A Planetary Nebula in Virgo

The star at the centre of this shell is more than twelve times hotter than the Sun, and most of what it emits cannot be seen at all.

Eight hundred light-years away

Abell 36 is a planetary nebula in Virgo, about 800 light-years from us, also catalogued PN A66 36. George Abell found it in 1955 while working through photographic survey plates, and the catalogue that carries his name is largely made of objects like this one: shells too faint and too spread out to have been picked up visually.

What is left of a star like the Sun

The nebula is material the central star has thrown off. It was a star not unlike the Sun; having run out of nuclear fuel, it shed its outer layers, and what remains is a core contracting and heating on its way to becoming a white dwarf. The shell is not an explosion — it is the star's own outer envelope, drifting outward and thinning as it goes.

Seventy-three thousand degrees

That contracting core now has a surface temperature above 73,000 K, against the Sun's 6,000. A star that hot radiates most of its output in the ultraviolet, which is why it looks unremarkable in a visible-light image while dominating everything around it in a part of the spectrum no camera here can record. The ultraviolet ionises the hydrogen and oxygen in the shell, and the shell re-emits at visible wavelengths. What the image shows is the star's invisible output, converted by the gas into something that can be photographed.

Ten hours of oxygen, three of colour

A CDK24 at f/6.5 from Rio Hurtado in May 2022, twelve hours and fifty minutes in total — but nearly ten of them are OIII alone, in ten-minute frames, and the three that remain are red, green and blue in frames of three minutes. That is not a balanced dataset, and it is not meant to be: the red, green and blue serve the stars and nothing else. There is one consequence worth stating plainly. The ultraviolet from the central star ionises hydrogen as well as oxygen, and no hydrogen frame was taken. What is visible here is the oxygen shell; the hydrogen component of the same nebula is not in this image, and adding it would change the shape of what you see.