Dr Mike Hobson from the Astrophysics group at the University of Cambridge
Part of the show Stars, Cosmology and the Beginning of the Universe
Kat - How do you weigh stars?
Mike - You weigh it dynamically. Gravity is a force and as the planets orbit the sun, that force keeps them circling. By measuring the distances, the masses and the velocities involved in the planets orbiting the sun, you can measure the weight of the sun. It comes out to be 10 to the power of 30 kilograms.
Kat - How do we measure how old the universe is?
There are a number of ways to do that. Firstly, you can measure the age of things in the universe. You can measure this using radioactive isotope decay rates, the age of the Earth, and one gets to about 5 billion years or so. Secondly, one can start measuring the age of stars. That's often done by measuring the stars themselves and understanding their characteristics in terms of their temperature and luminosity and so forth. We then get to 10 billion years or so for the age of the universe. For the universe itself, one can essentially measure the rate that it's expanding, and factor in all the matter in the universe that tries to slow the expansion down. If you wind time backwards to when everything collapses to a point, it ends up being around 15 billion years ago.