h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

1. How do scientists know the composition of the sun, let alone another star? It seems to me that the only ways of determining this is by examining the waste that comes out of the sun--the solar wind--which is, if I recall correctly, mostly alpha particles (a proton and a nucleus). Is my information correct? If so, how does this support the theories about the composition of the sun? And why should scientists assume that all other stars are just like our sun?

2. How long have scientists been observing stars? About five hundred years at the most. Two hundred years is as much as I would venture, because Galileo didn't invent the Hubble telescope. So what makes scientists think that it takes billions of years for a star to decay? And if it does, how can they plot out the life cycle of a star? They've only seen about one two-millionth of the span at most. It would be like plotting a normal animal life by viewing many animals at once for ten minutes.

3. Scientists can date the universe by the oldest stars they find, right? But how do they find the age of the star? Or they take the distance of the furthest stars they can see and multiply that by the speed of light, when they use the size of certain stars that are supposedly all the same size to guage the distance, but they haven't gone to those stars to see their sizes. It's all theories and guesses made to fit their notions on the age of the universe. And it brings me to the next point.

4. How do scientists determine the size of a star? By the type of star it is? But then how do they determine that? Is it by the size and color of the star?

~Gareis, ravenous eater of knowledge

First of all: I'm not a real expert, only a *solid* state physicist. But I'll try my best.

1. You can examine the spectrum of the solar light to find the absorption lines that are some sort of fingerprint for the different elements. (By the way, when physicists did this for the first time, they found lines of an element that was unknown at that time. Since it existed on the Sun, they called it 'Helium'.)

But by examining the spectrum, you only get the ingredients of the outest layer of the sun. The rest is competent extrapolation: In order to have a working fusion within the star, you need a certain amount of Hydrogen which has been partly converted to Helium. I think you cannot see these things directly, but otherwise the star wouldn't be stable or have another colour or even wouldn't be shining at all.

I don't know exactly what the solar wind consists of, but an Internet research should be simple. Alpha particles are Helium4 nuclei -- two protons and two neutrons *forming* a nucleus. But also beta particles (ordinary electrons) are in the solar wind.


2. I think it bases on thermodynamic calculations. You know, one thing is pretty easy to find out: The surface temperature. And as far as the Sun is concerned, we also know mass and radius. This is enough so make good assumptions about eg core pressure and temperature. Then you can determine how much Hydrogen is converted to Helium per year, and then you know when it will become critical for the star.


3. Well, actually we know the age of the universe by observing the movement of far galaxies. But your assumption with "measuring the distance to the furthest stars and multiply that by the speed of light" is also rather close to the truth I think.

The age of the star is known by mere guessing. Astrophysicists try to find the "way" that the star takes during his life, especially how its spectrum and its real brightness (in Watt) change. Then you can try to estimate the age of a given star.


4. It's very difficult to measure the diameter of a star directly. I don't know latest developments here. But most stars are mere dots in the telescopes and will remain so. However I know that eg Betelgeuse's diameter could be measured rather accurately. This was many years ago. Things have improved surely.

The rest is estimated by using eg the 'HR diagram'. I wrote an article about it: A645086.

Lol, thanks, similar to what I was going to say, had I been able to find the words.

I think it's also a good idea to add that gaining scientific knowledge purely by observation takes a long time, and in this instance is not at all practical. Looking at many of the scientific achievments of the human race you will find that very few of the more advanced theories have been devised through direct observation. It is, in essence, very difficult to do and would mean that we, as a species, would not have advanced very far at all.

Using your 'observing 100 animals for 10 minutes' metaphor- I believe it would be just as absurd to try observing just one animal for its entire lifetime. The ideal is a mixture of the two- observing what you can, and then extrapolating from that. Indeed, over time, what you observe may change your 'accepted' theory, but waiting for billions of years just because you cannot be 100% certain you are correct isn't very sensible and won't get you very far.

In short, a lot of science is 'guesswork', but it is all educated guesses- not just blind fumbling in the dark.

TK