STAR DIAMETERS ARE UNKNOWN
Reader of Robitaille Said That
SIRIUS A
Robitaille explained at youtube.com/watch?v=sEz71ULRqEA that white dwarfs are not super-dense. A reader @JoshuaTreeObservatory commented a little off-topic saying no star diameters have ever been resolved by any telescope. I asked Perplexity.ai if the reader’s statements is true as follows.
Isn't it true that “even in the biggest of telescopes, no angular diameter of Sirius A or B (or almost any star at all other than the Sun) can be resolved”? It answered below that Sirius has been resolved to an apparent diameter of ~6mas; mas = milliarcseconds. (PS, the Moon’s apparent diameter viewed from Earth is ~1,800,000mas.)
Answer
This statement is not entirely accurate. While it's true that most stars cannot be resolved by even the largest optical telescopes, Sirius A is an exception. Its angular diameter has been successfully measured using interferometric techniques. The angular diameter of Sirius A has been determined to be approximately 6.041 ± 0.017 milliarcseconds (mas)1,2. This measurement was made using interferometry, specifically with instruments like the Sydney University Stellar Interferometer (SUSI) and the Very Large Telescope Interferometer (VLTI)1,2. Interferometry allows astronomers to achieve much higher angular resolution than traditional single-telescope observations. By combining light from multiple telescopes or apertures, interferometers can effectively create a much larger virtual telescope, enabling the measurement of very small angular diameters. For context, Sirius A's angular diameter of about 6 mas is equivalent to the size of a tennis ball viewed from a distance of about 400 kilometers. While this is indeed a tiny angular size, it is measurable with current technology. It's important to note that while Sirius A can be resolved, Sirius B and most other stars remain unresolved even with interferometry due to their smaller apparent sizes3,5.
Then I asked: How was the .017 margin of error calculated?
Answer
The 0.017 mas margin of error for Sirius A's limb-darkened angular diameter was calculated through a weighted mean of two separate measurements:
The Sydney University Stellar Interferometer (SUSI) measurement of 6.048 ± 0.040 mas1,3.
A conventionally calibrated measurement from the Very Large Telescope Interferometer (VLTI) at 2.176 μm1,3.
The weighted mean of these two results produced the final value of 6.041 ± 0.017 mas1,3. This combined measurement achieves a higher precision (±0.28%) than either individual measurement due to the statistical advantage of using multiple independent observations. It's worth noting that this result excluded a second, globally calibrated VLTI measurement that was not in agreement with the other two1,3. By combining the most consistent and reliable measurements, the researchers were able to reduce the margin of error and increase confidence in the final result.
So Sirius A is the only star whose apparent diameter has perhaps been resolved. I’m not totally convinced that it really has been resolved, though. Some day, maybe I’ll try to see if 6mas equates to Sirius A being 4 lightyears away and if it’s about the same size as the Sun. I have reason to believe that most stars are much closer than conventionally stated.