Why is the HR diagram important to scientists?

The HR diagram shows the composition of stars. The HR diagram shows the shape of stars.
The HR diagram groups the stars by their sizes. The HR diagram groups stars by their position in the sky.

Question

Why is the HR diagram important to scientists?

The HR diagram shows the composition of stars. The HR diagram shows the shape of stars. 
The HR diagram groups the stars by their sizes. The HR diagram groups stars by their position in the sky.

anonymous · Answer

@iGreen

igreen · Answer

What are your other options?

anonymous · Answer

The HR diagram shows the composition of stars. 
The HR diagram shows the shape of stars. 
The HR diagram groups the stars by their sizes. 
The HR diagram groups stars by their position in the sky.

igreen · Answer

It plots stars by its color and temperature.