PLANETARY NEBULA ABELL 74
Photo Credit: T.A. Rector (University of Alaska Anchorage) and H. Schweiker (WIYN and NOAO/AURA/NSF): CLICK photo for larger view and see below for information and links.
Located approximately 2500 light years distant in the constellation Vulpecula is this spherically shaped ancient planetary nebula called Abell 74. It’s approximately 15 light years in diameter and at its center remains a white dwarf star cataloged as WD 2114+239. Let’s go into detail a little and see what a planetary nebula is exactly.
WHAT IS A PLANETARY NEBULA (PN)? On July 12, 1764 Charles Messier became the first person ever to view a planetary nebula; it was M27, the Dumbbell Nebula in the constellation Vulpecula. Antoine Darquier later stated that they resembled “fading planets” though that’s not where name planetary nebula came from completely.
The name itself 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 is now imposing 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.
The effort in photographing objects like these just simply does not get the credit that it deserves. Unlike photographs you see or take on a day to day basis here on Earth or even of the Moon; planetary nebula or any kind of deep sky object (DSO) for that matter take an incredible amount of desire, patience, painstaking alignment, hours upon hours of different length exposures through different settings, filters etc. Then comes the very time consuming and stressful part (because I’m horrible at it) of collecting all of your exposures, darks, flats, etc. and processing them to render a photo one-quarter the quality of this. Astrophotography; namely amateur astrophotography is not for the faint of heart or for those with a low tolerance for trial and error or low level of patience. All this of course is after you have dedicated much of your hard earned money to the purchase of the equipment that you need, packed it up, traveled out to a dark sky site, hoped for good seeing etc. That’s why I truly enjoy showing off the works of others because I personally know how difficult, expensive and time consuming DSO photography is and it doesn’t get the credit that it deserves. Everyone loves the incredible photos but very few know what it takes to create them.
NAME: Abell 74 with central white dwarf star WD 2114+239.
WHAT IS IT?: Planetary nebula.
HOW FAR AWAY IS IT?: Approximately 2500 light years distant.
HOW BIG IS IT?: About 15’ X 13’ arc minutes on the night sky which translates to about 16 X 14 light years in diameter at this distance.
APPARENT MAGNITUDE?: A pretty dim 12.2 with a central white dwarf of 17.
WHERE IS IT? (General): Constellation Vulpecula the “Little Fox”.
WHERE IS IT? (Exact RA/DEC J2000): RA 21h 16m 52s / DEC +24d 08’ 05”.
Dr. Travis Rector page for this photo: http://aftar.uaa.alaska.edu/gallery/details.cfm?img=341&type=#
NOAO page for this photo: https://www.noao.edu/image_gallery/html/im1165.html
Dr. Travis Rector astrophotography page: http://aftar.uaa.alaska.edu/gallery/
Messier SEDS planetary nebula info: http://messier.seds.org/planetar.html
George Abell’s Catalog of Planetary Nebula: http://www.astronomy-mall.com/Adventures.In.Deep.Space/abellpn.htm