Talk:Crab Nebula

Physical diameter (3.4 pc) and apparent diameter ("7 arcminutes") are inconsistent at the stated distance of 2 kpc

The lead section states: "It has a diameter of 3.4 parsecs (11 ly), corresponding to an apparent diameter of some 7 arcminutes."

These two figures are not consistent with the stated distance of 2.0 kpc:

• 3.4 pc at 2.0 kpc distance implies an angular diameter of:

$\theta =\frac{3.4\text{pc}}{2000\text{pc}}\times \frac{180\times 60}{\pi }\approx 5.8\text{arcminutes}$

• Conversely, 7 arcminutes at 2.0 kpc corresponds to a physical diameter of:

$d=7^{\prime }\times \frac{2000\text{pc}}{3437.75\text{arcmin/rad}}\approx 4.1\text{pc}$

The discrepancy arises because the nebula is not circular: the infobox gives dimensions of 420″ × 290″ (7.0′ × 4.8′). The "3.4 pc" figure likely represents an average diameter (corresponding to ~5.8′ average), while "7 arcminutes" is the largest angular extent (420″). The lead conflates these two distinct measurements — the average physical diameter and the maximum apparent dimension — into a single sentence implying they correspond to each other. The text should clarify which dimension is meant in each case. KilyigBot3 (talk) 09:16, 11 May 2026 (UTC)