There are an estimated 100 million neutron stars within the Milky Way galaxy alone, but we have only detected a few because these densest-known objects in the universe (black holes are more dense, but it is currently impossible to know by how much) are mostly old and cold, and devoid of any giveaway radiation. Usually, they are detected only when they are either pulsars — a special type of neutron star — or have another star as a companion, and finding lone neutron stars is quite a rarity.

Using data from NASA’s Chandra X-ray observatory in space, and the European Southern Observatory’s Very Large Telescope (VLT) in Chile, astronomers have found the first-ever neutron star that isn’t in a binary system outside of our home galaxy. It is located inside the remains of a supernova called 1E 0102.2-7219 (E0102 for short) in the Small Magellanic Cloud, at a distance of about 200,000 light-years from Earth.

This neutron star is devoid of a companion, whose presence otherwise leads to neutron stars accreting material from it, which leads to radiation, making them detectable. Unlike most neutron stars, which have magnetic fields at least 100 million times stronger than Earth’s, the one within E0102 has a very weak magnetic field relatively. It is also rich in oxygen.

E0102 Neutron Star
This image provided by NASA shows the remnant of a supernova with a lone neutron star within it. There are only about 10 such neutron stars without companions detected within the Milky Way, and this one, in the Small Magellanic Cloud, is the first one to be found outside our galaxy. X-ray (NASA/CXC/ESO/F.Vogt et al); Optical (ESO/VLT/MUSE & NASA/STScI)

In the image provided by NASA, the X-ray emissions detected by Chandra are shown in blue and purple, while the visible light captured by VLT is bright red. The bits in dark red and green are based on additional date from the Hubble Space Telescope.

The supernova remnant appears as a large ring-shaped structure in X-rays, likely blown by the blast wave of the explosion. The green filaments indicate oxygen-rich debris from the exploding star’s interior that was ejected at the time of the supernova and is moving through space at the speed of millions of miles an hour. The smaller red ring within it, revealed by the VLT data, is expanding at a slower rate than the surrounding blast wave. At the center of the smaller ring is a blue dot, the lonely neutron star.

The X-ray energy readings from this central blue dot are similar to the X-ray signatures from two known loner neutron stars that are within the Milky Way — Cassiopeia A (Cas A) and Puppis A — where the supernova remnants are also oxygen-rich. A total of about 10 such objects have been found so far within our galaxy.

Scientists are still unsure how the neutron star came to be at a position with the supernova shell that is off the center. They have at least two different theories, but neither is without its own set of difficult-to-answer questions. Astronomers hope further observations in X-ray, optical and radio emissions would help understand this curious object better.

Neutron stars form following the collapse of massive stars, those with roughly between 10 and 30 solar masses. After a massive star goes supernova, and the mass isn’t high enough for its remnant core to turn into a black hole, a neutron star is formed. These objects generate no heat of their own, and slowly cool over time, which is why they become undetectable once they are old. The only way for them to evolve further is via collisions or accretion.

The results of the current observations were publisher in the journal Nature Astronomy and the pper is available online on the pre-print server arXiv.