George's Random Astronomical Object

Object 90: UKS 1

Podcast release date: 23 January 2022

Right ascension: 17:54:27.2

Declination: -24:08:43

Epoch: J2000

Constellation: Sagittarius

Corresponding Earth location: Slightly over 2100 km west of Antofagasta, Chile, in the Pacific Ocean

UKS 1 is a globular cluster located in the constellation Sagittarius. Globular clusters are spheres of stars generally containing hundreds of thousands or millions of stars, they generally orbit the center of our galaxy outside the galaxy's disk, and they are also some of the oldest objects in our galaxy.

The UKS in the name for UKS 1 stands for the UK Schmidt Telescope, which was the telescope in Australia used to discover the cluster. The information about the object's discovery lacks some details, but it was found by two people with the last names of Longmore and Hawarden sometime in the late 1970s [1]. However, these two astronomers did not publish their discovery but instead let three other people describe their discovery in a science paper published in 1980 [1]. It's not really clear why Longmore and Hawarden didn't publish a paper about UKS 1 themselves. It probably would have been cited quite a few times by other researchers.

Anyway, one of the more interesting things about UKS 1 is that it was discovered in near-infrared images (where near-infrared means that the wavelengths of light are close to but slightly longer than the wavelenegths of light that we can see with our eyes) [1]. Many other globular clusters had been discovered decades or even centuries earlier in observations of the visible part of the electromagnetic spectrum. For example, the Messier Catalogue, which was compiled in the late 1700s, lists multiple globular clusters that were discovered by people observing the visible light from these clusters in relatively small eighteenth century telescopes. However, UKS 1 is virtually impossible to see in the visible part of the electromagnetic spectrum, so it could only be discovered after astronomers had developed the technology to make near-infrared images in the 1960s [2].

The reason why UKS 1 cannot be seen in the visible part of the electromagnetic spectrum is because of its location in the sky. As I mentioned before, the cluster is in the constellation Sagittarius, which is also the location of the center of our galaxy, and UKS 1 looks like it's on the far side of our galaxy's center, although exactly how far seems unclear [2, 3, 4]. The distance is so uncertain that it is even possible that it's just barely on our side of the galaxy [5]. In any case, we see the light from this cluster passing through the plane of our galaxy, which contains a lot of interstellar dust, and that really affects our ability to see UKS 1.

The density of interstellar dust in our galaxy is relatively low; on average, a volume of 1 million cubic meters in space (or a cube with sides that are 100 m long) contains only one dust grain. However, when we look through thousands of light years of this low density dust, it begins to look really thick, and it will obscure things behind it. Interstellar dust tends to either absorb or scatter shorter wavelengths of electromagnetic radiation before it absorbs or scatters longer wavelengths of radiation, so, for example, a yellow or blue star that is obscured by a moderate layer of interstellar dust may end up looking red. For reference, if you've been in Australia or the western United States when those areas have experienced extreme forest fires in recent years, you may have seen the sky turn red from the amount of smoke in the air. That phenomenon with smoke on Earth is similar to what happens with interstellar dust in space.

Since the light from UKS 1 is basically passing through a huge section of the inner part of our galaxy to reach us, the light passes through so much interstellar dust that all of the visible light is completely obscured, which means that we can only really see longer wavelengths of electromagnetic radiation in the infrared from the cluster. This is why it took astronomers so long to find the cluster. (By the way, if you're ever in a place on Earth where the smoke from a forest fire makes the sky look completely black, be very afraid and also try getting some infrared night vision goggles.)

Now, it might seem like, if a few astronomers wanted to study globular clusters, they would rather look at one of many other nearby, easy-to-see globular clusters instead of trying to look at something like UKS 1 that is on the far side of the galaxy and that is much harder to see. However, astronomers have actually spent quite a bit of time observing UKS 1 since its discovery.

The main reason why is because UKS 1 is one of very few globular clusters that appears to be associated with our galaxy's bulge [3, 6], although research is ongoing to confirm this. Most other globular clusters are associated with our halo, and many of those clusters seem to have come from outside our galaxy. If UKS 1 is associated with our galaxy's bulge, that would make it rather unusual among globular clusters, and it would also mean that astronomers could expect the stars in UKS 1 to be very similar to the stars in the bulge. It's very difficult to just look at any location in our galaxy and then try to figure out which stars are part of the bulge just because the bulge doesn't really have a well-defined boundary and because the bulge and disk of our galaxy intersect each other, but if UKS 1 is a cluster associated with our galaxy's bulge that contains just bulge stars, that would make it a go-to place for astronomers who want to study a sample of bulge stars in more detail. So, even though it's really hard for astronomers to see the stars in UKS 1, it's really worth the effort to study the cluster.


[1] Malkan, M. et al., Infrared studies of globular clusters near the Galactic Center., 1980, Astrophysical Journal, 237, 432

[2] Bica, E. et al., Globular Clusters in the Galactic Bulge, 2016, Publications of the Astronomical Society of Australia, 33, e028

[3] Fernandez-Trincado, Jose G. et al., The enigmatic globular cluster UKS 1 obscured by the bulge: H-band discovery of nitrogen-enhanced stars, 2020, Astronomy & Astrophysics, 643, A145

[4] Vasiliev, Eugene and Baumgardt, Holger, Gaia EDR3 view on galactic globular clusters, 2021, Monthly Notices of the Royal Astronomical Society, 505, 5978

[5] Baumgardt, H. et al., Mean proper motions, space orbits, and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data, 2019, Monthly Notices of the Royal Astronomical Society, 482, 5138

[6] Ortolani, S. et al., HST NICMOS photometry of the reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan 4, 2001, Astronomy & Astrophysics, 376, 878

Podcast and Website: George J. Bendo

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© George Bendo 2022. See the acknowledgments page for additional information.

Last update: 22 January 2023