A team of astronomers led by David Weintraub of Vanderbilt University in Nashville, Tennessee, studied six “Centaur” asteroids. Centaurs lie well beyond Jupiter, are between 100 and 400 kilometres in diameter and are usually bluish in colour.

The researchers made observations of the Centaurs using a 3-metre infrared telescope on Mauna Kea in Hawaii and a visible telescope at the Steward Observatory on Kitt Peak, Arizona.

At this week’s meeting of the American Astronomical Society in Winston-Salem, North Carolina, the team announced that when they compared the amounts of reflected visible and infrared light from two of the Centaurs, known as 5145 Pholus and 1995 GO, they found that they are redder than any other asteroids in the Solar System.

They are red, Weintraub says, because they are covered in organic substances and pristine minerals that have hardly changed since the birth of the Solar System. This suggests that the asteroids have recently fallen out of the very distant comet-rich disc beyond Pluto, known as the Kuiper Belt. Out there, they did not experience the kind of heating and collisions with other asteroids that can erase these surface materials.

“If we are right,” says Weintraub, “then red Centaurs are freshly ejected from the Kuiper Belt into unstable orbits in the outer Solar System.”

Weintraub adds that Centaurs have elongated orbits that occasionally bring them across the paths of Neptune, Uranus and Saturn. Every 10 million to 100 million years, a close encounter with a giant planet like Saturn ejects a Centaur into interstellar space or diverts it into the inner Solar System. So red Centaurs may have collided with the Earth in the past.

The red Centaurs “may have carbon and nitrogen-rich materials on their surfaces that may represent the building blocks from which life in our Solar System got started”, says Weintraub. However, it is not yet clear exactly what these reddish compounds are. “What form or flavour those organics are is completely unknown and would make for a really interesting study, if someone could figure out the right experiment to do.”

THE Galileo spacecraft may have caught a glimpse of molecules containing
carbon and nitrogen on the surface of two of Jupiter’s moons, Ganymede and
Callisto. The presence of these elements, which are vital for creating life,
adds these worlds to the growing list of places in the Solar System that may
have once harboured living creatures.

ĐÓ°ÉÔ­´´s announced the result at last month’s Lunar and Planetary Institute
Conference in Houston. The evidence comes from data gathered by the Near
Infrared Mapping Spectrometer (NIMS) aboard Galileo. The instrument analysed the
light reflected by the moons’ surfaces at near-infrared wavelengths, and from
these data researchers worked out which chemicals make up the surface.

Thomas McCord of the University of Hawaii, one of the scientists on the NIMS
team, says that four new substances came to light on both Ganymede and Callisto.
Three of the these—various water-bearing minerals, water ice and sulphur
dioxide—are not unexpected. The discovery that caused most excitement was
the presence of organic cyanides called nitriles. These contain a CN group: a
pair of carbon atoms linked to a nitrogen atom by a triple bond.

McCord says that the observations could point to several different molecules,
but that those containing CN are the most likely. “We’re saying that CN is the
best candidate—that doesn’t mean that there aren’t others.”

Galileo has not yet flown close enough to Jupiter’s moon Europa to see
whether nitriles exist there, too. “The signal level is much lower for Europa,”
says McCord. Europa is one of the key targets in the search for extraterrestrial
life (see “Jupiter’s odd bunch”). Galileo will be able to look for organic
molecules there later in the mission.

SO momentous was the sighting of Supernova 1987A that it turned nearly
everyone’s attention to the skies. But the real excitement, it turns out, was
going on underground in a tin-zinc mine in the Japan Alps near Kamioka, where
scientists had built instruments designed to detect neutrinos emitted by the
Sun. Shadow of a Star by Alfred Mann is a surprisingly engaging account
of how science triumphed in the nick of time to detect neutrinos from Supernova
1987A.

I say “surprisingly engaging” because, unless you’re a die-hard particle
enthusiast, you don’t expect a story about detecting neutrinos to compare to the
excitement of seeing a supernova or studying its tattered remnants.
Nevertheless, Mann, who is a physicist, pulls it off—not merely because
his writing is proficient but because the story is a cracking one. Shadow of
a Star is a captivating drama that illustrates why researchers will go to
the ends of the earth to understand the Universe.

Central to the story is the fact that the neutrinos from Supernova 1987A
might have gone undetected had Mann and his Japanese colleague, physicist
Masatoshi Koshiba, failed to get the Kamioka detector ready on time. Work began
in 1984, but it took more than two years to install the sensitive detectors and
eliminate background radiation from cosmic rays and radon gas. The work was
arduous and often aroused tensions between the Japanese and American physicists.
But dedication to the cause prevailed, and in late 1986 the detector was ready
pick up stray solar neutrinos.

Little did anyone realise, however, that Kamioka would detect 12 neutrino
events, not from the Sun, but from a supernova 160 000 light years away. The
neutrinos arrived a few hours before the first sighting of the star. Mann
describes how thrill turned to triumph when these neutrino events were later
confirmed by data from another large detector in a salt mine near Cleveland,
Ohio, which picked up eight neutrino events occurring almost simultaneously.

Of course, every story needs a proper substructure. But when your main
characters are exploding stars, neutrinos and particle detectors, the foundation
may seem overly complex. Fortunately, Mann, a believer in responsible science
popularisation, succeeds well in providing the necessary gory details about
particle physics and stellar evolution without turning off uninitiated readers.
Colour illustrations, a glossary and appendixes, including a fine explanation of
powers of 10 notation, complement the text.

For readers who want to learn more about how extreme things can
get—both in the Universe and in the pursuit of science—or for those
looking for a refresher course on Supernova 1987A, Shadow of a Star is
the book of choice.

MYSTERIOUS, fast-moving clouds of hydrogen gas that have baffled astronomers
since they were discovered in the 1960s may be leftover remains from the time
when the Milky Way and other nearby galaxies were forming, claims a group of
researchers from the US and the Netherlands.

Hydrogen is found throughout the Universe and is the most abundant gas in the
Milky Way. But in 1963, astronomers discovered clouds of hydrogen in space that
were moving much faster than normal cosmic hydrogen. Further observations
suggested that these high-velocity clouds were streams of gas either being drawn
out of the Milky Way or falling into it.

Leo Blitz of the University of California at Berkeley and colleagues from
four other institutes in the US and the Netherlands used two ground-based radio
telescopes to analyse the radiation emitted by the high-velocity hydrogen. They
also re-evaluated infrared data from the COBE satellite and the Hubble Space
Telescope.

By matching the data to computer simulations of the motion of the galaxies,
the researchers concluded that most, if not all, the observed properties of
high-velocity clouds could be explained if these clouds were orbiting the centre
of mass of a collection of about 30 galaxies known as the Local Group. This
group includes the Milky Way and the Andromeda Galaxy.

The researchers claim that the high-velocity clouds are nearby examples
of more distant clouds of hydrogen gas that have been detected spectroscopically
between galaxies. These distant clouds are thought to have been among the first
structures that formed in the Universe.

NO ONE will ever see a black hole, because light cannot escape from them. But
American researchers say they have found the most direct evidence yet of the
“event horizons”—the boundary beyond which matter and energy cannot return
—of four black holes.

Ramesh Narayan, Jeffrey McClintock and Michael Garcia from the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts,
examined satellite images of nine X-ray novae, in which stars orbit compact
invisible sources such as black holes or neutron stars.

Gas from the orbiting star flows steadily towards the more compact object in
a process called “accretion”, in which the gas is drawn into a disc around the
central object. Friction within the rapidly rotating accretion disc heats the
gas up to extraordinarily high temperatures, often up to a million million
degrees, which is why it radiates X-rays.

The researchers found that although all nine systems are accreting gas, four
of them are dimmer than the others. They claim that the dimmer novae have a
black hole at their centre, and that nearly all of their thermal energy is
vanishing across the black hole’s event horizon, rather than being radiated out
into space. In the brighter novae, they say, the thermal energy is being
reradiated from the surface of a neutron star.

When the superheated gas reaches the event horizon it disappears “down the
drain” into the black hole, taking its enormous thermal energy with it. “This is
the most direct evidence scientists have had that black holes are real,” claims
Narayan. And, he adds, it confirms that they have event horizons.

IT’S a rare galaxy that lacks a supermassive black hole at its centre,
according to an international team of astronomers. What’s more, says the team,
the mass of the supermassive black hole, believed to be a dormant quasar,
appears to match that of its host galaxy.

Using the Hubble Space Telescope and ground-based telescopes in Hawaii, Doug
Richstone of the University of Michigan and colleagues measured the wavelength
and intensity of the light coming from the central regions of 27 nearby
galaxies. The data revealed that there were supermassive black holes at the
centres of all 27 galaxies.

The researchers then worked out the masses of the black holes by measuring
the masses and velocities of the stars orbiting them. They found a strong
correlation between the mass of a galaxy and the mass of its central
supermassive black hole.

Richstone suggests that the central black holes are “fossil quasars. We
believe that most galaxies at one time burned brightly as a quasar.” He says the
development of the black hole probably depended on the development of its host
galaxy.

Richstone admits that his conclusions may not apply to all galaxies. He says
his group is suggesting only that supermassive black holes may be a common
feature among most galaxies. “All we can do is extrapolate and argue about the
physics. We think we’re starting to get a clue about the global picture [of
black holes and galaxies], and it looks simple. What we seem to be discovering
makes sense.”

Charles Steidel, an astronomer at the California Institute of Technology in
Pasadena and a researcher on the early Universe, says Richstone’s work is “very
interesting. Many people have come to believe over the last 10 years or so that
most galaxies probably harboured quasars at one time or another.”

Dark matter is virtually undetectable except by its gravitational influence
on galaxies. Samar Abbas and Asfar Abbas of the Institute of Physics at Utkal
University in Bhubaneswar, India, suggest that Earth could have encountered
dense clumps of dark matter in space, and that this produced large quantities of
heat in the planet’s interior. Over time, they say, the build-up of heat could
have led to catastrophic volcanic eruptions. The subsequent climate changes
would in turn have wreaked havoc on living species, possibly causing mass
extinctions. Their paper is available at the Los Alamos National Laboratories
Web site (http://xxx.lanl.gov/abs/astro-ph/9612214).

In the mid-1980s, scientists estimated that, given a uniform distribution of
dark matter throughout space, Earth could “capture” as many as 10¹⁸ particles
per second. Abbas and Abbas say this means that Earth would eventually gather so
many dark-matter particles in its dense core that they would begin to collide
with normal matter there, annihilating themselves in the process. These
annihilations could create 10 billion watts of extra heat in the lower
mantle.

But according to some cosmologists, dark matter is not uniformly distributed,
but is instead clumped throughout the Universe. The researchers calculate that
an encounter with a dense clump of dark matter could generate even more
heat—enough to cause magma to well up from the lowest regions of the
mantle.

Pulsars are neutron stars, which are themselves the collapsed cores of
supernovae. According to a new theory, the simplest explanation for their speed
is a subatomic drama in the collapsing core that culminates in a violent, and
slightly uneven, blast of neutrinos.

Most pulsars are between 20 and 30 kilometres in diameter, with intense
magnetic fields and rapid spins. In extreme cases, pulsars can spin more than
100 times a second. From Earth, they appear to pulsate because the beams of
radio waves they emit from their poles sweep past the Earth with every
revolution.

Since the discovery of the first pulsar in 1967, astronomers have struggled
to explain why they travel so fast. Although most researchers agree that the
high velocities of pulsars must be related to their origin in supernova
explosions, the exact cause of the pulsar’s “punch” has remained a mystery.

In a paper to be published in the 9 December issue of Physical Review
Letters, Alexander Kusenko and Gino Segre of the University of Pennsylvania
argue that the velocity kick that propels pulsars through space may be the
result of a flood of neutrinos rushing out from the collapsing core, but in a
nonuniform way, like the air from a balloon.

When a massive star explodes, it can temporarily outshine a galaxy of several
hundred billion suns. But even more staggering is the energy released in the
form of neutrinos, which can briefly exceed the luminosity of all stars in the
known galaxies. “Most of the energy involved with gravitational contraction of
the supernova core is converted into heat,” says Kusenko, “and 99 per cent of
that energy is carried away by neutrinos.” But the neutrino emission may not be
uniform—and this may be what causes the kick.

Neutrinos usually pass through normal matter. But the hot interior of a
recently-formed neutron star is so dense that neutrinos begin to interact with
it. The three types of neutrino—electron, tau and muon—interact with
the matter in slightly different ways, with the more massive muon and tau
varieties able to escape from deeper within the neutron star.

It is possible, the researchers say, that during this interaction neutrinos
may flip from one type to another: an electron-type neutrino may change to a tau
type, which could then quickly escape. It turns out that the distance over which
these flips occur depends on the direction of the magnetic field in that region
of the neutron star, so that on one side, far more tau neutrinos will emerge
from its hotter inner regions. “You open the door on one side and let out the
hot particles, but you don’t open the door on the other side,” says Kusenko.
“This is quite similar to how a rocket flies.”

According to Kusenko and Segre, a difference of just 1 per cent in the total
momentum of neutrinos emerging in either direction would result in a kick, or
“recoil velocity”, that is consistent with the measured average pulsar velocity
of 500 kilometres per second.