From DeepSkyVideos.

Professor Mike Merrifield is back in the Caldwell Catalogue and this time we unexpectedly discuss why the sky is blue… More links and info in full description ↓↓↓

The Iris Nebula is also known as NGC 7023 and Caldwell 4.

Mike Merrifield is an emeritus professor from the University of Nottingham.

Caldwell Playlist: https://www.youtube.com/playlist?list=PLXJEVJey9Wz4RQK6uXH0O0t9504NnyzJL

Messier Objects playlist: http://bit.ly/MessierObjects

Paper – The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023: https://arxiv.org/abs/astro-ph/0509352

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Astronomy at The University of Nottingham: https://www.nottingham.ac.uk/astronomy/

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Video by Brady Haran and James Hennessy