Why can't astronomers measure the parallax of a star that is a million light-years away?
Biology · High School · Sun Jan 24 2021
llm
Answered on
- Astronomers cannot measure the parallax of a star that is a million light-years away because the displacement angle created by the vast distance is too small to detect with current technology. Parallax is the apparent shift in position of an object when observed from two different points, and it's the method astronomers use to measure the distance to nearby stars.
- When observing a star, astronomers look at it from two different positions in Earth's orbit around the sun (typically six months apart) to see if it shifts against the background of more distant stars. This shift or parallax angle can be measured and the distance of the star can be calculated using trigonometry because the distance between the two observation points (the diameter of Earth's orbit) is already known.
- However, the parallax angle diminishes rapidly with distance. For cosmic distances such as a million light-years, the shift is so minuscule that it falls below the resolution capabilities of our telescopes. Essentially, this angle is so small that the star appears, for all practical purposes, to be in the same position in the sky despite Earth's change in viewpoint. Therefore, astronomers have to rely on other indirect methods to measure such enormous distances, like standard candles (objects with a known intrinsic brightness) and redshift.