I know that there is plenty of theory to predict the size of a neutron star: my question is whether or not there are any reliable size estimates based on observation. Please note that, to be reliable, such an observation would need to have also a good estimate for the neutron star's distance from us, which must necessarily be reckoned from some independent source, i.e., it must not be reckoned from the apparent size of the object, or from theory, otherwise one is guilty of circular reasoning!

  • $\begingroup$ Crossposted from physics.stackexchange.com/q/265006/2451 $\endgroup$ – Qmechanic Jul 14 '16 at 21:02
  • $\begingroup$ @Qmechanic Not an exact duplicate as currently worded. This focuses on measuring the radius, whereas the one on Physics has been constrained to mere observation. $\endgroup$ – called2voyage Jul 15 '16 at 14:32
  • $\begingroup$ @called2voyage: The Phys.SE question has been edited (see v4). $\endgroup$ – Qmechanic Jul 15 '16 at 16:29
  • $\begingroup$ @Qmechanic It has been rolled back to the original state which is different than this question. $\endgroup$ – called2voyage Jul 15 '16 at 16:31
  • $\begingroup$ @Qmechanic Plus: This discussion is now in a chat-room, titled "Discussion on question by PERFESSER CREEK-WATER" $\endgroup$ – PERFESSER CREEK-WATER Jul 19 '16 at 18:29

Yes, the neutron star RX J1856.5-3754 has an observed* radius of 17 km. After accounting for general relativity, its actual radius is calculated as 14 km. It is not the only neutron star whose radius is known.

* It's worth mentioning how astronomers arrived at this radius. Technically, fitting the X-ray data to a blackbody arrives at a calculation of a 5 km radius, which seems unusually small, so for a time it was believed that this star may be something more exotic, like a quark star. However, after atmospheric modeling based on temperature, a more accurate estimate of the emission radius was derived to be 17 km. Once gravitational redshift was accounted for, the value of 14 km was yielded. See Magnetic Hydrogen Atmosphere Models and the Neutron Star RX J1856.5−3754 - Wynn C. G. Ho, et al.

  • $\begingroup$ It might be worth including how this measurement was done: Not by direct imaging or course, but by considering the temperature and the brightness. $\endgroup$ – James K Jul 14 '16 at 21:33
  • $\begingroup$ @JamesKilfiger Added. This is not my subject area--I'm just reading the paper, so let me know if I have introduced any inaccuracies. $\endgroup$ – called2voyage Jul 15 '16 at 14:29
  • $\begingroup$ @JamesKilfiger I, too, must admit that much of the paper is "over my head" ... however, I get the idea that they made a lot of assumptions, re the composition of the objects atmosphere, magnetic-field strength, etc., with the goal of explaining away the initial finding of a 5km radius ----- which they characterized as "rather small for a neutron star" ... it's well-known that scientists have a TENDENCY to do this, so that their results fit the "accepted" numeric-values ..... $\endgroup$ – PERFESSER CREEK-WATER Jul 19 '16 at 18:58
  • $\begingroup$ @PERFESSERCREEK-WATER On the contrary, I find that there were more assumptions made for the 5km figure. $\endgroup$ – called2voyage Jul 19 '16 at 19:00
  • $\begingroup$ @called2voyage Well, I can't argue with you there, as I don't know enough about the details of either of the findings. $\endgroup$ – PERFESSER CREEK-WATER Jul 19 '16 at 19:08

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