APOD: Mu Cephei (2023 Oct 12)

[APOD Robot]

Mu Cephei

Explanation: Mu Cephei is a very large star. An M-class supergiant some 1500 times the size of the Sun, it is one of the largest stars visible to the unaided eye, and even one of the largest in the entire Galaxy. If it replaced the Sun in our fair Solar System, Mu Cephei would easily engulf Mars and Jupiter. Historically known as Herschel's Garnet Star, Mu Cephei is extremely red. Approximately 2800 light-years distant, the supergiant is seen near the edge of reddish emission nebula IC 1396 toward the royal northern constellation Cepheus in this telescopic view. Much cooler and hence redder than the Sun, this supergiant's light is further reddened by absorption and scattering due to intervening dust within the Milky Way. A well-studied variable star understood to be in a late phase of stellar evolution, Mu Cephei is a massive star too, destined to ultimately explode as a core-collapse supernova.

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[Ann]

Today's APOD is a very fine visible-light image of a truly fascinating star.

Mu Cephei. Image Credit & Copyright: David Cruz

---

Mu Cephei is known as the Garnet star, but it is certainly not garnet-colored. The way I remember it, when I was able to look at it through a 14-inch telescope, it was about this color: ███

Garnet would be about this color: ███

Still, it's certainly reddish as non-carbon stars go.

In the APOD, Mu Cephei is seen to be involved with a lot of nebulosity. And since this nebulosity is very red, it is easy to think that Mu Cephei is ionizing a red emission nebula. But that is impossible, because Mu Cephei is way, way, way too cool for that. Of course, it is certainly possible that Mu Cephei has previously shed a lot of matter, and that this matter is now being ionized by ultraviolet light from the O-type star HD 206267, which is ionizing the entire IC 1396 nebula.

IC 1396: Emission Nebula in Cepheus. Image Credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator. Color Composite: Davide De Martin (Skyfactory)

---

The blue-white star at the center of IC 1396, and responsible for ionizing it, is O-type HD 206267. It could be that the strong wind from HD 206267 is colliding with Mu Cephei's own wind, which might cause hydrogen alpha ionization near Mu Cephei. Or else, frankly, all the ionization may come from HD 206267.

Wikipedia wrote:

The distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.55±0.20 mas, which corresponds to an estimated distance of 1,800 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390±140 parsecs.[24]

Calculation of the distance from the measured angular diameter, surface brightness, and calculated luminosity leads to 641 pc. Averaging the distances of nearby luminous stars with similar reddening and reliable Gaia Data Release 2 parallaxes gives a distance of 940 pc

Well, if Gaia has measured the parallax of Mu Cephei, then the news of this has not come to Simbad.

The APOD caption said that the distance to Mu Cephei is nearly 3,000 light-years from planet Earth. Did Gaia tell you that, Robert Nemiroff and Jerry Bonnell, in private communication? Even though Gaia failed to inform Simbad?

Well, Gaia did tell Simbad that the parallax of HD 206267 is 1.3604 milliarcseconds (with an uncertainty of 0.2188 mas), which, if we disregard the uncertainty, corresponds to a distance of ~ 2,400 light-years. I take that to mean that Mu Cephei is more distant than HD 206267, and and that Mu Cephai may not be involved with nebulosity of IC 1396. Or is it? Inquiring minds want to know!

Do note the little planetary nebula to the upper right of Mu Cephei. Just follow the Mu Cephei diffraction spike that points upwards and to the right, and you'll see it.

I wonder what this little planetary nebula is called? I'm too lazy to find out.

Ann

[rstevenson]

Thanks, Ann. I was wondering if that little blob was perhaps a distant galaxy, but it does look a bit nebulish.

Do you happen to know if that scattering of bright blue stars around Mu Cephei are all related, that is, do thay form an open cluster? Or are they just coincidentally near each other?

Rob

[Christian G.]

Clusters and nebulae and galaxies are awesome sights, but sometimes there's nothing like observing closely a single star. I find it hypnotic.

[Don1442]

I'm confused about the 2 size comments. They seem very contradictory. If the star is 1500 times bigger than the sun, that gives a radius about 7.1 times larger (using the volume of a sphere formula). That won't even make it out to Mercury, never mind engulfing Mars and Jupiter. What am I missing?

[Chris Peterson]

I'm confused about the 2 size comments. They seem very contradictory. If the star is 1500 times bigger than the sun, that gives a radius about 7.1 times larger (using the volume of a sphere formula). That won't even make it out to Mercury, never mind engulfing Mars and Jupiter. What am I missing?

"1500 times bigger than the Sun" == 1500 times greater radius than the Sun.

[Ann]

Thanks, Ann. I was wondering if that little blob was perhaps a distant galaxy, but it does look a bit nebulish.

Do you happen to know if that scattering of bright blue stars around Mu Cephei are all related, that is, do thay form an open cluster? Or are they just coincidentally near each other?

Rob

I think we can be quite confident that the stars near Mu Cephei on the sky are just coincidentally near each other along our line of sight.

I chose a few of the stars near Mu Cephei, identified them with my software and used Simbad to find out their parallaxes (and a unit converter, Stellar Distance and Parallax Calculator), to find out the distance to them in light-years. I found that HD 234791, spectral class A1V, V mag 9.19, has a parallax of 2.5522 milliarcseconds and a distance of some 1,300 light-years. HD 239751, A2, V mag 9.22, parallax 4.0797, distance some 800 light-years. HD 239758, B2III, V mag. 9.43, parallax 0.7976 and distance some 4,000 light-years.

Frankly, most of these stars are not close to one another.

I googled "Mu Cephei membership" to see if Google would tell me that Mu Cephei is a member of a cluster. After all, if you google "Alcyone star membership", you will quickly be told that Alcyone is a member of the Pleiades. But the only "membership" that I could find for Mu Cephei was membership in the Cepheus OB2 association - but then, Mu Cephei really isn't a member of the Cep OB2 association!

Jim Kaler wrote:

Its distance, too far for parallax, is uncertain, but from its traditional membership in the Cepheus OB2 association of hot stars is around 2400 light years.
...
Oddly, one study shows the star NOT to be a member of the association.

Nothing else is said, as far as I could see, about Mu Cephei being a member of any cluster or association. I also have found no suggestion that there is a line-of-sight cluster in front of or behind Mu Cephei.

I might add that if Mu Cephei is a member of a cluster after all, then it is surely the odd man out of this cluster. It would be a giant star indeed in a cluster otherwise made up of stars that are modest in mass and size.

Well, there is at least one cluster where one star dominates its stellar siblings so totally that you can't even confidently point out any other members, and that is NGC 6169.

The bright star is O9.7Iab-type Mu Norma. Mu Norma belongs to cluster NGC 6169, but where are the other members? Credit: DSS2.

---

I'm reminded of loose cluster IC 4665, but in that cluster you can at least pick out the member stars with some confidence:

IC 4665. Credit: DSS2.

---

Ann

[rstevenson]

Thanks Ann.

[AVAO]

I wonder what this little planetary nebula is called? I'm too lazy to find out.

Ann

The planetary nebula PK100+04.1 / PM1-333 reminds me of a mix of The Necklace Nebula PN G054.2-03.4 and NGC 5189.


jac berne (flickr)


PN G054.2-03.4 Credit: ESA/Hubble & NASA, K. Noll


NGC 5189 Credit: NASA; ESA; Hubble, Gemini Observatory/AURA; Processing: Robert Gendler

[orin stepanek]

A super large Star; I'm glad it's very far! When it goes nova; the night
sky will brighten significantly!

[AVAO]

Today's APOD is a very fine visible-light image of a truly fascinating star.

Mu Cephei. Image Credit & Copyright: David Cruz

---

Mu Cephei is known as the Garnet star, but it is certainly not garnet-colored. The way I remember it, when I was able to look at it through a 14-inch telescope, it was about this color: ███

Garnet would be about this color: ███

Still, it's certainly reddish as non-carbon stars go.

In the APOD, Mu Cephei is seen to be involved with a lot of nebulosity. And since this nebulosity is very red, it is easy to think that Mu Cephei is ionizing a red emission nebula. But that is impossible, because Mu Cephei is way, way, way too cool for that. Of course, it is certainly possible that Mu Cephei has previously shed a lot of matter, and that this matter is now being ionized by ultraviolet light from the O-type star HD 206267, which is ionizing the entire IC 1396 nebula.

IC 1396: Emission Nebula in Cepheus. Image Credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator. Color Composite: Davide De Martin (Skyfactory)

---

The blue-white star at the center of IC 1396, and responsible for ionizing it, is O-type HD 206267. It could be that the strong wind from HD 206267 is colliding with Mu Cephei's own wind, which might cause hydrogen alpha ionization near Mu Cephei. Or else, frankly, all the ionization may come from HD 206267.

Wikipedia wrote:

The distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.55±0.20 mas, which corresponds to an estimated distance of 1,800 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390±140 parsecs.[24]

Calculation of the distance from the measured angular diameter, surface brightness, and calculated luminosity leads to 641 pc. Averaging the distances of nearby luminous stars with similar reddening and reliable Gaia Data Release 2 parallaxes gives a distance of 940 pc

Well, if Gaia has measured the parallax of Mu Cephei, then the news of this has not come to Simbad.

The APOD caption said that the distance to Mu Cephei is nearly 3,000 light-years from planet Earth. Did Gaia tell you that, Robert Nemiroff and Jerry Bonnell, in private communication? Even though Gaia failed to inform Simbad?

Well, Gaia did tell Simbad that the parallax of HD 206267 is 1.3604 milliarcseconds (with an uncertainty of 0.2188 mas), which, if we disregard the uncertainty, corresponds to a distance of ~ 2,400 light-years. I take that to mean that Mu Cephei is more distant than HD 206267, and and that Mu Cephai may not be involved with nebulosity of IC 1396. Or is it? Inquiring minds want to know!

Do note the little planetary nebula to the upper right of Mu Cephei. Just follow the Mu Cephei diffraction spike that points upwards and to the right, and you'll see it.


APOD 12 October 2023 detail.png


I wonder what this little planetary nebula is called? I'm too lazy to find out.

Ann

ThanX Ann

A lot of interesting facts. The area is also interesting in IR (WISE), dust pockets full with "strands" of new stars

Click to view full size image

jac berne (flickr)

[johnnydeep]

Cool star. So, given that Mu Cephei is 20x the mass of the Sun and 1000x its radius, how much would Saturn's orbit and matter be affected is Mu Cephei were to replace the Sun, and Saturn ended up orbing within its outer layers?

[alter-ego]

...

Wikipedia wrote:

The distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.55±0.20 mas, which corresponds to an estimated distance of 1,800 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390±140 parsecs.[24]

Calculation of the distance from the measured angular diameter, surface brightness, and calculated luminosity leads to 641 pc. Averaging the distances of nearby luminous stars with similar reddening and reliable Gaia Data Release 2 parallaxes gives a distance of 940 pc

Well, if Gaia has measured the parallax of Mu Cephei, then the news of this has not come to Simbad.

The APOD caption said that the distance to Mu Cephei is nearly 3,000 light-years from planet Earth. Did Gaia tell you that, Robert Nemiroff and Jerry Bonnell, in private communication? Even though Gaia failed to inform Simbad?

Well, Gaia did tell Simbad that the parallax of HD 206267 is 1.3604 milliarcseconds (with an uncertainty of 0.2188 mas), which, if we disregard the uncertainty, corresponds to a distance of ~ 2,400 light-years. I take that to mean that Mu Cephei is more distant than HD 206267, and that Mu Cephei may not be involved with nebulosity of IC 1396. Or is it? Inquiring minds want to know!

Do note the little planetary nebula to the upper right of Mu Cephei. Just follow the Mu Cephei diffraction spike that points upwards and to the right, and you'll see it.
...
Ann

Gaia DR3 lists µ Cep parallax = 0.119 ± 0.264 mas.
The uncertainty is excessively large since negative parallaxes don't make sense. However, the closer greater parallax (adding the uncertainty) does yield a statistically valid (closer) distance. For a parallax = 0.383 mas, the distance = 8500 ly and statistically there's a 66% probability that the distance to µ Cep is between 8500 ly and the mean distance, 27,400 ly. Bottom line, it's much farther than HD206267, and APOD description needs updating. I suspect the large parallax uncertainty is due to the µ Cep brightness: M_G = 2.45 for this star and is ~1/2 mag brighter than Gaia's bright star limit M_G = 3 mag.

By the way, Simbad does have the DR3 distance parallax for µ Cep. Unfortunately, the default parallax displayed on the main µ Cep page is from Hipparcos. Scrolling down to "Collection of Measurements", click the "plx" box to display the available present and past 5 parallaxes.
 
simbad.cds.unistra.fr/simbad

[Ann]

...

Wikipedia wrote:

The distance to Mu Cephei is not very well known. The Hipparcos satellite was used to measure a parallax of 0.55±0.20 mas, which corresponds to an estimated distance of 1,800 parsecs. However, this value is close to the margin of error. A determination of the distance based upon a size comparison with Betelgeuse gives an estimate of 390±140 parsecs.[24]

Calculation of the distance from the measured angular diameter, surface brightness, and calculated luminosity leads to 641 pc. Averaging the distances of nearby luminous stars with similar reddening and reliable Gaia Data Release 2 parallaxes gives a distance of 940 pc

Well, if Gaia has measured the parallax of Mu Cephei, then the news of this has not come to Simbad.

The APOD caption said that the distance to Mu Cephei is nearly 3,000 light-years from planet Earth. Did Gaia tell you that, Robert Nemiroff and Jerry Bonnell, in private communication? Even though Gaia failed to inform Simbad?

Well, Gaia did tell Simbad that the parallax of HD 206267 is 1.3604 milliarcseconds (with an uncertainty of 0.2188 mas), which, if we disregard the uncertainty, corresponds to a distance of ~ 2,400 light-years. I take that to mean that Mu Cephei is more distant than HD 206267, and that Mu Cephei may not be involved with nebulosity of IC 1396. Or is it? Inquiring minds want to know!

Do note the little planetary nebula to the upper right of Mu Cephei. Just follow the Mu Cephei diffraction spike that points upwards and to the right, and you'll see it.
...
Ann

Gaia DR3 lists µ Cep parallax = 0.119 ± 0.264 mas.
The uncertainty is excessively large since negative parallaxes don't make sense. However, the closer greater parallax (adding the uncertainty) does yield a statistically valid (closer) distance. For a parallax = 0.383 mas, the distance = 8500 ly and statistically there's a 66% probability that the distance to µ Cep is between 8500 ly and the mean distance, 27,400 ly. Bottom line, it's much farther than HD206267, and APOD description needs updating. I suspect the large parallax uncertainty is due to the µ Cep brightness: M_G = 2.45 for this star and is ~1/2 mag brighter than Gaia's bright star limit M_G = 3 mag.

By the way, Simbad does have the DR3 distance parallax for µ Cep. Unfortunately, the default parallax displayed on the main µ Cep page is from Hipparcos. Scrolling down to "Collection of Measurements", click the "plx" box to display the available present and past 5 parallaxes.
 
simbad.cds.unistra.fr/simbad
Simbad PLX Data Display.jpg

Thanks a billion, alter-ego! Yes, I can see that the brightness of Mu Cephei could be a problem for Gaia.

I immediately went to Simbad to check Eta Carina, because I have found it so frustrating that Simbad has said nothing about the parallax, and thus distance, of this iconic star. So I scrolled down to "Collection of Measurements" and looked for "plx". Guess what? There hasn't been a single measurement of the parallax of Eta Carina! Not one!

Why is that? Eta Carina, at a G magnitude of 4.034632, should not be too bright. Is it because Eta Car is enveloped in a nebula, the Homunculus?

Ann

[alter-ego]

...


Well, if Gaia has measured the parallax of Mu Cephei, then the news of this has not come to Simbad.

The APOD caption said that the distance to Mu Cephei is nearly 3,000 light-years from planet Earth. Did Gaia tell you that, Robert Nemiroff and Jerry Bonnell, in private communication? Even though Gaia failed to inform Simbad?

Well, Gaia did tell Simbad that the parallax of HD 206267 is 1.3604 milliarcseconds (with an uncertainty of 0.2188 mas), which, if we disregard the uncertainty, corresponds to a distance of ~ 2,400 light-years. I take that to mean that Mu Cephei is more distant than HD 206267, and that Mu Cephei may not be involved with nebulosity of IC 1396. Or is it? Inquiring minds want to know!

Do note the little planetary nebula to the upper right of Mu Cephei. Just follow the Mu Cephei diffraction spike that points upwards and to the right, and you'll see it.
...
Ann

Gaia DR3 lists µ Cep parallax = 0.119 ± 0.264 mas.
The uncertainty is excessively large since negative parallaxes don't make sense. However, the closer greater parallax (adding the uncertainty) does yield a statistically valid (closer) distance. For a parallax = 0.383 mas, the distance = 8500 ly and statistically there's a 66% probability that the distance to µ Cep is between 8500 ly and the mean distance, 27,400 ly. Bottom line, it's much farther than HD206267, and APOD description needs updating. I suspect the large parallax uncertainty is due to the µ Cep brightness: M_G = 2.45 for this star and is ~1/2 mag brighter than Gaia's bright star limit M_G = 3 mag.

By the way, Simbad does have the DR3 distance parallax for µ Cep. Unfortunately, the default parallax displayed on the main µ Cep page is from Hipparcos. Scrolling down to "Collection of Measurements", click the "plx" box to display the available present and past 5 parallaxes.
 
simbad.cds.unistra.fr/simbad
Simbad PLX Data Display.jpg

Thanks a billion, alter-ego! Yes, I can see that the brightness of Mu Cephei could be a problem for Gaia.

I immediately went to Simbad to check Eta Carina, because I have found it so frustrating that Simbad has said nothing about the parallax, and thus distance, of this iconic star. So I scrolled down to "Collection of Measurements" and looked for "plx". Guess what? There hasn't been a single measurement of the parallax of Eta Carina! Not one!

Why is that? Eta Carina, at a G magnitude of 4.034632, should not be too bright. Is it because Eta Car is enveloped in a nebula, the Homunculus?

Ann

[Ann]

Gaia DR3 lists µ Cep parallax = 0.119 ± 0.264 mas.
The uncertainty is excessively large since negative parallaxes don't make sense. However, the closer greater parallax (adding the uncertainty) does yield a statistically valid (closer) distance. For a parallax = 0.383 mas, the distance = 8500 ly and statistically there's a 66% probability that the distance to µ Cep is between 8500 ly and the mean distance, 27,400 ly. Bottom line, it's much farther than HD206267, and APOD description needs updating. I suspect the large parallax uncertainty is due to the µ Cep brightness: M_G = 2.45 for this star and is ~1/2 mag brighter than Gaia's bright star limit M_G = 3 mag.

By the way, Simbad does have the DR3 distance parallax for µ Cep. Unfortunately, the default parallax displayed on the main µ Cep page is from Hipparcos. Scrolling down to "Collection of Measurements", click the "plx" box to display the available present and past 5 parallaxes.
 
simbad.cds.unistra.fr/simbad
Simbad PLX Data Display.jpg

Thanks a billion, alter-ego! Yes, I can see that the brightness of Mu Cephei could be a problem for Gaia.

I immediately went to Simbad to check Eta Carina, because I have found it so frustrating that Simbad has said nothing about the parallax, and thus distance, of this iconic star. So I scrolled down to "Collection of Measurements" and looked for "plx". Guess what? There hasn't been a single measurement of the parallax of Eta Carina! Not one!

Why is that? Eta Carina, at a G magnitude of 4.034632, should not be too bright. Is it because Eta Car is enveloped in a nebula, the Homunculus?

Ann

Eta Carinae Parallax not Possible.jpg

Thanks again, alter-ego! Yes, as soon as I saw that there were no parallax measurements for Eta Carina, I checked a nearby O-type star, HD 303308. Its Gaia parallax is 0.4432 ± 0.0213, and, disregarding the uncertainties, that corresponds to a distance of some 7,300 or 7,400 light-years. So I guess that Eta Carina is at a similar distance.

Ann