Lesson: Redshift Physics • 9th Grade

In this lesson, we will learn how to calculate the radial velocity of a star or galaxy using the amount by which absorption lines in the spectrum of light from it are shifted.

Lesson Plan

Students will be able to

understand that if a galaxy is moving very fast away from Earth, the light emitted from it will be redshifted,
understand that if a galaxy is moving very fast toward Earth, the light emitted from it will be blueshifted,
understand that we can measure the redshift of a galaxy by looking at the positions of emission or absorption lines in its spectrum,
recognize spectral line patterns in an emission or absorption spectrum and determine whether they have been redshifted or blueshifted,
determine, based on spectral lines in an emission or absorption spectrum, whether a galaxy is moving away from or toward Earth,
use in all permutations,
use in all permutations,
use in all permutations.

Lesson Video

Lesson Worksheet

Q1:

The diagram shows the spectral lines in the light from two galaxies, as well as the same spectral lines in the light from a laboratory source.

How is Galaxy A moving relative to Earth?

How is Galaxy B moving relative to Earth?

Which galaxy is moving faster relative to Earth?

Q2:

The diagram shows the absorption lines in a spectrum from a distant star compared to those in a spectrum from a laboratory sample. How is the star moving relative to Earth?

Q3:

An astronomer looks at the absorption lines in the spectrum of light coming from a distant galaxy. He identifies the absorption lines of hydrogen, which makes up most of the galaxy. He then compares these lines to the same absorption lines from a laboratory sample. His results are shown in the diagram.

How is the galaxy moving in relation to Earth?