An interstellar self-luminous gas cloud that radiates visible light due to ionisation. The most common source of ionisation is high-energy photons emitted from a nearby hot star. There are several different types of emission nebulae, which include HII star formation regions. Another type is a planetary nebula where a dying star has thrown off its outer layers, which are then illuminated by ionisation from the left over hot core of the star.

M42 "The Orion Nebula" is a naked eye HII star formation region. Bright planetary nebulae include M27 "The Dumbbell Nebula" and M57 "The Ring Nebula".

M42 The Great Orion Nebula (credit:- NASA, ESA, M. Robberto, Space Telescope Science Institute)

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Sky Highlights - August 2017

Total Solar Eclipse
Total Solar Eclipse of August 21st

Meteor Shower
Perseids meteor shower peaks on August 12th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for August

Northern Hemisphere
Evening
West:- Mercury (mag. +0.4) (start of month)
Southwest:- Jupiter (mag. -1.9)
South:- Saturn (mag. +0.3)
East:- Neptune (mag. +7.8)
Midnight
Southwest:- Saturn
Southeast:- Neptune
East:- Uranus (mag. +5.8)
Morning
Southwest:- Neptune
South:- Uranus
East:- Venus (mag. -4.0)

Southern Hemisphere
Evening
West:- Mercury (first half of month)
Northwest:- Jupiter
North:- Saturn
East:- Neptune
Midnight
West:- Jupiter
Northwest:- Saturn
North:- Neptune
East:- Uranus
Morning
West:- Neptune
North:- Uranus
Northeast:- Venus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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