Sharpless 2-101: The Tulip Nebula...

Object: Sharpless 2-101 (Sh2-101)
Type: Emission Nebula 
Constellation: Cygnus
Distance: 6000 light years

Date: October 2nd, 2019
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 10 x 600s H-alpha, 10 x 600s OIII (2x2 binned), 5 each of 150s 2x2 binned for RGB star colour, no flats/darks (hot pixel removal in Astroart).

This image captures a section of the Milky Way just to the east of Eta Cygni, a third magnitude star in the neck of the constellation of Cygnus the Swan.  Within it is a HII emission nebula popularly called the Tulip Nebula, a glowing red cloud of interstellar gas and dust recorded by astronomer Stewart Sharpless in his 1959 catalogue as Sh2-101.

Stellarium sky map showing location of Sh2-101

The bright star near the centre of the nebula is HDE 227018, and is believed to be the primary source of ionising radiation that lights up the nebula.  Also framed in the field of view is Cygnus X-1, one of the strongest X-ray sources in our sky. Driven by powerful jets from a black hole accretion disk, its fainter visible curved shock front is just visible in the above image as a faint bluish arc to the west (right) of the Tulip's petals.

Cygnus X-1 is a binary star system and a strong source of X-rays that provided the first major evidence for the existence of black holes. The primary star, HDE 226868, is a hot supergiant star revolving around an unseen companion with a period of 5.6 days. Analysis of the orbit led to the finding that the companion has a mass of nearly nine solar masses. A star of that mass should have a detectable spectrum, but the companion does not.  From this and other evidence astronomers believe that it must therefore be a black hole.  The X-ray emission is understood as being due to matter torn from the primary star that is being heated as it is drawn to the black hole.

In the case of Cygnus X-1, perhaps little of that matter actually makes it into the black hole. In-falling gas may first collide not only with itself but with an accretion disk of swirling material surrounding the black hole.  The result may be a microquasar that glows across the electromagnetic spectrum and produces powerful jets that expel much of the in-falling matter back into the cosmos at near light speed before it can even approach the black hole's event horizon.  Confirmation that black hole jets may create expanding shells has come recently from the discovery of shells surrounding Cygnus X-1. 

A section of the main image is shown above, showing a trace of one such shell quite possibly created by the jet of microquasar and black hole candidate Cygnus X-1. The area within the circle has been stretched a bit in the blue channel to help the arc of hot gas stand out.

Seeing was not particularly good on this evening, and the guiding of the telescope was a bit erratic (around 0.9 RMS – the current autoguider settings had previously achieved 0.4 RMS). Sh2-101 is also a fairly faint object, and it could have done with more and longer subframes than I had patience for. 

The resultant narrowband image stacks were rather noisy and the stars a little bloated. The OIII emissions from the nebula are quite weak and I had to stretch the stack quite a long way to get the bluish colour to show through in the final image. Nevertheless, they cleaned up reasonably well in PSP and were RGB combined in PSP using the Ha stack as the red channel and the OIII stack as the green and blue channels. The RGB stars were added as a layer in “lighten” mode.

After all that I was quite surprised and pleased to have captured the shells from Cygnus X-1.