Astronomy Cast

Come on, admit it, you've had this question. If the Universe is expanding from the Big Bang, what is it expanding into? What's outside the Universe? Ask any astronomer and you'll get an unsatisfying answer. We give you the same unsatisfying answer, but really explain it, so your unsatisfaction doesn't haunt you any more.

Episode 27: The Third Question Show

The questions are piling up, so it's time to get through them. We've got a great collection this week. How can our eyes collect so many photons? What's the speed of gravity? Shouldn't the light from the cosmic microwave background radiation passed us by?

Episode 26: The Biggest Structures in the Universe

This week we continue the story of galaxy formation, learning how groups of galaxies come together to form the biggest structures around - galaxy superclusters. And when you look at the Universe at this scale, environment is everything.

Episode 25: How Galaxies Form

Our Milky Way is a complex and majestic barred spiral galaxy. But 13.7 billion years ago it began, like all galaxies, from the elementary particles formed in the Big Bang. How did our galaxy grow from nothing to the hundreds of billions of stars we see today?

Episode 24: Fermi Paradox

We live in a mind bogglingly big Universe filled with countless stars. We know intelligent life evolved here on Earth. It must be common across the Universe, right? But if there's life out there, how come we haven't been visited by aliens yet? Why haven't we even picked up signals from alien television stations? Where's all the life?

Episode 23: The Drake Equation

If you're wondering how many extraterrestrials there are in our galaxy, you just have to use a simple equation developed by astronomer Frank Drake in 1961. Just find out how many stars there are, how many support life, how many advanced societies form, and a few other details and we'll be set.

Episode 22: Variable Stars

Our Sun has been around for billions of years, and will last for billions more. We're lucky, it's pretty stable and regular as stars go, only changing in brightness a little now and then. But there are stars out there that change dramatically; astronomers call them variable stars, and they demonstrate just how bizarre and dangerous the Universe can be.

Episode 21: Black Hole Questions Answered

Our episode on black holes generated many many questions from listeners. We dip into this bottomless pool of questions and start dealing with them. Are really big black holes like the Big Bang? How can black holes evaporate? What would it look like to stand on a black hole? And just how large would a rock have to be before its gravity is so strong that a human can't escape?