Before we discuss what happens if the Sun is replaced by a neutron star, to keep in mind, the Sun will never be a neutron star.
As the Sun gets older, the star that becomes the center of the Solar System will also evolve. Now, the Sun is still in the combustion stage of hydrogen. In 5 billion years, the sun will expand into a red giant and eventually end its life as a cold white dwarf.
Long before the Sun became a white dwarf, life on Earth was extinct or even moved to another planet.
But, let's suppose for a moment. What if the Sun is replaced with a neutron star. Will the Earth and the planets remain as they are today? Before we discuss what happened with the Solar System, how about we get acquainted with the neutron star.
The neutron star is the star that formed at the end of the life of a massive star. Normal stars like the Sun only end up as white dwarfs.
But, a massive star usually ends his life by exploding as a supernova. The outer part will be ejected at the speed of tens of thousands of kilometers per second while the center of the star will collapse into a very compressed object.
During collapse occurs in the center of the star, a very strong force of gravity will continue to pull matter toward the center so that eventually the electrons penetrate the nucleus of protons and neutrons. Protons and electrons join together to form neutrons. As a result, formed a gas which rich neutrons.
In a very compressed condition (very high density), the pressure is also very high. As a result, the neutrons degenerate and give backpressures that stop the shrinking in the center of the star. Ultimately, neutron stars are formed with a diameter of only 20 km. But, its mass is equivalent to the Sun mass, that is 1.4 solar masses. Compare with the Sun 1.4 million km in diameter (and accommodate 1 mass of the Sun). That means all matter is compressed in a very narrow space.
Neutron Star in the Center of the Solar System
The attraction in this neutron star is enormous or about ~ 2 billion times stronger than Earth's gravity.
So, when we replace the neutron star as the center of the Solar System, the planets will be drawn by the gravitational force of a very large neutron star. As a result, the planets are also destroyed and the material is drawn toward the neutron star. Could be, splinter planets in the Solar System destroyed by the pull of this neutron star will form the accretion disk around the star.
The further an object of a neutron star is, of course, the pulling force that affects the object will be weaker. Had the sun replaced with a neutron star, then surely life on Earth would also become extinct.
If what we replace is not a neutron star but a massive star who will become a neutron star then the Solar System will not survive.
The inner planets will get closer to the star whose radius is large, and potentially starved when it evolves into a red giant star. When the star explodes and the center of the star collapses, the explosion will catapult planets from the Solar System.
So that's the fate of the Solar System if the sun is replaced by a neutron star.
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