THE BEAUTIFUL ABELL 7 & WHAT IS A PLANETARY NEBULA?
Photo Credit: Don Goldman: CLICK photo to view full size and see below for links and info.
This beautiful capture of planetary nebula; Abell 7 is yet another example of the violent beauty of the universe. As this star died, it released most of its material back to into the universe to be recaptured and recycled into new stellar life in the far future. Behind, all that is left is the white dwarf left alone to the universe. In this photo the progenitor star is hued slightly blue and it can easily be identified near the center of the structure.
WHAT IS A PLANETARY NEBULA (PN)?
Well the name is somewhat of a misnomer but let’s start with the words themselves; planetary and nebula. Frederick William Herschel devised the name because as he viewed these objects in telescopes they resembled the greenish disks of planets like Uranus which he discovered. The word “nebula” is Latin for mist or cloud so combined I suppose you could call them “misty/cloudy planets” though obviously they have no relation to actual planets.
As for the objects themselves they are very low density and comprised of mostly gaseous materials. They range in size depending on the mass of the dying star, composition, speed of material ejection and the amount of time the event has been taking place as well as under what processes it was undergoing while it was being formed. Density in PN’s are so low (Approximately a million atoms per cubic centimeter) that no vacuum on Earth can recreate these conditions. Though planetary nebulae were known about for over 200 years, nobody knew what they were or what their origins were besides the fact that they could look and see them.
Inside the core of a star pressures and temperatures are so great that a process called Thermo Nuclear Fusion can take place. It’s important to remember that stars don’t “burn” as burning is a chemical reaction. Thermo Nuclear Fusion is a nuclear reaction. Hydrogen atoms are being fused together to create helium atoms. When the two nuclei fuse together the combined mass is slightly less than the sum of the original nuclei and the difference is released as energy. As the progenitor star burns through all of its available fuel the star quickly begins to die. The key word here is “available” fuel as most of the hydrogen in a star will never be fused in the core. When this happens gravity begins to win over the outward pressure of the nuclear reaction. The star then begins to crush down onto itself from the extreme inward pressure that gravity has imposed on it.
Ironically enough, this very process of compression causes the core to become super-heated. Core temperatures rise to ten-times the temperature they were while in the main sequence phase. Once the temperature reaches 180,000,000 degrees the temperature will be hot enough to fuse helium into carbon. As this continues, the star will begin to quickly use up its available helium fuel. The massive temperatures cause the star to swell and the asymptotic giant branch aka, the infamous red giant phase begins. Vast layers or shells of gas are then released into the surrounding cosmos creating what’s known as proto-planetary or pre-planetary nebula (PPN’s). The furiously hot star whose inner layers are now exposed radiate massive amounts of ultraviolet radiation rendering the surrounding material aglow and that’s where the planetary nebula (PN) phase begins.
When the available helium begins to run dry, gravity once again begins to take over, and again the star is crushed in upon itself; this time down to about the size of the Earth. It is at this point when electron degeneracy pressure is enabled. This means that electrons themselves are being crushed together so tightly that their repulsive pressure on one another will hold the star up. That is where it will remain for billions of years as a white dwarf.
Most stars in the universe (about 95%) will end their lives this way; the rest will die as supernovae.
NAME (S): Abell 7.
WHAT IS IT?: Planetary Nebula (Dying Star).
HOW BIG IS IT?: Approximately 8 light years in diameter and 12.7’ arcminutes on the night sky.
HOW OLD IS IT?: Approximately 20,000 years old as we see it.
HOW FAR AWAY IS IT?: Approximately 1800 light years.
APPARENT MAGNITUDE: A pretty dim 13.2 or +13.2.
WHERE IS IT? (General): Constellation Lepus “The Hare”.
WHERE IS IT? (Exact RA/DEC J2000): RA 05h 03m 07.5s / DEC -15d 36’ 23”.
Don Goldman website: http://www.astrodonimaging.com/gallery/gallery.cfm?type=2
This photo is NASA’s APOD: http://apod.nasa.gov/apod/ap131205.html
Great Planetary Nebulae page: http://www.scholarpedia.org/article/Planetary_nebulae
Cosmotography Planetary Nebula page: http://www.cosmotography.com/images/planetary_nebula_formation.html