Peter von Schilling wrote: ↑
Thu May 23, 2019 4:54 pm
Hi I'm very new to this.
In the image of RS Pup I notice maybe around 1000 or 1500 small bright spots, which I assume are stars.
Most of these do not have any spikes. What I'm trying to find out: are these stars further away than RS Pup with attached Nebula,
or do they lie in front of RS Pup and Nebula?
If the answer is that they lie "behind" RS Pup, can I draw the conclusion that also in images of other Nebulae,
most of the stars pictured would lie behind the nebula, and thus the nebula must be very thin for these stars
to shine through the nebula in question?
Stars vary enormously in brightness. RS Puppis itself is very bright.
RS Puppis is a supergiant with a spectral classification of G2Ib, although its spectral type varies between F9 and G7 as its temperature changes.
The radius varies between 164 R☉ and 208 R☉, although the changes vary somewhat even from one cycle to the next. The temperature varies between a minimum of 4,640 K and 5,850 K, and the bolometric luminosity between 14,200 L☉ and 29,500 L☉.
A star's bolometric magnitude is its total emission of energy across the electromagnetic spectrum. Stars are always brighter in bolometric magnitude than they are at any specific wavelength. But because RS Puppis is a mid-temperature star, its absolute visual magnitude is not so different from its bolometric one. In short, RS Puppis probably produces about 10,000 to 20,000 as much visual light as the Sun does. That's bright.
The point to understand is that most stars are faint, but the intrinsically bright and reasonably nearby stars stand out very brightly in the sky. Stars as bright as RS Puppis are very rare. Surprisingly enough, even our own Sun is unusually bright as stars go. As a rule of thumb, we can say that 99% of the stars that stand out clearly in the night sky are intrinsically brighter than the Sun, but about 90-95% of all the stars in our galaxy are intrinsically fainter than the Sun. The nearest star to the Earth after the Sun, Proxima Centauri, is a tiny red dwarf star that is 18,000 times fainter than the Sun.
So are the stars that are seen in the apparent vicinity of RS Puppis bright background stars, "normal" stars at RS Puppis' distance, or are they faint foreground stars? That is impossible to say by just looking at the photograph. We may note that many of the small stars are reddish, which suggests either that they are intrinsically small and cool and therefore reddish, or else that they are far away and have been reddened by dust, such as the dust-rich nebula that RS Puppis is immersed in.
A very reasonable guess is that RS Puppis is the intrinsically brightest star that can be seen in this APOD. But even that is not certain. To know for sure we must measure the parallax of each star, which is the star's apparent motion back and forth across the sky as the Earth completes one orbit around the Sun. The spacecraft Gaia is doing just that right now, which is to say that it is measuring the parallaxes of and therefore the distances to, I think, a million - or was it a billion? - stars in the Milky Way. But it will take a long time for the Gaia mission to release its catalogue over the million (or billion) stars whose distances they have measured, and I for one can't imagine how I could browse that catalogue and find the stars that interest me.
So to summarize: Yes, those little bright dots are stars, but no, we can't say if they are background stars or foreground stars. The stars are obviously so faint-looking in the sky, probably around magnitude 15 or fainter, that there currently exists no catalogue than has any really useful information on them.