# Astronomical distances

#### jay

Hai..
One 'parsec' is the distance from a star which makes an angle of 1''(1/60 of a degree)with the sun and earth(1.5*10^8 km).So by measuring an angle of 1'' we can measure to a maximum of 2 parsec.
But this only about 7 light years and we are speaking of much much bigger astronomical distances.How these are measured.
Thanks...

Jay

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#### chroot

Staff Emeritus
Gold Member
A second of arc is actually one 1/60 of 1/60 of a degree -- 1/3600 of a degree.

Parallax can actually make measurements all the way out to nearly a hundred parsecs. You just need equipment that is capable of measuring angles in a sub-arcsecond range. An instrument that can only measure an arcsecond resolution would not be very useful, since only one two stars are that close.

Beyond parallax, we can use stars like Cepheid variables to measure distance. Cepheids have a particular relationship between the period of their variation and their total luminosity (how much total energy per unit time the star is producing). If you can measure the period, you can calculate the luminosity. If you know the luminosity (how bright the star is up-close) and the apparent magnitude (how bright the stars is from where we are), you can calculate its distance.

Even beyond that, we can use the redshift of light from distant galaxies to measure their distances. The farther away a galaxy is, the faster it is moving away from us, and the more its light is redshifted.

#### Stellar Tourist

V good explanation

#### Phobos

Staff Emeritus
Gold Member
The different methods for measuring distances (e.g., parallax for "close" stars, standard candles for more distant galaxies, etc.) have overlapping ranges where they can be applied. So each method can be used to calibrate/check the other. I forget the name of this procedure...the "cosmic distance ladder" or something like that.

The closer the object, the better the measurement. The more distant, the more uncertainty.

another good description...
http://www.straightdope.com/mailbag/mstardistance.html

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