by Ann » Wed Sep 09, 2020 6:42 am
It's so nice to see a beautiful RGB picture of the lovely blue Seven Sisters in the sky!
Well, the Sisters look like they usually do, immersed in their delicate blue nebulosity, so I'd like to call attention to the orange star at 9 o'clock in the image. Because it is in fact a mystery that the Seven Sisters are so blue, and that there is no red star among them. The Pleiades is a rich cluster, whose total mass according to
Wikipedia is about 800 solar masses, and it contains over 1,000 statistically confirmed member stars.
Such a rich cluster ought to have produced at least one star more massive than Alcyone, whose mass according to
Wikipedia is about 6 solar masses. A star of, say, 8 solar masses, ought to have evolved into a red giant by now, in view of the fact that the Pleiades is considered to be about 100 million years old.
Think of all the open clusters that are dominated by blue stars but contain at least one red giant:
M6,
M7,
M11,
M25,
M37,
M38,
M41,
M44,
NGC 457,
NGC 3293,
NGC 3532,
NGC 3766,
The Jewel Box,
The Double Cluster of Perseus... okay, I give up now, but you get my drift. Believe me, it's hard enough to find an open cluster that has shed its natal cloud and is dominated by blue stars, but still doesn't contain a red giant!
Well, one such cluster is
IC 2602, and it is also known as... (drum roll) the Southern Pleiades. I guess that is because IC 2602 and the Pleiades both lack red giants!
Or do they? What about that orange star at 9 o'clock in today's APOD, HD 23712? Could that star be the "missing" red Pleiad?
No. No, that star is not a member of the entourage of the Seven Sisters, because the proper motion of HD 23712 is extremely different from the proper motion of the Pleiades. The animation at right shows the fascinating proper motion of the stars of M45. As you can see, they carry no bright red star in tow as they tumble southeastwards in the sky.
Please, bystander, let me keep this animation in my post, even though its larger (but not that much larger ) than 400 KB!
Ann
It's so nice to see a beautiful RGB picture of the lovely blue Seven Sisters in the sky! 🤩
Well, the Sisters look like they usually do, immersed in their delicate blue nebulosity, so I'd like to call attention to the orange star at 9 o'clock in the image. Because it is in fact a mystery that the Seven Sisters are so blue, and that there is no red star among them. The Pleiades is a rich cluster, whose total mass according to [url=https://en.wikipedia.org/wiki/Pleiades#Composition]Wikipedia[/url] is about 800 solar masses, and it contains over 1,000 statistically confirmed member stars.
Such a rich cluster ought to have produced at least one star more massive than Alcyone, whose mass according to [url=https://en.wikipedia.org/wiki/Alcyone_(star)]Wikipedia[/url] is about 6 solar masses. A star of, say, 8 solar masses, ought to have evolved into a red giant by now, in view of the fact that the Pleiades is considered to be about 100 million years old.
Think of all the open clusters that are dominated by blue stars but contain at least one red giant: [url=https://apod.nasa.gov/apod/ap110906.html]M6[/url], [url=https://apod.nasa.gov/apod/ap160713.html]M7[/url], [url=http://annesastronomynews.com/annes-image-of-the-day-the-wild-duck-cluster/]M11[/url], [url=http://www.apod.ro/ap090831.html]M25[/url], [url=https://www.messier-objects.com/messier-37/]M37[/url], [url=https://www.messier-objects.com/messier-38-starfish-cluster/]M38[/url], [url=http://cs.astronomy.com/asy/m/starclusters/433196.aspx]M41[/url], [url=https://apod.nasa.gov/apod/ap140222.html]M44[/url], [url=https://media.sciencephoto.com/c0/33/50/45/c0335045-400px-wm.jpg]NGC 457[/url], [url=https://apod.nasa.gov/apod/ap150406.html]NGC 3293[/url], [url=https://en.wikipedia.org/wiki/NGC_3532]NGC 3532[/url], [url=https://en.wikipedia.org/wiki/NGC_3766]NGC 3766[/url], [url=https://apod.nasa.gov/apod/ap100817.html]The Jewel Box[/url], [url=https://apod.nasa.gov/apod/ap090103.html]The Double Cluster of Perseus[/url]... okay, I give up now, but you get my drift. Believe me, it's hard enough to find an open cluster that has shed its natal cloud and is dominated by blue stars, but still doesn't contain a red giant!
[float=right][img3="Animation of the proper motion of the Pleiades and stars in its vicinity in 400,000 years. Author: Alexander Meleg."]https://upload.wikimedia.org/wikipedia/commons/0/04/Astro_4D_m45_cr_anim.gif[/img3][/float]
Well, one such cluster is [url=https://en.wikipedia.org/wiki/IC_2602]IC 2602[/url], and it is also known as... (drum roll) the Southern Pleiades. I guess that is because IC 2602 and the Pleiades both lack red giants!
Or do they? What about that orange star at 9 o'clock in today's APOD, HD 23712? Could that star be the "missing" red Pleiad?
No. No, that star is not a member of the entourage of the Seven Sisters, because the proper motion of HD 23712 is extremely different from the proper motion of the Pleiades. The animation at right shows the fascinating proper motion of the stars of M45. As you can see, they carry no bright red star in tow as they tumble southeastwards in the sky.
Please, bystander, let me keep this animation in my post, even though its larger (but not that much larger ) than 400 KB!
Ann