RX J1856.5−3754

What is RX J1856.5−3754?

Neutron stars & pulsars· Distance ~400 ly. The closest known neutron star to Earth. It has no radio pulses and no companion — just a 600,000-degree surface glowing faintly in X-rays. With the Sun's mass packed into 20 km, it is a benchmark for the physics of neutron-star matter. It lies in Corona Australis.

Where RX J1856.5−3754 is in your sky right now: https://nowsky.org/solar/sky?sel=rx j1856 — location and time set automatically, no login.

Its place in the sky

A pulsar, in the direction of Corona Australis.

Spin, distance, discovery

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Discovery and name

Found in 1996, one of the nearest neutron stars (400 ly) — leader of the "Magnificent Seven".

Frequently asked

What kind of object is RX J1856.5−3754?

isolated neutron star, one of the thermally emitting "Magnificent Seven".

How long is the orbital period of RX J1856.5−3754?

7.06 s.

How far is RX J1856.5−3754 from Earth?

~400 ly.

When was RX J1856.5−3754 discovered?

1992 by ROSAT; identified as a neutron star in 1996.

Where in the sky is RX J1856.5−3754?

In the direction of Corona Australis.

Where is RX J1856.5−3754 in the sky right now?

Open the sky view for your location and time and it shows the position in the sky right now.

Vacuum birefringence, seen eighty years after it was predicted

In 2016 the European Southern Observatory's VLT observed the faint visible light of this neutron star in polarised light. In light of 26th magnitude a linear polarisation of 16 percent was measured, more than thermal emission from the star's surface alone could produce. The explanation was a phenomenon predicted from quantum electrodynamics by Heisenberg and Euler in 1936: in an extremely strong magnetic field the vacuum itself becomes birefringent and splits light.

Because the effect needs a magnetic field no laboratory on Earth can make, this star with a field of 10¹³ gauss served as nature's apparatus. The observation was published as the first evidence, with the caveat that ruling out other explanations entirely requires more precise polarimetry in X-rays. The nearest neutron star to Earth thus became a place for testing how empty space responds to light.

The neutron star in depth

RX J1856.5-3754, about 400 light-years away toward Corona Australis, is the closest neutron star currently known. It drifts through space alone — no supernova remnant, no companion — as an "isolated neutron star" that emits no radio pulses; only its surface, heated to roughly 600,000 degrees and slowly cooling, glows faintly in X-rays. It rotates once every 7.06 seconds, and astronomers group it with six similar quiet objects as the "Magnificent Seven".

The ROSAT X-ray satellite first caught it in 1992 and it was recognized as a neutron star in 1996. The Hubble Space Telescope later picked out its 26th-magnitude optical counterpart and measured its proper motion and parallax, pinning down the distance; it appears to have been kicked out of the Scorpius–Centaurus association when its parent star exploded about a million years ago. Because its bare surface is directly visible, measurements of its size and temperature serve as a benchmark for testing the equation of state of neutron-star matter.

Corona Australis is a small constellation just below Sagittarius, so from Korea it hangs low over the southern horizon on summer nights. Select RX J1856.5-3754 in the Sky view to mark its spot, and use the nearby-stars 3D view to see this star 400 light-years from the Sun alongside our other stellar neighbors. It is invisible to the eye, but it is worth knowing where the nearest dead stellar core lies.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.