Cookie consent

AstroBin saves small pieces of text information (cookies) on your device in order to deliver better content and for statistical purposes. You can disable the usage of cookies by changing the settings of your browser. By browsing AstroBin without changing the browser settings, you grant us permission to store that information on your device.

I agree
Getting plate-solving status, please wait...
Abell 35 - Stromgren Sphere in Hydra, 


            Gary Imm
Abell 35 - Stromgren Sphere in Hydra

Abell 35 - Stromgren Sphere in Hydra

Technical card

Resolution: 2496x2312

Dates:April 13, 2019April 14, 2019April 15, 2019

Astrodon Blue 31mm Gen2 I-Series: 30x60" (gain: 139.00) -20C bin 1x1
Astrodon Green 31mm Gen2 I-Series: 30x60" (gain: 139.00) -20C bin 1x1
Astrodon Ha 31mm 5nm: 30x300" (gain: 300.00) -20C bin 1x1
Astrodon OIII 31mm 3nm: 30x300" (gain: 300.00) -20C bin 1x1
Astrodon Red 31mm Gen2 I-Series: 30x60" (gain: 139.00) -20C bin 1x1

Integration: 6.5 hours

Avg. Moon age: 9.24 days

Avg. Moon phase: 68.86% job: 2698122

RA center: 193.414 degrees

DEC center: -22.871 degrees

Pixel scale: 0.782 arcsec/pixel

Orientation: 16.353 degrees

Field radius: 0.370 degrees

Locations: Backyard (Mag 20.8), Onalaska, Texas, United States

Data source: Backyard


This object, also known as Sh2-313, is a Stromgren sphere located 1200 light years away in the southern constellation of Hydra at a declination of -23 degrees. The low declination makes it hard for me to image this object clearly. The apparent size of the image is slightly larger than our full moon, at just over half a degree.

For most of its known existence, this object has been classified as an old planetary nebula. But looking carefully at the image, and comparing to many other planetary nebulae structures, it is clear that the structure of this object is different from a planetary nebula in many ways. The star at the center is very bright, and appears to be at least a double star. The structure is not circular in nature and has many arcing and straight line features which are typcially not seen in a PN. Finally, there is no symmetry apparent in the structure. Recent scientific work has concluded that this object is not a planetary nebula but is a Stromgren sphere. A Stromgren sphere is a region of ionized hydrogen (HII) surrounding a hot, young O-B star.

Interestingly, the bright star at the center of the image is not hot enough to ionize the nebula. Scientists have verified that there is an unseen, hot companion star that powers the nebula. My image shows the bright star to be a binary star, but I don't know if the smaller star is actually the "unseen" star powering the nebula.

My favorite part of the image is the bow shock OIII wave at the center of the image. The other horizontal ridge-type structures also seem to indicated some type of high speed disturbance. It seems that some of these impacts are due to the source star and some may be due to the movement of the nebula through the ISM.

This object joins a short list of interesting objects that look "somewhat like" planetary nebula, but may or may not be. I have compiled a catalog of such objects at this link.



Gary Imm
License: None (All rights reserved)

Sky plot

Sky plot


Abell 35 - Stromgren Sphere in Hydra, 


            Gary Imm