Why is space cold when it has so many stars?

Felicity Bedford spoke to Ryan McDonald from the Cambridge University Institute of Astronomy to find out more...

Ryan - The distance between the stars in the universe is absolutely phenomenal; we're talking just between our star and the nearest star - trillions and trillions of miles. And, although that means that we can be quite warm in space nearby to the sun, the majority of the universe is so far away from the rest of the stars in the universe that it is absolutely freezing. I mean we're talking the temperature of space in shadows or far away from stars - it's around -270 celsius - that's almost as cold as it's possible to get. Only about three degrees above the coldest temperature theoretically possible.

Felicity - How do we know about these extreme temperatures if they're so far away from our own experience of space?

Ryan - So, the temperature of just empty space, far away from any stars, is set by something we call the cosmic microwave background. So, basically, after the big bang took place, it emitted a huge amount of light - about 400,000 or so years after it and that light has gradually lost energy and cooled down over time. And so, this is just background light that fills the entire universe and it's really, really cold and so that's what sets the fundamental temperature of space itself, away from the stars.