Starship Asterisk*

Yesterday I complained about the old Voyager 1 color-enhanced picture of Saturn, because I had no idea what the colors represented. Today I'm not complaining! I know what the colors mean - blue means doubly ionized oxygen, OIII, ███, green means red hydrogen alpha, Hα, ███ and red means ionized sulfur, SII, ███. Blue means a higher degree of ionization and red means a lower degree of ionization.

So the meaning of today's APOD is very clear to me. Not only that, but I like the specific mapped colors here!

A great thing about today's APOD is that it called my attention to a nebula that I had never taken any interest in, the Loden 153 nebula. The fact that this nebula looks so blue in the OIII-Hα-SII image means that there is a very hot star in there that I had never heard of! I must find it, and I did!


The ionizing star is HD 91572! It is a star of spectral class O6.5V, which is still fusing hydrogen to helium in its core. It must be very massive to be as hot as spectral class O6.5, and the fact that it is still luminosity class V means that it is still fusing hydrogen to helium in its core. This is turn makes it very young as stars go. Perhaps, oh, 3 million years? Or even less?


The brightest, pale cyan region in today's APOD is the Gabriela Mistral Nebula, so named because it resembles the Chilean Nobel Laureate poet!



The Gabriela Mistral Nebula is ionized by one blisteringly hot O6V star, HD 92206A, and a pair of slightly less hot O-type stars, SAO 238266.


Finally, NGC 3293! Did you know that it is the same age as the Double Cluster in Perseus?


NGC 3293 and the Double Cluster in Perseus are contemporaries! Fancy that.

Ann

AVAO wrote: ↑Mon May 01, 2023 7:22 am

APOD Robot wrote: ↑Mon May 01, 2023 4:05 am Carina Nebula North

Indeed, interesting area up there.
I really like the open cluster NGC 3293, a wonderful treasure chest full of cosmic diamonds.


Comparison of NGC 3293 (ESO, La Silla-Observatorium) 7600-8400 ly away with Trumpler 14 (VLT infill) 8980 ly away at precisely the same scale.


biggg: https://live.staticflickr.com/65535/528 ... 74c6_o.jpg


Trumpler 14 / jac berne (flickr)

Absolutely hugely interesting, AVAO! NGC 3293 and Trumpler 14 are at reasonably the same distance from us, yet Trumpler 14 looks positively tiny compared with NGC 3293!

How can we explain it? I guess we're talking age here. NGC 3293 is 12 million years old, so not only have all the stars that used to be extremely hot O-type main sequence stars evolved into either cooler B-type blue giants or extremely cool M-type red giants, and this evolution has made them optically brighter than they were before. Yes, because O-type stars are "extreme emitters" of ultraviolet light, but (comparatively) "faint" at optical wavelengths.

So not only have the stars of NGC 3293 themselves become optically brighter over time, but the cluster itself has expanded and quite literally grown larger in size. According to Wikipedia, the radius of NGC 3293 is 13.2 light-years, but the diameter of Trumpler 14 is 6 light-years!

The reason why NGC 3293 is so much larger than Trumpler 14 is because the stars of NGC 3293 have "moved away from one another"! Because NGC 3293 is not massive enough to be gravitationally bound, and therefore the cluster is doomed to "evaporate" and disappear over time. NGC 3293 must have been much more compact and also optically fainter when it was as young as Trumpler 14!

The age of Trumpler 14 is believed to be no more than some 300.000–500.000 years, and even if we settle for the higher value, NGC 3293 is still at least 20 times older than Trumpler 14. The latter cluster contains, among others, multiple star HD 93129, consisting of three stars with spectral classes O2If+O3.5V+O3.5V((f))z, with the most massive component weighing in at 110 solar masses. And then there are six other O-type stars as well in Trumpler 14. There is none in NGC 3293!

https://en.wikipedia.org/wiki/Trumpler_14

https://en.wikipedia.org/wiki/NGC_3293

Goodness me! But at the tender age of Trumpler 14, its massive stars are unevolved and the cluster itself is compact and therefore physically "smallish" in size.

Ann