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Some Stars Do More Than Twinkle

July 30, 2008 on 2:05 pm | In Astronomy, Q & A | 18 Comments

And some readers ask really great questions that make writing easy and enjoyable. Dave (whose website is approaching 600 hits; show him some love!) asked me about twinkling stars, and more specifically, wanted me to explain this physorg.com article to him.

Ever look up at the night sky? Happen to live in the Northern Hemisphere? Then you probably know what the Big Dipper looks like, and maybe you even know how to find the North Star and the Little Dipper. The big dipper is one of the brightest constellations in the northern sky, and is alternately known as the plough (England) and the chariot (China); when I first looked at it I was sure it was called the Big Dipper because of its resemblance to the PAAS easter egg dye-kit’s wire egg-holder:

See those last two stars on the big dipper; the two farthest from the handle? They point the way to the North Star! Seriously, next time you see the big dipper in the sky, make an imaginary line with those two stars and follow it, follow it until you get to the next bright star:

The bright star that you get to is Polaris, also known as the North Star. There’s another constellation that starts with Polaris, which you can see if the sky is dark enough, called the little dipper; here’s what that looks like on a clear night:

Regardless of what shapes you see when you look up at these bright stars, they all appear to twinkle very brilliantly; flickering brighter and dimmer like candle flames. We know why they twinkle; it’s because of the fact that we have an atmosphere! As warm and cool air pockets move through the sky, they pass in between us and the stars we view, causing tiny fluctuations in the path that the light travels to reach our eyes. These tiny fluctuations appear as twinkling to us; you can tell a star from a planet by the simple fact that planets (much like the Moon) don’t twinkle. This is because the planets are so much closer to us, so the light from them doesn’t appear to come from a single point; instead, it comes from a small disk, but that disk is large enough that the little atmospheric fluctuations we have don’t affect the light we see enough to cause twinkling. And we know this is the cause, because when we look at stars through the Hubble Space Telescope, or when astronauts in space or on the Moon have looked at stars, there was no twinkling at all!

But the North Star is special. Not only because it’s so close to being directly over the North Pole of the Earth (which it is):

But because this is a star which has variations and fluctuations in its brightness all on its own. It’s a special type of star, known as a Cepheid Variable Star, which has intrinsic fluctuations. The really weird thing about the North Star is that they were measuring these variations very accurately, and they were disappearing; fading away. I’ll show you the graph, and you can see pretty easily that the fluctuations look to be dying away over the course of the last few decades:

And we thought that maybe this is what happens to all Cepheids; eventually these fluctuations calm down and the star reaches equilibrium, and it stops pulsing altogether. The North Star was the only Cepheid to do this; you can see that the variations had dropped from 10% to 2%, and these variations made a complete cycle every 4 days. So they made some detailed observations recently to see whether the variations had turned off completely. Here’s what they found:

The variations are ramping back up! We’ve never seen a star do either of these things before, either lose their variations or gain them back, and now a star that anyone can find without a telescope has been confirmed to be doing both. Keep your eye on the North Star, because it’s doing something that nobody has ever seen before!


Could It All End With A Rip?

July 28, 2008 on 1:06 pm | In Dark Energy, cosmology | 13 Comments

Here we are, nearly 14 billion years after the big bang, and we’re still trying to figure out where we’re headed. We know that the Universe is not only expanding, but that the expansion rate isn’t dropping to zero as the matter density drops. This, first off, is weird. After all, what determines the expansion rate of the Universe? Energy density, or the amount of energy you have in a given amount of space. As space expands, you’d expect that the energy density would go down, and it did for billions of years. This was because there was more matter in the Universe than anything else. But over the past three billion years or so, the matter density has dropped so low that we’ve discovered a new form of energy in the Universe, one that doesn’t appear to dilute the way matter does. This is what we call dark energy:

Now, one of the great unsolved mysteries of dark energy (other than what the hell it actually is) is how this dark energy density changes as the Universe expands. It could stay constant, which means as the Universe expands, the expansion due to dark energy stays the same, and the Universe will continue to expand at a rate that approaches a constant, of about 60 km/s for every Megaparsec (3,086,000 light-years) in distance. The data all point towards this as what’s going on.

But we don’t know for sure. As the Universe expands, we don’t have enough data to constrain how dark energy changes over time very well. It could, very slowly, dilute. So maybe when the Universe is 10 times the size it is now, dark energy will be 10% weaker. This is very different from matter, which will be 1000 times less dense when the Universe is 10 times the size it is now, but we don’t know whether dark energy really stays constant, or whether it decreases a little bit.

But there’s another possibility that’s really interesting: what if dark energy actually gets stronger as the Universe continues to expand? The Canadian Broadcasting Company did a radio show on this topic (and thanks to reader Brian for pointing this out), and what it would mean for the Universe. If dark energy gets stronger and stronger, that means the Universe will start to expand faster and faster. Instead of 60 km/s per Megaparsec, things can start expanding at 600, or 6000, or even 6,000,000 km/s for every Megaparsec they are apart.

Isn’t that faster than the speed of light? Yup. Because space doesn’t care about speed limits, it just expands based on the amount of energy it contains. You make the expansion rate large enough, and objects that were bound to each other fly apart. It’s a lot like spinning the Earth faster and faster: you spin the Earth fast enough and it starts to fly apart, much like this CD from mythbusters.

Dark energy, despite being a real form of energy, acts like a repulsive force. You increase a repulsive force and leave the rest of your forces (like gravity, electromagnetism, and the nuclear forces) the same, and eventually you overcome them. Galaxies fly apart into individual stars; solar systems lose their planets, individual planets are broken up into atoms, and eventually atoms themselves are destroyed as electrons are ripped off of their nuclei, and protons and neutrons are ripped apart into quarks and gluons. What a horrible ending to the Universe, and yet if dark energy is of this special type, called phantom energy, this is the fate of the Universe, called the big rip.

And then what happens to all that energy? Well, we don’t know, but if this happens, we can get back to the kinds of high energies that haven’t existed since… well… that other ‘big’ thing we all know…

Isn’t the Universe full of neat possibilities?


Weekend Diversion: I’m Going to be a Star!

July 26, 2008 on 12:37 pm | In Random Stuff | 6 Comments

By Protostellar Molecular Cloud Barnard 631 (Stolen from The Onion. Picture is of molecular cloud barnard 163; there is no 631.)

I know what you’re thinking, and it’s true: In this big, crazy universe, gaseous regions with the density and heat required to ignite deuterium fusion are a dime a dozen. Any wannabe can overcome internal pressure in order to initiate gravitational collapse. But you’re dead wrong if you think I’m going to let that stop me. I’m more than some molecular cloud with the potential to have an unstable core, and I won’t just be almost undetectable molecular hydrogen forever. I’ve got what it takes. Stardom, here I come!

Here I am, stuck in some podunk H II region not even visible to the naked eye. Do I let that get me down? No way. Sure, the Horsehead and Crab Nebulas have all the star-making rep, and if you’re a young and hungry mass of interstellar dust, gas, and plasma, they say it’s not who you are, but the supernovae you know. All I need is that one Big Break to show you what I’m really made of. Point me in the direction of some cataclysmic, entropic, destabilizing explosion, and look out, Milky Way!

Yes sir, once I get that fusion of heavy hydrogen underway, there’s no holding me back. After that, it’ll only take a couple hundred millennia before I’m on the scene. Then I’m gonna outshine everything for light-years around with a candlepower unseen in this galaxy. Look, the universe is just going to have to make room for this rising circumstellar disc. After all, when you’ve got that kind of electromagnetism, everything revolves around you.

I realize the outward pressure of the resultant radiation could slow me down—happens to the hottest stars out there. But I’ll be goddamned if I muddle around in the obscurity of gradual accretion just to end up as some pathetic, ancient black dwarf that no longer even registers in the visible spectrum. When I get there, I’ll work even harder, ceaselessly raining down my remaining cloud matter until everyone recognizes me. If I just fight through all the negative energy from the bipolar flow, the solar masses of the Bok globules I create will be higher than this quadrant has ever seen. You’ll see. Twinkle, twinkle, little star? Hardly. I’m going to be hot. The hottest.

I’m going main sequence, baby. The Big Time.

But when you’re as hot as I’m going to be, there’s bound to be a downside. I’ll have to deal with constantly having my picture taken and getting my radiation, temperature, and rotation velocity routinely measured. Other, lesser bodies will try to get in my orbit and share my intense light. Any binary relationships I may have with other stars will be placed under the telescope as well. But it will all be worth it, and every body that comes in contact with me will have to understand that when the time comes to expand exponentially—well, I can’t be held responsible for those destroyed.

I know that stars that hot only last one, three million years, tops. But it’s better to explode with 100,000,000,000,000,000 times the solar luminosity than it is to fade away.

Don’t cry for me, though: My legacy will extend far beyond your lifetime. After my spectacular collapse due to hot-and-fast living, you’ll look up, and I’ll be as bright as ever. No one will even know I’m gone.

Not for at least 600 million years, anyway.


Boys and Girls are Equal in Math Ability

July 25, 2008 on 12:37 pm | In Education | 29 Comments

Oh, but don’t take my word for it. Let’s look at the evidence to see what it tells us. After all, there have been many authoritative people contending that boys are better than girls at math. Think about how that could be possible. Could the mathematical ability of the human brain somehow be connected to the presence or absence of item 19 below?

Or, is it possible, that despite our physical gender differences, our brains’ mathematical potentials have negligible differences between them due to gender?

Previous studies (admittedly done 20 years ago) showed that boys scored higher on math standardized tests than girls did. Some argued that this proves the kindergarten insult, “Boys are better than girls!” Others argued that this was due to the fact that boys took greater numbers of advanced math and science classes in school, better preparing them to do well on the tests.

Well, it’s been 16 years since Mattel pulled the talking Barbie that said, “Math class is tough!” And despite the discouragement of ignorant educators and a society that may actually believe that girls aren’t as good at math as boys, modern girls now take just as many advanced math and science courses (except for high school physics; get working on that!) as boys do. This means that we can now test who was right: are boys better than girls at math, or were boys doing better because they were taking more advanced classes? How do we find out?

Well, let’s get the results of the study, led by Janet Hyde from my former employer, the University of Wisconsin at Madison. This study is based on the test results of 7 million students from 10 states:

The researchers looked at the average of the test scores of all students, the performance of the most gifted children and the ability to solve complex math problems. They found, in every category, that girls did as well as boys. (To their dismay, the researchers found that the tests in the 10 states did not include a single question requiring complex problem-solving, forcing them to use a national assessment test for that portion of their research.)

Wait a minute, the skeptic in you might say. Don’t boys still do better on the math section of the SATs? Ah, but their research has you covered there, too. Observe:

The study also analyzed the gender gap on the math section of the SAT. Rather than proving boys’ superior talent for math, the study found, the difference is probably attributable to a skewed pool of test takers. The SAT is taken primarily by seniors bound for college, and since more girls than boys go to college, about 100,000 more girls than boys take the test, including lower-achieving girls who bring down the girls’ average score.

On the ACT, another college entrance test, the study said, the gender gap in math scores disappeared in Colorado and Illinois after the states began requiring all students to take the test.

Hmm. So let’s sum up. Boys and girls exhibit equal math performances. They perform equally on math exams when they’ve had equal schooling, and when the same pools of boys and girls are chosen. To conclude, based on the evidence, it seems that they have the same aptitude and ability for mathematics and mathematically intensive careers.

Or, to put it more prosaically, in your face, sexism! And while I have your attention, check out this week’s Carnival of Space, where you can learn about the fact that days weren’t always 24 hours.


When is a Day not a Day?

July 23, 2008 on 1:07 pm | In Solar System | 2085 Comments

Lucas was telling me about a book he wants me to read, The Know It All. While going through it, Lucas saw something that prompted him to ask me this question:

Are days really getting shorter? Like a few million years ago a day would have been 28 hours (guessing here) and in a few million they will be 20?

As unbelievable as it sounds, Lucas is on to something here. Remember what makes a day happen: the Earth rotating. It takes about 24 hours for the Earth to make a complete rotation, and that’s why our day is that length.

But as any Jew who’s lived through Chanukkah can tell you, things don’t spin at the same rate forever. Start something spinning, and inevitably some force will slow it down.

For a spinning top, the frictional force between the top and the floor will slow the top down:

For a magnetic hovering globe (hint: my birthday is coming up a week from Sunday), even a perfectly frictionless magnet and globe will slow down due to the force from the air interacting with the globe:

What about the Earth? It rotates relatively undisturbed. There’s practically nothing around to mess with it. Nothing, that is, except for the Sun and the Moon:

But the Sun and the Moon both exert gravitational forces on the Earth, which rotates! These forces are not only responsible for the Ocean tides on Earth, but also cause a frictional force on the spinning Earth, causing it to slow down in its rotation!

The effect is tiny, so that every century, a day becomes 1.4 milliseconds longer. That means that if 100 years ago, a day was 86,400 seconds, today a day is 86,400.0014 seconds. Big deal, right? Except the Earth is old.

This means that when it was first created, 4.5 billion years ago, a day on Earth was only about 6 hours long, faster than any planet in the Solar System today. But don’t worry; this means that 100 million years from now, a day will still be about 24 hours; we’ll only have about an extra 20 minutes to contend with. Still, if the Earth were to live indefinitely, friction due to the Sun’s and Moon’s gravity would eventually slow it down so that it rotates just once per year, and the same side of the Earth would always face the same side of the Sun. This would take about 100 billion years. Thankfully, the Sun will explode long before that!


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