WhatTheStar

M 54 Deep sky · Globular cluster in Sagittarius · M 54 · NGC 6715

M 54

A globular cluster in Sagittarius that is not ours: it belongs to a small galaxy being torn apart by the Milky Way.

Finder chart of M 54: about 6° of sky with north up and east to the left, M 54 ringed at the centre, stars to magnitude 6.5, with Ascella named, and M 70 marked. M 54AscellaM 70
About 6° of sky, north up and east to the left, as you see it facing that way. Stars to magnitude 6.5.

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Type
Globular cluster
Designations
M 54 · NGC 6715
Constellation
Sagittarius
Apparent magnitude
7.7
Apparent size
12′
Position (J2000)
RA 18h 55m · Dec −30° 29′

The full Moon, about 31′ across, is 2.6 times as wide. The Sun passes 8° from it around early January, and for about 25 days of the year it sits within 15° of the Sun, lost in its glare.

Survey photograph of M 54: 20′ of sky, north up and east to the left, with the stars around it taken out
20′ of sky, north up and east to the left: the picture the sky map lays in as you zoom in, its stars taken out so the map’s own show through. Photo: DSS2 colour (STScI / Caltech / AAO; colour and HEALPix by CDS). Licence: DSS: © Caltech, AAO/UKSTU and AURA, by agreement; HiPS © CNRS/Unistra (ODbL).

When to see it

On the night of Sat 3 Jul 2027, when it is highest around midnight, on each city’s own clock:

FromRisesHighestSets
London GMT+1 22:21 01:10 · 8° 03:59
New York EDT 21:07 01:04 · 19° 05:02
Singapore GMT+8 19:19 01:15 · 58° 07:11
Sydney GMT+10 16:34 00:06 · 87° 07:38

Common questions

Can I see it from 50° north?
Only just. At its highest it is 10° above the southern horizon, so it needs a clear, flat view that way and clean air.
Can I see it from 35° south?
Yes, and well placed: at its highest it passes almost overhead, 85° up, above most of the air’s haze.
When is the best time of year to see it?
Around early July, when it stands opposite the Sun and is highest at midnight. It comes round about two hours earlier each month, so by early October it is highest in the early evening, and in early April not until the hours before dawn.
How bright is it?
Magnitude 7.7 in total, but that light is spread over 12′ of sky: on average 21.7 magnitudes per square arcsecond. Against a dark country sky that average stands out; a city’s glow drowns all but its brightest parts.
Naked eye, binoculars or telescope?
Binoculars. At magnitude 7.7 it is past the eye, but a 50 mm pair under a dark sky reaches about magnitude 11.0.

Discovery

It was the first globular cluster found outside the Milky Way — though nobody knew it until the late twentieth century, when the Sagittarius Dwarf Galaxy was found behind it.

Look for

It sits near Ascella, at the bottom of the Teapot: a small, bright, concentrated spot.

Look for

From 50° north it barely clears the horizon, at 10°; from 35° north it climbs to 25°; from the equator it climbs to 60°; from 35° south it climbs to 85°.

Look for

Magnitude 7.7 against the faintest star each can show: the eye under a city sky, 4.5 (3.2 short); the eye under a dark country sky, 7.0 (0.7 short); 50 mm binoculars under that dark sky, 11.0 (3.3 to spare); a 100 mm telescope there, 12.5 (4.8 to spare). Those limits are for a point of light; its light is spread over its whole size, so it needs some of that margin in hand. Through 7×50 binoculars, whose field is about 7.1° across, it spans 3% of the field’s width; through a 100 mm telescope at 50×, a field about 1° across, it spans 20% of the field’s width.

Words used here

Its constellation

Nearby in Sagittarius

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