At an elevation of 73° above the Galactic plane, NGC 5466 would seem somehow related to M53 and N5053. It is closer, 52,800 light years, and midway between them in total luminosity, 52,000 Suns spread across a 170 light year circle. Yet visually it is only 0.8 mag brighter than N5053 in a similar 9 arcmins diameter. That means it, too, is very loosely hung together. Its orbit plunged N5466 through our Galactic disc 50 million years ago at approximately at the same radius as the Sun lies today, 26,000 light years. (
See Fig 1 on p.3 here.) Had human evolution begun 50 million years earlier, we might be watching the show. The brightest N5466 stars are red giants, of which I can see a dozen or so in my 6-incher on an alt-az in the middle of South Africa.
With NGC 5053, background counts of distant galaxies and quasars were an important deduction to make if astronomers were to accurately calculate cluster members. No so with 5466. Go to the
Simbad 3 arcmin plot and we find an almost impenetrable thicket of blue stragglers and only a smatter of RR Lyrae stars (which are the most accurate yardsticks to its distance). Blue stragglers are important because they are a direct measure of a cluster’s early-epoch mass density. “BS” is the acronym astronomers apply to merged binary stars of unequal masses (and also what happens when they sit around having a beer, but they don’t tell you that). Unequal binaries are uncommon in globulars. In their first hundred million years, all the whirlygig motion in a globular core tends to make many multi-star combinations. Unequal-mass trinaries are a fabulous Keplerian family fight in which the least massive star (the red sheep in the family?) is hurled out of the cluster altogether. Astronomers have not cast the final vote yet, but unequal mass binaries likely result because most globulars undergo a two-generation star-formation process. The first generation is denser and hotter, hence makes more large stars. The second generation makes do with the first gen’s leftovers, and fewer, smaller stars result. Generational couplings occur. If the BBC made a series out of globular flings, we’d have Downton Abbey without the rush. The net result is that globular binaries trend toward equal masses. Binary ejection is the main cause of globular mass loss over the gigayears. Some of these binaries merge into a single star which is twice the mass at the same luminosity and so therefore appears bluer. A lot of BS (stars, that is) means the cluster has lost most of its smallest stars—60% in N5466. Since a given population of blue stragglers clumps into groups 2.4, 1.6, and 0.8 times the Sun’s mass, any BS you see are only the A-list.
There is another pointer to N5466’s oddball antiquity. Its colour-magnitude diagram shows a remarkably dense clump of blue-tip horizontal branch stars. These are blue for an entirely different reason. Stars of a present mass of less than 0.7 that of the Sun will not end in planetary nebulae and white dwarfs as our Sun will end. Instead, they will pass through a helium-burning core phase which burns at 5 to 6 times the temperature as the core of the Sun—100 million Kelvin. That heat eventually ejects nearly all the hydrogen in the stars’ outer envelope, which leaves the star practically naked out there. And hot: 100 million K makes a very blue star. Unhappily for the star, all that ejected hydrogen envelope was also the star’s dinner plate. The star needs hydrogen to fuse into helium. If the star has no more hydrogen, it too will end up a white dwarf, but a rare class called helium dwarfs. We can truly, if wryly, say that when it comes to numbers white dwarfs dwarf helium dwarfs.
All in all, we have three globulars in a small pip of the sky which have three different stories to tell. If they are so different and yet all the same age, what happened. There’s a marvelous tale to tell how these three all managed to arrive in such close proximity, but this post is windy enough as it is. Renowned astrophotographer Robert Gendler has
a good discussion of the subject—and he turns out to as good a writer as he is an image guru.
After all this, we are still left with a question: if M53 is the same age and remoteness as N5053 and N5466, why is it so bright and they so dim? Answer: because like N2419 way out in the middle of nowhere, M53 is made almost entirely of first-generation stars. Usually we see mainly second-gen stars when we look at a globular. In the early universe, occasionally conditions were right for a very large first star generation (roughly a million solar masses) and much smaller second. The technical explanation is the dynamic friction efficiency of high-concentration density profiles. The less lofty reason is that first gen globular stars were tough old buzzards, but they never much liked kids.
