How do they calculate star distance?

[Turtle]

If light is broadcast spherically from a star, the distance between individual photons must stretch as the light travels further from the point of origin. Not being coherent light like a laser, I can't see it not being the case.

 

But - seeing as the photons are getting more distant from each other, should we at some stage (if a star is VERY far away) reach a point where it'll appear as if the star is flickering, and only because the light received by the observer now consist out of a photon stream that's so diluted that there's no continuous constant 'image' being formed?

___The image gets fainter (the old inverse square of the distance formula) with the distance, but does not flicker. Furthermore, until the star dies & the last light arrives at the viewer, one views the light continuously at a dim level because photons continue to arrive. Additionally, light is not the only electromagnetic radiation we observe from stars with our artificial detectors. We see stars in ultaviolet, infrared, X-Rays, & looking to see them in gravity waves as well. :D

____On the diluted thing, we overcome that with long & repeated exposures with highly sensitive detectors. You can thank your astronomers for advancing CCD technology now found in most cameras. :)

[goku]

Not being coherent light like a laser,

i wonder if scientists test for that?

[Qfwfq]

because the light received by the observer now consist out of a photon stream that's so diluted that there's no continuous constant 'image' being formed?

Of course, it also depends on the aperture of your scope's objective lens!

 

As Turtle says, they use long exposures for the faintest objects, this goes for emulsion film as well. In some cases the exposure during a good clear night is counting a few photons for each pixel of the CCD device.

[Tormod]

If light is broadcast spherically from a star, the distance between individual photons must stretch as the light travels further from the point of origin.

 

Light travelling around the universe is not in the form of particles but electromagnetic waves, so there is not a distance between them as such. Only when we measure a wave do the photons appear as particles. Thus from very distant stars the light will be very faint but it is not 0.

[Turtle]

___This just out on a star explosion recently detected. The article is full of the terms & concepts we have under discussion in this thread:

http://space.com/scienceastronomy/050927_star_cracked.html

:)

[maddog]

how many of you actually think any of the methods work?

or do you just believe anything that a SCIENTIST says.

 

my point is, if the stars were that far away we would not be able to see them.

the farther away something gets the smaller it gets.

I know the methods I have studied work because I can do the calulations to

determine the values myself. I can do this because I studied mathematics, physics

and astronomy.

 

It depends on how far away and with what are you viewing. With the naked eye

you can only see down to about 5 on the visual magnitude or for an average star

about 800 or so light years away for an individual star. With a telescope as Tormod

said, it depends on how big. The best land telescopes are now with digital CCD

enhancements getting better than 25 visual magnitude or being able to resolve a

single star in either magellanic cloud near our galaxy. To do better would need a

space based telescope. This is because of poor seeing on Earth no matter where

you go. The best you can do is a mountaintop like Mauna Kea or Chilean Andes.

Now mind you I was only considering Main Sequence stars. For giants and super

giants (or even Hypergiants like Eta Carinae) we can resolve single stars in other

galaxies (like Cephids). This was how Hubble was able to determine the Hubble

Law.

 

I don't just blindly trust scientists. I am one. I can check the figures myself. This

is how science works.

 

maddog

[maddog]

i wonder if scientists test for that?

Yes, it is called a spectrograph that can capture the spectral absorbtion and emission

lines of a star. Each line is an indication of the elements found on the surface of that

star. The thickness is an indication as to how prominent that element is within that

particular star.

 

maddog

[Tormod]

I don't just blindly trust scientists. I am one. I can check the figures myself. This is how science works.

 

Touché. :Waldo:

[goku]

how accurate is GPS?

[Jay-qu]

I hear that they can get it down to within a few meters

[Tormod]

how accurate is GPS?

 

This is a different topic for a new thread, goku.

[goku]

my point is if GPS is off by a few meters, how far off is their stellar calculations?

GPS triangulation is teeny tiny compared to stellar triangulation.

[Tormod]

my point is if GPS is off by a few meters, how far off is their stellar calculations?

GPS triangulation is teeny tiny compared to stellar triangulation.

 

You're comparing apples to oranges. Please stay on topic.

[CraigD]

my point is if GPS is off by a few meters, how far off is their stellar calculations?

GPS triangulation is teeny tiny compared to stellar triangulation.

GPS has hardly any similarity to the various techniques used to estimate the distance to stars, so a comparison of sources and amount of GPS error doesn’t shed much light on sources and amount of error from these techniques.

 

GPS doesn’t use triangulation in the technical sense of the word. GPS transmitters and receivers do not measure angles (as the stellar parallax measuring technique does), but durations, which are used to calculate distances, which are then trilaterated to determine position.

 

If you could place a radio transceiver (or even just a mirror) near a distant star, you could get a very high-precision measurement of the distance to it (although it could take an impractically long time). Unless we discover technologically advanced aliens at distant stars who are willing to help us with it (unlikely stroke of luck that would be!) determining the distance to stars using simple timings (as GPS does) can’t be done.

[goku]

GPS has hardly any similarity to the various techniques used to estimate the distance to stars, so a comparison of sources and amount of GPS error doesn’t shed much light on sources and amount of error from these techniques.

it measures distances, inaccurately at that

GPS doesn’t use triangulation in the technical sense of the word. GPS transmitters and receivers do not measure angles (as the stellar parallax measuring technique does), but durations, which are used to calculate distances, which are then trilaterated to determine position.

maybe because triangulation would be far too inaccurate

[Erasmus00]

it measures distances, inaccurately at that

 

The way GPS measures distance between human and sattelite is different then the way that we estimate the distance to stars. The major source of error is because the ionosphere has a variable index of refraction, and hence light travels at a variable speed. These minor deviations from c constitute the major source of error in GPS. Now, none of the methods

 

maybe because triangulation would be far too inaccurate

 

Or maybe its because the problems are totally different. For the method of parallax triangulation, you have to actually see the object whose distance you want to determine. Obviously, for GPS this is unreasonable. GPS is a totally different set of calculations designed to handle a totally different problem. Your analogy is false.

 

For stars close enough to do triangulation with, this is a nice, accurate way to determine distance. The better you can determine the small top angle with a telescope, the more accurate the distance calculation. This, along with the use of cepheid variable stars, allows for distance to be determined fairly well.

-Will

[Qfwfq]

my point is if GPS is off by a few meters, how far off is their stellar calculations?

GPS triangulation is teeny tiny compared to stellar triangulation.

How far off stellar triangulation is depends on how far awy the star is. Obviously.

 

If you want to measure a distance of many light years, would you expect a precision of a few meters? If the distance to measure is a hundred thousand light years, what accuracy can you ask for? OTOH, if you have to measure a distance of ten metres across your front lawn and you couldn't get it to within 1 metre, you might be the laughing stock of the neighbourhood. GPS is measuring distances of hundreds of km and then applying geometry to determine position, I'd say it's accurate enough, it could only be improved by removing environmental troubles and using higher frequencies .

 

By the time astronomers are measuring the distance of quasars, the uncertainty is high and you can jolly well expect it to be. You find a better way, without travelling across this galaxy.