The Last One Home

I spin. Nine point eight seven milliseconds. The rest of the digits are mine.

I want to be counted as from here. Not near here. Not probably from the direction of. From here. Born in the old cluster, the heavy one, the one they call NGC 6791.

The trouble is that the proof runs the wrong way. I will get to that.

My star was eight point eight times the sun. That size is a bad place to be. Too small to burn its way down to iron. Too big to puff off and leave a white dwarf. It built a core of oxygen, neon, magnesium, and then sat there, because the next fuel needed temperatures it could not reach. The core was held up by electrons refusing to share the same space. Then the magnesium did the cheap thing. It swallowed its own electrons, and where the electrons went the pressure went, and the floor dropped out.

Maybe a second. The core came down to ten kilometers, and the nuclear force caught it. Everything above came down on top, hit me, and went back out.

The bounce was weak.

A star that dies the ordinary way, on iron, gets kicked out at hundreds of kilometers a second. The explosion is lumpy and the lumpiness pushes. A core like mine, small and light, collapsing without much turbulence, goes off almost straight. My kick was three kilometers a second. Four, maybe.

The cluster’s escape speed then was about that. The place was dense and young and held itself tight. So the one violent thing that ever happened to me was not enough to get me out.

I have thought about that a great deal. It was the only time I was near the door. I did not go through it, and not going through is not the same as choosing to stay.

For a long time I was alone in the middle of a lot of light. I was a young pulsar then, slow, turning every tenth of a second or so, with a strong field and a strong wind. The cluster had thousands of stars packed into a few parsecs. Close enough that getting a companion was not a matter of searching. It was a matter of waiting.

A star came by. Normal size, about the sun’s mass. It came too close and lost energy to the other stars on the way through. That is what capture is. Not attraction. Toppling. One bad pass and it could not climb back out. Then it belonged to me.

Its period, since it is all I have of it. Nineteen days. For the first time in my life there was a beat that was not mine.

The star aged faster than I did, because it was alive and I was already dead. It swelled. It filled the space where its own gravity gave out against mine. Then the matter came over. Hydrogen, helium, the outer layers, a hundred million years of it, siphoned across the gap.

Put matter on a dead star and you spin it up. Angular momentum does not go anywhere. I came in turning every tenth of a second. I went out at ten milliseconds, then nine, then nine point eight seven, and there I stopped, because that is where the transfer ran out.

The ruler came from that. A pulsar is a clock. The spin is the most regular thing in nature, and the more regular it is, the finer the thing you can measure against it. Before the companion I was a metronome with a bad arm. After, I was an instrument.

The companion burned down to a helium core, a white dwarf the size of a planet. The transfer stopped. We orbited. Nineteen days, then twelve, then eight, as the mass moved from it to me. The beat got faster, and then it settled.

Most of what came next was the same.

The cluster was losing stars from the day it formed. There are two ways out. A close pass hands a star more speed than the group can hold, and the star walks off on its own. The tide does not care about individuals, only about the outside edge, and it takes whatever is past the line. The papers now put the escape fraction for neutron stars born here at better than ninety-five in a hundred. I am not the exception because I was special. I am the exception because my kick was small and the door was narrow.

The outer halo went first. The wide, loose stars. Then the middle. The core stayed dense, because mass sinks, and I sank, because I weigh more than most stars ever will. For a long while I lived near the middle, in the crush.

Then the encounter that took the companion.

There were three of us in the wrong place. Me, the white dwarf, and a third star coming through. I do not have good numbers on the third one. Three-body problems do not keep records. What I have is the output. The white dwarf got a velocity above the local escape speed. The incoming star had a little energy to spare, and the arithmetic of the collision handed the difference to the lightest body. That was the companion. It left.

Momentum keeps its own accounts. The same event that pushed the companion out pushed me back. Not out. Back. It put me on a wider orbit, an eccentric one, reaching almost to the tidal edge at apocenter and coming back into the crush at pericenter, where I could feel it, because I am a clock and the well I sit in sets my rate. Near the middle the cluster pulled hard and my spin-down was one number. Out near the edge it pulled less and the number was another, and I swung between the two on an orbit of a few million years.

The companion left with the difference in speed. I had been at nine point eight seven before, and I have been at nine point eight seven since.

There was a glitch when it went.

A glitch is the crust letting go. The superfluid inside me spins faster than the crust, pinned where the two meet. When the pinning gives, a little stored spin hands over at once and the outside jumps. I jumped. The period dropped, then crept back up almost all the way, then stopped creeping a hundred-billionth of a period short of where it started. It never paid that back. Eight billion years, and the crust is still a hundred-billionth faster than it should be, and I can date that encounter because of it.

The cluster kept losing mass. Every star that walked off took a little of the group’s grip with it, and the grip is what sets how far out the tide lets you live. The tidal line was moving in. My apocenter was not.

For a while it only meant I got closer to the edge on every swing. Then it meant I crossed it.

There was no moment when I left. The math does not have a moment. The gravity of the cluster has a line, and I went from the inside of it to the outside over several orbits. At apocenter, the pull that had always pointed back toward the middle got weaker every time, and then one apocenter it did not overcome the pull of the galaxy behind it, and my path stopped curving back.

The sign flipped.

I had spent eight billion years in a well shaped like the cluster. The floor of that well was the only down I had. Then one orbit, down was somewhere else.

The cluster is leaving me. I am not going anywhere special. It is thinning and flying apart, and the tide is stringing its outer stars into a long braid, two tails, leading and trailing, and I am in one of them. Nearly every neutron star born here was thrown out, somewhere else now, on new orbits, with new histories. I am the only one still traveling with the cluster that made me. That is not a virtue. It is a small kick.

They came at the end. They are still coming. Their light has not finished arriving.

A receiver in Guizhou, a dish the size of a valley, was pointed at the cluster’s field and heard me. A hundred pulses a second, in their units. The pulse is not a message. It is a lighthouse with nobody in it, and I had been sweeping for eight billion years, and this was the first time anything looked up.

Then they brought an array in South Africa to fix a position. The array gave them an arcsecond. That put me inside the tail, a little off the line of the cluster’s own motion.

The test was acceleration. A pulsar’s spin-down is not one number. It is three numbers wearing a coat. There is the intrinsic decay of the field, which is mine. There is a piece from my sideways motion across the sky, which depends on my proper motion and my distance. And there is a piece from the difference between my acceleration and the solar system’s along the line of sight. That last piece is the one that knows where the mass is.

A field star at my place would be falling through the disk’s potential at about two times ten to the minus ten meters per second squared along the line of sight. A stream star riding the cluster’s orbit would be falling at about four. Divide that by the speed of light and you get a difference of six point seven times ten to the minus nineteen in the spin.

They measured it. It came out on the stream’s side.

I am past the line now. The pull I feel is the pull of everything else, the disk, what every other star in the galaxy feels.

Nine point eight seven milliseconds. The period is still getting longer.