APOD: Composite Messier 20 and 21 (2017 Jun 28)

[Ann]

The Johnson B-V of Achernar is (a lot) more negative than -0.2, but the Hipparcos B-V is well below (that is, a lot less negative than) -0.2. The difference between the Johnson and the Hipparcos color measurements is remarkable. And the most likely reason for it that I can think of is the oblateness of Achernar and the temperature variations that come with it.

Please explain the difference between the (ground based) Johnson B-V and the (space based) Hipparcos B-V.

(Ground based systems can't observe UV and 20,000K peaks in the UV.)

Sorry. Like Chris pointed out, Hipparcos didn't collect any B-V measurements, but another European satellite, Tycho, did. I think of the Tycho measurements as if they were made by Hipparcos, because my software, Guide, mentions the Hipparcos distance parallax and the Tycho color measurement in more or less the same go, as if they were two peas in a pod.

I do know that Tycho didn't measure exactly the same wavelengths as groundbased Johnson, so you would expect Tycho and Johnson to show slightly different colors for the same star. You will have to take me at my word when I say that I have used my software to compare the Johnson and the Hipparcos - that is, the Tycho - B-V color of hundreds if not thousands of blue stars. The color indexes very often differ a little, but usually just a little. The difference between the Johnson and the Tycho B-V color for Achernar is remarkable. My guess, for what it is worth, is that Achernar was in a relatively pole-on position when the Johnson measurement was made, and in a far more equator-on position when Tycho made its measurement.

Either that, or else the Johnson measurement was partly guessed at from the spectral class of Achernar.

Ann

[neufer]

For a blackbody, all you need are two measurements and that defines the shape of the curve, which tells you where the peak lies. That's one of the main uses of color indices.

That doesn't help much if most of the radiation comes from ~20,000K (with a small contribution from ~10,000K).

Since Gaia can only sense wavelengths longer than 320 nm it is not clear how well it can distinguish a ~20,000K spectrum from a ~10,000K spectrum even if one knew the orientation of the star (from rotational Doppler spreading say).

Maybe I'm not understanding you. You can perfectly tell the difference between a 20,000 K and a 10,000 K blackbody radiator using two measurements in the visible part of the spectrum, assuming that the radiator is a blackbody and that you have sufficient S/N in your recording.

I agree that if something (like rapid rotation) makes the source deviate too far from a blackbody, a pair of measurements will no longer suffice to determine its temperature (or temperature profile).

Blackbodies all have pretty much the same dull shape at long wavelengths...it's only at short wavelengths where they deviate in shape.

It would have been nice if Gaia could sense wavelengths shorter than 290 nm to detect the peak of the 10,000 K blackbody spectrum
(if not wavelengths shorter than 145 nm to detect the peak of the 20,000 K blackbody spectrum).

[Chris Peterson]

Maybe I'm not understanding you. You can perfectly tell the difference between a 20,000 K and a 10,000 K blackbody radiator using two measurements in the visible part of the spectrum, assuming that the radiator is a blackbody and that you have sufficient S/N in your recording.

Blackbodies all have pretty much the same dull shape at long wavelengths...it's only at short wavelengths where they deviate in shape.

Nevertheless, the B/V and U/V ratios are significantly different for 10,000 K and 20,000 K sources.

It would have been nice if Gaia could sense wavelengths shorter than 290 nm to detect the peak of the 10,000 K blackbody spectrum
(if not wavelengths shorter than 145 nm to detect the peak of the 20,000 K blackbody spectrum).

It would enhance the accuracy of the measurement. But we don't routinely look anywhere near the peak emission of hot stars to identify their temperatures. The standard (at least for surveys) remains ratios within the UBVRI photometric system (where the U is very near UV, still detectable with ordinary silicon sensors).

[Ann]

I'm home! So now I can start trying to figure out if M21 would be likely to contain a red giant if it was 8 rather than 4.6 million years old. (Spoiler: It probably wouldn't. I was wrong.)

But let's look at a few clusters with and without red giants.

Click to view full size image

Super star cluster NGC 3603 in three near infrared bands.
Contains no red giants, but note red giants nearby.
Age: 1 million years.
ESO/ISAAC instrument at the ANTU telescope

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NGC 6231: No red giants. Age: 3.2 million years old.
Photo: Josef Pöpsel, Stefan Binnewies.

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Westerlund 1: Several red giants. Age: 3.50 or 4-5 million years.
Photo: ESA, Hubble.

NGC 3603, one of the most massive clusters in the Milky Way, is only a million years old and therefore contains no red giants. Note the red giants nearby, however.

NGC 6231 and Westerlund 1 are both younger than M21. NGC 6231 does not contain any red giants - the yellow star at left is a foreground object - but Westerlund 1 does. Westerlund 1 is undoubtedly much more massive (63,000 M_☉) than M21, and also more massive than NGC 6231. It contains very massive and therefore particularly shortlived stars.

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NGC 3293. Contains a prominent red giant. Age: 8 million years.
Photo: ESO/G. Beccari.

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The Double Cluster of Perseus.
Several prominent red giants in the field.
Age: 12.8 million years.
Photo: Roth Ritter.

NGC 3293 in Carina and the Double Cluster in Perseus are both young, just a little older than M21. Actually, NGC 3293 may be the same age as M21, if we assume that M21 is really 8 million years. Both NGC 3293 and (in particular) the Double Cluster in Perseus are more massive than M21 and contain more massive stars. According to Wikipedia, the total mass of the Double Cluster (including the surrounding halo of stars) is about 20,000 M_☉.

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M47. The red giant to the right of M47 belongs to the cluster.
Age: 78 million years.
Photo: Wikisky.

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M6, the Butterfly Cluster.
Contains a prominent red giant. Age: 80-100 million years.
Photo: Sergio Eguivar.

M47 and M6, which may be more or less the same age, both contain one prominent red giant.

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Sparse blue cluster IC 2602. Red giants nearby don't belong to the cluster.
Age: 13.7 million years? 50 million years?

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Sparse blue cluster IC 2391. Age: 50 million years? 13.7 million years?
Photo: Celestial Image Co.

The long and short of this seems to be that a sparse cluster whose most massive members are not massive enough to ever have been O-type stars don't have to contain a red giant when they are 12-14 million years old, and certainly not when they are 8 million years. And M21 doesn't look very massive at all, and as far as I can understand, it has never contained any O-type stars.

But if a cluster very massive, it can contain red giants even if it is only 4.6 million years old.

Ann

Click to view full size image

NGC 3532.
Age: 300 million years.
Photo: ESO/G. Beccari .

Can't resist showing you this one before I go. NGC 3532, a rich cluster, 300 million years old, and full of red giants.

[Ann]

One more. Can't resist.

Wikipedia wrote:

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R136. NASA, ESA, F. Paresce, R. O'Connell.

R136 is thought to be less than 2 million years old.[8][9] None of the member stars is significantly evolved and none is thought to have exploded as supernova. The brightest stars are WNh, O supergiants, and OIf/WN slash stars, all extremely massive fully convective stars. There are no red supergiants, blue hypergiants, or luminous blue variables within the cluster.

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Galileo - Eppur Si Muove
Art by discouragedone

There are no red supergiants in R136??? But I can see one red supergiant there! Looks bright, too!

Okay... maybe the red supergiant doesn't belong to R136 proper, but to the larger association, NGC 2070. Lately astronomers have even found that R 136 appears to be merging with another cluster, but even here the red star is left out of the picture.

Perhaps the red star near R136 is the Betelgeuse of the R136 region. It isn't quite located where the action is taking place. It isn't as young as the wild bunch of the central cluster. It isn't as massive as the most massive ones of the blue bunch, because stars lose matter at an increased rate when they turn into red giants. Besides, the most massive stars may never turn into red giants in the first place, but instead they become Wolf-Rayet stars and luminous blue variables.

But hey, the Lone Red Ranger of the R136 wilderness sure is visible. And it may be the first of its R136 region kind, but it sure won't be the last.

Ann

[Ann]

I was reading about this amazing star just last night and was fascinated to learn that at its poles it may be 20000K while at its equator it could be around 10000K. That's blindingly blue at the top and a warm orange in the middle. That's quite a tremendous temperature difference.

The Johnson B-V of Achernar is (a lot) more negative than -0.2, but the Hipparcos B-V is well below (that is, a lot less negative than) -0.2. The difference between the Johnson and the Hipparcos color measurements is remarkable. And the most likely reason for it that I can think of is the oblateness of Achernar and the temperature variations that come with it.

Ann

Sorry to be quoting myself, but I need to set things straight. (I was writing the above post when I was in London, where I didn't have access to my software and was reduced to using my little IPAD and its tiny keyboard.)

The Johnson B-V of Achernar is -0.158 ± 0.007. The Tycho B-V of Achernar is ~-0.064, with an uncertainty that might be as large as 0.02. Still, there is a big difference between the Johnson and the Tycho color indexes for Achernar.

Click to view full size image

Constellation Scorpius. B1.5IV-type star Shaula is the bright component of
the "double star" at about 8 o'clock. Photo: Bill and Sally Fletcher.

Another star that lost much of its Johnson blue luster when Tycho measured its B-V is Shaula, Lambda Orionis. The Johnson B-V for Shaula is -0.231 ± 0.008. The Tycho value is -0.161 ± ~0.02. When I said that the Johnson B-V index for Achernar is more negative than -0.2, while the Tycho value is well below -0.2, I was thinking of the Johnson and Tycho color indexes for Shaula.

Why is Shaula so faintly blue according to Tycho compared with the Johnson measurement of its color? It just might have to do with the fact that Shaula is a multiple star with one very close companion.

Jim Kaler wrote:

Orbiting Shaula A with a period of just 5.9525 days is a far lesser star called "Shaula Ab" (rendering the principal star "Shaula Aa") that is hypothesized to be the origin of Shaula's highly unusual X-ray radiation. With an estimated mass of 1.8 solar, Shaula AB would orbit at a distance from Shaula Aa of only 0.15 AU, less than half Mercury's distance from the Sun. One would expect a small companion this close to have a circular orbit. Instead, Shaula Ab winds its way from as far as 0.19 AU from Shaula Aa to as close as 0.11 AU.

My amateur guess is that the very close companion might whip up a dust cloud near Shaula, reddening it. Just maybe the dust cloud wasn't there when the Johnson color measurement was made.

Ann

[Chris Peterson]

The Johnson B-V of Achernar is -0.158 ± 0.007. The Tycho B-V of Achernar is 0.064, with an uncertainty that might be as large as 0.02. Still, there is a big difference between the Johnson and the Tycho color indexes for Achernar.

Somewhat repeating my earlier comment, but specific to Tycho-2: this catalog does not include Johnson B-V information. Hipparcos used filters with different bandpasses. There are approximate conversions, but they can be quite far off, depending on reddening and on the spectral characteristics of the star. The Tycho reference material makes it quite clear that as a rule, it is best to directly utilize the B_T-V_T values and not convert to B-V. So the question is, what is your program reporting for the photometry with Tycho stars? The B_T-V_T values or calculated B-V values derived from those?

[Ann]

The Johnson B-V of Achernar is -0.158 ± 0.007. The Tycho B-V of Achernar is 0.064, with an uncertainty that might be as large as 0.02. Still, there is a big difference between the Johnson and the Tycho color indexes for Achernar.

Somewhat repeating my earlier comment, but specific to Tycho-2: this catalog does not include Johnson B-V information. Hipparcos used filters with different bandpasses. There are approximate conversions, but they can be quite far off, depending on reddening and on the spectral characteristics of the star. The Tycho reference material makes it quite clear that as a rule, it is best to directly utilize the B_T-V_T values and not convert to B-V. So the question is, what is your program reporting for the photometry with Tycho stars? The B_T-V_T values or calculated B-V values derived from those?

This is what Guide says on the matter:

The BT magnitude and VT magnitudes are blue and visual magnitudes as measured by Tycho. They correspond pretty closely to standard Johnson visual magnitudes and B magnitudes. The BT system has a peak at 435 nanometers; the VT, at 505 nanometers.

The reason why I brought up Shaula is that it seems to have a "cooler", less blue BT-VT color than nearby stars of similar spectral classes. For example, Nu Scorpii, which belongs to a very slightly cooler spectral class compared with Shaula (B2IV versus B1.5IV) has a slightly hotter and bluer Tycho BT-VT index than Shaula (~-0.20 versus ~-0.16). Mu1 Scorpii, which belongs like spectral class B1.5IV just like Shaula, also has a BT-VT index of ~-0.20. Mu2 Scorpii, spectral class B2IV, has a BT-VT of ~-0.22. Shaula seems to be the odd one out with a slightly "cool for its spectral class" color index. Interestingly, while the BT-VT index of Shaula is less blue than the ones of Kappa, Nu, Mu1 and Mu2, the Johnson B-V index of Shaula is bluer than any of those of Kappa, Nu, Mu1 or Mu2.

Two interestingly contrasting stars, both immersed in (faint) nebulosity, are Tau Scorpii and Zeta Ophiuchi. The BT-VT index of Tau Scorpii, spectral class B0V, is ~-0.23. But Zeta Ophiuchi, spectral class O9.5V, has a BT-VT index of ~+0.02. Of course we're talking reddening here.

Usually the Johnson B-V and the Tycho BT-VT agree relatively well, but sometimes they don't, for various reasons.

Ann

[Chris Peterson]

Somewhat repeating my earlier comment, but specific to Tycho-2: this catalog does not include Johnson B-V information. Hipparcos used filters with different bandpasses. There are approximate conversions, but they can be quite far off, depending on reddening and on the spectral characteristics of the star. The Tycho reference material makes it quite clear that as a rule, it is best to directly utilize the B_T-V_T values and not convert to B-V. So the question is, what is your program reporting for the photometry with Tycho stars? The B_T-V_T values or calculated B-V values derived from those?

This is what Guide says on the matter:

The BT magnitude and VT magnitudes are blue and visual magnitudes as measured by Tycho. They correspond pretty closely to standard Johnson visual magnitudes and B magnitudes. The BT system has a peak at 435 nanometers; the VT, at 505 nanometers.

Well, that's simply wrong. The two systems produce substantially different numbers that cannot ever be compared directly. For many unreddened, main sequence stars it's possible to apply a transformation to the Tycho values in order to derive approximate Johnson values. But it doesn't sound like Guide did that, so you should not, ever, compare Tycho color indices to Johnson color indices using that as a source.

[Ann]

I can't argue with your science savvy, Chris, that much is certain.

Guide doesn't provide any BT-VT indexes. It gives the BT and the VT magnitude of stars. I have then subtracted the VT value from the BT value for each star that I have taken an interest in. Usually I come up with a color that is quite similar to the Johnson B-V index, which is always given by Guide.

As for Achernar, its color and temperature are indeed hard to measure.

Jim Kaler wrote:

Achernar, a hot class B (B3) dwarf, is the hottest of the top nine, rather handily beating out Rigel in Orion. Yet surprisingly, for such a bright star, its temperature is not well known, various measures running from 14,500 to 19,300 Kelvin. From its distance of 140 light years (second Hipparcos reduction, the lower temperature gives a luminosity 2700 times that of the Sun, while the upper gives 5100 (the difference caused in part by different estimates of the amount of ultraviolet radiation). The radius then ranges between 8.2 and 6.4 times solar. Interferometer measures show the star to be distinctively flattened, the result of a minimum 225 kilometer-per-second rotation speed. The projected minor and major axes are respectively measured to be 7.5 by 11.6 Suns across (which gives a rotation period of under 2.1 days), which agrees better with that derived from the lower temperature. The higher temperature, however, is more in tune with that indicated by the spectral class.

There is clearly something weird going on with Achernar. To my amateur mind, it seems reasonable that the confusion as to its temperature and color is at least partly caused by the fact that different measurements of its color (Johnson and Tycho?) have produced quite different results.

Ann