The Rosette Nebula (NGC2237)

Rosette Nebula – 10th Feb 2021

The Rosette Nebula is a large area of nebulosity located in the rather dim constellation of Monoceros, to the left of the star Betelgeuse in Orion. The nebula is also known to amateur astronomers as Caldwell 49, whilst the star cluster at its centre is Caldwell 50.  The nebula is about 5,000 light years from Earth and is about 130 light years across with a mass 10,000 times that of our sun. This makes it considerably larger than the much closer and brighter Orion Nebula. The hollow appearance of the nebula arises because the stars that have formed in the centre of the gas cloud are now in the process of dispersing the gas, and after about 5 million years have largely cleared the central region.  As the gas gets blown outwards, it also becomes compressed, and this encourages more stars to form, so stellar formation will continue outwards before the gas is finally dispersed. The bright central stars are responsible for ionising the gas cloud, which then emits radiation at specific wavelengths of light, which depends on the chemical make-up of the gas. The most important of these wavelengths is H-alpha, emitted by ionised Hydrogen, which is at the extreme red end of the visible spectrum.  

The nebula is called Rosette partly because of its shape, which is slightly reminiscent of a rose, but also for the strong red colour of the H-alpha emission. Unfortunately, as I am using an unmodified DSLR, much of this red emission is not captured by the camera, as a standard DSLR has a filter specifically to blot out this red end of the spectrum. To fully appreciate the nebula therefore requires a modified camera with the red filter removed, but that is a step I have yet to take. In the meantime, an unmodified camera mostly captures the slightly blue-green colour that is produced by ionised Oxygen and Helium.  Although these elements are much less common than Hydrogen, they emit radiation which is more easily captured by the camera.  Further out in the nebula and the ionising radiation from the central stars is no longer strong enough to ionise Oxygen and Helium, so only a small fraction of the H-alpha emission is detected, giving a red outline to the nebula.

With an unmodified DSLR, this was quite a challenging object to photograph. Even after long sub-exposures of 5 minutes, there was no hint of the nebula in the individual images. It was only after stacking almost two hours  worth of exposures and doing extensive editing that the nebula began to emerge. Even so, I did not manage enough time on this target and the image remains rather noisy due to the amount of processing required to show detail in the nebula. This is definitely a subject to return to in the future, either to add more integration time, or to try again with better equipment that can capture more of the main H-alpha emissions.

Image Details

  • Date: 10th Feb 2021
  • Exposure Details: 20 x 300s, F6, ISO400
  • Total Integration Time: 1hr 40min
  • Camera: Canon EOS80D (unmodified)
  • Telescope: Altair Astro 72EDF at 432mm focal length.
  • Mount: Skywatcher HEQ5
  • Guide Scope: Altair Astro 60mm
  • Guide Camera: QHY5LII

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