The posts below, both the ones discussing the Hertzprung-Russell diagram and the news bulletin about Globular Clusters bring up a related topic, that of Stellar Populations. About the middle of the 20th century, astronomers came to divide the stars of the Galaxy into two distinct populations. With typical astronomer whimsy, they called them Population I and Population II.

Population I was the disk population, including the sun and most of the stars in the solar neighborhood. They rotated about the center of the Milky Way, rolling down the disc like cars on a freeway. Like cars on the turnpike, they were moving quite fast, but relative to each other they were only moving slowly. Population I stars seemed to have slow radial velocities (in the line of sight) and proper motions (across the line of sight). They also tended to be younger and metal-rich (“metal” is jargon for any element other than hydrogen or helium).

Pop II stars had relatively less metals in their spectra, and they all seemed uniformly old, that is, highly evolved. But remarkably, they seemed to have very high radial velocities and proper motions, as if their dynamical behavior was somehow related to their structure or evolution. Both of these observations were misleading. The Pop II stars weren’t moving all that fast, but their velocities were random, they seemed to be orbiting in random orbits around the galactic nucleus, as opposed to flowing along in the galactic disc. The Pop II stars were coming from above and below the disk, cutting across it at an acute angle, so they appeared to be all fast, as would automobiles crossing the turnpike at angle, as opposed to staying in the proper lanes. Pop II stars seem to surround the Galaxy in a vast spherical cloud (the Halo population) as opposed to Pop I (the disk population). They also appeared to be very old, perhaps as old as the Galaxy itself.

This latter deduction came from the high percentage of stars with little or no metals in their spectra. No doubt thermonuclear fusion had accumulated large amounts of metals in their cores, but little of it had been carried up to their outer layers where we could detect it. Pop I stars, on the other hand, had been born in molecular clouds in the disc, clouds already enriched with metals by outgassing planetary nebulae and supernovae of earlier generations of stars. Pop II, the galactic halo, was supposedly an artifact of the Galaxy’s earliest history; the stars born before or soon after the Galaxy had just formed, right after the Big Bang. Pop I is still in the process of being born today, Pop II was a window into the Galaxy’s youth.

The galactic disk is where the open clusters are, where the big gas clouds reside, and where open clusters are condensing even today. But the Halo was different, the Halo was dominated by Globular Clusters, big spherical balls of stars in random spherical orbits around the galactic nucleus. And the stars in these gas-free globular clusters seemed to be all Pop II. And this arrangement is not just a feature of our own Milky Way. It appears other galaxies have their own orbiting globulars, their own stellar populations. This architecture appears to be a result of how galaxies form.

So, this beautifully reasoned model now appears to be threatened by the new discoveries that suggest globulars may not be as old as we thought they were. And if the globulars can be young, and if they had access to the metals formed by the first generation of stars right after the big bang (now called Population III), they are now potential targets for SETI research.

That’s why that article is such a big deal.