Stellar Astronomy: Part 8 – Stellar Clusters and Populations

Stars often exist as binary pairs – two or more stars that orbit their common center of mass.  However, even larger numbers of stars can exist in close proximity to each other – a star cluster – which can consist of hundreds to hundreds of thousands of stars.  Single stars, binaries, and star clusters exist as part of a much larger structure – a galaxy – which can contain millions to trillions of stars.  Furthermore, the composition of stars varies slightly in a way that depends on where they are found.

Binary Stars

Most stars exist as part of a binary pair.  In this sense, our sun is somewhat unusual in that it does not.  But our nearest neighbor, Alpha Centauri, is a multi-star system.  At a distance of 4.3 light-years, Alpha Centauri is the third brightest star in our night sky but can only be seen from latitudes south of 30 degrees north.[1]  The brightest two stars, Alpha Centauri A and B, orbit close to each other and appear as one star to the naked eye (and hence share the name “Alpha Centauri”), but they appear as separate stars in a small telescope. They orbit each other with a period of 79 years at a distance that varies between 11.2 AU and 35.6 AU.   That’s about the distance from the sun to Saturn and to Neptune, respectively.  The brighter star, Alpha Centauri A, is a G-type star about 10% more massive than the sun and 50% more luminous.  Alpha Centauri B is class K, slightly smaller and cooler at 0.9 times the mass and half the luminosity of the sun.

A third star of this system, Alpha Centauri C, orbits much farther out at 13,000 AU, or 0.21 light-years.  It is a class M red dwarf and is too faint to be seen with the unaided eye.  But it is well separated from the other two in our night sky and is therefore often given a separate name: Proxima Centauri.  It is currently the closest star to the sun.

Some multiple star systems are much more complicated.  Consider Castor, a bright star in the constellation Gemini.  Although Castor appears as one bright star to the unaided eye, it actually consists of six individual stars.  Telescopes reveal three visual stars, two of which orbit each other while a third orbits their common center of mass.  However, each of these visual components consists of two stars in a very tight orbit.  To watch all six stars perform their celestial dance over time would be fascinating.

Star Clusters

Stars sometimes are found in groups called clusters.  There are two types of star clusters: open clusters and globular clusters.  Open clusters consist of a few hundred to a few thousand stars and have a random shape.  In our winter evening sky (or summer morning sky), two open clusters are easily visible to the unaided eye.[2]  These are the Pleiades and the Hyades.  The Pleiades appears like a tiny “little dipper” and should not be confused with the constellation Ursa Minor (The Little Dipper).  To the unaided eye, six or seven stars are visible very close to each other.  These are type B stars, and appear blue in color.  Binoculars reveal even more stars and a modest telescope reveals hundreds of stars.

The Hyades cluster appears close to the Pleiades but is larger and more spread out.  Find it by locating the bright red star Aldebaran.  Aldebaran is not actually part of the cluster but is directly in front of it from our perspective on Earth.  A star that is directly in front of a cluster but does not belong to it is called an interloper.  Think of Aldebaran as “photo-bombing” the Hyades star cluster.

Many other open clusters are visible in binoculars, such as the Beehive cluster in our winter evening sky, or the Ptolemy and Butterfly clusters in our summer evening sky.  A given open cluster may or may not be gravitationally bound.  In other words, should the Lord tarry, these stars will either remain together orbiting their common center of mass (bound) or will disperse into space (unbound).

Globular clusters are even more spectacular.  They consist of hundreds of thousands of stars and always have a spherical shape with the number density of stars increasing near the core.  The nearest globular clusters exist at much greater distances than the nearest open clusters and are not easily seen with the unaided eye.  Binoculars will reveal a globular cluster as a tiny fuzzy ball, but individual stars will not be visible.  It takes a telescope with a six-inch or larger diameter mirror to begin to resolve individual stars in the nearest globular clusters.  An eight-inch or ten-inch telescope works even better.  These telescopes will reveal hundreds of thousands of stars in a compact, spherical region.  Although it may look very crowded, globular clusters are often around fifty light-years in diameter on average.  Globular clusters are gravitationally bound – their stars orbit the common center of mass.

Nearly all globular clusters are found in the summer evening sky, with very few visible in the winter.  This is because globular clusters themselves orbit the core of our galaxy.  In the summer evening sky, we are looking toward the center of our galaxy and thus see the majority of globular clusters.  In the winter evening sky, we are looking in the opposite direction, and very few globular clusters orbit farther from the core than we do.  This was one of the first lines of evidence that led astronomers to realize that our solar system is not in the center of our galaxy.

Galaxies

The largest organized collection of stars is a galaxy.  Galaxies come in different sizes, ranging from a million stars minimum, to a few trillion stars.  The galaxy in which we reside, the Milky Way, is thought to have at least 100 billion stars.  Galaxies come primarily in two shapes: elliptical or spiral.

Elliptical galaxies have the form of a prolate ellipsoid (similar to an American football but with rounder tips).  Different elliptical galaxies will vary in terms of the eccentricity (“squashedness”) of their ellipse and are assigned a number between 0 and 7 following an “E” for “elliptical.”  Some are very nearly round, and appear spherical: E0.  Others can be quite elongated E7.  This number reflects the apparent eccentricity as seen in Earth’s sky, which may or may not reflect the true eccentricity of the galaxy.

The smallest galaxies are generally elliptical in shape and are referred to as dwarf ellipticals.  Dwarf ellipticals are often satellites of larger galaxies.  They orbit around the larger galaxies, or they would if they had enough time to do so.  However, the very largest galaxies in the universe are often elliptical as well, and are referred to as giant ellipticals.  M87, the massive galaxy at the heart of the Virgo cluster, is a giant elliptical with over a trillion stars.  Elliptical galaxies tend to have fewer of the high-temperature blue stars than spiral galaxies, and may therefore look redder in appearance.

Spiral galaxies are generally large and always disk shaped with a central bulge.  Their disk is organized into spiral arms with most stars in the arms and fewer in between.  The spiral arms have a higher proportion of hot, blue stars than the central bulge and therefore look bluer in images.  The spiral arms generally take one of three forms.  They can form two distinct primary arms that wrap around each other in a double spiral; these are called grand design spirals.  They can form arms which split into other arms; these are called multi-armed spirals.  The galaxy in which we reside is thought to be a multi-armed spiral.  Finally, some spiral galaxies have small bits of spiral-like arcs without having distinct, continuous arms; these are called flocculents.

In many spiral galaxies, the arms form a straight line segment near the core.  These are called barred spirals.  They are quite common and possibly outnumber the unbarred spirals.  Some astronomers believe that our galaxy has such a bar near the core.  Spiral galaxies tend to have a higher density of dust and gas than elliptical galaxies.

Spiral galaxies are classified by how tightly wrapped their spiral arms are, and how large their central bulge is relative to the disk.  These two effects correlate: galaxies with wide open spiral arms tend to have a small bulge, and vice versa.  The spiral galaxies with the largest bulge and tightest arms are class Sa.  Medium bulge galaxies are Sb.  And those galaxies with the smallest core and widest arms are Sc.  Sometimes the letters are combined for intermediate classes, like Sab or Sbc.  If the galaxy is barred, a capital “B” follows immediately after the “S.”  Our galaxy is though to be an Sb, Sbc, or SBbc.

There are two rarer types of galaxies that do not fit neatly into the above classes.  Lenticular galaxies have a small disk and enormous bulge.  But their disk is uniform with no evidence of spiral structure.  Lenticular galaxies are either like ellipticals but with a small disk or like a spiral galaxies but without spiral structure.  They are given the classification S0.

Finally, we have the irregular galaxies.  These tend to have a random shape.  They do not have a central bulge nor any definite spiral structure.  They are given the designation Irr.

On a clear evening in late summer, you can see that our galaxy is a spiral or at least is disk shaped.  Away from city lights, you can see a faint cloudy band reaching from the Northeast to the Southwest.  A telescope will reveal that this Milky Way is actually the combined light of billions of distant stars.  Our galaxy appears as a thick belt because it is disk shaped, and we are within the disk.  Dark patches where stars seem to be missing are actually clouds of dust and gas which obscure the light of stars behind them.

Stellar Populations

From spectroscopy, we know that stars are made primarily of hydrogen and helium gas – the two lightest elements.  But trace amounts of heavier elements are also found in stars.  Astronomers refer to elements heavier than helium as metals (regardless of whether they are truly a metal in the chemical sense of the term).  All stars have trace metals in their spectra.

Astronomers have discovered that stars come in two fairly distinct populations based on their fraction of metals (called metallicity).  Stars like the sun have a relatively high fraction of metals.  Spectroscopy shows that at the solar surface, about 1% of the sun’s mass comes from metals – elements heavier than helium.  That may not seem like much, but it is relatively high for a star.  Stars like the sun that have high metallicity are called population I (“population one”) stars – abbreviated as pop I.  They receive the Roman numeral I since they were the first population to be discovered due to the fact that our planet happens to orbit such a star.

Population II stars have a much lower metallicity, about 10% that of stars like the sun.  So, population I stars are metal rich, and population II stars are metal poor.  There is a range of metallicity within each population, but most stars fit rather naturally into one of these two groups.

Stellar populations are somewhat organized by location.  Most stars in the disk of a spiral galaxy are population I.  These would include the sun and all its neighbors.  Open star clusters contain mainly population I stars.  However, the central bulge of a spiral galaxy contains mainly population II stars.  Also, globular clusters contain almost exclusively population II stars.  And elliptical galaxies contain mainly population II stars.

Populations and Stellar Evolution

Secular astronomers have conjectured that the reason for the two stellar populations has to do with stellar evolution – how stars are said to change over millions of years.  Although I don’t believe such a conjecture has merit, it is worth discussing the idea and the evidence that appears to challenge it.  Secular astronomers believe that the two populations are due to different generations of stars.  They believe that population II stars are the older generation, and population I stars are the younger generation.  This is based on the idea of the big bang.

According to the most accepted secular origins story, the big bang is supposed to produce only the three lightest elements: hydrogen, helium, and trace amounts of lithium.  The conditions (density and temperature) are not right to produce any other elements.  Yet, we do find heavier elements in the universe.  So, secular astronomers believe that these were produced by nuclear fusion in the most massive stars.  When these stars explode in a supernova, the heavier elements form in the core and are then released into the universe, contaminating the hydrogen and helium gas with trace amounts of metals.  The next generation of stars would form from such gas, thereby having a small amount of metallicity.  These are the pop II stars.  Then when the heaviest pop II stars explode, they produce even more heavy elements and further enrich the surrounding gas with metals.  The third generation of stars would then have the most metals.  These would be pop I stars.

So, secularists believe that the sun is essentially a third-generation star, whereas population II stars are second generation.  In such a scenario, we might expect that pop II stars would generally be older than pop I stars.  But this wouldn’t always have to be the case for the same reason that an uncle, in some circumstances, can be younger than his own nephew.  Generation doesn’t always equate to age.

However, why there should be only two populations in such a scenario isn’t clear to me.  Are we to believe that there are only two generations of stars?  Based on their luminosity and available fuel, the hottest blue stars have a maximum lifespan on the order of millions of years.  If the universe were really 13.8 billion years old, then why haven’t there been hundreds of generations of the hottest, fastest burning stars?  Why only two?

More importantly, where are the first generation of stars?  If the big bang can produce only hydrogen, helium, and lithium, then the first stars would be comprised only of those three elements.  These would be called population III stars and were expected to be found in the most distant galaxies.[3],[4]  The coolest, red stars have sufficient fuel for lifetimes far in excess of the secular age of the universe.  Thus, many population III stars should still exist today.  So where are they?  The fact that the James Webb Space Telescope found metals in the most distant galaxies was shocking to the secular community.  But it is what creation astronomers predicted.

So, stellar populations seem to have far more to do with structure than with generation or age.  Biblically, I would think that all stars have about the same age since they were created on day four of the creation week.[5]  Yet, God has organized them by chemistry into various locations (star clusters, spiral arms, etc.) for His sovereign reasons.  Our joy is to discover what the Lord has created and, in some cases, to understand the reason.

 

 

 

 

 

[1] As such, it cannot be seen from the United States except for Hawaii, Florida, Southern Texas, and Southern Louisiana.

[2] References to summer or winter are given from a Northern Hemisphere perspective.  Readers located in the Southern Hemisphere should reverse these.

[3] This is because, in the secular view, we see the most distant galaxies not as they are today but as they were billions of years ago, shortly after the big bang.

[4] Note that the generation number goes inversely as the population number.  The first generation would be population III, the second population II, and the third population I.

[5] The only exceptions would be any stars that came into existence since then.  While I concede that this is theoretically possible (since a star is a ball of gas and not an organism with irreducible complexity), the conditions necessary for such an occurrence seem unlikely, particularly within the biblical timescale.