by Ann » Thu Aug 30, 2018 4:06 am
Chris Peterson wrote: ↑Wed Aug 29, 2018 6:08 pm
Ann wrote: ↑Wed Aug 29, 2018 12:16 pm
APOD Robot wrote:
An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?
Actually, the true brightness itself is completely irrelevant to the story here. Indeed, stating the true brightness might even confuse the actual point being made.
I was obviously grumbling and very much exaggerating. Nevertheless, I'm not backing down all the way. RS Pup is interesting not because it is a bright star, but because it is one of the brightest known examples of a pulsating star whose true brightness is intimately linked to the length of its pulsation periods. Therefore stars as RS Pup are used as standard candles for distance estimation.
Certainly it is interesting to know exactly how bright RS Pup is, given the fact that it belongs to a class of stars that are used as standard candles?
According to a source quoted by Art, the true (V?) magnitude of RS Pup is about -5. Interesting! If the true V magnitude of RS Pup had been -4 instead, then the pulsating Cepheids would have been different as standard candles. And if the true V magnitude of RS Pup had been -6, then again the pulsating Cepheids would have been another kind of standard candles. And maybe, just maybe, astronomy may have had to reassess its "cosmic distance ladder" a little bit, if it had turned out that RS Pup was either brighter or fainter than expected.
You might still argue that it doesn't matter exactly
how bright RS Pup is, as long as its brightness agrees with what astronomers had expected, so that the cosmic distance ladder doesn't have to be adjusted. Okay, fine.
I still don't see why the APOD caption couldn't have said, for example, that the true V magnitude of RS Pup when it is at its brightest is -5, which makes it some 10 magnitudes brighter than the Sun, which, I guess, makes it some 10,000 or so times brighter than the Sun in visual light. And then the caption could have added that this is how bright classic Cepheids can get, and to be that bright they have to have a period of over 40 days.
Ann
[quote="Chris Peterson" post_id=285388 time=1535566118 user_id=117706]
[quote=Ann post_id=285379 time=1535545017 user_id=129702]
[quote]APOD Robot wrote:
An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup[/quote]
Isn't it an intolerable act of cheating to tell us that the true brightness of RS Pup has been established, without telling us what the true brightness of RS Pup actually is?
[/quote]
Actually, the true brightness itself is completely irrelevant to the story here. Indeed, stating the true brightness might even confuse the actual point being made.
[/quote]
I was obviously grumbling and very much exaggerating. Nevertheless, I'm not backing down all the way. RS Pup is interesting not because it is a bright star, but because it is one of the brightest known examples of a pulsating star whose true brightness is intimately linked to the length of its pulsation periods. Therefore stars as RS Pup are used as standard candles for distance estimation.
Certainly it is interesting to know exactly how bright RS Pup is, given the fact that it belongs to a class of stars that are used as standard candles?
According to a source quoted by Art, the true (V?) magnitude of RS Pup is about -5. Interesting! If the true V magnitude of RS Pup had been -4 instead, then the pulsating Cepheids would have been different as standard candles. And if the true V magnitude of RS Pup had been -6, then again the pulsating Cepheids would have been another kind of standard candles. And maybe, just maybe, astronomy may have had to reassess its "cosmic distance ladder" a little bit, if it had turned out that RS Pup was either brighter or fainter than expected.
You might still argue that it doesn't matter exactly [i]how[/i] bright RS Pup is, as long as its brightness agrees with what astronomers had expected, so that the cosmic distance ladder doesn't have to be adjusted. Okay, fine.
I still don't see why the APOD caption couldn't have said, for example, that the true V magnitude of RS Pup when it is at its brightest is -5, which makes it some 10 magnitudes brighter than the Sun, which, I guess, makes it some 10,000 or so times brighter than the Sun in visual light. And then the caption could have added that this is how bright classic Cepheids can get, and to be that bright they have to have a period of over 40 days.
Ann