Archive for the ‘Stellar evolution’ Category

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So close and yet so far

August 24, 2010

Some of the folks on Cloudy Nights have really cool quotes in their sig files. Last night I came across this one, which crystallizes the elusive feeling that I catch on some nights and pursue on all the rest:

There is a moment after you move your eye away
when you forget where you are
because you’ve been living, it seems,
somewhere else, in the silence of the night sky.

I figured this had to be part of a poem, so I Googled it, and found the original source here. It’s a poem entitled “Telescope” by Louise Gluck, and it originally ran in New Yorker on January 17, 2005. Since it’s already out there on the intarwebz, I don’t feel bad about reproducing it with attribution.

Telescope

by Louise Gluck

There is a moment after you move your eye away
when you forget where you are
because you’ve been living, it seems,
somewhere else, in the silence of the night sky.

You’ve been stopped being here in the world
You’re in a different place
a place where human life has no meaning.

You’re not a creature in a body.
You exist as the stars exist,
participating in their stillness, their immensity.

Then you’re in the world again.
An night, on a cold hill,
taking the telescope apart.

You realize afterward
not that the image is false
but the relation is false.

You see again how far away
each thing is from every other thing.

I think it’s a smashing poem and I wholly agree with the sentiments expressed, right up until the last two stanzas. Then, as far as I’m concerned, it all goes straight to heck.

– – – – – – – – – –

M8 photographed by Rob Gendler

One of the things I like best about observing is that with very modest equipment, one can see most of the stages of the life cycles of stars. Turn to M8, the Lagoon Nebula, or M42, the Great Nebula in Orion, and you can see stellar nurseries. The nebulae are great clouds of gas and dust that are only visible because they are illuminated from within by the terrifying light and heat of newly formed stars.

This process cannot last forever. Even as the last few protostars of a nebula straggle into ignition, their older siblings are blowing away the nebular cocoon by the force of their stellar winds. Eventually the nebula will be entirely dissipated, and all that will remain is a cluster of young stars, all of similar ages and chemical compositions. These are open clusters–as opposed to the vast and ancient globular clusters that haunt the galaxy’s halo–and they include some of the sky’s most brilliant jewels, such as the Pleiades and Hyades, the Beehive, the Double Cluster, and thousands more, of many sizes, ages, and distances.

The Double Cluster photographed by Rob Gendler

Even the stars of open clusters are not destined to remain together forever. They may remain together for tens or hundreds of millions of years, but the lives of stars are measured in billions of years. As open clusters orbit the core of the galaxy, repeatedly passing through the galactic plane, being overtaken and left behind by successive spiral arms, their constituent stars are stripped away from their weak mutual gravitation embrace. Eventually the cluster is entirely dispersed, its constituents becoming the un-clustered field stars that make up most of the galactic disc. Almost all of the stars  that you can see with the naked eye are field stars, each pursuing its own course around the galactic core, forever sundered from their siblings. Lurking out there in the Milky Way are the long-lost sister stars of our own Sun, which we might identify now only by their chemical fingerprints.

Even stars do not last forever. Near the end of their lifespans, with most of the hydrogen fuel in their cores converted to helium, main sequence stars start fusing hydrogen in the shell around the helium core. The star’s interior heats up still further, and the outer layers expand into a vast tenuous envelope. The surface area of this envelope is much larger, in relation to the total energy passing through it, than the star’s old surface. Hence it is cooler, and the light emitted at the star’s surface is shifted toward the red. The star has become a red giant. Arcturus in the constellation Bootes and Aldebaran in Taurus are familiar examples, respectively the third and thirteenth brightest stars in the night sky.

Still larger stars start to fuse helium to carbon and eventually carbon into still heavier elements. These stars may become red hypergiants, so large that they could swallow the entire inner solar system. Betelgeuse in Orion and Antares, the glaring red eye of Scorpio, are red hypergiants, and respectively the eighth and sixteenth brightest stars as seen from Earth.

M27 photographed by Rob Gendler

Now we come to a fork in the road. In small and mid-sized stars, such as the sun, the process of helium fusion proceeds in fits and starts, alternatively heating and cooling the star’s outer envelope. As it expands and contracts, the gas in the outer envelope picks up enough kinetic energy to escape the star’s gravity and expand into space. This process repeats, and star comes to be surrounded by concentric shells of blown-off gas. The gas is still energized by the star’s radiation, and glows as a nebula. This type of nebula is called a planetary nebula, not because it has anything to do with planets (the planets of such a star will have been scorched to cinders or completely eroded by star’s late-stage pulsations), but because they are often round and looked something like planets in the small telescopes of early astronomers. M57, the Ring Nebula, and M27, the Dumbbell Nebula, are two of the best and brightest planetary nebulae. Even tiny NGC 7662, the tiny round glow Brian and I star-hopped to last week, could not be mistaken for anything else. After blowing off most of their mass, the cores of the former giants persist as white dwarfs, which glow not because they sustain fusion but because their matter is heated to fantastic temperatures by gravitational contraction. Even after their planetary nebulae dissipate, white dwarfs may shine feebly for tens of billions of years.

Stars over a certain size, just a few times larger than the sun, have a different destiny. Bigger, hotter, they sustain more rapid fusion, exhaust their hydrogen and other light elements in rapid succession, and then blow themselves apart as supernovae. The cores of the exploded stars persist as neutron stars and black holes. Although supernovae are frequently spotted in other galaxies, there hasn’t been a naked-eye supernova in the Milky Way in centuries. There are some fine supernova remnants, however, diffuse halos of material still expanding outward from the explosions that created them. M1, the Crab Nebula, is one, and the much older and larger Veil Nebula in Cygnus is another.

M1 photographed by Rob Gendler

The matter blown off by dying stars, slowly and gently in planetary nebulae or all at once and violently in supernovae, rejoins the vast, diffuse molecular clouds that clot the galactic disc. Eventually the clouds will be sufficiently compressed, by the pressure waves that form the spiral arms, or by the shockwaves of nearby supernovae, for knots of material to start to accumulate. As the gravitational force of these concentrations pulls in more and more material, they will pass a critical threshold: fusion reactions will start in their cores and they will become new stars, lighting the encircling nebula from which they were born. The circle is then complete.

– – – – – – – – –

It is often noted that we are made of stardust. This is true, but it has become such a cliche that I fear it has lost its visceral impact. Consider: every breeze that has ever cooled you, every bite you’ve ever savored, every caress you’ve ever felt, the blood in your veins, the brain that you think with, the pillow under your head at night, the plastic and metal on which you’re reading this–every atom you’ve ever perceived with any of your senses, and all of others in the universe that you have not perceived, were born in the hearts of stars (except for the hydrogen and some of the helium atoms, which formed in the cooling fires of the Big Bang itself). So, yes, you are made of stardust. And so is everyone and every material thing you know.

And we will be stardust again. In five billion years the expanding sun will envelope the Earth. Our atoms, having been through the planetary cycles dozens  or hundreds of times, and incarnated in countless organisms of which we are but a snapshot, will be blown off with the rest of the crust and outer mantle. For a while we will shine as part of the sun’s planetary nebula, before being dispersed into the interstellar medium. But our constituents will know still greater fires when they are taken up into new stars, and new life when they are incorporated into other worlds.

Ashes to ashes, dust to dust, light to light.

– – – – – – – – – –

That is why I disagree with the estimable Louise Gluck, former Poet Laureate of the United States. When I am packing up the telescope, I don’t see how far each thing is from every other thing. I am still charged by what I have seen, and by the knowledge that I have been a nebula and an open cluster, a red giant and a supernova remnant. Any stage of stellar evolution that I can see in the sky, my atoms have gone through–and will go through again. There is no distance separating me from the stars. There is only time.

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