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    Hello guys,

    I was just revising the topic 'The Big Bang'.
    I found it very hard to understand some stuff about the star's life history.
    I was wondering if someone could explain it to me (hopefully in least complex detail as possible as I'm a GCSE student)

    1. Why does a star lose gravitational potential energy? - isn't that to do with height. But, then I thought of it as since when the star contracts, it becomes smaller and in the 'red giant' stage - it loses its mass.

    So, it will lose gravitational potential energy because its mass decreases?

    2. Why does the core of a massive star, much bigger than the Sun, collapse?

    I wrote these two answers in my revision notes - but I'm not sure which one is correct, or if any are correct.

    This happens because there is no more fuel touse and no more pressure, so the gravity causes the atoms to get closertogether increasing the density and temperature of the star’s core – causing theatoms to collide and collapse.

    It can also happen because there is no nuclearfusion occurring so the core contracts, shrinking in size and eventually it canno longer maintain its size and the star collapses.

    3. My revision guide says 'Thestar is now visible and remains visible while there is enough hydrogen'.

    Does that mean when there is no hydrogen left, obviously no nuclear fusion can take place and the star dies - so it disappears (invisible)? I'm using my brain for these answers, so they are probably wrong :/.

    4. Why is the gravitational force of the black hole so large when its mass is smaller than the massive star that formed it?
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    (Original post by Chittesh14)
    Hello guys,

    I was just revising the topic 'The Big Bang'.
    I found it very hard to understand some stuff about the star's life history.
    I was wondering if someone could explain it to me (hopefully in least complex detail as possible as I'm a GCSE student)

    1. Why does a star lose gravitational potential energy? - isn't that to do with height. But, then I thought of it as since when the star contracts, it becomes smaller and in the 'red giant' stage - it loses its mass.

    So, it will lose gravitational potential energy because its mass decreases?

    2. Why does the core of a massive star, much bigger than the Sun, collapse?

    I wrote these two answers in my revision notes - but I'm not sure which one is correct, or if any are correct.

    This happens because there is no more fuel touse and no more pressure, so the gravity causes the atoms to get closertogether increasing the density and temperature of the star’s core – causing theatoms to collide and collapse.

    It can also happen because there is no nuclearfusion occurring so the core contracts, shrinking in size and eventually it canno longer maintain its size and the star collapses.

    3. My revision guide says 'Thestar is now visible and remains visible while there is enough hydrogen'.

    Does that mean when there is no hydrogen left, obviously no nuclear fusion can take place and the star dies - so it disappears (invisible)? I'm using my brain for these answers, so they are probably wrong :/.

    4. Why is the gravitational force of the black hole so large when its mass is smaller than the massive star that formed it?
    4. the mass is concentrated into a very much smaller volume so you get a very strong field strength at small distances from the BH... You might think there wouldn't be a difference between being deep below the star's surface, near the center of mass of a star and being near a black hole of similar mass. Inside a star (or planet) the gravitational field strength goes down as you move from the surface to the centre of mass - this is because if you are below the surface you have some of the stars mass above you.

    3. This sounds like a description of a star on the Main Sequence (see Hertzsprung-Russel diagram in your notes) - stars will fuse other heavier atoms when the hydrogen is used but this puts them into the Red Giant area of the HR diagram (high luminosity, low temperature). eventually there is no more energy available from fusion and a massive enough star might have a core collapse.

    2 & 1 Gravity is trying to compress the star, radiation pressure from fusion is trying blow the gas away from the core. For most of the stars life it's size is the size at which the forces are in equilibrium. There is a suprising amount of stored (gravitational potential) energy in a large star and if fusion ends abruptly the gas falling inward causes a shock wave violent enough to eject the electrons from the core and crush the neutrons together - which is the core collapse

    This is my attempt at explaining stellar evolution, core collapse, supernova, neutron star / black hole formation... the big bang is something different.
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    (Original post by Joinedup)
    .
    4. So basically, below the surface - you have the most gravitational field strength/attraction - and that's where the black hole forms or is present?

    3. Thank you, I do not know about the HR diagram as I'm still doing GCSE - maybe that's why. But I've learnt a lot in this topic, spent so many hours just reading articles to be honest about the topic and researching.

    2. What is radiation pressure?

    1. So, the star loses its gravitational potential energy when the shock wave crushes the neutrons together and the core collapses?

    Regarding the topic, it is covered under the Big Bang lol as it links to the Universe etc.
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    (Original post by Chittesh14)
    4. So basically, below the surface - you have the most gravitational field strength/attraction - and that's where the black hole forms or is present?

    3. Thank you, I do not know about the HR diagram as I'm still doing GCSE - maybe that's why. But I've learnt a lot in this topic, spent so many hours just reading articles to be honest about the topic and researching.

    2. What is radiation pressure?

    1. So, the star loses its gravitational potential energy when the shock wave crushes the neutrons together and the core collapses?

    Regarding the topic, it is covered under the Big Bang lol as it links to the Universe etc.
    4. no, the graph of gravitational field strength for a uniform solid sphere looks like this...



    zero at the centre,radius zero (r= 0)
    increases linearly between the centre and the surface (r<a)
    maximum at the surface (r=a)
    decreases between the surface and infinity following the 1/(r^2) law

    stars and planets aren't exactly uniform spheres but it's close enough

    a star doing it's steady business is in equilibrium - the pressure at the core is high but not high enough to make a neutron star or B/H.

    the gravity acting inwards is in equilibrium with the radiation pressure pushing outwards, radiation pressure is just pressure on the stellar gas caused by radiation from the fusion in the core.

    when the fusion stops due to the fuel being exhausted, the material the outer layers of the star are made of falls towards the centre under gravity and violently hammers the core making the pressure there greater than it ever was while fusion was taking place, only for a short time but it's enough. might be worth thinking about the difference between placing a hammer on top of a nail and then trying to push the nail into some wood and using the hammer properly by striking the head of the nail at some speed.
    having the momentum of the hammer head build up over a great distance and then stop abruptly creates a much greater pressure on the nail than just pushing on it steadily.

    note core collapse only happens in the largest stars - not stars like the sun.

    the HR diagram is central to understanding stellar evolution - it's in the astronomy gcse. if you're not being taught it I guess it's not going to be part of your physics exam, but you could always teach yourself if you're interested e.g. http://www.atnf.csiro.au/outreach/ed...n_hrintro.html
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    (Original post by Joinedup)
    4. no, the graph of gravitational field strength for a uniform solid sphere looks like this...



    zero at the centre,radius zero (r= 0)
    increases linearly between the centre and the surface (r<a)
    maximum at the surface (r=a)
    decreases between the surface and infinity following the 1/(r^2) law

    stars and planets aren't exactly uniform spheres but it's close enough

    a star doing it's steady business is in equilibrium - the pressure at the core is high but not high enough to make a neutron star or B/H.

    the gravity acting inwards is in equilibrium with the radiation pressure pushing outwards, radiation pressure is just pressure on the stellar gas caused by radiation from the fusion in the core.

    when the fusion stops due to the fuel being exhausted, the material the outer layers of the star are made of falls towards the centre under gravity and violently hammers the core making the pressure there greater than it ever was while fusion was taking place, only for a short time but it's enough. might be worth thinking about the difference between placing a hammer on top of a nail and then trying to push the nail into some wood and using the hammer properly by striking the head of the nail at some speed.
    having the momentum of the hammer head build up over a great distance and then stop abruptly creates a much greater pressure on the nail than just pushing on it steadily.

    note core collapse only happens in the largest stars - not stars like the sun.

    the HR diagram is central to understanding stellar evolution - it's in the astronomy gcse. if you're not being taught it I guess it's not going to be part of your physics exam, but you could always teach yourself if you're interested e.g. http://www.atnf.csiro.au/outreach/ed...n_hrintro.html
    Thanks a lot! . The information provided has helped me a lot and hopefully I remember it now!
    I will take a look at the HR diagram just to understand the stellar evolution :P
 
 
 
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