M13, the Great Globular Cluster in Hercules, is the finest globular of the northern sky, a dense sphere of old stars in the Galaxy’s halo.
At a glance
| type | globular cluster |
|---|---|
| distance from the Sun | 23,200 light-years (7.1 kpc) |
| its light left it | 23,200 years ago |
| size in the piece | about 140 light-years acrossmodel |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About M13
M13 is a globular cluster: a compact, roughly spherical swarm of stars that formed early in the Milky Way’s history and now orbits far from the disc, in the halo. Its stars are old and poor in heavy elements, and its core is so crowded that individual stars merge into a single glow in small telescopes.
Edmond Halley found it and described it as a little patch, and Messier later added it to his catalogue. Under dark skies it is just visible to the naked eye on one side of the Keystone, the quadrilateral at the heart of Hercules. Like other globulars it contains blue stragglers, stars that look too young for the cluster, probably formed when stars merged or one stripped another of mass.
It is also the target of the Arecibo message, a short radio greeting broadcast from Earth toward the cluster by the Arecibo radio telescope.
What you see in the piece
Its old stars give it a warm, yellowish light; from the Sun it is a soft point well above the plane of the Milky Way, and up close it resolves into a dense swarm.
Measured or modelled
The outline drawn for the cluster is a modelled extent; a globular’s outer stars thin out gradually into the halo.
Questions
How far away is M13?
M13 is about 23,200 light-years (7.1 kpc) from the Sun, so the light you see left it 23,200 years ago.
Where is M13 in the sky?
It lies in Hercules, on the western side of the Keystone, the four-star figure at the centre of the constellation.