Astronomical Distance Scale

[Martin]

the astronomical distance scale is established step by step in a kind of ladder

 

the lowest rung is parallax

and each lower rung is used to calibrate the rung above it

 

Wikipedia does not have an entry for "astronomical distance scale" yet. Wiki has a STUB. Someone should write them an article. the distance scale is very important and a lot of different kinds of physics and astronomy went into establishing and checking the various steps on the way up the ladder.

 

Many people know about the H-R diagram (Herzsprung-Russell) gauge of distance and the Hubble-law redshift gauge of distance, and the Type IA supernova standard candle gauge of distance.

 

but the thing is, those gauges have to be CALIBRATED using something more basic.

 

the crucial step in establishing the distance scale, that most people are not aware of, is the OPEN CLUSTER METHOD

that uses loose clusters of nearby stars like the Pleiades and Hyades.

 

This is a beautiful method and it is key to the whole scale.

Wiki DOES have something about this

http://en.wikipedia.org/wiki/Open_cluster

 

Check it out and look down to section 8 in the Table of Contents

"8. Open clusters and the astronomical distance scale"

 

http://en.wikipedia.org/wiki/Open_cluster#Open_clusters_and_the_astronomical_distance_scale

 

The point is that these small clusters are just barely near enough that you can survey distance to them with elementary trig. Using parallax and also a very elegant subtle extension Wiki explains.

 

And because these small clusters are COLLECTIONS of stars that ALL FORMED FROM THE SAME CLOUD AT ABOUT THE SAME TIME and are all THE SAME known DISTANCE AWAY (so we know their absolute magnitudes) it is possible to do Herz-Russ STATISTICS on them and fit a Herz-Russ curve to them. and in this way we can connect color and absolute magnitude and calibrate the next rung in the distance scale.

 

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the trig extension is like this. think of a formation of airplanes all flying away from you. Dots on a radar screen. Doppler tells you the radial speed. You see the dots getting closer together! but you know that the planes are in a set formation. The dots are getting closer together because the formation (like a large object) is getting smaller as it recedes into the distance.

 

so you have two measures of the radial speed: doppler shift and the rate the cluster appears to shrink, the rate the stars are getting closer to each other angular wise.

 

so you set those two measures of speed equal, and solve the equation for the distance to the cluster.

[ydoaPs]

the lowest rung is parallax

are you sure? i thought paralax was an effect that makes things look like they are in different places depending on which angle from which you are viewing it.

[Martin]

Ned Wright talks about

"ABC's of Distance"

http://www.astro.ucla.edu/~wright/distance.htm

 

 

He calls this method "Moving Clusters"

 

His method A is Trigonometric Parallax

and moving clusters is his method B

 

and then he goes on up the ladder with Herz-Russ diagram fitting

 

and with CEPHEID VARIABLES

and all that good stuff

 

But the thing to remember is that Cepheid variables are great but they had to be CALIBRATED at some point. And somewhere along the line, getting out to those cepheids, somebody had to do some trigonometry

[Martin]

are you sure? i thought paralax was an effect that makes things look like they are in different places depending on which angle from which you are viewing it.

 

hi yourda (oh highly improved one)

 

you are welcome to add something to the list that is lower than parallax

 

(like for inside the solar system determining the AU, the distance to the sun, "astronomical unit")

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when Astronomers describe the building up of the distance scale, they usually start with parallax

 

e.g. look at Ned Wright's website discussion. He is typical of astronomers.

 

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what they mean by parallax is a scale that depends on taking the AU as your unit, or else you have to know the AU in kilometers or light seconds or some other unit.

 

to do parallax you use the earth-sun distance, the AU, as the BASE OF A TRIANGLE

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the answer to your question "are you sure?" is Yes I am sure:-)

but you, if you have a different view, are indeed welcome to add something to the list. Do you by any chance know how the AU has been measured in meters? Want to talk about it?

[ydoaPs]

wait, are you talking about the distances used, or the way to measure certain ranges?

 

edit: i was under the impression that you were talking about distances, like: AU, light year, parsec,...

[Martin]

wait' date=' are you talking about the distances used, or the way to measure certain ranges?

 

edit: i was under the impression that you were talking about distances, like: AU, light year, parsec,...[/quote']

 

 

both, all,

 

before you can do a standard parallax you need to know the distance to the sun because that will be the base of your triangle

 

and you are limited how far parallax can go because the angles get too small to measure.

 

parallax has been pushed to a few hundred parsecs but it is dubious out that far

 

the next range of distance is covered by the MOVING CLUSTER method

 

check out Ned Wright's webpage on the astronomical distance scale. you are a smart lad and can surely figure it out for yourself

 

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a beautiful thing: Moving cluster method was applied to the Hyades cluster and gave a result of around 46 parsecs. then the Hipparchos satellite was put up to do parallax much more accurately than it can be done on earth surface. And Hipparchos parallax CONFIRMED that Hyades are about 46 parsecs.