Hello All,

This little planetary took all of the UK summer to collect data for. Its a small, but bright planetary nebula in the constellation of Hercules, between 4892 and 6849 lightyears away (depending on what source you read). The main body of the PNe is 26" x 25", but it is surrounded by a larger, but much dimmer halo. In this image north is to the bottom and east is to the right.

For the nebula, the narrow band channels have been independently masked stretched (in PixInsight) and then combined according to the following PixelMath formula: Red = N2, Green = 0.5*(Ha+ O3), Blue = O3.

Unfortunately the available sky time meant I couldn't capture any broadband data for the stars so theses are narrowband only and processed separately from the nebula. The star only images where combined according to the following PixelMath formula: Red = Ha+N2, Green = 0.5*(Ha+ O3), Blue = O3.

The nebula only and stars only images were processed separately and then recombined to create the final image. There were guiding issues with this project which has resulted in some elongated stars, plus the collimation wasn’t really spot on. So, this is not really my best image. Also, I'm not entirely happy with the stars but these are about as good as I can get them. 

I normally aim to capture more than 10 hours per channel and then cull back to 10 hours. However, the weather in the UK this summer has meant that although I got approx. 12 hours for O3, I barely got 8 hours for the other filters. What you see here is what WBPP thought was good enough. There was no time to capture RGB stars, so the star field is sparser than it should be and I have not been able to capture the central star for the PNe.

An unexpected issue with this PNe was just how bright it was. I normally use 10 minute exposures, but this made the inner nebulae far too bright. After some experimentation I settled on 2 minute exposures. The downside is that the dim outer halo came out rather noisy and required quite a bit of noise cancellation to bring it out.

The nebula itself is quite interesting, being rather chaotic in structure. But I suppose this is to be expected given this is a star falling apart! According to the paper “The five axes of the Turtle: symmetry and asymmetry on NGC 6210” by William J. Henney, J. A. Lopez, et al, December 2020, this object has at least 5 axes of symmetry. Typically, this PNe is very blue as there is a lot of O3 compared to the other filters. However, there is a lot of interesting structure in the inner part of the nebula in Ha and N2 that I wished to show, so I deliberately process the data to bring this out. I think it highlights the complicated and messy nature of this planetary nebula.

The PNe appears to have started approx. 3500 years ago, with an initial mass of ~2 solar masses. As we see it now, there is an outer intermediate shell of ejected stellar material surrounding an inner shell. The intermediate shell is expanding out at speeds of 20 to 30 km/s. The inner shell appears to be the gas of the intermediate shell closest to the central star being compressed by a faster expanding shell of later material (expansion speeds of 30 to 50 km/s). I believe in this image the inner shell is the inner most blue ring (based on the angular measurements given in the referenced paper for the size of the two rings). The fainter outer shell is then the ring surrounding the orange-coloured Ha/N2 signal that fills the centre. These shells are thought to be elliptical in shape, with their major axes being aligned along two different directions (accounting for 2 of the 5 axes of symmetry). The inner shells major axis lies close to the plane of the sky, while the major axis of the outer shell is inclined at 40 degrees to the plane of the sky.

Beyond the intermediate shell there is a set of lobes of gas, moving rapidly out from the central star (speeds 90 km/s). These form symmetrical pairs along three axes (one of which is shared with the major axis of the inner shell). The other two run from the (approx.) SSE to NNW direction. These latter two are thought to be a similar age or slightly older than the shells, suggesting they were blasted out as the star began its transition to a PNe.

The last axis is coming almost straight out of the plane of the sky towards and away from us. Along this axis lies knots of low ionisation N2 (I think it’s the brighter parts that can be seen between the two shells on the northern side). These knots are believed to be at the same distance from the central star as the lobes. They are receiving the same amount of ionizing radiation from the star as the lobes, but having a higher density, the knots have not been as strongly ionised. Studies suggest that the lobes are an example of Bipolar Episodic Jets (BRETS) and the knots are an exampled of Fast Low Ionisation Emission Regions (FLIERS).

Finally, we have the much fainter halo. This is a relatively smooth flow of gas reaching out to approx. 2.3 light years from the central star. It is probably the remnants of the gas outflow during the stars AGB phase. There are two sets of knots in this gas, both approx. 1.6 light years from the central star: one set lies to the NNE, the other to the ENE. The halo extends beyond these, but in my image it has become to diffuse to see.

Hope you enjoy the image.

Cheers,
Ian