Object of the Week December 13, 2015 – NGC 288

[deepskytraveler]

Object of the Week December 13, 2015 – NGC 288

NGC 288, Cl Melotte 3, GCl 2, Bennett 5

Constellation: Sculptor
Type: Globular Cluster, Class X
RA: 00h 52m 45s
Dec: -26° 34’ 57”
Distance: 37,000 ly
Size: 13.0’
Mag: 8.1


This week’s OOTW is the unusual and obscure globular cluster, NGC 288.

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Although it is moderately bright and large, it is located in a constellation relatively unknown to many Northern Hemisphere observers – Sculptor. Often the one and only deep sky attraction in Sculptor for many observers is the oblique spiral galaxy NGC 253, aka the Sculptor Galaxy. NGC 288 is located 1.8° southeast of the Sculptor Galaxy. In a 3° field of view NGC 288 rises from obscurity and shares the spotlight with NGC 253. Stephen O’Meara in Deep-Sky Companions: Hidden Treasures opines that “Few sights in the sky are as rewarding as seeing this dramatic pairing of deep-sky splendors.”

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At a declination of -26° NGC 288 may seem to be off limits to observers in the mid and northern latitudes – however it is not. In fact, this cluster was discovered from a rather northerly latitude of 51° by William Herschel in October 1785. Although NGC 288 culminates at the meridian in mid-October, early sunsets in the Winter provide an ample window of opportunity to observe at dusk, at least for a few more weeks. An absolute key to a successful observation is access to an unobstructed southern horizon.

William Herschel’s discovery notes for the cluster he logged as H VI-20 describe it as “Considerably bright, irregularly round, 8’ or 9’ in diameter. A great many of the stars are visible so there can remain no doubt that it is a cluster of very small stars.”

Unfortunately I have yet to log an observation of NGC 288, so instead I must rely on the DSF membership to share their observations and sketches, as they regularly do. Meanwhile I’ll share a few observational characteristics I garnered from The Night Sky Observer’s Guide, Volume 1 by George Kepple and Glen Sanner.

• [the cluster] is partially enclosed by a semicircular star chain open on the SW side
• the large dense core is embedded in a much less concentrated halo 7’ in diameter and irregular in outline
• outlying members spread noticeably further to the south and SW
• the loosely-structured Class X globular has a 9’ diameter but a star-poor halo around a dense 3’ diameter core
• the globular’s periphery is irregular in profile

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Hubble ACS image

What makes NGC 288 such an unusual globular cluster?

• the cluster travels in an oddball retrograde orbit
• it is a loose halo cluster whose members each contain, on average, anywhere from 1/13 to 1/20 as much iron as the Sun
• based on the large number of massive bright blue stars that have left the red giant phase and are being powered by helium fusion (on the so-called Horizontal Branch) – the age of NGC 288 has been calculated to be 13 to 14 billion years old, which places it among the very oldest globular clusters

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Now it’s your turn….don’t forget to share your observations and sketches of this incredible globular cluster.

As always,

"Give it a go and let us know!
Good luck and great viewing!"