I just finished submitting my first for-publication journal article entitled “The Origin and Physical Properties of Cometary Knots in the Helix Nebula NGC 7293.” I am second author, after my Astronomy Senior Thesis advisor, Dr. Eugene Capriotti. We submitted it to the Astrophysical Journal, one of the more prestigious journals in the Astronomy community. The review process, if all goes well, should take between 6 months and 1 year, though it varies greatly by journal and discipline. Either way, it ought to get published in mid-late 2006 if it’s accepted by the scientists that review our work. Anyway, it’s a very interesting paper, enough so that perhaps I can explain the basics here:
When smaller stars (those with masses less than about 8 times the mass of our sun, including our sun of course) reach the ends of their life-cycles, they go through a phase that many of us have heard of with respect to our planet’s eventual demise, known as the Red-Giant phase. Exhausted of hydrogen in the stellar core, helium fusion begins. Helium fusion is much more energetic than hydrogen fusion, and the formerly yellowish star expands to accommodate the extra heat, eventually reaching enormous dimensions and reddening in the process. The sputtering end to this red-giant phase results in unstable releases of energy that produce variable stars (of the Mira type) whose brightness waxes and wanes on a period of 80 - 1000 days, depending on the star. Eventually, the material at the edges of the star becomes accelerated so quickly by the fluctuating energy output of the star that large amounts of the hydrogen and helium gas are expelled from the outer regions of the star. These then expand away from the star very quickly (approximately, in the case of our Helix Nebula, at about 90,000 miles per hour, compared to the 40,000 miles per hour speed of Voyager 1, the fastest object created by man) and become large, diffuse clouds of gas and dust that themselves emit or reflect light from the now much smaller central star. After sometimes several large mass ejections, the central star ceases shedding its envelope and settles into the white dwarf stage of its life where it will cool slowly over the next several billion years.
Helix Nebula: The tiny white dot in the middle is the white dwarf that shed this beautiful cloud of gas and dust, and is still illuminating it today.
Okay, still with me? Our neblua, the Helix Nebula (New General Catalog (NGC) number 7293, seen above) ejected its last gasp of star-stuff around 6500 years ago. In that intervening time, the gas is continually pushed by the very high-energy radiation emitted by the cooling central star. As the gas is pushed, it is heated, this heating sends a shockwave through the nebular gas that compresses it into billions of pancake-like clouds. Depending on the conditions within the nebula, these clouds may find themselves quite a bit denser than the gas they are effectively sitting on top of (picture this: the star is at the bottom of your field of view, with very diffuse hot gas above that, and denser, colder, pancake-like clouds at the very top). Now, if you’ve ever poured water on top of oil, you know that the denser water wants to be at the bottom of the container, and thus the pancakes break up into smaller globules (still about the mass of Jupiter, and the size of the inner solar system) that then sink towards the central star (at least relative to the gas expanding around them, the globules are still being ejected from the stellar system, just less quickly). These globules are called cometary knots, and are shown in the picture below.
Cometary Knots in the Helix Nebula. The brownish, finger-like globs of material towards the bottom left of the image are the knots, clouds similar to those they came from are pinkish and at the top left. The general blue color is the color of the hot gas.
Our paper discusses how these globules form via instability processes known as Kelvin-Helmholtz and Rayleigh-Taylor types. Though these have been studied for the past few decades, major efforts at modeling their formation mathematically have not been undertaken, and our paper extends on work done by Capriotti in the late 1960s. It is a bit of a niche study, but our paper fills it well. The theory is robust and fruitful. It generates masses, sizes and locations of the cometary knots that match observations nearly spot-on, given a few assumptions about central star properties that themselves are all well within reasonable limits.
The next paper that Capriotti and I are publishing, which will be finished somewhere near January, will be about a similar type of instability process that may have led to the formation of Globular Clusters, such as the one pictured below. That paper is an extension of my senior thesis work done a year or so ago. I describe it poetically this way: Imagine a cluster of stars forming nearly 13 billion years ago, deep in the dark heart of a giant cloud of gas containing more mass than 100,000 of our own suns. These enormous O-type stars erupt into being, their combined brilliance forces back the gases around them. After several million years of constant pressure from this central stellar core, a large portion of the giant gas cloud has now been compressed into an enormous spherical shell of gas many hundreds of light-years across.
But, eventually this ponderously massive shell of gas begins to outweigh the pressure from the waning stars at its core, and very slowly, the shell collapses. The collapsing shell gradually accelerates inward upon itself, while the continued light from the aging central stars tries futilely to force the shell outward again. In a few cases, forces are just slightly out of balance such that the shell begins to break up, fracturing into tens to hundreds of thousands of gigantic globs of gas. In even few of these cases, the globs are the right temperature and size such that their own weight is enough to overcome their desire to disperse into the interstellar medium, and they begin their collapse.
Over the next few hundred thousand years, the shell of gas that had once glowed so brilliantly as it expanded goes dark. But, slowly at first, then much more rapidly, tiny points of light burst forth around the now dim central stars. First one, then hundreds, then finally thousands of points of light appear where there was only darkness. These newborn stars, all locked in an intricate gravitational embrace, grow steadily in brightness, and resume their place on the stellar main-sequence, which will be their home for the next few billion years. One day, a strange thing happens, the central stars, having eaten furiously through their hydrogen fuel, begin fusing helium, then carbon, nitrogen and oxygen are fused. But the stars’ appetites are too large, when their entire cores have fused into iron, fusion can no longer proceed, and they collapse in upon themselves. A series of supernovae briefly outshine the entire young cluster around them. But that was all ten billion years ago. Now in the much yellower cluster, only older stars remain, all serenely orbiting their combined center of gravity.
M22, a globular cluster near the center of our galaxy. See copyright information.
For each paper that I publish, I will attempt to describe its significance in a straightforward manner. I think that this is extremely important, for, if someone cannot explain their science in physical terms, or at least by close analogy to the physical world, then how can the general public be excited about their research? And, since you all fund my research, it is my job to get you excited!
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