Starship Asterisk*

by **Ann** » Sat Oct 27, 2018 8:38 pm

BDanielMayfield wrote: ↑Sat Oct 27, 2018 3:19 pm
Ann wrote: ↑Sat Oct 27, 2018 4:05 am
BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm It could have just in fairly recent times transitioned into being the Red Giant that it is today.
Gamma Cas is not a red giant!

Of course! Please pardon my Sol-centric slip. O and B stars never become red giants.

Well, they do. Only the most massive stars, like Eta Carina, may blow their tops without ever becoming red giants. But stars that still belong to spectral classes O and B are never red giants.

Do A class stars also bypass the red giant phase when they exit the MS?

Bruce

They do indeed! Take Pollux, a red giant whose luminosity in yellow-green light is only some thirty times Solar, which is very puny for a red giant. I'm just guessing, but I wouldn't be entirely surprised if Pollux started out as an early F-type star, of spectral class F0 or F1. Pollux was definitely never a B-type star! And Arcturus, one of the really easy-to-find bright stars in the sky (because you just follow the handle of the Big Dipper until you find it) has a mass of only about 1.5 times Solar according to Jim Kaler, and it may possibly have started out its main sequence life as another Vega.

In any case, A-type main sequence stars do indeed become red giants. So do F-type stars, and Procyon is on its way to becoming one. The Sun, too, is expected to become a red giant.

I should perhaps comment on what you said about stars that "bypass the red giant stage when they become red giants". When stars leave the main sequence, they may keep their temperature (more or less) and just become brighter for a while. But astronomers believe that they definitely become red giants after that, unless they are massive enough to become Luminous Blue Variables like Eta Carina. The stars stay on the red giant branch for a moderately long time, although the red giant stage is much shorter than the star's main sequence lifetime. During the red giant stage the stars belong to spectral class K, or so I believe.

I believe that after some time, the stars start fusing helium in their cores. During this stage the stars are a little bit hotter and less red than before. But then they exhaust the helium in their cores, and now they enter the aymptotic giant branch, when they become brighter and redder than they have ever been before. I believe that stars at this stage are spectral class M.

Ann

by **BDanielMayfield** » Sat Oct 27, 2018 3:29 pm

Tragic Astronomy wrote: ↑Sat Oct 27, 2018 7:24 am Is it a mean streak to a certain degree
Come whistling down the crookedest street
A mean streak in the ghost state
Between the certain and the hesitating

Why don't you write me
A letter would brighten
My loneliest evening

Mail it today
If it's only to say
That you're leaving me

Monday morning, sitting in the sun
Hoping and wishing for the mail to come
Tuesday, never got a word

Wednesday, Thursday, ain't no sign
Drank a half a bottle of iodine.

Friday, woe is me
Gonna hang my body from the highest tree.

Why don't you write me?

Don't do that! It would truly be tragic.

There there now. See, someone has written you.

Bruce

by **BDanielMayfield** » Sat Oct 27, 2018 3:19 pm

neufer wrote: ↑Sat Oct 27, 2018 2:35 am
BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm Perhaps it wasn't just one of its minimums that caused it to go unnoticed and unnamed.
It appears that the Arabs were often more interested in naming asterisms than in naming individual stars;
e.g., Dziban : adh-Dhi'ban : "the Two Wolves or Jackals" for the dim doublet: Psi Draconis.

Hence both:

Shedar : as-Ṣadr : "the Breast" and
Ruchbah : ar-Rukbah : "the Knee"
may, actually, each represent 3 star asterisms.

And these asterisms only make sense if they also observed Gamma Cas.

Thanks Art. That makes sense and clears things up.

Ann wrote: ↑Sat Oct 27, 2018 4:05 am
BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm It could have just in fairly recent times transitioned into being the Red Giant that it is today.
Gamma Cas is not a red giant!

Of course! Please pardon my Sol-centric slip. O and B stars never become red giants.

It is a hot blue star whose blue light is certainly reddened by a lot of dust and disks of its own making. But it isn't more reddened than it still looks blue-white in the sky.

Gamma Cas is transitioning into a giant, and it has shut down the hydrogen fusion in its core. But it is still hot and blue, with a temperature of, say, almost 30,000 K. This makes it much, much hotter than the Sun, whose temperature is, if I remember correctly, some 5,700 or 5,800 K.

Red giant stars are cooler than the Sun, usually around 4,000 K or cooler.

But I agree that Gamma Cas may have become noticeably (or considerably) brighter when it started transitioning into a giant star.

Ann

Do A class stars also bypass the red giant phase when they exit the MS?

Bruce

by **Tragic Astronomy** » Sat Oct 27, 2018 7:24 am

Is it a mean streak to a certain degree
Come whistling down the crookedest street
A mean streak in the ghost state
Between the certain and the hesitating

Why don't you write me
A letter would brighten
My loneliest evening

Mail it today
If it's only to say
That you're leaving me

Monday morning, sitting in the sun
Hoping and wishing for the mail to come
Tuesday, never got a word

Wednesday, Thursday, ain't no sign
Drank a half a bottle of iodine.

Friday, woe is me
Gonna hang my body from the highest tree.

Why don't you write me?

by **Ann** » Sat Oct 27, 2018 4:05 am

BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm

Perhaps it wasn't just one of its minimums that caused it to go unnoticed and unnamed. It could have just in fairly recent times transitioned into being the Red Giant that it is today.

Also, it's amazing that it is rotating so fast that it is sort of unraveling itself, decreting to use a term my spell checker didn't like. Due to conservation of angular momentum things rotate slower as they inflate, so it must have been spinning like crazy when it was back on the main sequence.

Bruce

Gamma Cas is not a red giant! It is a hot blue star whose blue light is certainly reddened by a lot of dust and disks of its own making. But it isn't more reddened than it still looks blue-white in the sky.

Gamma Cas is transitioning into a giant, and it has shut down the hydrogen fusion in its core. But it is still hot and blue, with a temperature of, say, almost 30,000 K. This makes it much, much hotter than the Sun, whose temperature is, if I remember correctly, some 5,700 or 5,800 K.

Red giant stars are cooler than the Sun, usually around 4,000 K or cooler.

But I agree that Gamma Cas may have become noticeably (or considerably) brighter when it started transitioning into a giant star.

Ann

by **neufer** » Sat Oct 27, 2018 2:35 am

BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm
Ann wrote: ↑Fri Oct 26, 2018 5:11 pm
... Gamma Cas may have been faint enough a couple of thousand years ago that the Arabs didn't pay any attention to it. That does not seem impossible, because as a variable star, Gamma Cas may have hit one of its "minimums" when the Arabs named the stars of the sky.
Perhaps it wasn't just one of its minimums that caused it to go unnoticed and unnamed.
It could have just in fairly recent times transitioned into being the Red Giant that it is today.

It appears that the Arabs were often more interested in naming asterisms than in naming individual stars;
e.g., Dziban : adh-Dhi'ban : "the Two Wolves or Jackals" for the dim doublet: Psi Draconis.

Hence both:

Shedar : as-Ṣadr : "the Breast" and
Ruchbah : ar-Rukbah : "the Knee"

may, actually, each represent 3 star asterisms.

And these asterisms only make sense if they also observed Gamma Cas.

by **BDanielMayfield** » Sat Oct 27, 2018 1:44 am

MarkBour wrote: ↑Fri Oct 26, 2018 10:12 pm
BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm
Ann wrote: ↑Fri Oct 26, 2018 5:11 pm And "decretion disk"! Seriously! I had certainly never heard that word before, either.
... Also, it's amazing that it is rotating so fast that it is sort of unraveling itself, decreting to use a term my spell checker didn't like. Due to conservation of angular momentum things rotate slower as they inflate, so it must have been spinning like crazy when it was back on the main sequence.

Bruce
"Decretion disk". That's new to me, too. I like it!

I looked briefly at some titles that a Google search pulled up. I'd have to read a lot more carefully to determine how much of the content of those papers was theoretical versus observational. They discussed binary systems with decretion disks as well as models for single stars that evolved to a point where they formed decretion disks. I wonder if a binary system can merge into a single star with very high rotational velocity.

Indeed binary stars can and do merge, and it is a fairly common occurrence too Mark, as binary stars in close orbits are relatively common, and all main sequence stars eventually swell when the hydrogen supply in their cores run out. I think your implied guess that Gamma Cas once once was a binary pair in a tight orbit is a good one.

Bruce

by **MarkBour** » Fri Oct 26, 2018 10:12 pm

BDanielMayfield wrote: ↑Fri Oct 26, 2018 8:01 pm
Ann wrote: ↑Fri Oct 26, 2018 5:11 pm And "decretion disk"! Seriously! I had certainly never heard that word before, either.
... Also, it's amazing that it is rotating so fast that it is sort of unraveling itself, decreting to use a term my spell checker didn't like. Due to conservation of angular momentum things rotate slower as they inflate, so it must have been spinning like crazy when it was back on the main sequence.

Bruce

"Decretion disk". That's new to me, too. I like it!

I looked briefly at some titles that a Google search pulled up. I'd have to read a lot more carefully to determine how much of the content of those papers was theoretical versus observational. They discussed binary systems with decretion disks as well as models for single stars that evolved to a point where they formed decretion disks. I wonder if a binary system can merge into a single star with very high rotational velocity.

by **BDanielMayfield** » Fri Oct 26, 2018 8:01 pm

Ann wrote: ↑Fri Oct 26, 2018 5:11 pm ... Gamma Cas may have been faint enough a couple of thousand years ago that the Arabs didn't pay any attention to it. That does not seem impossible, because as a variable star, Gamma Cas may have hit one of its "minimums" when the Arabs named the stars of the sky.

And "decretion disk"! Seriously! I had certainly never heard that word before, either.

Perhaps it wasn't just one of its minimums that caused it to go unnoticed and unnamed. It could have just in fairly recent times transitioned into being the Red Giant that it is today.

Also, it's amazing that it is rotating so fast that it is sort of unraveling itself, decreting to use a term my spell checker didn't like. Due to conservation of angular momentum things rotate slower as they inflate, so it must have been spinning like crazy when it was back on the main sequence.

Bruce

by **Ann** » Fri Oct 26, 2018 5:11 pm

BDanielMayfield wrote: ↑Fri Oct 26, 2018 4:22 pm
Great post Ann. It prompted me to seek more info on Gamma Cas. After seeing the photo you included I wondered 'doesn't Gamma Cas have a more common name?' and I found this in its Wikipedia article:
Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual magnitude that varies from 1.6 to 3.0, it has no traditional Arabic or Latin name.
After reading that surprising fact I wondered why it wasn't named; could it have been dimmer back when Arab astronomers were naming all the bright stars? I read further:
Gamma Cassiopeiae is an eruptive variable star, whose apparent magnitude changes irregularly between +1.6 and +3.0. It is the prototype of the class of Gamma Cassiopeiae variable stars. In the late 1930s it underwent what is described as a shell episode and the brightness increased to above magnitude +2.0, then dropped rapidly to +3.4.[11] It has since been gradually brightening back to around +2.2.[12] At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3).

Gamma Cassiopeiae is a rapidly spinning star with a projected rotational velocity of 472 km s−1, giving it a pronounced equatorial bulge. When combined with the star's high luminosity, the result is the ejection of matter that forms a hot circumstellar disk of gas. The emissions and brightness variations are apparently caused by this "decretion" disk.

The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated.[13] It has 17 times the Sun's mass and is radiating as much energy as 34,000 Suns. At this rate of emission, the star has reached the end of its life as a B-type main sequence star after a relatively brief 8 million years. The outer atmosphere has an intense effective temperature of 25,000 K, which is causing it to glow with a blue-white hue.
So apparently Gamma Cas was dim enough back in the day that the Arabs didn't notice it enough to name it. It is also rotating so fast that it has a "decretion" disk! That is something new that I've never heard of in more than three decades of avidly reading about astronomy.

Thanks Ann! You helped me find a hidden gem that wasn't even inside the view of today's APOD.

Bruce

Thanks, Bruce, and that's a great post of your own! Imagine that Gamma Cas may have been faint enough a couple of thousand years ago that the Arabs didn't pay any attention to it. That does not seem impossible, because as a variable star, Gamma Cas may have hit one of its "minimums" when the Arabs named the stars of the sky.

And "decretion disk"! Seriously! I had certainly never heard that word before, either.

: Click to view full size image

Gamma Cas and the dark monster. Photo: Rogelio Bernal Andreo.

Well, I have just discovered another thing that I didn't notice the first time I found the picture by Rogelio Bernal Andreo. Have you seen the huge, huge dragon-like black monster with a terrible gaping mouth, jutting its head in from the left and opening horrible jaws to engulf Gamma Cas?

The "monster", of course, is a large dark nebula. I have to wonder if Gamma Cas itself was born from this dark cloud, and if it was, if the strong stellar wind from Gamma Cas has been responsible for blowing a large hole in this cloud, making it look like a gaping monster.

Whatever the cause of the appearance of the black cloud, it makes for another fine astrophoto for Halloween, doesn't it?

Ann

by **BDanielMayfield** » Fri Oct 26, 2018 4:22 pm

Ann wrote: ↑Fri Oct 26, 2018 5:58 am Today's APOD is a truly beautiful portrait of two nebulas associated with the star Gamma Cas. The brilliant RGB portrait brings out the true nature of these two nebulas: They both have an emission (red) and a reflection (blue) component. The red emission light dominates at the edges of these clouds, where highly energetic ultraviolet photons from Gamma Cas slam into hydrogen atoms of these nebulas and kick the electron of the hydrogen atom into a higher orbit, causing the electron to emit red light when it falls back into its original orbit again. But the blue component dominates the light of the nebulas away from their edges, where dust particles reflect the blue light from Gamma Cas itself.

It is clear from Ken Crawford's image that IC 63 is both brighter and redder than IC 59. Clearly IC 63 bears the brunt of the ultraviolet onslaught from Gamma Cas. As for IC 59, it is possible that one part of it is shielding another part of it from the ultraviolet light of Gamma Cas, and perhaps we are seeing the part of it that is mostly shielded from from the direct onslaught of the harsh ultraviolet light.

Click to view full size image
Constellation Cassiopeia with Gamma Cas and IC 63 and 59.
Photo: Rogelio Bernal Andreo.
Can't resist showing you a wide angle picture of constellation Cassiopeia with Gamma Cas in the middle and IC 63 and 59 located to the upper left of Gamma Cas (in this picture). If you think that the labeling gets in the way of your view of IC 63 and 59, here in an unlabeled version of the picture. In that picture, you can really see how IC 63 and 59 are just the brightest parts of a mini version of "Barnard's Loop", made by Gamma Cas itself.

I love Gamma Cas. For me as a northerner, this star is the apparently and intrinsically brightest of the "easiest to find" really hot blue stars in the heavens, apart from the hottest and brightest blue stars in Orion, of course.

Ann

Great post Ann. It prompted me to seek more info on Gamma Cas. After seeing the photo you included I wondered 'doesn't Gamma Cas have a more common name?' and I found this in its Wikipedia article:

Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual magnitude that varies from 1.6 to 3.0, it has no traditional Arabic or Latin name.

After reading that surprising fact I wondered why it wasn't named; could it have been dimmer back when Arab astronomers were naming all the bright stars? I read further:

Gamma Cassiopeiae is an eruptive variable star, whose apparent magnitude changes irregularly between +1.6 and +3.0. It is the prototype of the class of Gamma Cassiopeiae variable stars. In the late 1930s it underwent what is described as a shell episode and the brightness increased to above magnitude +2.0, then dropped rapidly to +3.4.[11] It has since been gradually brightening back to around +2.2.[12] At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3).

Gamma Cassiopeiae is a rapidly spinning star with a projected rotational velocity of 472 km s−1, giving it a pronounced equatorial bulge. When combined with the star's high luminosity, the result is the ejection of matter that forms a hot circumstellar disk of gas. The emissions and brightness variations are apparently caused by this "decretion" disk.

The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated.[13] It has 17 times the Sun's mass and is radiating as much energy as 34,000 Suns. At this rate of emission, the star has reached the end of its life as a B-type main sequence star after a relatively brief 8 million years. The outer atmosphere has an intense effective temperature of 25,000 K, which is causing it to glow with a blue-white hue.

So apparently Gamma Cas was dim enough back in the day that the Arabs didn't notice it enough to name it. It is also rotating so fast that it has a "decretion" disk! That is something new that I've never heard of in more than three decades of avidly reading about astronomy.

Thanks Ann! You helped me find a hidden gem that wasn't even inside the view of today's APOD.

Bruce

by **bystander** » Fri Oct 26, 2018 2:06 pm

Hubble Captures the Ghost of Cassiopeia (IC 63)

by **De58te** » Fri Oct 26, 2018 9:52 am

Just a thought for food, or is it food for thought. What a coincidence that it looks like a ghost because the top is towards the North. The pillars of creation are arranged the same direction. But we have heard that in space there is no up or down. Imagine if the picture was upside down or that South was up. Then it wouldn't look like a ghost. It would look more like flowers in a vase or something. Humm. I recall seeing an article on Yahoo about an upside down world map from Australia. Australia was at the top of the map, upside down of course, and Europe and North America were at the bottom. Down under.

by **Boomer12k** » Fri Oct 26, 2018 6:34 am

Very nice image...where is Casper when you need him...

:---[===] *

by **Ann** » Fri Oct 26, 2018 5:58 am

Today's APOD is a truly beautiful portrait of two nebulas associated with the star Gamma Cas. The brilliant RGB portrait brings out the true nature of these two nebulas: They both have an emission (red) and a reflection (blue) component. The red emission light dominates at the edges of these clouds, where highly energetic ultraviolet photons from Gamma Cas slam into hydrogen atoms of these nebulas and kick the electron of the hydrogen atom into a higher orbit, causing the electron to emit red light when it falls back into its original orbit again. But the blue component dominates the light of the nebulas away from their edges, where dust particles reflect the blue light from Gamma Cas itself.

It is clear from Ken Crawford's image that IC 63 is both brighter and redder than IC 59. Clearly IC 63 bears the brunt of the ultraviolet onslaught from Gamma Cas. As for IC 59, it is possible that one part of it is shielding another part of it from the ultraviolet light of Gamma Cas, and perhaps we are seeing the part of it that is mostly shielded from from the direct onslaught of the harsh ultraviolet light.

: Click to view full size image

Constellation Cassiopeia with Gamma Cas and IC 63 and 59.
Photo: Rogelio Bernal Andreo.

Can't resist showing you a wide angle picture of constellation Cassiopeia with Gamma Cas in the middle and IC 63 and 59 located to the upper left of Gamma Cas (in this picture). If you think that the labeling gets in the way of your view of IC 63 and 59, here in an unlabeled version of the picture. In that picture, you can really see how IC 63 and 59 are just the brightest parts of a mini version of "Barnard's Loop", made by Gamma Cas itself.

I love Gamma Cas. For me as a northerner, this star is the apparently and intrinsically brightest of the "easiest to find" really hot blue stars in the heavens, apart from the hottest and brightest blue stars in Orion, of course.

Ann

by **APOD Robot** » Fri Oct 26, 2018 4:05 am

IC 59 and IC 63 in Cassiopeia

Explanation: These bright rims and flowing shapes look ghostly on a cosmic scale. A telescopic view toward the constellation Cassiopeia, the colorful (zoomable) skyscape features the swept-back, comet-shaped clouds IC 59 (left) and IC 63. About 600 light-years distant, the clouds aren't actually ghosts, but they are slowly disappearing under the influence of energetic radiation from hot,luminous star gamma Cas. Gamma Cas is physically located only 3 to 4 light-years from the nebulae, just off the top right edge of the frame. Slightly closer to gamma Cas, IC 63 is dominated by red H-alpha light emitted as hydrogen atoms ionized by the star's ultraviolet radiation recombine with electrons. Farther from the star, IC 59 shows proportionally less H-alpha emission but more of the characteristic blue tint of dust reflected star light. The field of view spans about 1 degree or 10 light-years at the estimated distance of gamma Cas and friends.

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