Keystone

The Keystone is an asterism, a star pattern that is not one of the 88 official constellations, made of four stars in Hercules. Its brightest star is ζ Herculis (magnitude 2.81), and it is best seen in the evening in July.

Free to use with credit: Atlas of the Sky (constellationmap.org), CC BY 4.0 · More

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<a href="https://constellationmap.org/asterisms/keystone/"><img src="https://constellationmap.org/img/asterisms/keystone-star-map.png" alt="Keystone star map: the four stars ζ Herculis, π Herculis, η Herculis and ε Herculis connected by lines and labeled, with the surrounding constellations" width="800" height="800" loading="lazy"></a>
<br><small><a href="https://constellationmap.org/asterisms/keystone/">Keystone</a> · Atlas of the Sky (constellationmap.org), <a href="https://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a></small>

How to use it

The globular cluster M13 lies on the western side of the Keystone, about a third of the way from Eta to Zeta Herculis.

How to find the Keystone

From 40° N (the USA, southern Europe, Japan) it is highest in the south, about 85° above the horizon, in July evenings. From 35° S (Australia, South Africa, Argentina) it reaches about 20° in the north.

Stars of the Keystone

Star	Designation	Magnitude	Distance	Constellation
—	ζ Herculis	2.81	—	Hercules
—	π Herculis	3.16	—	Hercules
—	η Herculis	3.53	—	Hercules
—	ε Herculis	3.92	—	Hercules

Questions about the Keystone

Is the Keystone a constellation?

No. The Keystone is an asterism: its stars belong to the constellation Hercules, one of the 88 constellations defined by the International Astronomical Union.

What is the brightest star of the Keystone?

ζ Herculis, with an apparent magnitude of 2.81.

When can I see the Keystone?

It is highest in the evening sky in July.