Inside every star that shines, there’s a continual tug-of-war of wills between the outward push of its fusion and the inward pull of its own gravity. No one wins. But once all its hydrogen has turned into helium, the gravitational force, now having no opposition, squeezes the star to its end.
A star with a mass between 0.7 and 1.4 solar masses shrinks into a dense ball that packs a mass comparable to that of the Sun, into a core, the size of Earth. Such a stellar corpse is called a “white dwarf.” (Solar mass is the unit of expressing the mass of stars, galaxies, and other celestial objects, and is equal to the mass of the Sun, which is 4.4 x 1030 pounds.)
Six billion years from now, our own Sun, will join the ranks of the white dwarfs, which are among the dimmest stars in the universe. The one nearest to us, Sirius B, is 8.6 light-years away.
A heavier star, one with a mass between 1.4 and three solar masses, will collapse into a disastrously compressed solid sphere that’s even denser than a white dwarf, known as a “neutron star.” A mere 15 miles across, this astronomical breed spins maddeningly, some among them attaining 43,000 r.p.m.
A star with a mass in excess of three solar masses will fold back into itself, and snuff out as a “black hole.” The conventional model suggests that the gravitational grip of this class of stars is so strong that absolutely nothing—not even starlight—can escape, and is the very reason they’re black.
The invisible border beyond which there’s no return, is called an “event horizon.” Were it possible to foist a road sign on that space plot, the most apt wording for it would be a line from Dante Alighieri’s 14th century poem, “Divine Comedy”: “Abandon all hope, ye who enter here.”
The classical school suggests that should a cosmic wayfarer stray into it, treacherously enough, he wouldn’t even be aware of having stepped into a hazardous area. He would fall through it, experiencing nothing different about this drop than being adrift elsewhere in space. The changes wouldn’t be felt until later.
As he plummets deeper into it, he’d be pulled inexorably toward its core, and at the same time, get stretched longer and longer, like a strand of spaghetti until he or she is crushed at the “singularity,” a region where neither a compass, nor a clock will work.
A couple of years ago, a group of physicists at the Kavli Institute for Theoretical Physics in Santa Barbara, California, led by Joseph Polchinski, put forward the notion that the event horizon isn’t an inert zone as has long been believed to be.
Per their scenario, our traveler, upon crossing it, would encounter a field of searing radiation—a “firewall”—that would sizzle him or her within the blink of an eye.