(1) Celestial bodies maintain shapes through the balance between self-gravity and outward pressure gradient. Interestingly, the planets, stars, white dwarfs, and neutron stars in this figure have different pressure origins and are distributed at different inclinations.
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(2) For planets, the outward pressure is the Coulomb repulsion of atoms and ions. For stars, it is the gas and radiation pressure originating from the energy generated by nuclear fusion. As the mass increases, the radius of the celestial body also increases and expands.
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(3) In the case of Coulomb repulsion, approximating the distance between particles, and thus the density, is almost constant; the mass M is proportional to the cube of the radius R. In the case of stars, considering their microscopic physics, M is only proportional to R.
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(4) On this log-log diagram, the constant density is an upward rightward line with slope 3. The right side of the line is less dense, and the further to the left, the denser it is. So a star with a large mass is fuzzier than a planet and less dense as a whole.
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(5) Planets and stars are distributed in an upward rightward direction, whereas white dwarfs, which are supported by the degeneracy pressure of electrons, are distributed in a rather downward rightward direction.
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(6) This is because as the mass increases, the star cannot be supported without increasing the density by decreasing the radius and increasing the degeneracy pressure to compensate for the increased gravity.
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(7) The leftmost one, neutron stars, is just as dense as an atomic nucleus, also regarded as a giant nucleus. The pressure originates from the nuclear force and the degeneracy pressure of neutrons. It is slightly downward to the right, but rise steeply at a radius of about 12 km.
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(8) On the far left is a black hole. They are already collapsed, so there is no pressure to resist gravity. The boundary of the black hole is drawn with the Schwarzschild radius. This means that the mass M and the radius R are proportional.
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(9) In terms of the evolution of stars, as mass accretion continues, stars move upward where the mass is larger. Eventually, when the central density becomes high enough, the probability of particle collisions increases, and nuclear fusion ignites above the hydrogen burning line.
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(10) In the case of a low-mass star, the outer layers are scattered, and finally, the star's central part becomes a white dwarf. This is a transition from left to right regions.
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(11) In more massive stars, when nuclear fusion ends, there is no outward pressure. They undergo gravitational collapse and move from right to left, making a neutron star or a black hole. This is a supernova explosion or gamma-ray burst.
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(12) We can also see from this figure that as the mass increases in a black hole, the density, evaluated from the Schwarzschild radius, is lower than some of other celestial bodies.
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(13) The 1.4 solar masses is the maximum mass of a white dwarf (Chandrasekhar limit mass), and about 3 solar mass is the estimated maximum mass of neutron stars. The horizontal lines depict these.
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(14) I tweeted these as a caption of this figure. This diagram has been presented in several textbooks, but I reworked it in my own way. That was fun.
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キューブサット衛星NinjaSatでX線天文学
宇宙線で月の水探しと月面天文台へ
X-ray astronomer / Citizen science / Collective power of science!