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Star Clusters Meet a Star Cluster! Nature’s Helpful Gift! The stars in a given cluster are all: Co-eval (formed at the same time, near enough) Formed of the same material (formed from an enormous At the same distance from us (so differences in apparent cloud of thoroughly mixed-up material: no significant compositional differences, star-to-star) brightness are meaningful! If one star looks brighter than its neighbour, it really is! It’s not an accident of distance.) How Do We Use Clusters? You can’t watch a single star cluster evolve (or any of the individual stars within it) but you can still intercompare the properties of stars within different clusters, and eventually identify clusters of different ages. This provides us with critical snapshots of stellar evolution. There are Two Kinds of Clusters 1. OPEN: relatively young often sparse (a few hundred stars) scattered about, no obvious symmetry Example: the Pleiades 2. GLOBULAR: contain the oldest known stars very rich (up to a million stars) centrally concentrated in a round ball – like a swarm of luminous bees! Inside a Globular Cluster Of course the stars are all moving! (If they were at rest, they would fall together under the mutual pull of gravity.) On the other hand, there is no dominant central object that they orbit around. Instead, each star moves in response to the combined gravitational effects of all the others. Think of a swarm of luminous bees! Watch this animation: https://www.spacetelescope.org/videos/heic1017a/ The Reality of Clusters Could they be chance clumpings of stars? There are good reasons to think not: Statistics (there are far too many stars in one area to be a chance clumping. Look at a globular cluster!) The stars share motions in common, moving ‘as a group’ Moving Clusters Visit http://www.astronexus.com/node/28 and watch the animations under `3D Universe’ (especially in the Orion region) Now Consider Their H-R Diagrams Pick an interesting cluster. Now measure the brightness and colour (= temperature indicator) for every star in the cluster. Plot these data as an HR Diagram. What should we expect? The first thing to note is that you will not see the same thing for all clusters. But why not? What Do We Expect? For a young cluster: we expect to see some brighter blue stars, some intermediate yellow (sun-like) ones, and some fainter red ones – all of them recently formed They will be spread out along the main sequence, reflecting the birth of stars of a variety of masses On the Other Hand For older clusters: we should no longer see any bright blue stars near the top of the main sequence This is because such stars are the first to use up their hydrogen fuel! The question is: what will they have turned into? Two Real Clusters: The Hyades and The Pleiades Their HR Diagrams [‘B-V’ is a measure of colour, from blue to red] Now Plot Them in the Same Figure as apparent magnitudes Compare Stars of Identical Colour [say, B-V = 0.4] These are main sequence stars, so those in the Hyades must be essentially identical to those in the Pleiades. A First Striking Difference A More Significant Difference What Else? The Hyades contains four red giant stars; there are none in the younger Pleiades. That is what the bright blue stars have turned into! ‘Turnoff’ Ages A cluster starts with stars all along the main sequence, but gradually the bright blue ones disappear. The point above which none are left is called the ‘turnoff.’ In this figure, there are no O and B stars left, only A stars and cooler. That defines the ‘turnoff age’. Two Examples, with Ages A Globular Cluster There once were main sequence stars in the region indicated by the blue line. They have all evolved away, and we also see lots of red giants! How old must this cluster be? All the stars originally hotter than the Sun are gone! One Important Clarification: Red Giants Aren’t Forever As we have seen, once stars exhaust their hydrogen in the core, they turn into red giants – but they don’t stay like that for long! This is just one relatively brief stage on the way to stellar demise. The globular cluster in the figure just above has a lot of red giants, continually forming from evolving stars near the turnoff. But there were originally many stars that were even more massive, that became red giants for a time, and that have moved on to a different final form. The cluster contains a huge number of ‘stellar remnants.’ [Details to follow!] Myriads of Clusters Our understanding is not based on a simple comparison of just a few clusters of stars! There are literally tens of thousands of star clusters in the Milky Way galaxy, and we have lots of observational evidence to compare with our theoretical expectations! Various Clusters Superimposed (showing a variety of turnoff points) Let’s Do a Specific Test of Our Theoretical Understanding Hypothesize: a gas cloud a million times the mass of the sun, mostly H and He gas; and assume: that it gravitationally condenses to form a globular cluster containing a million stars (a small number of extremely very massive ones, very many of lower mass), each of which then undergoes known sets of nuclear reactions The Crucial Question What kinds of stars would we expect to see in such a cluster after, say, 10-15 billion years? What would its HR diagram look like? Use the Computer! Calculate the nuclear reactions in each star (the pp cycle, the CNO cycle, etc) Work out the rate of fuel consumption (faster in the more massive [hotter] stars, etc) Calculate how the stars will change in internal structure and external appearance Predict what we would see! Meet the Target – 47 Tuc https://www.eso.org/public/videos/eso1302a/ Real Data Frames the raw stuff of astronomical research The Observed HR Diagram for the Cluster Compare with What Theory Predicts Great Success! One Final Question: Did the Sun Itself Form in a Cluster? ..or is it perhaps even now in a loose cluster? (suggested reading: Nightfall, by Isaac Asimov) Stars Do Form in Rich Complexes But Must They Do So? There are small dark clouds of gas and dust that may form single stars in isolation. It’s A Challenging Question Analogy: without extra info, you don’t know if your adult friend was a single child, or part of a really big family. Similarly in astronomy; even if born in a cluster, stars can later wind up alone, for a couple of reasons: 1. 2. Clusters can slowly‘evaporate’ (fast-moving stars leave); or Clusters can be slowly disrupted (tidal forces ‘pull’ stars out) So the sun may indeed have been in a cluster at one time. We don’t know.