Title: Life and Death in Orion
Podcaster: Christopher Crockett
Organization: Lowell Observatory
Description: The constellation Orion is a highlight of winter evenings, never failing to impress with its brilliance and distinctive pattern. The Egyptians saw these stars as the celestial home of Osiris, the God of the Underworld. Just as Osiris traversed death and was reborn, the stars of Orion present a tapestry of the stellar life cycle from the nursery of the Orion nebula to the aging giant Betelgeuse. In this installment, we will explore what the stars of Orion can tell us about the birth, life, and death of stars.
Bio: Christopher Crockett is a UCLA graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him. For more info see: http://www.lowell.edu/users/crockett/
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Wilfred Nijman, a 54-year old astronomy student who would like to remind you that it is never too late to start your studies by distance learning at http://www.studyastronomy.com
Hello. This is Christopher Crockett from Lowell Observatory.
Winter is a great time to go out and star gaze. The winter sky is home to some of the most brilliant stars and well-known constellations. The most striking and most famous of these is the great hunter, Orion. Its seven bright stars create a distinct hourglass shape in the sky that is easily visible from even the most light polluted of skies. In February, from the Northern Hemisphere, Orion sits high in the southern sky during the early evening hours placing it perfectly for some after-dinner viewing. The three nearly aligned stars of the hunter’s belt mark the center of the constellation and are an easy target for even a first-time stargazer.
Orion is one of the oldest constellations with a history dating back to the ancient Sumerians who saw in this collection of stars their epic hero Gilgamesh battling the Bull of Heaven. The Greeks saw Orion, the great hunter. Orion is seen, like in the Sumerian legend, battling the bull Taurus with his two faithful hunting dogs, Canis Major and Canis Minor, at his side. While various Greek mythologists provide differing accounts for Orion’s death, they all have one element in common: the lethal sting of a scorpion. It is for this reason, the Greeks tell us, that Orion and the constellation Scorpius are placed on opposite sides of the sky. As the scorpion rises in the east, Orion escapes below the horizon in the west, doomed to spend eternity fleeing from his nemesis.
As an astronomer, the most interesting description to me comes from the Egyptians who marked this as the celestial home of Osiris, the God of Death and the Underworld. Osiris was brutally murdered by his brother, Set. Set carved Osiris into a number of pieces and scattered them along the banks of the Nile. Osiris’s wife, Isis, collected the pieces and made her husband whole again. Osiris was then given watch over the land of the Underworld and receives every Pharoah who passes into death. It is remarkable that the Egyptians chose this constellation as a celebration of death and resurrection for within its boundaries, astronomers find the celestial cycle of life playing out through the birth and death of its constituent stars.
Our tour of Orion starts at a fuzzy patch of light just below the three belt stars (or just above if you’re listening to this in the Southern Hemisphere). To the Greeks, this was Orion’s sword. To modern astronomers, this cloud-like apparition is one of the most photographed and intensely studied areas of the sky: the Orion Nebula.
The word nebula comes from the Latin word for cloud; in modern parlance, the word is reserved for the clouds of molecular gas and dust that roam the space between the stars.
The Orion Nebula is a place of birth and renewal, an active stellar nursery! At nearly 1500 light years away, it is one of the closet such nebulae to the Earth. The part of the nebula we can see with unaided eyes is just a small part of the much grander Orion Molecular Complex – a massive cloud of predominately hydrogen and dust grains spanning hundreds of light years in diameter. It is within these molecular cocoons that stars are born.
The path to becoming a full-fledged star begins with the collapse of pockets of gas within the nebula. This collapse is triggered by some external event: perhaps by passage through one of the spiral arms of the Galaxy or the shockwave from a nearby supernova. Regardless, once the cloud has been perturbed its self-gravity quickly takes over forcing it to begin collapsing on itself. As the cloud contracts, it fragments into a hierarchy of ever smaller clumps. Eventually the smallest of these clumps begin to glow as heat from the collapse radiates into space. The collapse does not continue unabated, however; something interferes! The rising temperatures and densities deep in the cores of these clumps of gas pass a threshold that allows hydrogen nuclei to overcome their mutual repulsion and fuse together to form helium. The onset of nuclear fusion releases a massive amount of energy that builds up the outward gas pressure and halts the gravitational contraction. A new star has been born!
At the core of the Orion nebula, the most massive of these infant stars have already begun to shine. It is these stars, known collectively as the Trapezium, which are responsible for lighting up the nebula. Through a modest backyard telescope, one can see this as a blue haze enveloping the collection of stars. One can also make out the four brightest stars of the Trapezium nestled tightly together. At temperatures exceeding 50,000 °F, these stars emit predominately ultraviolet light that ionizes the surrounding gas, shearing electrons off of their parent atoms only to have them crash back down again. As the now separated electrons and atoms recombine, photons of light are released thus giving the nebula its ethereal glow.
Within the confines of this nebula, astronomers have also discovered creation of another kind. Around stars dimmer and less massive than the behemoths of the Trapezium, swirling disks of gas and dust have been found. The material within these disks is spiraling down onto their hosts, marking the final stages of star formation. But within these disks, the seeds of future planetary systems are being planted. Out of the countless collisions of dust and debris, new planets – possibly new Earths – are being built piece by piece!
Elsewhere in Orion, a very different phase of the stellar lifecycle is ongoing. Marking the upper right shoulder of Orion is a red star: the massive supergiant, Betelgeuse. The red color comes from its relatively cool temperature – a mild 6000 °F. Betelgeuse is one of the largest stars known. Were a group of mischievous aliens, as part of some interstellar fraternity prank, to replace our Sun with Betelgeuse, its outer most layers would extend to roughly the orbit of Jupiter.
The star is probably less than 10 million years old, but weighing in at over 20 times the mass of the Sun has forced Betelgeuse to burn through its nuclear fuel at a prodigious rate releasing the energy equivalent of 135,000 Suns! As a result, despite its relatively young age, Betelgeuse is a star nearing the end of its life. Stars as massive as Betelgeuse don’t fade away quietly (like our Sun); when they die, they release their energy in one of the most cataclysmic events in the Universe: a supernova! As massive stars age, they fuse progressively heavier elements in their cores: Hydrogen fuses to form Helium, Helium forms Carbon, Oxygen and Neon, and so on. The total mass of a star determines how hot and dense its core can become which in turn sets a limit on which elements it can create. The cores of the most massive stars can fuse elements as heavy as iron. And this is when things get really interesting.
Unlike every lighter element, iron nuclei don’t release energy when they fuse together; they instead absorb energy. This is a problem for the star. The energy released by thermonuclear fusion is needed to support the star’s massive bulk against collapse. Once one of these stellar heavyweights starts to fuse iron, it’s robbed of this counteracting pressure. Without this energy supply, gravity starts to win in the precarious balance of forces that defines a star’s life. The star begins to implode!
REDO: As the outer layers hurdle down at speeds approaching 1/5 the speed of light, they eventually crash into the now hyperdense core and effectively “bounce” sending a supersonic shock wave back up through the star that literally blows the star apart. The core is left behind to become either an exotic neutron star or, if the star is massive enough, a black hole! One supernova can release as much energy in a few seconds as the Sun will radiate in its entire 10 billion year lifetime! Seen from across the Universe, a single supernova can temporarily outshine its host galaxy! The conditions present during the explosion are such that all of the heavier, more rare, elements on the periodic table can be created. The gold in your wedding band was forged in the fires of a nearby supernova billions of years before you were born!
Astronomers estimate that Betelgeuse could become a supernova within the next thousand years. Of course, given the 640 light year distance between us and the star, one is lead to the tantalizing possibility that Betelgeuse exploded long ago and that the supernova shockwave is already en route to Earth. When the light from this supernova reaches us, we’ll have front row seats to one of the most spectacular shows the Universe has to offer. For a time, the shoulder of Orion could glow as brilliantly as a full moon, bathing the nighttime landscape in its cool glow. After a few months, she will fade away and leave Orion with one less star.
But long after the light from the supernova has ebbed and the shape of Orion is forever altered, the effects of the explosion will continue to be felt. The shockwave will ripple through the gas and dust between the stars and trigger new waves of star formation possibly forming a second Orion Nebula. The new elements forged deep in the interior of Betelgeuse, now sent hurdling through the cosmos, will become incorporated in these new nurseries and seed the next generation of stars, planets, and perhaps intelligent creatures.
Spend a few moments some time this week and gaze at Orion. As you do, consider the dozens of cultures throughout human history that have celebrated these very same stars. Consider the hundreds of light years the light has traveled to reach your eyes. Consider the stars that are being born and the stars that are about to die. And consider that out of the fires of countless supernovae and one special swirling nebula you, and everything you know, came to be.
365 Days of Astronomy
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