How far is that star?

[Sparkey]

Any insight regarding how astronomers determine distance for celestial objects? I've heard about paralax and how we can measure geometrically by observing the change of angle as we look at a star from pictures taken 6 months apart (from both sides of earth's orbit around the sun). There are 360 degrees in a circle and each degree has 60 minutes of degrees and each minute is comprised of 60 seconds of degree. Proxima Centauri (our closest neighboring star) when viewed from opposite sides of earth orbit is less than 1 second (.7 seconds) of paralax. But that small offset is enough to establish distance from it to us.

The question that I have though is how can we determine the distance for objects that are further away? I've understood only the most basic concepts and should need to take some classes on the subject soon as it is very interesting but wondered is anybody here had some easy to understand insight to how we go about it. I've heard analogies that compare pulsations of stars to the frequencies of lighthouses but don't understand how we can for instance use that information to determine distance to a pusating star in Orion's Belt.

Coloring book edition replies are appreciated. Ignorant here. :nod

Oh, I should mention that the doppler effect (as it applies to light) is familiar so "red shift" concept can be introduced but I'd be interested to know if we have precise formulae to show actual distance not just velocity. (Thanks)

 

[Pizzaguy]

THey use one or more of several methods, including measuring light echos off nebulas (yea, really!) As you have guessed, the parallax thing just does not work for really distant objects.

But, anyway, We have guys over at thespaceport.us that can answer that one.
We even have two real astronomers over there that show up a few times a week!
I can post the question for you, but you'd get more out of it if you did it yourself!

If ya join, let me know and I'll alert the admins that you are a real person - we have terrible spam troubles over there.

I found some links just now, but posting links here is SUCH a hassle...

Universetoday.com and badastronomy.com are good places to start.

Howstuffworks.com can be VERY "elementary" - almost to a point of being inaccurate, but you can try it.

ALSO, try scienceforums.com

 

[Deavonreye]

There's also physicsforums . com

 

[reba]

you guys don't even speak english :dunno

 

[Pizzaguy]

Got some answers for ya:

Yev says:
For stars less than 400 light years, we use parallax. If you look at a star today and then again six months from now, there will be a difference in the viewing angles of the two because the Earth has moved and you are no longer in the same place. Consider those two viewing points as the legs of a triangle (and the target star is the third). Some very simple Trigenometry will provide the distance.

For over 400 LY, we use color-brightness, where by understanding the relationship between the color spectrum of the star and it's brightness, distance can be determined.

(Refer them to apparent magnitude, absolute magnitude, and luminosity, plus Herzsprung-Russel - that's plenty for the layman)

rvastro says:
For example, on earth to find an unknown distance to an object, we start with a baseline of know distance. We look at the background behind the object and measure the angle that object makes with respect to that background. Using some trigonometric formulas, we can compute the distance to that object using the baseline length and the angles.

We do the same thing for stars. Our baseline is the diameter of Earth's orbit-about 300 million km or so. We note the angle the star makes with respect to the background stars and use close to the same routine as the Earthly system. For stars farther out, this method fails and we must turn to more complex methods such as spectroscopic parallax (using the star's spectrum to determine the distance)to compute the distance.

When our resident meteor researcher shows up, I'm sure he'll have something to say. I'll post it when he drops by.

 

[Pizzaguy]

Proplyd said:

The Ceipheid Variable helps with those greater distances. We time the change from its brightest to its dimmest. That change will correspond to the star's absolute brightness. We then measure how bright it looks. The ratio will the square root of the starndard distance to a star. If it looks a quarter as bright as it should look at that standard distance, then it is twice as far. This works out to about 300 million light-years. After this distance, the star is to faint to see any change in brightness.

(I said I'd get an answer, didn't say we'd understand the answer).

MeteorWayne responded with:

Sure, I was sleeping, and still am (Just got up to fire the wood stove, since it's 9 degrees outside), so a short response for now.

Further out than what prop described, a type of supernova (1A?) is used. They always have the same peak brightness and decay curve, and can be seen in distant (billions of light years) galaxies. So how dim they appear to us here, tells us how far away they must be.

ZZzzzzz MW

 

[Sparkey]

Proplyd said:

The Ceipheid Variable helps with those greater distances. We time the change from its brightest to its dimmest. That change will correspond to the star's absolute brightness. We then measure how bright it looks. The ratio will the square root of the starndard distance to a star. If it looks a quarter as bright as it should look at that standard distance, then it is twice as far. This works out to about 300 million light-years. After this distance, the star is to faint to see any change in brightness.

(I said I'd get an answer, didn't say we'd understand the answer).

MeteorWayne responded with:

Sure, I was sleeping, and still am (Just got up to fire the wood stove, since it's 9 degrees outside), so a short response for now.

Further out than what prop described, a type of supernova (1A?) is used. They always have the same peak brightness and decay curve, and can be seen in distant (billions of light years) galaxies. So how dim they appear to us here, tells us how far away they must be.

ZZzzzzz MW

Thanks! I've read about Ceipheid Variables before but didn't understand that there was a simple relationship between the period of variation and distance. They time the change (Delta) in brightness to determine the "standard brightness" then they check how bright it looks *now* and compute distance by taking the square root of the difference (if it looks 1/4 as bright as it 'should' look) then it is twice as far. The square root of 1/4 is 1/2 so it is twice as far?

I can accept that because it applies to my understanding from flash photography and how brightness of the flash falls off by the square root of the distance - something 2 feet away doesn't get 3 times as much light as something 6 feet away. From what I've read about cepheid stars, there is a well defined relationship or at least it is currently considered to be a well defined relationship -- between the period of the pulse and the luminosity that lets astronomers use them as "standard candles" of known brightness to help determine distance. We have developed classifications for various type of that let us try to figure how how variable stars relate to the sun in terms of luminosity.

If I understand correctly it has to do with the opacity of helium at different tempertures. What I don't understand is how we can figure out what other types of elements are in the distant stars, how much metal, for instance? That would effect brightness too. In addition, it seems that we are making some plus/minus factors in our "standards" and I'd have to wonder if there isn't some type of "uncertainty principle" hidden to our sight that effects the observations. But I'm talking off the top of my hat here, no way to determine "hidden" things, no substance to that type of speculation. Thanks again for carrying my question to the other forums.

~Sparrow

 

[Pizzaguy]

The square root of 1/4 is 1/2 so it is twice as far?

That's how I read it!

I am still waiting for a deeper answer from MeteorWayne... sometimes it takes time for him to answer - busy guy.

Your welcome, I was wondering what kind of answers we'd get.