Superbubble in the LMC

This bubble is three hundred light-years across and it is mostly empty. What the image shows is its wall.

Blown from the inside

Henize 70, also catalogued N70, is a superbubble in the Large Magellanic Cloud, some 170,000 light-years away and around 300 light-years across. A small group of very hot O and B stars sits at its centre, and their combined stellar winds have pushed the surrounding gas outward into a shell. There is evidence that an old supernova contributed to the shape as well, though that has not been confirmed.

Several million degrees, and invisible

A superbubble is not a cloud but a cavity. The gas inside it has been heated to several million degrees by the winds and explosions of stars above eight solar masses — hot enough that it radiates in X-rays and gives off almost nothing at visible wavelengths. Everything in this image is the wall: the swept-up material at the edge of the cavity, compressed and lit from within.

Hydrogen on the rim, oxygen in the middle

The H-alpha emission is strong across the shell, which is consistent with a supernova having passed through it. The OIII is intense too, and it sits in the central region rather than at the rim — the blue-green part of this image. That distribution is the useful thing: the two lines are not tracing the same gas, and where each one is strongest says something about what heated it.

Twenty-eight hours, twenty-two of them narrowband

The CDK17 was working at f/6.8 with adaptive optics ahead of the sensor, from Rio Hurtado in February 2022. Of twenty-eight hours, eleven and a half went to H-alpha and ten and a half to OIII — almost an even split, which is unusual. On most emission nebulae OIII is the weaker line by a wide margin and gets a fraction of the time; here it is strong enough in the interior to be worth as much as the hydrogen. The remaining six hours across luminance and colour do one job: they give the surrounding stars their real colours, which narrowband cannot. What none of it reaches is the inside of the bubble, which is where the energy is. Martin Pugh gathered the data at El Sauce.

HOO palette: H-alpha in the red channel, OIII in green and blue. The nebula's colours are not its true colours; the star colours are, taken from the broadband frames.