1. Why is the rate of expansion of the gas shell in a planetary nebula often not uniform in all directions? 2. What kinds of stars would you monitor if you wished to observe a supernova explosion from...

1. Why is the rate of expansion of the gas shell in a planetary nebula often not uniform in all directions?

2. What kinds of stars would you monitor if you wished to observe a supernova explosion from its very beginning? Look up the tabulated lists of the nearest and brightness stars in Appendix Tables E-5 and E-6. Which, if any, of these stars are possible supernova candidates? Explain.