Physical Science:

A neutron star is a type of compact star that is incredibly dense, with a diameter of only about 10-20 kilometers and a mass of around 1.4 to 3 times that of the Sun. Neutron stars [Figure 13.2.3] are formed when a massive star undergoes a supernova explosion, and the core of the star collapses under its own gravity, making it so dense that protons and electrons combine to form neutrons. The extreme density of a neutron star causes its gravitational field over a billion times stronger than Earth’s gravitational field. It spins several times per second which can produce intense magnetic fields, making neutron stars some of the strongest magnets in the universe. The strong magnetic fields can also produce beams of radiation that are visible as pulsing signals, leading to the term pulsar [Figure 13.2.4]. Neutron stars are incredibly hot, with surface temperatures estimated to be around 1 million degrees Celsius (1.8 million degrees Fahrenheit), and they emit radiation across the electromagnetic spectrum, from X-rays to radio waves. They are also believed to be a source of heavy elements in the universe, as they are thought to be involved in the production of many of the elements beyond iron.

The current understanding is that if a white dwarf star exceeds a mass of about 1.4 to 3 times the mass of the Sun, known as the Chandrasekhar limit, it will collapse to become a neutron star.

Pulsars [Figure 13.2.4] are believed to be rotating neutron stars that emit beams of radiation from their magnetic poles. As the neutron star rotates, the beams of radiation sweep across space like a lighthouse, producing a regular pattern of pulses that can be detected by telescopes on Earth.

A black hole [Figure 13.2.5] is a region of space where the gravitational pull is so strong that nothing, not even light, can escape. Black holes are formed from the remnants of massive stars that have undergone a supernova explosion and collapsed under their own gravity. If the collapsing object is larger than about 3 times the mass of the Sun, the gravitational forces become so strong that not even neutrons can resist the collapse, and a black hole is formed. They have no surface or structure, but instead are characterized by their mass, spin, and electric charge. The mass of a black hole can range from a few times that of the Sun to billions of times that of the Sun. The region around a black hole where the gravitational pull is so strong that nothing can escape is called the event horizon. Once matter crosses the event horizon, it is trapped by the black hole and cannot escape. The point at the center of a black hole where the gravitational pull becomes infinite is called the singularity, and our current understanding of physics breaks down at this point. Black holes are invisible, as they do not emit any light or other forms of radiation. However, their presence can be inferred from the effects of their gravity on nearby matter. For example, if a black hole is in a binary star system, its gravity can cause the visible star to wobble, and the mass of the black hole can be estimated from the size of the wobble. As a black hole pulls matter into it, the matter gets accelerated and emits radiation that can be detected by telescopes. This radiation can come from the accretion disk around the black hole [Subsection 14.1.7], where matter gets heated up and emits X-rays and other high-energy radiation.