Star Clusters

Two Main Types of Cluster 1. OPEN:  relatively young  often sparse (a few hundred stars)  scattered about, no obvious symmetry Example: Pleiades Example: Pleiades

The Pleiades

…and the Other 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!

A Globular Cluster A Globular Cluster

Inside a Globular Cluster Of course the stars all move! (If they were at rest, they would fall together under the mutual pull of gravity.) But there is no dominant central object that they orbit around: instead, each one moves in response to the combined gravtiational effects of all the others. Think of a swarm of luminous bees! Watch this animation:

There are also smaller Stellar Associations [not gravitationally bound: the stars will drift apart ]

The Reality of Clusters Proven by: Statistics (there are too many stars in one place to be a chance clumping. Look at the globular cluster!) Statistics (there are too many stars in one place to be a chance clumping. Look at the globular cluster!) Common motions of the stars Common motions of the stars Their Hertzsprung-Russell diagrams (as we will see) Their Hertzsprung-Russell diagrams (as we will see)

Singles? Couples? Groups?

Common Motions

Moving Clusters Visit and watch the animations under `3D Universe’ (especially in the Orion region) (especially in the Orion region)

Nature’s Helpful Gift! The stars in a given cluster are all: Co-eval (formed at the same time, near enough) [consider analogy to twins] Co-eval (formed at the same time, near enough) [consider analogy to twins] Formed of the same material (formed from an enormous cloud of thoroughly mixed-up material: no significant compositional differences, star-to-star) Formed of the same material (formed from an enormous cloud of thoroughly mixed-up material: no significant compositional differences, star-to-star) At the same distance from us (so differences in apparent brightness are meaningful! If one star looks brighter than its neighbour, it really is! It’s not an accident of distance.) At the same distance from us (so differences in apparent brightness are meaningful! If one star looks brighter than its neighbour, it really is! It’s not an accident of distance.)

Their H-R Diagrams Pick an interesting cluster. Now measure and plot the brightness and colour (= temperature indicator) for every star in the cluster. What do we expect to see?.

What Should We See? For a young cluster: some brighter blue stars, some intermediate yellow (sun-like) ones, and some fainter red ones They will be spread our along the main sequence, reflecting the recent birth of stars of a variety of masses

On the Other Hand Somewhat older clusters: should no longer have bright blue stars near the top of the main sequence [Why? Because such stars are the first to use up their hydrogen fuel!] [Why? Because such stars are the first to use up their hydrogen fuel!] But what will they have turned into? But what will they have turned into?

Hyades and Pleiades

HR Diagram for the Pleiades

HR Diagram for the Hyades

Now Compare Them Now Compare Them

One Obvious Difference

Another 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!

Notice the Red Giants in the Hyades

Another Example, with Ages

A Globular Cluster: Very Old Indeed! All the main sequence stars hotter than the sun have evolved away. There are lots of red giants! How old must this cluster be?

Various Clusters Superimposed (a confusing figure!)

Many Examples 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!

A Specific Test of Our Theoretical Understanding Start with: 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 nuclear reactions

The Crucial Question Then What kinds of stars would we expect to see after, say, billion years? What would its HR diagram look like? What kinds of stars would we expect to see after, say, 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) 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) 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 Calculate how the stars will change in internal structure and external appearance Predict what we would see! Predict what we would see!

Meet the Target – 47 Tuc

Collect the Data

Near The Cluster Itself: Lots of Stars!

Farther Out Some ‘Field’ Stars

The Observed HR Diagram for the Cluster

Comparison to Theory (the lines): Great Success!

Did the Sun 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

…Such as Here …Such as Here

But Must They Do So? There may be small dark clouds of gas that will form single stars in isolation.

It’s Hard to Know the Birth Scenario because: Clusters can slowly ‘evaporate’ (fast-moving stars leave) Clusters can slowly ‘evaporate’ (fast-moving stars leave) or can be slowly disrupted (by tidal forces) or can be slowly disrupted (by tidal forces) So the sun may indeed have been in a cluster at one time.