NGC 6440 and NGC 6445 in Sagittarius

June 2026 - Nebula and Cluster of the Month

June brings with it the glories of the Summer Milky Way, but also the shortest, lightest nights of the year. It should not be assumed, however, that astronomical observation cannot take place during the period known as Astronomical Twilight. With light pollution as endemic as it is, most of us live under permanently twilit skies anyway, and yet we still observe.

The greatest glory of the Milky Way lies towards its centre, in the constellation of Sagittarius, sadly low in British skies. Given a good sky and a decent southern horizon, however, the two objects that form this month’s article should be moderately easy. These two objects are just about 20’ apart, so should both fit into the field of a good, wide-field eyepiece.

The brighter and more southerly of the two is NGC 6440, a ninth magnitude globular cluster first spotted by William Herschel on the night of 28 May 1786. The description he gave was Considerably bright, round, very gradually much brighter towards the middle, about 1½’ in diameter. Herschel considered this a bright object and placed it in his first class as 150.HI.

On the same night, he first saw our second object, the planetary nebula NGC 6445. He thought this a bit fainter, placing it in his second class as 586.HII. He described it as Pretty bright, small, with an irregular figure.

The brighter of the two objects is undoubtedly the globular cluster NGC 6440. This cluster lies towards the galactic centre, about 27,000 light-years from us. It is especially rich in millisecond pulsars. Eight radio pulsars have been discovered, plus two accreting X-ray pulsars. These two objects are being ‘spun-up’ by material being accreted onto the pulsars from companion stars.¹

Interestingly, one of the radio systems is a so-called ‘black widow’ pulsar – a rapidly rotating neutron star that is gradually ablating a very low-mass companion star through energetic radiation and particle winds.² This is almost the reverse process than that occurring with the X-ray pulsars.

In the eyepiece (again, given good skies and favourable light-pollution), NGC 6440 appears as a small, almost woolly-looking ball. It is much brighter in the centre, and the edges fade away gradually. I have never achieved resolution of this globular cluster, but those blessed with better skies than me may have a good chance of doing so. The brightest stars are of thirteenth and fourteenth magnitude.

20’ slightly east of north from NGC 6440, lies our second object, the planetary nebula NGC 6445. This is a curiously-shaped nebula, the brightest portions of which form a rectangle, or box. Deeper images reveal a wealth of inner and outer structure. The nebula was formed by a solar-mass star ejecting its outer layers at the end of its red giant phase. What remains is a dense, white dwarf star surrounded by the ejected material. The structure of this ejected matter tells us a great deal about the processes that occur at this point in a star’s life. In NGC 6445, the structure is particularly complex.

A major area of recent research into NGC 6445 concerns this intricate structure. High-resolution optical and infrared observations show that the planetary nebula contains a bright inner ring embedded within extended bipolar lobes. Researchers have interpreted these features as evidence for multiple episodes of mass ejection from the dying central star, possibly influenced by binary interactions or asymmetric stellar winds. Studies of comparable nebulae using the James Webb Space Telescope have reinforced the view that many bipolar planetary nebulae are shaped by highly collimated outflows and complex dynamical processes rather than simple spherical expansion.

Visually, NGC 6445 is less prepossessing than its deep images suggest. It’s quite faint at magnitude 11.2, but an OIII filter (or Nebula filter) should suffice to bring it out from the background. Moderately high power and some careful use of averted vision can, on occasion, reveal the rectangle, though this is likely to appear as an elongated ring.

Seen together in a wide-field view, these two objects provide a fascinating contrast, both in their visual appearance and in the wild differences in their natures.

Patrick Maloney

Object	RA	Dec	Type	Magnitude
NGC 6440	17h 48m 53s	-20° 22’	Globular cluster	9.1
NGC 6445	17h 49m 15s	-20° 00’	Planetary nebula	11.2

Reference:

1. Discovery of a 205.89 Hz Accreting Millisecond X-ray Pulsar in the Globular Cluster NGC 6440, Altamirano, D. et al., The Astrophysical Journal Letters, Volume 712, Issue 1, pp. L58-L62 (2010).
2. Discoveries and timing of pulsars in NGC 6440, L Vleeschower et al., Monthly Notices of the Royal Astronomical Society, Volume 513, Issue 1, June 2022, Pages 1386–1399

If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Cluster of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.