Caldwell 86 — Globular Cluster in Ara

NGC 6397

Globular Cluster Showpiece (91/100)

Magnitude 5.7^m GlobularCluster Ara Visible

Credit: NASA/ESA and the Hubble Heritage Team (AURA/STScI). License: Public domain. (Wikimedia Commons)

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About C86

Description

NGC 6397 is one of the nearest globular clusters to Earth, at about 7,800 light-years away in Ara. At magnitude 5.7, it is visible to the naked eye from dark sites. It has an unusual core-collapsed structure, meaning its center has become extremely dense.

Observing Tips

A superb target visible to the naked eye as a fuzzy star. Binoculars show a granular ball. Any telescope resolves it into a beautiful field of stars. Notable for its lack of a bright concentrated core — a hallmark of core collapse. Best in summer from southern locations.

History

Discovered by Nicolas Louis de Lacaille in 1751 from South Africa. It is one of the two nearest globular clusters (along with M4) and has been extensively studied by the Hubble Space Telescope.

Fun Facts

NGC 6397 has undergone core collapse, a gravitational process where the most massive stars sink to the center, creating an extremely dense core. Hubble images revealed a population of faint blue stars in its core — helium-core white dwarfs in binary systems.

Observe

1Properties

Magnitude 5.7

Angular Size 15.3′

Distance 7,500 ly

Description

Globular Cluster [Distance: 7500 ly]

Position & Identifiers

RA 17h 40m 42.0s

Dec -53° 40' 26.4"

Constellation Ara

Catalog C86

Also known as NGC 6397

Physical size

13 light-years across — tens of light-years across — wider than the solar neighbourhood

2How easy to spot?

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Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Easy	Easy	Easy
150 mm Newton 150mm Newt.	Easy	Easy	Easy
Celestron C8 (203 mm SCT) C8 203mm	Easy	Easy	Easy

Easy Medium Hard Very hard Impossible

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

Easy on Seestar S50

3Visibility

Set a location in User Settings to see visibility data.

Best season May – Jul (peak: Jun)

4 Eyepiece View

Telescope Eyepiece

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50x TFOV: 1.0° Lim. mag: 13.6

C86 · 15.3′ diameter · N up, E left

5 Best Magnification

6Metallicity

[Fe/H] = -2.02 — these stars formed from gas about 105× poorer in iron than the Sun.

7Concentration class

Shapley-Sawyer class I — extremely centrally concentrated core.

Explore

8 Classification Decoder

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9

Light Travel Time Machine

10

Relativistic Travel

Community Photos (1)

Credit: NASA/ESA and the Hubble Heritage Team (AURA/STScI). License: Public domain. (Wikimedia Commons)

Credit: NASA/ESA and the Hubble Heritage Team (AURA/STScI). License: Public domain. (Wikimedia Commons)

Skybred Mar 2, 2026

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.

Eclipsing binary, period 3.2d

Variable Star mag 5.92 1.4° Ara

Rich open cluster

Open Cluster mag 6.9 4.5° Ara

Globular cluster — dense ball of ancient stars

Globular Cluster mag 8.2 5.8° Ara

Globular cluster — dense ball of ancient stars

Globular Cluster mag 9.2 5.9° Telescopium

Globular cluster — dense ball of ancient stars

Globular Cluster mag 6.9 9.0° Scorpius

Globular cluster — dense ball of ancient stars

Globular Cluster mag 9.2 9.9° Scorpius

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