Tag: fusion

xkcd asks, "what if…?"

By Phil Plait | July 12, 2012 9:57 am

If you read this blog, then you probably already know about xkcd, the web comic by the geektastic Randall Munroe. What you may not know is that Randall really is just that smart, with a keen interest in physics and math. He likes thinking about big-picture stuff, including taking what might seem like silly ideas and running with them to see where they lead.

So I’m really excited to see he’s started a blog called "what if?" He takes crazy questions from readers and answers them, following the logic wherever it may lead.

The inaugural post asked, what if you threw a baseball at very nearly the speed of light? I have seriously thought about this as well, and while I found myself smiling at Randall’s explanation – his thinking followed mine very closely – he took a turn I hadn’t thought about: atoms in the air undergoing nuclear fusion with the baseball. Huh.

The second post, which just went up, is about how well you’d score if you answered SAT questions randomly, and somehow due to Randall’s machinations all the US Presidents and 75% of the the cast of Firefly get electrocuted by lightning.


As usual, this is clever, funny, odd, and just plain cool. You’ll feel smarter – you’ll be smarter – after reading it.

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CATEGORIZED UNDER: Cool stuff, Geekery, Humor, Science

The Ghost in the Shell

By Phil Plait | July 5, 2012 7:00 am

Stars are one of the fundamental building blocks of the Universe. Huge, hot, and powerful, they emit energy that can be detected across vast reaches of space. For as long as they live (so to speak) they glow with a fierce luminosity.

And even when they die they can announce their presence in weird and wonderful ways.

Meet U Camelopardalis, just such a dying star about 1400 light years from Earth:

[Click to doomsdaymachinenate.]

U Cam is a red giant, a star that was once like the Sun but has gone much further along its evolutionary path. Our Sun is fusing hydrogen steadily into helium in its core, providing warmth and light for us. U Cam ran out of hydrogen in its core long ago, and began fusing helium into carbon. Then it even ran out of helium as a fuel! The core is now essentially an extremely hot ball of carbon, squeezed by pressure to within an inch of its life. There’s still helium outside the core, and gets so much heat from the core’s radiation that it’s fusing in a thin shell. Think of it like a very hot skin around an orange.

This helium fusion is ridiculously dependent on temperature. Increase the heat just a wee bit and the fusion rate increases madly, generating huge amounts of energy, which get dumped into the outer layers of the star on very short timescales. And by "short" I mean like years. Not millions of years. Just years.

When this happens the star swells immensely and ejects its topmost layer, like a solar wind on cosmic steroids. This event doesn’t last long, maybe a century or so, then it subsides. But that shell of ejected gas expands out from the star, eventually dissipating over millennia.

And that’s where U Cam is right now. Not long ago its core underwent one of these paroxysms, and the star blasted out the shell of material you can see in this Hubble Space Telescope image. Measuring the expansion rate, it looks like this shell was ejected about 700 years ago, and the event only took 50 years to unfold! Because these events don’t last long compared to the life of the star, it’s rare to see them. U Cam is one of the best of only a handful of such stars known.

In the image you can see how thin the shell is, indicating the event happened rather quickly (if it took a long time the shell would be thicker). I’ll note that the total mass of the shell is only about a tenth the mass of the Earth! It’s spread out over so much volume of space that it’s barely more dense than the vacuum surrounding it.

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CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

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