Torn apart
The story of a TDE begins at the heart of a galaxy, near the edge of a supermassive black hole millions or billions of times the mass of the Sun. Astronomers now think that practically every large galaxy has one such black hole at its center. These gravitational monstrosities play a key role in the formation of their host galaxies and wield huge influence on their surroundings.
Black holes are famously so dense that even light cannot escape their gravity. But despite popular conceptions, they don’t actually suck material in any more than the Sun sucks in the planets that orbit it. For example, if the Sun were to suddenly compress into a black hole, it would shrink to just 4 miles (6 kilometers) across, yet the planets would continue to orbit as they currently do because its mass wouldn’t change.
The gravity of a black hole works the same way. When astronomers look to the center of our own Milky Way Galaxy, we see over a dozen stars orbiting a common point where our galaxy’s supermassive black hole, called Sagittarius A* (Sgr A*), resides. In fact, astronomers have been observing Sgr A* for so long that they have seen the innermost star, S2, complete one full orbit, which takes 16 years. After establishing S2’s orbital parameters, researchers applied Kepler’s third law of planetary motion to calculate the mass of Sgr A*, which clocks in at a whopping 4 million solar masses. While the final calculation was simple, the work to get the data over so many years was not — in fact, it won astronomers Andrea Ghez and Reinhard Genzel the 2020 Nobel Prize in physics.
S2 looks like it’s on a stable orbit for now, but researchers estimate that thousands of stars, including stellar remnants like neutron stars and white dwarfs, also orbit Sgr A*. When two of these objects have a close encounter, their gravity perturbs each other’s orbits and they head out on new, altered trajectories. Most of these orbits remain stable, or perhaps fling the star outward from the center of the galaxy. But on rare occasions, a star’s new orbit sends it inward on a collision course with disaster.
