RX J1856 in Optical Light
This optical image of RX J1856.5-3754 portrays a crowded region of star formation. In comparison, the Chandra X-ray image shows that RX J1856 outshines all of the other sources in the field, indicating it is both extremely hot and very small.
(Credit: European Southern Observatory Very Large Telescope)

Size Comparison of RX J1856 to Neutron and Quark Stars
This artist's rendition shows the diameter of RX J1856.5-3754, determined by data from NASA's Chandra X-ray Observatory, is too small to be a neutron star. The data are consistent with predicted size for a strange quark star, an object never before seen in nature.
(Illustration: CXC/M. Weiss)

Neutron Star/Quark Star Interior
In a neutron star (left), the quarks that comprise the neutrons are confined inside the neutrons. In a quark star(right), the quarks are free, so they take up less space and the diameter of the star is smaller.
(Credit: Illustration: CXC/M. Weiss)

Illustration of relative sizes of Grand Canyon, neutron star and quark star
The Grand Canyon is 18 miles rim to rim. A neutron star is about 12 miles in diameter, and a quark star is about 7 miles in diameter.
(Illustration: CXC/D. Berry)

Chandra X-ray Image of 3C58
Chandra observations of 3C58, the remnants of a supernova noted on Earth in AD 1181, reveal that the neutron star in the core has a temperature much lower than expected. This suggests that a new state of nuclear matter might exist inside the star.
(Credit: NASA/CXC/CfA/P.Slane et al.)

Chandra X-ray Image with Scale Bar for 3C58
Scale bar = 1.4 arcmin
Credit: NASA/SAO/CXC/P.Slane et al.