James B. Kaler

Department of Astronomy, University of Illinois

First published in the Proceedings of the 34th Annual GLPA Conference, Nashville, IN, October 21-24, 1998. Reprinted by permission.


The best news is that we will not be destroyed soon by an asteroid. Beyond that, we reveled in ice on the Moon, water on Mars, Galileo discoveries, bodies outside the orbit of Neptune, and newly-found planets orbiting other stars. We saw the creation of a lovely gallery of planetary nebulae by Hubble, a possible explanation of gamma ray bursts, and viewed farther into the Universe with more understanding, resulting in a feeling that we are beginning to understand some of its structure.

Asteroids and impacts

I've usually begun with a set of highlights, but I thought I'd revert and start with the Earth, because this year WE were special. Remember the dreaded 1997XF11? This mile-wide asteroid was first calculated to come within 45,000 kilometers. The orbital errors included the Earth. How wonderful! It was supposed to come in 2028, at which point I would be ninety years old. I would even be able to see it go by in the sky with the naked eye, or bow out of this Earth in absolutely glorious fashion. However, a recalculation showed that it's going to miss by about a million kilometers; it's just another boring old asteroid. But the incident showed that the asteroids are out there and that they are going to do some interesting things to us over the years.

One of the other interesting discoveries was the finding of a different kind of asteroid, 1998DK36, which is nowhere near as dramatic a name. (Anything that has X in it always sounds dangerous). There are three classes of dangerous asteroids that either approach or cross the orbit of the Earth. This one orbits entirely inside our own. We have no idea how many there are, but the class increases the number of potentially dangerous asteroids. Here is the latest example of a correlation between mass extinctions and asteroid impacts. If you know any statistics, you know that if you take two aperiodic functions you will commonly find some relation. I think this one is highly suspect, but then again, maybe it does happen. That's the state of the art, which isn't much of an art at this point. We're also discovering that our nuclear weapons may not blast asteroids apart, especially if they're rubble piles that just absorb the impact. We're not entirely sure we're going to be able to get rid of them, at least all that easily.

Why would such mass extinctions occur? There have been many theories as to why periodic infalls of comets occur. From a few years ago you may remember "Death Star" or "Nemesis," wherein the Sun was thought to be orbited by a small red dwarf star that periodically went through the Oort Comet Cloud and brought showers of destructive comets upon us. Nobody could find the thing and it probably doesn't exist. Last year I commented that tidal forces raised by the Galaxy could disturb Oort-Cloud comets. Today it is that the movement of the Sun in and out of the Galaxy's spiral arms may cause the periodic infalls. The spiral arms are filled with giant molecular clouds. They are the most massive things in the Galaxy, with masses up to a hundred thousand times that of the Sun. Every few tens of millions of years, they could produce tidal interactions that disturb the Oort Cloud and send in rains of comets. The giant molecular cloud theory has been around for quite awhile, but it's the movement of the Sun in and out of the spiral arms, where they reside, that may in fact do it. That's beginning to make some sense. The Sun would have moved in and out of the spiral arms in its 250 million year orbit around the center of the Galaxy. This is a part of one message I've tried to get across to students over the years. We're not an isolated species. We're a part of the whole Galaxy and cannot separate ourselves from it.

If you want a real hazard, try predicting the Leonid meteor shower! You have three choices. You can say it's going to come and then it doesn't, you can say it's not going to come and then it does, or you can just waffle on it and say "I have no idea." In any case, you are going to get a certain segment of the population rather annoyed with you. This is a wood cut from the 1833 shower, the greatest one of all time. I think the current estimate is 100,000 meteors per hour. The 1866 shower was predicted and it came pretty much on time. The 1900 shower fizzled in spite of a great deal of press coverage. People disdained astronomers after that for several years. It was the "Comet Kohoutek" at the turn of the century. Charles P. Olivier, the dean of American meteor scientists, commented that the lack of the 1900 shower was "the worst disaster ever to befall American astronomy" because of the public reaction. I have no idea if it is going to hit again. You might want to get up early on the 17th or 18th. For us it occurs at 1:00 PM central time on the day of the 17th. There are two models: one says we're going to get a lot of meteors, the other one says we're not going to get any. So just get up and see for yourself. Odds are probably against it, but I didn't say that.

New telescopes

This beautiful picture is of the Hobby-Eberly Telescope at the McDonald Observatory in Texas. It's a 9.3 meter meridian spectrographic telescope and an el-cheapo system. They can move the detectors to follow a star for about an hour as it culminates. It's expected to take huge numbers of spectra over the next several years. In fact, they're building a companion to it in the Southern Hemisphere. It's one of those telescopes that does not get a lot of press because the public and media do not understand spectra very well, which of course are the heart and soul of all astronomy.

The digital sky survey is up and running. It will survey a quarter of the sky in the next five years and will do spectra on bad nights. Over the next several years it should take spectra of somewhere around a million galaxies and 100,000 quasars to obtain red shifts, and will revolutionize the survey industry. Here's an example, the beautiful spiral NGC 6070. The survey will have much better resolution and depth than was obtainable photographically. But the older photo surveys will not be passe, because they recorded the sky at a specific time and will always be useful.

Astronomers have such imaginations. This magnificent instrument is being constructed in the Southern Hemisphere by the Europeans. It's a very large telescope, so what do they call it? The VLT, the Very Large Telescope. There are actually four 8-meters in kind of an L-shape. They will not only feed their combined light into one point for the combined effect of a 16-meter telescope, but they can all be used separately. Ultimately, they will be used as a multiple interferometer to give very high resolution. This will undoubtedly be the best instrumental set-up in the world. They've seen first light with one of the telescopes, here a beautiful bi-polar nebula. The system should be completed by 2002.

The Moon

Somebody said to me, said, "Looking forward to your talk. Are you going to tell us if there's water on the Moon?" The answer to whether there is water on the Moon is "Yes, No, Yes, No, Yes, No, Yes." the Lunar Prospector contains a neutron spectrometer that can detecting hydrogen. The argument is whether the hydrogen is in water or is free, captured from the solar wind. The latest is that it really is water, buried in the regolith at the poles in shadowed craters, up to a few billion metric tons of it. It probably accumulates from cometary impacts. In going back to the Moon we don't have to drag the water along with us, and we should be able to use it for rocket fuel. You don't go pouring water in your gas tank, hoping to get anywhere, but you can break it down into hydrogen and oxygen with sunlight. There's probably water on the poles of Mercury for similar reasons. But not much. Though water is ubiquitous in the planetary system, the inner part is still pretty dry. The oceans are not much more than a film on the surface of the Earth.

The other interesting thing about the Moon is that they're still working with Apollo moon rocks and have used tungsten and hafnium isotopes to date the Moon at 4.51 billion years old. I'm just amazed at the precision. From meteorites, the best estimate we now have of the age of the Solar System are 4.53 to 4.54 billion years, so the Moon formed within a few tens of millions of years after the Earth developed. It was a very quick thing, still believed to be caused by an impact between a large protoplanet and the Earth, the debris forming the Moon. The theory is still contended, but still seems to be the one that holds up the best.

The Moon, of course, is responsible for producing this beautiful eclipse of the Sun. Of course there's lots of science that will come out of it. I just want you to look at it and feel very sorry for me because this crossed the island of Antigua, and up until six months before the eclipse, my daughter lived there. Then she moved to Chicago, so I had no place to stay.

The Sun

You know the old line, "The King is Dead, Long Live the King, The King Lives". It reminds me of SOHO, the solar satellite that's in one of the Earth's Lagrangian points. It's done a wonderful job of continuous coverage of the Sun. SOHO died, went out of control, and they brought it back as of a week or two ago. It gives testimony to marvelous NASA engineering.

This picture looks like a ProAm tournament played at night with radium golf balls. In fact, it is a dramatic rendering of the so-called "magnetic carpet" of the surface of the Sun. We see a few Sunspots here and there. SOHO discovered this immensely dense network of low-level magnetic flux lines that exit the Sun and that keep popping up and going away. We think now that it is the collapse of these enormous numbers of flux tubes, the "magnetic carpet" of the Sun, that produces the heat of the corona.

The Sun rings like a bell. It oscillates with a principal five minute period. But, there are dozens, hundreds, of other periods that are involved. You can use a theory for the internal structure of the Sun, which changes as a result of solar evolution, and then use the vibrations of the Sun to determine its age, and you come up with 4.5 billion years. All ways of measuring the Solar System's age give consistent values, whether using meteorites, the Sun, the Moon, even the Earth, factoring in the cooling time. The oscillations seem to be driven by intense downdrafts between the bright granules.

There has been an increase of 0.04% in the solar luminosity over the last decade. This raises a problem. If you haven't been confronted with it before in public speaking, you probably will be at some time. What is causing the heating of the Earth? It's clear that the Earth's temperature has gone up over the last century. Is it the greenhouse effect caused by ejection of greenhouse gases into the air? Is it the Sun? The oil companies would really like it to be the Sun. A letter circulated by a well- known scientist that pretended to be a scientific article and promoted by the oil industry suggested that global warming was all caused by the Sun. Nobody really knows. It may be a combination of the two. The Earth has suffered times of great heating in the past, probably as a result of solar activity. But nobody really knows. The issue is politically charged and will continue to be. We'd hate to assume that the cause is just the Sun years down the line be wrong. Then what do you do with the CO2?

The Sun has taught us a great deal about basic physics. This almost looks like abstract art, but is a picture of Super Kamiokande Japan, the detector of solar and other neutrinos. There are billions of them passing through you right now from the Sun, and it doesn't matter. In the middle of the night, middle of the day, you get the same number. The Earth is transparent as glass to them. We still have the old problem that we find only about half the number of expected solar neutrinos, these a by-product of thermonuclear fusion in the Sun's core. It has long been suspected that the problem is with the neutrino, not the Sun. It looks like neutrinos do, in fact, have mass, which allows them to change from one kind that is detectable to another kind that is not. Super Kamiokande, for the first time, has measured the masses of neutrinos produced in the Earth's upper atmosphere by impacts of cosmic rays. The Sun therefore was the first to show us something about this basic property of the neutrino. It's important because the Universe is filled with neutrinos as a result of the Big Bang, and they may add considerable mass to the Universe as so-called hot dark matter.

Planets and satellites

Move outward to the planets. The big news about Mercury and Venus is that they were in conjunction with each other not too long ago. I hope everybody saw it. I don't know what this means astrologically. Maybe it means you're going to fall in love pretty fast.

Move right along to the god of War. Global Surveyor shows us Pathfinder's location. You can't quite see it. But there are the Twin Peaks. The Pathfinder scientists continue to manipulate the images. There's the South Peak again at very high resolution. You can see a water terrace, or what appear to be water terraces. Almost all of the data that have come back imply water, water, liquid water. Liquid water is not there anymore; whatever water exists is locked in the polar caps or is in the form of subterranean ice. It was certainly there at one time. Then you have the picture of a big volcanic rock, so there's been clearly volcanic activity, which we all knew of course. You are looking here at Valles Marineris with the Orbiter, and you see rock terraces. It looks for all the world as if they're sedimentary and carved out, or maybe it's just volcanic layering, nobody really knows. Mars is clearly a body that was very active geologically, mostly in the dim, distant past.

Here we are looking into what appears to be a lake bed with playa deposits, chemical deposits. You see this sort of thing on Earth, implying that the bed was filled with water at one point. Here is an impact crater made on a frozen bed of some sort. The impact liquefied the ice and it ran. These cracks here are taken as little erosion channels coming from the water off the crater. Again, it's just water, water, water everywhere. Is there life? That seems to be what everybody wants to know. We don't really know. The meteorite ALH84001 has been pretty much debunked now except by the people who discovered the alleged life forms. The features within just seem to have been caused by geological processes. So many times discoveries are announced in the papers and then they evaporate and are never really mentioned again. The media do not like to talk about discoveries that become undiscovered. Water aside, Mars still looks so much like the Earth in many ways; here, for example, we see wind-blown sand dunes.

The Galileo craft continues to function. Here is a picture of Io with its sulfurous deposits. Between two visits of the Galileo craft, this new feature developed, which shows how active the body is. It appears to be a volcanic flow with very high silicon content, or a plume. The sulfur volcanoes, of course, are not pure sulfur, but silicate volcanoes with a high sulfur content. Nobody's going to land here. Not only are there active volcanoes, but the radiation level from Jupiter's radiation belts is somewhere far beyond the lethal level for a human being. I can't imagine a more dangerous place in the Solar system, other than the Sun.

Here is Europa, which apparently has an ocean beneath the ice. Where there's an ocean, there has to be life, right? (Or so the fund-raising goes). Some of these deposits have the reflection characteristics of salt, so there's some indication that there is a briny ocean of some sort. To those who speculate, there's maybe more evidence that, in fact, there is life on Europa, but I wouldn't bet the farm on that. It's a pretty dangerous place, also within the radiation belts.

You're looking at Callisto, which everybody thought was a dead body, but the gravity measurements show that it's structured body. This may be an ice bulge produced from the inside. It's certainly icy out there. Callisto has a great deal of ice. There's suggestions now that even Callisto is magnetically heated and also may contain a warm water ocean underneath. It's far enough away from Jupiter that you might actually be able to land on it. Certainly you would be able to send a robotic craft there at some time and bring back a little piece.

Farther out now to the planet Uranus. Not that it's Earth-shaking news, but the little critter has more moons than we thought. It's probably got even more than that. We've just discovered numbers number 16 and 17. They're not very big, 80 and 160 kilometers across, in retrograde orbits maybe ten million kilometers from the planet. They are probably captured bodies. Uranus probably has many more moons than this. We have evidence of minor bodies, captures and collisions all throughout the planetary system. Do you remember reading about the "discovery of new rings around Jupiter," or the origins of them? They seem to be produced by collisions between small bodies and the inner satellites. That's probably what the rings of the planet Uranus and Neptune are all about as well.

Don't take this too literally. This is not going to happen over the next several Mercury orbits, but there have been suggestions that funny things could happen in the Solar System that involve examinations of its stability. This is one possible scenario of the Solar system losing planets. The orbits are chaotic in nature. You shift the Earth a millimeter, and after a hundred thousand years, Jupiter and Pluto are some place else. It's that sensitive. It's conceivable, at least, that if you continue to slowly change the eccentricity of Mercury's orbit, which is pretty eccentric already. The orbit reaches a critical point at which it starts changing very quickly, zooms by Venus, and is ejected from the planetary system. It's not that it's going to happen tomorrow and may not happen at all, but it does go to the possibility that the Solar System at one time had more planets than it does now. One astronomer has actually suggested that planets ejected from other stars are a major constituent of the dark matter that controls the speeds of stars around the center of the Galaxy. That seems to be a bit of a stretch, but hold this thought. We will come back to this at a later time.

Comets and asteroids

I saw a beautiful picture of Hale Bopp last night: Hale Bopp in Russia. Here we're passing through the orbital plane of the comet, and see the dust sheet, which the comet ejects, edge on. It looks like a spike headed down toward the Sun. As much as anything I wanted to show this because it looks a lot like the first comet I ever saw, Arend-Roland of 1957. Everybody remember that one? Hale-Bopp was still ejecting an awful lot of dust. The theoreticians have explained the X-rays from comets. They arise from charge exchange between cometary water and heavy ions in the solar wind. Who would ever have expected anything like that? Nobody ever predicted it.

This is a picture Rob Landis produced. You can actually image the asteroid Ceres with the Hubble Space Telescope. You are seeing it rotating, the spots on it moving around. We've also been able to make better measurements of the masses of asteroids. They're about a percent of the Moon. Ceres's is about one percent that of the Moon, Pallas and Vesta about half a percent. Ceres's density is only about two grams per cubic centimeter. The outer satellites of Jupiter are around 1.9. That implies there's a lot of water in these asteroids. Rob commented that it might provide the possibility of sending a spacecraft there. It's certainly a good step along the way toward exploring the outer planetary system. Several other asteroids have had their densities measured as well. They range from this to about four or so grams per cubic centimeter for Pallas and Vesta, showing that the asteroids are not just one set of bodies but consist of many different kinds of bodies whose orbits are separated, yet all mixed up at the same time. Jupiter keeps them pretty well stirred up. The asteroids out beyond the main belt toward Jupiter, however, are different from those in toward the Sun.

Speaking of asteroids, these are all by-products of images made by the Hubble Space Telescope. Someone started searching for asteroid tracks and lo and behold they found truckloads of the darn things. The paths appear curved because of the motion of the spacecraft in orbit around the Earth during the time of the exposure. You can pick them out right away. They estimate that somewhere around 300,000 of these things that would be detectable by the Hubble Space Telescope. The last count I had on asteroids is the detection of somewhere around 100,000, with known orbits now on 40,000. The rate goes up at about 5,000 per year as a result of the dedicated asteroid surveys that are good enough to detect a body not much more than 10 meters across if it passes between the Earth and the moon. The sizes of the asteroids are "going down," shrinking down the size of meteoroids, which of course, are really just small asteroids with a different name.

There are many groups of minor bodies. The asteroid belt, of course, is tucked way in here. It's kept stirred up by Jupiter. It was really a set of bodies that were never allowed to develop into a planet as a result of the gravity of Jupiter, at least so we think. We're also recovering many more cometary-like bodies. For the past several years I've looked at the increasing number of Kuiper Belt objects. The Kuiper Belt is a thick disk of icy bodies out beyond Neptune that seem to feed the periodic comets. Comets like Temple-Tuttle, the one that produces the Leonid meteor shower, is in a retrograde orbit and probably don't fit. But that's neither here nor there.

The number of Kuiper Belt objects is now up to 60. They fall into two groups, those with red and blue colors. The red color seems to come from carbon compounds that have been altered by solar ultraviolet light. The colors don't seem to relate to orbital groups. Nobody seems to understand what's going on out there. A good fraction, maybe a third of them, have pretty much the same orbit as Pluto and are now known as plutinos. Pluto is just the biggest of them, and Pluto should properly be considered a Kuiper Belt object rather than a planet in its own right, or maybe a crossover body. You get into semantics with this, which is probably less than useful.

Star formation

These planets were produced out of a spinning disk of gas that was in orbit around the early Sun, the idea introduced a couple hundred years ago by Immanuel Kant. It's amazing how a prediction that old has held up with time, because that's apparently about what happened, except that the planets accumulated not so much from gas but from dust particles, the giant planets then attracting the gas. Within only a few tens of million years you've got yourself some planets. All of that is supported by the fact that we do see disks around new stars. This is HK Tauri, a T Tauri star, a brand new star with flaring in the ultraviolet as gas crashes onto its surface as the star accumulates mass from its surroundings. Many of these are surrounded by disks. HK Tau provides a beautiful example of a such a disk. You can almost imagine planets forming in there as long as the disk is not disrupted in some way or other.

Stars form whole out of the interstellar medium. Planets are accumulated upward out of dust from a circumstellar environment. Then there are the things in the middle: the brown dwarfs, which according to current theory, formed whole out of interstellar matter but don't have enough mass to radiate like a star, that is, they cannot turn on the full thermonuclear fusion chain. Astronomers have been looking for them for the last fifteen or twenty years with great numbers of discoveries and then undiscoveries. The first one was recently found orbiting Gliese 229. With a surface temperature of only 1000 Kelvin, it had to be a brown dwarf, although the mass itself has not been measured. However, we now have some superb infrared instruments that are beginning to pick them up. There's not much doubt that brown dwarfs exist. This is a picture taken with the instrument called 2Mass, which is designed to look in the infrared for faint stars. Here is what appears to be a brown dwarf, seen in the infrared but not even there in the optical.

A whole new spectral classification was invented for these. You all teach OBAFGKM for spectra classes, and now we can add spectral class L, an unused letter. Actually the original Harvard classification of 100 years ago had a class L, but it got merged into F, so was not used long. Remember the M stars? They have titanium oxide bands, but when you get down into the L stars, they go away as the temperature is so cool. These are probably all brown dwarfs, not real stars, although it's really hard to know; there's going to be an overlap. We don't really know how brown dwarfs overlap planets. The two are built by different processes and there is also a dividing line of 13 times the mass of Jupiter, at which point you can no longer fuse the natural deuterium in a brown dwarf, which powers the thing for a little while. Are there submassive brown dwarfs below 13 Jupiter masses? Are there planets of greater mass that (like brown dwarfs) can fuse deuterium? We don't really know. Are there planets that can form whole out of interstellar medium? Are there stars that can accumulate out of dust? We don't know.

Extra-solar planets

We are certainly seeing what we think are planets. The 51 Pegasi flap seems to have died down. It's the first planet in orbit around another G star. It seems to have a mass of about that of Jupiter with an orbital period of only a few days; it's really close. How you get a Jupiter-mass body that close is not clear. It was thought for a while that it wasn't even a planet, that the spectral shifts were due to oscillations in the stellar surface that were mimicking the planet, but they are too regular; 51 Peg really does seem to have a planet.

We now have a whole new way of looking at planetary systems. We always used our own as a paradigm of what we are going to find out there. As somebody pointed out, that is a violation of the Copernican Principle, which says we are not special. Look at the difference. In 51 Peg we see a "Jupiter" right next to the star. How did it get in there? It's too hot in there to have formed a planet like Jupiter with all of its volatile gases and ices. The idea is that it formed way out, but before the disk out of which it was made dissipated, and then the planet migrated inward. The kind of planetary system you get is what is left after the disk dissipates. Maybe our disk was thin, so the Jupiters got stuck way out there. Other disks were thick, so the planets migrated inward by viscous friction or by the gravity of waves in the disk. The disk of 51 Peg dissipated leaving Jupiter in tight orbit. Our planetary system may be just one of a myriad different kinds of systems. All we're doing now is really discovering the big Jupiters in orbit. There are sixteen planets known now that are below the 13 Jupiter mass limit -- and 11 brown dwarfs above the limit, all detected by radial velocity oscillations of the star. These are all lower limits, because we don't know the orbital tilt, so everything is still pretty much up for grabs. There are in fact astronomers who think that these aren't planets at all.

Here are the Hyades and Aldebaran. There appears to be a planetary body in orbit around Aldebaran, the first one to be discovered around a giant star. It may of course be a brown dwarf. We don't really know. But there seems to be something there with a mass greater than 11 Jupiters. Besides, I like the Hyades so I thought I'd show a picture. Here is another discovery that seemed to come and go, TMR1 C. It was purported to be the first planet actually seen, one seemingly attached by a ribbon of gas to its star. But again we don't really know. It may be just a distant star mimicking a nearby planet.

Look at a graph of the eccentricities of companion orbits plotted against their periods. The "planets" have the same distribution as stellar companions, suggesting that they are all brown dwarfs and not planets at all. This is, after all, the fun of astronomy, isn't it? Looking out there for something that you don't understand. We are just getting our foot in the door in this subject.

Circumstellar disks

Beta Pictoris had its picture taken again during the past year. This is the edge-on disk. With the Hubble Space Telescope, you can see the warping of the disk, and you can see it from the ground as well. There is also something of a hole in the middle of the warping that's suggestive of gravitational disturbances by a planet that might have formed out of the disk, though we have never seen it.

Infrared observations show radiating infrared disks around other class A stars, notably Vega and Fomalhaut, but also around Denebola and others. Recent observations of radiation from the dust disk around Fomalhaut also shows a hole in the middle. Are there planets? We don't know. Everybody tries to jump on the bandwagon of the planets, of course. We want them to be there, I guess, just maybe to think there's somebody else like us out there.

Here is Proxima Centauri. I've looked at this picture over and over to see what the authors meant and can't. One of these is a "satellite." There seemed to be something going around Proxima Centauri. I bring this up because it may have hit the papers that Proxima Centauri might have a planetary or brown dwarf companion. Unfortunately, that all seems to have disappeared. It's just an artifact of observation. You've got to be very careful about interpreting marginal observations. The evaporating discoveries of planets over the last forty years has certainly demonstrated that. Not until the instrumentation got good enough to detect the speed of a world class sprinter did they finally start coming up with the planets, if indeed that is what they are.

Stars and stellar mass loss

Hubble took a picture of the great globular cluster 47 Tucanae. These circles represent "blue stragglers," which sound like something you find some place in the Bowery. They've long been a puzzle in astronomy. They are stars that are above the turnoff point in a cluster's main sequence, that is they are too massive for the cluster. The cluster's old enough to have burned down the main sequence below this point and yet you have overly bright main sequence stars. Where did they come from? Apparently, two normal main sequence stars merge together in the dense core by collision or by binary merger to form a massive star that suddenly appears above the main sequence. You're dealing with some pretty wild stuff here.

All of these stars die. And I thought I would show you rather a nifty gallery of planetary nebulae, since they are my first research love. These pictures, all taken with the Hubble Space Telescope, are just so beautiful. We'll go backwards in time. We are working on a slide set for ASP that will produce a nice gallery of planetary nebulae for you to show. This is NGC 2440 in Puppis. There's the central star. These stars are actually some of the brightest in the Galaxy. They do, however, produce most of their light in the ultraviolet where you can't see it. If you had ultraviolet eyes, you'd see the central stars of planetary nebulae all over the place. Many are thousands of times brighter than the Sun. The one in NGC 2440 hold the temperature record of 240,000 Kelvin, compared to the Sun's 6,000 Kelvin.

A little bit earlier in time is NGC 6537. It has an incredible structure around it. Planetary nebulae are produced by ionization of the old winds of giant stars that have been compressed and disturbed by a hot wind that pours off the planetary's current central star, which is the giant's old nuclear burning core. What this image shows is the asymmetry of mass loss from the giant star that produced it. Giant stars slough off almost all of their outer envelopes leaving the nuclear burning core behind, and for about a hundred thousand years the nuclear burning core lights up the departing wind. One as extreme as this might be produced by a double star, one star, or maybe a brown dwarf in orbit around the giant that stirred up the mass loss.

We tell our students that planetary nebulae have nothing to do with planets. Herschel just named them that because they were disk-like. Maybe, at least, the structures of planetary nebulae are produced by orbiting planets. What's going to happen with 51 Peg when it begins to expand into a giant star with that Jupiter tucked up right next to it? It's going to disturb the wind and maybe produce a weird looking planetary nebula. In some cases planetary nebula structures might be produced by planets that are going to be destroyed by the expansion of the giant star.

Let's go back earlier in time. This critter is called Henize 401. There is a funny looking disk right in the middle with plumes coming out of it. It's an extreme case. Here's another, a "protoplanetary nebula" called the Red Rectangle. (How's that for imagination)? There's a disk running right across it. The mass lost by these giant stars condenses into dust, ultimately producing most of the interstellar dust. Why can't that dust accumulate to form new planets? Maybe, for a period of time, you can get new planets in a developing planetary nebula. The idea is hardly confirmed, but Herschel would have loved it, especially since he thought that planetary nebulae were stars in the process of developing.

Here's an odd picture of Betelgeuse. This was taken in the radio, showing its pattern of mass loss. Big blobs of stuff seem to be coming off that may relate to the convection cells that the Hubble Space Telescope appears to have seen. No one understands such asymmetric mass loss. These supergiants will develop into supernovae. Wouldn't Orion look strange without Betelgeuse? If it exploded it would cast strong shadows on the ground.

Supernovae and "hypernovae"

This is Supernova 1987a in the Large Magellanic Cloud. We see a ring of mass lost from the supergiant's earlier wind illuminated by the light from the explosion. Theorists predicted that at some point the blast wave from the supernova would hit the ring and sure enough, it's just starting to do that. You should see that whole ring light up again over the next few years as the blast wave hits all of it, again showing the power of supernovae.

Some supernovae may develop black holes. The few black hole candidates are X-ray stars that seem to have black holes orbiting binary companions. Orbital measurement shows the invisible bodies to have high masses, not just above the neutron star limit of two or three solar masses, but much higher. There are seven with seven solar masses and one with fifteen solar masses, so one astronomer said, "There must be two groups." Only astronomers would say that.

Black holes sure can blast it. This is a sort of a movie of outward moving jets being produced by what appears to caused by the infall of matter into a black hole. We can estimate how far away the system is, and then find the blobs to be moving outward by twice the speed of light. Such "superluminal" motion is rather easily interpreted as an optical illusion, a relativistic effect caused by jets coming almost at the observer and at almost light- speed. The actual motion here is about nine-tenths the speed of light, showing great violence. We surmise a star feeding mass into a black hole. You overload the accretion disk. The stuff in the inner part of the disk drops into the black hole, and magnetic fields eject some of it away. You can't quite see it here, but you wind up with blobby stuff being shoved away from the black hole at high speed. Even centers of galaxies may work this way to produce the lumpy structures that you see in jets coming from galaxies like M 87.

These are gamma ray bursts, the issue brought up in past talks as well. We had no idea where they were coming from. The bursts were discovered by one of the first Star Wars satellites, which were designed to look down at to detect nuclear explosions. Instead, they found celestial gamma ray bursts. The gamma ray satellite shows about one to two of these events every day. Theories have ranged from the Oort comet cloud out to the most distant reaches of the Universe. The amazingly smooth distribution of the bursts has strongly suggested that they are coming from very far away. If they are, they are the most energetic phenomena that we know of, far outstripping the power of a supernova.

Some gamma ray sources are, in fact, Galactic. There is a set of three or four "soft gamma ray repeaters" that give off X-ray pulses. They are assumed now to be very supermagnetic neutron stars more powerful than the pulsar in the Crab Nebula. The Crab pulsar has a magnetic field of a trillion or so gauss, a trillion times stronger than the magnetic field of the Earth. These objects are thought to have fields 100 times as great. What blew me away was the recent report of an outburst from one of these that produced so much radiation that it strongly affected detectors aboard Earth-orbiting satellites and partially ionized the Earth's upper atmosphere! And it's thousands of light years away! Are we part of the Galaxy or what? You wouldn't want one of these things going off a thousand light years away, or heaven forbid a hundred light years away. It would probably produce mass extinctions on Earth.

The actual daily extragalactic gamma ray bursts may now be explained. At least, there are viable theories for them, which is something we couldn't say last year. Here is a ground-based picture in the optical of a gamma ray burst. A surrounding blob has a red shift of about 3.5, which implies that the burst took place in a distant galaxy. If that's the case, it's producing an amazing amount of energy. That is far more than what the standard theory up to now, the merger of neutron stars, can produce. The idea was that two neutron stars in orbit around each other spiral together, and in their merger create gamma rays. But the amount of energy in this gamma ray burst is 100 times greater than can be produced by a merger of neutron stars. A tipoff as to what is going on is given by this huge bright supernova, the most energetic supernova known. They measured gas flows coming out at almost the speed of light. Typical supernova velocities are more like 10-20,000 kilometers per second. There is some suggestion that there may have been a gamma ray burst associated with it as well, although it is not pinned down.

Theoreticians are suggesting "hypernovae," though superdupernova sounds better to me. The idea is that you have a rapidly rotating nuclear core, which collapses to produce a black hole instead of a neutron star. Mass falls into the black hole and the thing grows so fast that it produces tremendous jets. They may be very rare. Clearly there is more out there than we begin to be aware of.

These hypernovae may be enough to produce this phenomenon. I showed you this slide last year as indicating a mystery. M 108 has a huge cloud of gas around it called a hyperbubble. These things are usually thought to be produced by supernovae that eject plumes of gas perpendicular to the disk that then cool and flow back into the disk. But this one is too big to be produced by ordinary supernovae. The theory last year was that it may be pristine intergalactic gas that is falling into the galaxy. The hypernovae, however, could by powerful enough to do the job.

The Galaxy

And speaking of our Galaxy, here it is, expressed by the Milky Way. You can see the Great Rift, the Southern Cross, the Coalsack, and Alpha and Beta Centauri. After diligent observation of enormous numbers of radial velocities of stars, some astronomers found that there was a whole set of stars near Sagittarius that did not share the motions of Galaxy. It looked as if these stars actually belong to another galaxy that is passing through our own. And there it is: the Sagittarius Dwarf. It's the closest galaxy to us because it's inside of ours, and it's going through to the other side. It will probably eventually be disrupted, because, in orbit around us, it will probably make this passage several times. This tells why it is so hard to come up with a theory for the evolution of the Galaxy. Our Galaxy seems as much as anything to be the result of mergers with other galaxies that are in different stages of evolution. In fact, I'm sure there are people in here who have observed the well-known globular cluster M 54 from the back yard. M 54 belongs to the Sagittarius Dwarf, not to our Galaxy, though it someday will. Mergers bring in clusters that formed under different conditions. It's no wonder that things are so fouled up, and why it's difficult to come up with some monolithic theory for the origin and evolution of the Galaxy. The Galaxy did not form in strict isolation. It developed and evolved on its own, yes, but it also evolved partly as a result of mergers.

M 67 is an old open cluster about five billion years old. The ages of the Galaxy's globular clusters have been brought down to at least twelve billion years, which makes them younger than the Universe. Thank goodness. There are a lot of variables, of course, that will be adjusted over time, maybe are even being adjusted as I speak. These measures would make the halo of the Galaxy somewhere around twelve billion years old. But the oldest open clusters are about ten billion years old. That means that the collapse that formed the disk happened pretty quickly. They are still arguing about these ages of course, and they may increase again. The new ages are the result of the new Hipparcos parallax distances and better theory. But be aware that additional developments may change these new ages overnight, and that not all agree that the globulars are that young.

Up to this year, the globulars were thought to be fifteen to sixteen billion years old, and if the Universe is closed and with the Hubble Constant up around 75 or 80 or so, the globulars were older than the Universe, which was considered by some to be a crisis in astronomy. Well, hardly. As far as I am concerned, if you get the two to within a factor of two of each other, you're doing pretty well and showing consistency.


That's a naked black hole in a galaxy. I'm still trying to figure out from literature exactly what it is; nobody quite said. It could be the illuminated disk, but it looks like it might be one of the first views of the bright gas surrounding a supermassive black hole in the center of the galaxy. This is NGC 6251.

There is also some evidence of what the relativists call frame dragging. I find this astonishing, the idea that as a black hole spins, it actually hangs onto space-time and drags space-time around with it. Of course, we're all under distortion of space-time in the gravitational field of the Earth, but here the black hole is actually whipping it around. The measures of velocities in a couple of sources have suggested it.

Dark matter has been a problem in the Universe as a whole. More and more we are thinking that some of it may just be in the form of dim galaxies. This is a whole galaxy that's only about 1% above sky level. Such galaxies may actually dominate the Universe. It's not the M 31s and the M 51s or the bright dwarfs, but these big things in which star formation has gone on at a slow pace, or in which much of it is over with.

The blue dwarf galaxies, which were so much discussed several years ago, appear to be not so much young as old. Here's an example of one with a lot of red giants in it.

We may be zeroing on the origin of the quasars. These are pictures of giant infrared galaxies. They are very bright in the infrared, and they're very distant. One theory suggests that the mergers of these large infrared galaxies create, for a while, a rapid infall into a black hole that produces the quasars. That has been a growing theme in these talks as well, where Hubble has shown quasars with stuff around them, with companions that feed matter into the quasars. The quasar dies down after a time, perhaps becoming a quasi-normal active galaxy of the type we have nearby. We may have a little quasar at the center of our own Galaxy that is now a relatively quiet million solar mass black hole.

Look at this Einstein Ring, the result of near-perfect gravitational lensing. It is scientifically irrelevant. We knew they existed, but somebody actually went out and found one. Here we see a quasar with a galaxy or another quasar exactly in back of it. The light is bent around the nearby quasar through the distortion of space-time by the closer galaxy. This is all the image of the quasar in back, which has been distorted into this perfect ring.

Here is a theoretician's view of what intergalactic space might look like, and shows the distribution of gas in intergalactic space, gas that is almost undetectable. There's a growing feeling that this intergalactic gas may also contribute to at least a certain percentage of the dark matter. As the universe expands, and you get more and more shock waves in it, the stuff actually heats up with time and you get to see it less and less.

A talk like this would not be complete without the next redshift record. This is a distant galaxy measured with a redshift of 5.34. That makes it about twelve billion light years from here. Shortly after I got this slide, somebody found yet another one, so it's now up to about 5.64. That is light redshifted by 5 1/2 times, so you're really looking at the far ultraviolet light from the galaxy shifted into the optical band.

The Universe

This is a distant galaxy. The little blob next to it is a Type 1a supernova. Most supernovae that we teach about are core collapse critters, arising from very massive stars in which the iron core collapses and the star blows up. We don't usually like to talk too terribly much about the other kind of supernovae, the kind produced by white dwarfs because we don't really quite know what makes them. We're almost certain that white dwarfs are involved. The standard idea is that of a main sequence star or an ageing giant star in orbit with a white dwarf onto which the larger star feeds mass. The white dwarf is near the Chandrasekhar limit of 1.4 solar masses, beyond which it can't support itself. The increased mass causes the thing to flame (in the nuclear sense) and collapse, and that produces the supernova. We know that something like that must exist, because these supernovae take place in galactic halos where there aren't any massive stars, those that develop iron cores. The beauty of white dwarf Type 1a supernovae is that, with rare exceptions, they all seem to have the same maximum brightnesses. Once you get something of reliable luminosity you've a natural standard candle with which to measure the distances of galaxies. Suddenly we're getting the distances of galaxies out to billions of light years. The only problem is to calibrate the brightness of the Type 1a supernova on a nearby galaxy whose distance you can get with Cepheid variables, one of Hubble's key projects.

There has long been a fierce argument between the east coast and the west coast about the Hubble constant. The east coasters at one point were claiming that it was up to 100 kilometers per second per megaparsec, which gives you an age of the universe of less than ten billion years and well below the ages of globular clusters, whereas Sandage and his gang out on the west coast were saying, no, it was around 50, which gives you a twenty billion year lifetime, which is fine for the globulars. Sandage is turning out to be closer to the mark. From the Type 1a supernovae, the Hubble constant is coming in around 65, much closer to the old value, giving you an older age of the Universe.

In fact, the velocities suggest that the Universe may be open and not just barely closed, as has so long been assumed, and that it will expand forever, most astronomers converging on that view. Not only that, the distant Type 1a supernovae seem to be somewhat fainter than they ought to be, which suggests that Einstein's expansive cosmological force (which he developed at a fairly early age and considered his greatest mistake) is real and that it may be accelerating the Universe. Relativists and cosmologists are taking that quite seriously. Whether that's true or not we won't know for some time, final decisions requiring more data.

That the Universe is open and ever expanding is supported by observations of the existence of very distant clusters of galaxies. The openness of the universe is required to have large clusters of galaxies form very quickly, which fits in very nicely with what we see with the Type 1a supernovae.

We can look yet farther and farther away. These are COBE results where everything known is removed. What remains is a faint infrared background that may be light from the first stars of the Galaxy absorbed by intergalactic dust and reradiated back to us. Maybe even deeply redshifted stuff. It looks like this infrared background may contain as much energy as all the visible stars that we see, and is clearly an important part of the energetics of the Universe.

This is actually a picture of the Hubble Deep Field made with an infrared telescope, which gives a very different view. You see a bright infrared blob here that may be among the earliest galaxies formed, with a redshift between 2 and 4. Here we have a galaxy with stars where the starlight is being absorbed and re-emitted by dust. It's about the only way you can get this infrared radiation coming out. The whole idea is that stars and their galaxies formed very quickly. Perhaps the first stars that formed and seeded the Universe with heavy metals did not even belong to galaxies, that is, were created even before the galaxies themselves began to form, explaining the so-called population III stars, those with no metals whatsoever, those made from the stuff of the Big Bang itself, which continued only hydrogen, helium, and a little lithium.

The Hubble Deep Field was re-taken with the Hubble's Nicmos camera. I'm not going to offer an explanation because the people look at this and know what they're doing can't offer one either. We see a whole bunch of infrared galaxies here, some of which match up with the Hubble Field, some of which don't. Look at all of these different structures and forms. We have little idea what's going on. This is a primitive galaxy; it may be twelve billion light years away. The red color might be coming from deeply redshifted light or it might be coming from light that's simply been absorbed by dust and reradiated. The fragmentary nature of it and the odd structures suggest that galaxies really did look different back then. We're able to use the Hubble Space Telescope as a natural time machine to look out into the distant universe and see what happened way back then.

As a fitting end, instead of stopping with a picture of the Earth as I often have, I want to finish with a picture of the Deep Field, the optical deep field, the 100 hour exposure that looks into distant space. Hubble is doing a second deep field in the southern hemisphere in Tucana to see how well it matches up with this with 1900-galaxy northern view. Isn't it marvelous to look at this tiny field that is only two minutes of arc across and see everything there is, stars, galaxies, billions of stars, maybe billions of planets, who knows, maybe billions of other creatures, some perhaps looking back and who see us at the distant edges of their Universe?

Thank you.


[Question on whether Betelgeuse will become a supernova and when]

It's probably about 10-12 solar masses and that should be enough to produce a supernova. There's a distance for it measured from Hipparcos. Give it a hundred million years maybe. Nobody can tell what's going on inside a supergiant, however. It could be in a stable helium-burning phase, which is going to take millions of years. One theoretician, however, thinks Betelgeuse may just miss being massive enough to make a supernova and may instead end its days as a rare oxygen-neon white dwarf (most are carbon- oxygen).

[question asking why the water on the Moon doesn't evaporate]

It's apparently buried in the regolith. It's not pooled, but is in the form of ice mixed in with a heavily insulating soil. There is nothing much to heat it, as it is in shadowed craters, so it just stays cold. Cometary impacts adds more and it just stays there.

[question asking more about the Sagittarius dwarf passing through our Galaxy]

Isn't that a neat thing? The core of it is evidently dense enough to get through our own Galaxy and out to the other side. It will probably oscillate many times through our Galaxy before it is finally disrupted, much of it merging with our own Galaxy. This sort of thing has been thought to have gone on over the last many billion years. A big problem in galactic research is that there are no good, smooth correlations between globular cluster metallicities and other parameters. The merger theory begins to take care of that. Our Galaxy not only developed on its own, but also consists of many other galaxies that have merged with it.

[question asking what is the mass of the Sagittarius dwarf compared with the Magellanic Clouds]

It's less I think, but I don't remember by how much. It's a dwarf galaxy. I'd have to look that up. It's just so intermingled with our own, and you can't tell unless you study one star after the other after the other, looking at the motions.

[question about the primordial stars]

The idea is that the first filaments of matter that formed out of the Big Bang could have formed massive stars very quickly. The Big Bang produced hydrogen, helium, and a smattering of lithium, and that's all. Deuterium, yes, but none of the heavy stuff. All of the heavy stuff is thought to come from stars. If you look out at the very distant Universe, you see carbon lines that had to come from somewhere. But there are no zero metal stars in the Galaxy. The accepted limit is one ten-thousandth of the solar metallicity, but not zero. Where are the zero metal stars that started it all? It's been a continuing problem. The accepted idea is that they were all very massive and blew up, so you don't see them any more. The idea explains the patterns in the abundances of the very low metal stars, patterns produced by rapid neutron capture, which is a signature of supernovae. Such stars may have formed before there were actual galaxies. The idea now of galaxy formation is bottom- up. You start with small stuff and build to the large with mergers, the process still going on.

[question asking if hypernovae could form elements heavier than uranium]

Even ordinary supernovae should do that. In rapid neutron capture, you build up an extremely heavy, highly radioactive nucleus that decays upward to produce uranium, thorium, and a variety of lighter elements. Uranium and thorium can only be produced this way. You should also easily be able to produce plutonium, probably neptunium as well. In fact, it wouldn't surprise me if you got all the way up to californium, which is the last of the more or less stable radioactive elements. Beyond that, they're gone so fast that it would not much matter. Observationally, there seem to be daughter products of plutonium in meteorites that suggest the natural decay of plutonium in our own Solar System. That is the best evidence that we have that our Sun was produced by the compression of the interstellar gases by a local supernova.

[question asking whether characteristic spectra are seen for these elements]

Well, you'd never see it. There's too little of it. Uranium has never been seen in stellar spectra either, including in the Sun, but it has to be there. Neither have radium absorption lines been seen. What you do to get the "solar" abundance of such scarce elements is to measure meteoric compositions. One of the great triumphs I think of 20th century astrophysics is the replication of the abundance patterns in meteorites, and in the Sun for that matter, with the theory that takes into account the ejection of matter from various nuclear processes from giant stars, supergiant stars, and supernovae, and trucks them into an interstellar cloud, collapses it, and distills it out to form the Earth and meteorites. You have the patterns that you expect theoretically, within certain limits anyway.

Closing remarks

I want to thank you for your attentiveness. I think this is the tenth year I've done this. It's a terrific honor to be asked to come back year after year. I want to thank you for the opportunity that you have given me to keep up with astronomy. It is among the most exciting work I've ever done.

Let me also take this time to say that you have been instrumental in moving me on a more personal level into astronomy education. You have been a powerful force because I've seen the way you work. It's true, I feel like I'm coming home to visit you. I have been working to develop some educational web sites that you might be interested in and that are outlined below. Tell your students. Get into them; you might enjoy them. You might be able to use them for instructional work. Feel free to use the sites and give me suggestions as to what else you might like to see.

Once again my deep thanks to you for what you have done for me and for all those who love astronomy.
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