1. Hydrogen and helium are the two lightest elements in the periodic table. What fraction of the mass in the known universe is made up of hydrogen and helium?
2. Why did it take billions of years after the big bang for planets like Earth to form?
3. What is meant by the term "heavy elements?" List three examples of heavy elements that are mentioned in the passage.
4. What is a supernova? How are supernovae connected to Earth's formation and our own existence?
5. Why are heavy metals like iridium so rare in Earth's crust?
6. The passage states that comets and asteroids are "pristine remnants of material that made up the original solar nebula." What does this phrase mean and how does the composition of these bodies differ from that of Earth's crust?
7. Why is iridium so useful for determining that an asteroid struck Earth more than 65 million years ago?