Sophia Nasr is currently pursuing her undergraduate degree in astrophysics at York University. Actively engaged in astronomical events and scientific outreach, Sophia holds a research position at the York University Observatory, occasionally co-hosts on York Universe Radio, and is the Vice President of the York University Astronomy Club. In an effort to help peak public interest in science, Sophia writes articles about astronomy and physics for Facebook’s “All Science, All the Time” and “Science That.”
Black holes are formed when stars of at least 20 solar masses die. Solar mass is an astronomical unit of mass, equal to the mass of the Sun, used for expressing the mass other celestial objects. Stars continuously fuse lighter elements into heavier ones during their lifetime. During the process of fusion, there is a constant inward pull due to gravity and outward push due to pressure, such that the two balance each other out. If a star is massive enough, it will fuse elements into iron. As more energy goes into fusing iron than is produced, this is the end of the line, and the star runs out of fuel. At this point, the star’s outer layers explode in an extremely violent supernova. Gravity wins the battle against pressure, causing the core to collapse under its own weight. When such a core is more than 2.5 times as massive as the Sun, the inward pull of gravity is so immense that the core continues collapsing upon itself, resulting in the formation of a black hole.
The mathematics describing black holes shows that the volume of the singularity—the center of a black hole—is zero, and thus the density of a black hole is infinite. With so much matter packed into an infinitesimally small space, black holes possess gravity so strong that not even light can escape its pull—if it crosses the event horizon, that is. The event horizon is the boundary beyond which nothing—not even light—can escape. This is because the escape velocity of a black hole is greater than the speed of light. As a result, we know nothing beyond a black hole’s event horizon—it marks the boundary beyond which no information can be obtained. Because black holes do not emit any light, they are not visible.
What follows is a discussion on how black holes “die.” Perhaps “evaporate” is a more suitable word in this case. The process by which black holes evaporate is called Hawking radiation, or sometimes, Bekenstein-Hawking radiation. In 1972, physicist Jacob Bekenstein introduced an idea stating that black holes should have a finite temperature and entropy. Two years later, renowned physicist Stephen Hawking formulated a complete theory describing black body radiation in black holes.
In Hawking radiation, a continuous production of “virtual” particle-antiparticle pairs occurs near the event horizon of a black hole due to fluctuations in energy. Normally, such a pair will collide and annihilate, which is why these particles are referred to as “virtual” (they exist only for a limited time). However, the force of gravity exerted by the black hole can pull the negative antiparticle in, while the positive particle escapes. Because the positive particle essentially dodges annihilation and is left in space when the negative antiparticle is sucked into the black hole, it is no longer “virtual”—it is now real. By this process, the black hole appears to have emitted a positive particle. This is what is known as Hawking radiation.
Over time, the continuous addition of negative antiparticles to the black hole adds negative energy, resulting in a gradual decrease in the black hole’s mass. This will in turn cause a black hole’s size to gradually decrease. With the decrease in size, the black hole’s temperature increases to such an extent that the black hole vanishes in an extreme burst of gamma radiation, sometimes including all kinds of energetic particles. This marks the end of the black hole.
That being said, the time it takes for a black hole to evaporate can be extremely long, depending on its size. Smaller black holes evaporate faster than do larger black holes. For example, microscopic black holes would evaporate very quickly, but a black hole with the mass of our Sun would take 10^67 years to evaporate—that’s a 1 followed by SIXTY-SEVEN ZEROS. This clearly makes detection of Hawking radiation in space problematic. While a group of physicists in Italy conducted a laboratory experiment that may have produced Hawking radiation in 2010, it has not yet been detected in space.
Follow Sophia on Twitter: https://twitter.com/Pharaoness
Follow Science That on Twitter: https://twitter.com/Science_That
Science That website: http://sciencethat.com/
Science That on Facebook: https://www.facebook.com/sciencethat
All Science, All the Time on Facebook: https://www.facebook.com/AllScienceAllTheTime
Image Credit: Ute Kraus / Space Time Travel