PDA

View Full Version : Mass and luminosity discussion moved from feedback



Jeff Root
2016-Jan-01, 11:53 PM
The idea that the pressure must be higher in the center of more
massive stars than it is in the center of less massive stars is just
an extension of the fact that the pressure *is* higher in the center
of more massive bodies than it is in the center of less massive
bodies, in a lower mass range. The only reason the pressure
*isn't* higher in more massive stars is that the star is so bloated
that a significant portion of its mass is lifted higher in the star's
gravity well, where it has less weight. That's the opposite of what
happens in smaller bodies, like planets.

-- Jeff, in Minneapolis

Ken G
2016-Jan-02, 02:35 AM
The idea that the pressure must be higher in the center of more
massive stars than it is in the center of less massive stars is just
an extension of the fact that the pressure *is* higher in the center
of more massive bodies than it is in the center of less massive
bodies, in a lower mass range.The important thing about science is that it is expressly designed to be a kind of antidote to what seems like it ought to be correct, but isn't. However, you have to actually follow the rules of science to get the benefits of science. The rules are, you don't just go with what sounds right, you go with what tests out. So you have to actually do the test. When you do the test, you find out that more massive main-sequence stars (the kind of stars that that "explanation" is supposed to hold for) have less core pressure than low-mass stars. In fact, this is quite an important thing to know about high-mass stars, it is part of the reason why radiation pressure is more important for them, and it is also part of the reason why degeneracy pressure is less important for them.

It's actually very easy to see that the core pressure of more massive stars has to be less than lower mass main-sequence stars. It's all explained above, early in the thread.

But The only reason the pressure
*isn't* higher in more massive stars is that the star is so bloated
that a significant portion of its mass is lifted higher in the star's
gravity well, where it has less weight. That's the opposite of what
happens in smaller bodies, like planets.Er, no.

Jeff Root
2016-Jan-02, 05:26 AM
No to what? The word "only"? The idea that the star is so bloated
that a significant portion of its mass is lifted higher in the star's
gravity well, where it has less weight? That that's the opposite of
what happens in smaller bodies, like planets?

-- Jeff, in Minneapolis

Jeff Root
2016-Jan-02, 08:56 AM
I just realized this is in "Feedback" rather than some other area.

-- Jeff, in Minneapolis

slang
2016-Jan-02, 11:02 AM
Some posts from this feedback thread (http://cosmoquest.org/forum/showthread.php?154582-Correcting-errors-in-the-quot-Explore-quot-encyclopedia) moved to a new thread, in Astronomy rather than Q&A as it's more a discussion than a question.

Ken G
2016-Jan-02, 02:28 PM
No to what? I now see that I misread your point-- I thought you were presenting an argument that core pressure is actually higher in higher mass stars. Instead, you were explaining the source of that error! I should not have read over the "in a lower mass range" part of your statement, that made all the difference because the lower mass range brings in very different physics (basically, degeneracy and intermolecular forces). So I now see what you meant, and I completely agree with you. The contrast between what happens with rocky planets (which have a nearly fixed density), and main-sequence stars (which have a nearly fixed core temperature but very different density), is very much the source of the wrong reasoning, you're right. I should have read more carefully! I'm glad slang moved this discussion, my misreading of your point would have seriously distracted from that other thread. But your original post could be moved back into that thread, it is perfectly apropos.