Nebulous, but no nebula

This Hubble Picture of the Week shows Messier 28, a globular cluster in the constellation of Sagittarius (The Archer), in jewel-bright detail. It is about 18 000 light-years away from Earth.

As its name suggests, this cluster belongs to the Messier catalogue of objects — however, when astronomer Charles Messier first added Messier 28 to his list in 1764, he catalogued it incorrectly, referring to it as a “[round] nebula containing no star”. While today we know nebulae to be vast, often glowing clouds of interstellar dust and ionised gases, until the early twentieth century a nebula represented any astronomical object that was not clearly localised and isolated. Any unidentified hazy light source could be called a nebula. In fact, all 110 of the astronomical objects identified by Messier were combined under the title of the Catalogue of Nebulae and Star Clusters. He classified many objects as diverse as star clusters and supernova remnants as nebulae. This includes Messier 28, pictured here — which, ironically, is actually a star cluster.

Messier’s mistake is understandable. Whilst Messier 28 is easily recognisable as a globular stellar cluster in this image, it is far less recognisable from Earth. Even with binoculars it is only visible very faintly, as the distorting effects of the Earth’s atmosphere reduce this luminous ancient cluster to a barely visible smudge in the sky. One would need larger telescopes to resolve single stars in Messier 28. Fortunately, from space Hubble allows Messier 28 to be seen in all its beauty — far more than a faint, shapeless, nebulous cloud.

Credit:

ESA/Hubble & NASA, J. E. Grindlay et al.

About the Image

Id:potw1910a
Type:Observation
Release date:11 March 2019, 06:00
Size:4098 x 3993 px

About the Object

Name:Messier 28
Type:Milky Way : Star : Grouping : Cluster : Globular
Distance:18000 light years
Constellation:Sagittarius
Category:Star Clusters

Image Formats

Large JPEG
7.0 MB
Screensize JPEG
692.1 KB

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609.7 KB
1280x1024
953.4 KB
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1.3 MB
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1.4 MB
2048x1536
1.9 MB

Coordinates

Position (RA):18 24 35.12
Position (Dec):-24° 52' 37.46"
Field of view:2.71 x 2.64 arcminutes
Orientation:North is 49.3° left of vertical

Colours & filters

BandWavelengthTelescope
Optical
C
390 nm Hubble Space Telescope
WFC3
Optical
B
435 nm Hubble Space Telescope
ACS
Optical
r
625 nm Hubble Space Telescope
ACS
Optical
V
606 nm Hubble Space Telescope
WFC3
Optical
NII
658 nm Hubble Space Telescope
ACS
Optical
I
814 nm Hubble Space Telescope
WFC3
Optical
H-alpha
656 nm Hubble Space Telescope
WFC3

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