[KBT2017] Gaia 1, A Sirius Star Cluster
Canis Major
Open Cluster
RA 06:45:53.8
DEC -16:43:59
Mag +8.3???
Size 13"-15”
Dist 15,000 l-y
Part I – Discovery, Confirmation, and Study
On December 19, 2013 – almost 34 years after the launch of the HST and 8 years before the JWST – one of the most sophisticated telescopes ever made was launched. Built by the European Space Agency, the observatory is named Gaia and consists of two rectangular, primary mirrors 1.45 x 0.5 meters across. It was designed to collect astrometry and photometry with unprecedented precision…and it has!
One-thousand days later (September 14, 2016), the first Gaia data release (DR1) was made public [1]. And within just a few years, it would be hard to not feel its effect in all corners of professional astronomy. Even the humble amateur would feel its contribution as it began decisively answering such question as “how far is that star” and “is this a true open cluster or an asterism”.
As for myself, it was in the February 2023 issue of Sky & Telescope (The Gaia Revolution, by Govert Schilling) where I learned of a fascinating open cluster it had uncovered. Within the first six months of the DR1, Sergey Koposov and two other colleagues identified 259 candidates of high significance using a computer algorithm designed for overdensity detection [2]. Of them, all but 15 had a clear association with a known star cluster or galaxy. They named the most intriguing of their new finds Gaia 1 after concluded that it was a previously unknown star cluster of high-mass and luminosity [3]. Not only that, they estimated its diameter at over 12’.
Why wasn’t it found earlier in imaging surveys? Because it lies a mere 11’ east of Sirius, the brightest star in the night sky! Its location places it in the glare of all telescopes where detecting and confirming any kind of star cluster is a huge challenge. Also, astronomers working on previous ground-based surveys usually automatically excluded overdensities near bright stars as nothing more than CCD saturation, ghost reflections, diffraction spikes, ect.
1567214640133-ESA_Gaia_Cluster_Gaia1_625.jpg
Wide-Field Infrared Survey Explorer (WISE) image with Gaia 1 to the left of Sirius
(Copyright: Sergey Koposov; NASA/JPL; D. Lang, 2014; A.M. Meisner et al. 2017)
Using what little data was available to them, Koposov et al concluded in their discovery paper that it was “a star cluster, most likely of globular variety.” That quickly got people’s attention! So much so that three more papers were published by three different groups almost before 2017 was over [4]. It was quickly found that while it is fairly massive for an open cluster (60 light-years wide and at least 22,000 solar masses) [5], it’s more likely a very luminous open cluster [6]. However, being roughly 15,000 light-years away in the direction of Sirius does place it in the outer disk of the Galaxy and more in the realm of captured clusters. But the idea of it having an extragalactic origin was considered and so far rejected by a lack of evidence [6].
Gaia_1_with_Sirius.jpg
Wikimedia Commons
Part II – Observations
Last February, with unusually good “seeing”, I decided to set up my now 40-year-old 10-inch Meade SCT in the evening to do some deep-sky observing. The only problem…there was a 13 day old Moon in the sky! I figured this wouldn’t hurt me, though, since one of my two goals was to observe a mere 11’ from a -1.44 magnitude star. The chart I had in hand was something I created using the AAVSO’s Variable Star Plotter (VSP) and it worked admirably.
Using one of my new “110°” eyepieces, I was able to achieve 590x with an 11’ field. This allowed the cluster to be at the center and Sirius 5.5' outside of the field, but the glare from it was still noticeably brightening it all. With work, I was able to make out 3 or 4 of the bright member stars at its center and possibly a faint glow of unresolved stars among them. Two weeks later, I observed the unique star cluster with my 16-inch under Moonless skies. At 440x, it was easier to see those few stars than it had been in my 10-inch, but I saw no addition ones.
Gaia 1 OC AAVSO - Copy (2).jpg
AAVSO VSP Chart
I look forward to hearing observations by others…especially with a refractor! So, please,“Give it a go and let us know!”
[1] Gaia DR1 (https://www.cosmos.esa.int/web/gaia/dr1)
[2] Koposov et al 2017 (https://ui.adsabs.harvard.edu/abs/20...2702K/abstract)
[3] [KBT2017] Gaia 1 (http://simbad.cds.unistra.fr/simbad/...ntYear#lab_bib)
[4] Mucciarelli et al 2017 (https://ui.adsabs.harvard.edu/abs/20......7M/abstract)
Simpson et al 2017 (https://ui.adsabs.harvard.edu/abs/20...4087S/abstract)
Koch et al 2018 (https://ui.adsabs.harvard.edu/abs/20.....13K/abstract)
[5] Koposov et al 2017 (https://ui.adsabs.harvard.edu/abs/20...2702K/abstract)
[6] Koch et al 2018 (https://ui.adsabs.harvard.edu/abs/20.....13K/abstract)
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