Black Hole Questions? We Got Answers

Artist depiction of matter spiraling into black hole XTE J1650-500 – NASA
Click on this NASA video to get a feel for what it’s like to circle a black hole.

Earlier this month, scientists discovered the smallest black hole ever. Called XTE J1650-500, it’s located in the southern constellation of Ara the altar and is orbited by a companion star. A disk of gas, siphoned from the companion by the hole’s enormous gravity, glows brightly as it spirals down into its gaping maw.

XTE J1650-500 contains 3.8 times as much stuff as the sun and packs it into black disk only 15 miles across. Astronomers are excited because that’s very close to the predicted size limit for these objects. Every time a new discovery confirms a prediction made by a scientific theory, it tells scientists they’re on the right track in their search for a clearer picture of nature’s workings.

Another artist’s view of the new tiny black hole (center) with its companion star – NASA

If you’ve ever watched a rocket launch on TV, you were probably impressed with all the energy it took to get the rocket off the ground and into space. To completely free itself from Earth’s gravity, a rocket has to speed away at 25,000 miles per hour. That’s our planet’s "escape velocity." Larger, heavier planets have even higher escape velocities. 

The escape velocity also depends on how far you are from an object’s center. The closer you are, the higher it climbs.

Right now we stand about 4000 miles from the center of the Earth. If you could pack the entire Earth into a marble-sized sphere 3/4" across, you’d be able to stand less than an inch away from its center. The escape velocity would now be so high that nothing could escape gravity’s grasp. Not even light.

Earth would suddenly disappear into blackness like Harry Potter under his invisibility cloak. We would become a black hole.

The only thing left to betray Earth’s presence would be its gravitational pull. That means the moon would still revolve around the planet but what an eerie sight it would be. Observers looking at the Earth-moon system from afar would see the moon orbiting around an invisible spot in space. Using basic mathematics, they could easily calculate how much matter the invisible thing must contain to keep the moon in orbit.

That’s one way astronomers discover likely black holes in the first place. They find stars revolving rapidly around massive things that shed little light. 

Let’s keep going. Crush the sun down to a 3.7 mile diameter sphere, and it would collapse into a black hole too. The Earth would still remain in its orbit, but there’d be no more heat and light from the sun. Everything we once called the sun would be pulled down into the hole and compressed into a point of infinite density called a singularity.

Dust ring around the suspected black hole in
the galaxy NGC 4261. Bright spot at center
is hot gas ablaze as it’s devoured by
the hole. — NASA

Don’t worry about the sun going to the dark side though. That only happens to the biggest of stars, called supergiants. They live relatively brief lives burning hydrogen and other elements in their fiery cores. Eventually they run out of fuel, and their nuclear furnaces shut down. With no heat and pressure to push back against the ever-present, inward pull of gravity, the star’s core experiences a runaway, nonstop collapse into a black hole. At the same time, its outer layers rebound and explode as a supernova.

That’s what happened to our tiny black hole XTE J1650-500. It was once a grand, shining star but in the end, it had no defense against the dark arts of gravity.

Special note: Althought I’ve heard some people referred to as "black holes", you’d actually have to squeeze someone down to the size of an atom for that to happen. Be nice now.