If you live in the northern hemisphere and go outside in the late summer, you’ll see a bright blue jewel of a star high in the sky. Named Vega, it’s one of the brightest stars (if it sounds familiar, it was the star Ellie Arroway, Jodie Foster’s character in Contact, detected alien signals from).
Vega is critically important to astronomers. Being bright and high overhead for many observers, it’s become a "standard star", a target you can use to calibrate your instruments. It was used in such a manner for years by astronomers around the world. I don’t think it’s used directly any more, but many astronomical brightness measurements are in some way based on Vega.
It therefore surprised astronomers years ago when it was discovered that Vega had way too much infrared light coming from it. It was quickly realized that the star was surrounded by a disk of dust. Heated by the star, the disk was warm, and emitted infrared light (just like you, a warm human, emit IR, which can be detected using heat-sensitive cameras).
But there have been some problems. Compared to similar stars, Vega appears to be too bright. Worse, high-resolution spectra seem to show anomalous features, what you might expect from a rapidly rotating star. But Vega shows no signs of rapid rotation.
Now a new paper puts all that into a tail spin. Literally.
Using interferometry, an amazing technique that allows incredibly high-resolution data to be taken, astronomers have discovered that Vega indeed spins quickly– very quickly. They took advantage of the fact that a star spinning really quickly will flatten out near the equator due to centripetal force; the same force that keeps water in a bucket as you swing it around. In a sense, this force acts against gravity, so if you were to stand on the equator of a spinning object, you’d feel like you’d weigh less (this is true on the Earth, too– you weigh about 0.3% less on the Equator due to the Earth’s spin).
In a star, this balance of forces makes the star cooler at its equator than at its poles, so in optical light its not as bright at the equator. Normally, stars are way too far away to detect this difference, but interferometry can make extremely high-resolution observations, and the astronomers were actually able to see this difference in Vega.
They determined we see Vega nearly pole-on– like we’re looking right down over its north (or is it south?) pole. The polar region is hot, while the equator is cooler. You can see that in this graphic:
The orange "plus" marks Vega’s pole the &subsolar point"* , the center of Vega’s disk as seen from Earth (incidentally, it’s known that the debris disk around Vega is circular in appearance, which matches the idea that we are "looking down" on an actual circle-shaped disk; if we saw it at an angle it would appear elliptical, like the rim of a glass as seen form an angle) , and hotter regions are in blue while cooler are in red. They also determined that to give this degree of uneven heating, Vega must be spinning really fast: about 275 kilometers/second at its equator– 620,000 miles per hour! If the Earth spun that fast, our days would be 90146 seconds long!* Incredible. In fact, if Vega spun much faster, the centripetal force would be stronger than gravity, and the gas on the equator would fly off. Vega would tear itself apart.
This affects a lot of calculations astronomers use, and it will be interesting to see how this new data will be assimilated into the body of knowledge. After reading the paper, my first thought, oddly enough, was not so much the impact on astronomy, but on the movie "Contact"– for a brief moment, we see Vega as Ellie Arroway stops there on her way to meet the aliens. We see Vega as a beautiful spherical blue star… but if it’s spinning as fast as the observations indicate, it would not be spherical at all: it will be highly flattened, like a basketball someone is sitting on. It will be 25% wider through the equator than through the poles.
It figures: a new observation comes along that affects almost all of observational astronomy, and I wonder how it’ll affect how I watch a movie.
Tip o’ the dew shield to Larry Klaes for the heads-up on this one.
* I made a couple of unrelated errors in this entry for some reason, which is annoying. The period error was because I didn’t convert from miles to kilometers! The thing with the orange plus was just misreading the plot. Duh.
March 22nd, 2006 at 2:33 am
I’m French and maybe my English is not so good, so bear with me. It seems to me that “centripetal” is incorrect here, and that it should be “centrifugal” (or something). “Centripetal” etymology is “peto”, latin for “to go to”. So “centripetal” describes a force pulling TO the center, no AWAY FROM it. “Centrifugal” would be appropriate, as “fugo” is latin for “to flee away from”.
Now I learned from my years of college physics that the centrifugal force is not a real force, it’s just an artifact appearing from conservation of impetus when you describe movement in a referential which is not static (or in uniform motion), but attached to a point in circular motion. On the contrary, for a point to be in circular motion, it must be subjected to a centripetal force.
This is not contradictory: a molecule of gas at the surface of Vega about its equator is subjected to a real centripetal force: gravity. The high speed of rotation means it has a large impetus which tends to make it move away from the center of rotation. When considered in its own referential, this appears AS IF it was under a CENTRIFUGAL force.
Unless I missed something…
Jean-Denis
I
March 22nd, 2006 at 6:06 am
Umm, you can’t really compare the speed of two different sized objects spinning unless you state it in angular velocity, in units of rad/s. Giving a speed in m/s is useless when you’re talking about something rotating, especially if you don’t know its radius.
March 22nd, 2006 at 6:38 am
My question would be how the heck did it get so spun up? I haven’t seen anyone doing battling tops in space.
March 22nd, 2006 at 6:48 am
Does it’s super fast spin mean that it doesn’t have planets or is it so young that it might be throwing off stellar material? There has to be some reason for this guy to be spinning so darn fast!
March 22nd, 2006 at 7:16 am
BB:
275km/s is likely to be an estimation of the tangential velocity (probably at the equator), and they are also as likely to have an estimation of the radius of the star. It would have been safer to express the comparison in terms of “if the surface of the Earth was moving as fast as the surface of Vega.” I’m pretty sure that’s actually the comparison being made (otherwise, it would be pointless to make the differentiation between Vega and Earth).
Also, you could express the angular velocity in terms of seconds/revolution, or in revolutions/hour, or in degrees/minute, or in millibobs per angstrotempo.
OK, so I made that last one up. But… having a velocity in m/s *is* useful *when* you know the radius, which is what you said, but it isn’t what you said.
March 22nd, 2006 at 7:33 am
I think Jean-Denis is right about using “centripetal” instead of “centrifugal”.
I don’t think the orange “plus” indicates a pole on the star. I think it indicates the center of the disk and shows its apparent closeness to the pole.
Do you suppose that Vega may, over millions of years, transfer some of that angular momentum to the debris in orbit?
March 22nd, 2006 at 8:20 am
To answer your question, Phil, Vega isn’t used as a standard photometric star anymore, because it’s too darn bright. The Johnson UBV system is based on ten primary standard stars (which IIRC were derived from Vega originally, but are now defined–sort of like moving from the speed of light being measured to the length of the meter being measured) and 47 secondary stars. The primaries are all around magnitude 2.5-5. I didn’t check the *entire* uvby or MK list, but even if Vega’s on those it doesn’t have any special status.
March 22nd, 2006 at 8:56 am
Here’s my question. Phil mentions that we weigh 0.3% more at the equator. 0.3% more than what? What we weigh where we live? What we weigh at the poles?
J. D.
March 22nd, 2006 at 9:44 am
The speed I quoted as at the equator. I should have been more clear.
Centripetal and centrifugal forces are the same thing. They are just seen in different reference frames. People argue endlessly about them, which I find very endlessely amusing. They are both real forces.
March 22nd, 2006 at 9:46 am
My question - why did they pick blue as hotter and red as cooler?
March 22nd, 2006 at 10:02 am
Jean-Denis: The paper says centrifugal, but I go with BA on his explanation of his own terminology.
BB & Pyrich: The paper gives their estimate of equatorial radius as 2.873 x R-sun. The sun’s radius is about 695 500 km, making its circumference (x 2 pi) = 12.55 e6 km. The value of 275 (actually, 272 km/s) is in fact the velocity of the surface at the equator. That makes the rotation time about 46 200 s, or 12.8 h. So it takes about a half a earth day for Vega’s equator to rotate, vs the sun at about 50 times that.
Mack: I _think_ that 0.3% is equator vs pole. If I have a spare moment I’ll figure that. Part of the effect is that you’re rotating, and part is that you’re farther from the center of the earth (which is slightly oblate).
Irishman: Yeah, why not green! (St Pat’s day just past :). I suspect for the same reason we have red shifts and blue shifts: the redded part of the spectrum is lower energy than the blue. Or maybe it makes a neater looking graphic.
March 22nd, 2006 at 10:05 am
That’s “redder part”. Pardon the typo. And “its circumference” refers to Vega, not the sun. My typing and brain weren’t quite in synch today.
March 22nd, 2006 at 10:42 am
“Centripetal” and “centrifugal” as words having opposite meanings (at least in common French, and from etymology), if they designate the same force, then I feel it is very much lacking in clarity. I am glad I attended my physics classes in French: there was no room for confusion in the vocabulary
March 22nd, 2006 at 11:01 am
Phil,
College physics was a long time ago for me. In an example of a child spinning a rock on a string, I distinctly recall that centripetal force was the force keeping the rock on the string from flying away–ie it was the force exerted on the rock by the string. That’s the opposite of centrifugal force, which is the force pushing the rock away from the child.
March 22nd, 2006 at 11:06 am
In stars, blue is hotter and red is cooler. I wondered if someone would ask about that…
March 22nd, 2006 at 11:51 am
When you are inside a rotating reference frame, it feels like like something is pushing everything away radially, this is called “centrifugal acceleration”. When you are outside of the rotating reference frame looking at it it seems like there is a force holding the material in the reference frame from flying off in a straight line. This is the centripetal acceleration. They are the same thing as was said. I’ll have to look at who coined the terms but I halfway remember it was Laplace or somebody like that, so I actually think this distinction is also made in France Jean-Denis, could be mis-remembering though.
March 22nd, 2006 at 1:29 pm
After Einstein and Co. pointed out the importance of reference frames, back about a century ago, the way people talk about classical mechanics has gradually been changing. The modern textbooks I’m familiar with take the standpoint that when you pick a frame of reference, it comes with a set of forces. The particular forces you use to describe an object (or multiple objects) in motion depend upon the reference frame you pick, and how your vantage point is moving relative to the bodies you’re studying.
I haven’t taken a classical mechanics class here in France, but since the physicists in this neck of academia seem to use the same concepts as those back in the United States, I expect they’d think about la force centripète and la force centrifuge the same way.
The details get horribly messy, and no two books describe rotating bodies in exactly the same way. The “centrifugal” or “centripetal” forces are not the only things at work. For example, imagine standing at the center of a merry-go-round and trying to walk to a point on the outer edge. In a reference frame which rotates along with the carousel, a point on the edge is fixed with respect to the center: walking from the center to the edge would look like a straight-line path. But to a person standing outside the carousel, feet planted firmly on the ground, you would be moving in a spiral trajectory.
According to Newton’s Laws, if a body in motion changes its speed or direction of movement, a force must be acting on it. Joe, standing beside the carousel, sees Moe on the carousel changing his direction of motion, and so Joe deduces that a force must be acting on Moe, a “Coriolis force”.
Things can get even more fun when you consider motion on a spinning planet. There are centripetal effects and Coriolis effects, and like I said, no two books want to talk about it the same way. This is a ready source of problems used to torture college physics students: “A cannon at the North Pole fires a projectile at 500 meters per second. If the planet were not rotating, the acceleration of gravity would be 9.8 meters per second squared, but the planet is rotating with an equitorial velocity of 1600 kilometers per hour. Where will the cannonball hit the surface of the planet?”
Really, too much brainpower has already been wasted arguing back and forth between “centripetal” and “centrifugal” this-or-that. If demon gnomes snuck into all the libraries one night and swapped the two words everywhere they appear, it wouldn’t make a difference. (People who care about etymologies might get a little confused, but we already have plenty of words in science which go “the wrong way”: positive and negative electric charge, for example. Cathode and anode are also turned around, in consequence.) We should be allowed to use whichever word we prefer when speaking in loose or qualitative terms, since everybody knows perfectly well what we’re talking about.
The dispute over petals and fugals is the science-class equivalent of the arguments about splitting infinitives or ending sentences with prepositions. It’s all the sort of nonsense up with which we should not put.
March 22nd, 2006 at 1:30 pm
I’ll add to the nitpicking. :p
Using Google as a calculator, and Nineplanets.org for dimensions:
275 km/sec = 615157 mph
Earth’s circumference = 24901 mi
615157 / 24901 = 24.7 revolutions per hour
1 hour / 24.7 = 146 seconds per revolution, not 90.
According to http://www.solstation.com/stars/vega.htm, the actual rotational period is 12.5 hours, which “suggest that the star is rotating at about 92 percent of the speed (angular velocity) that would cause it to physically fly apart”.
The orange “+” represents the “subsolar point”, according to the paper at the link Phil posted. Doesn’t that mean a line between the core of Vega and us passes through that point? Which in turn means it’s the center of the disc, as seen from here?
Finally, about the centripedal/centrifugal thing: Not sure how it’s “officially” defined, but it seems logical to me to use them for the correct forces. Since centripedal means “to the middle”, it should refer to the major body pulling inward on the minor body (whether major/minor be star/planet, planet CoG/planet particle, or person/bucket of water). Since centrifugal means “away from the middle”, it should refer to the minor body pulling outward on the major body. Both are real, opposite forces that are actually present in any reference frame. The direction may seem to change (so which is which might be hard to define without a defined reference point), but they are still different forces. The inertial effect is what gives the forces meaning (otherwise they’d both just instantly accelerate each other to infinity–or impact).
Like in a car: the centripedal force is the tires pulling the car towards the center of the corner, the centrifugal force is the car pushing the tires towards the edge of the road. The inertial effect determines how much force the tires must overcome to keep you from smashing into a tree.
March 22nd, 2006 at 1:43 pm
P.S. The Physics FAQ does a pretty good job of treating the whole centri-whatever issue.
March 22nd, 2006 at 1:58 pm
Jean-Denis your English is just fine, far better than some Americans’
English!:)
March 22nd, 2006 at 2:17 pm
If the following points are clarified in previous comments, I apologize for not finding those clarifications. Note: There doesn’t seem to be such a word as “centripedal” in my dictionary.
Your weight at sea level at the equator is about 0.5% lower than it is at the poles. About half of the difference is due to the centrifugal effect and about half is due to the spheroidal shape of the earth, also caused by the centrifugal effects. Thus, because you are farther from the center of the earth, the effective force of gravity is lower at sea level on the equator than at the poles. The acceleration of gravity (which by convention, includes centrifugal effects) changes by about one part in a million for every every ten feet change in altitude. Thus, you also weigh about 0.3% less at the summit of Mt. Everest (not that I’ve been there) than you do at sea level at the same latitude. Gravity provides the centripetal force toward the center of the earth. This, of course, is far larger than the force needed to overcome the tendancy for the rotation to throw you off the earth.
March 22nd, 2006 at 2:32 pm
Michael, thanks. I corrected the orange plus thing.
I’ll check my math again when I get a chance.
March 22nd, 2006 at 3:06 pm
[…] Phil Plait “The Bad Astronomer” gets into a spin about Sirius. […]
March 22nd, 2006 at 3:48 pm
BA sez: “Centripetal and centrifugal forces are the same thing. They are just seen in different reference frames. People argue endlessly about them, which I find very endlessely amusing. They are both real forces.”
As I learned it back in high school physics (slightly after the Earth cooled), centrifugal force is cause by centripital acceleration. The radial acceleration is towards the center causing a force away from the center.
- Jack
March 22nd, 2006 at 3:58 pm
Phil: Don’t worry about Vega as depicted in “Contact.” After all, if Ellie Arroway was traveling the worm hole express from Earth, woudn’t she likely pop out along the Sun-Vega line? That means that she’d be looking at one of the poles of Vega, from which vantage point it still looks round.
I thought it interesting that the producers even put a debris field around the star.
- Jack
PS - If you have the DVD of the movie, watch the special feature on the making of the opening “pullout” sequence. On the image of Mars, the FX guys embedded Hogland’s face. Well, not his actual face; it’s the image he’s been making a living off of for the past decade or so. Just another in-joke.
March 22nd, 2006 at 4:26 pm
I was wondering if, because of its very fast rotation, Vega did not emit an intense solar (vegan) wind. I imagine a lot of atmosphere would be ejected and then cool down as it leaves the star. Is there a possibility that the infrared we see could be emitted by this cooling ejected atmosphere instead of a disk of dust? And even if there really is a disk of dust, wouldn’t it be blown away by the intense wind?
March 22nd, 2006 at 4:28 pm
Not a big criticism, but we poor, benighted souls south of the equator CAN see Vega too! Lower in the sky, naturally, but still very visible. It gets a bit annoying when everything astronomically worthwhile is assumed to be only visible above the equator.
March 22nd, 2006 at 4:45 pm
Mark, that’s why I said “high in the sky”. It can be seen from pretty far south, but not high in the sky.
March 22nd, 2006 at 5:32 pm
With a little research and calculation, it looks like the total difference in acceleration of gravity between the poles and the equator, at sea level, is about 0.6%. In this value about 0.34% comes from the rotational acceleration and 0.26% comes from the difference in mean sea level distance from the center of the earth. Actually, its not just the distance but includes the effect of the distribution of the earth’s mass. A gravity change caused by the difference between the polar and equatorial radii would result in about a 0.66% effect by itself.
Back near the middle of the last century, I gave many talks on the subject of measuring the acceleration of gravity and I used to always use the 0.5% approximate value with about a 50/50 split between the two effects. It was not quite true.
March 22nd, 2006 at 6:13 pm
Note that I said in the entry that 0.3% is due to the Earth’s spin. I didn’t want to complicate it by adding in anything else.
I also corrected my math error. I didn’t convert miles to km! Amazing. Good thing I don’t plan any Mars probes.
March 22nd, 2006 at 9:49 pm
BA said: “Good thing I don’t plan any Mars probes.”
Mixing pounds with newtons, that was waay bad!
March 23rd, 2006 at 2:11 am
I was thinking about the possibilities that arose regarding the current understanding of stellar evolution and how it applies to Vega’s extremely rapid angular spin. Apparently, some rotational force, some shock or another provides enough torque to cause a cold molecular cloud to condense and begin spinning. Perhaps Vega was located close to a supernova, and the energy imparted upon its inception resulted in the high rate of angular motion.
Does anyone else have any theories regarding the possible reason for Vega’s high rate of spin? Perhaps we can have an informal discussion, because I doubt there are any simulations that present Vega’s stellar origin in regards to this new data.
March 23rd, 2006 at 8:36 am
Another reason for Vega’s spin might be that it’s in the process of collapsing, or perhaps coalescing, which will be interesting for us, being so close. Although that’s not predicted by its place on the stellar life chart, is it?
March 23rd, 2006 at 10:32 am
I have a question tangential to the centri[fug|ped]al debate.
You mentioned that the apparently nearly circular disk of Vega’s system is evidence that we are looking nearly pole-on to a circular system — taken from the fact that if we looked obliquely at a circular system we’d see an ellipse.
Okay, so how do we know we aren’t looking at an elliptical system obliquely, yielding an almost-circle?
March 23rd, 2006 at 3:54 pm
My physics prof at UVa hated the word centrifugal and didn’t care too much about centripetal. His ideas were that centrifugal was just a result of inertia and that centripetal referred to too many separate forces, i.e. many things provide a centripetal force: it’s not “one” special force. I think these are teaching subtleties to help students understand.
I agree with my prof. and think that centrifugal is truly a fictitious force because if suddenly the centripetal force were to disappear, then the object flies off in a straight line at constant velocity until another force acts on it. If the centrifugal force was real, then the object should move in the direction of that force, i.e. if the gravitational force holding the star together (and providing the centripetal force) were to disappear, I don’t think particles would fly off toward the outside.
I do however understand that it’s sometimes beneficial when you’re thinking about a rotating frame of reference to use “centrifugal” but deep down, I feel that it’s fictitious.
March 23rd, 2006 at 3:56 pm
In order for that to be the case, we would need to be aligned with the major axis of the ellipse. But there is no reason the major axis would stay constant. The distant objects would remain distant as they orbit, so the ellipse would rotate around Vega. Thus we would not see a circular projection for very long, it would quickly* become an ellipse.
*Quickly in astronomical terms. I don’t know how fast the disk material is orbiting.
March 23rd, 2006 at 4:23 pm
Blake Stacey Said:
“It’s all the sort of nonsense up with which we should not put.”
I always liked Winston Churchill!