A team of astronomers at the Gemini Observatory in Hawaii has taken a stunning image of the planetary nebula Sharpless 2-71.
Discovered in 1946 by Rudolph Minkowski, Sharpless 2-71 (Sh2-71) is located in the direction of the constellation Aquila and visible in amateur telescopes.
For most of its recorded history, astronomers assumed that the nebula formed from the death throes of an obvious bright star (a known binary system) near its center. Arguments against that claim, however, have turned this case into a classic mystery of uncertain parentage.
The new image shows the long-assumed central star shining as the brightest object very close to the center of the nebula’s beautiful gas shell. But new observations have shown that the nature of a dimmer, bluer star – just to the right, and a bit lower than the obvious central star – might provide a better fit for the nebula’s “birth parent.”
The uncertainty arises from the fact that the brighter central star doesn’t appear to radiate enough high-energy (ultraviolet) light to cause the surrounding gas to glow as intensely as it does, whereas the dimmer, bluer star likely does.
On the other hand, the brighter star’s binary nature would help explain the nebula’s asymmetrical structure. Astronomers do not yet known if the dimmer, bluer star also has a companion.
Another unresolved issue is whether the brighter star’s unseen companion might be hot enough to excite the gas to glow. If so, this pair might be able to hold on to its parental connection to the nebula.
“At the assumed distance to the nebula – roughly 1 kiloparsec or about 3,260 light-years – the faint star has about the right brightness to be the fading remnant of the nebula’s progenitor star,” said Dr David Frew of the Macquarie University in Sydney.
Then again, the brighter binary star is an uncommon one that shows strong and broad hydrogen-alpha emission, which are seen in some planetary nebulae. According to Dr Frew, this star is also unlikely to be a chance projection or alignment with the nebula. “So there could be at least three stars in this system,” he said.
Putting aside the complex issue of which star or stars formed this object, the nebula’s striking morphology also poses difficult questions.
“The nebula presents a multi-polar structure and several pairs of bipolar lobes at different orientations,” said Dr Luis Miranda of Spain’s Instituto de Astrofísica de Andalucía, who has also studied this object. “These lobes most certainly formed at different times and likely involved a binary progenitor – in particular with mass-transfer and multiple episodes of mass ejection along an axis where the orientation changes with time.”
“Noting that the nebula’s structure is difficult to explain without a binary pair for parents,” Dr Miranda said. “The chaotic morphology of Sh2-71 implies that very complex processes have been involved in its formation.”