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IC 4665 — Open Cluster in Ophiuchus

Open Cluster Excellent (65/100)
Magnitude 4.2m OpenCluster Ophiuchus (Oph) Visible
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1Properties

Magnitude 4.2
Angular Size 24.6′
Cl, co

Position & Identifiers

RA 17h 46m 27.2s
Dec +05° 38' 55.3"
Constellation Ophiuchus (Oph)
Catalog IC 4665
Physical size
8.3 light-years across — about 1.9× the Sun-to-Alpha-Centauri distance

2How easy to spot?

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Telescope Bortle 3 Bortle 4 Bortle 5
80mm refr. Easy Easy Easy
150mm Newt. Easy Easy Easy
C8 203mm Easy Easy Easy
Easy Medium Hard Very hard Impossible

Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs

Easy on Seestar S50
At 150mm under B5 skies you should resolve about 57 of 167 members.

3Visibility

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Best season May – Jul (peak: Jun)

4 Eyepiece View

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50x TFOV: 1.0° Lim. mag: 13.6
N E

IC 4665 · 24.6′ diameter

5 Best Magnification

6Where this cluster sits in time

1 Myr 10 Myr 100 Myr 1 Gyr 10 Gyr NGC 2362 Pleiades Hyades M67 NGC 188 IC 4665 33 Myr

Open clusters span more than four orders of magnitude in age — from newborn OB associations to ancient, metal-rich survivors.

7 Colour-Magnitude Diagram

A cluster's colour-magnitude diagram reveals its age: the bluer the turn-off point where the main sequence bends into red giants, the younger the cluster.

Loading member data…

Each point is a Gaia-DR3 member. Colour encodes spectral type; size reflects membership probability.

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8 Classification Decoder

Nearby in the Sky

Other targets within a few degrees — pan your scope a little and keep exploring.

Visibility scores assume a 150 mm Newton at Bortle 4.

NGC 6426
Medium
Globular cluster — dense ball of ancient stars
Globular Cluster mag 11.2 2.7° Ophiuchus
Collinder 350
Easy
Open cluster
Open Cluster mag 6.1 4.4° Ophiuchus
Collinder 359
Easy
Open cluster
Open Cluster mag 3.0 4.6° Ophiuchus
NGC 6572
Medium+
Small bright planetary nebula
Planetary Nebula mag 9.0 6.5° Ophiuchus
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