June 2024 - Picture of the Month
NGC 6164 in Norma
This month I bring you an emission nebula some 4,200 light-years distant in the southern skies that's sometimes known as the Dragon's Egg. I think you'd agree that Martin's image of this object is spectacular enough to be a Picture of the Month.
It's designated as both NGC 6164 and NGC 6165 since it was first observed by John Hershel in 1834 as two unconnected bars of nebulosity separated by about 5 arc-minutes. These nebulae are the brightest edges of that broadly rectangular opening that's being created by the central star. NGC 6164 is the bar at the bottom left (actually the NW), and NGC 6165 is at the top right (or the SE).
Photographically it looks rather like a planetary nebula doesn't it? And that's what it was considered to be by Karl Henize in 1959, but by the early 1960s it had been established as an emission nebula. The cause for some of this confusion lies at the middle of the nebula in the form of an unusual type of star. It's the one that you can see shining brightly in the centre of this image at visual magnitude 6.7: HD 148937, a spectroscopic binary.
The components of HD 148937 are huge and hot O-type stars that have been in existence for 3–4 million years and will probably last the same again before ending life as a supernova. Yet one of the pair displays magnetic fields, and since stars as hot as these don't have convective interiors they're not meant to be magnetic. On top of that this magnetic star is about 1.5 million years younger than its companion.
As for the nebula itself, including the extensive blue coloured halo, it is significantly younger, at about 7,500 years, than either of the stars at its centre. Additionally, observations by ESO have confirmed that it contains relatively large quantities of carbon, nitrogen and oxygen, not the kind of thing expected from the outer layers of a star, but rather its core. So it's not likely that the nebula was formed from gas ejected by these young stars as had been supposed.
Recent observations by ESO have provided confirmation for an alternative hypothesis by Frost et al leading to a paper being published in April 2024 in Science. It seems likely that this was once at least a three star system, but that two of the companions were close enough to merge after 1.5 million years, forming a star with a magnetic field and ejecting the matter to create the nebula.
James Whinfrey - Website Administrator.
