MadSci Network: Astronomy

Re: How do astronomer measure the distance of a star from earth.

Date: Thu Jun 22 02:44:10 2000
Posted By: Bryan Mendez, Grad student, Astronomy and Astrophysics, University of California at Berkeley
Area of science: Astronomy
ID: 961223598.As


Actually, the distances to stars are NOT measured using the redshift of their light and the Hubble constant. That method only works for measuring the distances to distant galaxies that are receeding away from us due to the expansion of the Universe. In that case we are looking for the redshift of the combined light of billions of stars from within the galaxies. The features in the spectrum of light are a result of emission and absorption of very specific wavelengths (colors) of light from different chemical elements. The pattern of emission and absorption "lines" produced by each element (and ion, and isotope, and molecule, etc.) is unique. It's like a finger print for the elements. When we look for redshifts, we are actually looking for the shifts of those line patterns from what they are at rest in a laboratory. The temperature of a star affects how much light is produced at any given wavelength (color) of light. This can affect which lines from an element will be visible, but it is something that astronomers take into account when they study the spectrum of light from a galaxy or star.

The distances to stars within our own Galaxy are measured more directly. For the most nearby stars, we use the parallax of the stars to triangulate their positions. Parallax is the shifting of an object with respect to a background when viewed from different angles. An example is when you hold your thumb out at arm's length and look at it with one eye shut. Now close that eye and look at it with the other eye. The position of your thumb shifts. It shifts more the closer it is and less the farther away it is. Astronomers use the Earth's orbit around the Sun. We look at a star at one time during the year and then wait 6 months when the Earth is on the other side of its orbit and look at the star again. It will shift by an angle inversely proportional to its distance. This method works for stars out to about 1500 light-years using space telescopes.

The measurement of distance beyond that gets quite a bit more tricky and complicated. Distance measurement is actually one of the most difficult measurements to make in astronomy. Measuring Doppler shifts is a piece of cake in comparison.

Hope that helps,


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