🌟 First, What Are Neutron Stars And Pulsars?
Neutron stars are the super-dense leftovers of big stars that exploded. Imagine squeezing the Sun into a city-sized sphere—so dense that a teaspoon would weigh as much as a mountain. Some neutron stars act like cosmic lighthouses, sweeping beams of radio waves past Earth as they spin. We call these pulsars. Because their pulses arrive like perfectly timed ticks, pulsars are fantastic natural clocks.
Sometimes pulsars orbit another star. When that happens, their clock-like signals can reveal the details of the orbit—and even let us weigh the stars involved.
🌀 The Clever Trick: Watching An Orbit Slowly Turn
Planets and stars in space often move in stretched-out circles called ellipses. In very strong gravity, that ellipse doesn’t stay fixed—it slowly rotates, like a hula hoop turning in slow motion. This effect is called the advance of periastron, and it’s the same kind of subtle shift first seen in Mercury’s orbit and explained by Einstein’s general relativity.
Pulsars help here because their pulses act like stopwatch ticks. By tracking exactly when pulses arrive over months and years, astronomers can measure how fast the orbit’s ellipse rotates. The faster this “turn,” the heavier the system. That measurement gives the total mass of the two stars together. It’s a bit like weighing a pair of dancers by how quickly their dance floor twists under their steps.
🔭 Meet The Two Pulsar Duos
Researchers timed two binary pulsar systems and measured how their orbits slowly rotate each year.
PSR B1802−07: This pulsar lives in a crowded star cluster. It circles its partner every ~2.6 days in a slightly stretched orbit. The orbit’s ellipse turns by about 0.06 degrees per year—tiny, but measurable. That points to a total mass of about 1.7 times the Sun’s mass. The data suggest the companion is likely a white dwarf weighing roughly one-third of the Sun, while the pulsar itself is around 1.4 Suns.
PSR B2303+46: This pair takes about 12.3 days to orbit, and the path is quite elongated. The orbit’s rotation is slower—about 0.01 degrees per year—but the measurement is very precise. The total mass comes out to about 2.53 Suns. That strongly hints both objects are neutron stars, each roughly between 1.2 and 1.4 times the Sun’s mass.
📈 The Big Picture: Neutron Stars Have A “Favorite” Mass
Putting these results alongside many others, a pattern stands out: most neutron stars seem to cluster around 1.3–1.4 times the mass of the Sun. Even though physics allows a wider range, nature appears to build neutron stars that are surprisingly similar in weight.
That consistency tells us something important about how massive stars die and collapse, and about the ultra-dense matter packed inside neutron stars. It also provides a reality check for theories of nuclear physics under extreme pressure.
🚀 Why This Matters (And A Fun Origin Story)
Precise pulsar timing is one of the best ways to test gravity in strong fields and to probe the inner workings of dead stars. These measurements:
- Sharpen our estimates of how heavy neutron stars really are
- Constrain models of how matter behaves at mind-bending densities
- Help map the family tree of binary systems, including those that later merge and create gravitational waves
One of the pairs, PSR B1802−07, likely formed through a dramatic close encounter in its crowded cluster—imagine a bumper-car arena of stars. That kind of encounter can leave behind an odd, more stretched orbit and a white-dwarf partner, exactly what the measurements suggest.
As astronomers keep timing more pulsars—and combine results with gravitational-wave observations—we’ll keep refining the cosmic scale for some of the universe’s most extreme objects.
Source Paper’s Authors: S. E. Thorsett, Z. Arzoumanian, M. M. McKinnon, J. H. Taylor
PDF: https://arxiv.org/pdf/astro-ph/9303002v2