Even the NASA website refers to stars as "big ball of gas". It's a common statement that's popularized by movies and popular culture. However, stars are technically not in a gaseous state - they're in a state of plasma.

So do stars qualify as being made of "gas" as the proverb would suggest, or is that considered a misconception?

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    $\begingroup$ It would be pedantic to do so. Most astrophysicists do not worry about the strict definition of a plasma. We know the Sun contains charged particles - but it does (almost) obey ideal gas laws. The "burning" phrase is more troublesome. Burning is a chemical oxidation reaction and does not really describe nuclear fusion. $\endgroup$ – Rob Jeffries Jan 27 '16 at 20:07
  • $\begingroup$ I'm voting to close this question as off-topic because you are asking about semantics not astronomy $\endgroup$ – James K Jan 27 '16 at 20:23
  • $\begingroup$ Related: Why does the Sun really shine? $\endgroup$ – pela Jan 27 '16 at 20:43
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    $\begingroup$ @JamesKilfiger I'm done editing. I want to be clear that I am not asking about semantics. My question really boils down to whether the physical state of matter in stars qualifies as gaseous. $\endgroup$ – JSideris Jan 27 '16 at 20:54
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    $\begingroup$ Yes, that is my point. Definitions = Semantics. You already know that stars are formed of plasma, and they they are powered by nuclear fusion. Poetic language does allow for flexibility in English. Scientific language tries for precision. Would you use "big balls of gas" in an MSc thesis? No. On a popular website? Yes. But that is a question of the use of English, not astronomy. $\endgroup$ – James K Jan 27 '16 at 20:57

While I agree that it boils down to semantics, I actually disagree on the scientific use of the term in the comments. In astronomy, we know there's a difference between gas and plasma, but we almost always use the term "gas" when talking about what's in stars. E.g. "the fraction of gas locked up in stars" (as opposed to in the interstellar medium). We also talk about the metallicity of gas, meaning both gas and plasma. I think we only use the term plasma when specifically discussing properties that are unique to plasma, like being "frozen" in magnetic fields. And googling ["ionized gas" -plasma] (the '-plasma' is to get rid of articles explaining what plasma is) returns 243,000 hits, of which by far the most seem to be scientific papers and websites.

So I think the answer is: "No, stars aren't made of gas, they're made of plasma, but go ahead and call it gas, you won't offend any astronomer."

However, I agree with Rob Jeffries that we don't like when you call it burning gas. It's nuclear power! It's a million$^\dagger$ times more powerful than fire!

$^\dagger$Why "a million"? Because nuclear reactions are measured in MeV, while chemical reactions are measured in eV.

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