near the midplane of a protoplanetary disk adsorb onto small dust grains very quickly. This helps in three ways: (a) by preserving gas that might otherwise be lost to photodissociation, so that you have it later to make air and water and icy things, (b) by making grain surfaces stickier, and (c) by hastening the approach to masses large enough for gravity to take over.

Second, laboratory dust, made to simulate interstellar and disk grains, aggregates more efficiently than had previously been thought. The investigators used micron-size particles of silica, silicon carbide, diamond, and enstatite and found that irregular shapes, porosity (fluffiness), and small electrical charges all made the particles stickier. And these are all characteristics that we expect in interstellar and protoplanetary grains on the basis of how they interact with light.

Index papers: Y. Aikawa and E. Herbst, November (I) 1999 Astronomy and Astrophysics; T. Poppe et al., April 10th Astrophysical Journal.

Stellar Collisions and Mergers

Don't expect the Sun to hit another star! Locally, the waiting time is about 2,000,000,000,000,000,000 years, and we have only 4,500,000,000 ahead of us. But in a denser environment, like the core of a star cluster or near a central black hole in a galaxy or quasar, stellar collisions will happen often enough to affect individual star lives and perhaps even the evolution of the system as a whole. Bigger stars (ones just forming or red giants) and slow motions also enhance collision rates. And for many binary pairs, mergers are just about guaranteed once they have shed some angular momentum in winds or gravitational radiation.

Earlier generations of astronomers thought that a collision or close encounter might have formed our planetary system by dragging gas out of the Sun. And there have always been a few enthusiasts since; but the numbers have now grown until a meeting last May at the newly refurbished Hayden Planetarium in New York City--the first to be held there--filled one of its auditoriums.

What do the collision products look like? Some are long-known but long-puzzling friends, like the blue stragglers--rare stars in clusters, especially globular clusters, too blue and too bright to be as old as the rest of the cluster. Merge a binary or colliding pair and--voila!--you have a star of larger mass that would otherwise long since have vanished from the cluster. Other products are more subtle, like the most massive stars near the center of the Milky Way and elsewhere and the central stars of a few odd, old planetary nebulae.

But collisions and mergers can also result in fireworks; think of two subcritical masses of uranium or runaway trucks! For instance, bring together two white dwarfs whose summed mass exceeds 1.4 times that of the Sun (the Chandrasekhar limit) and you will probably get a nuclear explosion of carbon and oxygen fusing to iron. This is one of several ways that Type Ia supernovae (the kind used for cosmological distance measurements) might be made. Making them this way would explain why they are all rather similar, even if they happen in galaxies with stars of different ages and compositions.

Let the input be two neutron stars, and you have enough gravitational energy to make the sort of gamma-ray burster that lasts less than a second and for which we have not yet seen afterglows or host galaxies. The recently launched HETE 2 mission should permit finding the afterglows, source redshifts, and hosts if they exist. Keep tuned for more on this and other SCAMs (and I have not yet decided whether the May conference organizers chose a title with this acronym deliberately or not).

Index paper: E. Rodriguez and M. J. Lopez-Gonzalez's catalog of SX Phoenicis stars, the subset of blue stragglers that are also pulsating, Cepheid-like variables, July (II), Astronomy and Astrophysics.

Stellar Activity Cycles (Spots Among the Stars)

Stellar (and solar) activity means the whole brigade of chromospheric and coronal phenomena as well as the radio, X-ray, and line emissions that tell us about them. We have to care about the solar case. Immediate effects of solar flares and coronal mass ejections include shortwave-radio fade-outs, failure of over-the-horizon radar, electric power-grid failures, and satellites lost temporarily to changes in the ionosphere or permanently to extra air-drag when our atmosphere gets puffed up. Longer-term effects seem to include changes in Earth weather and climate. The classic example is the Maunder Minimum in sunspot numbers and the simultaneous Little Ice Age--remember those Dutch paintings of people skating on the canals? You'd need water wings now.

A strong motivation for studying activity in other stars is, therefore, the hope that their behavior may let us get a better grip on what the Sun might do over the next few decades and centuries and the implications for global warming, cooling, drying, drowning, or whatever worries you most. Data that might be relevant abound. Faced with such richness, what does an astronomer do? She draws a log-log plot, that is, looks for correlations of one stellar trait with another that might make physical sense. Promising traits in this case include how much activity there is; whether it is periodic and, if so, over how many years; the ages of the stars, their rotation periods, and the depth of the surface layers in which the gas boils up and down in convective flows.

What have we learned? Perhaps most of all that we are lucky to be supported by a relatively staid, middle-aged star. Younger, faster rotators not only have many more flares and such, the activity is also much more erratic, with, typically, no well-defined period. What is there still to learn? Most critically, what puts the energy up there in the chromosphere and corona? Is it the convective motions themselves, waves for which magnetic-field energy is important, or both? A recent answer (too beautiful to be entirely true) is that every star with a convective surface has a sort of basal metabolism of convective (or acoustic) energy input, while stars with rapid rotation and strong magnetic fields have a second power source for their activity.

A critical test comes from looking at rapidly rotating stars and brown dwarfs that are so cool that convection goes all the way to the center, implying lots of "basal metabolism" but, supposedly, little magnetic-field input. Curiously the few examples studied so far seem to have not steady-state chromospheres (and so forth) but do experience occasional rather weak X-ray flares. Not quite what you might have expected, but, on the other hand, we are lucky not to have to depend on one of these very cool, faint stars, either!

Index paper: M. Rodono et al., June (II) Astronomy and Astrophysics. It dealt with a particular close binary, II Pegasi, which displays at least three different periods in its level of activity.

Making and Breaking: The Life and Death of Dwarf Galaxies

"The-most-important-unsolved-problem-in-modern-astrophysics," said as if it were all one word, is a pretty good description of trying to make the matter as lumpy as it is in superclusters of galaxies, voids, and all, while keeping the cosmic microwave background radiation as smooth as it is. And, no, the problem wasn't solved last year.

But two possible, partial solutions to a subproblem have appeared. The subproblem is that the "best buy" models for galaxy production tend to make many more little galaxies (dwarfs) than we see. The Local Group should have a couple hundred rather than a couple dozen. Or, at least, it should have a couple hundred dark-matter halos that could hold enough gas to make dwarfs. Thus one possibility is that the gas somehow never got there and is drifting around in intergalactic sheets and filaments. Just that can happen if the really big galaxies and quasars started shining early enough to fill the universe with ultraviolet radiation before a redshift of 5 or so --a time when the universe was about a fifth its present size. Then most small halos (the ones that would have hosted galaxies with star motions of 15 to 40 km per second, like the dwarfs we do see in the Local Group) never got any gas at all, and intermediate halos (meant to be galaxies with star speeds of 20 to 55 km per second) are partially inhibited.

In this first case the naked dark halos would have become part of the total dark-matter supply of the Local Group, and we would never know that they were supposed to be here without theorists to tell us.

Or, alternatively, as it were, the dwarfs may have formed and we just don't recognize them. What does the Local Group have several hundred of? Why, globular clusters, of course, with about 150 belonging to the Milky Way and perhaps more associated with M31, the great Andromeda Galaxy. You say that they don't look much like dwarf galaxies? Well, you are probably right. They are not far from being the right mass, but their stars are concentrated into smaller volumes and seem to be all the same age and composition, at least in most clusters. On the other hand, Omega Centauri, the largest, brightest of the Milky Way globulars, does actually include stars with a range of heavy-element abundances and, possibly, ages. That is, it may be the nucleus of a partially disrupted dwarf galaxy orbiting the Milky Way. The stars lost from its outskirts would long since have mingled with other stars in the halo, as the stars of the dwarf spheroidal galaxy in Sagittarius are doing right now (well, right this billion years anyhow).

Could all or most globular clusters be stripped dwarf cores? This would provide the additional satisfaction of telling us when and how the clusters formed, which has always been a bit of a mystery, in the sense that many astronomers know the answer, but they "know" different ones. You still don't think this scenario sounds very likely? Perhaps not, but the best analogy I've come up with is that if you are missing a bunch of tadpoles, you shouldn't refuse to look for them among the smaller frogs.

Is the poor stripped core or cluster at least now safe from the rapacious tidal grasp of our galaxy? No, not even that. Twenty images of 20 different globulars, including Omega Cen as well as less massive, less compact ones, reveal that every one is dragging a tail of stars behind it. The discovery was a triumph of a relatively new way of looking at images, called a wavelet transform. And, while Omega Cen has lost only about 1 percent of its stars in the last billion years, half or more have been dragged out of Messier 10 (NGC 6254), which may not survive for more than another orbit or two around the Milky Way.

Index papers: E. Pancino et al., May 1st Astrophysical Journal Letters; T. Kitayama and S. Ikeuchi, February 1st Astrophysical Journal; S. Leon et al., July (III) Astronomy and Astrophysics.

Black Holes for Fun and Profit

At last count, there were three kinds of black holes--small, medium, and large, with action on all three fronts. The large ones live in most galaxies, power quasars (etc.), and have masses that are closely proportional to the masses of the bulge or spheroidal parts of their hosts (S&T: October 2000, page 28). For the medium ones, the main story is increasing evidence for their existence. But my "random-paper algorithm" picked out two items about the small sort, formed by collapses of cores of relatively massive individual stars.

Cygnus X-1 was the very first X-ray binary where astronomers were largely convinced, from 1974 onward, that hot X-ray-emitting gas was orbiting a black hole rather than a neutron star. The primary evidence was, and is, that the principal star has about six solar masses, twice the maximum possible for neutron stars. Indeed, your present author wrote one of the last, wrong, dissenting papers on the subject, back when the conclusion depended on knowing the brightness and so the distance of the system. It no longer does. The claim was that the hot, principal star might be a faint, low-mass one, very late in its evolutionary path, rather than a young, massive, very bright one. In this case, the X-ray emitter could be a neutron star, and observer colleagues quickly set to work to disprove the suggestion.

That issue was quickly settled, but puzzles connected with the details of the light, radio waves, X-rays, and all that we see --and whether there are "black hole signatures" in the spectrum --persist to the present. In focus last year was the millimeter-wave emission. With unusually steady brightness and flat spectrum (meaning about the same amount of energy at each wavelength over a broad band), it is the best case around for continuous, steady outflow of gas from an X-ray binary. Unfortunately, just how the millimeter photons are emitted remains unclear, a poor condition for a signature.

Index paper: R. P. Fender et al., March 11th Monthly Notices of the Royal Astronomical Society.

The gamma-ray bursters are so varied in their brightnesses, durations, spectra, and all the rest that they can surely find homes for at least a half dozen models (out of more than 100 published over the years). Close encounters of the neutron-star kind (see page 55) are strong candidates for the bursts that last less than a second. The longer-lasting kind, which includes all those for which we have optical and/or radio counterparts and redshifts, now clings tightly to a bandwagon in which the core of a single, massive, rapidly rotating star collapses to a single, not so massive, even more rapidly rotating black hole. The ensuing mess provides a happy home for jets, disks, shocks, magnetic fields, and whatever else you might need to power not only the GRB itself but also an associated supernova, hypernova, or failed nova. Enough papers have explored aspects of this scenario to constitute not just a cottage industry but a castle.

Index paper: C. E. Brown et al., July New Astronomy.

Meanwhile, near the gatehouse, a few other astronomers are examining a picture in which the burst energy comes from an ordinary neutron star (think how Fritz Zwicky would have rejoiced in that phrase!) transforming itself into a strange quark star (arguably Zwicky's object Hades). Both strange quark matter and the transformation remain hypothetical, but keep your distance just in case. Once the process starts it may consume any normal material within reach, and your friends would be most surprised if you appeared at the next star party in the form of a lump of strange matter. (Oh, so your friends already think ...)

Index paper: I. Bombaci and B. Datta, February 20th Astrophysical Journal Letters.
Matter and energy in the universe: seen and unseen,
familiar and unknown. The good news is that there's
definitely enough of the stuff to reach so-called
critical density and thus support the leading,
inflationary-universe theory of the Big Bang.
Sky & Telescope illustration.

Baryonic matter:                  5%
(stars, nebulae, brown dwarfs,
atoms, molecules)

Non-baryonic
dark matter:                     25%
(unknown particles)

All matter in the
universe:                        30%

Dark energy:                     70%
(Cosmological
Constant

Note: Table made from pie chart.


VIRGINIA TRIMBLE oscillates at 37 nanohertz between the University of California, Irvine, and the University of Maryland, College Park. She is Chair of the Historical Division of the American Astronomical Society. Editor Emeritus Leif Robinson tweaked the author's text after her husband, physicist Joseph Weber, died and she was unable to see the manuscript through publication.
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Date:Feb 1, 2001
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