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Segue 3: a captured, disrupted outer halo globular cluster, 


Segue 3: a captured, disrupted outer halo globular cluster
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Segue 3: a captured, disrupted outer halo globular cluster

Acquisition type: Lucky imaging

Technical card

Imaging telescopes or lenses: Orion Skyquest xx16g Skyquest xx16g

Imaging cameras: ZWO ASI294MC PRO ASI294MC PRO

Software: SharpCap 3.2  ·  Fitswork4 Fitswork 4.4.7

Filters: ZWO IR Cut Filter ZWO IR/UV Cut Filter

Accessory: Explore Scientific HR Coma Corrector

Dates:Sept. 6, 2020

Frames:ZWO IR Cut Filter ZWO IR/UV Cut Filter: 7x480" (gain: 320.00) -5C

Integration: 0.9 hours

Darks: ~66

Flats: ~100

Avg. Moon age: 18.64 days

Avg. Moon phase: 83.91%

Bortle Dark-Sky Scale: 5.00

Temperature: 23.00

Basic astrometry details

Astrometry.net job: 3832398

RA center: 21h 21' 29"

DEC center: +19° 6' 32"

Pixel scale: 0.507 arcsec/pixel

Orientation: 352.186 degrees

Field radius: 0.217 degrees

Resolution: 2581x1674

Data source: Backyard


I was looking for interesting objects and came across some unusual in the UNGC catalog (Updated Nearby Galaxy Catalog) and came across Segue 3, an odd little object of (initially) uncertain type in Pegasus. It was discovered back in 2010 during a search of Sloan Digital Sky Survey data and has since been confusing succeeding rounds of astronomers attempting to determine its nature, although I think the latest investigations have finally figured it out.

In the 2010 discovery paper, it was determined to be a a distant star cluster of unknown type in the outer halo of our own galaxy: Bigfish, little fish: two new ultra-faint satellites of the Milky Way:

Then, in 2011, this paper came out: Segue 3: an old, extremely low luminosity star cluster in the Milky Way's halo.

Then, in 2013, this paper came out: Segue 3: the youngest globular cluster in the outer halo.

Another paper came out in 2013 looking for RR Lyrae variables in the cluster to measure its distance better - they couldn't find any!

Then, in 2017, this paper came out: A multi-wavelength study of the Segue 3 cluster.
This last paper built on the earlier data and got a lot more to add in and came up with the following:
"Our results indicate that Segue 3 resembles the LMC clusters, and we support [earlier paper's] claim that it is the youngest globular-like cluster in the [Milky Way], although spectroscopy is needed to confirm its nature, if we are to rule out that it could be an old sparse [Open Cluster]. In either case, its location in the outer halo and its youth argue against it being a cluster native to our Galaxy."

So, astronomer's thinking now is that its less than 3 billion years old, its stolen from another galaxy, and is in the process of being disrupted by the Milky Way! I guess the youth of the cluster is why the authors of the 2013 paper couldn't find any RR Lyrae variables, which are evolved older stars. The cluster is roughly 78 light years across and 95,000 light years away, so its a Milky Way galaxy diameter out into our galaxy's halo. (For reference, M13 is just 25,000 light years away).

I thought it would be a good subject to hunt down under bright moon conditions a couple nights back. The cluster is about 1 minute across, roughly the same diameter of M57. Not as pretty as M57, but the backstory is fascinating. Despite the Moon, the limiting magnitude in the image is around 21st magnitude, due to the good seeing helping out faint stars overcoming the Moon glow.

Version B has image annotations showing the location of the cluster - (its small!



License: None (All rights reserved)


  • Final
    Segue 3: a captured, disrupted outer halo globular cluster, 


  • Segue 3: a captured, disrupted outer halo globular cluster, 




Description: Annotated version showing cluster location and a few stars with SDSS g filter magnitudes noted.

Sky plot

Sky plot


Segue 3: a captured, disrupted outer halo globular cluster,