: : Mr. Zephram,please tell me how scientists know how far are stars or galaxies? Ex: Virgo, 78000000 light years. Thank you very much.

There are various methods used for measuring astronomical distances. For nearby stars trigonometric parallax is often used. This is just a determination of the distance using the motion we perceive the star to undergo across the sky that is due to the earth's motion around the sun.

For wide cluster groups distance can be determined from the stars' velocities which can be determined from their tangential motion and Doppler shifting giving their radial motion.

Cephides are often used in Doppler shift methods for distant galaxies.

For sources at very large distances and where gravitational lensing occurs, the degree of lensing can be used to get the distance to the source.

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