For the radial speed of an astronomical object to be determined, what must the object’s spectrum contain?

Either absorption or emission lines

Explanation:

For the radial speed of an astronomical object to be determined with a Doppler shift, what must we be able to see in the object's spectrum, Either absorption or Emission lines.

Asorption and Emission line explained below;

The object's spectrum must contain a Doppler shift

Explanation:

This is because, the radial speed of the object is determined from the absorption and emission spectral lines.

The object can either be "blue shifted" or "red shifted". This blue or red shift helps us determine the radial speed of the object.

If the astronomical object moves towards us, it is blue shifted and if it moves away from us, it is red shifted.

This extra shift in its wavelength can thus be used to determine the radial speed of the astronomical object.

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