How do they know it?

[Airbrush]

I love science programs, but my pet peeve is when they just state facts without giving a clue HOW they know it. For example, they will always say our Sun will last for a few more Billion years before swelling into a red giant and destroying the inner planets, probably including Earth. How do they know the sun will last so long? Maybe they are way off, and the Sun will become unstable in Millions or even thousands of years from now.

Edited February 2, 2012 by Airbrush

[Schrödinger's hat]

I love science programs, but my pet peeve is when they just state facts without giving a clue HOW they know it. For example, they will always say our Sun will last for a few more Billion years before swelling into a red giant and destroying the inner planets, probably including Earth. How do they know the sun will last so long? Maybe they are way off, and the Sun will become unstable in Millions or even thousands of years from now.

 

One way of determining this (not necessarily the only way) is by using empirical measurements and observing other stars.

 

If you look at enough of them, you can generate something called a Hertzsprung Russell diagram

http://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram

 

There's a lot of info not on one of these that you can get from the spectrum of a star, but suffice to say for now that we know that our sun is on the main sequence.

 

We then go and look at various star clusters.

There's a range of ways of finding the age of a star cluster (we know from some simple energy equations, and measuring luminosity, that big stars burn fast, for example). You can measure the ratios of large to small stars, count the number of supernovae (and remnants thereof) and stellar formation events, and so on.

 

Once you do that, you start to notice that stars follow a pattern based on their age.

Depending on their mass (and I don't feel like going into the mass calculations right now) and metallicity (basically how much of the star is made of heavier elements) they follow a fairly standard pattern as they age.

 

Stars above a certain mass get brighter and hotter as they run out of fuel, and then finally start burning Helium and turn off of the main sequence. You can also predict where on the main sequence a star of given mass and metallicity will be at a given age from your HR diagram.

(more here) http://en.wikipedia.org/wiki/Main_sequence#Evolutionary_tracks

 

 

There are plenty of things I have skimmed over, or ignored here feel free to ask more on any point I've glossed over.

This is a subject somewhat bigger than one forum post can contain. As such most documentaries tend to leave it at 'because that guy said so'. Adding a little bit more would be 'because of blah, because that guy said so' so there's not really much point.

The only alternative is to assume your watcher has a large amount of patience and can follow a more technical explanation while you explain the whole lot (and probably take a lot longer to do so).

Sadly documentary writers seem to be expecting less and less of their audiences recently.

[Airbrush]

Fantastic answer SH! Thanks for your reply and insight. I need more time to study your presentation.

 

I'm wondering if it is possible to summarize the chain of reasoning to arrive at a certainty that our Sun will last a few Billion more years. You are correct about the "reason is blah, blah, blah..." but it must be possible to put it into simpler, more understandable terms, so the non-expert can get a good idea about how they know this, in a FEW words, before you lose their attention.

Edited February 3, 2012 by Airbrush

[Schrödinger's hat]

Fantastic answer SH! Thanks for your reply and insight. I need more time to study your presentation.

 

I'm wondering if it is possible to summarize the chain of reasoning to arrive at a certainty that our Sun will last a few Billion more years. You are correct about the "reason is blah, blah, blah..." but it must be possible to put it into simpler, more understandable terms, so the non-expert can get a good idea about how they know this, in a FEW words, before you lose their attention.

 

Any simplification (including the one I gave) leads to another point where you can point and say, 'how do you know that?'. You could say something along the lines of 'by looking at other stars we know that....', but that doesn't say anything about why we think the sun will act in a similar way to other stars.

[Widdekind]

Is it not true, that predicted stellar lifetimes, are based upon human computer simulations ?? Surely, no human has ever "sat and watched the star grass grow" for millions to billions of years, and "stopped their stop-watch" when the star exits MS ??

[swansont]

Is it not true, that predicted stellar lifetimes, are based upon human computer simulations ?? Surely, no human has ever "sat and watched the star grass grow" for millions to billions of years, and "stopped their stop-watch" when the star exits MS ??

 

Yes, it is true. Your point?

 

I'm wondering if it is possible to summarize the chain of reasoning to arrive at a certainty that our Sun will last a few Billion more years. You are correct about the "reason is blah, blah, blah..." but it must be possible to put it into simpler, more understandable terms, so the non-expert can get a good idea about how they know this, in a FEW words, before you lose their attention.

 

Depending on the level of detail you want, it might require a few hours or a few dozen hours to explain. Some of the things we know are based on having studied the subject in college and graduate school. Much of the detail is decidedly not simple, so the expectation that a detailed explanation can be made simple isn't realistic.

 

edit to add a video that Schrödinger’s hat had pointed to some time ago:

[StringJunky]

Airbrush

 

I don't know if you might find "Stardust" by John Gribbin a useful start on your question...it's a popsci book on stellar recycling...I enjoyed it as a layman and it's not expensive in paperback or ebook.

Edited February 4, 2012 by StringJunky

[D H]

I love science programs, but my pet peeve is when they just state facts without giving a clue HOW they know it.

I feel your pain. One problem here is that giving a solid clue as to "how they know it" requires a basic understanding of the underlying science. People only start getting that basic understanding in the first year of a graduate program.

 

Another problem with many of those popular science programs is that they often overstate things as fact. How are you to know that what you just saw on a TV show presented as fact is in fact they a highly conjectural consequence of one of but many interpretations of quantum physics? (Note that no interpretation of QM of has yet been successfully merged with general relativity.) I'm writing here of the "physics as mystical woo" nonsense presented on shows that feature the likes of Sean Carroll, Brian Cox, Brian Greene, or Michio Kaku as commentators. A noted exception is Neil deGrasse Tyson. He's usually pretty good on distinguishing conjectural from mainstream science.

 

When you see some of this "physics as mystical woo" nonsense and yell at the TV "HOW to they know that? They can't know that to be fact!": You are absolutely right. This mysticizing of physics, astronomy, and cosmology and intermingling fact, conjecture, and fiction as one in my opinion does more harm than good.

 

 

For example, they will always say our Sun will last for a few more Billion years before swelling into a red giant and destroying the inner planets, probably including Earth. How do they know the sun will last so long? Maybe they are way off, and the Sun will become unstable in Millions or even thousands of years from now.

In this case the case is extremely strong that this is true. Scientists have a fairly good understanding of what goes on inside stars, of how stars are born, how they change as they age, and how they ultimately die. It took quite a bit of work to come up with this understanding. Theoretical physicists, experimental physicists, and astronomers contributed to the effort. Even geologists and biologists played a role. Physicists scoffed when geologists and biologists first started coming up with numbers in the billions of years for the age of the Earth. That evidence from geology and biology was incontrovertible. It forced physicists and astronomers to reevaluate their theories. The current theory is quite deep and is very consistent with all of these multiple lines of evidence.

Edited February 4, 2012 by D H

[IM Egdall]

My pet peeve on popular science TV programs (and in some popular science books) is that they talk about new unsubstantiated physics without emphasing there is as yet no supporting evidence for these theories, e.g. string theory.

[swansont]

My pet peeve on popular science TV programs (and in some popular science books) is that they talk about new unsubstantiated physics without emphasing there is as yet no supporting evidence for these theories, e.g. string theory.

 

I think that much of science journalism shares this trait.

[John Cuthber]

I think that much of science journalism shares this trait.

[Widdekind]

Yes, it is true. Your point?

 

I recall Prof. Icko Iben telling our class, that stellar ages are predicted, from 1970s-era, 1D, spherically-symmetric, hydro-dynamics simulations; incorporating no magneto-hydrodynamics; incorporating no stellar rotation; incorporating mass-loss as an artificial factor, numerically imposed as if by "numerical fiat". From Carroll & Ostlie's Intro. to Mod. Astrophys., I learned that, when simplifying assumptions are involved, astrophysical calculations are expected to be accurate, only to within "a factor of a few". Er go, if a simple 1D star code estimates some stellar life-time to be, e.g. 10Gyr; then I would expect that the corresponding actual star life-time to be 3-30Gyr. Are star-codes more accurate, than in my stated understanding ?

[swansont]

I recall Prof. Icko Iben telling our class, that stellar ages are predicted, from 1970s-era, 1D, spherically-symmetric, hydro-dynamics simulations; incorporating no magneto-hydrodynamics; incorporating no stellar rotation; incorporating mass-loss as an artificial factor, numerically imposed as if by "numerical fiat". From Carroll & Ostlie's Intro. to Mod. Astrophys., I learned that, when simplifying assumptions are involved, astrophysical calculations are expected to be accurate, only to within "a factor of a few". Er go, if a simple 1D star code estimates some stellar life-time to be, e.g. 10Gyr; then I would expect that the corresponding actual star life-time to be 3-30Gyr. Are star-codes more accurate, than in my stated understanding ?

 

Do Carroll & Ostlie explain to which models they refer in their claim?

[Realitycheck]

How can we know that the density of some stars are a hundred thousand pounds per square meter or so? Because that's how dense they would have to be in order to last that long? Because that's how much you could fit in that given area with that much mass and gravity bearing down on it? At what point does it all add up and agree with each other that you convince yourself? All because a bomb produced an explosion of size x?

[imatfaal]

How can we know that the density of some stars are a hundred thousand pounds per square meter or so? Because that's how dense they would have to be in order to last that long? Because that's how much you could fit in that given area with that much mass and gravity bearing down on it? At what point does it all add up and agree with each other that you convince yourself? All because a bomb produced an explosion of size x?

 

 

The fact that it all fits together and makes reliable predictions - we have not found any experimental results that makes us think it might be wrong. Gut instinct that "this just cannot be the case" is trumped every time by experimental proof that it is. we get the experimental proof by various means - in talking about density and mass of stars, we have never stuck the sun or any of the planets on a set of weighing scales, yet our theories allow us to compute intricate orbits and trajectories that do end up with probes landing on mars, satellites orbiting moons of saturn, pairs of spacecraft setting themselves up on exactly opposite sides of the sun...