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Wednesday
Oct202010

Made Of Star Dust

A star sustains itself through a delicate balance between two competing forces: an outward pressure created by the nuclear reactions in its core that consume its fuel - typically Hydrogen; and the gravitational inward pull due the star's weight. When the star exhausts its fuel, gravity wins out and it undergoes a catastrophic collapse. The process of the collapse may heat up the star enough to start secondary nuclear reactions that burn the by-product of the previous burning stage. But this depends on the original mass of the star. For an average star, like our Sun, the collapse ensues after the star's Hydrogen fuel is consumed and transformed into Helium, and the star never gets hot enough to start a new life. Instead, it goes through a momentary expansion in size becoming a so-called Red Giant (go Giants?). During this phase, it spews part of itself in its neighborhood - very sad but nevertheless true… it then collapses back into an uninteresting rather dim object, a white dwarf - a ball of Helium and perhaps Carbon held by the pressure of a gas of electrons. For larger stars, the core gets hot enough for new nuclear reactions to follow - lighting up the star once again as it burn other heavier elements. Eventually however, all stars will end in a total collapse. The more massive ones may form neutron stars - balls of neutron particles packed together very very (very) densely ! or more interestingly, they may end up punching a hole in space by forming a black hole (see previous post on black holes)…

For average stars, the last explosive event they undergo generates beautiful patterns of dust and gases - called planetary nebula - around the dying star. The typical size of such a pattern is about one light-year; that is it takes about a year for light to travel across the nebula. Different materials in the cloud of dust glow with different colors in the veil of the strong UV light coming from the star at the center, and they form elaborate patterns on the dark background of the cosmos. The star ends its life by painting a last spectacular masterpiece with the stuff it was made of across the sky. This material eventually seeds other stars and planets. It is said we are all made of stardust...

If you're over 21, get a good glass of red wine, sit back, and watch the 3 minute slideshow I've prepared from imagery of planetary nebulae from NASA. The soundtrack is "Prayer of St. Gregory" from the Armenian-Scottish composer Alan Hovhaness. If you're under 21, use root beer instead.

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Reader Comments (2)

Thanks for the show of planetary nebulae, which are indeed very beautiful. As an amateur astronomer they are among some of my favorite targets. The astute reader may be wondering however, what do they have to do with planets? Very little actually, as the name is an historical anachronism. Back in the early days of visual astronomy when astronomers didn't know the difference between a nebula and a galaxy or the true size or distance of things, visual appearance was the main attribute of an object. Messier made his famous catalog of objects (M31, M57, etc.) as a list of objects not to confuse with the visually similar comets! Anyway, these nebulae formed at the end of the red-giant phase of a star's life have a visual appearance through a small telescope that resembles the round or oval disk of a faint planet. Thus the name "planetary nebula". It is an unfortunate name in retrospect, since it has the potential to engender confusion with a planet-forming nebula, thought to accompany the birth of a new solar system, rather than its death. Oh well, science is full of such infelicitous naming conventions, but we seem to live with them!

One last comment... the "event" that gives rise to a planetary nebula from an average-sized star is very gentle and gradual as compared with a supernova explosion. I'd be tempted to describe it more as "evaporating" than exploding.

- GL

October 20, 2010 | Unregistered CommenterGreg Lyzenga

Thanks Greg for the correction and clarifications. Maybe on a next post, I'll do a slideshow of how things look like when a star goes through a full blown explosion...

October 20, 2010 | Registered CommenterVatche Sahakian

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