NEBULAE
The Crescent Nebula
— NGC 6888 —
Processed by Nicolas Rolland, Paris. 3.3 hours gathered at IC Astronomy Observatory, Oria, Spain.
The star at the centre is destroying the gas it lost four hundred thousand years ago, using the gas it is losing now.
A star throwing itself away
WR 136, also catalogued HD 192163, is a Wolf-Rayet star: a massive star far enough into its life to have exhausted the hydrogen in its core and to be shedding its outer envelope wholesale. It is losing the equivalent of the Sun's mass every ten thousand years, carried off by a wind moving fast enough to leave the star behind for good. Stars of this kind are rare and short-lived — the phase lasts a few hundred thousand years, which is why only a few hundred are known in the whole galaxy.
A shell built by two winds
This is not the star's first ejection. Around four hundred thousand years ago it was a red giant, and it shed a slower, denser wind that drifted outward and is still out there. The fast wind blowing now has caught up with it. Where the two meet, the fast gas piles into the slow gas and compresses it, and the resulting shell is about 25 light-years across — close to the full width of this frame at 5,000 light-years. The crescent shape is that collision boundary, seen from outside.
Two shocks, and an ending
A collision like this produces two shock fronts, one running outward into the old wind and one running back into the new. Between them the gas reaches millions of degrees and radiates in X-rays, which is to say the hottest part of this object emits nothing an optical telescope can record — it was found by Chandra, not by anything working in visible light. None of it will last. WR 136 is expected to explode as a supernova within the next million years, and the blast will overtake and disperse the shell it spent that time building.
Three hours, two filters
AAn Officina Stellare ProRC 700 with an FLI PL16803, from Oria in August 2020 — three hours and twenty minutes in five-minute frames, half in H-alpha and half in OIII, with no broadband at all. On a shell of magnitude 7.4 that is enough for the crescent itself, which is bright and sharply bounded. It is not enough for what surrounds it. Deep exposures of this object show a much larger and far fainter OIII shell extending well beyond the visible crescent, and reaching it takes tens of hours on the oxygen line alone. What this frame holds is the bright boundary between two winds; the outer edge of the older one is beyond it.
