[Photo by Joe Bauman]
On an ice-cold desert night late last October, I was trolling around a star cluster with my telescope. I wasn’t paying attention to which one, just watching its hundreds of stars slide by. A blob passed. I stopped and scrolled back up, star after star, the blob, more stars. I slid back and stopped on the blob. It was a larger solid circle in the midst of a field of stars.
Intrigued, wondering if I had stumbled across a new comet, I took a dozen photos. As they were coming onto the computer I concluded it was the planetary nebula called the “Eskimo Nebula,” which looks like a face with a parka’s ruff around it. But back home I saw differences; it wasn’t the Eskimo.
Obviously it was a planetary nebula, layers of material puffed off a dying star and flying into space, spreading out so far that the remains look like a vast round planet. But which one?
I should have paid more attention to where my telescope was pointing. I began searching through hundreds of planetary nebula photos on the Internet. Eventually I stumbled upon the remarkable “Planetary Nebulae Observer’s Home Page,” which is maintained by an observer named Doug Snyder of Palominas, AZ.
Among Snyder’s many informative pages are thumbnail photos of some of the best planetaries, arranged by season. I went to the page for those visible in the winter in the northern hemisphere and studied its 25 thumbnails. One showed a familiar pattern with stars lining up in a distinctive V shape. I had found my target. It is called NGC2438.
In the background is a star cluster dubbed M46, about 5,400 light-years from us. The open cluster with the planetary make a lovely combination, but this is a grim picture too.
The nebula is just over one arc-minute across, according to Snyder. To put that in perspective, Patrick Wiggins, NASA solar system ambassador to Utah, noted that if you measure all the way around the horizon, that’s 360 degrees. Each degree is 60 arc-minutes. The total number of arc minutes going around the horizon is 360 times 60, that is, 21,600. Call it 21,600 parts. This planetary is the size of one part.
I was impressed to learn that its progenitor star, a tiny blue dot in the center, is extremely dim. It’s only magnitude 17.5 or 17.7, yet here it is in my view.
Snyder provides a good description of planetary nebulae, of which he says more than 1,000 are known. They are “formed when old stars of a particular size, similar to our Sun’s size, have consumed most of their hydrogen fuel after billions of years. The hydrogen has mostly been converted to helium, and the star has expanded to become a Red Giant.”
Scientists say that will happen to our sun in about 5 billion years. When it expands its outer reaches will encompass the orbit of the planet Mercury. Without hydrogen, for a time it will fuse helium but then it will use up the supply of that element. It will start to collapse, then re-expands until it swallows Earth’s orbit.
“The Sun will not be very stable at this point and will lose mass. This continues until the star finally blows its outer layers off,” says a web site maintained by NASA’s Goddard Space Flight Center. This is the planetary nebula stage. “The core of the star, however, remains intact, and becomes a white dwarf.”
I had always thought the central star of a planetary was a white dwarf. But Goddard says planetary nebulae seem to mark a transitional state of the star from red giant to white dwarf. Stars the size of our sun will become white dwarfs “within 75,000 years of blowing off their envelopes,” the site adds.
The remnants of our sun will become a white dwarf only about the diameter of the late planet Earth, but so compact that it will be one of the densest objects in the galaxy. Only neutron stars and black holes will be denser. Over eons, the carbon will cool down and the sun will become a black cinder.
Stars the size of ours aren’t large enough to explode in a supernova that lights up a galaxy. They just swell up, blow off their gases, and slowly cool down.
So this is the way the world ends, not with a bang but a cinder.
My photograph is a view of our future, 5 billion years from now. Maybe a planetary system with civilizations was enveloped when the central star of NGC2438 puffed out.