RX J1856.5−3754


RX J1856.5−3754 is a nearby neutron star in the constellation Corona Australis. It is the closest neutron star to Earth yet discovered.

Discovery and location

RX J1856.5−3754 is thought to have formed in a supernova explosion of its companion star about one million years ago and is moving at 108 km/s across the sky. It was discovered in 1992, and observations in 1996 confirmed that it is a neutron star, the closest to Earth discovered to date.
It was originally thought to be about 150–200 light-years away, but further observations using the Chandra X-ray Observatory in 2002 indicate that its distance is greater—about 400 light-years.
RX J1856 is one of the Magnificent Seven, a group of young neutron stars at distances between 130 and 500 parsecs of Earth.

Quark star hypothesis

By combining Chandra X-ray Observatory and Hubble Space Telescope data, astronomers previously estimated that RX J1856 radiates like a solid body with a temperature of 700,000 °C and has a diameter of about 4–8 km. This estimated size was too small to reconcile with the standard models of neutron stars, and it was therefore suggested that it might be a quark star.
However, later refined analysis of improved Chandra and Hubble observations revealed that the surface temperature of the star is lower, only 434,000 °C, and respectively the diameter is larger, about 14 km. Thus, RX J1856.5-3754 is now excluded from the list of quark star candidates.