I looked at today’s APOD and wondered “how big is that galaxy?”
The article said “the arcing structures form tenuous loops extending more than 150,000 light-years from the narrow, edge-on spiral,” so a thumbnail measurement told me that the disc was around 150,000 light-years across… assuming the loops and the disc were all the same distance from the camera. Clicking on the links in the article gave me a bunch of information that did not include helpful things like “this galaxy has an estimated diameter of…”
But they DID tell me that it was 40,000,000 light-years away*, and was 12.8 arc-seconds in diameter as observed from Earth. It’s been a long time since I took any trigonometry, but I remembered enough to know that I had all the information I needed to calculate diameter. I just needed to google the formula.
The formula, so you know, is D (diameter) = (2pi*(distance)(angular diameter))/360, where angular diameter and “360” are both expressed in degrees. Thus “degrees/degrees” drops that unit from the result, leaving only the unit of measure used for “distance” behind to be applied to the diameter (or in this case “length” since the object being observed is does not present a disc)
Rounding to the comma, the galaxy in question is 146,000 light-years in diameter.
(*Note: I didn’t ask how astronomers measured the distance to the galaxy in question, but I assume it has to do with luminosity, red-shift, and other stuff. Obviously if the distance figure changes as a result of new measurements, the calculated diameter of the object will also change.)