One of the true pleasures related to writing scientific news is that I get to look at some of the most amazing (and beautiful) images taken by the Hubble Space Telescope. Today’s image is no exception.
The image accompanying this article was taken by the Hubble on February 6, 2007 of an object known in astronomical circles as NGC 2440 (the 2,440th object in the New General Catalog of deep space objects). The object itself is classified as a “planetary nebula” and was formed when an average-sized star similar to our sun reached the end of its life cycle and collapsed into what is known as a “white dwarf” star while simultaneously shedding most of its atmosphere.
The term “planetary nebula” is misleading in that it was first used in the mid to late 18th century by astronomers who noted the existence of objects that “looked like planets” (they appeared similar to the outer planets of our solar system in the low-technology telescopes of the era) but did not change position over time. We know now that these objects are located far beyond the solar system and that they are not planets nor are they likely to become planets.
A planetary nebula is formed by a star that began its life cycle with a weight between 0.9 and 1.4 solar masses (1 solar mass = the mass of our sun). As the star gradually fused its available supply of hydrogen into helium, it reached a point where it began to fuse helium into carbon, nitrogen, and oxygen (known as the CNO Cycle or “helium burning”). Since the amount of heat produced by the CNO Cycle is dependent on the inward gravitational collapse of the star’s atmosphere there is a period of alternating expansion and contraction of the star’s atmosphere, it eventually reaches a point where most of the atmosphere is lost into space as the star itself collapses into a white dwarf star.
White dwarf stars possess about half the mass of their parent stars but this mass is compressed into a volume not much larger than the Earth. White dwarf stars are estimated to have densities approaching 10 billion kilograms per cubic meter. Although white dwarfs are extremely dense they are not dense enough to cause elements heavier than oxygen to fuse. When fusion ceases, the star is then radiating heat that is “left over” from its previous energy production. After about 100 million years this residual heat is lost and the star becomes a “black dwarf” at a temperature equal to the space surrounding it.
We have no idea when the star that gave birth to NGC 2440 became a white dwarf. We do know that since it is about 3,500 light-years distant what we now see was the condition of NGC 2440 at the time that the Middle Kingdom of ancient Egypt was drawing to a close.
For those that know how to use astronomical coordinates to locate celestial objects, NGC 2440 can be found at RA 7:42, Dec -18:13, magnitude (nebula) 10.8, (central white dwarf) 17.5, distance ~ 3,500 light years.
More Images of NGC 2440
An earlier image (published on July 30, 2000) of NGC 2440 was first displayed on the Astronomy Picture of the Day web site at http://antwrp.gsfc.nasa.gov/apod/ap000730.html.
Another early image (with fine detail) is at http://www.emjc3.com/universetour/sld068.htm.
This press release, http://hubblesite.org/newscenter/archive/releases/1992/12/image/a/, from the Hubble Space Telescope web site contains the first image (taken in 1992) of NGC 2440 by the Hubble.
For More Information about the Life of Stars and Planetary Nebulae
Although a significant per centage of teachers, instructors, and professors will not accept the Wikipedia entry on any subject, it is still one of the better online resources for the average Internet user. I recommend the following to anyone wanting to learn more about the Life of Stars and Planetary Nebulae:
· You should start with the entry “Stellar Evolution” located at http://en.wikipedia.org/wiki/Stellar_evolution. This page is a bit “heavy” in places, but there are links to less technical pages as well as pages that are really math-intensive.
· Next, visit is the entry “White Dwarf” stars at http://en.wikipedia.org/wiki/White_dwarf.
· In the early universe only hydrogen, helium, and traces of lithium were present. To learn how other elements came into being, read “Stellar Nucleosynthesis” at http://en.wikipedia.org/wiki/Stellar_nucleosynthesis.
· Finally, read the entry for “Planetary Nebula” at http://en.wikipedia.org/wiki/Planetary_nebula.