A Most Distant Globular Cluster

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NGC-2419
This image shows the cluster’s luminous, highly evolved red giant stars as decidedly orange/ yellow along with the blue stragglers appearing as a combined blue cast in and around the core. Image credit: this author

Well, I did it! Using the college’s 0.63m flagship instrument, I, along with colleagues, visually observed the second most distant globular cluster associated with our Milky Way galaxy!

I’m fascinated with relics as they have a story to tell, a deep story of something very old, of a long-gone epoch, a bridge, if you will, back to a time when the universe was much different. Globular clusters are populated with very old stars, each a relic unto itself with a story to tell of a time when the universe was much younger.

Given that distinction [of being the second most distant globular cluster associated with the Milky Way] and as an object of research and study during the completion of my graduate work, observing this intergalactic stepping stone held a special interest and fascination for me. Located in the northern circumpolar constellation of Lynx and known as the Intergalactic Wanderer, NCG-2419 is at least 40 Kpc distant (40,000 parsecs/ 130,000 light years) or almost 10% of the distance to the Great Spiral Galaxy in Andromeda, M-31!

With Lynx almost at the zenith, high in the North-Northwest, the moonless sky on the night of January 28th – 29th was brilliantly clear and dark, one of the most transparent in recent memory.

To get a sense of the scale involved, in time, space and distance, it needs to be understood that the cluster-member stars that we observed and the only stars directly observable with a telescope of this aperture (for such an object at this distance), are evolved, red giant and supergiant stars, stars such as Antares, Betelgeuse and, to a lesser extent (in that it is *only* a giant), Arcturus. Those familiar stars are bright and friendly in our own sky but, when observed at this distance, similar stars are at the limit of detection with averted vision, the brightest presenting at a visual magnitude between 16.5 – 17, barely observable as fleeting points of light.

Having evolved off the Main Sequence, these luminous beacons beyond the boundaries of our galaxy and above the Galactic Halo are now using the Helium produced during their 10 billion year Main Sequence lifetime to produce energy. They have been burning bright for over 10 billion years and are among the first stars that formed, dating back to a time when the universe was much younger and quite different. At this cluster’s tremendous distance, any stars that still remain on the Main Sequence would be below the detection capability of this telescope and the telescope of comparable size used to acquire the image below, obtained during my graduate work. As such, the only representative stars available for study would include luminous Horizontal Branch, Asymptotic Giant Branch and Red Giant Branch stars (see HR Turnoff diagram below). As we indicate above, no Main Sequence cluster stars are detectable at this distance with a telescope in this size-class. The sun would be feeble, presenting at visual magnitude +23.5, well beyond the threshold of detection. Given the ancient age of the cluster, the only stars that would still remain on the Main Sequence would be late G (> G5) stars and cooler.

In order to properly understand stellar evolution, a technique used by astronomers is to observe how stars evolve as a group; in this way, we’re able to extrapolate a common origin and, hence, their age. We then compare that result with current models to gauge the model’s accuracy and veracity as well as constrain the age of the universe and the ages and evolutionary trends of galaxies and clusters of galaxies. Globular cluster stars are among the oldest stars in the universe and since NGC-2419 is so distant, well outside of the galactic halo (where most globular clusters reside) and the galaxy proper, this cluster holds a unique opportunity to better understand galactic evolution and how the member stars evolved in an isolated, extragalactic environment.

By comparing the cluster stars brightness at the given distance to where it would normally be if it were a “normal”, Main Sequence star, we can “age” the star and, hence the cluster since it has to be as old as its oldest member stars. The “turn off” point is where the star’s brightness diverges or “turns off” relative to where it should be on the color-brightness diagram, known as the HR Diagram. Since no main sequence stars were observed (Sirius, so bright in our sky, would present at visual magnitude +20), the stars represented in this study are luminous giant and supergiant stars; they are the oldest, most evolved in the cluster with ages “above” the 10 billion year turnoff point for the cluster. When first building this diagram (see below), it wasn’t shaping up to “look like” a typical globular cluster turnoff diagram until I realized there were no “normal”, main sequence stars represented!

The following image was produced by a 0.61 meter Ritchey-Chretien telescope located in the dark, desert skies of Rodeo, New Mexico, a telescope in the same size class as that used to visually observe the cluster.

NGC2419 (2047×1319). The brightest cluster stars, appearing as “specks” around the cluster’s “globe”, are red giant stars such as Arcturus, warm, bright and inviting in our sky but appear as faint specks at this enormous distance. This observation serves as an excellent object lesson between distance, brightness and luminosity.

NGC-2419 As Imaged by this author
This image shows the cluster’s luminous, highly evolved red giant stars as decidedly orange/ yellow along with the blue stragglers appearing as a combined blue cast in and around the core. At the tremendous distance to this cluster, the individual blue stragglers are below the resolving power for this telescope and, thus, we see their combined light as a blue cast in and around the cluster’s core (click to render the full-res image). Blue Stragglers are a class of star found almost exclusively in these types of clusters and are more luminous than typical Main Sequence stars. The brightest cluster stars in this image are the red giant stars and are fainter than visual magnitude +16; they have exhausted the compliment of hydrogen fuel in their cores and are now using the helium that had been built up over the preceding 12 billion years of normal hydrogen burning. The multitude of faint stars that appear as a halo and surround the central agglomeration have a visual magnitude of +20 and are at the limit of detection with the exposure and observing configuration used. Many of these threshold stars are classified as Horizontal Branch stars (see corresponding B – V Color Diagram, below). As stated earlier, they are of a comparable luminosity (brightness) as the brightest appearing star in our sky, Sirius, a Main Sequence (normal) star that is 25 times more luminous than the sun.

NGC2419_B-V_ColorDiagram
The brightest, most luminous stars are the AGB (Asymptotic Giant Branch) stars followed closely by the RGB (Red Giant Branch) and HB (Horizontal Branch) stars. All these stars are highly-evolved, helium burning giant and supergiant stars. The only way they are visible at such a tremendous distance is almost entirely due to their enormous size, in many cases greater than 100x the radius of the sun.


Imagination is more important than knowledge

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