Messier 46, also numbered New General Catalogue (NGC) 2437, is an open cluster, a cluster of about 500 stars that formed from the same molecular cloud and are similar in age, estimated to be 251.2 million years old.  M46 is located in the Milky Way, approximately 4930 light years away, and it has an apparent size of 25.3 arcmin, so it is approximately 36 light years across.  

I chose to photograph this open cluster not only because it is a Messier object I haven’t imaged yet but because there is a planetary nebula in front of it, so I was getting to image two objects at the same time. 

But when I processed the image and removed the stars, I got a surprise.  There wasn’t just one planetary nebula – there were two!  I was delighted and felt the joy of discovery.  I hadn’t known there was a second planetary nebula there, and yet there it was!  I immediately used PixInsight to annotate the image to see if the small planetary nebula was known, and of course it had already been discovered.  But it was still super fun to find something unexpected in my image.

The planetary nebula I had been intending to image, NGC 2438 or PK 231+04.2, is located in the Milky Way, approximately 1377 light years away, and it has an apparent size of 1.173 arc min, so it is approximately 0.5 light years across.  Because it had a different relative motion relative to us than M46, it is not considered to be part of the open cluster.  Before I learned that, I had thought it would be cool if it were part of the cluster and that possibly (since cluster stars are generally of similar age) multiple stars in the cluster could generate visible planetary nebula at the same time.  Wouldn’t that be a sight to see?

My surprise planetary nebula, Minkowski M1-18 or PK231+0.41, is located in the Milky way, approximately 14500 light years away, and it has an apparent size of 0.507 arcminutes, so it is approximately 2.1 light years across.  So further away and larger than NGC2438.

In this image, the stars came from images using red-green-blue filters, and the two nebulas came from images using Hydrogen alpha (mapped to red) and Oxygen iii (mapped to turquoise) filters.  The two nebulas were processed separately to maximally enhance each, which means that they are more equal in brightness in the image than they are in reality (M1-18 is much dimmer).

Have you felt the joy of discovery lately?

Camera geek info – Narrowband:

• Williams Optics Zenith Star 73 III APO telescope
• Williams Optics Flat 73A
• ZWO 2” Electronic Filter Wheel
• Antila SHO and RGB filters
• ZWO ASI183MM-Pro-Mono camera
• ZWO ASiair Plus
• iOptron CEM40
• Friendswood, Texas Bortle 7-8 suburban skies

Frames:

• November 23, 2024
  • 135 60 second Gain 150 Ha lights
  • 30 1 second Gain 150 Ha flats
• December 19, 2024
  • 97 60 second Gain 150 Oiii lights
  • 30 0.5 second Gain 150 Oiii flats
• December 20, 2024
  • 89 60 second Gain 150 Ha lights
  • 30 1 second Gain 150 Ha flats
• December 31, 2024
  • 100 30 second Gain 150 Green lights
  • 30 0.02 second Gain 150 Green flats
  • 100 30 second Gain 150 Blue lights
  • 30 0.02 second Gain 150 Blue flats
  • 100 30 second Gain 150 Red lights
  • 30 0.05 second Gain 150 Red flats
• 30 Flat Darks from library
• 30 Darks from library

Processing geek info:

• PixInsight
• BlurXterminator
• NoiseXterminator
• StarXTerminator
• NBColourMapper
• Generalized Hyperbolic Stretch