Stellar Physics 1a: Star Formation
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- เผยแพร่เมื่อ 13 ก.ย. 2024
- Stellar formation from a collapsing dust cloud. This is the first video in the Stellar Physics series. In this video I go over:
- Summary of what will be covered in the Stellar Physics series.
- What is a star?
- The Jeans instability (Jeans mass and Jeans length).
- The Virial Theorem
- Maximum and minimum star masses
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Just a heavy equipment mechanic here and really have no understanding of the math but I was just so amazed that I watched the whole video.
Well thanks for watching! And for the feedback. As this was the first video in the series, I wanted to open with some interesting results, so I didn't delve too deep into deriving where some of the quantities come from. The subsequent videos are more thorough in deriving where the math comes from (at least that was my intention).
Very nice! Thank you very much. Although I wished to see a derivation of the virial theorem, the quality of video is really good! Wish you more subs and views, don't give up.
Thanks for the comment! You got the first comment of the channel 🎉. I'll plan on a virial theorem derivation for a future video, because it pretty long.
BTW I just up loaded a video on the sun's binding energy, showing that the Virial Theorem holds at least for a monatomic gas and gravity:
th-cam.com/video/Bf9S1spAvFM/w-d-xo.html
Thanks for the videos, they are very helpful! I love watching them on my daily walks!
You’re welcome! Doing astrophysics while you walk?! Very impressive.
Superb lecture. I am looking forward to seeing how this class progresses. Most of the material on astrophysics I have found on TH-cam is gee whiz stuff and not good for developing a real understanding of stellar physics. Good for you for making these videos! Thank you…
Thanks! Let me know what you think once you've gotten deeper into the series. I look forward to the feedback.
Great intro to a fascinating topic!!
Glad you liked it! Hopefully you'll enjoy the rest of series as much 🙂
Good video. Good explanations.
Thanks for the feedback... and for watching!
Very nice. Had to watch twice. So once fusion starts, the radiation pressure will cause expansion upward toward the surface. Force of gravity will increase as it goes up toward the surface until either it achieves an equilibrium between radiation pressure and gravity, or it reaches the surface and the star self-destructs. (This is NOT a super nova!)
The force of gravity increases as the star contracts (since the radius decreases). But the gas also heats up, increasing the pressure which counteracts gravity. Eventually the temperature is hot enough for fusion to start. This will provide the energy to support the star. The hotter the star, the higher the fusion rate. The star will settle to a stable point where the pressure balances out the gravitational force. This is covered in chapter 3 where I go over energy and stability: th-cam.com/video/EW3KB-_64YA/w-d-xo.html . Hope this helps!
@@physicsalmanac But is it possible for gravity to be low enough so that it is unable to balance the fusion pressure so that the fusion reaches the star surface? Or maybe fusion would not be sustainable.
@@edweinb if the mass is too low (less then about .05), then fusion will not occur, as the core will never get how enough. Once fusion occurs the star will find a stable equilibrium. Fusion only occurs in the stellar core, as it requires very hot temperatures. But the energy release by fusion makes its way out of the star, and is eventually emitted as light.
This was great! One addendum I would like to see is how the minimum temperature for fusion relates to the charge repulsion force between nuclei.
Thanks for the comment! I actually covered this in detail (at least qualitatively) in a later video on nuclear fusion basics:
th-cam.com/video/ADU5Qxh-b9g/w-d-xo.html
Hope you find it helpful!
Just a note that ħ is the reduced Plancks constant. Not the constant itself.
What do you mean when you say "the cloud will go unstable if the sound crossing time is greater than the free fall time"? When determining the free fall time, why are you using kinetic and potential energies? I am assuming that the kinetic energy of the gas is working against gravitational potential. Is that correct? If you don't mind me saying it would be nice if a few additional moments were given to explaining how and why specific equations are being used. Good to see a series of this kind, as there really isn't much of this on You Tube.
The sound crossing time is the time it takes a sound wave to cross the cloud. The free fall time is the time it would take an object at the edge to fall into the center. If it takes longer for a sound wave to move across the cloud then for objects at the edge to fall in, the cloud will not have time to adjust and re-equilibrate to perturbations at its edge... i.e. by the time info about the disturbance gets to the other side, the edge will already have fallen in. So it will be unstable. Hope this helps!
You are correct that the gravitational and kinetic energy are in equilibrium. But they are both essentially zero, at this point, as this cloud is very loosely bound. Technically the cloud is bound, meaning the total energy is slightly negative, but you can pretty much treat this cloud as having 0 temp and 0 gravitational potential energy to a good approximation. If you want to know more about energy considerations you check out the videos in chapter 3 of the series which deals with binding energy th-cam.com/video/EW3KB-_64YA/w-d-xo.html
@@physicsalmanac thank you
@@tomaffatigato1498 Thanks for watching! And for your comments!🙂
Thank you so much for such phenomenal lecture series!!
Just one minor thing which I might be wrong: at 16:00, when you solve for T, would the coefficient to be:
`4/3 me G^2', instead of
`12 me G^2'?
Yes I believe you’re right. Good catch. That would change the answer by a factor of about 5, but it’s a rough estimate anyway.
Wouldn't the outward pressure counteracting gravity be the gas pressure rather than radiation pressure?
It depends if the star is gas dominated or radiation dominated. The more massive the star the more radiation pressure it has. Small stars are almost entirely gas pressure. Generally it’s a combination of radiation and gas pressure. Check out the next video in this series on luminosity or my video on polytropes. I discuss this in more detail in both of these.
Can't particle bump onto other particles faster than the speed of sound?
I’m not sure I fully understand your question, but particles bumping into one another is what sound is.
Can you make a video about some book recommendations on stellar physics ? Such as star formation, star collapse I mean on the topics you talk about ?
You got it.