Meet The Black Hole

The universe appears as a vast expanse, resembling an empty ocean with occasional islands in the form of galaxies. However, this is merely an illusion. Galaxies contain only a small fraction of all atoms, while the majority is believed to drift in the intergalactic medium, the space between galaxies. Gas extends from each galaxy, resembling the roots of a massive tree, with gravity guiding fresh mass into this dense cosmic forest. Within the intergalactic medium, the building blocks of creation are present: hydrogen and helium, interwoven into sheets and filaments that flow into galaxies, eventually giving rise to stars.

Yet, upon closer inspection, it becomes evident that quasars hold dominion. Quasars, the most powerful objects in existence, inhabit the centers of certain galaxies. Despite their minuscule size compared to the vastness of the universe, they radiate with the intensity of a trillion stars, unleashing immense jets of matter that reshape the cosmos around them. Their power is so great that they can even "kill" a galaxy. But what exactly are quasars, and how do they shape the universe?

In the 1950s, astronomers discovered enigmatic radio waves emanating from various points in the sky. Dubbed "quasi-stellar radio sources" or "quasars," these sources appeared as star-like dots in radio waves rather than visible light. Their characteristics were peculiar: some flickered, others emitted high-energy X-rays alongside radio waves, and all appeared exceptionally small. Their high speeds, exceeding 30% of the speed of light, suggested they were incredibly distant, with their apparent motion resulting from the universe's expansion moving them away from us. These enormous distances implied that quasars were not mere stars but the active cores of galaxies billions of light-years away.

To exhibit such remarkable brightness and intensity over vast distances, quasars had to rely on feeding supermassive black holes. Although the exact process of their formation remains uncertain, it appears that every galaxy harbors a supermassive black hole at its center. The light emitted by quasars does not originate from within these black holes but from the surrounding space—an accretion disk composed of gas. Quasars utilize matter as their fuel, similar to stars, but black holes prove to be the most efficient engines for converting matter into energy in the universe. The energy released by matter falling into a black hole can be sixty times greater than that generated by nuclear fusion in a star's core, as it stems from gravity rather than nuclear reactions.

When matter approaches the event horizon of a black hole, it accelerates to nearly the speed of light, carrying an extraordinary amount of kinetic energy. This energy becomes observable only when matter falls into the black hole in a specific manner, forming an accretion disk—a rapidly spiraling mass that heats up due to collisions and friction, reaching hundreds of thousands of degrees. Within a region comparable in size to our solar system, the central region of a galaxy can emit energy surpassing that of all its stars combined. This is the essence of a quasar—a supermassive black hole feasting on matter.

Quasars consume vast amounts of gas, with typical quasars devouring between one and a hundred Earth masses of gas per minute. Around ten billion years ago, when the universe was much younger, quasars were more prevalent, coinciding with the peak of galaxy and universal youth. During this time, the intergalactic medium was more compact, allowing the gas filaments surrounding quasars to serve as a bountiful banquet, resulting in prodigious emissions of light and radiation.

The brightest quasars produce jets that twist the magnetic field of surrounding matter into a concentrated beam. Similar to a particle accelerator, these jets propel enormous streams of matter, piercing through the circumgalactic medium and forming vast plumes that span hundreds of thousands of light-years. The scale of these phenomena is almost unimaginable—a minuscule point within a galaxy shaping regions of the universe extending hundreds of thousands of light-years.

However, quasars cannot sustain their feast indefinitely. Their ravenous nature ultimately disrupts their host galaxies by overheating them, hindering star formation. Hot gas lacks the necessary conditions for stars to emerge, as atoms move rapidly and collisions exert pressure that counters gravitational collapse. Cold gas, in contrast, provides an ideal environment for star formation as it readily collapses into stars without resistance.

Furthermore, quasars expel gas from their galaxies, depleting the raw materials required for new stars to form. Although this outcome may seem disheartening, it can be seen as a positive factor for the potential development of life. An excess of stars formed in quick succession leads to subsequent supernova explosions, potentially sterilizing planets.

Nonetheless, the intricate dynamics of galaxies reveal that every component influences and relies upon others within the galactic environment. While quasars and supernovae tend to expel gas from galaxies, shockwaves and quasar jets can compress gas, facilitating short-term star formation. Gas that exits the galaxy mixes with incoming gas, ultimately recycling it back into the galactic system. In general, a certain level of equilibrium and moderation is necessary for our existence today.

This brings us to the question of whether the Milky Way had its own quasar in the past. The history of galaxies is challenging to preserve, as the constant churn of cosmic events erodes the traces of their past. It remains uncertain if the Milky Way underwent a quasar phase, which could have allowed our supermassive black hole, Sagittarius A*, to grow to its current mass, around 4 million times that of the Sun. Unfortunately, the ancient history of our galaxy remains a mystery. Nevertheless, even in its dormant state, Sagittarius A* holds the potential to transform into a quasar in the future.

In a few billion years, the Milky Way will merge with the Andromeda galaxy, and during such galactic collisions, fresh gas is provided to fuel the central black holes. While the occurrence of this event is uncertain, witnessing it would undoubtedly be an awe-inspiring spectacle. Perhaps beings in the distant future will have the privilege of observing this grand cosmic merger and marveling at its magnificence. However, one need not wait that long to explore fascinating phenomena. Our planet already offers countless wonders to be explored, provided we possess the knowledge to comprehend them.