Stalking the Fires of
Creation:
Gould’s Belt and
the Astrophysics of Our Part of the Milky Way
Popular
descriptions of our place in the galaxy usually make it sound as if the Sun sits in some
out-of-the-way nook in the Milky Way, halfway through it’s life span, average and
ordinary, except for us, evidently, and with nothing much to worry about, except for us,
evidently, for around 5 billion more years. This
picture is accurate enough and indeed, the high drama of extreme astronomy fortunately
occurs far enough away to keep us safe, although it seemingly means “nothin’
ever happens round here”. But really,
it’s more the time factor than our location that gives us our “Dullsville”
address. Get a handle on that, and all that
fascinatin’ rhythm about star formation and supernova blasts and compression waves
and shock fronts becomes the song of our stars:
the ones we see shining brightly in our night skies, here and near in our own galactic
neighborhood. Recent discoveries and old
observations combine to show how the incredibly dynamic processes of star birth, death and
reincarnation are indeed going on all around us and are even powerful enough to alter the
history of life on Earth, (even us, evidently!).
Just a few million years ago, long after the extinction of the dinosaurs and well into the age of mammals, barely a blink of galactic time ago, our region of the Milky Way underwent a sudden burst of star formation. The skies of Earth 30 million years ago would have been overrun with bright giant heavyweight stars and every few thousand years, an intense supernova or two. The light show began and was most intense in the northern skies in the direction of the constellation Perseus. While the ignition source remains obscured and debated, obliterated by the tremendous forces it unleashed, evidence of its intensity remains in the stellar associations and supernovae it spawned, reflected through time as many of the stars and nebulae amateur astronomers know best.
“In the elevated position and clear atmosphere of Cordoba
[Argentina]… few celestial phenomena are more palpable there than the existence of a
stream or belt of bright stars, including Canopus, Sirius and Aldebaran, together with the
most brilliant ones in Carina, Puppis, Columba, Canis Major, Orion, etc, and skirting the
Milky Way on its preceding side. When the opposite half of the Galaxy came into view, it
was almost equally manifest that the same is true there also, the bright stars likewise
fringing it on the preceding side, and forming a stream which…comprises the
constellation Lupus and a great part of Scorpio, and extends onward through Ophiuchus
toward Lyra. Thus a great circle or zone of
bright stars seems to gird the sky, intersecting with the Milky Way at the Southern Cross,
and manifest at all seasons, although far more conspicuous on the Orion side than the
other. …This zone of bright stars may be traced with tolerable distinctness through
the entire circuit of the heavens, forming a great circle as well defined on the heavens
as that of the galaxy itself.”
That
was Benjamin A. Gould addressing the American
Association for the Advancement of Science 130 years ago.
What we now know is that his “stream” of suns is the remnant ring of
stars, disintegrating molecular clouds, and star forming regions, in a plane roughly 2000
light years across, expanding outward from a point about 600 lys from the sun and tilted
at about 20 degrees to the galactic plane, which dips well below the Milky Way in its
outward direction, toward Orion, and rises just above it in the direction of the near end
of our galaxy’s central bar. It is similar in form, though two orders of magnitude
smaller in extent, to ring galaxies, like Hoag’s Object,
or the famous Cartwheel
Galaxy. It is also a very recent phenomena, in
galactic terms, the expansion having begun just 30 million years ago, and is centered on
the alpha
Perseus (Mirfak) moving cluster. This ring
of creation includes some of the most famous objects in the sky: the Pleiades, the Great
Orion Nebula, the Horsehead, the
California Nebula, the Coal Sack, and the colorful and often photographed rho
Ophiuchi region near Antares. The majority of stars in the constellations of Orion and
Canis Major, Puppis, Vela, Centaurus and Scorpius formed at the leading edge of this wave.
With some famous exceptions for stars that are very bright because they are very
close neighbors, like Sirius, Vega and Alpha Centauri, or whose hyper-luminosity make them
bright from thousands of lightyears away, such as Deneb, Tau Canis Majoris or Zeta-1
Scorpii, most of the blue-white stars visible to the naked eye in the Belt are very
luminous giants and supergiants, many hundreds of light years away, and only visible to us
because of their tremendous power output. They
are nearly all daughters of the initial episode of massive star formation and destruction.
That
episode has been traced back to the location in space of the Perseus OB3 association,
(a.k.a. the alpha Perseus moving cluster). The
theory is that there was a period of intense compression of a massive cloud of cold
interstellar gas and dust in this area, either from a passing galactic density wave, or
possibly the collision with and subsequent ingestion of such a giant cloud of gas by the
Milky Way. The resultant gigantic molecular
clouds make very luminous heavyweight stars which have intense ultraviolet radiation and
very short lives; and about 30 million years ago many of these began exploding in
supernovae. The expanding pressure waves from
these winds and detonations have been compressing gas into stars in the vicinity since
then, and tracing the ages of the stars in Gould’s Belt confirm this. As you go eastward through Orion, away from Perseus
and the source of the expansion, the stars in Orion get younger. Studies of
One of
the most photogenic areas of the sky is here and is called the rho Ophiuchi region. Infrared observations show new stars being born
within that cloud of dust and gas. Unlike the
massive molecular clouds in Orion, which spawn brilliant and violent heavyweight stars,
this region’s low density clouds collapse to form sun-sized stars, and has provided a
nearby window on the physics of forming stars and solar systems like our own.
This activity is does not only push outward. As individual
molecular clouds collapse and become star clusters, their stellar winds push against the
interstellar medium in all directions. In this
way, even as the fires of Gould’s Belt recede from our immediate neighborhood they
can still have dramatic effects on Earth and its inhabitants. As the bubble of gas around the giant
Scorpius-Centaurus Association expands under pressure from its stars, it is pushing some
thin gas clouds, dubbed the Local
Fluff, toward the solar system. While the
heliosphere is already in contact with the tenuous outer portions of this cloud, the
densest parts are estimated to lie only 50,000 years in our future. Astrophysicists
suspect that direct encounters like that may perturb the Oort cloud, raining comets down
upon the inner solar system. It is unknown what effects such an interaction would have on
the heliosphere, which serves to deflect a large percentage of interstellar dust and
potentially harmful cosmic-rays away from the inner solar system.
The mysteries of cosmic-ray interaction with our atmosphere and
their drastic effects on its weather—some believe increases in their number can start
and/or stop Ice Ages, for instance--may be connected with newly identified nearby high
energy gamma-ray sources in Gould’s belt. There
is evidence from isotopic ratios found in ice cores that the solar system has encountered
dust clouds dense enough to allow some interstellar material direct access to Earth twice
in just the last 60,000 years. Some
short-lived byproducts produced in large quantities by supernovae are known to be very
harmful to “surface” life, such as Arsenic and Nickel-46, and high energy cosmic
rays. Some researchers indicate the derived
time of the nearby Vela supernova remnant coincides with many estimates for the date of
the Biblical flood story, about 11,000 years ago, and it is certainly interesting that
this is also the time of the rain-drenched end of the last Ice Age, and the subsequent
rise of both sea level and civilization. Could
this be a case of a supernova actually benefiting life on Earth? Much remains unknown.
British astronomers
Viktor Clube and
Bill Napier have analyzed the effects of the sun’s entry into the region of the
Belt, which they estimate to have been between 6 and 9 million years ago, and have
concluded that the gravitational effects of the matter there, primarily of the molecular
clouds, combined with passing though the galactic plane and at the same time exiting the
Orion arm, have put the Oort cloud into a period of great stress, and that the risk of
comet catastrophe may well be greater than commonly perceived. In their books The Cosmic Serpent and the Cosmic Winter they have described evidence for
a period of much greater flux of comets, including giant comets unknown today, into the
inner solar system, beginning a couple of million years ago. They suggest the visits
continued even into prehistoric times, giving rise in their view to the fantastic mythic
stories and religions of ancient mankind, and that the Taurid
meteor stream, Comet
Encke, the 1908 Tunguska event, many of the Earth-crossing Apollo asteroids, and the
Zodiacal Light are all remnants of this recent bombardment. And they warn we are not
out of danger yet, estimating “that further debris from the zodiacal cloud will
intersect the Earth in the period 2000-2400 AD.” The effects of an “Impact
Winter” have been widely described. Clube
and Napier insist the risks are higher than usually calculated. One is tempted to believe that correlation of
geological, genetic, archeological and astronomical data may yet show the direct effects
of cometary inflow, gamma-ray flux, and “local” supernovae on the development,
evolution, and future of life on Earth.
The visual observations Dr. Gould made from
Per O. Lindblad, Adriaan Blaauw, and other radio astronomers
beginning in the 1950’s found large amounts of neutral hydrogen being compressed
along the inside edges of the Belt, and eventually assembled a picture of a ring of
molecular clouds expanding at about a mile per second.
Ultraviolet imagers on the Mariner spacecraft identified a set of sources
clearly following the Belt in 1967. That the
emission and absorption nebulae occupy two distinctly observable zones is indeed obvious
from a careful look at most Milky Way panorama images. At larger distances they are
confined to the galactic disk, but as is apparent from Beverly Lynds’ 1962 survey of
interstellar dark clouds in her Atlas of Dark
Nebulae, and 1965 sequel, the Atlas of Bright
Nebulae, the nearest of these arc through the heavens in a slow twist following the
Gould Belt that confirms Hubble’s observations of four decades before.
In recent years studies of the galactic magnetic field also show
local deviations of field direction in conformity with the contours of the Belt. Infrared
investigators using the IRAS orbiting telescope have detected large quantities of
sub-micron sized dust particles among the cool, compact interstellar clouds of the Belt
and are detailing the current states of the many stellar nurseries within its confines. Graphs developed from the Hipparcos catalog showing
the proper motions of nearby
"As possible relics of supernova explosions
in the Gould belt, the gamma-ray sources highlight how dynamic the cycle of matter through
star formation and death is in our neighborhood -- a lively drama that eludes us because
of the stillness of the optical sky to our naked eyes.”
It seems clear then that in galactic terms, the
solar system is literally in the thick of things. It’s our own puny lifetimes that
make it seem so quiet ‘round here. And
yet of course, it is also our own puny brains that have discerned the disquieting truth.
In
closing, here’s a famous astronomer’s description of Gould’s Belt:
“[There’s a] zone of large stars which is marked out by the brilliant
constellation of Orion, the bright stars of Canis Major and almost all the more
conspicuous stars of Argo, the Cross, the Centaur, Lupus and Scorpio. A great circle
passing through Eta Orionis and alpha Crux will mark out the axis of the zone in question,
whose inclination to the galactic circle is therefore about 20 degrees, and whose
appearance would lead us to suspect that our nearest neighbours in the sidereal system (if
really such) form part of a subordinate sheet or stratum, deviating to that extent from
the general mass which seen projected on the heavens forms the Milky Way.”
That observation, which describes the modern
conception of Gould’s Belt with great accuracy and uncanny insight, was made in
What would the Belt look like from outside?
Observing Gould’s Belt from within is one thing, but what would it look like
from without? The search for analogues to our
little starburst begins, as all good self-examinations do, at home. The nearest similar
concentration of OB associations and molecular clouds is in the Carina-Crux arm but has
been identified only by studies of Cepheids, so there’s
not much there to look at. It has been
suggested that the famous Double Cluster, lying as it does some distance above the thin
disk of the galaxy, might belong to one end of a similarly tilted and detached segment of
the Perseus arm, most of which remains hidden behind the Orion arm. One is tempted to
consider the Great Sagittarius starcloud as an obvious example, but it seems that this
fabulous complex is in fact the majority of the Sagittarius spiral arm in that direction,
fortuitously unobscured from Earth as we look down its great length. Many spiral and
irregular galaxies exhibit patchy, fragmented arms with gaseous spurs and flaps like
Gould’s Belt. The giant stellar association 30 Doradus is one such, but it’s
location in the Large Magellanic Cloud keeps it out of sight for Northern Hemisphere
observers. Another Local Group galaxy,
IC10, is now known to be a dwarf starburst galaxy, but its position behind the Milky
Way in Cassiopeia deeply veils its dramatics. The example cited most often is the famous NGC
206, the luminous stellar complex in M31, two-thirds of a degree southwest of the
nucleus. The similarities are notable: it is
not responsible for the spiral arm structure in its vicinity and lies along the inner edge
of one, it appears to have a small tilt relative to the plane of its host, and the ratio
of its size to Mama Andromeda is approximately that of Gould’s Belt to the Milky Way.
It would seem to have somewhat brighter and younger stars, which may indicate it is closer
to the beginning of its starburst episode. Many
similar regions appear in Paul Hodge’s Atlas of Andromeda, available online at:
http://nedwww.ipac.caltech.edu/level5/ANDROMEDA_Atlas/frames.html
which plots 164 stellar associations, and a small
percentage of them may be analogous to the Belt. The
huge stellar complexes, or superassociations, NGC 595 and 604 in M33 are very large and
suffused with both neutral and ionized hydrogen, but not unlike Gould’s Belt
otherwise in comprising genetically related “families” of
Here is a brief bio on B. A. Gould Photo
Benjamin
Apthorp Gould (1824—1896) was one of the most important American astronomers of the
19th century. He was the first
American to earn a PhD in the field, his teacher being the great Carl Friedrich Gauss. He
was among the first to realize the importance of both telegraphy and photography to
astrometry, the precise measurement of star position and magnitude, and for the
determination of longitude and time, and was the first to photograph Mars. A hard working, diligent mathematician, in the days
when the tedious and voluminous calculations inherent in astronomy were all done by hand,
he also played a major role as founder, in 1849, and editor until his death, of The Astronomical Journal in codifying the many
varieties of nomenclature then in use among astronomers of the day, and promoting both the
cutting edge of the science in America and the cooperation of colleagues worldwide. He was
embroiled in a bitter public controversy over who should control the science being done at
the observatories then being built by the wealthy barons of the day. His side, which argued that science should guide
scientists, not the whims of rich but eccentric donors, ultimately prevailed, though at
great personal cost to Dr. Gould, and all humanity is in his debt as a result. His public
antagonism of the powers that be meant he would never be granted a major post in his
country’s astronomy establishment, but this did not deter him much, as he carried on
publishing The Astronomical Journal, helped found the National
Academy of Sciences, and then braved the wilds of Argentina for 15 years to build Cordoba
Observatory and produce his century’s greatest catalog of the southern sky,
completing the visual charting of the heavens with unprecedented precision of all stars
down to magnitude 7.5, and photographing and measuring scores of open clusters in the
southern Milky Way.
He also re-discovered something as big as all outdoors:
today’s astronomers call it Gould’s Belt, in his honor.
Primary Source material:
The Guide
to the Galaxy by Nigel Henbest and
Heather Couper; 1994Cambridge U.
A View of
the Universe by David Malin;
The Gould Belt by Per O. Lindblad, Stars and the
Milky Way, ed. L. Mavridis
Nature,
The Local System of Stars, by Otto Struve,
Sky and Telescope, 1963
Space.com
Results of Google search for: Gould's Belt
Results
of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8 at the
American
Journal of Science and Arts, Nov. 1874
The Cosmic
Winter, by V. Clube and S. Napier 1990 Basil
Blackwell
Extreme
Stars, by James Kaler 2001
Dust in
the Galactic Environment by DCB Whittet 1992
Institute of Physics Publishing
