2020-09-04

解析天坛座的两个球状星团

https://sandandstars.co.za/2016/08/29/the-two-ara-globulars/

Two Ara Globular Clusters

29th Aug 2016

NGC 6397 ARA

Globular star clusters are among my favourite deep-sky objects. These beautiful swarms of ancient stars are among the most stirring and beautiful sights in our skies.

Globular clusters, more than any other objects give me a sense of movement. Not only because of the dizzying chaos of stars that radiate out in all manner of arcs and curves and lines and bows and curls, but also that while each globular cluster moves as a whole in our galaxy, within the cluster the stars also move individually and randomly – almost like molecules in a gas. It seems to make globular clusters a more organic object, full of wondrous light and movement.

George Djorgovski wrote this about globular clusters: “The simple beauty of globular clusters is that they combine an elegant spherical symmetry with the glittering richness of thousands of individual stars.”

He could have been talking specifically about NGC 6397, that gorgeous globular in Ara. The Hubble photo shows a “close up” of the globular’s glittering richness of thousands of individual stars. (For the observations I used magnifications of 64x, 90x, 144x, 230x, 288x.)

NGC 6397 Globular Cluster

17h 40m 41.36s −53° 40′ 25.3″ mag 6.8 diam 31’

DSS image

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Hubble image credit ESA/Hubble

Image credit ESA/Hubble

This globular cluster is an observer’s dream… it is the fourth brightest globular cluster in our skies (Omega Centauri, 47 Tuc and M22 being brighter). It is the second closest globular to the sun (M4 is closer by a paltry 700 light years). It ranks first for brightest stars (10th mag) and resolvability (its horizontal branch magnitude is 12.9.)

Through 10×50 binoculars it is lovely – a big and conspicuous glow of soft and gentle light, like silk in candlelight. With averted vision I can see a gradual brightening toward the centre, a sort of silkier glow upon a silky glow.

At low power, it is a breathtaking beauty. Strings of mixed stars curve away from the core, which itself is absolutely gorgeous… a tight little circular core that is brilliantly bright. The strings of stars taper off into the surrounding field in a way that no other globular does.

There are three lovely orangey stars in the southern region. At medium power it is even more gorgeous, a dizzying turmoil of stars exploding outwards. I can see dark speckled dark areas between the strings of stars that spiral out. At high power the core of diamond dust resolves into tiny pinpoints that glitter like crushed diamonds.

Herschel mentioned a double star at the northern border, but I forgot to look for it; no matter, another night, another look, another treat.

NGC 6352 Globular Cluster

17h 25m 29.16s – −48° 25′ 21.7″ Mag 8.87 diam 9′

DSS image

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Hubble Space Telescope image of NGC 6352. Image credit ESA/Hubble

Image credit ESA/Hubble

A beautiful little cluster. At low power a bright irregular round hazy glow, not very dense; a gorgeous sight against the rich background field. At medium power the glow became a haziness, a sort of haziness enveloped in a silky glow. At high power I could see a brightness towards the core but not a single star resolved. There is a hazy granular busyness of stars toward the southwester area, and a hint of grainy glints scattered around the hazy periphery. When I back the power down, the entire globular cluster takes on the frosted look of sand-blasted glass. A hazy opacity of light.

What a beautiful pair of globular clusters, both of them absolute delights: “…the glittering richness of thousands of individual stars…”

Oh, astronomy! I’m so glad I devote so much of my life to this. I wouldn’t have it any other way.