Now you see it ... now you don't ...
Mars and the Great Comet
So, what's new in astronomy? Mars exploration continues to top the headlines. In 1997, using Pathfinder and its Sojourner rover, planetary scientists explored the red planet's weather and geology. Image after spectacular image showed a valley where rivers of water once raged across the landscape.
Far above the tortured surface of the planet, Mars Global Surveyor has been mapping dusty valleys, cratered plains, layered rock structures, and dry lake beds. Vividly characterizing the "now you see it ..." syndrome, Surveyor also found a global magnetic field. Since such fields are produced by electrically conducting interiors, scientists thought that Mars (by analogy with Earth) might have a still-hot iron core. It turned out, however, to be a fossilized magnetic field left over from the past.
In the "you never did see it" realm, Surveyor waded into the gulf between real science and pseudoscience with a high-resolution image of the infamous Face on Mars (above), allowing a comparison with the 1979 Viking image. The feature is an eroded mesa, not a sculpted signal built by a race of long-gone aliens (and it definitely isn't Elvis).
In the same vein, there wasn't really a flotilla of UFOs accompanying Comet Hale-Bopp. The real science coming from Hale-Bopp's visit to the inner solar system was far more exciting, involving a little chemistry and a bit of X-ray astronomy. First, astronomers detected a record number of different molecules such as methanol, ethanol, and formaldehyde in the gas boiled off the comet's icy nucleus. Second, they found Hale-Bopp sporting a new kind of tail, one made of sodium atoms. Third, given its chemical makeup, Hale-Bopp must have formed--as theorists expected--in the solar system's cold outer regions, where it spent eons in a cosmic deep freeze before making its trip into the realm of the inner planets.
Finally, following the discovery of X-rays from Comet Hyakutake in 1996, researchers used the Extreme Ultraviolet Explorer satellite to study X-rays from Hale-Bopp. These emissions appear to be created when energetic particles from the Sun slam into the comet's gaseous ejecta.
Planetary scientists will have to wait nearly 2,500 years to see Comet Hale-Bopp again. But their next close-up view of Mars is only a year away. (See page 15 for a look at the next wave of robotic Martian explorers.)
The Moon and the Sun
An interesting drama has unfolded on the Moon. Radar observations suggest that water ice lies hidden in the forever-dark and cold shadows of the deep Aitken basin at the lunar south pole. However, another study showed that the Moon doesn't have any water ice. Which is right? Thanks to the Lunar Prospector satellite, astronomers are now confident that the Moon does have frozen water, though only in tiny amounts at both poles ("now you see it ..." in reverse).
Other lunar news came from improved dating studies of Moon rocks returned by the Apollo astronauts in the late 1960s and early 1970s. We've thought the Moon was about 4.3 to 4.5 billion years old. These latest studies give us 4.51 billion years as a more definite lunar age. It now appears that the Moon formed about 75 million years after the Earth itself (apparently in a giant glancing collision with a body bigger than Mars).
Turning our attention to the Sun, the new solar cycle has begun, the increasing numbers of sunspots marching across the face of the Sun heralding the increase in solar magnetic activity. As if to celebrate, solar physicists used the European Space Agency's Solar and Heliospheric Observatory satellite to solve the puzzle of how the solar corona (outer atmosphere) is heated. Magnetic loops form with sunspots, and they have counterparts in myriad small but intense fields that form a "magnetic carpet" over the solar surface. Continuous short-circuiting sends magnetic energy aloft, raising the temperature of the pearly solar corona to two million degrees Kelvin.
The Major and Minor Planets
Jupiter and its swarm of natural satellites continue to give up secrets to the Galileo spacecraft, which has been orbiting the giant planet since late 1995. Of particular interest are Jupiter's four largest moons, which appear more and more like bona fide planets. Ice seems to cover a deep water ocean on Europa, keeping the remote possibility of life on that moon in the news. Europa, Io, and Ganymede also have iron cores. In fact, Ganymede has a related magnetic field that comes close to rivaling that of Earth.
Before reaching Jupiter, Galileo passed through the asteroid belt, where it imaged the tiny worlds Gaspra and Ida. Another asteroid, whose name--1997 X[F.sub.11]--sounds vaguely weaponlike, made news in early 1998. Preliminary orbital calculations showed that the object appeared to be drawing a bead on us. It would pass too close for comfort, and possibly even strike the Earth, in the year 2028. Fortunately, additional observations settled the issue by showing that 1997 X[F.sub.11] will miss us (now you see it ... now you don't ...) by nearly a million kilometers (600,000 miles). Whew!
It used to be easy to tell the difference between comets and asteroids. Comets were characterized as dirty snowballs sprouting tails of gas and dust when they got close enough to the Sun to feel its warmth. Asteroids were simply inert chunks of rock. Now life is more confusing, though perhaps more interesting. A rocky "asteroid," 1996 PW, appears to have come in from the Oort comet cloud beyond Pluto, and "asteroids" with tails have been seen in the main asteroid belt between Mars and Jupiter. The deeply dark asteroid Mathilde (thought to be a rocky body) was found by the Near Earth Asteroid Rendezvous (NEAR) spacecraft to have a density more like that of an icy comet. Alternatively, it could be a fused pile of interplanetary debris left over from earlier asteroidal collisions.
Even classic asteroids are not similar in appearance, density, or composition. Ceres, with a newly measured mass of 1 percent that of the Moon, has an average density only about two-thirds that of Earth rock, whereas Vesta's density is considerably higher.
The growing complexity of the solar system has been reinforced by telescopic observations beyond Pluto, where astronomers have turned up a previously unknown population of minor planets. The object known as 1996 TL66, for example, orbits in the Kuiper Belt. Its average distance from the Sun is twice that of Pluto's. The Kuiper Belt--a ring of material beyond Neptune --has been identified as the source of comets with relatively short orbital periods. So is 1996 T[L.sub.66] a comet? It's hard to tell. Its average distance from the Sun is twice Pluto's, and it never gets close enough to develop a tail.
Pluto itself, a body half the size of the contiguous United States, has been given the title "King of the Kuiper Belt." It's too big to be a comet, too small for a planet (at least according to some astronomers). It is joined by about 60 other bodies, including two dozen "plutinos" that, like Pluto, are caught in a gravitational resonance, orbiting the Sun three times for every two orbits of Neptune.
Meanwhile, Uranus is threatening Saturn's lead in having the most natural satellites. The discovery of two previously unseen bodies orbiting the blue-green planet brings its total to 17; Saturn remains ahead by only one or two.
Where Did Planets Come From?
At least one "now you see it ... " category has been put to rest by researchers studying stellar nurseries. It has been long accepted by astronomers that dark dust clouds spawn stars. Theoretically those clouds should also produce "substars," so-called brown dwarfs whose masses are less than 8 percent of the Sun's. These are too cool inside to fire up their nuclear furnaces. Over the years, numerous "discoveries" of substars have been made that later turned out to be erroneous. At last supercool Gliese 229B, with a surface temperature of only 1,100[degrees] K (compared to our Sun's 5,800[degrees] K), broke the logjam in 1995. Several other confirmed substellar objects have been found recently. It now appears that brown dwarfs (unlike Elvis) live!
What's the difference between large planets and small brown dwarfs? As far as we know, planets are built from the dusty debris of a disk-shaped nebula surrounding a new star, while brown dwarfs (like stars) are created whole by condensation from interstellar clouds. A brown dwarf is also able to fuse its natural deuterium (hydrogen with a neutron attached), which requires it to have a mass of at least 13 Jupiters.
Much smaller objects--believed to be true planets--seem to have been found around several nearby stars. So far about 10, some the size of Jupiter itself, have been detected. If they really are planets, some are exceedingly strange. The prototype is a roughly Jupiter-size body circling the star 51 Pegasi at a radius much smaller than Mercury's distance from the Sun. It took a while, but theorists finally came up with a couple of ways to get such a large planet so close to its parent star, where--according to our understanding of our own solar system's development--it shouldn't be.
Dusty disks seen around other stars could contain the building blocks of future planets. To those around Vega, Fomalhaut, and Beta Pictoris we add another around a double star and yet another with a central "hole" in which planets might already roam.What wonders will we find when we have a more complete inventory of planetary disks and systems?
From Starbirth to Stardeath
Interstellar dust is a crucial ingredient in the recipe for making a star. Gathered within great clouds of interstellar gas, the dust acts like a refrigerator that blocks out starlight and allows the clouds to chill to near absolute zero and form complex molecules.With little internal heat, condensations in the clouds can collapse to produce stars. The dust also makes the solid seeds from which planets might eventually form.
Cosmic dust is produced by a wide variety of dying stars. Of particular interest are the cool, distended Mira variables. These objects, with diameters as large as planetary orbits, are pulsating red giants whose brightnesses change regularly over months or years. They blow fierce winds into interstellar space, and some of the material in these winds condenses to form dust grains.
As large as Mira variables are, they are so far away that they have been seen only as points of light--until now. High-resolution observations made with the Hubble Space Telescope (HST), as well as from the ground with sophisticated imaging techniques, show that even the most fundamental assumption about Mira giants--that they are round like the Sun--is wrong. The stellar disks are not spherical but seem to take on elongated shapes. Usually such a shape comes about as a star spins rapidly. The faster it spins, the more it appears to flatten. However, Mira stars spin slowly and still retain their oblong appearance. Very odd!
Like many other stars, Miras end their days as planetary nebulae--beautiful objects created when dying giant stars blow off most of their outer envelopes, revealing burned-out cores. Fast winds from the remaining stellar envelopes sculpt the blown-off matter into intricate-looking shells that are then illuminated by the blistering hot cores.William Herschel, who discovered these objects late in the 18th century, named them for their disklike appearance through his telescope.
Current Hubble observations reveal complex structures within planetaries, supporting the idea that a small amount of asymmetrical mass loss (no surprise from a star that is not round!) can create hourglasses and other strangely symmetrical structures.
The shapes of some planetary nebulae may also arise from the gravitational interactions between the dying giants and nearby companion stars. As the two orbit each other, the outer shells of gas are sculpted into complex shapes.
It's also possible that orbiting planets, not companion stars, help produce the distorted shapes of some planetary nebulae, making the name far more apt than Herschel ever dreamed.
After Mira variables (and other stars) have exhausted their atmospheres, their cores turn into white dwarfs about the size of Earth.When very high-mass stars die, the situation is quite different. Their cores continue to fuse elements until they form iron. Then the cores collapse catastrophically as supernovae, while the outer envelopes of the stars explode away. The debris, enriched in heavy elements formed in the heat of detonation, spreads throughout the galaxy.
Chief among the candidates for such destruction is the star Eta Carinae, a 100-solar-mass star that we are only beginning to understand. New Hubble observations reveal the details of an extraordinary flow of debris cast off in an outburst that took place in the last century.
The recently discovered "Pistol Star," found by Hubble near the center of the Milky Way galaxy, may be the record-holder for massive stars, starting off at nearly 200 Suns. It, too, is shedding mass, filling the space around it with concentric shells of gas.
The imploded core of a supernova typically becomes a neutron star, a ball of neutrons packed into an area of space about the size of a small town. These are commonly referred to as pulsars because they seem to pulse in a regular rhythm. The pulses are actually the result of rapid rotation. The stars beam energy out from a tilted magnetic axis and act like spinning airport lights. If the beam happens to sweep across the Earth, we see a blast of radiation (though in the case of most pulsars, it's radio radiation, not visible light). Now, for the first time, a bare neutron star has been identified, one from which we see no pulses at all. This object has set an all-time stellar temperature record of more than a million degrees Kelvin.
A huge stream of antimatter was recently discovered near the center of our galaxy. It may be caused by strings of supernovae or by a jet of matter emerging from the massive black hole believed to lie at the Milky Way's core. Observations of the antimatter with NASA's Compton Gamma Ray Observatory even raised some concern in the popular press that, even though it's 20,000 light-years away and extremely thin, the jet might hit and melt us! In any event, the real excitement lies in answering the question of where the flow originates--a quest that will energize galactic studies for some time to come!
More and more we find that this galaxy of ours is not a simply evolving monolithic structure but that it was assembled in part from collisions with other galaxies in different states of evolution. These collisions explain our galaxy's complexity. The newly discovered Sagittarius Dwarf galaxy is even now crashing into the Milky Way, bringing its own stellar populations with it, including the well-known globular cluster M54. In the far distant future, our home galaxy could look quite different indeed! There is a growing sense that most large galaxies possess central black holes. New studies of the speed of material swirling around the black hole in M84 indicate that it contains the mass of more than a billion Suns. In addition, Hubble observations of the center of NGC 6251 seem to reveal a nearly bare black hole whose bright surroundings illuminate a circulating disk of material spiraling into the ever-growing central "singularity" (the heart of the black hole, where the matter has seemingly shrunk into a point).
The most curious manifestations of black holes are quasars. They have long been suspected of being early galaxies with ultrabright nuclei resulting from the infall of matter into central black holes. Observations of "fuzz" around the cores seem to support the existence of disks of material spiraling in from the surrounding galaxies to feed these monsters, or even from matter invading from collisions with other galaxies.
In addition to harboring black holes, some distant galaxies are also suspected as sources of the mysterious gamma-ray bursts, flares of high-energy radiation detected by orbiting satellites at a rate of more than one a day. Optical counterparts of a few bursts have been detected and associated with distant fuzzy-looking clouds.
The light from one such cloud has a high redshift, meaning that the cloud is speeding away from us. In the 1920s astronomers found that the universe is expanding and that the more distant a galaxy, the faster it moves away from us and the more its light is shifted toward "redder" (longer) wavelengths. The cloud in question must therefore be a distant galaxy. If it is truly remote, the gamma-ray burst that erupted from it was one of the most powerful explosions known in nature. The only phenomena believed capable of producing such energies are collisions between such exotic and massive objects as neutron stars or black holes.
Way Back When
Peering billions of light-years into space, the Hubble Space Telescope also looked far back in time to when the light from distant galaxies first began its journey. All those billions of years ago galaxies looked different, appearing like ragged shards of matter rather than the stately spirals and ellipticals we see around us today.
Galaxies have a strong tendency to cluster together. Large-scale clustering seems to have taken place only a billion or so years after the Big Bang, almost too fast for theory to explain, leaving us with more mystery.
And how long ago was this Big Bang? Cosmologists have long wrestled with the fact that globular clusters seem older than the universe itself. (Globular clusters are dense collections of very old stars--often older than the galaxies to which they are attached.) To solve the problem we need to know how fast the universe is expanding. The expansion rate is defined by a number called the Hubble constant, which tells the increase in expansion speed for each million light-years in distance. The greater the Hubble constant, the shorter period of time it took for a galaxy to get to a certain distance, and the younger the universe.
New studies of galaxy distances made using measurements from the European Space Agency's Hipparcos satellite have moved the galaxies outward, making the universe seem older. At the same time, new studies of the distances to globular clusters make them seem younger, helping to reconcile the discrepancy. From the current Hubble constant we derive an age for the universe of about 15 billion years, in rough agreement with, and perhaps a bit more than, the ages of globular clusters.
However, just to prove that nothing is simple, remember that the universe contains mass, which drags on it and slows down its expansion rate. The Hubble constant thus decreases with time. Thus, the real age of the universe must be less than that found from the current Hubble constant. The relationship between the simple age found from the Hubble constant and the real value depends in a complex way on how much matter the universe contains.
If the universe has enough mass to "close itself," that is, to make the expansion someday come to a halt, then it's only 10 billion years old and we still have a problem with globular cluster ages. However, new measurements of the relationship between expansion and distance have been made by looking at distant supernovae. These studies show that the universe is not slowing down as quickly as it would if it were closed. There's even a hint that the expansion is accelerating! The universe therefore seems destined to expand forever. As a result, the real age of the universe is indeed not much different from the ages of the globular clusters. The actual universe can indeed contain its stars.
However, as in all other aspects of astronomy, it pays to be wary of what we think we understand. Like our view of the expanding universe, our understanding is changing. Remember, "now you see it ... now you don't."
JAMES KALER writes about astronomy and studies planetary nebulae at the University of Illinois.
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|Title Annotation:||Astronomy In Review|
|Author:||Kaler, James B.|
|Date:||Jan 1, 1999|
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