ASTRONOMY UPDATES

ASTRONOMY UPDATE 1999

James B. Kaler

Department of Astronomy, University of Illinois

First published in the Proceedings of the 35th Annual GLPA Conference, Kalamazoo, MI, October 20-23, 1999. Reprinted by permission.

Abstract

The year was dominated by discoveries at both "ends" of the Universe. Within the Solar System we found out more about the Moon and Mars and watched Uranus begin to develop more clouds. In the great distance we set yet another redshift record, seemingly pinned down the Hubble constant, improved our knowledge of the age of the Universe, and perhaps even detected its acceleration. In between we reveled in discoveries about new planetary systems that will someday relate back to our own.

Hazards

Thanks. What a delight to be here once again.

I would have loved to have begun this talk with danger, but there were no serious threats from impacts as there have been in preceding years. The greatest danger now seems to be from having to predict what will happen with this year's Leonid shower! Last year's at least provided some nifty fireballs. A defense satellite even saw the streaks of Leonids from above, clearly showing their parallel tracks, which are not evident from the ground. The Leonids are nothing, however, compared with the danger to astronomy from flares and radio emission from the Iridium satellites. At least Motorola has agreed to help the radio astronomers.

The greatest danger is perhaps from an ever-increasing amount of new data from the new telescopes that are, and will be, coming on line (at least if asteroids and errant satellites don't get us first). How ever will we keep up with all the information?!

Telescopes, telescopes, telescopes

It's Yahoo for Subaru. The Japanese 8-meter (actually, at 8.3 meters, the biggest monolith around) is open for business. Here we see a galaxy group 300 million light years away in Hydra, the telescope equipped with a half-degree CCD. Then it's on to Gemini north, which while not to be completed until 2002, saw first light. With its adaptive optics, it will resolve to 0.08 arcseconds in the infrared. And wait until you see what is planned: a hundred-meter! The OWL. Well, the term Very Large Telescope is used for the quartet in Chile, so the OWL is for the "Overwhelmingly Large Telescope!" Where do we go from there in both size and names?

Out in space, Chandra saw first light in its high orbit. This huge satellite will provide exquisite X-ray images, as it did here for the supernova remnant Cassiopeia A. The bright point in the middle may be the neutron star, the stellar remains of the supernova.

At the other end of the spectrum, plans are moving forward for the 1HT -- the "one hectare telescope" that uses multiple small dishes and will be used both for SETI and for straight radio astronomy as we build 'em bigger and build 'em cheaper at the same time.

The Moon

And what will we observe with them? Not the Moon of course, which is best explored directly by satellite and probe. Lunar scientists hoped to see more evidence for water by crashing the Lunar Prospector and looking for the vapor. Not too surprisingly, none was found, though that does not damage the idea of polar ice. Not only does our satellite have some water, but it also now seems to have the long-sought iron core, as determined from gravity measures. It may be only 600-900 km in diameter, and only about 2-3% of the lunar mass, but it is there and supportive of the collision theory for lunar origin.

More directly important for you might be that the old "blue moon" mystery has been solved. The term was originally used in the Maine Farmer's Almanac for a fourth Moon in an ecclesiastical season. The usual three had popular and descriptive names, but the occasional fourth did not. Sky and Telescope mistranslated the idea into a second full Moon in a month. At least now we can answer the public question as to what it means.

The Sun

The Moon of course is illuminated by the Sun (the best segue I could come up with). The solar disk is covered with small convective granules, but there is also a larger superimposed "supergranule" pattern. The high-speed solar wind seems to emerge from the edges of the supergranules.

We also seem finally to be able to predict coronal mass ejections, from the prior appearance on the solar surface of s- shaped figures called "sigmoids." And we had better be able to do so, because space weather, the particle and high energy radiation environment created by the Sun, will be bad in the coming years because of the solar cycle maximum.

The Planets

(All of which are deep within the solar wind.) But first how about that great planetary lineup in the spring! What a wonderful sight that was. Of all the planets, however, it was Mars that got the greatest attention, Mars and the MGS. MOLA, the Mars Orbiter Laser Altimeter, managed the best surveying of the planet to date, with accurate elevations. Hellas is clearly seen to be the biggest impact basin known, 4000 km across, 9 km deep, bigger than the lunar Aitken basin at the Moon's south pole, and located on an 8-km-high southern plain. Mars also displays evidence for continuous running water, and a more reasonable shoreline has been found for the purported ancient northern ocean. Hubble also imaged the planet and found evidence for hydrated rock. At the poles, permanent ice (incuding both water and dry ice) sits on the plateaus, but nowhere near enough water ice to cover the planet with a deep ocean. Nobody really knows where the water went. But if one is lost, another is found, as the planet really does seem to have an iron core, perhaps 2000 km in radius. Though Mars now has no magnetic field or tectonic processes, magnetic striping similar to that found in the Atlantic basin has been seen, suggestive of activity similar to our sea-floor spreading. The planet on the whole looks more and more earthlike.

Even Mars's little moons get in the act. The temperature of Phobos has been measured on both its sunlit (200 K) and dark (161 K) sides. The satellite is blanketed with three-foot-thick dusty debris from itself. Small meteoroids continually hit it, and with Phobos's low gravity, knock dust into orbit around Mars. Phobos then sweeps it up.

Off to the outer Solar System and Jupiter, in which we now see towering thunderhead clouds 1000 km across and lightening that dwarfs our own. Jupiter's faint ring also now rather clearly seems to come from particles blasted off the small inner four satellites, the particles collectively going into temporary orbit. Moreover, the inner big moon, Io, has aurorae, not just at the poles, but also where the giant current ring connects it with Jupiter. The satellite is also seen to be quite hot, 1200- 1700 Kelvin in the flowing volcanic ejecta, sulfur piled everywhere. Even the "atmosphere" (such as it is) is sulfur dioxide. Terribly thin atmospheres have been found around the other satellites too, carbon dioxide for Europa and Ganymede, oxygen for Callisto (but at a trillionth the Earth's don't try to breathe it).

Moving farther out (and my apologies to those who suffered through this at MAPS), "the rain on Titain falls mainly as methane." Sorry. But Titan, Saturn's largest satellite, which rivals Ganymede in size, really may rain hydrocarbons from clouds that cover about ten percent of the surface, though seeing through the thick haze is difficult.

Uranus's weather seems to be stirring up as the Sun marches steadily toward its equator. Hubble is seeing more and more clouds. We are also seeing more satellites, the count up to 18. Neptune's weather is heating up too. Want a vacation? Triton has warmed to 39! From 38. Complaining about the heat? That's Kelvin, folks.

I think the most startling news about the planetary system involves the migration of planets. Recent orbital simulations show that in ancient times, as the planets gravitationally scattered the thick leftover debris of formation (comets and the like), they themselves were moved around. Jupiter migrated somewhat inward, and Saturn, Uranus, and Neptune moved outward. Neptune went from 22 to 30 AU and in the process picked up Pluto! The little planet is in a gravitational resonance with Neptune, and orbits three times for every two Neptunian years. Neptune also moved it into its present strange elliptical and eccentric path. This ancient activity goes strongly to the formation of other planetary systems and to the odd positionings of the planets now being found orbiting other stars.

Asteroids and stuff

The planets are hardly everything in the life of the Solar System. The NEAR spacecraft had a near miss (sorry again) with its target Eros because of a rocket problem, but will catch up with it again next year. The probe did image the asteroid, however, and found a 33 X 13 lump of rock with a density of 2.5 grams per cubic centimeter. Most fascinating, real liquid water (in VERY small amounts) has been found in meteorites. Liquid water must therefore present in at least some asteroids.

And who can forget Hale-Bopp, which is still merrily bopping along over 7 AU from the Sun and still kicking out volatiles such as carbon monoxide, methanol, HCN, and other good stuff. It contains argon, but no neon. From the condensation temperatures of these noble gases, it is possible to say something about the temperature at which the comet formed, between 20 and 35 Kelvin, which places its formation around Neptune, just that expected for it now to inhabit the Oort comet cloud (the outer planets having scattering the ancient comets to great distances).

Farther out yet, dedicated researchers are finding more and more objects in the Kuiper belt out around, and beyond, Pluto. The count is up to 200 and rising. Slim evidence in the perturbation of some comet orbits also re-raises the possibility of a larger, yet unseen, body that may inhabit the Solar System (but don't count on a real Planet X just yet).

Star and planet formation

Disks disks disks! Everywhere we seem to look now around new young stars we see dusty disks that are reminiscent of our planetary system, rather of the predecessor to our planetary system, the so-called "solar nebula." The disks should dissipate if planets form in them, and that is sort of what we see, as the youngest young stars, those under a million years old, have thicker disks than the older young stars. Rings set within a pair of the dusty disks also are suggestive of buried planets, the planets -- if they exist -- gravitationally sweeping or herding the grainy gas.

If we could just see the planets! We do see them in a way, of course, through their gravity, which shifts the parent stars back and forth along the line of sight. The count is now up to 21 below the 13 Jupiter-mass "limit" (not necessarily a limit of formation, but a planet-brown dwarf crossover limit where the interior becomes hot enough to fuse deuterium, heavy hydrogen, into helium). The 11 more above the limit may be brown dwarfs -- stars too small to run the full fusion chain -- but who knows.

One of new discoveries set a record with a Jupiter-like planet that has a period around its star of a mere 3.5 days! Another, Iota Herculis, has one with a period of a year. The close "jupiters" apparently are formed at a good distance from the planet and then migrate inward as a result of orbiting in the thick dusty gaseous disk. They stop migrating when the disk dissipates (say that fast 20 times). The biggest news in this subject perhaps is that Upsilon Andromedae appears to have a family of at least THREE planets that ranges from one with a mass greater than 0.7 of a Jupiter mass (with a period of 4.6 days) to a greater-than-4.6 Jupiter mass with a period of 3.5 years. (Since we do not know the inclinations of the orbits, we can get only lower limits to orbital velocities and masses). BUT Rho-1 Cancri has a dust shell in addition to a planet, and from the clear tilt of the shell we can get the inclination and a real mass of 1.9 Jupiters. In addition, good old Epsilon Eridani, only about a billion years old, has a nice dust shell about the size of our Solar System. Does it too have planets?

We can't wait to see them. We thought we had one, a seeming dim companion to TMRC-1 (a young star in the Taurus Molecular Cloud) connected by some kind of filament. Alas, the "discovery" evaporated, as the "planet" is just a background star, showing how hard such observations are to make.

Planets may be less than benign. Some stars, exemplified by Omicron Aquilae, may pop huge flares vastly larger than those seen on the Sun, flares that would be dangerous to life. Even the giant star Enif (Epsilon Pegasi) is suspected of having done so. Close planets may disturb the stars and make such activity possible -- pure speculation really, again showing our ignorance in a newly developing field.

Given giant planets, are there earths, and is there anything on them? SETI -- the "search for extraterrestrial intelligence" -- continues to gallop forward, one project developing ways to look for interstellar laser beams in case extraterrestrial technology is beyond mere radio. But it is still galloping into emptiness, as nothing of course has ever been found by any technique. I doubt it would be hidden on the back page of the newspaper if it were.

Stars

Never mind Y2K. Are you ready for L2K, for L stars? For T stars? For OBAFGKMLT? Add L and T to your teaching notes, because they are here. New infrared telescopes -- DENIS, 2MASS, and others -- are turning up hosts of faint red stars. Off the end of the classic main sequence, they are dominated by spectra not of oxides (titanium oxide strong in M stars), but of hydrides and alkali metals. To accommodate these stars, a new spectral class L, attached to the end of the classic century-old spectral sequence (OBAFGKM) and continuous with M, was born. The L stars have temperatures between around 1500 and 2000 Kelvin, L0 coming right after M9.5. The L stars seem to be a mixture of real stars at the end of the main sequence (which runs down to 0.08 solar masses, below which the full hydrogen proton-proton chain cannot run) and brown dwarfs, which (down to 13 Jupiters) can fuse only deuterium.

The first real brown dwarf found was REALLY cool, so cool, about 1000 Kelvin, that it has methane in its spectrum! Several similar stars have been found as well. Instead of the term "methane brown dwarf," "class T" is becoming popular for them. (In accord with the rest of the spectral sequence, the letters that indicate class do not, and are not supposed to, have anything to do with the features found in the spectra; they are meant to be neutral.) Here is a picture of an L dwarf. Here is another of a T dwarf. They look a lot alike don't they? There may be twice as many L and T dwarfs as M dwarfs (which make up 70% of the stars of the standard OBAFGKM sequence). Yet their masses are so small that they make no significant impact on the "dark matter problem," the problem that there is gravitating mass in the Galaxy that we cannot see (stars in the outer part of the Galaxy moving much too fast for the observed stars and nebulae).

There are two kinds of clusters in the Galaxy, open (like the Pleiades) and globular (M 13, Omega Centauri). All clusters evaporate under tidal action from the Galaxy, which strips off stars that have migrated to the clusters' outer parts. The open clusters dissolve quickly, so there are few old ones. But the globulars are so densely packed that they dissipate only slowly, so we still see original systems that were born at the Galaxy's beginnings. Here, though, is an example of such tidal stripping, in the globular cluster NGC 6712, which comes within 1000 light years of the powerful galactic center. The lower mass stars are leaving. But here, also close to the galactic center, are two newly-found spectacular young open clusters seen by Hubble. Ten thousand solar masses each, the "Arches cluster" has some ten percent of known massive Galactic stars! The more we look, the more we find, from the dimmest of stars to the brightest.

Stellar evolution

And just as these stars are all born and live their quiet (for the most part) main sequence lives, so too must they die. More and more we are leaning how planetary nebulae are formed, as new technology allows us to probe into the dusty shells around "protoplanetary nebulae," which are the old dusty ejecta of once- Mira variables. Within the protoplanetaries are heating stars that are little more than the old burned cores of main sequence and giant stars. The buried stars' high-speed winds will someday compress the ancient ejecta, and then when the interior stars are hot enough, they will ionize the surrounding gas to make the glowing rings of bright nebulae such as the Ring, the Helix, and myriad more. Most mysterious are the opposing high-speed jets that flow within the planetaries (look at the "ansae" on the Saturn Nebula, NGC 7009). No one still knows where they come from. But you can see planetary nebulae in all their glory in (ta-daaa) the new slide set produced for the Astronomical Society of the Pacific by Kaler and Bruce Balick (strictly non-profit by all concerned). Bruce did a masterful job of massaging Hubble images to out-do Hubble.

A different view, specifically of the Helix nebula, has been made with emissions from carbon monoxide, velocity information allowing the astronomers to pick it apart and see how it is constructed. We seem to be looking down the throat of a broad bipolar flow. The most amazing thing perhaps about this very old nebula (relative to other nebulae -- none of them are "old," the phase being quite ephemeral) is how both dust and molecules can survive the onslaught of radiation from the immensely hot star, which shines at around 125,000 Kelvin. I doubt we could do that!

The planetary nebulae, rather their central stars, can do some odd things. As Mira variables evolve and brighten, before they produce planetary nebulae, they undergo a series of internal "helium shell flashes," in which the helium-fusing shell that surrounds the deep carbon core violently turns on (later to fade away as the outer hydrogen-fusing shell begins to fuse to helium in a quieter fashion). As the condensed central stars of planetary nebulae finally cool to become white dwarfs, there lurks within a potential for a final helium-shell flash that can swell the star back into a giant again, allowing it to produce a planetary within a planetary. Abell 30 and 78 are the classic examples, and here is Sakurai's Object, caught in the act.

The events all seem puny compared with the results of evolution of the most massive stars. Eta Carinae stays in the news. This evolving star (or stars, it may be binary) began life at around 100 solar masses, about as much as a star can have. It keeps ejecting vast amounts of matter. The huge dusty lobes, made famous by the Hubble Telescope, were produced in the great outburst of the 1840s, in which Eta Car reached beyond first magnitude to become one of the brightest stars in the sky as it ejected a solar mass! A visibly smaller brightening event in 1890 (apparently as violent but partially hidden by dust) seems to have produced the "skirt" that splits the big lobes. We have actually been able to watch the nebula expand. It is not clear, however, whether we are dealing with one star or two. The nebula is nitrogen-rich, but the visible star is not, and some astronomers expect that the action is caused by a somewhat lesser but more evolved companion. If so each began life with over 100 solar masses. Whether single of double, watch out, as Eta Car is going to blow at some point as a supernova, perhaps one that will produce a black hole. Even now it is active, the star having brightened from below naked-eye vision to about fifth magnitude. The brilliance of the inner star or stars are hidden by the thick dust of the ejecta).

The most massive stars now seem to be the culprits in making black holes. The lesser massive stars (those over about 10-12 solar masses, but less than the really huge ones, and no one knows the cutoff) make neutron stars when their iron cores collapse to make the Type II supernovae. (Type Ia supernovae, made by white dwarfs, will surface later in this talk.) We've just passed a milestone, 1000 known pulsars, rotating neutron stars that spray immense tilted bipolar beams. Since we see pulsars only when they are energetic and only when their beams sweep past the Earth, there must be huge numbers of neutron stars around us.

The Crab pulsar makes the news once again, as its motion in space has been found to be along the axis of the nebula, evidence that may someday lead us to understand more about off-center blasts within the exploding supernovae that can kick pulsars to such high speeds that they can leave the Galactic disk. No one knows why a symmetrical star should produce an off-center detonation.

"Ordinary" pulsars pale beside the rarer "magnetars," in which the magnetic field strengths can reach 100 trillion times that of Earth's. Last August, the radiation from an internal adjustment on one such magnetar (SGR 1900+14 in Aquila), a "soft gamma ray repeater" that also produces X-ray pulses, and which is 20,000 light years away, actually set the electronics of Earth satellites reeling and partially ionized the Earth's upper atmosphere. Are we or are we not in the cosmic environment? Don't get too close to one of these anytime in the next few billion years!

Gamma rays

Increasing the violence yet more, gamma ray bursts, which occur about once a day from deep in the cosmos, remain mysterious. One, that took place in a distant galaxy with a redshift (see below) of 1.6, a galaxy nine billion light years away, was not only recorded visually but hit ninth magnitude. You could have seen it with large binoculars. Neutron star mergers, one of many theories, have been suspected. A burst 10 billion light years away was too energetic even for that unless the radiation is somehow beamed, that is, the energy concentrated into tight flows rather than broadcast spherically into all of space. Beaming reduces the energy requirements. The bursts may also be produced in neutron stars by the release of powerfully wound-up magnetic fields. Or not. At least we now know where the gamma ray burst progenitors are, even if we do not yet know what causes the bursts themselves. What would astronomy be without mystery?

But remember hypernovae? Gamma ray bursts are thought by some to be caused by ultra-energetic supernovae. The collapse of rapidly rotating stellar cores drag in matter around them to create black holes, and in the process generate huge quantities of gamma rays. In support, one old supernova has near-light- speed ejecta. Newly found supernova remnants in the spiral galaxy M 101 in Ursa Major, 800 light years across, also seem to contain 10 times more energy than "ordinary" supernova remnants. Hypernovae may really exist. Whatever will we call anything greater? Overwhelminglyhypernovae? Yet other astronomers think that not even hypernovae are energetic enough to make the gamma ray bursts either, again unless the bursts are beamed.

The Galaxy

Though the gamma ray bursts are at cosmic distances, their origins in a sense must lie close to home, among the same kinds of objects that are found within our own Galaxy, so by returning here and studying the stars and other objects of our own system we may someday understand how the bursts are made.

The Milky Way Galaxy seems like a relatively quiet place. Nevertheless, there is spectacular activity at its center, as seen here in this 90-cm radio-wave image taken with the Very Large Array in New Mexico. The view includes orbiting gas, supernova remnants, and the center of the Galaxy itself, Sagittarius A (one of the brightest radio sources in the sky and one of the first discovered).

At the center of Sagittarius A is a pointlike source, Sagittarius A* (A-star), which is most likely the actual Galactic nucleus itself. It is thought to be a million-plus solar mass black hole illuminated by matter falling into it and simultaneously ejected from its immediate environs. Sagittarius A* seems finally to have been resolved from its previous pointlike nature, at 0.4 thousandths of a second of arc, which at its distance of over 25,000 light years translates into 3.6 astronomical units.

Galaxies and dark matter

Large galaxies all seem to have such bright, pointlike nuclei. That belonging to the Andromeda Galaxy, M 31, appears unique, as it is doubled, two bright sources shining at the galactic center. The problem seems to have been resolved. The bright sources are made of an elliptical ring of stars. We see them piling up where the ellipse is farthest from the purported black hole ("apoblackholicum") and where they whip the fastest at the point closest to the black hole ("periblackholicum"). I don't know how important this is, but it gave me the chance to make up some nifty words that, who knows, might catch on.

Hubble has been focussing on galactic bulges, on where they come from and when. There seem to be two scenarios. The bulges of "early" galaxies, in the jargon those with tightly wound arms and large bulges, formed quickly, along with the galaxy. The bulges of "late" galaxies, those with open arms and smaller bulges, were built up slowly by means of the action of a central bar. (The terms "early" and "late" have nothing to do with age or evolution, but with the positions of the galaxies in the famed Hubble classification diagram).

Dark matter remains, well, dark, mysterious, even though several candidates can be rejected (such as brown dwarfs). The existence of dark matter in galaxies is known through the motions of the galaxies' bright matter, which moves too fast, and is known within clusters of galaxies through the movement of the individual galaxies relative to the common center of mass. Smaller galaxies seem to have relatively more dark matter. Are there then near-totally dark galaxies that are made of little else, galaxies we can't see?

The Universe

What would a GLPA talk be without a new redshift record? The "redshift," "z," tells to what degree the spectrum lines of a galaxy are shifted to the red end of the spectrum due to the expansion of the Universe. (The factor by which spectrum lines are shifted to the red is z+1. If the wavelengths are increased by a factor of 1.6, then the redshift is 0.6. "z+1" is also the factor by which the age of the Universe has increased since the light left the distant galaxy). New technologies allow us to keep looking farther and farther back into time. I first had z = 5.6 in my notes, and then came along an even bigger record, a whopping z = 6.68. All others move over. The galaxy is relatively bright, suggesting early and rapid star formation, a general theme now in looking at the most distant galaxies. Star formation came along very early indeed. Even the quasars set a record, this one depicted here having a redshift of z = 5.

If you adopt a uniform expansion (or for that matter, a known expansion) of the Universe, you can turn the problem around and use galaxy redshifts to indicate galaxy distances. Surveys to plot the Universe continue to bound forward. Here is a partial result from the AAT (Anglo-Australian Telescope) survey with 40,000 redshifts, showing what has been seen before, a filamentary structure in which galaxies and their clusters fall into clumps and walls. The survey will eventually include 250,000 galaxies and will be compared with the results from cosmic background radiation observations (the "cosmic background radiation" the remnant radiation from the Big Bang fireball). Such surveys require multiple-object spectrographs and dedicated telescopes.

Perhaps, just perhaps, we are zeroing in on some of the nature of the Universe. Many astronomers are convinced of it. One key has been supernovae that come from white dwarfs, the so- called "Type Ia" supernovae. (The scenario involves a white dwarf in close mutual orbit with another star from which it tidally feeds. If the deposited matter sends the white dwarf over the "Chandrasekhar limit" at 1.4 solar masses, the white dwarf explodes; merging orbiting white dwarfs could produce the same effect.) These supernovae all seem to have similar maximum brightnesses, and therefore serve as "standard candles" in the quest to measure galaxy distances. Type Ia supernovae are so bright that they can be seen for immense distances and to redshifts that are large enough that local movement due to local gravity effects are not important. The brightnesses of Type Ia supernovae are calibrated through one of the Hubble "key projects," in which distances to closer galaxies (some of which have displayed the supernovae) are measured from Cepheid variable stars. The most distant Type Ia supernovae are too faint for their redshifts, suggesting (with the aid of theory) that the Universe was expanding more slowly in the distant past. The implication therefore is that the expansion has sped up. To explain an acceleration one needs to invoke Einstein's old idea of an expansive "cosmological force" (which he once called his greatest mistake). We may be dealing with the positive pressure of the vacuum itself. The idea fits the concept of inflation (in which early the Universe expanded exponentially and flattened itself). Indications now are that the age of the Universe, at perhaps 14 or so billion years, is reconciled with the age of the globular clusters.

The local expansion rate, the Hubble constant, is now in good shape too. The two "definitive" values from two teams, though different, have overlapping error bars. One team gets 70 km/sec/megaparsec, the other (Sandage's) finds 58, both to 10%. They overlap at 64. Even this is too high though to reconcile with a closed Universe.

The view now is that the Universe is flat and Euclidean with a positive cosmological constant. But there are still doubts about the acceleration. We may not know as much about Type Ia supernovae as we think; maybe they were somewhat different in the distant past, to which we look at such great distances. Another suggestion is that "gray dust," dust that does not much change the color of distant objects, is fooling us into thinking that the Universe is accelerating. Another group suggests that Cepheid distances may not be as good as think. The problem, if it is one, is with M 106, in which water masers that orbit near the galaxy's center give a distance (derived from the comparison of their movements across the line of sight with their radial velocities) 20% closer than do the Cepheids. Clearly the discrepancies should be reconciled with more and better observations (providing significant employment opportunities in astronomy!) The most promising of cosmic ventures involves precise measurement of the fluctuations in the cosmic background radiation, which will give hosts of needed parameters, allowing us to attack the problem from both ends, from the galaxies themselves and from the primitive fluctuations in the primordial soup that eventually made them.

Caution, as always, is advised. Though the accumulation of galaxies is thought to be controlled by huge amounts of "cold dark matter," we have little clue as to what it even is. We do know, however, that stars came along very fast and that early galaxies are cauldrons of star formation that built metals very quickly. A look into the vast distance in the Hubble NICMOS deep field shows some galaxies that might be 12 billion light years away, in forms and shapes that, though still dimly seen, are not understood.

We seem to know a great deal, yet at the same time seem to be so ignorant about all that is out there. One thing we know is where to find the Center of the Universe, which is written right here on this water tank in Philo Illinois. One thing that we really DO know is that many surprises await us as we probe deeper. Another is that we are a grand part of it all, that the Universe not isolated from us. It is as much a part of us as we are a part of it.

Thank you all once again for the opportunity to wander through the year's events in wonderfully exciting science.

Questions.

[I know they are using occulting disks for the extra solar planets, but about how many arc seconds out are these planets from the stars?]

Jupiter would be about four seconds of arc out if it were orbiting Alpha Centauri. That's easy to resolve. An amateur could go out and separate them. The problem is the contrast between the dim planet and the brilliant star. In addition, these stars are farther away and their planets closer. So it's impossible right now. The Next Generation Space Telescope might do the job. It also helps to go into the infrared where the planet gets brighter and the star (if solar type) fainter.

[You talked about a white dwarf that reignited a helium burst and produced a second planetary nebula. How did it get the mass to re-ignite?]

Nuclear burning doesn't entirely shut down in white dwarfs. Most nuclei of planetary nebulae, which eventually turn into the white dwarfs, come off the second-ascent giant branch (the AGB, along which stars have carbon-oxygen cores) as hydrogen burners, but some of them might be helium burners. In a second ascent giant star a helium-burning shell is nested inside a hydrogen-burning shell, and they turn on and off in sequence. When the overburden gets removed, you can therefore have a helium-burning central star or a hydrogen-burning central star. The residual envelope of a developing planetary nucleus is eaten away by nuclear fusion from below and by winds at the top, and therefore heats and eventually lights its nebula. A hydrogen-burning shell adds helium to the inner helium-rich shell. After the star turns the bend and begins to cool, you may then have a critical mass in the helium shell, which has the potential to re-ignite, which expands the star and sends it back to the giant branch where it can lose more matter and create a smaller planetary nebula. The evidence is that in the interiors of these "born-again" planetaries, the new nebula has no hydrogen, since the matter is coming from a star that has already lost its hydrogen-rich envelope. I think five of these are now known.

[Would a brown dwarf would be in the L and T spectral classifications you were talking about?]

Brown dwarfs radiate as a result of gravitational contraction; they are also deuterium burners, the deuterium coming from the Big Bang before the star formed. Deuterium burns at a million Kelvin or so; the process starts at the second point in the proton-proton chain. As a result, brown dwarfs can be reasonably bright (about the same as the lowest mass stars). They therefore mix in with real stars in class L. Class T should contain nothing but brown dwarfs. It is important to emphasize that the spectral classes are based not on physical characteristics but strictly upon the appearance of the spectra; M stars are characterized by TiO bands. You can't have an M star without them. At lower temperatures, the titanium actually precipitates into solid grains. But in class L, hydrides become very strong, so you have to create another spectral category. In class T you see methane bands. Physical characteristics come later. The beauty of the spectral classification system, which is over a hundred years old, is that it is continuously expandable. And think of the fun you can have in your classes making up new mnemonics!

Once again, thank you very much.