Download In the icy near-vacuum of interstellar space are seething

In the icy near-vacuum of interstellar space
are seething cauldrons of stellar genesis.
The breeding grounds are becoming known,
Trying to describe the formation of a
star with the fragmentary evidence
now available to astronomy, University of California astronomer George
H. Herbig once remarked, is very much
like trying to reconstruct the plot of a
movie from just a few still frames. The
problem is that the gestation period of a
star is so long—ranging from a few tens
of thousands of years for certain giant
stars to hundreds of thousands of years
for stars like the sun—that astronomers
can never hope to observe any one star
through the transformations it undergoes
between conception and birth.
Instead, Herbig and other astronomers
find themselves forced to capture instantaneous images of young stars at different
stages of their growth. From these isolated data points, they try to work out the
beginning, middle and end of the stellar
formation process. That there are beginnings, as well as middles and ends, to
stellar odysseys has long been known.
But in and among these trimesters are
many, many uncertainties.
There was far less uncertainty 75 years
ago, or at least so it seemed. By the end of
this century's first decade, a Danish and
an American astronomer had each independently come up with a new way of
classifying stars diagrammatically. By
plotting temperatures against luminosities, Ejnar Hertzsprung of Denmark and
Henry Norris Russell of Princeton
University determined that most stars fell
along a gently undulating, ordered and
orderly curve, from bright and hot to
faint and cool.
The diagram plotting the curve came
to be known as the Hertzsprung-Russell
Stellar nursery. Spiral galaxy NGC 5194, in
Canes Venatici, and the regions along the
trailing edges of its spiral arms where infrared
data suggest new stars are being formed.
28
MOSAIC MAY/JUNE 1 978
diagram, and the gently undulating curve
as the main sequence. The latter was
thought to reflect the principal stages in
the life of a fully developed star, after it
had completed the processes of genesis
and before it began its inevitable decline.
There were, to be sure, two major star
types that didn't quite lend themselves to
superposing on the main sequence. These
were the red giants (quite luminous but
cool) and white dwarfs (dim but extremely hot). Astronomers at first believed that the giants might be infant
stars, too young and too unconsolidated
to be on the main curve, and that the
dwarfs were superannuated stars, on
when coupled to a telescope, generated
an electric signal proportional to the intensity of the light falling upon it.
These and subsequent technological
advances (see "Catchers of the Sky" in
this Mosaic) enabled astronomers to
measure with some precision the apparent brightness (magnitude) and color
of many stars. The revelations were often
astounding. Stars that had been thought
to be young were found, on the contrary,
to be quite old; the red giant Betelgeuse,
for instance, proved to be not a vigorous
young adult star, but rather a doddering
oldster, nearing the end of its life.
Moreover, the theoretical astronomers
Supergiants
Giants
Mam sequence
White dwarfs
J—
L_
Stellar sequence. The famous Hertzsprung-Russell diagram, an early attempt to describe stellar
evolution. Stellar evolution is actually far more complex and far less well understood, even now.
their way to the stellar boneyard.
This schema, so plausible and so logical, was widely accepted by the
astronomical community as a definitive
description of stellar evolution. It was,
unfortunately, wrong. It is still useful,
but for much more limited purposes than
were originally intended.
A number of astronomical developments in the second and third decades of
this century revealed the weaknesses of
this premature evolutionary theory.
These were principally advances in instrumentation that made possible insights that Hertzsprung and Russell
could not have had. These advances included better photographic plates and
emulsions for the then-operating telescopes, and the photoelectric cell which,
and physicists were also contributing to
the ferment of the time. In the nineteentwenties, Harlow Shapley demonstrated
that the sun and its planets lie on the outskirts of the Milky Way galaxy. At about
the same time, Edwin Hubble disclosed
both the existence of untold numbers of
other galaxies in the universe and the expansion of the universe (see "One
Universe, Indivisible" and "The Extragalactic Ferment" in this Mosaic). In
the late nineteen-thirties, Hans Bethe
showed that the incredible outpouring of
energy from a star was the result of thermonuclear reactions deep in its interior.
Progress continued into the nineteenforties and nineteen-fifties. The photomultiplier tube enabled astronomers in
the late nineteen-forties to establish firm
magnitude standards for faint stars. And
in the early nineteen-fifties, astronomers
were busily building models of stars and
stellar processes and predicting properties appropriate to those models. The
five-meter Hale telescope on Palomar
Mountain had gone into operation in
southern California; by then it was providing observational data against which
the new theories could be checked. Not
all celestial objects—-those hidden by
clouds, for instance—are visible in ordinary light, however, and it began to
become apparent that traditional, optical
astronomy had inherent limitations.
"All this early work was being carried
out optically," says Herbig, who is also a
staff astronomer at the Lick Observatory
on Mount Hamilton, some 50 miles
southeast of San Francisco. "It was being
done with conventional telescopes at
conventional wavelengths. But people
recognized at the time that the very
earliest stages of star formation must
have taken place when the objects were
very cool [and surrounded by cool dust
and gas] and probably undetectable in
visible light.
"If only astronomers could penetrate
these dense dust clouds where stars were
thought to be forming, they thought, they
might be able to see stars in their earliest
stages," Herbig continues. "So it became
apparent that infrared
and
radio
astronomy,
particularly
microwave
astronomy, were the way to go; these
radiations can penetrate the clouds and
provide information on the very initial
stages of stellar formation."
Unswaddling the infants
And that was the way astronomy
went. Throughout the nineteen-sixties
and nineteen-seventies—using principally the radio telescopes and interferometers at the National Radio Astronomy Observatory at Green Bank, West
Virginia, NRAO's millimeter-wave radio
telescope in Arizona, the 300-meter fixed
radio antenna at Arecibo, Puerto Rico,
and optical and other equipment at the
Kitt Peak National Observatory in
Arizona and the Lick, Owens Valley and
Hat Creek Observatories in California—
astronomers working in radio and infrared frequencies gathered a great deal
of information about young stars and the
dusty, gassy, interstellar medium that is
the stellar environment.
"Let me say," Herbig told a 1976 international symposium in Geneva, Switzerland, on star formation, "how struck I
MOSAIC MAY/JUNE 1978
29
am by the delicate symbiosis that exists
between the stars and the interstellar
medium, how each is nourished by the
other and how the galaxy as we know it is
entirely a consequence of that balance
and interplay."
Astronomers have long known that
there are huge, cloudlike collections of
dust and gas swirling through the interstellar regions of a galaxy; they discovered these clouds as a result of the
reddening effect that dust (as well as
radial velocity—see "One Universe, Indivisible" in this Mosaic) has on starlight:
the more dust there is between a star and
an observer, the more that star's visible
radiation will seem to be reddened, as is
the light of the sun at sunset. Only recently, however, have they come to appreciate the "delicate symbiosis" between these clouds and the formation of
new stars.
The dust is particularly important. It
shields the gas within a cloud's deep interior from the effects of radiation from
older, adjacent stars; dust grains provide
surfaces on which chemical reactions can
take place and dust radiates energy from
the cloud during the star's early, formative stages.
And yet these tiny grains—of uncertain composition, though graphite, silicon, carbide, and iron are possible constituents—contribute less than one percent to a cloud's total mass. Their effect
becomes appreciable only because of the
great size of the clouds—some of them up
to 100 light-years across.
The screening effect is perhaps the
most important function served by interstellar dust. Like an opaque filter
placed between a heat lamp and a bowl of
setting Jell-O, the dust blocks out
ultraviolet radiation from stars surrounding the cloud. It thus enables
chemical reactions inside the cloud to go
forward and complex, delicate molecules,
formed from the atomic constituents of
the gases, to survive.
The first molecular specimen to be
found in the interstellar medium was discovered 40 years ago. Astronomers then
regarded it as something of a cosmic
freak, a chemical oddball amidst the
overwhelming profusion of atomic elements like hydrogen, helium and carbon.
Today, of course, astronomers are aware
of approximately 40 molecular species in
the clouds among the stars and along the
arms of spiral galaxies. These include
molecular hydrogen (H 2 ), hydroxyl radical (OH), formaldehyde (HCOH), water
vapor (H 2 0), ammonia (NH 3 ) and car-
30 MOSAIC MAY/JUNE 1 978
2 light-years
Orion Nebula
(Ionized gas)
Stellar womb. The Orion
Molecular Cloud (above) and
the location within it where
Don Hall locates the protostellarBecklinNeugebauer infrared object.
The process and key components of such a sfeliar
birthplace are depicted (after
Zuckerman) at left.
to Earth
Trapezium
cluster
More
molecular
cloud
bon monoxide (CO), among others. New
ones are constantly being discovered.
The dust grains, and the molecules
thought to form on their microscopic surfaces, absorb the radiation given off by
the slowly contracting envelope of gas
and dust at the center of the cloud. They
then reradiate it at their own characteristic wavelengths. Without this cooling by reradiation, the collapse of
molecular clouds would be much less
likely.
Clouds collapse
By radiating, explains Gillian R.
Knapp, a senior research fellow in radio
astronomy at Caltech, the molecules sap
the cloud of the heat energy that has been
keeping it inflated. "As a cloud cools,"
she says, "the balance between its internal heat and its gravitational forces is
upset, and it starts to collapse."
The collapse of a cloud is a key step in
star formation. But it is a process that is
poorly understood. It once was thought
that a cloud was more or less spherical
and that when it fell in upon itself, it did
so radially, like a deflating balloon. The
current view is that a cloud may break up
into many fragments during the collapse.
"Why they form," Knapp says of the
fragments, "how many there are, how
massive they are and how they're dis-
tributed throughout the cloud are all
questions for which we really don't have
good answers at the moment."
There are, however, some hints from
infrared data: Small clouds appear to
decompose into only two or three fragments, each of which may be some two or
three times as massive as the sun. At the
other extreme, there are huge clouds—
cosmic overcasts—which disintegrate
into a dozen pieces or more. Some of the
pieces are the equivalent of ten solar
masses, some are a few thousand solar
masses and some are as large as several
hundreds of thousands of solar masses.
Current calculations by radio and infrared observational astronomers, says
David N. Schramm, a University of
Chicago theorist, indicate that motions
within the collapsing cloud cause the individual, spinning fragments to take the
shape not of a deflating balloon, but
rather of a rotating disk.
Moreover, Schramm says, based on
observations by Martin Cohen of the
University of California at Berkeley and
others, the disks seem to be thinner close
to their central cores than they are
Cloud-to-protostar. An artist's conception of
the process by which a collapsing cloud
takes the spinning, scooped-out disk shape of
a star in formation. At stage III, it is radiating
energy at its poles.
around their perimeters. This gives them
something of a scooped-out appearance
on top and bottom. Thus, if a disk were
seen edge-on, all that would be observed
would be a thick, obscuring envelope of
dust. Seen from above or below, however, the disk would appear to be a core
surrounded by dust of graduated density:
thinner in the immediate vicinity of the
center and thicker farther out toward the
rim.
Critical densities
This structure allows the formative
star to radiate energy from its polar
regions out into the interstellar medium,
while continuing to draw in matter along
the equatorial plane of the disk. The signature of these processes, says Knapp, is
t h e p r e s e n c e of c e r t a i n m o l e c u l a r
wavelengths. "Hydrogen cyanide and
formaldehyde can r a d i a t e , " she says
"only when the density, the number of
particles per cubic centimeter, is above a
critical value. So, when we see one of
those, for example, we know that the
density in that region of the cloud is
roughly ten times what it is in a region
where we see only CO or hydroxyl radical."
Once collapse begins, gravitational
forces dominate the process. The disk
continues to shrink and its density to in-
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MOSAIC MAY/JUNE 1978
31
crease until, at last, the center is generating heat faster than it can radiate it away.
Temperatures begin to rise and the radiation given off changes from the longer
wavelengths of infrared to the shorter.
Any dust particles still in or near the core
are either blown off or sublimated; only
gas remains at the center, and it is now
beginning to glow.
When this stage is reached—when the
center of a collapsing fragment is dense
enough and hot enough to emit the
shorter wavelengths of infrared—it can
be said that a protostar is born (or protostars, since the evidence suggests that
dark molecular clouds bear stars in litters
rather than singles).
It may take a collapsing cloud a few
tens of thousands of years to reach the
protostar stage. Thereafter, depending
upon its mass, it may be anywhere from a
million to ten million years before it
bursts into the flame of thermonuclear
fusion. Then its radiation moves into the
visible and ultraviolet part of the
spectrum. Until that time, it exists as an
infrared object.
A star is born
Back in 1966, Gerry Neugebauer and
Eric Becklin of Caltech noticed a prominent infrared source in the Orion
Molecular Cloud. The cloud, in Orion's
sword and some 1,500 light-years from
the earth, is a complex structure containing a variety of objects: the Orion
Nebula, a sheet of ionized gas; the Trapezium cluster of young, hot stars that
are responsible for the ionization of the
nebula; a dense layer of neutral molecular gas some 500 times the mass of the
sun, and a group of protostars that are
very bright in the infrared part of the
spectrum but still not visible in the optical region.
Becklin and Neugebauer focused their
attention on one of these infrared objects.
It was compact, glowed 10,000 times as
luminous in infrared as the sun, had a
temperature of 600 degrees Kelvin and
appeared to have a diameter of not quite
200 astronomical units, roughly 18 billion miles. These characteristics, the
Caltech astronomers suggested, made the
object a prime candidate for identification as a pre-main-sequence star—a star
about to be born.
In January 1978, a team of astronomers looked closely at the BecklinNeugebauer object with the four-meter
telescope at the Kitt Peak National Observatory, rigged for infrared. The radiation
from the Becklin-Neugebauer object collected by the telescope was fanned out in a
special spectrometer designed and built by
team members Donald N. B. Hall and
Stephen T. Ridgway of Kitt Peak.
Hall and his colleagues—besides
Ridgway there were F. C. Gillett of Kitt
Peak and Susan G. Kleinmann of the
Massachusetts Institute of Technology—
focused their attention on two spectral
lines in the object's infrared spectrum.
These were lines that were emitted by hydrogen atoms at different levels of excitation. Through a series of complicated
backtracking equations, the astronomers
were able to deduce the kind of object
that would produce such patterns.
" O u r observations revealed," Hall
says, "that the object is a B-class star in its
very earliest stages of evolution." A Bclass star is large and hot (only O-class
stars are hotter); Rigel, the brightest star
in Orion, is a B-class star.
The Kitt Peak astronomers also have
inferred that the Becklin-Neugebauer object had first rapped at the door of the
main sequence within the past 1,000
years or so; it is now running on the
energy of thermonuclear fusion and is
emitting visible light. At the moment, the
newborn star is probably throwing off its
swaddling layers of gas and dust.
Nevertheless, because of its distance
(1,500 light-years), it could be anywhere
from a few hundred to a few thousand
years before the corroborating evidence
of this activity—photons of visible
light—are received here on earth. It may,
a team of Japanese astronomers has suggested, almost be ready to take on the
characteristics of the next, and only
slightly more advanced, kind of star: the
troublesome T Tauri.
The T Tauri class, so called because the
first such stellar object was discovered in
the constellation Taurus (others have
since been found all over the sky), is
among the more interesting, and puzzling, of star types. These stars are visible
in both optical and infrared wavelengths
and are found clustered in dark clouds or
in the turbulent regions sandwiched between the cloud and an adjacent bright
nebula. Astronomers believe the T Tauri
stars to be further along than the BecklinNeugebauer source but still in the premain-sequence process of collapsing
gravitationally.
The T Tauri stars are noted for their
variability. Back in 1936, a faint star that
is believed to have been a T Tauri, of a
type known as FU Orionis, went from the
16th to the 10th magnitude, an increase
in luminosity of 250 times, in about a
year. It was considered then to have been
Stellar midwivery. Shock fronts from the death-blast of a star-cum-supernova are among prime
candidates for inducers of the collapse of clouds into protostars. Mixing at the interface gives second-generation stars their complement of heavy elements.
.- Shock Front
. \ SHOCK Front
\
5-
Gas Mi/'ng
at Interface -~-.^/\
t
\
V.
Proicsciar nebula
(interstellar gas
and dust grains;
32 MOSAIC MAY/JUNE 1978
Grains Penetrate •"*" |. 1_^_
interface
" *" \j
(Shrapnel'i •
""""'J\X
) Collapse
\
Begins
T ..
^ • f\
s
C3j/
Supernova Gas
and Grabs
Presoiar Nebula
[interstellar Gas and Grains
- Supernova Grains and Gaj
/
A.
an arcane object. Some astronomers
believe now, however, that it was observed in the process of blowing off the
remnants of its cloud as, it has been suggested, the Becklin-Neugebauer protostar
is expected to do in the not-too-distant
future.
Just in the last few months, however,
Jeremy Mould of Kitt Peak and others
have obtained infrared spectra of FU
Orionis stars that suggest a more precise
mechanism for t h e brightening. The
spectra appeared to show the stars to be
rotating fast enough to be casting off
mass from their equatorial regions. As
the stars redistribute their mass to compensate for the loss, Don Hall explains,
"they could brighten considerably." That
model, the astronomers caution, "appears the more promising" of several
possibilities, though it cannot be "strong-
ly preferred" from the present data.
In any event, the interest that the
Becklin-Neugebauer star has attracted is
due more to its proximity—a mere 1,500
light-years—than to its age or its singularity; there may be others as young or
younger, and there are known to be at
least several dozen other young or forming stars embedded in thick interstellar
dust clouds here and there about the
universe.
Indeed, says Roger Ulrich, chairman
of UCLA's astronomy department, there
is probably as much interest in galactic
regions, where dust clouds and their
associated stars and infrared objects appear to be concentrated, as there is in the
sequential development of an individual
star. There seem to be processes at work
within these regions that cause the clouds
to fragment, collapse and produce stars.
The mechanisms, at this junction, are
more important than the consequences.
Processes a! work
It is no accident that the BecklinNeugebauer object is in the Orion
Molecular Cloud. That cloud is just such
a region as Ulrich has in mind. Judging
from the large number both of young
visible stars and infrared objects recently
discovered there, it appears that this
complex cloud, two light-years in diameter, with its Great Nebula and HerbigH a r o objects—tiny, shock-heated hot
spots in the dust, named for Guillermo
Haro of the Mexican National Observatory and for George Herbig—are to stars
what Scammon's Lagoon in Baja California is to gray whales: a breeding ground.
It is, however, just one of several
different cosmic niches where stars seem
to form. Some of these sites are concentrated along the trailing edges of a
galaxy's spiral a r m s ; others can be
detected in and around the hub of a
galaxy.
The question: Where? appears to be
being answered. But not the questions:
Why? or How? What causes the dust and
gas in interstellar space to aggregate and
form dark clouds? What perturbs these
clouds and sets the collapse process in
train? What is the relationship between
the lee of a spiral galactic arm and other
stellar breeding grounds?
Spiral illusion. Differential rotation of elliptical
density waves around a galaxy core has been
proposed as an explanation of the illusion of
spiral arms. "Windrowing" of interstellar
material behind the density waves can provide the environment for the coalescing of gas
and dust into protostars.
So far, the forces at work within the
spiral arms of galaxies come closest to
producing a theory of cloud collapse and
star gestation. At least there is one explanation that currently finds favor
among many scientists: the density wave
theory credited to the Swedish astronomer Bertil Lindblad and American
astronomers Chia-Chiao Lin of the Massachusetts Institute of Technology and
Frank Shu of the University of California
at Berkeley.
According to this theory, the stars,
dust and gas of a galaxy are initially more
or less evenly distributed throughout the
rotating galactic disk. But a density wave,
something like a cosmic analogue of a
sound wave, ripples through the disk.
This wave rotates around the galactic
core less rapidly than does the rest of the
galactic matter and presents an obstacle
to the overtaking gas, dust and stars.
These, then, bunch up like a windrow
behind the slow-moving density wave.
In each of these elliptical log-jams of
galactic matter, for the length of time that
matter bogs down behind the density
wave, the density of the galactic matter itself builds up, and conditions become
right for clouds to aggregate and stars to
begin to form.
Although the exact mechanism of the
density wave remains to be elaborated, it
is thought that the compressional effects
of the wave cause small variations in the
densities of stars washing up behind it.
These variations lead to small warpings
in the gravitational fields extending out
from the stars. These warpings, in turn,
have an impact on the gas and dust particles drifting about in that region.
"You might think of it," says Michael
W. Werner of Caltech, "as the density
wave causing little 'holes' of gravitational
anomalies into which the gas and dust
fall." This would aggregate the gas and
dust into a cloud, the first step toward
star formation.
(A series of such galactic bottle-necks,
created by crests of anomalous density
sharing a common hub and rotating at
different velocities around the galactic
core, has been proposed by Jay Pasachoff,
director of the Hopkins Observatory,
Williamstown, Massachusetts, as an explanation of the spiral appearance of
some galactic disks. The spiral appearance may be no more than an illusion, the proposal holds, created as the
ellipses, sharing a hub and rotating at
different speeds, approach but don't cross
each other at a sequence of points that
trace an apparent spiral.)
MOSAIC MAY/JUNE 1978
33
Other processes
But there are other processes seemingly at work as well: F. J. Vrba of the University of Arizona and Karen M. Strom
and Stephen E. Strom of Kitt Peak carried
out a survey of stellar rookeries between
1974 and 1976. They used the 4-meter,
2.1-meter and 1.3-meter telescopes at Kitt
Peak, equipped for infrared. By measuring the polarization of the radiation
being emitted by young stars, the trio was
able to determine something about the
magnetic fields surrounding the stars.
This was possible because magnetic fields
cause a systematic alignment of dust particles within a cloud, and the dust particles affect the polarization of light emitted by the stars.
Vrba's and the Stroms' findings indicated that magnetic fields play a role in
the collapse of some clouds, either by
channeling the gas along field lines into a
cloud center or by mapping the general
flow directions for the gas. One cloud
they studied seemed to have a magnetic
sinkhole; matter poured down along the
lines of force and stars were forming at
the bottom. A second type of cloud, having weak magnetic fields, seemed to be
undergoing a relatively quiet collapse as
the result of gravitational attraction
alone. A third and a fourth cloud type,
however, gave indications of more dynamic forces at work, the former because
of a shock wave ripping through it and
the latter because of a collision with
another cloud.
What the three astronomers showed is
that the efficiency of the star-forming
process in a cloud is coupled to the vigor
of the collapse process and that, whatever
the role of magnetic fields, it gets washed
out by more energetic forces. In the first
two cloud types, those under the influence of the magnetic sinkhole and simple gravity, only one to two percent of the
original cloud mass was converted into
new stars. The shock-wave cloud, in contrast, made about 8 percent of its original
mass into new stars, while the colliding
clouds yielded 28 percent of their masses
in the form of young stars.
Shock waves are, at the moment, one
of the more hotly pursued topics in stelNew stars and hydrogen. Reading hydrogen
distribution at two central velocities around
new stars in the direction of Cepheus (right),
Assousa and Herbst identify portions of the
sheii ejected in our direction—dashed lines—
and estimate initial rest velocities of matter
ejected into the shell'—solid lines. The distribution of ionized hydrogen in the red-sensitive photo plates (above) indicates star formation.
34 MOSAIC MAY/JUNE 1 978
lar astronomy. Astrophysicists William
Herbst and G. E. Assousa of the Carnegie
Institution of Washington have suggested
that the shock waves from a supernova—
the cataclysmic explosion of a huge star—
act as triggers for the collapse of dark
molecular clouds.
Such might have triggered the birth of
the solar system. A Caltech group, including Gerald Wasserburg, Dimitri
Papanastassious, Typhoon Lee (now at
the University of Chicago) and Malcolm
McCulloch, has advanced evidence to
support such a contention. They contend
that a supernova occurred in the vicinity
of the solar system some 500,000 to a
million years or so before the sun and the
planets formed (see "Stellar Ontogeny:
. . .to Ashes" in this Mosaic). And, the
Caltech researchers propose, along with
Schramm, Harvard University's A. G. W.
(Al) Cameron and others, that a supernova was instrumental in the system's
formation.
The occurrence of a supernova can
only be described as a colossal event. The
responsible star, at least six times as
massive as the sun, after having burned
brightly for perhaps ten million years,
collapses within a span of just a few seconds. The outer shells of the star are
ejected with great force and velocity by
shock waves generated in the implosion.
It is just such shock waves, according to
Schramm and his theoretical group at
Chicago, that interact with the elements
in those outer shells to spawn such
heavier elements as magnesium and
aluminum and spray them through the
interstellar space.
Herbst and Assousa have found a
group of newly formed stars around the
edge of an expanding shell of gas that
they have traced to a supernova remnant
in Canis Major, the constellation that includes the star Sirius. They have also
detected an expanding shell of neutral
hydrogen in the constellation Cepheus,
as well as a clutch of new stars there. Both
the shell and the stars appear to be
around 430,000 years old.
Based on these associations and the
occasional presence of rare elements in
some stars (elements not thought to be
synthesized by the slow, thermonuclear
fusion in stellar cores, but only in the
very violent crucible of supernovas), the
two Carnegie researchers contend that
stellar explosions such as these could be
major producers of hot young stars in
spiral galaxies.
Rare elements also figure in the
Caltech group's assertion that a super-
nova was instrumental in the formation
of the solar system (see "Stellar Ontogeny: . . .to Ashes" in this Mosaic).
Wasserburg and his colleagues detected
an unusually enriched concentration of a
telltale element in the Allende meteorite,
a two-ton carbonaceous chondrite that,
left over from the primordial solar
nebula, fell to earth in 1969.
The element was magnesium-26, a
substance created when aluminum-26
decays radioactively. Since aluminum-26
has a half life of 700,000 years (meaning
that half of a given amount of aluminum-26 will decay to magnesium-26 and
other forms in that time), the fact that
there is so much of its decay product in a
fragment 4.6 billion years old is startling.
It can only mean that there was a lot of
aluminum-26 around when the meteorite,
the sun and the planets all condensed.
Could the early sun have produced
this element? It's possible, says Wasserburg, but then the planets, as well as
meteorites, would have picked up some
of this radioactive element—and melted.
The heat released during decay would
have done the job within about 300,000
years. Since the planets have lasted at
least 4.6 billion years, this hypothesis is
unlikely.
Instead, Wasserburg and his Caltech
associates argue, the aluminum-26 came
from a nearby supernova that exploded
anywhere from five billion to perhaps
seven billion years ago. The element was
synthesized as the shock wave from the
Planetary possibility. The circumstellar disk
around MWC 349 could well represent the
condensation of planets out of a protostellar
structure. The temperature of the disk
decreases with distance from the star.
explosion ripped through the outer shells
of the dying star and fired, like a spray of
pellets from a shotgun, into an adjacent
cloud. The wave would have initiated the
collapse of the cloud and elements like
aluminum-26 would have been captured
by the dust particles in the cloud.
Planetesimals and planets
At a symposium on protostars and
planets, held in early January at the
University of Arizona, Cameron called a
supernova "the most straightforward explanation" for all of the trace elements
like aluminum-26 now known to be included in meteorites.
The explosion would have caused the
primordial cloud to disintegrate into
fragments, one of which eventually
became the solar disk. Initially, says
Cameron, the disk grows in radius by
drawing in the matter from the collapsing cloud. As a simile, a large, roughly
spherical lump of runny clay spinning on
a potter's wheel and gradually flattening
into a disk suggests an image of the process; matter is flowing inward near the
center of the disk and outward along its
perimeter. Most of the disk's mass would
radiate away or be spun off within a
fairly short period of time—perhaps less
than 100,000 years or so—leaving only a
MOSAIC MAY/JUNE 1978
35
thin plane of dust particles circling the
central region.
There are two possible ways for these
particles to accrete, or grow together, to
form planets, according to William K.
Hartmann of the Planetary Science Institute in Tucson:
• The dust rains down out of the extended gaseous nebula or fragment
toward the midplane of the galactic disk;
like raindrops sliding down a windowpane, the dust clumps with other
dust particles contacted during the fall. In
the midplane of the galaxy, the particles
and clumps are ultimately so closely
crowded that groups of them aggregate
by gravitational attraction into kilometerscale bodies.
• The grains collide individually,
sometimes bouncing apart, sometimes
sticking together and sometimes breaking
up aggregates previously formed. But
each collision bleeds a little velocity from
the interacting particles; the next time
they strike each other they tend to remain
closer. Eventually, when relative speeds
are low enough, colliding grains are held
together by their own gravity.
"Most people feel that the planets
formed as either a mix or sequence of the
two processes," Hartmann says. But once
the second process, called "collisional accretion," began, then planets would have
agglomerated within a fairly short period
of time. The time required might be as little as 10 to 100 years for the first
planetesimals, a few tens of kilometers
across. Calculations by Hartmann and his
colleagues indicate growth of a few
bodies to 1,000-kilometer dimensions
within as few as 20,000 years. For all
these rocky objects to have been swept up
into the solar system's so-called terrestrial planets—Mercury, Venus, Earth
and Mars—however, it could have taken
tens of millions of years.
The asteroids that lie between the orbits of Mars and Jupiter are thought to be
remnants of that long-ago process. "It
could be," says Hartmann, "that no
single asteroid ever got big enough to
carry the process of collisional accretion
through to sweep up all the rest. It could
also be that Jupiter may have disturbed
the belt so much that the objects never
slowed down enough to accrete."
Other solar systems
Are there other solar systems, either
in this galaxy or in external galaxies?
Astronomers have long believed that
planetary systems should be something
of a commonplace, even though they
36
MOSAIC MAY/JUNE 1978
have not yet developed the techniques
needed to demonstrate that this is actually the case. Barnard's Star, some six
light-years away from earth, may or may
not have two huge planets roughly the
size of Jupiter and Saturn revolving about
it (see, "Life Among the Stars," Mosaic,
Volume 8, Number 2). There may be
others; the data depend principally on
astrometric techniques and are, at the
very least, ambiguous.
More recently, Peter Strittmatter and
Rodger I. Thompson of the University of
Arizona reported finding a greater-thananticipated amount of visible light coming from a star known by its catalog number of MWC 349. From this, and from a
body of infrared data, the Arizona investigators believe that MWC 349 is a star
surrounded by a preplanetary disk structure. Continued studies of this object may
reveal whether, indeed, the sun and its
planets have cousins elsewhere in the
vastness of the universe.
Not forever?
There is a sense of continuity in the
birth and evolution of stars, as there is in
the propagation of h u m a n life. A
biologist once remarked that human
beings were only temporary repositories
of DNA, deoxyribonucleic acid, the
molecular basis of life. It may be that
something of the sort is true of stars.
At this moment, according to Beatrice
M. Tinsley of Yale University, about 80
percent of the known mass of the Milky
Way galaxy is bound up in stars, living
and dead. Even in death, stars return
some of the matter that once was theirs to
the interstellar medium, via the agency of
explosions like novas and supernovas.
Young stars draw upon these used
materials to build their own new stellar
furnaces, and the cycle goes on.
Not forever, however. As long as the
universe continues to expand (see "One
Universe, Indivisible" in this Mosaic), a
time will come—in perhaps 40 billion
years or so—when too much matter will
have been irrecoverably buried in dead
or dying stars for the universe to give rise
to new stars. No young stars will be born,
and any observer alive then would have
the melancholy privilege of watching the
last few points of light sputter, flare, fail
and finally gutter out. Perhaps, at least in
an eternally expanding open universe,
the birth of a star should be regarded
with sadness as, philosophically, the
Chinese traditionally regarded the birth
of a human. They saw it as bringing
closer the moment of death.®