On July 8th, 2018 we learned about

Triple star system demonstrates that gravity is agnostic to the density of neutron stars

To test an idea scientifically, you should aim to have an experimental condition that you can compare to a control. So for example, if you want to see if food coloring helps your flowers grow faster, you’d want to grow the same kind of flowers without food coloring for comparison. Now, setting up experimental conditions isn’t so tough when you’re talking about small plants in a flowerbed, but what if you’re interested in something that could only be observed in some of the most extreme conditions in the universe? Maybe something like the nature of gravity in ultra-dense neutron stars? In that case, you may need to get lucky and find PSR J0337+1715, a star system apparently ready-made for exactly those kinds of questions.

Different types of dying stars

The unfortunately-named PSR J0337+1715 is a triple star system some 4,200 light years from Earth. Two of these stars are white dwarfs, meaning they’re the decaying cores of larger stars from long ago. They no longer have the temperatures needed to enable nuclear fusion and push their mass outwards, and as such are collapsing back into themselves. This means that a white dwarf with the mass of our Sun would be only have the volume of Earth, making these stars significantly more dense than what’s in our solar system, although a lot of their shrinkage is thanks to material being lost altogether.

While this is clearly different from our Sun, in the case of PSR J0337+1715 the white dwarfs are providing the normal baseline for researchers’ observations. The neutron star, on the other hand, is where the major questions lie. Instead of slowly degrading like a white dwarf, a neutron star is the result of a much larger star collapsing all at once. It packs a lot more mass into a lot less space, leaving it with an incredible density, gravitational and magnetic fields. The gravitational field is so strong that an object falling from three feet above a neutron star’s surface would instantly fall at over three million miles per hour while also being spaghettified due to tidal forces. There was also a chance that these extremes would cause the whole neutron star to interact with gravity as a whole in a different way than less-dense objects, such as some conveniently-positioned white dwarf stars.

Detailed tracking of next to no deviation

As it happened, the neutron star of PSR J0337+1715 is paired with one of the white dwarf stars in its orbit around the second white dwarf. This means that researchers could track how both each of these dead stars moved around the same object, looking for differences in their acceleration that would point a difference in how gravity was affecting the neutron star. If that weren’t convenient enough, this particular neutron star was actually a pulsar, meaning it was emitting a strong blast of radio waves 366 times per second. This broadcast could then be used like a tracking device, allowing the Green Bank Telescope to follow the neutron star’s movement with incredible fidelity. Even though it was over 4,000 light years away, the neutron star’s location could be tracked to within a few hundred feet. While they couldn’t say that the neutron star’s orbit behaved in absolutely the same manner as its white dwarf partner, researchers could at least be confident any variation would be less than three parts per million.

As perfect as this triple star system was for measuring this kind of movement in a neutron star, none of this data is particularly surprising. While there have been proposals that super-dense objects like neutron stars would bend the rules of gravity, the observations from PSR J0337+1715 basically support predictions Albert Einstein made in his general theory of relativity. Known as the Equivalence Principle, the idea is that all objects interact with gravity in the same way, even if the mass involved is dense enough to crush and strip an object into goo at over three million miles an hour.

Source: Even phenomenally dense neutron stars fall like a feather by Green Bank Observatory, Science Daily

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