What would the sky look like if you could stand on the surface of a pulsar?
— Kathryn S., Seattle
A blur, mostly.
A pulsar is a fast-spinning neutron star: the remnant of a star that has fallen in on itself under the pull of its own gravity, collapsing down to a dense, city-size orb of subatomic particles. The fastest known pulsar, PSR J1748-2446, spins at a rate of 716 times per second, equal to about 43,000 miles per hour.
A neutron star’s gravity is not as strong as a black hole’s, but it’s close. Light can escape a neutron star, but not much else can. Thanks to the massive gravitational pull, no ordinary matter can survive there, certainly not a fragile human body; landing on the sun would be easier.
If you tried to stand on a pulsar, your molecules would be instantly torn apart by gravity and turned into a film of particles spread out across the star.
But if you could somehow stand there and look up, the sky would look strange. The main thing would be the absence of stars — you would be spinning too fast to see them. A “day” on a pulsar lasts only a few milliseconds, so the stars would rise and set hundreds of times per second, moving too fast for your eye to perceive their motion. Instead, they would blur together into long, thin, barely visible lines.
A neutron star’s gravity bends light, which distorts your view, just like what happens when you look through the surface of water. I asked Katie Mack, an astrophysicist at North Carolina State University, what effect this would have on the sky. She said that on a neutron star, you would actually be able to see objects that are a little bit over the horizon, because the light leaving the object curves around the star on its way to your eye.
“It’s not so much the light that’s bending as the space the light moves in,” she said. “With such intense gravity, the star is turning space itself into a lens.”
The star’s gravity would also change the shape of the sky, squishing it together, as if you were looking at it through a fisheye lens. On Earth, you can see only half the sky at once; from the Southern Hemisphere, you can’t see Polaris, the North Star, because it is below the horizon. But if you stood on a pulsar’s equator, you would easily be able to see the north and south poles of the sky at the same time.
The stars would also look a little more blue than usual, because the light that reached your eyes would have gained extra energy from its long fall to the surface. But the sky might not look more blue over all; the blue shift could be canceled out by faint infrared stars shifting into the visible spectrum, adding new dim red stars to the mix.
Jocelyn Bell Burnell, the astronomer who discovered pulsars, mentioned in a recent lecture that if you dropped a ball onto a pulsar from space, it would be moving at half the speed of light by the time it hit the surface. Dr. Mack pointed out that even if you dropped the ball from just waist height, it would be moving at 1 percent of the speed of light — 7 million miles per hour — when it hit the ground.
I asked: “So playing baseball on a pulsar would be tough, and dunking a basketball would be out of the question?”