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A sky-high accessory

January 25, 2011
By Aileen O’Donoghue

Orion's brilliant white belt, an asterism known to every culture on Earth, is with us again to accompany us through the winter nights. Orion first rises in the morning sky in September as celebrated in the chorus of Leo Kretzner's "Bold Orion on the Rise":

"Bold Orion, mighty hunter, rising in a clear, cold sky

"See the summer fall before him, Bold Orion's on the rise."

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Acknowledging that the home we share with him is winter, the last verse of the song says,

"Cut the wood and stack it high now, stoke the fires in your home,

"Burnin' nightly send the smoke up to the sky,

"Keep the winter at your door and keep the summer in your heart,

"Drink a toast to bold Orion on the rise."

And yet, I write of this in Puerto Rico, where 30 faculty and students from institutions across the United States have come to study and observe at the Arecibo Observatory. Here, Orion rises almost straight up from the horizon over tropical forests lush with leaves, lizards and the little frogs, the coquis, that sing throughout every night. It was disconcerting, almost disorienting the first time I saw Orion from a place where it felt like summer. Such are the deep associations most of us make with this spectacular constellation.

But Orion is not the only bright constellation of the winter. As the diagram shows, the hunter stands in the center of the "Winter Hexagon" asterism of bright stars made up of Rigel (Rye'-jel), his left knee, Aldebaran (Al-deb'-uh-rahn) in Taurus, Capella (Kah-pel'-ah) in Auriga, Castor and Pollux in Gemini (Castor is closer to Capella, Pollux closer to Procyon), Procyon (Pro'-see-on) in Canis Minor and the brightest star in the sky, Sirius (Seer'-ee-us). Seen when the cold air snaps with freshness, these and the surrounding stars can seem close enough to touch and invite us to pause, as we leave a restaurant or take out the trash, to marvel at the sky.

As well as delighting us with its beauty, this region of the sky also teaches us of the origin, evolution and deaths of the stars. The youngest stars are found in the Orion nebula at the end of the hunter's sword. Even with a small telescope, a quartet of bright stars, the Trapezium, is easily visible against the glowing gas of the nebula. These stars formed about a million years ago in cocoons of molecular hydrogen (H2) that contracted and heated under the relentless pull of gravity until their cores became hot enough, 10 million degrees, to fuse hydrogen into helium. It takes four hydrogen atoms to make one helium atom, but the helium weighs 0.7 percent less than the collection of hydrogen atoms did. This miniscule mass is converted according to Einstein's famous equation, E = mc2, to the energy that comes to us as sunshine and starshine. Only this nuclear fusion reaction can stop the contraction of the cocoon and hold the star up against gravity. The energy created by fusion is so great that particles - primarily protons and electrons - are blown off the outer surface of the star at tens of millions of miles per hour. This stellar wind blows the remaining cocoon away from the star, revealing it to the universe. The Trapezium stars, clearing their cocoons, created a large cavity in the larger nebula that broke through the side facing Earth. Thus we see into an inner chamber of this stellar nursery where there are actually upwards of a thousand stars crowded into a volume about four light years across. Most of the stars in the nebula are small red dwarfs that are only visible in infrared as they are still shrouded by gas and dust. In images from the Hubble Space Telescope, some show disks of gas and dust that may be forming planets.

The dust forming those planets is, itself, a product of the stars. During their lives, stars create the lighter elements of which our world is made. When the first stars formed about 500 million years after the Big Bang, the material in the universe was 75 percent hydrogen and 25 percent helium. The stars then fused more hydrogen to helium, but also fused helium into heavier elements such as carbon, oxygen, neon, magnesium, sodium and silicon, up to iron. All the carbon of which we are made, the oxygen and nitrogen in each breath, and the calcium in our bones and teeth was fused in the core of a star!

The length of a star's life depends entirely on its size. Massive stars evolve very quickly from hot blue giants to bloated red giants in their death throes. The stars of Orion's belt are blue giants 20 to 40 times the mass of the sun. Since they are only a few million years old, they are still vigorous and youthful. Rigel, the hunter's foot, is almost 20 times the mass of the sun and on the verge of old age at 10 million years. Betelegeuse, about the same mass and age as Rigel, is already an aged star. As the fuel for nuclear fusion runs out, complex reactions cause the star to swell to an immense size. As it does, the outer layers cool to "red hot" from the "white hot" of the younger star, creating a red giant. Betelgeuse is such a giant, with an outer surface that would reach the orbit of Jupiter if it were to replace our sun.

The deaths of stars also depend upon their masses. Sun-sized stars expand as red giants a couple of times and, after the second, lose their grasp on their own outer layers that expand away to become what is known as a planetary nebula. They have nothing to do with planets, but were give this name because they appear small and round in a telescope. The Eskimo Nebula is one of these. The naked core of the star then remains as a white dwarf that slowly cools to a black dwarf.

Giant stars such as Betelgeuse and Rigel die in supernova explosions that emit as much energy as the sun will over its entire life and can glow brighter than a galaxy for a few weeks. They explode after the iron in their core begins to fuse. Unlike the lighter elements, the fusion of iron absorbs energy so the core cools and can no longer hold the layers of the star up against the relentless pull of gravity. The star begins to fall in on itself. The central density increases rapidly and creates a shock wave that moves outward with explosive force, destroying the star. The explosion destroys the iron core but, in its immense power, creates more iron and all the naturally occurring heavy elements. So the iron carrying oxygen to your muscles and brain, the iodine regulating your metabolism and the silver and gold in your jewelry were all forged in the violent deaths of stars that seeded the nebula from which the sun and Earth formed. We truly owe our lives and our world to the stars - they are our ultimate parents!

This event took place in 1054 in the constellation of Taurus and was noticed by the astronomers of the Sung Dynasty of China and the Anasazi of Chaco Canyon in what is now New Mexico. The remnant of this supernova is the Crab Nebula, first discovered by Charles Messier in 1758, named by the Earl of Rosse who observed it in the 1840s from Birr Castle in County Offaly, Ireland, and visible in modest telescopes.



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