<p>Yes, the neutron star <a href="https://en.wikipedia.org/wiki/RX_J1856.5-3754" rel="nofollow">RX J1856.5-3754</a> 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 <a href="https://en.wikipedia.org/wiki/Category:Neutron_stars" rel="nofollow">neutron star</a> whose radius is known.</p>

<hr>

<p>* 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 <a href="https://arxiv.org/pdf/astro-ph/0612145v1.pdf" rel="nofollow">Magnetic Hydrogen Atmosphere Models and the Neutron Star RX J1856.5−3754 - Wynn C. G. Ho, et al.</a></p>

CC BY-SA 3.0
2016-07-14T21:33:40.930Z
It might be worth including how this measurement was done: Not by direct imaging or course, but by considering the temperature and the brightness.
James K
CC BY-SA 3.0
2016-07-15T14:29:35.687Z
@JamesKilfiger Added. This is not my subject area--I'm just reading the paper, so let me know if I have introduced any inaccuracies.
called2voyage
CC BY-SA 3.0
2016-07-19T18:58:51.227Z
@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 .....
PERFESSER CREEK-WATER
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2016-07-19T19:00:30.600Z
@PERFESSERCREEK-WATER On the contrary, I find that there were more assumptions made for the 5km figure.
called2voyage
CC BY-SA 3.0
2016-07-19T19:08:59.697Z
@called2voyage Well, I can't argue with you there, as I don't know enough about the details of either of the findings.
PERFESSER CREEK-WATER
CC BY-SA 3.0
2016-07-13T21:15:05.480Z
2016-07-15T14:28:50.627Z
2016-07-15T14:28:50.627Z
called2voyage
<p>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 <em>not</em> be reckoned from the apparent size of the object, or from theory, otherwise one is guilty of circular reasoning!</p>

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2016-07-14T21:02:54.430Z
Crossposted from http://physics.stackexchange.com/q/265006/2451
Qmechanic
CC BY-SA 3.0
2016-07-15T14:32:07.547Z
@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.
called2voyage
CC BY-SA 3.0
2016-07-15T16:29:50.457Z
@called2voyage: The Phys.SE question has been edited (see v4).
Qmechanic
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2016-07-15T16:31:53.767Z
@Qmechanic It has been rolled back to the original state which is different than this question.
called2voyage
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2016-07-19T18:29:03.777Z
@Qmechanic Plus: This discussion is now in a chat-room, titled "Discussion on question by PERFESSER CREEK-WATER"
PERFESSER CREEK-WATER
<observational-astronomy><neutron-star>
Are there any reliable optical measurements of the radius of a neutron star?
PERFESSER CREEK-WATER
# Are there any reliable optical measurements of the radius of a neutron star?

(Asked by: username_0 on 2016-07-13)

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!

**Tags:** observational-astronomy, neutron-star
> **Comments:**
> **username_1**: Crossposted from http://physics.stackexchange.com/q/265006/2451  
> **username_2**: @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.  
> **username_1**: @called2voyage: The Phys.SE question has been edited (see v4).  
> **username_2**: @Qmechanic It has been rolled back to the original state which is different than this question.  
> **username_0**: @Qmechanic Plus: This discussion is now in a chat-room, titled "Discussion on question by PERFESSER CREEK-WATER"  

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## 1 answers

### Answer by username_2 on 2016-07-13. Score: 4

Yes, the neutron star [RX J1856.5-3754](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Category:Neutron_stars) 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.](https://arxiv.org/pdf/astro-ph/0612145v1.pdf)

> **Comments:**
> **username_3**: It might be worth including how this measurement was done: Not by direct imaging or course, but by considering the temperature and the brightness.  
> **username_2**: @JamesKilfiger Added. This is not my subject area--I'm just reading the paper, so let me know if I have introduced any inaccuracies.  
> **username_0**: @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 .....  
> **username_2**: @PERFESSERCREEK-WATER On the contrary, I find that there were more assumptions made for the 5km figure.  
> **username_0**: @called2voyage Well, I can't argue with you there, as I don't know enough about the details of either of the findings.  

---

