How do they calculate star distance?

[goku]

how do they calculate star distance?

[Jay-qu]

One such technique I know of is using parallax - Fairly detailed explanation ->

 

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

[goku]

okay, but how do they know the measurment of the small top angle?

[Buffy]

They don't. Parallax is only useful out to a few thousand light years away, making use of the full width of the earth's orbit (e.g. measure one angle in January and another angle in July and triangulate).

 

For distances beyond that its possible within our own galaxy to use brightness/spectrum comparisons to do approximations. When you jump out to other galaxies, the primary method is to use the fact that there's a specific type of star known as a Cepheid, that is a variable star whose frequency is exactly in ratio to its apparent brightness, thus you can measure its frequency and its brightness and compute how far away it is. Every galaxy has a bunch of them, so its actually quite easy to map fairly accurate distances of all the galaxies we can see. Amazing visual maps of the universe have been generated by this method to show objects such as the "Great Wall", etc.

 

Check out the VRML link on this page: http://www.pbs.org/wgbh/nova/universe/tour_ggs.html

 

Cheers,

Buffy

[goku]

http://www.pbs.org/wgbh/nova/universe/tour_ggs.html

pretty pictures

a car is in the distance with it's radio and headlights on.

the brihgtness of the headlights is 10 candle power,

the decible level of the radio is 4.

how far away is the car?

[Qfwfq]

Clarification:

that is a variable star whose frequency is exactly in ratio to its apparent brightness

I'm sure Buffy meant absolute brightness, which can then be compared with the apparent brightness so as to compute how far away it is by the inverse square of distance rule.

 

The sequence of methods used for calculating larger distances, each based on information gleaned from measurements of less far distances, is called the cosmic ladder. It starts with parallax between simultaneous observations from different places on Earth. Once Earth's orbit was determined, the next steps were those Buffy mentions. Knowing the distances of many galaxies, Hubble noticed the proportionality of it to spectral redshift.

[goku]

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.

[Erasmus00]

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.

 

It is true,the farther away something gets, the smaller it gets. However, how far must something be before you can no longer see it at all? On Earth, things tend to fade away in the distance because of atmospheric effects (the light gets scattered before it gets to you). Consider that you can see planets fairly well with a decent telescope still under the atmosphere. Now, stars are much larger then planets, and look to be just pin points in the night sky.

 

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

or do you just believe anything that a SCIENTIST says

 

I find it far easier to believe that these methods work then that there is a global conspiracy of scientists out to distort things just for the hell of it.

-Will

[Turtle]

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.

___Not true; the smaller it appears is the case. Your engagement of science appears to me to have no interest in adding to knowledge, either your own or others. I take the view that science is always ammendable & you make it clear that your view never is. Why do you bother? If you have in mind you will save me (us) in the name of some untestable belief, I offer you this: The last chosen is the first to go. I'll get in line when & if I choose.

[Tormod]

the farther away something gets the smaller it gets.

 

This is an interesting point of view.

 

Do a scientific experiment.

 

Bring a friend and stand back to back. Then leave your friend where he is and walk, say, 100 yards.

 

Has your friend shrunk? Have you? From your point of view, it is (or should be) obvious that YOU have not shrunk. Now, the question is: How do you know that your friend has not shrunk?

[CraigD]

… 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 think you’re being confused by 2 very different phenomena that both go by the term “seeing”.

 

The first, and most common kind of seeing (angular) is the ability to distinguish an object from its background at a particular distance. Factors that determine the ability to see in this sense include:

• Angular resolution. Depending on the frequency of the light radiating from the target object, and the geometry of the optics (eg: your eye), there is an absolute limit to the distance at which one can see the object. A standard explanation of this can be found at the wikipedia article on “angular resolution”.
  
• Available light. By definition, one cannot see in the absence of light. In this kind of seeing, the source of the light is usually different than the target object – that is, the light radiating from the object is reflected.
  
• Distinctness from the background. A large white disk may be easy to see against a black background, but impossible against a white one.
  
• Intervening objects. An opaque object between observer and target clearly makes seeing impossible. Smoke, mist, and other less than opaque substances may make it difficult or impossible.

Examples of this kind of seeing range from spotting other cars while driving, to identifying features the size of a breadbox on the surface of the Earth from an orbiting satellite, to identifying football-field size features on the surface of other planets.

 

A second kind of seeing (photometric) is the ability to detect light from a light source. Although some of the factors determining the ability to see in this sense are the same as in the first sense (intervening objects), Optical resolution is not. Even an object that takes up nearly no visual arc can be visible at a great distance, if it is emits a lot of light. Except when observing our own sun, or when using very powerful telescopes, this is the kind of seeing used to perceive stars.

 

The following experiment may help to clarify the difference between these 2 kinds of seeing.

1) Remove cap from a mini MagLite flashlight, exposing the grain-of-rice-sized light bulb

2) Remove the batteries from the mini MagLite, so that the bulb does not light

2) Place it 100 meters or so away, concealing the body of the flashlight so that only the bulb is visible (a lawn with medium-length grass would be good for this)

3) Using sunlight, lamplight, or another source of reflected light, attempt to see the bulb.

 

You will likely fail.

 

Now, replace the batteries in the mini MagLite, and repeat the experiment at light. Although it is no optically “bigger” at the same distance, you will easily be able to spot the intensely glowing bulb.

 

So, your original point, that if the stars were very far away we would not be able to see them, would be true if we were attempting to see them in the first sense, using reflected light. We see stars, however, in the second sense, by their emitted light, making it possible to see them, expecially the very bright ones, at tremendous distances.

 

Size still matters with stars, thought. It’s the total light energy that determines the distance at which we can see them, not their brilliance. A large, relatively dim star may be visible at a greater distance than a small, dazzling one.

 

To get an intuitive sense of this, try an outdoor experiment with a large lamp, such as a square-batteried “camp light”, and a small, very bright one like the mini MagLite. You may be surprised to discover that you can see the larger, dimmer lamp at a greater distance than the smaller, brighter one.

 

:) BTW, your skepticism of accepting science just because SCIENTISTs say it, is commendable. Don't listen to scientists, be a scientist! A good way - arguably the only way - is to experiment.

[Tormod]

:) BTW, your skepticism of accepting science just because SCIENTISTs say it, is commendable. Don't listen to scientists, be a scientist! A good way - arguably the only way - is to experiment.

 

Hear, hear. ;)

[goku]

the light from a star is emitted in all directions, spherically.

there is dust in space

this two facts leave me douting star distance.

[Tormod]

the light from a star is emitted in all directions, spherically.

there is dust in space

this two facts leave me douting star distance.

 

There really are two options: Stay ignorant or find out how star distances are measured. If you can't buy our explanations, read an astronomy book and find out for yourself.

 

Distances in the universe are measured using, basically, the speed of light and various properties of it. It is not that hard to do, really.

[goku]

Distances in the universe are measured using, basically, the speed of light and various properties of it.

using this equation V=D/T?

[Tormod]

There are many techniques, like triangulation, seconds of arc, redshift etc.

 

http://www.astronomycafe.net/qadir/q2278.html

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970415c.html

http://www.astro.uiuc.edu/~kaler/sow/star_intro.html

http://www.johnpratt.com/items/astronomy/notes/notes10.html

[Boerseun]

Interesting question, although Buffy and the rest have answered most of it (to the best of my knowledge).

 

However, I want to throw in another point:

 

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?