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In the formula E = mc 2, what does E represent? 

The polarity of sunspot magnetic fields:

Which layer of the Sun do we normally see?

Consider an atom of gold in which the nucleus contains 79 protons and 118 neutrons. What is its atomic number and atomic weight? 

A light-year is:

Study this H-R diagram. Which of the following stars has (or had) the longest hydrogen burning lifetime? (The red arrows help you locate these stars on the diagram.)

This photo shows an object located in the halo of our Milky Way galaxy. What kind of object is it?

Open clusters and young stars are generally found only in the disk of the galaxy and not in the halo.

How does the light-collecting area of an 8-meter telescope compare to that of a 2-meter telescope?

Which type of binary can have their sizes measured directly by photometry?