Stellar black hole

A stellar black hole is an incredibly dense object formed by the gravitational collapse of a massive star. These objects are defined by their immense gravity, which prevents anything from escaping once it passes a certain point. They are much smaller than the supermassive black holes found at galaxy centers. Instead, they represent the final stage of life for certain massive stars. Scientists categorize them by their mass, which typically ranges from about 5 to several tens of solar masses. Understanding these objects helps astronomers map the life cycles of stars and the evolution of galaxies.

The formation of a stellar black hole is a specific sequence of physical events. It begins when a massive star exhausts all its internal stellar energy sources. Without this energy to provide outward pressure, gravity takes over and causes the star to collapse inward. This process is known as gravitational collapse. The final outcome depends heavily on the mass of the collapsing core. If the mass is below the Tolman–Oppenheimer–Volkoff limit, or TOV limit, the collapse stops. This results in a compact star like a white dwarf or a neutron star. However, if the mass exceeds the TOV limit, the crush continues indefinitely. The matter collapses until it achieves zero volume, creating a black hole at that point in space.

There are several important mass limits that scientists use to distinguish between different types of stellar remnants. The TOV limit represents the maximum mass a neutron star can have before it must become a black hole. In 1939, this limit was estimated to be 0.7 solar masses. By 1996, researchers updated this estimate to a range between 1.5 and 3 solar masses. We can see the upper end of neutron star masses in objects like PSR J0740+6620, discovered in September 2019. If a compact object exceeds 3.0 solar masses, it is likely a black hole rather than a neutron star. This distinction is vital for astronomers studying X-ray binary systems.

Stellar black holes also exhibit specific physical properties described by the no-hair theorem. This theorem suggests a black hole can only have three fundamental properties: mass, electric charge, and angular momentum. Angular momentum refers to the rotation of the black hole. This rotation is caused by the conservation of angular momentum from the original star or the objects that formed it. While black holes are often invisible, we can detect them in binary systems. In these systems, a black hole pulls matter from a companion star. As this matter falls toward the black hole, it heats up to hundreds of millions of degrees. This process releases massive amounts of energy in the form of X-rays.

Astronomers have identified specific mass gaps where black holes are predicted to be rare or non-existent. The lower mass gap is a suspected range between 2 and 5 solar masses. Scientists notice a scarcity of candidates in this range, though some might be created by merging neutron stars. The upper mass gap is predicted by models of late-stage stellar evolution. This gap occurs because of a process called pair-instability supernovae. In stars between 130 and 250 solar masses, energetic gamma rays can collide with atomic nuclei. This creates electron-positron pairs, which reduces the internal pressure supporting the star. The resulting explosion can blow the star apart completely, leaving no remnant behind. This creates a gap in the expected masses of black holes.

We have discovered many different examples of these objects across the universe. In our own galaxy, the system 2MASS J05215658+4359220 holds the smallest known stellar black hole. It has a mass of 3.3 solar masses and a diameter of only 19.5 kilometers. In September 2015, gravitational waves revealed a massive rotating black hole of 62 solar masses. This object formed during a merger event between two smaller black holes. Other notable candidates include Cyg X-1, which has a mass of about 21.2 solar masses. We also find candidates outside our galaxy through gravitational wave detections, such as GW150914. These observations provide concrete evidence of how black holes behave and interact.

Stellar black holes are part of a much larger cosmic hierarchy. They are distinct from intermediate-mass black holes found in globular clusters. They are also much smaller than the supermassive black holes that reside in the centers of galaxies like the Milky Way. Studying stellar black holes allows scientists to test the theory of general relativity. It also helps us understand how energy is distributed in the universe through X-ray emissions and gravitational waves. By observing how they pull matter from companion stars, we learn about the fundamental forces of gravity. Each discovery brings us closer to understanding the complex life cycles of the stars that populate our universe.