standing on a planet orbiting a pulsar

[wilfulton]

I'm writing a fic involving a planet orbiting a pulsar that was ejected from the galaxy. The planet is by its lonesome, and the accretion disk it coalesced out of is long since gone. The planet is a little bigger than Mars, and has a 16 day orbit around the pulsar, so I figure it's probably tidally locked with it.

My question is, what would the pulsar look like from the ground? Would anything even be visible at all (to the unaided eye) because neutron stars give off most of their energy in the x-ray and gamma ray spectrum, and are probably too small to see at that distance to begin with.

Next off, would you be able to build a base on the "night" side of the pulsar?

Or should I tidally lock this planet a bit farther out than a 16 day orbit, because that seems shorter than all the other planets we've observed so far orbiting pulsars, that I'm aware of.

[Surlethe]

If it gives off any visible light at all, it's very dim and redshifted because of the neutron star's immense gravitational field. You're also going to have to deal with the effects of the star's electromagnetic field. I don't know how much of the radiation would penetrate to the other side of the planet, and I'm interested to find out how much the neutron star's radiation heats the planet.

[King Kong]

From some quick calculations using Kepler's 3rd Law and the angular size relationship (assuming your neutron star is as big as they come: 20 km radius with 1.35 solar masses), I found that the neutron star will have an angular size of about 0.4 arc seconds on the sky (An arcsecond is one sixtieth of an arcminute, which is one sixtieth of a degree. The view from horizon to horizon is 180 degrees).

IIRC, the human eye's resolving limit is about 1 arc minute. So even if this neutron star is bright enough to be seen, it will just appear as a point of light (like a star).

[Kuroneko]

It's likely that the only kind of planet that could survive the formation of a neutron star would be the core of a gas giant, possibly with some remnants of the gas on it. As such, it would likely be small but dense. As King Kong points out, the pulsar itself appear pointlike, but the natural spectra of such objects is unclear. For older neutron stars, it is probably blackbody, but I'm not very confident in that assumption.

If you wanted a realistic description and if your planet still has a thick atmoshere remaining after most of the gas giant was destroyed, then Rayleigh scattering will protect the planet from soft and moderate X-rays while making the sky an almost uniform blue, except possibly for a slight glow around the pulsar's position in the sky. Unfortunately, Rayleigh scattering would not apply to γ-rays, but that can be solved by having an older, colder, and (relatively) slower pulsar that radiates very little in that spectrum. That would make even the 'day' side of the planet a rather dim and cold environment.

[wilfulton]

I figured the planet would be airless, or very nearly so, and it would only be a little bigger than Mars. As for age, it would probably be an older pulsar, as it has been ejected from the galaxy (a few tens of millions of years perhaps).

I didn't exactly the planet to be any kind of paradise, and the story takes place on the night side, because it would be perilous building on the side constantly being blasted by gamma rays from the pulsar. It doesn't sound like tidal heating would be much of an issue, although I suppose gamma ray bombardment could heat up the "Day" side.

Meh! It's a pulsar base! You've gotta have those if only for the cool factor!

[Kuroneko]

That's certainly very realistic for an airless planet. But then the pulsar wouldn't look like anything from the night side of a tidally locked planet. By Kepler's third law, the semimajor axis of the planet is r = [2T²GMπ]^{1/3}/[2π], where T is the orbital period and M is the mass of the pulsar, with r = 2.05e10m under King Kong's assumption of size. The luminosity of neutron stars in the tens of millions of years is more than a bit unclear, but 1e21-1e23W is probably reasonable [1]; this large window makes me less hesitant over the lack of data specifically in the visual band. Assuming these parameters, the total irradiance with on the planet with the pulsar directly overhead will be between 0.2 and 20 W/m². This is small but respectable; the apparent magnitude at the surface of the planet will be between -17 and -22. Just for a visual comparison, that is between 1/70 and 1/7000 as bright as the Sun [from Earth, of course] and between 60 and 6000 times as bright as the full moon. You have considerable leeway there.

[Andras]

You might want to try to find a novel called Dragon's Egg. It's about man encountering life evolved /on/ a neutron star.

Robert L. Forward is the author, published 1980 by Del Rey/Ballentine.

[Alan Bolte]

What does space look like outside a galaxy, anyway? Are there many galactic clusters that give off enough light to appear like stars in our sky? Or would this pulsar be the only star visible in the sky?

Between tidal forces and radiation, how hot could you make it?

[wilfulton]

What does space look like outside a galaxy, anyway? Are there many galactic clusters that give off enough light to appear like stars in our sky? Or would this pulsar be the only star visible in the sky?

Between tidal forces and radiation, how hot could you make it?

Supposedly the sky would appear very black, with the only light being given off from nearby galaxies. If you are fortunate enough to live in an area where you can see the Milky Way at night, you can get a feel for about what you'd see. Most galaxies are too faint to resolve with the unaided eye.

[Surlethe]

If you're traveling at about 0.1c relative to the galaxy (a quick back-envelope calculation shows the Sun is traveling at 0.004c relative to the galactic core, for comparison), and you're ten million years out of the galaxy, then you're some million light years away from the galaxy. Assuming the galaxy is a spiral one hundred thousand light years in diameter, and you're directly above it, it's going to subtend an arc of about 0.1 radians = 5 degrees in the sky in both directions; that means it will take up four hundredths of one percent of the night sky, if I've done my math correctly. That's far larger than the neutron star itself will appear.

[Ariphaos]

What does space look like outside a galaxy, anyway? Are there many galactic clusters that give off enough light to appear like stars in our sky? Or would this pulsar be the only star visible in the sky?

Between tidal forces and radiation, how hot could you make it?

This really depends on the galaxy in question. The Milky Way is fairly dim for its size, with comparitively little star forming going on compared to its neighbors. For Surlethe's calculations, the Milky Way at an absolute magnitude of -20.5 would appear as an extremely faint disc with an apparent magnitude of ~1.9 or so. A few other prominant galaxies would still be visible, along with, perhaps, some field stars.

One interesting bit of sci-fi I often thing about writing is a small field star, host to sentient life, trapped so far into a void they don't even know other stars exist, much less galaxies.

[Sriad]

If you're traveling at about 0.1c relative to the galaxy (a quick back-envelope calculation shows the Sun is traveling at 0.004c relative to the galactic core, for comparison), and you're ten million years out of the galaxy, then you're some million light years away from the galaxy. Assuming the galaxy is a spiral one hundred thousand light years in diameter, and you're directly above it, it's going to subtend an arc of about 0.1 radians = 5 degrees in the sky in both directions; that means it will take up four hundredths of one percent of the night sky, if I've done my math correctly. That's far larger than the neutron star itself will appear.

For refrence with our experience, the Andromede galaxy is 178x63 arc minutes in size, and has a magnitude of 3.4 We view it at an angle of about 30 degrees from the perpendicular, and it's 2.2 million light years away. I'm guessing the neutron star in this case was ejected more like 60-90 degrees up from the galactic plane.

If the planet orbits such that the neutron star is interposed between it and the galaxy, there'd probably be some cool gravitational lensing every couple weeks, much more spectacular than the star itself would appear.

[Surlethe]

I'm guessing the neutron star in this case was ejected more like 60-90 degrees up from the galactic plane.

That's what I meant when I said I was assuming you're looking straight down on the galaxy: you're perpendicular to the galactic plane.

[Kuroneko]

If you're traveling at about 0.1c relative to the galaxy (a quick back-envelope calculation shows the Sun is traveling at 0.004c relative to the galactic core, for comparison), and you're ten million years out of the galaxy, then you're some million light years away from the galaxy.

That's unrealistic. The Sun's velocity relative to the galactic core is 217km/s, which is 7.24e-4c. The fastest star ever detected was only travelling at 670km/s nearly perpendicular to the disk, more than than enough to escape the galaxy. Assuming an average speed of 0.0020c over fifty million years, the star will only be a hundred thousand lightyears away, or 6.3e9AU, making it about 53° of the sky. Assuming a galaxy of about 3e8 stars of solar luminosity on average, it will appear 3e8/(6.3e9)² = 7.5e-12 as luminous as the Sun, making its apparent magnitude -26.74 - log[7.5e-12]/log[100^{1/5}] = 1.1. Or somewhat dimmer, due to interstellar dust.

For Surlethe's calculations, the Milky Way at an absolute magnitude of -20.5 would appear as an extremely faint disc with an apparent magnitude of ~1.9 or so. A few other prominant galaxies would still be visible, along with, perhaps, some field stars.

Under Surlethe's distance of 3.1e6 parsecs, the apparent luminosity is -20.5 + 5[log10(3.1e6)-1] = 7.0. If a luminosity of 3e8 suns is assumed instead (as I have above), it will be 6.1. Either way, it will not be visible to the human eye and still about two orders of magnitude away from your estimate.

[Surlethe]

If you're traveling at about 0.1c relative to the galaxy (a quick back-envelope calculation shows the Sun is traveling at 0.004c relative to the galactic core, for comparison), and you're ten million years out of the galaxy, then you're some million light years away from the galaxy.

That's unrealistic. The Sun's velocity relative to the galactic core is 217km/s, which is 7.24e-4c. The fastest star ever detected was only travelling at 670km/s nearly perpendicular to the disk, more than than enough to escape the galaxy. Assuming an average speed of 0.0020c over fifty million years, the star will only be a hundred thousand lightyears away, or 6.3e9AU, making it about 53° of the sky. Assuming a galaxy of about 3e8 stars of solar luminosity on average, it will appear 3e8/(6.3e9)² = 7.5e-12 as luminous as the Sun, making its apparent magnitude -26.74 - log[7.5e-12]/log[100^{1/5}] = 1.1. Or somewhat dimmer, due to interstellar dust.

I was attempting to go for a lower limit on size. Apparently, I neglected to mention that, as I should have; mea culpa.

[Ariphaos]

Under Surlethe's distance of 3.1e6 parsecs, the apparent luminosity is -20.5 + 5[log10(3.1e6)-1] = 7.0. If a luminosity of 3e8 suns is assumed instead (as I have above), it will be 6.1. Either way, it will not be visible to the human eye and still about two orders of magnitude away from your estimate.

He said a million light years (1e5 parsecs), not a million parsecs. -20.5 + 5[log10(3.16e5)-1] = ~2.0 (I used a flat 3e5 for my first calculation).

[Ariphaos]

He said a million light years (1e5 parsecs), not a million parsecs. -20.5 + 5[log10(3.16e5)-1] = ~2.0 (I used a flat 3e5 for my first calculation).

Err, ghetto edit, 3.16e5 parsecs - brainfart.

[Kuroneko]

He said a million light years (1e5 parsecs), not a million parsecs. -20.5 + 5[log10(3.16e5)-1] = ~2.0 (I used a flat 3e5 for my first calculation).

Aa. Of course. I probably misremembered his ten million years as light-years. Mea culpa.