two bits from SCIENCE-WEEK October 20, 2000

From: Eugene Leitl (
Date: Sat Oct 21 2000 - 19:54:50 MDT

From: "Science-Week" <>



     The death of supermassive stars must result in collapse,
since no known force can resist gravity in such stars once their
nuclear energy sources are exhausted. For the most massive stars,
the result of this inevitable collapse is the "*black hole", at
the present time the most exotic astronomical object in the
Universe. In recent years, astrophysicists have come to
distinguish ordinary black holes and supermassive black holes,
and these latter objects now reign as the supreme cosmic exotica,
with masses 1 million to nearly 10 billion times the mass of the
Sun, and an origin completely unknown. The center of our own
Galaxy is apparently a supermassive black hole, and observations
indicate that supermassive black hole growth and galaxy formation
in general are closely linked. Supermassive black holes are thus
becoming an integral part of our understanding of galaxy
formation and galaxy dynamics. One important approach is to
estimate the properties of a galactic-center black hole from
measurements of the gravitational behavior of nearby objects.
... ... A.M. Ghez et al (5 authors at University of California
Los Angeles, US) now report the first measurements of
accelerations of stars orbiting the supermassive black hole that
apparently forms the center of our Galaxy. The authors make the
following points:
     1) Recent measurements of the velocities of stars near the
center of the Milky Way Galaxy have provided the strongest
evidence for the presence of a supermassive black hole, but the
observational uncertainties make the properties of the black hole
ill-defined. More precise information concerning the position and
mass of the central black hole can be obtained by determining the
accelerations of stars in their orbits around the center.
     2) The authors report measurements of the accelerations of
three stars located approximately 0.005 *parsecs (projected on
the sky) from the central radio source Sagittarius A*. The
authors report these accelerations are comparable to those
experienced by the Earth as it orbits the Sun. The authors
suggest their data increase the inferred minimum mass density in
the central region of the Galaxy by an order of magnitude
relative to previous results, and localize the dark mass to
within 0.05 +- 0.04 arcsec of the nominal position of Sagittarius
A*. In addition, the authors suggest the orbital period of one of
the observed stars could be as short as 15 years, which would
provide an opportunity in the near future to observe an entire
... ... In a commentary on the above work, John Kormendy
(University of Texas Austin, US) states: "With these
measurements, the authors help to identify Sagittarius A* as the
supermassive black hole at the center of the Milky Way. They also
test -- and provide support for -- fundamental assumptions about
the dynamics of the Galactic center and our understanding of
violent activity in [*active] galactic nuclei."
A.M. Ghez et al: The accelerations of stars orbiting the Milky
Way's central black hole.
(Nature 21 Sep 00 407:349)
QY: A.M. Ghez:
John Kormendy: Galactic rotation in real time.
(Nature 21 Sep 00 407:307)
QY: John Kormendy:
Text Notes:
... ... *black hole: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable.
... ... *parsecs: 1 parsec equals 3.262 light-years, or 30.86 x
10^(12) kilometers.
... ... *active galactic nuclei: (AGN) Some galaxies are known to
have very "active" central regions from which enormous amounts of
energy are emitted each second. These "active galactic nuclei"
are probably powered by accretion of matter into a supermassive
black hole of 10^(6) to 10^(9) solar-masses.
Summary & Notes by SCIENCE-WEEK 20Oct00
For more information:
Related Background:
Recent observations have led to the conclusion that at the center
of many galaxies there is an object producing effects
characteristic of a supermassive *black hole. Alexei V.
Filippenko (University of California Berkeley, US) reviews
current research on black holes, the author making the following
points concerning the apparent massive black hole at the center
of our own Galaxy:
     1) Some galaxies are known to have very "active" central
regions from which enormous amounts of energy are emitted each
second. These "active galactic nuclei" are probably powered by
accretion of matter into a supermassive black hole of 10^(6) to
10^(9) solar-masses. The center of our own Galaxy exhibits mild
activity, especially at radio wavelengths: so-called "nonthermal
radiation" characteristic of high-energy electrons spiraling in
magnetic fields is emitted by a compact object at the Galactic
center known as *Sagittarius A*. Does the center harbor a
supermassive black hole? One approach is to determine whether
stars in the central region are moving very rapidly, as would be
expected if a large mass were present. During the past 5 years,
two teams have obtained high-resolution images of our Galactic
center, each team on several occasions, so that temporal changes
in the positions of stars could be detected. The observations
were conducted at infrared wavelengths, which penetrate the gas
and dust between Earth and the Galactic center (a distance of
approximately 25,000 light years) much more readily than optical
light. In summary, the data are in excellent agreement with the
conclusion that the gravitational potential of the central region
of our Galaxy is dominated by a single object. The derived mass
of this object is (2.6 +- 0.2) x 10^(6) solar-masses, and the
mass density within a radius of 0.05 light-years is at least 6 x
10^(9) solar-masses per cubic light-year, effectively eliminating
all possibilities other than a black hole.
     2) Although our Galaxy provides the most convincing case for
the existence of supermassive black holes, observations of the
centers of a few other galaxies bolster the conclusion. For
example, very precise measurements of the galaxy NGC 4258 reveal
a central compact object with a derived mass 3.6 x 10^(7) solar-
masses. On somewhat larger scales, spectra obtained with the
Hubble Space Telescope show gas and stars rapidly moving in a
manner consistent with the presence of a supermassive black hole.
The most massive existing case, that of the giant elliptical
galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover,
x-ray observations of some active galactic nuclei reveal emission
from a hot disk of gas apparently very close to a black hole,
since extreme relativistic effects are detected. In general, it
now seems that a supermassive black hole is found in nearly every
large galaxy amenable to such observations.
     3) The author concludes: "In the last decade of the 20th
century, black holes have moved firmly from the arena of science
fiction to that of science fact. Their existence in some *binary
star systems, and at the centers of massive galaxies, is nearly
irrefutable. They provide marvelous laboratories in which the
strong-field predictions of Einstein's general theory of
relativity can be tested."
Alexei V. Filippenko: Black holes in the Milky Way galaxy.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9993)
QY: Alexei V. Filippenko []
Text Notes:
... ... *black hole: See notes in main report.
... ... *Sagittarius A*: Sagittarius A is a prominent radio
source in the constellation Sagittarius, coincident with or close
to the center of our Galaxy. It is a highly complex region
consisting of a central core approximately 50 light-years in
diameter. Sagittarius A* is a compact component at the heart of
the central core of Sagittarius A. Sagittarius A* is an intense
source of radio waves, and is apparently unique in our Galaxy:
while everything else in our Galaxy is on the move as they follow
their orbits, Sagittarius A* is absolutely stationary and must
therefore lie exactly at the Galaxy's center. Many astronomers,
in fact, use Sagittarius A* as the "Greenwich Meridian" of the
... ... *binary star systems: Binary stars are a pair of stars
revolving around a common center of mass under the influence of
their mutual gravitational attraction, and apparently the
majority of stars in the Universe are binaries and not singlets.
In some cases the binary system is resolvable into two
components, and in other cases the presence of a second star is
inferred by perturbations in the motion or emitted radiation of
the first star. If the binaries are close enough, they may share
stellar material, and this results in a particular kind of
stellar evolution. In the black hole-binary systems mentioned in
this report, matter transfers from a relatively normal star
(known as the "secondary star") to a dark compact object (the
"primary"). Recent comparisons of x-ray and optical brightness
suggest that in many cases the dark primary in such a binary
system is a black hole.
Summary & Notes by SCIENCE-WEEK 15Oct99
For more information:


     The Near Earth Asteroid Rendezvous (NEAR), a Discovery
spacecraft launched on 17 February 1996, was designed to carry
out the first detailed orbital investigation of an asteroid.
After a flyby of asteroid 253 Mathilde in June 1997, and a
swingby of Earth in January 1998, the spacecraft carried out an
unintended flyby of the asteroid Eros, approaching within 3800
kilometers on 23 December 1998. During this flyby, data important
to the detailed planning of the subsequent orbital mission, such
as mass, dimensions, and spin state, were obtained. On 3 January
1999, a burn was executed to slow the speed of the spacecraft
relative to Eros from approximately 1 kilometer per second to
only 10 meters per second, resulting in a year-long gradual
return to the asteroid. The spacecraft was successfully inserted
into an initial orbit around Eros on 14 February 2000.
... ... J. Veverka et al (33 authors at 10 installations, US)
present an analysis of data obtained from the NEAR mission to
Eros, the authors making the following points:
     1) Discovered in 1898, Eros was the first asteroid found to
cross the orbit of Mars, and was the first asteroid detected to
show periodic brightness fluctuations, which were soon attributed
correctly to its elongated shape and a rapid rotation period of
5.27 hours. The orbital path of Eros takes it from 1.13
*astronomical units (AU), close to Earth's orbit, to beyond the
orbit of Mars at 1.73 AU, every 1.76 years. Spectrally, Eros is
classified as an S-type asteroid, the type common in the inner
portions of the asteroid belt, the surface mineralogy of which is
apparently dominated by silicates (pyroxene and olivine) and Fe-
     2) Eros is a very elongated (34 kilometers by 11 kilometers
by 11 kilometers) asteroid, most of the surface of which is
saturated with craters smaller than 1 kilometer in diameter. The
largest crater is 5.5 kilometers across, but there is a 10-
kilometer saddle-like depression with attributes of a large
degraded crater. Surface lineations, both grooves and ridges, are
prominent on Eros; some of these probably involve planes of
weakness produced by collisions on Eros and/or its parent body.
Ejecta blocks (30 to 100 meters across) are abundant but not
uniformly distributed over the surface. *Albedo variations are
restricted to the inner walls of certain craters and may be
related to downslope movement of *regolith. On scales of 200
meters to 1 kilometer, Eros is relatively bland in terms of color
variations, and spectra are consistent with an *ordinary
chondrite composition for which the measured density of 2.67 +-
0.1 grams per cubic centimeter implies internal porosities
ranging from approximately 10 to 30 percent.
... ... In a commentary on the above report, Richard P. Binzel
(Massachusetts Institute of Technology, US) states: "No longer
just "star-like" points in the sky, asteroids have become the
focus of dedicated geological and geophysical studies aimed at
gaining insights into planetary formation and addressing
practical concerns for the long-term safety of our planet. These
studies are yielding new links to meteorites and planetary
J. Veverka et al: NEAR at Eros: Imaging and spectral results.
(Science 22 Sep 00 289:2088)
QY: J. Veverka, Cornell University, Ithaca, NY 14853 US
Richard P. Binzel: Asteroids come of age.
(Science 22 Sep 00 289:2065)
QY: Richard P. Binzel:
Text Notes:
... ... *astronomical units (AU): 1 AU = the mean distance from
the Sun to the Earth = approximately 93 million miles, and
exactly 149,597,870 kilometers.
... ... *Albedo: In this context, the fraction of total sunlight
falling on an asteroid that is reflected from it. In general, the
albedo is equal to the amount of light reflected divided by the
amount of light received.
... ... *regolith: In general, surface layers of rock that overly
... ... *ordinary chondrite: Meteorites are divided roughly into
3 main classes according to their composition. "Iron meteorites"
consist of an alloy of iron and nickel; "stony meteorites"
consist of silicate minerals; and "iron-stony meteorites" are a
mixture of the two previous types. The stony meteorites are
further divided into "chondrites" and "achondrites". Chondrites
contain small spherules of high-temperature silicates
("chondrules") and constitute more than 85 percent of recovered
meteorites. The achondrites range in composition from rocks made
up essentially of single minerals (e.g., olivine) to rocks
resembling *basaltic lava. Each category is further subdivided on
the basis of chemical composition. "Carbonaceous chondrites" have
little or no metal but abundant carbon, and display evidence of
chemical alteration by water; they have the highest proportion of
volatile elements and are the most oxidized. "Ordinary
chondrites" (the most common type) are intermediate in volatile
element abundance and oxidation state.
... ... *basaltic lava: Basalt is a dark gray to black igneous
rock of volcanic origin that cools rapidly. "Igneous rocks" are
rocks that have congealed from a molten mass.
Summary & Notes by SCIENCE-WEEK 20Oct00
For more information:
Related Background:
Asteroids (also called "minor planets") are small rocky objects,
most of which orbit the Sun in a belt between the orbits of Mars
and Jupiter. A few asteroids follow orbits that bring them into
the inner Solar System, and several asteroids occasionally pass
within a few tens of millions of miles of Earth. Some asteroids
are located in the orbit of Jupiter, and some asteroids have been
detected as far away as the orbit of Saturn. Concerning asteroids
in general, approximately 200 of these objects are more than 100
kilometers (60 miles) in diameter, and more than 2000 asteroids
are more than 10 kilometers in diameter. There are believed to be
approximately half a million asteroids with diameters greater
than 1 kilometer. The consensus view is that asteroids are
composed of material that failed to build a planet at a distance
of 2.8 *astronomical units (AU) from the Sun, perhaps due to the
influence of massive Jupiter just outside the asteroid belt
[*Note #1]. Until recently, the shapes and surface features of
asteroids were a matter of conjecture; during the past decade,
however, significant direct observations of asteroids have been
relayed back to Earth from spacecraft.
... ... Erik Asphaug (University of California Santa Cruz, US)
presents a review of current research on the geology of
asteroids, the author making the following points:
     1) The dominant geological process affecting asteroids is
collision with other asteroids, most of these collisions
producing craters formed when material is blown explosively from
the impact site. The most violent collisions are catastrophic and
produce families of smaller asteroids. At lower impact
velocities, colliding asteroids can merge, and this is relevant
for an understanding of the formation of planets from the swarm
of *planetesimals (proto-asteroids) in the early Solar System.
Because catastrophic collisions and cratering reduce the mass of
an asteroid, one research problem has been to provide an
explanation of the survival of asteroids and a description of how
planetesimals could have accumulated into planets.
     2) All known apparent asteroid densities are quite low,
suggesting these bodies are porous. This porosity might be due to
a fragmented internal structure or to a structure consisting of
loosely bound piles of material. Many asteroids have giant
craters which are half their total size or larger, and since
craters of that size cannot form in solid rock without shattering
the parent body to pieces, their existence has been a puzzle.
     3) Computer models and laboratory experiments have
demonstrated that because a composite asteroid transmits impact
energy so poorly, it is much more difficult to break up than a
monolithic asteroid, so the internal composition and structure of
an asteroid are important factors determining the effect of a
     4) A recent hypothesis by Housen et al (2 installations,
US), based on experimental data and dimensional analysis
(scaling) arguments, suggests that highly porous asteroids might
accumulate rather than lose mass during collisions. These authors
show that giant craters could be formed by compression of a
highly porous target, rather than as a result of ejected
material. Thus, high porosity might be a transient feature of
certain asteroids, which become more dense with each impact. This
is good news for researchers who model planet formation and who
need a mechanism for planetesimal mass accumulation. But it is
bad news for planners of protection of the Earth against asteroid
impacts, because the implication is that instead of an asteroid
on a collision course with Earth being diverted or fragmented by
a missile, the asteroid could simply absorb explosive energy.
Erik Asphaug: Survival of the weakest.
(Nature 11 Nov 99 402:127)
QY: Erik Asphaug []
K.R. Housen et al: Compaction as the origin of the unusual
craters on the asteroid Mathilde.
(Nature 11 Nov 99 402:155)
QY: Kevin R. Housen []
Text Notes:
... ... *astronomical units (AU): 1 AU = the mean distance from
the Sun to the Earth = approximately 93 million miles, and
exactly 149,597,870 kilometers.
... ... *Note #1: The figure "2.8" is based on Bode's law
(Titius-Bode law), an intriguing relationship between the
distances of the planets from the Sun, named after Johann Titius
(1729-1796) (who formulated the law in 1766) and Johann Bode
(1747-1826) (who published the law in 1772). Take the sequence
0,3,6,12,24,..., where each number (except the 3) is twice the
previous number, add 4 to each number and divide by 10. The
resulting sequence (0.4, 0.7, 1.0, 1.6, 2.8, 5.2,...) is in good
agreement with the actual distances in astronomical units of most
planets, provided that the asteroids are included and considered
as one entity at a mean distance of 2.8 AU. Although the law
fails to predict the correct distances for Neptune and Pluto,
some researchers believe the law may have some significance with
respect to the formation of the Solar System. The law did predict
the distances of the 6 planets known by Titius and Bode, and led
Bode to predict the existence of an undiscovered planet between
Mars and Jupiter.
... ... *planetesimals (proto-asteroids): Planetesimals are
bodies with dimensions of 10^(-3) to 10^(3) meters that are
believed to form planets (or asteroids) by a process of
accretion. The term "accretion" refers to an aggregation, an
increase in the mass of a body by the addition of smaller bodies
that collide and adhere to it, provided the relative velocities
are low enough for coalescence. As the mass of the agglomerate
increases, so does the rate of accretion, and this accretion
process is believed to generally occur in a disk of debris
surrounding a star (stellar accretion disk), the disk a swarm of
dust grains that evolve into planetesimals and then planets.
Summary & Notes by SCIENCE-WEEK 7Jan00
For more information:



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