The Abell catalogue is an all sky catalogue of galaxy clusters. It contains 4,073 rich clusters with a nominal redshift of z less than or equal to 0.2. Each galaxy cluster must meet certain criteria of richness, compactness, distance and galactic latitude to be included.

For example, to meet the richness criteria a cluster must contain at least 30 galaxies within a set magnitude range. There are six groups defined:
Group 0: 30-49 galaxies
Group 1: 50-79 galaxies
Group 2: 80-129 galaxies
Group 3: 130-199 galaxies
Group 4: 200-299 galaxies
Group 5: more than 299 galaxies

The original Northern Survey was published in 1958 by George O. Abell and contained 2,712 clusters of rich galaxies. A later Southern Survey added a further 1,361 clusters. The catalogue is of interest to amateur astronomers with large aperture scopes since it contains challenging galaxies that require dark locations.

Hubble Frontier Fields view of Abell 2744 (NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI))

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Sky Highlights - May 2017

Mercury
Mercury reaches greatest elongation west on May 17, 2017

Meteor Shower
Eta Aquariids meteor shower peaks on May 5th and 6th, 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for May 2017

Northern Hemisphere
Evening
West:- Mars (mag. +1.6)
South:- Jupiter (mag. -2.4)
Midnight
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.2)
Morning
South:- Saturn
East:- Venus (mag. -4.7)

Southern Hemisphere
Evening
West:- Mars
North:- Jupiter
Midnight
Northwest:- Jupiter
North:- Saturn
Morning
Northwest:- Saturn
East:- Venus, Mercury (mag. +2.5 to -0.3), Neptune (mag. +7.9)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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