Abell 39: Its symmetry is unusual
It looks like a gigantic soap bubble floating in space. This is Abell 39, a remarkably beautiful object that could help astronomers solve problems about the chemical composition of stars.
The object is an example of a planetary nebula, a shell of material puffed off by certain stars towards the end of their lives. The term planetary nebula is a misnomer; early astronomers mistakenly thought these objects to be planets.
The symmetry of Abell 39, one of the largest such spheres ever discovered, enables scientists to make an accurate estimate of how much material is actually absorbing and emitting light. This will allow its composition to be determined.
At present, there is a disagreement among astronomers about the exact chemical make-up of dying stars that produce these planetary nebulae. Abell 39 may help solve this problem.
"The truly spherical nature of this beautiful nebula helps us eliminate a common confusion concerning the actual three-dimensional geometry of most nebulae," said George Jacoby, director of the Wisconsin-Indiana-Yale-Noao Observatory.
Not so central star
The nebula is about five light-years across, and the thickness of the spherical shell is about a third of a light-year (a light-year is about 10 million, million km or six million, million miles).
Located deep in the constellation of Hercules, Abell 39 is about 7,000 light-years distant.
Most planetary nebulae are only roughly spherical, or more commonly a butterfly shape with winding filaments and clumps of gas. These shapes make it tricky to model the interactions between the expanding gas shell and the surrounding medium.
Abell 39 is one of the rare cases in which the geometry is much simpler. The remarkable image reproduced here was taken with a 3.5-metre (138-inch) telescope, but in order to make detailed measurements of its spectra - from which its composition can be deduced - the astronomers had to use a larger telescope.
Precise measurements indicate that the central star is slightly off-centre, by about a tenth of a light year. Astronomers do not understand why. There is also evidence of a puzzling faint outer halo around the entire planetary nebula.