HOW COME? If the sun exploded, we wouldn't hear it

In 1979, the film "Alien" was promoted with a now-familiar phrase: "In space, no one can hear you scream." And no one can hear a star scream -- er, blow up -- either.

Why? Despite all the stars we see in the night sky, the planets that may orbit them, the gas clouds left over from exploded stars and the stray atoms adrift between stars, space is mostly an empty vacuum. That doesn't bother light. Light is composed of weightless particles called photons that zip right through empty space at a staggering 186,000 miles a second.

But sound waves are different. Sound waves need some material to travel through: a dense-enough gas (like the air around us), liquid (like water) or solids (like an apartment ceiling). Unlike light, with sound the medium is literally the message: What we hear as sound is simply matter, vibrating.

So when your upstairs neighbor clomps across his floor, your ceiling begins to shake, the vibration passing into the air below it. As the air molecules compress and spread out, a wave of molecules passes into your ear. Your eardrum begins to vibrate, too. Your brain interprets the sound ("My noisy neighbor, at it again!").

But in the vacuum of space, it's eerily silent. Unlike in the movies, even if you were floating in a spacesuit near a passing rocket, you wouldn't hear its engines firing (although you would be dazzled by the fiery glow). An explosion in space would be a silent visual spectacle, like fireworks that are all light and no boom.

(Luckily, since radio waves are just a kind of light we can't see, spacewalking astronauts can communicate using radio transmitters and receivers built into their suits.)

Scientists say the sun will continue to shine for another 5 billion years before it swells into a red giant, enveloping much of the solar system. By then, humans may have managed to depart for a more hospitable planet. So there's no known danger of the sun suddenly exploding. But if it did, astronomers say, we wouldn't hear a thing.

According to astronomer Lynn Carter, who works at the Smithsonian Institution, the trace gases in space aren't dense enough to carry sound waves that our ears could detect. And gasses released by an exploding star -- even one much bigger than our own -- would expand so swiftly into the vacuum of space that any sounds carried would be too faint to hear on Earth. Even if Earth were as near to the sun as Mercury, Carter says, the sun's explosion would be silent.