Looking Up: A butterfly dies in distant space
This is NGC 6302. Its shape resembles a butterfly, but sometimes it is just called the "Bug Nebula." It’s probably fitting that this insect-like object is in the constellation Scorpius – the scorpion.
It was first identified back in 1888. However, the earliest, serious study of the object was conducted by the great visual astronomer Barnard, who examined and drew an image of it in 1907.
What is it?
When most stars die they do so because they have run out of fuel, which is needed to keep the nuclear reaction going in their core. This nuclear reaction creates a "push out" force from the center of the star, which is balanced nicely by the "push in" effect of the star's mass, after it has reached its long term eco balance. Our own star, the sun, reached that point about 4.6 billion years ago and has about another five billion years before its fuel is exhausted.
When this happens the relation between the "push in" and the "push out" changes dramatically. The star leaves the state of equilibrium and becomes unstable. Its mass begins to be jettisoned or "blown off" into space. The resulting shell of gas surrounding the star continues to expand until there is no more mass left to eject.
Well, almost no more. There is a small, residual amount left over, and that collapses down to form a very dense, cool "corpse" called a white dwarf star. Our own sun will someday be a white dwarf. Before that the hot gas it puffs off will go well out into the solar system, past the orbit of the earth – thus destroying it with heat.
This stage in the life of a star is called a "planetary nebula." It is not accurately named since it’s a stellar event and has little to do with planets (except to destroy the nearby ones). The resultant appearance of the outward expulsion of the gaseous debris from the dying star takes different shapes. Usually its round, but in the case of NGC 6302 it looks somewhat like a butterfly.
Note the very thin, dark line running through the center between the two "wings." The gas in this ring is particularly thick, and it was the first material expelled from the surface of the dying star. It formed an early, dense belt around the star's equator, and later material being expelled was forced to follow a "polar" pattern of outflow from the star. Thus the "Butterfly" is said to be a "bi-polar" planetary nebula.
The small, central star is not visible in the largest, land-based telescopes in the world, largely because it is hidden inside the thin, dense belt around its waist. Although the Hubble has detected it.
The star has not yet passed to the final cool, white dwarf stage. In fact, right now, it is among the hottest stars ever detected in the galaxy with a surface temperature around 250,000 degrees Celsius.
Before it began its death throes this was one very large star. In its full grown "adult stage" the star had a mass about five times larger than our sun but has since shrunk down to about 0.64 the size of the sun. When the final expulsion of its material takes place, it will be even smaller.
The size and shape of the Butterfly took only about 2,200 years to form, which is a very short time on the clock of the universe. Likewise, it will eventually dissipate into space rather rapidly, thus leaving only a tiny "white dwarf" in its place.
Wolfe is a member of the Everglades Astronomical Society. Organized in 1981 it serves the Naples community, providing information in all aspects of amateur astronomy. Its goals include educating the public, school children and other groups to the wonders of the universe. The society meets at 7 p.m. every second Tuesday of the month at the Norris Center (public invited). Regular viewing visits to a special, dark sky site in the Everglades are held each month, allowing the public to observe the night sky though telescopes under pristine conditions. For more information, visit the website at http://naples.net/clubs/eas.