Why is an absorption spectrum especially useful for astronomers?
A. It has dark lines in it that allow astronomers to determine what elements are in the star
B. It has bright lines in it which allow astronomers to determine how bright the star is
C. It helped astronomers to understand the rainbows we see on Earth after storms
D. It shows that the stars are transparent: we can see right through them
E. An absorption spectrum is not useful to astronomers at all. When they see one, it means they cannot learn anything about the stars that produced the annoying absorption.

Electromagnetic waves is a propagating medium used in all communications device to transmit data (messages) from the device of the sender to the device of the receiver.

An absorption spectrum can be defined as an electromagnetic spectrum transmitted through a substance (absorbing medium) giving rise to dark lines due to a decrease in the intensity or level of radiation at specific wavelengths and frequencies.

Basically, the substance absorbs energy i.e photons from the electromagnetic field.

An absorption spectrum is especially useful for astronomers because it has dark lines in it that allow astronomers to determine what elements (gases) are in the star. This is mainly due to the corresponding frequency of the electromagnetic spectrum.

A star is a giant astronomical or celestial object that is comprised of a luminous sphere of plasma, binded together by its own gravitational force.

It is typically made up of two (2) main hot gas, Hydrogen (H) and Helium (He).