Globular cluster in Sagittarius
M28
In 1987 this cluster gave up the first millisecond pulsar ever found inside a globular — a dead star spinning better than three hundred times a second, wound up by material stolen from a neighbour. Eleven more have been found in here since.
Millisecond pulsars turn out to be a globular speciality: the crowding is so extreme that stars capture each other, and a dead neutron star that acquires a companion can spin itself back up on the material it takes. What looks like a quiet ball of ancient stars is, in radio, one of the busiest places in the galaxy.
- Catalogue
- M28
- Type
- Globular cluster
- Constellation
- Sagittarius
- Distance
- 17,900 light-years
- Apparent magnitude
- 6.8
- Surface brightness
- 20.6 mag/□″
- Apparent size
- 11′
- Right ascension
- 18h 25m
- Declination
- −24.87°
- Found by
- Charles Messier, 1764
Small enough that its 6.8 magnitudes arrive concentrated rather than smeared: 20.6 per square arcsecond, which is why it stands out from the sky better than its brightness alone suggests. Why surface brightness is the number that matters.
Could you see it?
Straightforward with the naked eye from Bortle 2 or darker; with 10×50 binoculars from Bortle 6.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 2 21.7 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 3 21.4 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 4 20.9 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 5 20.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 6 19.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 8 18.1 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
Small and compact, sitting a degree from M22 and comprehensively outclassed by it. At 200mm the edges just begin to grain. Everything interesting about this cluster is invisible and was found with a radio telescope, which is worth remembering while looking at it.
When, and from where
Highest in the middle of the night in June. Overhead at 25°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 65°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 5° | Low — in the murk |
| 51°N London, Calgary | 14° | Low — in the murk |
| 40°N New York, Madrid | 25° | |
| 30°N Cairo, Houston | 35° | |
| 0°N the equator | 65° | |
| 33°S Sydney, Cape Town | 82° |
Nearby in the sky
The whole catalogue →2 more of the catalogue sit within five degrees, close enough to take in on the same night without moving far. Everything below is measured from M28 itself, not from the middle of Sagittarius.
- M22 Globular cluster 2.9° apart Sagittarius · magnitude 5.1 · binoculars from Bortle 5 or darker
- M8 Lagoon Nebula 4.7° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 3 or darker
- M21 Open cluster 5.1° apart Sagittarius · magnitude 6.5 · binoculars from Bortle 9 or darker
- M20 Trifid Nebula 5.3° apart Sagittarius · magnitude 6.3 · binoculars from Bortle 3 or darker
Terms on this page
- Neutron star A city-sized remnant so dense that a sugar cube of it would weigh as much as a mountain.
One of 110. Positions J2000; the sky is the final authority.