APOD: The Massive Stars in Westerlund 1 (2017 Jun 20)

[APOD Robot]

The Massive Stars in Westerlund 1

Explanation: Star cluster Westerlund 1 is home to some of the largest and most massive stars known. It is headlined by the star Westerlund 1-26, a red supergiant star so big that if placed in the center of our Solar System, it would extend out past the orbit of Jupiter. Additionally, the young star cluster is home to 3 other red supergiants, 6 yellow hypergiant stars, 24 Wolf-Rayet stars, and several even-more unusual stars that continue to be studied. Westerlund 1 is relatively close-by for a star cluster at a distance of 15,000 light years, giving astronomers a good laboratory to study the development of massive stars. The featured image of Westerlund 1 was taken by the Hubble Space Telescope toward the southern constellation of the Altar (Ara). Although presently classified as a "super" open cluster, Westerlund 1 may evolve into a low mass globular cluster over the next billion years.

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[Chris Peterson]

Wow. It's the Phoenix Asteroids.

phoenix-asteroids_3962326_GIFSoup.com.gif (426.88 KiB) Viewed 4446 times

[starsurfer]

Seems like my prediction was wrong.

[Coil_Smoke]

Wow. It's the Phoenix Asteroids.

phoenix-asteroids_3962326_GIFSoup.com.gif

https://en.wikipedia.org/wiki/Westerlund_1-26 is the total opposite of a 'Dark Star'

[Boomer12k]

Ummmmm..... Isn't that how "War of the Worlds" starts??? A big meteor crashes, brightens up the night sky... mayhem ensues?????

Would LOVE to go Meteorite hunting...Um.... in JANUARY...it is too hot right now...been to Arizona.

:---[===] *

[douglas]

Wow. It's the Phoenix Asteroids.

phoenix-asteroids_3962326_GIFSoup.com.gif

Excellent reference.
https://en.wikipedia.org/wiki/Dark_Star_%28film%29

Now all we need is a theoretician to claim Wunderlund is gravitationally assisted in its self-binding by a swarm of small, invisible black holes .. and it's Off! to the Races ...



/sarc

[BDanielMayfield]

The link in the Explanation Westerlund 1 may evolve goes to a non existent paper. Perhaps the suggestion that globular clusters can still form in today's universe has been disproved to the extent that a paper suggesting such an idea has been retracted?

Bruce

[BDanielMayfield]

I hope Ann is ok. Such a magnificent collection of brilliant blueness, and no comment yet from our intrepid color commentator. It's been nearly half a day, and no massive discussion yet? Perhaps something really huge is in the works.

Bruce

[bystander]

The link in the Explanation Westerlund 1 may evolve goes to a non existent paper. Perhaps the suggestion that globular clusters can still form in today's universe has been disproved to the extent that a paper suggesting such an idea has been retracted?

There seems to be a problem with the SAO/NASA Astrophysics Data System (ADS) link, however, I'm confident this is the paper that is supposed to be linked (a search for the title on ADS gives the same broken link).

The very low-mass stellar content of the young supermassive Galactic star cluster Westerlund 1 - M. Andersen et al

• Astronomy & Astrophysics 602:A22 (June 2017) DOI: 10.1051/0004-6361/201322863
  arXiv.org > astro-ph > arXiv:1602.05918 > 18 Feb 2016

[BDanielMayfield]

Thanks bystander. The last sentence in the abstract of the paper states this:

Westerlund 1 is sufficiently massive to remain bound and could potentially evolve into a low-mass globular cluster.

So, quite possibly, the period in which globular star clusters can still form is not quite completely over. Interesting.

Bruce

[Chris Peterson]

Thanks bystander. The last sentence in the abstract of the paper states this:

Westerlund 1 is sufficiently massive to remain bound and could potentially evolve into a low-mass globular cluster.

So, quite possibly, the period in which globular star clusters can still form is not quite completely over. Interesting.

It's also possible that such a "globular cluster" wouldn't be a globular cluster at all. That is, something might morphologically appear to be a globular cluster but have no connection at all in terms of formation and evolution to the satellites of galaxies that we typically refer to as "globular clusters". Two completely different routes to the same apparent end.

[Ann]

I hope Ann is ok. Such a magnificent collection of brilliant blueness, and no comment yet from our intrepid color commentator. It's been nearly half a day, and no massive discussion yet? Perhaps something really huge is in the works.

Bruce

I'm just back from Estonia! Been there for my nephew's wedding to a Russian girl whose family lives in Estonia. My nephew Erik and his Alina have been a couple for four years, and now they got married in eastern Estonia. It was a beautiful though somewhat vodka-drenched wedding, not on Erik's or Alina's parts - nor on mine!!!! - but Erik's father-in-law probably drank four bottles of vodka himself. I'll tell you about it some other time.

Click to view full size image

Westerlund 1 with Westerlund 1-26, red hypergiant.
VLT Survey Telescope image/Lithopsian.

Well, back to business. Hey, I've been gone for four days and everybody is making jokes about the great stars of Westerlund 1 instead of taking them seriously? :pouts:

Wikipedia wrote:

Westerlund 1-26 is classified as a luminous supergiant. With its surface temperature of about 3000 K, it occupies the upper right corner of the Hertzsprung–Russell diagram. The cool temperature means it emits most of its energy in the infrared spectrum. It also shows huge mass loss of atmospheric material, suggesting that it may further evolve into a Wolf–Rayet star. Westerlund 1-26 has been observed to change its spectral class during several periods, but it has not been seen to change its luminosity.

Another source quoted on the same Wikipedia page says that the temperature of Westerlund 1-26 is 3,600 K. Whatever.

In October 2013, astronomers using the European Southern Observatory's Very Large Survey Telescope (VST) discovered that Westerlund 1-26 is surrounded by a glowing cloud of ionized hydrogen. This is the first ionized nebula to have been discovered around a red supergiant star through its optical emission lines, and follows the discovery of an ionized nebula around NML Cyg in 1982.[13][14] The nebula extends 1.30 parsecs from the star.

That is amazing!!! There is hardly any emission nebulosity at all in Westerlund 1, but most if not all of what is there is centered on this humongously large cool red star! And check out what Wikipedia wrote about the colors of Westerlund 1-26:

U−B color index −7.445
B−V color index +5.000

Can you believe it? I am a color index nerd, and I have never heard of a U-B index of minus seven!!! Coupled with a B-V index of plus five!!!

Click to view full size image

Thor's Helmet, NGC 2359, with its Wolf-Rayet central star.
Photo: P. Berlind & P. Challis (CfA), 1.2-m Telescope, Whipple Obs.

It would seem that Westerlund 1-26 is ultraviolet-bright, for some hugely strange reason. I can't think of a way that this could happen to a cool red star, but the ultraviolet brightness of Westerlund 1-26 would explain the ionized nebula close to the star. Of course, the fact that Westerlund 1-26 is an unstable hypergiant shedding a lot of mass also explains why there is a gas cloud close to the star in the first place.

Perhaps Westerlund 1-26 is really going to evolve into a blisteringly, blisteringly hot but kind of smallish blue star of the Wolf-Rayet kind. These critters are extremely ultraviolet-bright, too.

Ann

[BDanielMayfield]

The last sentence in the abstract of the paper states this:

Westerlund 1 is sufficiently massive to remain bound and could potentially evolve into a low-mass globular cluster.

So, quite possibly, the period in which globular star clusters can still form is not quite completely over. Interesting.

It's also possible that such a "globular cluster" wouldn't be a globular cluster at all. That is, something might morphologically appear to be a globular cluster but have no connection at all in terms of formation and evolution to the satellites of galaxies that we typically refer to as "globular clusters". Two completely different routes to the same apparent end.

True enough, and I did qualify my comment with a "quite possibly". Could have left off the "quite".

Certainly Westerlund 1 isn't a traditional globular made of metal poor stars. Also, what about its location and orbit? I assume (since that's where the dust is) that it has formed inside our galaxy's disk, whereas true globulars are members of the halo.

Globular imposter!

Bruce

[Chris Peterson]

That is amazing!!! There is hardly any emission nebulosity at all in Westerlund 1, but most if not all of what is there is centered on this humongously large cool red star! And check out what Wikipedia wrote about the colors of Westerlund 1-26:

U−B color index −7.445
B−V color index +5.000

Can you believe it? I am a color index nerd, and I have never heard of a U-B index of minus seven!!! Coupled with a B-V index of plus five!!!

It would seem that Westerlund 1-26 is ultraviolet-bright, for some hugely strange reason. I can't think of a way that this could happen to a cool red star, but the ultraviolet brightness of Westerlund 1-26 would explain the ionized nebula close to the star.

Bear in mind that the U-B measurement is very uncertain. This star is almost obscured by dust, which results in a very large amount of reddening. And the u filter on the camera leaks red light. So an unusual (and more error prone) method of calculating color indexes was used, one which depends upon spectroscopic measurements.

It does seem likely that something odd is going on with this star, but it might not be as extreme as these reported indexes would suggest.

[BDanielMayfield]

I hope Ann is ok.

I'm just back from Estonia!

Glad your safely back Ann.

U−B color index −7.445
B−V color index +5.000

Can you believe it? I am a color index nerd, and I have never heard of a U-B index of minus seven!!! Coupled with a B-V index of plus five!!!

It is not possible to understand why you are so worked up about these color numbers if we don't know what they mean. Could you explain index numbers please?

[Ann]

I hope Ann is ok.

I'm just back from Estonia!

Glad your safely back Ann.

U−B color index −7.445
B−V color index +5.000

Can you believe it? I am a color index nerd, and I have never heard of a U-B index of minus seven!!! Coupled with a B-V index of plus five!!!

It is not possible to understand why you are so worked up about these color numbers if we don't know what they mean. Could you explain index numbers please?

Let me give you an example. Mu Columbae is a very blue star, and above all it is very unreddened. Its spectral class is O9.5V, although it's also classified as B0V. According to Jim Kaler, the temperature of Mu Columbae is 33,700 Kelvin. And according to French Wikipedia, the B-V index of Mu Columbae is -0.28. That's a very blue color index. Any B-V index that is negative is blue, and indeed, some stars with non-negative color indexes are pretty blue, too. Go out one night when you can see both Jupiter and A0-type B-V = 0.00 star Vega, and make sure you're carrying at least a pair of binoculars. Start by checking out Jupiter through your binoculars and then check out Vega. You'll be surprised at how blue it is. Indeed, through a telescope Vega usually looks blue even if you don't compare it with Jupiter.

Click to view full size image

Zeta Puppis, artist's illustration by Kryptid.

Okay, so the B-V index of Mu Columbae is -0.28. What is its U-B index? It's -1.06. Again, that's very blue. Or rather, that's very ultraviolet! I don't know any stars that are a lot more ultraviolet. Zeta Puppis, a star of spectral class O4if, an O4-type supergiant, has a B-V of -0.27 and an U-B of -1.09. Zeta Puppis is intrinsically hotter than Mu Columbae, but it is more reddened, which is why its B-V index is less negative than Mu Columbae's. But the U-B index of Zeta Puppis is more negative than that of Mu Columbae, and Zeta Puppis, as the hotter star, is undoubtedly the more ultraviolet.

My point is that I don't know of any star that has a more negative U-B index than ~-1.1, even though I suppose that there are many blisteringly hot white dwarfs that are more ultraviolet.

Click to view full size image

T Lyrae, a red carbon star.
Photo: Palomar Observatory.

T Lyrae is a rather famous and extremely red star. Its B-V varies, but it is around +5.5. I have seen one such star through a telescope, V Aquilae, and it was red! V Aquilae is the only red star I have ever seen, because all the other "red" stars have just been yellow! Well, Mu Cephei, the Garnet Star, was actually faintly coppery.

What about the U-B index of T Lyrae? My software, Guide, unfortunately does not inform me what its U-B index is, but it must be pretty red, too.

So, to summarize: A B-V index of +5.5 is very very red, and it indicates that the star is very red indeed. A U-B index of -7 is so ultraviolet I can't believe it, and it indicates that the star is either absolutely crazily hot or crazily ultraviolet compared with how blue it is. I guess Chris is right, there must be something wrong about the estimated U-B index of Westerlund 1-26.

Ann

[douglas]

There is hardly any emission nebulosity at all in Westerlund 1, but most if not all of what is there is centered on this humongously large cool red star! And check out what Wikipedia wrote about the colors of Westerlund 1-26:

U−B color index −7.445
B−V color index +5.000

It would seem that Westerlund 1-26 is ultraviolet-bright, for some hugely strange reason. I can't think of a way that this could happen to a cool red star ..

Bear in mind that the U-B measurement is very uncertain. This star is almost obscured by dust, which results in a very large amount of reddening.

It does seem likely that something odd is going on with this star, but it might not be as extreme as these reported indexes would suggest.

Ann's point that it is extremely bright in the ultraviolet suggests it is interacting with a companion. Are 'individual' stars as "crowded" as the cluster's center?
"Due to the relatively large distance to Wd1 all binaries are unresolved."
https://arxiv.org/abs/1602.05918

Question: Do Wolf-Rayets ever display spectra that are anomalous or appear mutated?

There has to be some data points that stand out as unusual, varying in some way that could indicate a reason.

[rstevenson]

... It is not possible to understand why you are so worked up about these color numbers if we don't know what they mean. Could you explain index numbers please?

And if you really want to get into it, start at the Color Index Wikipedia page. A short quote will get you started...

To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light... . The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U-B or B-V color index, respectively.

(There are also R and I filters.)

Rob

[BDanielMayfield]

Thanks Rob.

[Ann]

Ann's point that it is extremely bright in the ultraviolet suggests it is interacting with a companion. Are 'individual' stars as "crowded" as the cluster's center?
"Due to the relatively large distance to Wd1 all binaries are unresolved."
https://arxiv.org/abs/1602.05918

That is a possibility. There could be a hot companion orbiting Westerlund 1-26. The gas cloud being ionized would then be provided by Westerlund 1-26 itself, because it is shedding a lot of mass, and the ionization could be provided by the hot companion.

Ann

[MarkBour]

Thanks for the tutorial, Ann!

Ann's point that it is extremely bright in the ultraviolet suggests it is interacting with a companion. Are 'individual' stars as "crowded" as the cluster's center?

Is there any variability, I wonder?

[Ann]

Thanks for the tutorial, Ann!

Ann's point that it is extremely bright in the ultraviolet suggests it is interacting with a companion. Are 'individual' stars as "crowded" as the cluster's center?

Is there any variability, I wonder?

I found this on the net, for what it's worth:

Talk: Westerlund 1-26 wrote:

New data from ESO shows the ionized nebula in Westerlund 1-26. However, the star will be way too cold to heat up the ionized nebula. Does W1-26 have a binary companion, heating the nebula?

==Johndric Valdez (talk) 09:43, 20 October 2013 (UTC)==

Unknown. All that is known is that there is an (apparent) red supergiant and an ionised nebula. A red supergiant shouldn't be able to ionize a nebula, hence something else is going on. There are a lot of stars nearby perfectly capable of producing such a nebula (W25 is a blue supergiant), but they seem a little distant to be producing this one, hence speculation that there is a very hot star hiding in plain sight. There are no other observations to suggest such a star, and it would have to be very faint to not show up. There are several other, less glamorous, explanations for the ionization of the nebula, and a hot companion to W26 seems the least likely. Lithopsian (talk) 14:33, 20 October 2013 (UTC)

So no hot companion of Westerlund 1-26 has been found.

I haven't found anything about any variability of Westerlund 1-26, although it almost goes without saying that such a monstrously large star is going to be unstable and therefore variable.

Ann