However, when they pull their arms in (and thus decrease their radius), they will spin faster. If the skater starts to spin with his or her arms outstretched, they will spin slowly. The best analogy for this physical effect that you are probably familiar with is that of a spinning ice skater. Conservation of angular momentum tells us that the rate of angular velocity (how fast an object spins) multiplied by its radius is a constant. There is another peculiar property of neutron stars beside their density, and that is their rotation rate. That discovery was made in the 1960s using observations in a different part of the electromagnetic spectrum.Ĭredit: Space Telescope Science Institute However, this was not the first neutron star known. If you want to make the same comparison and ask when the first neutron star was observed optically, the answer is in the 1990s by the Space Telescope. The first white dwarf star to be observed was Sirius B when it was resolved and separated from its companion (Sirius A) in the 1860s by Alvan Clark. The best estimates are that neutron stars have an upper mass limit of about 3 M Sun, but that value is uncertain. That is, what is the mass limit above which even neutron degeneracy pressure is not strong enough to resist gravitational collapse? There is no easy answer to this question, though.
You may be curious to know the equivalent of the Chandrasekhar limit for neutron stars. If you calculate the density of a neutron star, it is astounding-one sugar-cube-sized lump has a mass of half a trillion kg, which weighs about 1 trillion pounds in Earth's gravity. The typical size of an object of this sort is only about 10 kilometers in radius, but it can contain more than 1.5 solar masses of material. The remnant of the core has become essentially one giant atomic nucleus made up of neutrons. When the neutrons have been created, they begin to resist further compression, exerting “neutron degeneracy pressure,” which halts the collapse of the core. This inverse beta-decay reaction between electrons and protons creates both neutrons and neutrinos (this is the source of the neutrinos that helped blow off the outer layers of the star in the supernova explosion). Therefore, the electrons get compressed to the point that they merge with the atomic nuclei in the core of the star. Inside the iron core of a high mass star, the electrons cannot exert enough electron degeneracy pressure to resist the collapse. Neutron stars are the second type of compact object we will study in this course. The next logical question to address is: What remnant is left behind (if any) after these supernova explosions? When the core of a star collapses at the beginning of a Type II supernova explosion, a neutron star is created.
We have seen that we expect massive stars to explode in core collapse supernovae, and that white dwarfs in binary systems may also explode in supernova explosions. Since the Chandrasekhar limit is 1.4 M Sun, this means that the more massive stars in the range of, say, 4 - 8 M Sun must lose most of their mass so that the white dwarf they leave behind is less than this limit. For stars less than approximately 8 solar masses, the remnant of the core that is left behind after stellar evolution is complete is the white dwarf.