measuring stellar distances without parallax

My first thought is that the wavefront from even the closest star would be almost a perfect plane by the time it gets to Earth. There would not be any place close to enough curvature to be able to measure it. Keep in mind that Proxima Centauri has a distance of

4.2 light years. With a radius of 4.2 light years the angle that you could reasonably subtend from the Earth is going to approach 0. And remember that Proxima Centauri is close. Any other stars would just get worse.

Bill

On a sunny day (Sat, 18 Jun 2011 04:01:27 -0700) it happened Jamie wrote in :

Clever idea :-)

Hi,

Ya, the measurement precision might make it impossible to do even if the star's wavefront could be isolated to begin with. Maybe there is a way to "amplify" the wavefront a known amount with optics to make it less planar a precise amount before doing the intensity over distance measurement?

I don't know enough about optics to know if something like that exists.

cheers, Jamie

If we are talking about ordinary stars, I do not understand how this could form a wavefront. After all, typical stellar radiation consists of black body radiation by individual sources at certain temperature, with the typical spectrum distribution.

Regarding laser or actually astronomical maser sources, eVLBI stations on different continents would certainly be able to do such measurements, provided that individual cycles could somehow be distinguished by some significant change in radiation.

Maybe, one of these daze, you might learn something about physics. *IF* and when that happens, you will know the (plural) stupidities in your "query".

SORRY; there is a star that is MUCH closer - in fact it is about eight light minutes away from us. And that is far enough to give us a rather planar wave.

...try that on the somewhat closer star i mentioned previously...

How do you measure curvature with only one point measurement? If you measure two points, isn't that an analog to parallax?

Well, yes, but I don't think the OP was thinking of our sun.

Bill

Better ways to measure distance to a star is to identify its type for particular variable stars there is a known period luminosity relationship - so measure how quickly it varies and you can work out how bright it really is. Cepheids were the first such standard candles:

Measure how bright it appears you you get your distance. This is also true for Type Ia supernovae which are visible over greater distances:

You don't. The experiment has already been done to measure apparent stellar diameters of the brightest stars as seen from Earth.

First by Michelson & Pease in 1921 on the Hale 100" and later by Hanbury-Brown & Twiss of Jodrell Bank using intensity interferometry which is in effect a sensitive way of determining deviation of the stellar wavefront from being an unresolved point source.

Optical astronomers are now applying the tricks and mathematics of radio astronomy to optical wavelengths with excellent results.

If you measure them simultaneously then it is interferometry.

It doesn't depend on the Earth's orbit diameter and is best suited to measuring bright supergiants like Betegeuse where the usable baselines of 20m - 200m are able to make a sensible determination.

Basically it is easier in general to let the satellites like Hipparcus do the job of cataloguing of stellar positions.

Regards, Martin Brown

Too bad; his "prose" suggests that he does not know how to think in the first place. As a weak "defense" it also seems that most people do not consider our sun as a star...

Hi,

There's no such thing as a stupid question, you agree?

cheers, Jamie

That line is one of my pet peeves. There are a whole lot of stupid questions--probably a good quarter of the ones that get asked, if you define stupid questions as those that the askers could have figured out for themselves with a moment's thought.

Saying that there are no stupid questions is a fiction designed to get people to talk in classroom settings--it's a promise that even if you say something dumb, nobody will put you down for it. Getting people to talk is the first requirement for a discussion, and even stupid questions can help by eliciting better ones.

(It doesn't apply here, for all manner of reasons.)

Cheers

Phil Hobbs

Parallax is a consequence of wavefront curvature, so measuring parallax is one way of sensing wavefront curvature.

Your wavefront curvature sensor would probably do a sterling job, as long as it was wide enough to capture several degrees of curve. Do the math for how big it has to be for your target distance, and order up some girders!

Yup.

A perfect plane wave would follow an inverse square law with the distance set to infinity. A wavefront with low curvature will follow the inverse square law, although for the relatively puny distance changes that we can currently achieve (even if we wait half a year for the earth to move 2 AU) you may not be able to see it.

Sure. How far can you get the remote sensor? Got a rocket?

Yup.

Could it? Look up "diffraction" on Wikipedia, then tell us.

Yup. Of course, if you just flew to the subject star trailing a tape measure you could measure the distance, too.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
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Hi,

Don't forget stupid answers too :)

cheers, Jamie