Naming the place and placing the name

“By the way, Cathleen, is there any rhyme or reason to that three-object object‘s funky name?  I’ve still got it on Old Reliable here.”

PSR J0337+1715

“It’s nothing like funky, Sy, it’s perfectly reasonable and in fact it’s far more informative than a name like ‘Barnard’s Star.’  The ‘PSR‘ part says that the active object, the reason anyone even looked in that system’s direction, is a pulsar.”

“And the numbers?”

“Its location in two parts.  Imagine a 24-hour clockface in the Solar Plane.  The zero hour points to where the Sun is at the Spring equinox.  One o’clock is fifteen degrees east of that, two o’clock is another fifteen degrees eastward and so on until 24 o’clock is back pointing at the Springtime Sun.  Got that?”

“Mm, … yeah.  It’d be like longitudes around the Earth, except the Earth goes around in a day and this clock looks like it measures a year.”

“Careful there, it has nothing to do with time.  It’s just a measure of angle around the celestial equator.  It’s called right ascension.

“How about intermediate angles, like between two and three o’clock?”

“Sixty arc-minutes between hours, sixty arc-seconds between arc-minutes, just like with time.  If you need to you can even go to tenths or hundredths of an arc-second, which divide the circle into … 8,640,000 segments.”

“OK, so if that’s like longitudes, I suppose there’s something like latitudes to go with it?”

“Mm-hm, it’s called declination.  It runs perpendicular to ascension, from plus-90° up top down to 0° at the clockface to minus-90° at the bottom.  Vivian, show Sy Figure 3 from your paper.”Right ascension and declination“Wait, right ascension in hours-minute-seconds but declination in degrees?”

“Mm-hm.  Blame history.  People have been studying the stars and writing down their locations for a long time.  Some conventions were convenient back in the day and we’re not going to give them up.  So anyway, an object’s J designation with 4-digit numbers tells you which of 13 million directions to look to find it.  Roughly.”


“That’s what the ‘J‘ is about.  If the Earth’s rotation were absolutely steady and if the Sun weren’t careening about a moving galaxy, future astronomers could just look at an object’s angular designation and know exactly where to look to find it again.  But it’s not and it does and they won’t.  The Earth’s axis of rotation wobbles in at least three different ways, the Sun’s orbit around the galaxy is anything but regular and so on.  Specialists in astrometry, who measure things to fractions of an arc-second, keep track of time in more ways than you can imagine so we can calculate future positions.  The J-names at least refer back to a specific point in time.  Mostly.  You want your mind bent, look up epoch some day.”

“Plane and ship navigators care, too, right?”

“Not so much.  Earth’s major wobble, for instance, shifts our polar positions only about 40 parts per million per year.  A course you plotted last week from here to Easter Island will get you there next month no problem.”

Old Reliable judders in my hand.  Old Reliable isn’t supposed to have a vibration function, either.  Ask her about interstellar navigation.  “Um, how about interstellar navigation?”Skewed Big Dipper

“Oh, that’d be a challenge.  Once you get away from the Solar System you can’t use the Big Dipper to find the North Star, any of that stuff, because the constellations look different from a different angle.  Get a couple dozen lightyears out, you’ve got a whole different sky.”

“So what do you use instead?”

“I suppose you could use pulsars.  Each one pings at a unique repetition interval and duty cycle so you could recognize it from any angle.  The set of known pulsars would be like landmarks in the galaxy.  If you sent out survey ships, like the old-time navigators who mapped the New World, they could add new pulsars to the database.  When you go someplace, you just triangulate against the pulsars you see and you know where you are.”

If they happen to point towards you! You only ever see 20% of them.  Starquakes and glitches and relativistic distortions mess up the timings.  Poor Xian-sheng goes nuts each time we drop out of warp.

~~ Rich Olcott


Rhythm Method

A warm Summer day.  I’m under a shady tree by the lake, watching the geese and doing some math on Old Reliable.  Suddenly a text-message window opens up on its screen.  The header bar says 710-555-1701.  Old Reliable has never held a messaging app, that’s not what I use it for.  The whole thing doesn’t add up.  I type in, Hello?

Hello, Mr Moire.  Remember me?

Suddenly I do.  That sultry knowing stare, those pointed ears.  It’s been a yearHello, Ms Baird.  What can I do for you?

Another tip for you, Mr Moire.  One of my favorite star systems — the view as you approach it at near-lightspeed is so ... meaningful.  Your astronomers call it PSR J0337+1715.

So of course I head over to Al’s coffee shop after erasing everything but that astronomical designation.  As I hoped, Cathleen and a few of her astronomy students are on their mid-morning break.  Cathleen winces a little when she sees me coming.  “Now what, Sy?  You’re going to ask about blazars and neutrinos?”

I show her Old Reliable’s screen.  “Afraid not, Cathleen, I’ll have to save that for later.  I just got a message about this star system.  Recognize it?”

“Why, Sy, is that a clue or something?  And why is the lettering in orange?”

“Long story.  But what can you tell me about this star system?”

“Well, it’s probably one of the most compact multi-component systems we’re ever going to run across.  You know what compact objects are?”

“Sure.  When a star the size of our Sun exhausts most of its hydrogen fuel, gravity wins its battle against heat.  The star collapses down to a white dwarf, a Sun-full of mass packed into a planet-size body.  If the star’s a bit bigger it collapses even further, down to a neutron star just a few miles across.  The next step would be a black hole, but that’s not really a star, is it?”

“No, it’s not.  Jim, why not?”

“Because by definition a black hole doesn’t emit light.  A black hole’s accretion disk or polar jets might, but not the object itself.”

“Mm-hm.  Sy, your ‘object’ is actually three compact objects orbiting  around each other.  There’s a neutron star with a white dwarf going around it, and another white dwarf swinging around the pair of them.  Vivian, does that sound familiar?”

“That’s a three-body system, like the Moon going around the Earth and both going around the Sun.  Mmm, except really both white dwarfs would go around the neutron star because it’s heaviest and we can calculate the motion like we do the Solar System.”

“Not quite.  We can treat the Sun as motionless because it has 99% of the mass.  J0337+1715’s neutron star doesn’t dominate its system as much as the Sun does ours.  That outermost dwarf has 20% of its system’s mass.  Phil, what does that suggest to you?”

“It’d be like Pluto and Charon.  Charon’s got 10% of their combined mass and so Pluto and Charon both orbit a point 10% of the way out from Pluto.  From Earth we see Pluto wobbling side to side around that point.  So the neutron star must wobble around the point 20% outward towards the heavy dwarf.  Hey, star-wobble is how we find exoplanets.  Is that what this is about, Mr Moire?  Did someone measure its red-shift behavior?”PSR J0337+1715Cathleen saves me from answering.  “Not quite.  The study Sy’s chasing is actually a cute variation on red-shift measurements.  That ‘PSR‘ designation means the neutron star is a pulsar.  Those things emit electromagnetic radiation pulses with astounding precision, generally regular within a few dozen nanoseconds.  If we receive slowed-down pulses then the object’s going away; sped-up and it’s approaching, just like with red-shifting.  The researchers  derived orbital parameters for all three bodies from the between-pulse durations.  The heavy dwarf is 200 times further out than the light one, for instance.  Not an easy experiment, but it yielded an important result.”

My ears perk up.  “Which was…?”

“The gravitational force between the pulsar and each dwarf was within six parts per million of what Newton’s Laws prescribe.  That observation rules out whole classes of theories that tried to explain galaxies and galaxy clusters without invoking dark matter.”

Cool, huh?


~~ Rich Olcott