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The Spin of a Star Reveals Its Age

Unknown Lamer posted more than 2 years ago | from the forgive-my-arthritic-axis dept.

Space 67

eldavojohn writes "Some soon-to-be-published research on gyrochronology has yielded a possible method for more accurately determining a star's age. While determining the age of stars in clusters has been done using the patterns of its color and brightness, singular stars are much more difficult. By comparing established age information from clusters and analyzing the spin of stars, the researchers have established a defined relationship between color (mass), spin and age giving them the beginning of a guide of 'stellar clocks.' This was accomplished after four painstaking years of collecting data from 71 single dwarf members of the open cluster NGC6811 and establishing a model using data from Kepler."

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How do they know this is remotely valid? (1, Insightful)

Anonymous Coward | more than 2 years ago | (#36233236)

Given that astronomy, in its modern form, only goes back a few hundred years at most (and even then, most of the knowledge was obtained within the past century), how can these scientists feel secure measuring events that span billions of years?

Re:How do they know this is remotely valid? (1)

Aquina (1923974) | more than 2 years ago | (#36233334)

Thats a bit to much philosophic for my taste. Science has concepts involved. Just think what we achieved so far. Things can be proven -- or the opposite.

Re:How do they know this is remotely valid? (1)

decipher_saint (72686) | more than 2 years ago | (#36233368)

Well, that's the joy of science. You work using models you can't disprove, until they are disproved, then you work under new models.

Re:How do they know this is remotely valid? (1)

Anonymous Coward | more than 2 years ago | (#36233406)

Because of lots of hard work, math, observations, and more math. The paper is available in the first link in the summary if you want to read it and come up with legitimate criticisms instead of going with navel-gazing one-liner.

If the technique Meibom et al. have proposed continues to hold up under further observations -- and it looks like there has already been quite a lot of testing -- this will be an awesome tool for astronomers. The age of a star tells you quite a lot, both about the star and about other objects it's associated with. There are already measures for this, of course, but they have limited precision and work under limited conditions. This technique is limited too, but the limits are different. Having multiple techniques which apply at different distance scales lets you play the strengths of one technique against the weaknesses of another. Meibom's technique will potentially help us get much more accurate data about a large volume of the universe.

Observation, not math, is what's important. (0)

Anonymous Coward | more than 2 years ago | (#36233508)

That's the GP's point exactly. While the math may work out, and maybe they've done a lot of it, it's never a replacement for real observation. In this case, it's just not possible at this time to perform proper observation, given that none of these scientists were around to directly witness the creation of these stars. Furthermore, we can't directly observe the creation of new stars today, and also ensure that millions or billions of years from now we can predict the age accurately.

Anything they come up with is no better than religion. It requires a slightly different sort of faith to believe in it, but it still requires faith nonetheless.

Re:Observation, not math, is what's important. (0)

Anonymous Coward | more than 2 years ago | (#36233544)

That's the GP's point exactly. While the math may work out, and maybe they've done a lot of it, it's never a replacement for real observation. In this case, it's just not possible at this time to perform proper observation, given that none of these scientists were around to directly witness the creation of these stars. Furthermore, we can't directly observe the creation of new stars today, and also ensure that millions or billions of years from now we can predict the age accurately.

Anything they come up with is no better than religion. It requires a slightly different sort of faith to believe in it, but it still requires faith nonetheless.

If this is all you have to say in response to what was answered, then I feel that you don't really understand science as a whole at all.

Re:Observation and math are both important (0)

Anonymous Coward | more than 2 years ago | (#36238590)

No, it's you that apparently doesn't understand science. Science should never be wielded like dogma. Scientific process is essentially based on empirical data collection and observation, testing and retesting, utilizing control sets for reference and a large enough sampling of data to adequately represent whatever is being tested. Seeing through a miniscule fraction of a time slice that is something like 0.0000000004 % (their 4 years of observation compared to say, the 10 billion year life span of a typical yellow dwarf star) obviously has some limitations with regard to both observation and a large enough sample of collected data.
A real scientist knows that his/her model or theory can be proven wrong someday, even if today it fits quite nicely with current observations - especially if the data is more theoretical or mathematical than observational. They could be right, but they could also be very wrong. .

Re:Observation, not math, is what's important. (2)

kurokame (1764228) | more than 2 years ago | (#36233568)

Good science is testable. Meibom's paper is an example of this. Your post and 36233236 are not; they are nothing more than a bare denial that anything in the universe is knowable. Yours additionally contains several factual errors, which would be more of an issue if the overall thesis wasn't a denial of the potential existence of true knowable facts.

Re:Observation, not math, is what's important. (1)

aristotle-dude (626586) | more than 2 years ago | (#36233642)

Good science is testable. Meibom's paper is an example of this. Your post and 36233236 are not; they are nothing more than a bare denial that anything in the universe is knowable. Yours additionally contains several factual errors, which would be more of an issue if the overall thesis wasn't a denial of the potential existence of true knowable facts.

You are still missing the point. Much of what we consider to be "modern" science is still a house of cards which is not built on a known quantity. In the classical scientific method, you have to have a known value that has been verified to be true. Mathematical models are not an adequate replacement for known values derived from direct observation.

Re:Observation, not math, is what's important. (2)

kurokame (1764228) | more than 2 years ago | (#36233682)

The scientific method is a toy model thrown together by philosophers to attempt what scientists do when they're doing "good science." Very few people in philosophy or science actually subscribe to it. The fact that it's taught as gospel in primary schools is somewhat depressing.

Your other objections are simply not a realistic depiction of modern science or of Meibom's study.

Re:Observation, not math, is what's important. (1)

aristotle-dude (626586) | more than 2 years ago | (#36235292)

The scientific method is a toy model thrown together by philosophers to attempt what scientists do when they're doing "good science." Very few people in philosophy or science actually subscribe to it. The fact that it's taught as gospel in primary schools is somewhat depressing.

Your other objections are simply not a realistic depiction of modern science or of Meibom's study.

Really? How fascinating. So what is the name of your science based religion? So you do not "believe" in controls in clinical trials then? Are you sure that you are not confusing philosophy with science?

Re:Observation, not math, is what's important. (0)

Anonymous Coward | more than 2 years ago | (#36236622)

It's more the fact that he made the mistake of assuming modern philosophers engage in reason instead of devising ever more wonderful recipes for hashbrownies and discussing the genius of Deepak Chopra.

Re:Observation, not math, is what's important. (4, Insightful)

doublebackslash (702979) | more than 2 years ago | (#36234448)

You are still missing the point. Much of what we consider to be "modern" science is still a house of cards which is not built on a known quantity.

Such as? I can think of string theory (a large unknown with no testable predictions and thus not used for any real science as of yet), dark matter (though we can observe the effects so we know it to be there, they just don't know what it is yet. Lots of very bad theories coming out of this one. I can say that because all but one of them is wrong, and we might not even have the one yet!), and dark energy (which recently got a big boost in observational evidence!)

In the classical scientific method, you have to have a known value that has been verified to be true.

No. Not always. The periodic table of elements was envisioned before there was enough evidence to prove that model correct. Ditto for gravity, "germs", electricity, relativity (general and special), and quantum physics. Those models predicted things that had yet to be observed based on mathematics and known quantities. Their predictions have been tested and proven right. Theories that were wrong are less remembered because they were wrong and abandoned. Steady state, for example.

Mathematical models are not an adequate replacement for known values derived from direct observation.

No, they compliment them. Models predict, observations confirm or contradict. Some models work well only sometimes, like gravity, but within their realm of usefulness they advance science to the point that they break down and science happens all over again. Constant testing and verification. This, just like every other useful model, will be tested mercilessly.

If you think that math and science are strange bedfellows then I encourage you to attempt science without mathematical models. I just don't see how a lack of modeling is an advantage. Clarify if you are willing, but I'm just not seeing it. Mathematics have become more important and much more the focus but it works and it has worked well. Saying that the models don't yield observable quantities nor come from observable quantities is just wrong. Some models take decades. DECADES (I can't stress this enough. Some of Einsteins predictions are still being tested today that have never been tested before such as frame dragging) before their predictions are testable let alone proven.

Re:Observation, not math, is what's important. (1)

aristotle-dude (626586) | more than 2 years ago | (#36235314)

Interesting that you should mention string theory. It is a perfect example of philosophy being referred to as "science". The problem with relying heavily on models is that researchers may be unconsciously steering design their experiments and interpreting their data with a bias in favour of their pet model so you end up with a similar phenomenon to how people can see "shapes" in cloud formations. People have a tendency to see patterns in what is chaotic data points.

In a nutshell, the research might be seeing what they expect to see rather than actual phenomenon that matches the model.

Re:Observation, not math, is what's important. (2)

doublebackslash (702979) | more than 2 years ago | (#36235414)

I even said that string theory is a large unknown. It is most likely bunk. Might not be, but that is not my area of expertise.

You mentioned that the models are "unconsciously" steering their experiments. They aren't. They are 100% fully aware of how the models are steering their experiments. That is how it works. Gotta test the model, how else do you test a model other than devise a test for it?

More importantly that is how plenty of models die. The tests show them to be wrong. Creativity certainly has its place in science, that is how the big leaps are made. Then they go and check and re check and publish and others do all that all over again.

Science is repeatable. If I can do something, so can you! Not only that but science intermingles. If someone comes up with the wrong idea but don't manage to prove it wrong then other pieces won't fit. Not only are the habits of scientists self-verifying but the science itself is too.

Most importantly if a scientist is wrong they will admit it. There will be whining, some gnashing of teeth, and all the human drama to some degree, but in the end it is either accept the evidence or be shamed out of a career. Truth (of this variety) has a way of becoming pervasive.

Again I ask, because I think this is key: What things specifically do you disagree with outside of those I mentioned? Some examples? Anything other than undirected mistrust?

Re:Observation, not math, is what's important. (1)

Thing 1 (178996) | more than 2 years ago | (#36261676)

Dark matter is our competition's Dyson spheres. Or Matrioshka Brains. They're well underway; we'd better get started, soon!

Re:Observation, not math, is what's important. (2)

TapeCutter (624760) | more than 2 years ago | (#36235330)

Mathematical models are not an adequate replacement for known values derived from direct observation.

I think you have misunderstood the last 500yrs science, mathematical models are not supposed to be a replacement for observations, they are a way to predict observations because you cannot observe what has not happened yet. No amount of past observations or scientific theory can give you a 100% guarantee about anything, if you want that kind of dogmatic certainty join a church.

Calculations are not direct observations. (0)

Anonymous Coward | more than 2 years ago | (#36233718)

No scientist has observed the creation and lifespan of even just one star. After all, that's not currently known to be possible, with entire human lifetimes being a mere 70 to 80 years, while stars apparently have a much, much longer lifespan.

Anything less than direct observation, including mathematical calculations, is a very poor form of science, if we can ever consider it to be science. While such theories may have a better foundation than, say, religion, they still can't truly be tested properly. Proper testing requires, at a minimum, repeatability and observability. If we don't have those, we have little more than speculation and assumptions. Neither of those are science.

Re:Calculations are not direct observations. (2)

martin-boundary (547041) | more than 2 years ago | (#36233880)

According to you, one can't know anything about a person unless one has observed and mathematically calculated their whole life 24/7 from the moment they were born to the moment they died.

You're a freak. Go troll the religious boards, as you obviously know nothing about Science.

Re:Calculations are not direct observations. (5, Insightful)

empiricistrob (638862) | more than 2 years ago | (#36233884)

That's not even remotely right.

Here's how science works (as it applies to astronomy):
- You form a hypothesis. In the case of astronomy this would most likely be a concrete mathematical model.
- Your model has predictions which you test.
- If the predictions are valid you look for more ways of testing the model. If not, you scratch it.

Observing the creation and death of one star is *not* necessary to test these models. There are an astronomical(!) number of stars to observe. You have plenty of stars in different stages of development to test the model with.

Certainly the model could be wrong, even if the data are consistent with it, but that does not make it unscientific.

Re:Observation, not math, is what's important. (3, Insightful)

martin-boundary (547041) | more than 2 years ago | (#36233820)

That's ridiculous. If you meet a person in the street, you don't need to have been present in the hospital when they were born to estimate how old they are.

Age can be measured as a consequence of physical behaviour of stars. The difficulty is only knowing exactly which quantities to combine to get the best estimate.

I won't comment on your other ignorant claim that we can't witness the birth of stars today.

Re:How do they know this is remotely valid? (0)

Anonymous Coward | more than 2 years ago | (#36233720)

How can you feel secure questioning their results? Are you an astrophysicist? If not, then the chances are any concerns you might have been able to raise have already been quashed by the scientists in question.

Re:How do they know this is remotely valid? (1)

damnfuct (861910) | more than 2 years ago | (#36234110)

If you point a telescope into deeper space you can effectively look further back in time. Maybe not the same thing as watching a single star get older, but it's better than sitting around for 6 billion years and watching paint dry. Hell, you might even be able to measure rotational period accurately enough to verify a theory.

Re:How do they know this is remotely valid? (4, Insightful)

Kjella (173770) | more than 2 years ago | (#36234206)

If you took photos of billions of people - but only one per person - could you get a decent idea of how humans age? Same with stars there's roughly 10,000,000,000,000,000,000,000 of them and there's many in every phase of life even though we pretty much only see a snapshot of each one. Take supernovas for example, a very short and rare event we haven't seen since 1604 in the Milky Way even though it has 200-400 billion stars. And yet we find 2-500 of them each year because there's so insanely many galaxies to look at. We won't have observed one star birth to death, but we will have observed everything from baby stars to stars in death throes many, many times.

Re:How do they know this is remotely valid? (0)

Anonymous Coward | more than 2 years ago | (#36237884)

The photo idea seems logical....until you get to the picture of Dick Clark!

Re:How do they know this is remotely valid? (0)

Anonymous Coward | more than 2 years ago | (#36261188)

There are strong maths to back up what they are doing. A lot of science uses math to show equation 'Q' must be correct and then spend millions of dollars looking for measurement data that are in line with those predictions.

Re:How do they know this is remotely valid? (1)

jon_doh2.0 (2097642) | more than 2 years ago | (#36234932)

Their measurements are not based on tracking these events as they unfold, like measuring distance with a perambulator. Thus the amount of time modern astronomy has been around for is irrelevant. Devising theories based on inferences from data, using logical reasoning, and attempts at falsification is a widely used method in science.
Shall i risk the obligatory comparison?
If i had a car...
No.
How about: Evolutionary biology has only been around for 150 years, how can these scientists feel secure measuring events that span millions of years?

Re:How do they know this is remotely valid? (1)

SanityInAnarchy (655584) | more than 2 years ago | (#36236366)

Evolutionary biology has only been around for 150 years, how can these scientists feel secure measuring events that span millions of years?

While I don't know the AC who made this particular anti-science rant, I would imagine the typical response to this would be "Yes! See? Evolution is wrong!"

The underlying issues is a lack of understanding and confidence in science. The only way I know of to deal with the lack of confidence is to work on the understanding.

Re:How do they know this is remotely valid? (1)

Unkyjar (1148699) | more than 2 years ago | (#36240316)

So how many years does a science need to be around before you feel secure in its ability to measure events that span millions of years? 200 years? 500 years? 1000 years? 10,000 years?

Re:How do they know this is remotely valid? (1)

khallow (566160) | more than 2 years ago | (#36236054)

Given that astronomy, in its modern form, only goes back a few hundred years at most (and even then, most of the knowledge was obtained within the past century), how can these scientists feel secure measuring events that span billions of years?

Light, for example, is the same, no matter what the age of the source or how long it took to get to Earth. They aren't measuring events that spanned billions of years, they're measuring things like light. The interpretation follows very sound observations.

Re:How do they know this is remotely valid? (2)

rgbatduke (1231380) | more than 2 years ago | (#36237432)

Because they build a chain of reasoning based on what is actually enormously well-known, mundane physics. We have only known about radioactive decay for just about one hundred years, but at this point we completely understand the process, understand how and why it represents a possible consistent clock, can validate the clock by looking back in time as we look at the light of stars given off in the remote past, and use it routinely to date things like the Earth itself, the fossil record, and the age of the solar system. There are over forty independent radioactive clocks used in radiometric dating, valid for overlapping ranges of dates from billions of years to thousands of years, and where they overlap they by and large agree.

We've known semiconductor physics for just about fifty years (a bit more, but only barely so). How can we feel secure that your computer, built on top of semiconductor physics, will actually work? Yet the physics that describes its operation -- being pure quantum physics involving things like crystal band structure, fermi surfaces, and so on is much more difficult than the physics of blackbody radiation, the fact that the intensity of light drops of like 1/r^2 with distance, and the relationships between power, temperature, and the size of a radiating object. How can we feel "secure" about Maxwell's Equations and the laws of thermodynamics? Because every minute of every day there are a few dozen powers of tens of confirmations of these equations visible to your naked eye, whether or not you know it. If the laws of thermodynamics were egregiously violated, if Maxwell's equations were not classically "true" in the classical limit, if quantum theory ceased to function reliably at the microscopic level, the existing structure of the Universe would catastrophically alter in ways you could hardly miss in the few nanoseconds before your brain chemistry (based on all of these things) ceased to function.

So if your question was intended -- as it seems to be -- to cast doubt on the entire chain of reasoning leading us to believe e.g. that our sun is between 4 and 5 billion years old, that the earth is just a bit younger, so gee -- perhaps the Universe is really only 6007 years old and the Bible is right after all, well, get a grip, you Anonymous Coward you! If it was a genuine question about how "these scientists" in general arrive at sound, reproducible conclusions that represent our best possible state of belief (so far) given the data and the entire network of evidence-supported, consistent beliefs known as "the laws of physics", "the laws of chemistry", etc, well, a good intro book on Astronomy would be happy to answer your questions and Enlighten you. I'd be happy to recommend one. Or you can easily answer your questions using the Internet -- wikipedia contains most of the answers if you start an astronomy wiki-romp and read a few articles on stars, the age of stars, and so on.

The answer to your question is basically that the Hertzsprung-Russell diagram (itself constructed using physics and parallax of the nearer stars) gives us a sound yardstick for Universal distances, one we leverage using e.g. Cepheid variables. Looking back in time (speed of light and distance) and using our knowledge of the fusion process and gravitational process that produces the energy radiated by stars we can make sound inferences about their age, so that the various star types in the HS diagram are themselves also a clock of their relative ages. Given a preexisting clock of stellar ages. "synchronizing" a new clock based on rotational angular momentum of stars of one type or another is straightforward, if tedious.

rgb

Re:How do they know this is remotely valid? (1)

arisvega (1414195) | more than 2 years ago | (#36237624)

Simple answer; By looking at numerous similar objects, each being at a different stage on its order-of-billion-years life, and by assuming the laws of physics are the same everywhere (and everywhen).

Seems commonsense in retrospect. (2)

PhxBlue (562201) | more than 2 years ago | (#36233394)

The thesis makes a lot of sense. If stars lose mass as they age (stellar winds), that's going to affect their spin. Of course, the tricky part is (a) doing the math and (b) proving it.

Re:Seems commonsense in retrospect. (1)

amRadioHed (463061) | more than 2 years ago | (#36233420)

That makes sense if you know the stars initial spin. I'm curious how that is known.

Re:Seems commonsense in retrospect. (0)

Anonymous Coward | more than 2 years ago | (#36233476)

Um...because they watch sunspots move across the surface of our star. And since sunspots can't walk they must be moving with the surface. Also there seems to be a general trend of things either revolving or orbiting or typically both in space....just hazarding a guess.

Re:Seems commonsense in retrospect. (1)

taktoa (1995544) | more than 2 years ago | (#36233542)

AFAIK, there's no way to detect sunspots from that far away. Additionally, sunspot position is linked to magnetic field changes more than stellar rotation.

Re:Seems commonsense in retrospect. (1)

taktoa (1995544) | more than 2 years ago | (#36233566)

Re:Seems commonsense in retrospect. (0)

Anonymous Coward | more than 2 years ago | (#36252558)

Additionally, some stars are so large, that some surface (=Photosphere) features can actually be "seen" with a sufficiently large telescope. The topmost picture in this article [discovery.com] shows some surface features of Betelgeuse. I don't know if this has ever been used to confirm the spin of any star.

Re:Seems commonsense in retrospect. (1)

element-o.p. (939033) | more than 2 years ago | (#36234430)

I understood PP to be saying "That makes sense if you know the star's initial spin characteristics (i.e., velocity, etc.)." -- not "That makes sense if you know that the star was initially spinning."

Re:Seems commonsense in retrospect. (1)

kurokame (1764228) | more than 2 years ago | (#36233580)

I haven't had time to read the paper yet, but if I was doing it, I'd use stars whose ages can be well-determined via other methods to form a statistical model.

Re:Seems commonsense in retrospect. (1)

devilspgd (652955) | more than 2 years ago | (#36233606)

Possibly as interestingly, is initial spin constant across all stars? All types of stars?

Still potentially interesting research.

Re:Seems commonsense in retrospect. (0)

Anonymous Coward | more than 2 years ago | (#36233726)

That's what I'm wondering also. Is there a minimum "ramp up" spin required?

Re:Seems commonsense in retrospect. (1)

Fractal Dice (696349) | more than 2 years ago | (#36233980)

I would expect the angular momentum of a star to be inherited from the shape/distribution/motion of the gas cloud that collapsed to form it. Given the variety of nebulas we see, I would not expect this to be consistent.

Re:Seems commonsense in retrospect. (3, Funny)

grcumb (781340) | more than 2 years ago | (#36234492)

That makes sense if you know the stars initial spin. I'm curious how that is known.

Well, in the case of Madonna, we were able to look at her early videos. Based on this, we calculated her age as 1.1873 x 10^16 years.

[insert Big Bang joke here]

Re:Seems commonsense in retrospect. (0)

Anonymous Coward | more than 2 years ago | (#36236468)

It's not so much about the maths as about the observations. The paper shows that for stars from a cluster, which presumably all have the same age, the scatter plot of colour vs. spin lies on a tight curve. This implies that the spin rate is a function only of age and initial mass. If you can calibrate that function using clusters of known age then you can use it to estimate the age of stars from their spin and colours. You can estimate the age of stars in clusters using conventional stellar evolution calculations and fitting to the HR diagram, so this takes you a long way.

(Actually _modelling_ the rotational evolution of stars is pretty hard.)

Obligatory: (0)

Anonymous Coward | more than 2 years ago | (#36233404)

Charlie Sheen must be ancient!

Re:Obligatory: (1)

newcastlejon (1483695) | more than 2 years ago | (#36233644)

I suppose that might be funny, but TFA tells us that stars' spin decreases with age.

Perhaps his supermassive ego is significant and your post is actually hilarious, but I always took Sheen for a big-headed, yet vacuous twit.

Sun Spin (1)

Obble (1680532) | more than 2 years ago | (#36233526)

So they assume an age based on the size / color of the star and then based on that assumption they will use the spin of the star as a guide to its age. This assumes that the older stars will always spin at different speeds from younger stars. Given that older stars will expand, and therefor be larger so will spin slower to keep it's angular momentum this raises the question

So how would they know the starting spin of the star?
if we have 2 "young" stars, 1 spins 1 per year, the other 1 per month, when they get old they will obviously spin at different speeds.

Also, does anyone know how long it takes the Sun to rotate? I remember it has 2% of the angular momentum of the solar system but the bulk of the mass,

(Warning, my christian troll views coming up, cover your hears if you don't like whats coming up)
Something to make you think:
Why does the Sun have 2% of the angular momentum if by the gas cloud theory says that it should be in the center of the solar system and therefor like a ballerina when it's arms are in, have the most angular momentum, but the Sun almost have none. the only theory I could think of is if the cloud wasn't spinning, but in that scenario, the planets would just fall into the Sun. Note: Angular momentum isn't effect by the size / mass of the Sun. The sun could be heavy or larger, the spin will be slower but the angular momentum would be the same.

Excellent. Now go repeat with another cluster (1)

syousef (465911) | more than 2 years ago | (#36233534)

...otherwise how do you know the relationship holds true outside of that one cluster. Perhaps there was something special about the conditions in which that cluster formed. These people aren't amateurs and probably have good reason to believe these relationships hold true elsewhere, but since we're talking science, credentials mean squat and it should be repeated.

Re:Excellent. Now go repeat with another cluster (1)

kurokame (1764228) | more than 2 years ago | (#36233662)

From the paper, they were using it to determine what stars were cluster members and what stars were field stars (i.e. stars which just happened to be in that direction). It has already been tested as a classifier to determine cluster members versus non-members. Whether it holds up as an absolute measure is another question, but it already holds "outside that one cluster" to a useful error rate.

Can it see through plastic surgery? (1)

VortexCortex (1117377) | more than 2 years ago | (#36233548)

Does this mean famous people won't be able to lie about their age anymore?

Re:Can it see through plastic surgery? (0)

Anonymous Coward | more than 2 years ago | (#36235676)

With all the spin from their press agents? Who could tell?

Where is the angular momentum going? (1)

Michael Woodhams (112247) | more than 2 years ago | (#36234168)

I'm happy enough with the idea that a star's mass determines its initial angular momentum (or, more likely, vice versa.) While not obviously true, it is certainly plausible. But once its angular momentum is set, how can it slow down? Here are all the possibilities I can think of:
* It expands (radius increases) and so it can spin slower for the same angular momentum. However, this would be very uninteresting - if we have luminosity and temperature, we already know the radius. Adding an extra measurement which correlates with radius would give us no new information. Also, main sequence stars change their radius very slowly. (But I'm not so sure about very young MS stars.)
* It redistributes its angular momentum from its envelope (which we observe) to its core. But this is the reverse of what I'd expect - the core would spin faster than the envelope, so any coupling between them would (rotationally) accelerate the envelope at the expense of the core.
* It sheds angular momentum via its stellar wind. But main sequence stars shed very little mass in their winds. (From memory, for the sun it is on the order of 10^-14 solar masses per year.) Even if some strange effect caused the wind to be expelled in the best direction for shedding angular momentum, I don't think this could give any appreciable slowdown. (Again, I'm not sure about very young stars.)
* It sheds angular momentum via its magnetic field interacting with its planetary accretion disk. This has the best chance, but still seems unlikely to me. My gut feeling is that even for a young star, the magnetic field won't be strong enough. Also, you need a large quantity of ionized gas close to the star for the magnetic field to interact with.

Can anyone help supply a plausible mechanism?

(I have an astronomy degree, but I've been out of the field for over a decade.)

Re:Where is the angular momentum going? (3, Informative)

Michael Woodhams (112247) | more than 2 years ago | (#36234402)

OK, I've done some cursory research (abstracts and intros of a few papers.) I didn't find a review, however it seems that there has been quite a bit written about such angular momentum transfers, and the age-rotational period-mass relationship for stars. (So this result is a step in an already developed field, not a breakthrough.)
There was mention of interactions with a magnetized solar wind (i.e. a combination of my points 3 and 4 above) and also something called the Tayler-Spruit dynamo, which I think is about angular momentum transport between the star's core and envelope. For a young star, you'd expect the core to rotate faster than the envelope (conservation of angular momentum during the contraction) but the sun rotates like a solid body - same rotation period for all depths (or as far as we can probe by helioseismology.) The Tayler-Spruit appears to be a possible explanation for how a middle aged star like the sun can rotate like a solid body.

Really? (0)

Anonymous Coward | more than 2 years ago | (#36236280)

I can't believe no one has realized that Gyrochronology to determine a stars age is futile. "How many gyros have you eaten mr. star?" "I don't know."

Perhaps Gynecology would be better. "Can you spread your legs for me?"

How old am I?

I read the article to understand ... (1)

Alain Williams (2972) | more than 2 years ago | (#36236854)

but it made my head spin. I know that I: am getting old; am red in the face; and am my father's son. Is this what they were talking about ?

James Jeans (0)

Anonymous Coward | more than 2 years ago | (#36237032)

Ya I know its a funny name.

But maybe the editors of the paper should try to read some of his literature...

Correlating a spin of a hydrogen cloud -> baby star -> full grown star and thus the age of the object goes back to the end of the 19th century and was described by him (and probably others) - though he missed a lot of other concepts like fusion processes at the time.

So no, this is not really news but probably something a lot of todays kids are simply not aware of.

Re:James Jeans (1)

cavreader (1903280) | more than 2 years ago | (#36237944)

It makes me happy to know that there are people in the world today that are not only capable of theorizing about and testing the universe around us but there are also those who will fund these types of activities without expecting a normal return on their investment.
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