The evolution of a star is dictated by its initial mass and chemical composition . The more massive a star is, the faster it depletes its energy sources. It is as if each star had their own ID. Today there are computer numerical codes that are capable of reproducing stellar evolution from the initial gravitational collapse to the final phases: white dwarfs, pulsars or black holes, depending on the initial mass. Here we will focus on the evolution of the most massive stars.
Stellar evolution can be summarized in three phases:
1) Pre-main sequence: The energy source comes from the gravitational collapse. These stars are relatively cool and have a very large radius. Thousands of years to millions of years can pass in this phase, depending on the initial mass. They are the so-called star babies.
2) Main sequence: As the star contracts, the temperature in the core increases, until it is high enough for hydrogen fusion. The star will spend a large part of its life in the main sequence, since there is enough fuel for it (on average, 70% of the stellar mass is made up of hydrogen).
3) Final phases: A massive star progressively burns hydrogen, then helium, carbon … forming increasingly heavy elements inside. Its structure resembles an onion and each layer has a different chemical composition. Upon reaching iron, it is not possible to extract more energy from nuclear fusion processes. The remaining iron stellar core must bear the weight of the upper layers and contracts to form a neutron nucleus. The outer layers fall on this neutron nucleus and when it touches it a rebound takes place that sends this material to the outside with very high speeds. We have a supernova! The associated energy is enormous: on the order of 10 ** 51 ergs. Much of the energy from a supernova explosion is carried off by neutrinos. Only 1% of the energy is in the optical range!
The by-product of a supernova explosion in the 8 to 20 mass range will produce a neutron star.
Curiosities of a neutron star
Radius: of the order of 10 km (depends on its mass)
Mass : of the order of the solar mass
Density in the center : 10 ** 15 g / cm3
Center pressure: 1 ** 36 dyn / cm2
Very high magnetic fields: There are isolated neutron stars and in binary systems.
Observation: They can be observed in radio, in the visible, neutrinos, gravitational waves, x-rays, gamma, etc.
The first confirmed exoplanet detection was made in 1992, with the discovery of several planets orbiting the PSR B1257 + 12 pulsar.
The so-called Hulse-Taylor pulsar (PSR B1913 + 16) is a system of two neutron stars where gravitational waves were first detected ‘indirectly’ in 1974.
Quark stars
For now, they only exist from a theoretical point of view. They are also called “strange stars” and are basically made up of quarks of the type u, d and s.
For almost 40 years, neutron stars only existed on paper, but after their discovery in 1967, today they constitute one of the most advanced fields of astrophysics. Quark stars, if detected, may follow a similar trajectory, opening an even wider window into the physics of compact objects .
Antonio Claret is a theoretical astrophysicist at the Instituto de Astrofísica de Andalucía of the CSIC in Granada.
