The Naked Scientists

The Naked Scientists Forum

Author Topic: How do astronomers measure how far it is to distant stars?  (Read 3678 times)

Offline Sorcerer

  • First timers
  • *
  • Posts: 7
    • View Profile
I am interesting follow question. How do astronomers and astrophysicists determine distance between our solar system and other stars? I think it concerned with red shift. But I donít know how.
« Last Edit: 23/08/2008 18:55:15 by chris »


 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
The distance to the stars has nothing to do with red shift that only applies to the distance to rather remote galaxies.  Stellar distances are known quite accurately but it is quite a long chain of measurements but here goes.

Firstly we can measure the distance to the moon by watching exactly where the moon travels through the sky against the background of the stars (parallax) from different spots on the earth's surface t.  We can also do this for the distance to the planets.  Sending out spacecraft now allows us to measure the size of our solar system and notably the earth's orbit very precisely (down to a few feet!)by measuring round trip transit times of signals. 

Next we can measure the distance to the nearest stars in terms of the earth's orbit by measuring their parallax against the more distant stars.  Some years ago a satellite called Hipparchos allowed us to measure the distance to many thousands of stars.

This gives us a measurement of the distance and therefore the brightness of many stars.  Fortunately most typical stars lie on a line known as the main sequence where the colour and spectrum of the star quite precisely defines their brightness.  This can also be seen by measuring star clusters.  This allows us to inferr the distance of most normal stars from their spectra.  the very brightest stars are a bit of a problem but reasonable estimates can be made from comparason with other similar stars. 

Stars can now be resolved and measured in nearby galaxies and this gives a reasonably accurate measurement of their distance too and allows us eventually to estimate the distances to more remote galaxies and prove the red shift law that allows us to measure distances out into the farthest regions of space.

There is also another ore recent way of measuring the distances to remote galaxies that is using type 1a supernovae.  These are a rather special exploding star where matter from a nearby star is deposited onto a white dwarf star and gradually increases its mass eventually it reaches a critical point where it explodes violently in a sort of fusion bomb.  these explosions should be very similar and also very bright because the star outshines the whole galaxy for a few days.
« Last Edit: 20/08/2008 22:25:20 by Soul Surfer »
 

lyner

  • Guest
Quote
Firstly we can measure the distance to the moon by watching exactly where the moon travels through the sky against the background of the stars (parallax) from different spots on the earth's surface t.  We can also do this for the distance to the planets.  Sending out spacecraft now allows us to measure the size of our solar system and notably the earth's orbit very precisely (down to a few feet!)by measuring round trip transit times of signals.
For objects as close as the Moon and Planets, you can't use orbital parallax because they are moving about as much if not more than we are. You don't need to, in any case. You can use triangulation / parallax from two separated points on the Earth's surface. (Just like a huge rangefinder) If you know the distance separating them and can measure the relative positions against the star background you can find the distance 'easily'. They did quite a good job of measuring planetary distances even before they believed that the Earth was moving around the Sun - just using triangulation.

For stars beyond the distance where parallax methods are possible, the Cepheid Variables give a good measure of distance.

But, basically, the further away you try to measure, the less accurate you can be. What the hell - it's pretty damn clever, in any case.
 

Offline daveshorts

  • Moderator
  • Neilep Level Member
  • *****
  • Posts: 2583
  • Physics, Experiments
    • View Profile
    • http://www.chaosscience.org.uk
For some objects inside the solar system you can also use effectively use a radio telescope as a radar dish and measure the time the radio waves take to get there and back. Since the Apollo astronauts put a retroreflecting mirror on the moon you can do the same thing even more accurately with a laser for the moon.
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8132
  • Thanked: 53 times
    • View Profile
You can use triangulation / parallax from two separated points on the Earth's surface. (Just like a huge rangefinder)

Re:Stellar parallax...
recording a star's position at different times of year gives a longer baseline and consequently a more accurate measurement of the star's distance,
i.e. taking measurements at different points in the Earth's orbit around the sun rather than different points on the Earth's surface.

Quote
Stellar parallax is most often measured using annual parallax, defined as the difference in position of a star as seen from the Earth and Sun, i. e. the angle subtended at a star by the mean radius of the Earth's orbit around the Sun. The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond. Annual parallax is normally measured by observing the position of a star at different times of the year as the Earth moves through its orbit. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars.
http://en.wikipedia.org/wiki/Parallax#Stellar_parallax
« Last Edit: 21/08/2008 10:56:12 by RD »
 

lyner

  • Guest
Quote
Re:Stellar parallax...
recording a star's position at different times of year gives a longer baseline and consequently a more accurate measurement of the star's distance,
i.e. taking measurements at different points in the Earth's orbit around the sun rather than different points on the Earth's surface.
Yes - a very effective (and the normal) method for not -too- great distances. Two photographs, taken six months apart can give you an excellent, almost stereoscopic view, if you present one to each eye.  The nearer ones are displaced more than the farther ones and will actually appear closer.

If there is any significant extra angular motion of an object, though, (e.g. a planet) it upsets the measurement.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums