ASTRONOMY UPDATE 2000
James B. Kaler
Department of Astronomy, University of
First published in the Proceedings of the 36th Annual GLPA
Conference, Chicago, IL, October 11-14, 2000. Reprinted by
The year ranged from wonderful views of the sky -- a lunar
eclipse, a planetary gathering, a nova -- to the discovery that
the Universe may be flat after all. In between, we saw new
discoveries about how the Sun heats its corona, looked close-up
at an asteroid, admired new planets orbiting other stars, delved
further into gamma ray bursts, and sometimes seemed to know less
rather than more.
How fine to be at Adler Planetarium, one of the great icons
of astronomy. The first actual article I saw on astronomy when I
was eight years old was illustrated by nothing less than the
sight of Adler's great dome from down the street.
And casual stargazing from the planetarium or anywhere else
on Earth was, in theory at least, no better than the wonderful
gathering of planets we had in May of this year, when Jupiter,
Saturn, Mars, Venus, and Mercury were all in a tight bunch.
Unfortunately, they had to include the Sun in their midst, making
the whole thing quite impossible to see. Unless you used the
SOHO spacecraft, which showed it all against the daytime sky.
Shortly afterward, on May 31, Jupiter and Saturn passed each
other for their "grand conjunction," which takes place every 20
years. It is my third grand conjunction since I have been in
this business. And in spite of some predictions, no buildings
fell down under the planetary influences; probably because of the
power of planetarium astronomers!
Not only was planetary disaster averted, but so was "OH
radio" disaster, as the Iridium satellite system, which was to
have interfered with radio astronomy, went bankrupt, and we don't
have to listen to the "noise" anymore. Danger was also averted
in Kansas, where the Big Bang first went bust and was then
rescued by the election of a new school board.
The telescopic picture -- of the instruments needed to learn
the wonders of the Universe -- is mixed. Europe's Very Large
Telescope, while not finished, is partially up and running and
producing spectacular results. On the other hand, the highly
successful Compton Gamma Ray Satellite was brought down as a
result of gyro failure and the worry that another failure would
render the machine out of control, a sad and contentious day for
the high-energy people, as there was considerable argument about
it. And of course we weep for the demise of the Mars Climate
Orbiter and the Polar Lander, which never even had their chance,
thanks to NASA failures (let's see, if the speed is 50 furlongs
per fortnight...). But on the positive side, the HST was again
upgraded and 2MASS, the 2 Micron All Sky Survey, is not only
pouring out the data, but all the data are on the Web for all of
us to use. Even better, the Next Generation Space Telescope, an
8-meter to be optimized for the infrared, is on track for 2009
even though the design has not yet been set.
Back to the long-wave folks, the Green Bank 100-meter radio
telescope, with microwave capability, is done, and the Square
Kilometer Array is underway, 30 interferometric stations with a
total collecting area of (guess what) a square kilometer (how
many acres is that?), is now planned for 2010.
It's nice, though, to have something that we can see with
the naked eye too. How about that wonderful eclipse of the Moon
last January! Apparently some Leonids were seen to strike the
dark side last November.
And the Sun
Without the Sun (segue here), we could not see the Moon at
all. We passed the solar maximum sometime between last January
and April, and are now on the downside of the solar cycle. What
a sight SOHO and other spacecraft have provided for us. The
increased ultraviolet radiation seems to affect the ozone layer,
and for reasons that are still mysterious affects climate and the
Earth's global temperature as well. Another spacecraft, the
remarkably successful TRACE, has been able to disentangle the
solar loop structures into narrow threads, and is leading the way
in showing how the corona is heated, apparently from below. Most
remarkably, solar astronomers have been able to use the myriad
solar oscillations to "image" what is on the BACKSIDE of the Sun,
allowing us to know of active areas that will soon be visible on
the front side. Since these active areas can strongly affect the
Earth through coronal mass ejections and flares, such
observations give us warning and more time to prepare. This
technique cannot help but improve with time.
And its Planets
The Sun was of course the background for the transit of
Mercury last November, which I missed because of clouds, OF
COURSE. (I did however, get to see the lunar eclipse, and one
out of two in the midwest "ain't bad.") The near-grazing transit
was also nicely visible against the chromosphere and the corona
Features on Mercury were actually seen for the first time
this year reasonably well from the ground, using multiple imaging
techniques, in which the best of a great number of individual
pictures were combined. The result was the discovery of features
on a portion of Mercury not yet seen by spacecraft.
And then there is Mars. In spite of spacecraft failures,
new Martian data continue to pour in. The Mars Global Surveyor
finds evidence for "recent" (at least geologically) water flows,
from now-dry watercourses inside and over craters. If we ever
arrive, the water does seem to be there for us to use. There is
also some evidence for "recent" volcanism (both in landforms and
from uncratered areas), suggesting that the planetary god of war
still has some life in him. Though of little research value, it
was also intriguing, and certainly instructive, to see a picture
of the shadow of Phobos against the Martian terrain, knowing that
a solar eclipse was taking place below.
The outer planetary system is no less intriguing. Stability
has been on many minds. Orbital simulations continue to suggest
that Neptune formed much closer to the Sun than its present
distance, and then moved outward under the action of Jupiter and
Saturn (or by gravitationally scattering comets), which makes
sense in terms of formation, as it seems that there was not
enough raw material where Neptune is now ever to have formed a
planet. To some degree the same is true for Uranus.
Formation problems still exist for Jupiter and Saturn too.
One side contends that the two giant planets formed (4.5 billion
years ago) by accumulating dense cores and then gravitationally
attracting hydrogen and helium from the surrounding solar nebula;
the other side suggests that the planets formed whole out of
instabilities in the circulating protosolar disk. We are far
from knowing all the details of planetary formation.
Jupiter, the big boss of it all (excepting the obvious Sun),
is now seen to produce massive lightning bolts over a thousand
kilometers long, perhaps one way in which the giant planet
removes its internal heat. Jupiter's Io is, in its own way, just
as violent. Its volcanic energy swamps what we have on Earth,
the volcano called Loki, at 1800 Celsius generating more volcanic
energy than what we see on Earth. Activity is monumental, with
even non-volcanic mountains over 15 kilometers high. The Galileo
craft shows red to yellow color changes in lava flows that
represent the transition of molecular sulfur from one form to
another. Europa is hardly left behind. We see fractured tidal
bumps 30 meters high that apparently move across the satellite's
surface, demonstrating the existence of a warm ocean below.
(Imagine such a thing coming at you across a cornfield!)
These satellites have more company than we thought. I
recall a time when Jupiter had but 12 satellites (or so we
thought), then the count went up to 16, and now number 17
(1999UX18 = S/1999 J1) enters the ranks, a little bitty thing
about 2 km across that takes two years to orbit the planet
retrograde (and is probably a captured asteroid). Uranus seems
to have hit 20 satellites!
Vastly farther out lies Pluto, which is eerily akin to
Neptune's Triton, both of them captured by Jupiter (Triton
directly, Pluto in a resonant orbit in which it circuits twice
for Neptune's thrice, a word you don't get to use much.) From
the estimated impact rates from the bodies of the outlying Kuiper
belt of comets (to which Pluto sort-of belongs, though it is
distinctly NOT a comet), Triton's surface appears relatively
young, perhaps only 100 or so million years old, suggesting a lot
of internal heat and activity, and a capture only 1-2 billion
years ago (tides from the capture heating the satellite).
Pluto itself has ethane in its snow-cover, mixed in with the
nitrogen, methane, and carbon monoxide. Nice place. Of course
no argument ever seems settled. Some think that the whole gang,
Pluto, Charon (Pluto's satellite), and the "plutinos" that share
Pluto's 2:3 orbit might all be collision relics. Some 300 of
these Kuiper Belt Objects are now known and the number will
surely increase rapidly.
Asteroids and Stuff
The great collection of collision relics of course is the
set of asteroids. A big new carbonaceous chondrite fall (500
pieces) in Canada (Tagish Lake) will tell us more about
asteroids' parents. Chondritic meteorites (the ones with small
round chondrules in them, inclusions that have been flash-heated
and cooled), have been found to contain such things as amino
acids and diamonds, and now of all things both liquid water and
halite, which suggest the existence of larger water inclusions or
perhaps a collision with a comet.
The "big guy," the Willamette meteorite, is now secure at
the Hayden planetarium after the planetarium and the Clackamas
tribe in Oregon have agreed on the meteorite's sacred
NEAR is also safely orbiting, and is observing Eros,
revealing craters and an old surface. A complete map is even
available. Someone calculated that, given the odd shape of the
asteroid, you could actually fall up a hill.
The real water supply in the little stuff is in the comets,
The fragile things eventually disintegrate under sunlight, Comet
LINEAR simply falling apart, the destruction well-viewed by
Hubble. Sublimation of the ice leads to immensely long comet
tails. Until recently no one knew exactly how long. The Ulysses
spacecraft (which is observing the solar poles) went through the
tail of Comet Hyakutake at a distance of 3.8 astronomical units!
Interstellar Medium and Star Formation
Ethyl alcohol, water: these and a hundred other molecules
are found in the dark clouds of interstellar space. And now,
Sweetheart, SUGAR, a form called glycolaldehyde. Now if someone
will discover interstellar lemon juice...
The intense cold of the dark clouds, which makes these
molecules possible, also breeds stars. Dense blobs of
interstellar matter collapse under their own weight, and as the
stars form in the middle, they spin out to produce dusty disks,
from which assemble the planets. And they are there -- planetary
systems -- in abundance. At this moment we know of 46 stars with
planets, plus 13 others above the 13 Jupiter-mass brown-dwarf
limit (at which point the interior temperature is high enough to
fuse the natural deuterium that was given to the body at birth).
The radial velocity technique used to find these planets (we
observe the star moving back and forth along the line of sight as
a result of a planet's gravity) gives only lower limits to the
planetary masses, as we do not necessarily know the orbital
tilts. Remnant disks have been seen around 55 Cancri and Rho
Coronae Borealis, and their clearly defined inclinations give
actual respective masses of 1.9 and 1.5 Jupiters. Better yet,
the planet of HD 209458 was observed to cross directly in front
of its star, right on schedule, beautifully nailing down the tilt
and giving a mass of 0.69 Jupiters. Lower yet in mass, 79 Ceti
was found to have a planet with a lower mass limit of only 0.35
Jupiters, in the neighborhood of Saturn. We also established a
new nearness record to the Earth (11 light years, maybe close
enough to see our TV programs), when good old Epsilon Eridani
(the subject of many earlier studies) was found to have a planet
in the neighborhood of 0.8 to 1.6 Jupiters orbiting at 3.4
astronomical units from the parent star. The Astronomy Picture
of the Day actually used my photo of Eridanus. Everybody else
photographs Orion, the Big Dipper. I do Eridanus. (It was the
only one they could find).
On the negative side, TMR-1C does not have a visible planet.
The faint body, which seemed to be "attached" to the star by some
kind of filament, is just a background (or foreground) object.
Planets did not show up in the core of 47 Tucanae either. With
all those stars, we should have seen some transits, and none were
observed, suggesting that these stars do not have planets. Maybe
the stellar density is too great, or maybe the stars need higher
metal contents, implied by the finding that many of the stars
with planets have above-average metal abundances.
Speaking of stars (the good segues never stop, do they), the
faint ones keep making news. There may be twice as many L and T
dwarfs as M dwarfs. L dwarfs are "later," cooler, than M dwarfs,
and have hydrides in their spectra; they are a mixture of low-end
stars and brown dwarfs. T dwarfs have methane! and are all brown
dwarfs down near 1000 Kelvin or below; the new cool record is
only 750 Kelvin, for a brown dwarf 19 light years distant. There
are not enough of these faint bodies to account for much "dark
matter," however. There are some suggestions that brown dwarfs
might be created below the upper mass limit for bodies we call
"planets," stars so light that they cannot even fuse deuterium.
Do planets (formed presumably from the "ground up" by
accumulation of dust and gas), and "stars" (formed whole and in
place by condensation) overlap?
The physics of these cool stars is not well understood. The
coolest of M dwarfs should be fully convective, leaving no
radiative core on which to anchor magnetic fields (so goes
theory). They should therefore not produce solar-like flares
(which result from local magnetic field collapse), but they do.
So here is the brown dwarf LP 944-20 Fornacis popping a big
flare. Nobody knows how or why.
Stars, stars, stars everywhere. A quick look at the Orion
Nebula reveals the Trapezium and a few fainter stars. The
infrared view by 2MASS reveals hosts of stars, cascades of them,
this region a densely packed cluster in which the Trapezium
stands at the top.
Even normal stars can do funny things. Delta Scorpii just
brightened by half a magnitude as it apparently turned itself
into a Be (class B emission-line) star, necessarily ejecting some
matter as it did so. The "funniest" thing to see might be a
collision between stars, at least as long as we are not part of
it. Students commonly ask about it, and are told that in our
part of the Galaxy it just does not happen, as stars are too far
apart compared with their diameters. Not so in the centers of
globular clusters. Here the stars collide and merge to produce
the "blue stragglers," stars that lag behind their cousins in
evolution and appear above the turnoff point in the HR diagram
where the giant branch joins the main sequence. At least again
so goes the latest theory.
Lower Mass Star Death
All these stars, whatever they are, are doomed, as their
initial fuel supplies are sure to run out. Those like the Sun
will eject their outer layers as planetary nebulae. We thought
we understood them pretty well until we looked at them closely (a
common astronomical complaint). Look, for example, at MyCn-18,
the "Etched Hourglass." There is an inner "hourglass" inside the
outer one, and the star is not centered. Neither observation is
understood. Nor are the concentric rings around the "Cat's Eye,"
NGC 6543. Nor is the formation of the World's Largest Planetary,
KjPn 18, which is 13 light years long and has another planetary
in the middle. One suggestion is that two stars produced two
planetaries at about the same time, which seems highly unlikely.
And how about those titanium carbide nanocrystals, anyway.
The tiny things have been observed in the ejecta of carbon stars
(which create carbon-rich planetary nebulae). To get them, the
wind flows had to be a lot larger than previously assumed,
further confusing the issues of how the nebulae form.
And how about too that Nova in Aquila last year? Naked-eye
novae are not all that common, and it was fun to see another.
Novae result from the thermonuclear explosions of matter
deposited on white dwarfs by tidally stretched companions, while
white dwarfs are the end of the line for intermediate mass stars,
and the successors of the planetary nebulae. In badly written
books, white dwarfs turn into invisible "black dwarfs." However
the Universe is not old enough for that ever to have happened,
making the end of the white dwarf sequence a good indicator of
Galactic Age. Long thought just to get redder and redder as they
cool, the coolest can actually get bluer (for a time), the result
of the formation of molecular hydrogen in their atmospheres.
Best not to tell this to beginning students.
Higher Mass Star Death
High mass stars do not make white dwarfs. Instead, they
explode as supernovae that make neutron stars or even black
holes. It seemed to be getting clearer that the leading
candidate for a Galactic supernova (100-solar-mass Eta Carinae)
is a binary, and that it might be a Wolf-Rayet companion that in
the last century belched the "homunculus" cloud that now
surrounds the star. (Wolf-Rayet stars are massive stars with
stripped hydrogen envelopes and weird high-nitrogen or high-
carbon compositions.) The companion is inferred from spectral
changes, and is purported to have an eccentric orbit and a 5.5
year period, winds from the two stars fiercely colliding. Other
astronomers, however, hold out for the single-star model, and say
that the 5.5 year period is caused by oscillations. Controversy
Further controversy involves the actual masses of these
great stars, the "O3 stars" at the top end of the main sequence.
The mass-luminosity relation found from double stars does not
observationally extend all the way to the stars with the highest
luminosities. Theoreticians have suggested that these stars may
have masses as high as 120 times that of the Sun. However, the
accumulation of observational data suggest that the upper mass
limit may is much lower, maybe 55 solar masses or so. No
explanation or resolution is yet available.
We do know that such stars indeed blow up, as witnessed by
the Large Magellanic Cloud's Supernova 1987A, which came from a
class B (Ia) supergiant. The expanding debris has hit the
surrounding ring of previous ejecta, which is continuing to
brighten. It is not a good idea to let such an explosion go off
too close to the Earth (like we can stop it). Ocean sediments
contain the iron-60 isotope, a by-product of high-temperature
stellar explosions. Its existence on Earth shows that a
supernova went off less than 100 light years away maybe only four
or so million years ago. Close explosions might damage the
atmosphere and affect life on the ground.
Lower-mass high mass stars (which really does make sense),
those between say 10 and some limit well above that, seem to make
the neutron stars and pulsars (obliquely rotating neutron stars).
Explosions are off center, which kick the resulting pulsars away
at high speeds. Observations of the Crab and Vela pulsars show
the directions of motion to be along the rotation axes. Again,
nobody knows why.
As the pulsars radiate their energy away, they slow, and
eventually disappear. The observational cutoff for the "pulses"
(the rotation period) is around 5-6 seconds. As a surprise,
nature gives us an 8.5 second pulsar, with of course no
explanation to go with it.
The highest mass stars should provide black holes, although
no one can prove it. Our observational knowledge of these
strange bodies, from which light cannot escape, comes from black
holes in X-ray binaries in which mass falls from the companion
into a heated disk surrounding the hole. One, X-ray nova Nova
Scorpii 1994, a leading candidate for a binary with a black hole
companion, has a visible star contaminated by supernova debris,
rather nicely showing that black holes really might be made by
supernovae. The nearest such candidate is only 1600 light years
away, about the distance of Deneb.
The "collapse of rotating massive stars into black holes" is
now a leading theoretical candidate for the explanation of the
gamma ray bursts that Compton observed for so long coming from
great distances far outside the Galaxy. The gamma rays might be
sent outward in tight beams, reducing the energy requirements
(which are huge if that much energy has to be radiated over an
Galaxies (Including Ours)
Globally, our Galaxy more and more seems to be a barred
spiral, though little is known about the bar itself. Ours has
also long seemed to have a massive, million solar mass black hole
at its center, the source of the Sagittarius A* radio source. In
fact, all large galaxies may have central black holes, which are
inferred by size and mass measurements. There are now enough
data for statistical studies. The masses of the central black
holes seem to correlate not with the masses of the prominent
galactic disks, but with the masses of the galactic bulges, the
masses of the black holes about 0.2% that of the bulge masses.
The disks have nothing to do with it.
The black holes may be older than the galaxies that host
them, and may be among the first things formed in the Universe.
The accumulation of such black holes and their associated
galaxies and quasars may also be responsible for the general X-
ray background of the Universe (quasars being distant,
ultrabright galactic nuclei in which the surrounding faint
galaxies are very difficult to observe).
The galactic black holes create huge jets that were thought
to be magnetically confined. Observations of the famed M 87 jet
(which pours from a three billion solar mass black hole) with
high-resolution very-long-baseline-interferometry now confirm
that suspicion, the jet seemingly wrapped in a spiral magnetic
sleeve. Presumably, the magnetic field is made by the rotation
of the disk surrounding the black hole, the disk that feeds the
"monster in the middle." M 87 might also be the source of very
high-energy cosmic rays. How, nobody knows (I seem to be saying
that a lot, which of course is what makes astronomy the fun it
Most dark matter is confined to vast halos around galaxies,
the so-called "dark matter haloes." Merged images of galaxies
taken in the Sloan Digital Sky Survey allow the study of the
distribution of matter in such dark matter haloes. A typical
spiral would have 5 trillion solar masses of the stuff within a
volume a million light years wide centered on the galaxy.
Yet for all the mapping, we have no idea what the stuff is.
It certainly is not an accumulation of brown dwarfs, unless we
are missing an awful lot of them. Could it still be white dwarfs
in the galaxy haloes? Or just ordinary molecular hydrogen? The
search goes on. And on. And on.
And Now... the Universe
We have to have a new record; it would not be a "year"
without one. This one is a quasar with a redshift of 5.8. (That
is, spectrum lines are shifted in wavelength to the red by a
factor of 5.8, showing the light to be coming from the quasar
when the Universe was 1/6.8 its present size). There is even a
faint suggestion of a galaxy of some sort with an unconfirmed
redshift of 12. Hold that in abeyance, however.
The great AAT (Anglo-Australian Telescope) redshift survey
goes on, the point of it to map the nearby Universe to see the
distribution of galaxies. So far they have bagged 160,000
redshifts out of a planned 250,000 (a few more than your Messier
Marathon). The purpose of the study is to be able to compare
actual galaxy distributions with the originating fluctuations in
the Cosmic Background Radiation.
Not quite finally, we have a "best" value for the Hubble
Constant. Of course we did last year too, and the year before...
Now it is 74 plus or minus 7 kilometers per second per
megaparsec. Maybe. There are some concerns. From observations
of eclipsing double stars, the Large Magellanic Cloud, which is
the basis for the calibration of the distance scale, appears 12
percent closer than usually adopted, which raises the Hubble
constant by a like fraction. On the other hand, the Cepheids
observed in other galaxies, Cepheids that allow the calibration
of the Type Ia supernovae that are used to probe the distant
Universe, might be contaminated with unresolved stars. (A Type
Ia supernova is caused by the explosion of a white dwarf in a
double-star system after it is somehow pushed beyond the limit at
which it can support itself.) The Cepheids would then be
fainter, the host galaxies would be farther away, and the Hubble
constant would go down. Then again, observations of water masers
in the galaxy M 106 (the comparisons of radial velocities against
proper motions, the latter motions being those across the line of
sight) suggest that the galaxy is 20 percent closer than
indicated by the Cepheids, so the Hubble constant would go back
up. All these issues must be clearly resolved before there is a
true definitive value.
Observations of the distant Type Ia supernovae suggest that
the Universe's expansion is accelerating, but there are lingering
doubts that the distant supernovae are really like those nearby,
from which we learned the supernova characteristics. We are
going to have to go farther out yet (past a redshift of 1) to
know what is really happening. We do know that off in the
distance, which is also distant in time, that galaxies formed
quickly, some astronomers suggesting only 300 million years after
the Big Bang.
Perhaps the most exciting discovery in this field is that
from the Boomerang balloon experiment. The fluctuations in the
Cosmic Background Radiation suggest that the Universe is indeed
"flat," that Omega (the density of the Universe vs. the density
needed to close it) is 1. But the observed density of matter
(which provides but 1 percent or so of closure) plus
gravitationally detected dark matter (which provides 25 percent)
is insufficient. We also need "density" in the form of energy
from the vacuum, that which seems to be making the Universe
accelerate. It all rather fits together, yet the uncertainties
are enough to give one pause as to whether we have really found
the truth. Time only -- combined with further investigation --
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