When You See a Pulsar with Two White Dwarfs…

A unique way to investigate the effects of gravity in a triple star system.

This week thousands of astronomers have converged on Washington D.C. for the winter American Astronomical Society meeting. This is a huge confab where scientists who really DO know a thing or two about the stars gather to discuss and share their work.

So, what are they talking about AAS?  Pulsars, for one thing. You  may have heard of these strange cosmic beasts. They are rapidly spinning neutron stars, left over after some supermassive stars destroy themselves in huge explosion called supernovae. They’re fascinating objects in their own right, but when you find one orbiting with two white dwarf stars, it’s a cosmic jackpot of interesting things to study.

So, today, the folks at the National Radio Astronomy Observatory presented a first look at such a jackpot. Most of the time when you think of triple star systems, you probably think of “regular” stars, not necessarily a grouping of such unusual objects. The questions that immediately sprang to my mind when I heard about this system were, “How did they get together in such a configuration?” “What’s their gravitational field like?” “Is this a stable system?”

Turns out these are questions still to be answered. Think about the ramifications of such a combo of stars. None of the members of this trio are what you might call “regular” stars. They are all products of star death. White dwarfs are formed as the end stage of stars similar to the Sun. They’re very dense, slowly cooling objects.  A neutron star is what’s left after a supernova explosion destroys a supermassive star. It spins on its axis very quickly, and is so dense that a fistful of it would be around the density of Earth. So, you have three objects that are very dense and one of them is spinning like a top and giving off a signal. Moreover, if you put them in our solar system, they’d be doing their orbital dance inside the orbit of Earth.

Dense objects have strong gravity, so that means they’re constantly interacting with each other gravitationally. Astronomers wanted to study this system and figure out the mass of each object, but had to contend with that strong gravitational field, which  makes things difficult.  Also, they wanted to measure the gravitational interactions these objects have with each other.

But how to do that? One clue to revealing such details about this system lies with the neutron star. It gives off a pulse of radiation as it spins. Astronomers call that a pulsar. It spins nearly 366 times per second! The best way to study the pulsar was to use as many telescopes as they could. The Green Bank Telescope in Virginia, the Arecibo Telescope in Puerto Rico, and the Westerbork Synthesis Radio Telescope in the Netherlands, all studied radio-frequency signals from the pulsar. Astronomers also used data taken by the Sloan Digital Sky Survey, the GALEX satellite, the WIYN telescope at Kitt Peak in Arizona, and the Spitzer Space Telescope, to get a handle on other characteristics of the system, such as brightnesses.

All that data allowed them to use the millisecond pulsar signal to probe the gravitational influence each star in the system has on the others. Essentially, they observed the pulsar and timed the arrival of each pulse at Earth and then used that data to calculate the geometry of the system and also the masses of each of the stars very precisely. Essentially, what astronomers have in this system is a natural laboratory where they can now test theories of gravity.

This is the kind of science that doesn’t always make the news a lot of the time because it doesn’t produce flashy pictures. Yet, much of astronomy research goes in in realms of light we can’t see—such as radio. Objects in the universe radiate “light” across the entire electromagnetic spectrum. Each frequency or wavelength that streams from an object such as a star or a planet tells us something about that object. The visible light we see radiating from a star tells us only a small part of that star’s life story.  We need the rest to fill in the entire saga of an object’s life. And, that’s where studies like this one, done by a collection of radio, infrared, and visible-light instruments, are helping to uncover the full story of a trio of stars, held in a complex orbital dance by their combined gravitational fields.

Stay tuned for additional astronomy news from the AAS. We’ve got extrasolar planets, space dust, gravitational lenses, and much, much  more!