VISUAL PHYSICS ONLINE

8.3 FROM THE UNIVERSE TO THE ATOM

SPECTRA OF STARS

P83 1902

The spectra of two stars X and Y as observed on Earth as shown with the wavelength in nanometres.

(A)

What type of spectra are shown?

(B)

What can you conclude about the motion of the two stars?

(C)

What can you conclude about the composition of the stars?

(D)

Give an explanation in terms of physics principles the features shown in the figure.

View solution below only after you have completed answering the question. The solution is not in a form that you would answer in an examination. The answers are often in more detail to help improve your appreciation and understanding of the physics.

Solution

(A)

Absorption spectra

(B)

The wavelengths of the absorption bands are at different wavelengths and they have been shifted by the same increment. Therefore, the stars have different relative motions to the Earth.

(C)

The two stars have the same absorption spectra. Therefore, we can conclude they have similar chemical composition.

(D)

This motion of stellar objects can be determined by looking at their spectrum. Because of the Doppler effect, objects moving towards us are blueshifted, and objects moving away are redshifted. The wavelength of redshifted light is longer, appearing redder than the source and the wavelength of blueshifted light is shorter, appearing bluer than the source light

A redshifted absorption or emission line will appear more towards the red end of the spectrum than a stationary line. Conversely, a blueshifted absorption or emission line will appear more towards the blue end of the spectrum.

Absorption lines are produced by atoms whose electrons absorb light at a specific wavelength, causing the electrons to move from a lower energy level to a higher one. This process removes some of the continuum being produced by the star and results in dark features in the spectrum.