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 rest of a star that has fallen on itself during the pull of its own gravity and collapses down to a dense, city-wide sphere of subatomic particles. The fastest known pulsar, PSR J1748-2446, spins at a speed of 716 times per second, equal to about 43,000 miles per hour.
If you tried to stand on a pulse, your molecules would immediately be torn apart by gravity and transformed into a film of particles scattered across the star.
But if you could somehow stand there and look up, the sky would look strange. The most important thing is the absence of stars – you spin too fast to see them. A “day” on a pulse lasts only a few milliseconds, so the stars would rise and set hundreds of times per second, moving too fast for your eye to understand their movement. Instead, they would be welded to long, thin, barely visible lines.
A gravitational pull from a neutron star bends the light, which distorts your vision, just like what happens when you look through the surface of the 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 could actually see objects that are slightly above the horizon, because the light leaving the object curves around the star on its way to the eye.
“It’s not so much the light that bends when the space the light moves in,” she said. “With such intense gravity, the star transforms space into a lens.”
The gravity of the star would also change the shape of the sky and squeeze it together, as if you were looking at it through a fisheye lens. On earth you can only see half the sky at one time; from the southern hemisphere you cannot see Polaris, the North Star, because it is below the horizon. But if you were on an equator of a pulse, you could easily see the north and south poles in 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 received extra energy from its long case to the surface. But the sky may no longer look blue over everyone; the blue shift could be interrupted by faint infrared stars moving into the visible spectrum and adding new faint red stars to the mixture.
Jocelyn Bell Burnell, the astronomer who discovered pulsars, mentioned in a recent lecture that if you dropped a ball on a pulse from space, it would move at half the speed of light when it hit the surface. Dr. Mack pointed out that even if you dropped the ball from the waist height, it would move at 1 percent of the speed of light – 7 million miles per hour – when it hit the ground.
I asked, “So it would be tough to play baseball on a pulse, and pounding a basketball wouldn’t be the issue?”