What was the age of the universe when it became transparent and photons could travel without running into anything?

What percentage of visible matter is made of Hydrogen?

What percentage of all the matter and radiation density is made of Dark Energy?

What percentage of the overall density of the universe is made up of dark matter combined with regular matter and energy?

Use the following table to answer the question. Star Apparent Magnitude (m) Absolute Magnitude (M) m-M Spectral Type 1 -1.46 1.46 0 F0 2 -0.72 -2.5 1.78 A1 3 0.12 -8.1 8.22 B8 4 0.41 -7.2 7.61 M2 5 -0.27 4.4 -4.67 G2     In which star’s spectrum are the hydrogen lines the strongest?

Which of the following would be considered a Reflection Nebula?

What is the minimum mass requires for a star?

The question asked in Olber’s Paradox of cosmology is

Which of the following types of galaxies are reddest in color?

Which is the classification given to a galaxy with a small nuclear bulge, and loosely wound arms with no bar through the nuclear bulge?