Re: [asa] New fruit fly threat in Southern California

"Same type of system" in the sense that they're all stars. & they're "set
up" in different ways - different masses, sometimes different compositions &
environments - when we start to observe them. Of course we can't know for
sure how they were in the past but we can make reasonable inferences.

E.g., it's standard practice to assume that the stars in a cluster were all
"born" at roughly the same time. That seems reasonable in the case of a
cluster in which all the stars are moving toward about the same point
because that suggest that they were formed from a cloud of material that was
moving that way & aren't just a random collection of stars (like the
Hyades). In cases of that type we can measure distance by cluster parallax
& thus get absolute luminosities (& thus absolute magnitudes). Spectra of
course give us info on surface temperature, composition & other properties &
if there are binaries we can get masses (or we can use even more indirect
methods involving pressure broadening of spectral lines &c for mass
estimates). Then we can see if & how different variables are correlated.

We can construct the H-R diagram ( magnitude vs. spectral type) for the
cluster & then compare such diagrams for different clusters. There are
significant differences that can be interpreted in terms of differences in
cluster ages. This is obviously a non-trivial step & I'm leaving a lot out.
That's clear from the fact that 70 years ago or so it was thought that stars
evolved from red giants to the main sequence instead of the other way
around, as we think now. But once we get it right (!) we can get some
understanding of stellar evolution. There is certainly a good deal of
theoretical inference in this but that's the case with all experiments. &
note that much of what we're doing is applying physical principles that have
a great deal of experimental & theoretical support in terrestrial physics.

Now is this, as you suggest, a circular argument? Are we assuming that all
the stars in a cluster were formed at about the same time in ordet to
"prove" that they all have the same age? No. We assume - & not without
some good reason - that they were formed from the same interstellar cloud at
about the same time. From that & other input we are able to work out things
like the mass-luminosity relation, ideas about stellar energy sources &
stellar evolution &c, some of which we can check against further
observations. This eventually does give us an estimate for the age of the
cluster we started with - but also a lot more. It's not a circular but a
helical argument - we go in a circle but also make progress.

The parallel here with what geophysicists do with radioactive dating should
be fairly clear.

Shalom
George
http://home.roadrunner.com/~scitheologyglm

----- Original Message -----
From: "Bill Powers" <wjp@swcp.com>
To: "George Murphy" <GMURPHY10@neo.rr.com>
Cc: "Cameron Wybrow" <wybrowc@sympatico.ca>; "asa" <asa@calvin.edu>
Sent: Sunday, August 02, 2009 9:55 AM
Subject: Re: [asa] New fruit fly threat in Southern California

> George:
>
> You say:
>
> " Start with astronomy. There are 2 things to note about this science.
> 1st,
>> we can't do experiments on stars or galaxies - it would be completely
>> beyond the physical ability of any foreseeable technological civilization
>> to manipulate the conditions of even our closest star. & 2d, this
>> science is historical - all the stars & galaxies we observe are in the
>> past, & in most cases the distant past. It's an historical science.
>> However, there are billions of stars & of galaxies. We can observe stars
>> of varying masses, chemical compositions & ages, in different
>> environments. (Of course all those things are inferred but so are all
>> our observational data.) In a quite real sense, the experiments on
>> stars have been done for us. It's as if we walked into a huge lab &
>> found billions of experiments on the same type of system set up under
>> different conditions."
>
> How do you know that it is the same type of system *set up* under
> different conditions? This appears to be a conclusion that is beyond the
> data, one that we "naturally" make today.
>
> I suppose we might view it as abductive. We try the presumption out for
> size. We hope to find confirming or supportive evidence that is
> consistent with the presumption. How does that work and not be circular?
>
> Got to go.
>
> bill
>
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>>>>
>>>> To unsubscribe, send a message to majordomo@calvin.edu with
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>>>>
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Received on Sun Aug 2 17:16:16 2009