Ou 4 and Sharpless 129

Ou 4, the so called Giant Squid Nebula, was discovered by the French amateur-astronomer Nicolas Outters in 2011. Although it seems to be part of the remote weak HII region Sharpless 129, it was primarily believed that Ou4 is a regular Planetary Nebula located in the foreground of the emission nebula. Sharpless 129 is about 2,300 light-years away and is sometimes called the “Flying Bat Nebula”. It is illuminated by a triple system of massive stars, catalogued as HR8119. The mass of its brightest companion is 32 times that of our sun.

New investigations suggest Ou 4 really is part of the HII region and was created by an outflow of gas launched about 90,000 years ago from the central system of stars. Ionisation of Ou 4 could be mainly driven by shock waves when the ejected matter collides with the surrounding gas clouds. If Ou 4 is associated with Sharpless 129 it would be about 50 light-years across – a really giant bipolar nebula.

Taking pictures of Sharpless 129 and Ou 4 is a real challenge for astro-imagers, especially on light-polluted sites. On the Ou4 one hand both objects are rather large – Ou 4 is spanning more than 1° on the sky, whereas Sharpless 129 diameter is at least 2.5°. On the other hand the characteristic of the emitted light is very special: The weak emission nebula Sharpless 129 emits mainly Hα light, OIII emission is limited to only the brightest areas of the nebula. Ou 4’s spectrum is dominated by the light of two-times ionised Oxygen (OIII), but its radiation is extremely weak and is easily washed-out by domestic glowing of the atmosphere.

Therefore, not only fast optics with short focal lengths and cooled cameras in combination with narrowband filters are required to successfully shoot these objects, you also have to consider very long exposure times.