Uploaded October 2025 | Updated September 2026, 2 weeks ago

---

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: youtube.com/playlist?list=PLXJEVJey9Wz4RQK6uXH0O0t9504NnyzJL

Messier Objects playlist: bit.ly/MessierObjects

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

Back us on Patreon: patreon.com/deepskyvideos

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

Twitter: twitter.com/DeepSkyVideos
Facebook: facebook.com/DeepSkyVideos

Video by Brady Haran and James Hennessy