ASTRONOMY UPDATE 2001
James B. Kaler
Department of Astronomy, University of
Illinois
First published in the Proceedings of the 37th Annual GLPA
Conference held jointly with the Southeast Planetarium
Association, Richmond, KY, June 26-30, 2001. Reprinted by
permission.
Abstract
Discoveries over the year range from Earth to the most distant
quasar, from 2478 BC (sort of) to yesterday. Waves of
astronomical data engulf astronomers, Mars loses a little water,
a spacecraft landed on an asteroid, yet more extrasolar planets
were discovered (some really odd), and oh by the way we have
solved the nature of the Universe, which is flat and accelerating
There is the small matter, however, of what it is made of, which
remains quite mysterious.
From Earth
The talk this year is a bit confounded, as it's been moved from
October to May, and it's been just seven months since the last
GLPA gathering. But since SEPA is in joint meeting with GLPA,
and since this talk is billed as the "year in astronomy," I
thought it best to go back over the whole year, so this year's
update will overlap some with last year's. And since this is my
first talk to SEPA, I thought perhaps I'd go back to the
beginning.
So let's start at that great year 2478 BC. The Great Pyramid at
Giza was apparently aligned close to north using Kochab and
Mizar. They split the difference between them when the two were
at the same altitude. The mis-alignment as a result of
precession gives you something close to the beginning of the
construction time. If you don't believe that, Dave Leake at
Staerkel will crank the Zeiss back 4479 years. He won't
mind.
Now in 2477 B.C. the Kansas School Board said the Big Bang did
not happen. Here's a picture of Kansas taken from space. But in
the last year the good guys won when the old Board was voted
out.
Going to 2476 B. C., Iridium went bankrupt for the first time.
Been on and off ever since. Now they are back; they did not de-
orbit and the system is still alive.
OK, enough of that, let's move to the present year.
We are seeing great observational advances. 2Mass (Two micron
all-sky survey) is now, so far as it is complete, on the Web. Go
to http://www.ipac.caltech.edu/2mass/ In space, the cleverly-
named NGST (for Next Generation Space Telescope) has been shrunk
from 8 meters to 6, and should fly around 2009. Somewhat to the
dismay of optical astronomers, it will be optimized for the
infrared so as to observe high-redshift galaxies. Still, it will
be a great advance over what we have now. Back on Earth, not
only is the fourth (again cleverly named) VLT (Very Large
Telescope) now in operation, but the interferometers of both it
and the Keck (in which the telescopes are linked) have locked
onto fringes. The Keck has a baseline of 85 meters; the VLT
measured the angular diameter of Alphard at 9.3 thousandths of a
second of arc.
At long wavelengths, the 100 meter radio telescope at Green Bank
(NRAO) is done and in operation. The focus of what optical
astronomers would call a "shiefspiegler" or Herschellian
telescope is off to the side, eliminating the interference of the
focal apparatus. The Square Kilometer Array is also underway.
It's a huge 30 element interferometer with a total area of 1
square km, and should be under construction by 2010. Back to the
optical, the US Naval Observatory is completing the 50-million-
star southern segment of a huge catalogue of star positions (0.02
seconds of arc accuracy to 14th magnitude). We've come a long
way from Tycho's 777.
Most amazing of all perhaps is the creation of the national
"Virtual Observatory," which will combine the databases of
several immense surveys at different wavelengths that include
2Mass, Sloan, and many others. There is so much data that
"observers" can link into the "VO" to carry on large research
projects. The database can only increase: one upcoming
technique, which allows measure of intensity, wavelength, and
arrival time can take data at a gigabyte per second!
Moon and Sun
Lunar rays (which darken with time) may not all be young; it
depends on local chemistry. This is not all that interesting,
but it's the only thing I could find to say about the Moon. The
Sun is better. Great eclipse on the day of the summer solstice.
For the rest of us who did not see it, there was the great naked-
eye sunspot group of last March (if you dared look at it). More
important, the TRACE spacecraft now has evidence that solar
magnetic loops are heated from below rather than uniformly. The
heating of the corona, while known to be magnetic, has been a
continuing puzzle. While this discovery does not solve the
puzzle, it will help lead the way.
Mission to the Planets
We have to start with Mars. The news spills in faster than
you can write it down. The red color, thought to be iron-
weathered soil, has an alternative explanation: it might come
from iron in the meteorites that have battered the planet for the
past few billion years. Volcanism, which wipes away the impact
craters, may not be entirely extinct; there is some evidence for
recent activity, though "recent" takes on a geological meaning.
Sorry to say too that the great volcano Olympus Mons has been
reduced in size to 21.3 km. No, it hasn't fallen in -- the Mars
Global Surveyor laser altimetry just gives a better idea of the
actual size of the Martian ellipsoid, a sort of "sea level" with
no seas. The touted water may also have not been there in such
abundance: truckloads of olivine, which weathers easily in wet
conditions, cover the planet; the layered deposits might be
water, but they might also be formed just from dust blown by
winds in an ancient thick atmosphere; the gullies seen at crater
edges might also be caused by falling dry ice. And ALH 82001 has
surfaced yet again. Some see new evidence for microscopic life
forms, others say nonsense. In none of these instances will we
really know until we get there.
Say hi again to Jupiter: the Cassini spacecraft took some great
pictures while passing by on its way to Saturn. This giant
planet's surface seems quite varied with lots of up and down
motion and clear dry hot spots, one of which was hit by the
Galileo probe. The satellites are as intriguing. More evidence
for Europa's ice-covered ocean comes from finding that Europa
disturbs Jupiter's magnetic field by a secondary field that
Jupiter induces in the saltwater. Even Ganymede might have such
an ocean, indicated by valleys seemingly covered with once-slushy
ice. And we certainly have more moons to explore. I remember
when Seth Nicholson discovered number 12 in 1951. Wow, 12
satellites. Latest count is 28. (Most of the satellites are
captured, or are fragments of once-bigger ones)! Saturn, however
(whose rings might just be all ice by the way) beats it at 30.
Even Uranus comes in at 21. Don't count on these numbers staying
constant.
Leaving the planetary system, NASA left Pluto (or IS it a planet)
in the lurch by sadly canceling Pluto Express. Perhaps the
public, who seem to love the little guy, will rise up and say NO.
Leftovers
But important ones, as they tell us so much about how the Solar
System was assembled. We lucked out with the big new Tagish Lake
Meteorite fall in the Yukon, much of which was quickly frozen.
Curiously, the stones contain fewer amino acids than the rocks of
the more famed Allende fall. The chondrules in chondrites (hence
the name) remain mysterious. These little round inclusions bear
witness to flash heating and freezing. A new theory suggests
that they were formed at the sunward edge of the early solar
nebula by solar flares and then ejected by magnetically-driven
winds. There is no lack of theory. While life forms may not be
entirely safe from meteorites, the great Willamette meteorite at
the Rose Center in New York is safe from us. It can remain
there, and the Native Americans on whose land it once lay will
have the right to enter and visit their sacred object.
Thankfully, most asteroids (which produce the meteorites) remain
in space, so if we want to study them, we have to go there. The
NEAR landing on Eros was just spectacular. It set down at about
walking speed, and the last image gives a resolution of an inch!
Here we find uncratered "sand ponds" and decaying rocks that
nobody understands. It seems that the more data we accumulate
about an object, the more confused we get. There certainly are
plenty of the critters to study. We just passed the 20,000th
asteroid for which a good orbit has been computed; the total
discovery count hovers around 100,000! More than expected are
binary, and nobody knows why.
Could it be asteroids that brought water to Earth rather than
comets? Comets seem to contain too much deuterium (the heavier
form of hydrogen) relative to Earth's waters. While the news
goes back a ways, it's still fun to witness the disintegration of
Comet LINEAR. (The disintegration of Biela's Comet in the 19th
century lead to the discovery that meteors are cometary debris).
That's all ok, since there is a new Comet LINEAR trekking the
skies. And how about that Comet Hale-Bopp? Thirteen
astronomical units from the Sun, well beyond Saturn, it remains
active.
The short-period comets come in from the Kuiper Belt around and
beyond the orbit of Pluto. While Pluto's origin remains
mysterious, the planet is clearly related to the other Kuiper
Belt Objects. Nearly 400 KBOs are now known, a substantial
fraction in the same kind of resonant orbit as Pluto (which goes
around the Sun twice for every time Neptune goes thrice). Deep
observations suggest that the Kuiper Belt may end rather abruptly
(at least for larger objects) at 55 AU. And a few are really
large, two in the 600-900 km diameter range, comparable to the
large asteroids and a quarter the size of Pluto itself. Pluto
may just be the biggest.
Interstellar Medium and Star Formation
All of this stuff, the Sun, planets, and debris, came from the
dusty gases of interstellar space. The dark, cold molecular
clouds that are stellar birthplaces contain some (and probably
far more than) 120 species of molecule, including one of the
latest, interstellar sugar (a simple one, glycoaldehyde).
Where there is one planetary system (ours), there should be
others. And there are. Planets now seem to be at least in part
a natural outgrowth of star formation. The count now hovers at
46 extrasolar planets plus another dozen that fall above the 13
Jupiter-mass brown dwarf limit, at which the body begins to fuse
its natural deuterium. Real brown dwarfs (made by condensation)
and planets (made by accumulation of dust and gas in a
circumstellar disk) may overlap in mass; know one yet knows.
Several of the newly-found planetary "systems" (?) are special.
Epsilon Eridani was finally added to the group as the nearest of
them, a mere 10.5 light years away (but probably not close enough
to watch their soap operas on "Eps-TV.") Gliese 876 has two
near-Jupiter planets (at least 0.6 and 1.9 Jupiter masses) in
resonant orbit with periods of 61 and 30 days. HD 82943 (which
hovers in Hydra at the edge of human vision) also has a pair of
eccentric resonators, while HD 80606 in Ursa Major has a planet
with an orbital eccentricity of 0.93. Such resonant and
eccentric orbits ought to be unstable, yet there they are.
Another way of detecting planets is by their transits across
their stars, which produce shallow eclipses. Such an eclipse was
detected for HD 209458 (whose planet was known from Doppler
shifts). A look at 34,000 stars of the globular cluster 47
Tucanae, however, showed none at all. The low metal content of
47 Tuc, and the finding that many stars with planets are metal-
rich, suggests that high metal content (and thus relative youth)
are required to produce planets.
Stars
Astronomers have long sought the "stellar mass function," how the
number of stars varies with mass. The number increases
dramatically down the main sequence from class O (70 percent of
the classic spectral sequence, sans L and T, are class M), but
had long been thought to drop of somewhere within class M. Now
it looks like it may remain "flat" through M even into class L
and maybe cooler. There may be huge numbers of T dwarfs, and
star formation may continue to 3 Jupiter masses, though no one
knows for sure.
This is a good time to bring up the new spectral classes once
again. Low mass red stars are turning up in the new infrared
surveys (2Mass and the like). Their spectra contain not the
usual titanium oxide (which defines class M), but hydrides and
alkali metals. In the true spirit of the century-old Harvard
classification system, a new class had to be invented, and the
letter "L" was least confusing. The L stars have temperatures
between around 1500 and 2000 Kelvin. Even lower is class T with,
of all things, methane (just like Jupiter)! There seems to be
huge numbers of them, but not enough to make an impact on the
dark matter problem. A large fraction of the L stars, and all of
the T stars, are brown dwarfs.
M stars (including Proxima Centauri) are known for sudden
magnetically-induced magnitude-or-better flares (which are
observed from the radio to the X-ray). The solar magnetic field
is thought to be anchored at the base of the convection layer,
which extends about a third of the way inward. Cooler M and L
dwarfs, however, are fully convective, and should no one quite
knows how they should generate magnetic fields. Yet a brown
dwarf was seen to pop a flare.
Though we know all the chemical elements are present in stars and
in the Sun, not all are in sufficient abundance to be
spectroscopically detectable. It was greatly satisfying,
therefore, to finally find the spectral signature of uranium in
an ancient metal-poor halo star. A comparison with the abundance
of thorium (which has a different half-life) leads to an age of
12.5 billion years, similar to that found from globular clusters
and from the Hubble constant.
At the high mass end, Eta Carinae, considered among the most
massive stars in the Galaxy, may (from X-ray observations) really
be a double, with components of 30 and greater than 80 solar
masses. The argument goes on and on, and is not yet resolved.
Star Death
And finally, they all die. In doing so the stars produce some of
the most beautiful of celestial sights. Planetary nebulae are
among the grandest. Look at the weird filigree pattern, for
example, within the "Spirograph Nebula," IC 418. (The silly
names seem to be getting out of hand, enough so that the
planetary nebula research community recently discussed trying to
put a stop to it. But then it does no harm, and charms the
public, and as we all know there is little more important than
public support.) I like Menzel-3 here even better (and I can't
recall the silly name). No one yet understands the reason for
these bipolar flows (as in Mz 3), or why planetaries like IC 418
do not have them. Perhaps the dying stars (the nebulae were
ejected when the stars were giants) have orbiting companions,
even orbiting planets, that cause the phenomenon; perhaps
rotation or magnetic fields are responsible. Next, look at the
strange multiple rings surrounding the "Cat's Eye," NGC 6543,
showing episodic ejection from the predecessor giant. The
protoplanetary "Egg Nebula" and others show similar features.
NGC 6543 also emits powerful X-rays from the spaces between
optical features, where shocks have heated the gas.
At the end of low-mass evolution we find the white dwarfs. Many,
as expected, are in double systems. Hubble discerned some new
ones, 14 Aurigae and 56 Persei (both multiple systems) containing
white dwarfs. And look at this marvelous picture of Sirius. No,
the bright one is Sirius B! It's an X-ray image. In spite of
its small size, Sirius B's high temperature (27,000 Kelvin)
causes it to emit more X-rays than Sirius A (9400 K).
High-mass death is quite different, and leads to supernovae and
either to neutron stars or black holes. Some astronomers now
suggest that the collapse of the iron core within a high mass
star (which produces the supernova) creates bipolar jets driven
by rotation or magnetic fields. In any case, such explosions
seem to be off-center, which drives the resulting pulsar away at
high speed. A binary companion may also then escape at equally
high speed to create a "runaway star." The Crab and Vela pulsar
both show that the ejection is along the rotation axis (who knows
why). Runaways like the classic AE Aurigae and Mu Columbae,
however, were most likely ejected from the Trapezium cluster when
two high-mass binaries encountered each other. Iota Orionis --
an eccentric double -- was left behind.
Pulsars may not be understood as well as we might like. One
found in the remnant of the supernova of AD 386 has a spin-down
age (found from the pulse period and the lengthening of the
period) of 24,000 years! Something, somewhere is quite wrong.
Binaries that contain pulsars or black holes are apparently
discriminated by their X-rays when in a quiescent state, as the
black holes simply absorb the incoming gas, whereas neutron stars
make an X-ray fuss when the gas lands on them. Black holes
really do seem to exist.
Gamma ray bursts provide a link between stars and the distant
Universe. All seem to be very distant, the record now (from the
spectral redshift) 12 billion light years. They may be caused
"hypernovae," in which excessively massive supernova explosions
are beamed toward the observer, or perhaps neutron star
collisions. Yet the fastest of the bursts seem clumped in the
sky, not randomly distributed as they would be if all very far
away in distant Universe. Are some local?
The remains of supernovae are seen as expanding clouds of gas or
as shells of interstellar medium swept up by the blast waves.
Among the most famed of supernova remnants is the (shock-heated)
two-degree-wide Cygnus Loop, which may be younger than thought.
From its rate of expansion, it seems to be only 5000 years old
(if as close as 1500 light years).
Galaxies (Including Ours)
Black holes come in two distinctive varieties, stellar and
galactic (that is, mega-solar-mass black holes at the centers of
galaxies). Stellar orbits near the center of our own have raised
the mass of the central black hole (invisible optically because
of interstellar dust, but brilliant in radio radiation) to 2.6
solar masses.
So how about that Stefan's Quartet? Thought it was a Quintet,
did you? So did I. Seems that NGC 7320 is only a foreground
galaxy a mere 35 million light years away, 13 percent as far as
the others. As long as we are admiring galaxies, take a look at
superimposed (not colliding) NGC 3314. One is 20 million light
years away, the other 140. The backlighting of the foreground
object allows a different sort of examination of its interstellar
medium. You can get this picture as a screensaver!
Spirals (like these) have bulges, and it is the masses of the
bulges that seem to correlate with the masses of the central
black holes, not the masses of the entire galaxies. At least
that is some progress in understanding the origins of the
systems.
Dark Matter
Sure is dark, isn't it. Except that 15 percent is matter of the
same sort found in stars and in us (most not identified) and that
the rest is something else (exotic particles?), that's about all
we know. The amount of ordinary matter is derived from the
abundances of the elements (hydrogen, helium, lithium) that came
out of the Big Bang (that is, from the density required to make
these elements), and the total amount of dark matter from the
gravitational action of the dark matter on the motions of stars
and galaxies.
Finally the Universe
Sure is big, isn't it. The redshift of the most distant quasar
(there has to be a new record every year!) is 6.2. The light
left when the Universe was only a seventh the present size
(really, "scale," as nobody knows how big it actually is). How
about some more neat statistics? The largest supercluster now
known, in Leo, 6.5 billion light years from here, is 600 million
light years across. So far it is the largest known structure
other, of course, than the Universe itself. MACS, the "Massive
Cluster Survey," has turned up 100 massive clusters over 5
billion light years away. Within the vast structure of our
Universe, the Anglo-Australian Telescope redshift survey is well
on its way to the measurement of a quarter-million redshifts,
which will be compared with Cosmic Background Radiation (CBR)
fluctuations. The Chandra X-ray observatory produced its own
version of the Hubble Deep Field, taking a one-million second
exposure of a 1/4 degree-wide-field in Fornax that shows central
black holes of galaxies all over the place. One feels
breathless.
At the center of old-fashioned (but very much still current, if
that makes sense) cosmology is the classic determination of the
Hubble constant, a Key Project of the Hubble Space Telescope.
Using a variety of methods for distance determination, including
the precise use of Cepheid variables and the determination of
distant galaxies using Type Ia supernovae, the team has come up
with a "definitive answer," 72 +/-8 kilometers per second per
megaparsec, which yields an age to a flat universe of 14 billion
years. (Type Ia supernovae are most likely caused by the
transfer of matter from a star in a binary system onto a white
dwarf. If the white dwarf is sufficiently massive to start with,
it overflows the white dwarf limit of 1.4 solar masses and
explodes.) "Definitive" or not, the figure will probably still
be argued a bit. But what an improvement from only a few years
ago.
Which brings us to the final result. From the fluctuations in
the CBR, the Universe is flat and Euclidean, making Omega (the
ratio of the density of the Universe to that required for
closure) equal to 1. Yet all the matter in the Universe,
including both kinds of dark matter, yield an Omega of only about
0.35. Something else has to make up the difference. The clue to
its nature is found by examining high redshift galaxies that
contain Type Ia supernovae, whose distances can be found with
some accuracy. Their redshifts and distances strongly show the
Universe to be accelerating rather than decelerating as one would
expect under the action of gravity alone. The acceleration is
being produced, so it is thought, by a mysterious "dark energy,"
whose equivalent mass makes up the difference in the Omega, that
is, to bring it to One. Perhaps it is the energy of the vacuum
itself. No one yet knows. The most distant Type Ia supernova
even reveals the early era of deceleration when global gravity
was stronger than the expansive force.
Stars, it seems, the glory of both the visible heavens and of the
planetarium dome, constitute less than one percent of the
Universe's mass! Stars, however, can be readily seen, cascades
of them in the Milky Way providing one of the grandest sights
nature has to offer. They also trace out all the other stuff,
and lead us onward to learning what kind of Universe we really
live in. Return then to the perpetual stars. To see them, clear
skies to all.
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