The Sun makes up about 99% of the total mass of the Solar System. Find out what the sun is made of, how hot it is, and why humans couldn’t survive without it. Dr. Grace Wolf-Chase is an astronomer with Chicago’s Adler Planetarium and a senior research associate in the Department of Astrophysics at the University of Chicago; Chris Lintott is an astrophysicist with Oxford University and the co-host of the BBC’s astronomy program, "The Sky at Night."