Question

what does an astronomer know about a star if they know its classification in the morgan - keenan spectral classification system?
a method of classifying stars by their temperatures and compositions
a method of classifying stars by their distance from the sun
a method of classifying stars by their age
a method of classifying stars by their age and distance from earth

Explanation:

The Morgan - Keenan Spectral Classification system categorizes stars based on their surface temperatures and compositions. It uses spectral lines in a star's light to determine these characteristics.

Answer:

a method of classifying stars by their temperatures and compositions