Concept Exchange Society


Wednesday, July 10, 1996 9:30 pm

At Phil Sullivan's Observatory.

Presentation: Phil Sullivan

An Astronomical Evening:
Birth And Death Of Stars



Pre-Meeting Notice:

Here's what Phil says about our coming meeting:

I'm hoping we'll have a good clear night. The summer milky way should give us plenty of objects on the theme of "birth and death of stars" and also the structure of the milky way itself. We'll see several galactic clusters, globular clusters, planetary nebulae, and multiple stars. If the group stays around until it's dark enough, we'll see several external galaxies. I'll see tonight if comet Hale-Bopp will be out of the trees by the time we meet. I'm sure we could see it with binoculars by walking about a little if the trees block the view from the observatory. I don't think we'll see any planets.
It isn't really dark even at 9:30, but I think that would be a good time to set as the gathering time. We could talk a little about the telescope and other equipement as the sky darkens. If people are interested, we could review the constellations. Then look at some of the brighter objects (multiple stars). By about 10:15 I think we could see the fainter, more interesting objects (globular and open clusters, planetary nebulae). Galaxies should come last, when it's really dark.


How are astronomers able to know so much about stars only by looking at them? The answer is that they interpret what they see using the physics that applies here on earth. And the results yield a consistent picture. This physics - the laws of nature - seem to be valid on the scale of the entire visible universe.

By a carefully chosen set of telescopic observations Phil gave us some insight into the process of deduction used in astronomy.

Phils little observatory is impressive. The nine of us present for this event just managed to squeeze into it. His 14 inch reflector telescope is mounted on a cement pier which reaches from four feet underground to the second floor where we stood. The hip roof of the building sits on a set of rails. It can be rolled back by hand exposing everything to the open sky.

He first pointed his telescope just under Vega in the constellation Lyra. This constellation is high in the night sky at this time of year. To illustrate the color of stars he located a binary pair . One of the pair was a clear and definite blue, the other a yellowish white. The first is hotter than the second. Coloration reveals the surface temperature of the star just as it does that of a flame. A blue flame is hotter than a yellow one. White heat is hotter than red.

How much energy per unit time an object radiates from each unit area of its surface is determined by one thing and one thing only - its temperature. Together with the actual surface size of the radiating body its temperature accounts entirely for its luminosity; the total amount of light energy emitted per unit time.

The light energy per unit time per unit area that reaches us here on earth from a star is called its brightness. The two are intrinsically related:

where d is the line-of-sight distance between the star and us. This distance is not easily known.

Often stars appear in clusters. Phil showed us several. One was a galactic cluster consisting of several hundred stars. It is in one of the spiral arms of our milky way galaxy. Another was one of the globular clusters which reside in the halo around our galaxy. A globular cluster consists of several hundred thousand stars.

Click here to see some images that Phil took with his new CCD camera.

Astronomers assume that the stars in any such cluster are all at approximately the same distance from us; that they all have the same d. That hundreds of stars form a cluster in a region of the celestial sphere but are strung out along the line of sight seems too unlikely to contemplate. Looking at a cluster through the telescope one acquires the feeling that these stars must surely all have the same d. So for those in a cluster the relative brightness (what we see among them) is an accurate measure of the relative luminosity (what is there among them).

Furthermore such a cluster is expected to have evolved from a common origin. They are all about the same age. So from studying clusters much can be deduced about how stars evolve.

To extract order from star surveys two astronomers independently proposed a particular way of plotting star features. It has become the standard thing to do. Ejnar Hertzsprung of Denmark and Henry Norris Russell of the U.S fathered the Hertzsprung-Russell diagram in the early 1900's. They plotted luminosity on the vertical axis against color on the horizontal for all the stars in a cluster. Since color represents temperature this is fundamentally a luminosity vs. temperature diagram. The colors are ranged traditionally from deep blue through yellow to orange and then red so that in fact temperature is plotted backwards - from hot toward the right is cold. And both axes are scaled logarithmically.

Remarkably the overwhelming majority of stars scatter around a single curve on a Hertzsprung-Russell diagram. This curve has come to be called 'the Main Sequence'.
Here is what the diagram looks like. I've only plotted two stars on this one - Vega and our sun. The sun is represented traditionally by a circle with a dot in it.

Because the wealth of information gleaned from H-R diagrams is enormous I will here mention only one interesting finding. The Main Sequence curve practically overlays the curve representing all stars with one solar diameter. Most stars have a diameter not less than one tenth (red dwarfs) nor more than ten solar diameters (red giants). Stars have a very limited range of size! And our star, the sun, is right in the middle of this range.

Click here if you're interested in the simple mathematics of this conclusion.

Phil asked me how one is to understand red giant star evolution off the main sequence where a star growing bigger drops in temperature. He expected that 'growing bigger' (increasing volume, V) should be associated with a temperature rise (larger T).
Here is my thought on the matter.

According to the ideal gas law, Phil's is, indeed, the expectation for an isobaric expansion - one that takes place at constant pressure. An isobaric expansion follows the rule, T/V = constant = P/nR which says that as T increases (as the temperature rises), V increases (the volume grows) with it. But the expansion of a star is not isobaric.
A reddening giant follows a law closer to the one for adiabatic expansion, VTb = constant, which says, since b>0, that when V goes up (expansion), T goes down (cooling).
When you compress a gas, say in a compressor, its temperature rises. When you let it expand suddenly - as from an aerosol can - it drops in temperature. That's why the can gets cold. This effect is used in some refrigeration schemes. In these cases the pressure, P, is not maintained constant. The changes in V and T do not take place isobarically. So one does not expect heating with expansion.

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© m chester 1996 Occidental CA