ASTRONOMY UPDATE 1998
James B. Kaler
Department of Astronomy, University of
First published in the Proceedings of the 34th Annual GLPA
Conference, Nashville, IN, October 21-24, 1998. Reprinted by
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
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.
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
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
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.
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.
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
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
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
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
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
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.
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
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.
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
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
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
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
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
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
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
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
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.
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
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.
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
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?
[Question on whether Betelgeuse will become a supernova and
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-
[question asking why the water on the Moon doesn't
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
[question asking more about the Sagittarius dwarf passing through
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
[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
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
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
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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