Stalking the Fires of Creation:

Gould’s Belt and the Astrophysics of Our Part of the Milky Way

 By Matthew Terry

  

     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. 

   The visible result from this most recent episode of galactic fireworks in our neighborhood is today called Gould’s Belt. It is the size on the sky, if not quite the prominence, of the Milky Way, some part of it is visible to the naked eye and informed brain any clear night, and some of the sky’s greatest showpieces call it home.   So what is it? Here’s its namesake’s description: 

“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 OB associations in the opposite direction, toward Lacerta, Cephus and Cygnus show the same pattern.  The portion of the belt nearest to us is toward the giant Scorpius--Centaurus OB association.  These stars lie at an average of about 450 light years, three times closer than Orion, and have provided a nearby stage to watch star formation in detail.  The shock wave triggering star formation first reached the region of Upper Centaurus and Lupus about 15 million years ago. It then spawned more stars in the direction of   Lower Centaurus and Crux about 3 million years later, and then in Upper Scorpius just 5 million years ago, and that in turn is creating new stars as gas clouds there and elsewhere are compressed, swirled and rearranged all along the expanding edge of  Gould’s Belt.

    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 Argentina in the 1870’s remained mostly a curiosity, and many doubted there was anything other than chance alignment going on.  The first indications that his ‘stream of stars’ were indeed related came when photography revealed the different quantities of light being emitted by the stars in Gould’s Belt as compared with those without.  The vast majority were much brighter in the blue and near-ultraviolet than in the yellow-green part of the spectrum our eyes are most sensitive to, and therefore brighter on photographic plates than to the eye.  And as is usually the case in astronomy, every time a different portion of the spectrum has come on-line, new information has followed, and the truth of Gould’s inspiration has been repeatedly confirmed. Edwin Hubble noted in 1922 that both the diffuse bright nebulae and the largest and most opaque dark nebulae are concentrated along two belts, one the Milky Way, and the other approximately the belt of bright helium stars “which defines the local cluster”, the term then used to describe Gould’s Belt.  That same year Harlow Shapley clearly demonstrated that stars of spectral type B and A brighter than magnitude 5.25 are aligned with the plane of the Belt, and that those fainter than magnitude 7 preferentially follow the galactic disk. Modern studies of the results from Hipparcos mission’s measurements of B and A stars within 1500 light years show the same.

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 OB association stars and young B and A stars within 1500 light years of the sun clearly follow the 20 degree tilt of Belt. The newest discovery is perhaps one of the most surprising: the EGRET gamma-ray detectors on the Compton Gamma Ray Observer satellite have led a NASA team headed by Neil Gehrels to discover an entirely new category of more than 80 “weak” gamma-ray sources aligned within the Belt. These too are distinguished from a broader population of more luminous but more distant emitters confined to the galactic plane. Known sources of these emissions include supernovae remnants, pulsars, black holes and cosmic rays interacting with molecular clouds.  Another investigator of gamma sources in the neighborhood, Isabelle Grenier, has this to say in the March 23, 2000 issue of Nature magazine about the latest discovery:     

"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 Africa when Benjamin Gould was still a schoolboy, and then lost to astronomy, apparently to await an American in Argentina.  The commentator was Sir John Herschel.

                         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 OB associations and HII regions. Owners of big scopes might also identify some similar regions in M81’s southern reaches, and perhaps hints of them in profile in the nearby active edge-on spirals, M82, NGC 253 and 891. The face-on spiral M101 has several superassociations among its subordinate NGC regions, in particular NGC 5447, 5461, and 5462, although these are integral to its spiral arms.  Numerous studies cite M83 as a good candidate for professional investigation of Gould Belt analogues, as many astronomers believe it most closely resembles our own among the nearby galaxies. 

 

                                  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;   Cambridge U. and Sky Publishing

The Gould Belt   by Per O. Lindblad,  Stars and the Milky Way, ed. L. Mavridis

Nature, March 23 2000:  Gehrels, et al; and I. Grenier

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 Cape of Good Hope,  by J.F.W. Herschel    Smith, Elder and Co, London 1847

American Journal of Science and Arts, Nov. 1874

Dudley Observatory of Union College, Schenectady, New York

The Cosmic Winter,  by V. Clube and S. Napier   1990  Basil Blackwell

Extreme Stars,  by James Kaler  2001  Cambridge U Press

Dust in the Galactic Environment  by DCB Whittet    1992   Institute of Physics Publishing