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

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

Postby APOD Robot » Wed Jun 28, 2017 4:07 am

Image Composite Messier 20 and 21

Explanation: The beautiful Trifid Nebula, also known as Messier 20, lies about 5,000 light-years away, a colorful study in cosmic contrasts. It shares this nearly 1 degree wide field with open star cluster Messier 21 (top left). Trisected by dust lanes the Trifid itself is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky, with newborn and embryonic stars embedded in its natal dust and gas clouds. Estimates of the distance to open star cluster M21 are similar to M20's, but though they share this gorgeous telescopic skyscape there is no apparent connection between the two. M21's stars are much older, about 8 million years old. M20 and M21 are easy to find with even a small telescope in the nebula rich constellation Sagittarius. In fact, this well-composed scene is a composite from two different telescopes. Using narrowband data it blends a high resolution image of M20 with a wider field image extending to M21.

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

Postby Ann » Wed Jun 28, 2017 5:00 am

It is risky to post on my Ipad (can not find the apostrophe, among other things). But I doubt that M21 is 8 million years old. The Universe Today link said that M21 is 4.6 million years old, which makes more sense. I would expect at least one red giant in an 8 million year old cluster of massive stars.

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

Postby rstevenson » Wed Jun 28, 2017 11:55 am

From the M21 article in Wikipedia...
M21 is a relatively young cluster of a mere 4.6 million years of age. It is tightly packed but contains about 57 stars. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars.

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

Postby neufer » Wed Jun 28, 2017 12:46 pm

Ann wrote:
I doubt that M21 is 8 million years old. The Universe Today link said that M21 is 4.6 million years old, which makes more sense. I would expect at least one red giant in an 8 million year old cluster of massive stars.

You doubt that M21 is 8 million years old :?:
You would expect at least one red giant in an 8 million year old cluster of massive stars :?:

If you are going to claim special insights then you should at least provide some reasons, Ann.

All I personally know about astrophysics is what I read in the funny papers... and everyone else currently seems to claim that M21 is 4.6 million years old so I too accept that as true; but I certainly have no gut feelings on this matter. (My role is to play the Shakespearean fool and try to keep our Starship Asterisk experts 'umble.)

Today's APOD is repeating a 3 year old APOD that referenced a 21 year old Astronomical Journal article giving a contraction age of (8±3) Myr based upon color. This recent APOD also referenced the very same Universe Today link that states that M21 is 4.6 million years old...but that particular UT article has been recently updated. We should gently remind the APOD writers when they too should update.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby Newtownian » Wed Jun 28, 2017 1:12 pm

The link from M21 goes to this site https://www.universetoday.com/31935/messier-21/ which discusses rapidly spinning Be stars of which Achernar is an extreme example. As a result of this spin the star is reportedly greatly flattened and as a result its surface varies in temperature.

I wondered how this oblateness was measured. The answer was given here http://www.sciencemag.org/news/2003/06/flattest-star as interferometry.

This set me wondering if there was an alternative way to measure oblateness/fast spinning stars, the measurement of the irradiance spectrum deviation from that of slow spinning stars.

With a slow spinning star like the sun you might expect the spectrum to approximate that of a black body. This is illustrated here
https://en.wikipedia.org/wiki/Sunlight# ... rum_en.svg

But if the star had greatly varying temperature zones there would tend to be a movement away from the black body shape - essentially spreading out?

Does this make sense, has it been done, would irradiance spreading be detectable, has it already been done?

Just speculating. Does anyone have any answers.

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

Postby Ann » Wed Jun 28, 2017 1:39 pm

neufer wrote:
Ann wrote:
I doubt that M21 is 8 million years old. The Universe Today link said that M21 is 4.6 million years old, which makes more sense. I would expect at least one red giant in an 8 million year old cluster of massive stars.

You doubt that M21 is 8 million years old :?:
You would expect at least one red giant in an 8 million year old cluster of massive stars :?:

If you are going to claim special insights then you should at least provide some reasons, Ann.

All I personally know about astrophysics is what I read in the funny papers... and everyone else currently seems to claim that M21 is 4.6 million years old so I too accept that as true; but I certainly have no gut feelings on this matter. (My role is to play the Shakespearean fool and try to keep our Starship Asterisk experts 'umble.)

Today's APOD is repeating a 3 year old APOD that referenced a 21 year old Astronomical Journal article giving a contraction age of (8±3) Myr based upon color. This recent APOD also referenced the very same Universe Today link that states that M21 is 4.6 million years old...but that particular UT article has been recently updated. We should gently remind the APOD writers when they too should update.


Good point, of course.

I base my doubts and my insights (for what they are worth) on what I think I know about the typical appearance of young clusters. Most of them contain at least one red giant (NGC 3293, the Jewel Box cluster), and even in the case where there does not seem to be a red giant, like in M46 (or maybe that should be M47), where you can not see a red giant in the cluster, there sure enough is one nearby in the sky, with a very similar parallax and proper motion. One cluster that really, really does not seem to have any red giants even nearby is the Pleiades, and I have often wondered why. This lack of red giants in a cluster as old as the Pleiades is really rare.

My doubts about the 8 million year age for M21 could be unfounded, to be sure. The Pleiades prove that red giants do not always date a cluster.

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

Postby Chris Peterson » Wed Jun 28, 2017 1:45 pm

Newtownian wrote:But if the star had greatly varying temperature zones there would tend to be a movement away from the black body shape - essentially spreading out?

Does this make sense, has it been done, would irradiance spreading be detectable, has it already been done?

It's a very good idea, but I think it probably wouldn't work too well because in fact, stars are not very good blackbodies. That's because the photons they emit are produced over a range of their outer regions, and there is a temperature gradient across that emission range. So their spectrum already consists of a superposition of different blackbody curves.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby JohnD » Wed Jun 28, 2017 2:53 pm

The trouble with high resolution pictures us that they remove the mind's ability to visualise - is that paradoelia?
No doubt, in a more fuzzy picture of the Trifid, you can say, yes, tri-fid (not triffid, SF carniverous plant) split in three.
Now all I can see is - er, um, how can I put this? - a sphincter.
Just as well, the name **** h*l* Nebula would never have been adopted.

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

Postby Figuratively Listening » Wed Jun 28, 2017 4:57 pm

neufer wrote: We should gently remind the APOD writers when they too should update.


This is the west,sir...

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

Postby Newtownian » Wed Jun 28, 2017 10:00 pm

"I think it probably wouldn't work too well because in fact, stars are not very good blackbodies. That's because the photons they emit are produced over a range of their outer regions, and there is a temperature gradient across that emission range. So their spectrum already consists of a superposition of different blackbody curves."

Thanks for the response Chris. I take your points though the finer details of why there would be this spread are beyond my physics level. And my guess is that if it were possible it would already have been checked for but not reported because it was a non result. you are right.

Still the real litmus would be checking the emission patterns or a quick and dirty suck and see exercise.

I wonder if it would be possible still to quickly check if there is any noticeable effect at least with Achernar as a reference. It even looks like something amateurs could explore using a simple diffraction grating setup. Not ideal but even comparisons of relative emissions could conceptually work?

Another issue is resolution. It might be that only very fast spinning stars showed a useful black body spread emission signal. Still that might be an interesting exercise.

A final thing I jsut remembered. Achernar is deep in the southern hemisphere (fine for me near Sydney but not for the US).

So perhaps the first step is to look at reference emission spectra that could be checked and compared against Achernar for starters. Do you know of where this database might be?

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

Postby Chris Peterson » Wed Jun 28, 2017 10:13 pm

Newtownian wrote:"I think it probably wouldn't work too well because in fact, stars are not very good blackbodies. That's because the photons they emit are produced over a range of their outer regions, and there is a temperature gradient across that emission range. So their spectrum already consists of a superposition of different blackbody curves."

Thanks for the response Chris. I take your points though the finer details of why there would be this spread are beyond my physics level.

I may not have been clear what I meant about "outer regions". I'm talking about the fact that photons are produced at different depths. It doesn't take much physics to understand that there is a temperature gradient with depth, including quite close to the surface where photons can be produced and released without being captured.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby Newtownian » Thu Jun 29, 2017 12:40 am

Chris Peterson wrote:
Newtownian wrote:"I think it probably wouldn't work too well because in fact, stars are not very good blackbodies. That's because the photons they emit are produced over a range of their outer regions, and there is a temperature gradient across that emission range. So their spectrum already consists of a superposition of different blackbody curves."

Thanks for the response Chris. I take your points though the finer details of why there would be this spread are beyond my physics level.

I may not have been clear what I meant about "outer regions". I'm talking about the fact that photons are produced at different depths. It doesn't take much physics to understand that there is a temperature gradient with depth, including quite close to the surface where photons can be produced and released without being captured.


Again thanks. I take your point about non ideal black body emission. What made me still wonder though were these points:

- To judge by the solar spectrum https://en.wikipedia.org/wiki/Sunlight# ... rum_en.svg black body emission still doesnt look too bad an approximation especially on the long wavelength side (maybe its just the scale though).

- The spread of temperatures for Achernar at least between pole and equator appears to be extraordinarily large https://en.wikipedia.org/wiki/Achernar#Namesake - and hence detectable???

- Slowly rotating comparable B6 stars could provide reference spectra which might account for photons coming from different depths and allow stars not being ideal black bodies to be accounted for. Other star spectra could also be extrapolated.

- In the case of Achernar I understand there is this independent estimate of its oblateness, such that any value derived from an irradiance distribution v. oblateness model could be checked (and if this has legs Achenar could act as a calibration/noise reference).

It may not be that the oblateness of many stars is sufficient to be measured. But if a useful relationship could be derived then spectra from space telescopes (Gaia?) might provide a means of estimating the oblateness of fast spinning stars (would Gaia be able to provide measurements of oblateness for nearby stars by virtue of its resolving power?).

I presume there are physical models already that relate star spin rate (measured from blurring of emission lines?) to oblateness but to my naive view it looks like direct oblateness measurements may be much less available. Perhaps this could be a way of identifying populations of very fast spinning stars?

Maybe I should email a query to the Gaia people - give them something else to do with their data that doesnt appear to be on the current TTD list?

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

Postby neufer » Thu Jun 29, 2017 11:07 am

Newtownian wrote:
It may not be that the oblateness of many stars is sufficient to be measured. But if a useful relationship could be derived then spectra from space telescopes (Gaia?) might provide a means of estimating the oblateness of fast spinning stars (would Gaia be able to provide measurements of oblateness for nearby stars by virtue of its resolving power?).

I presume there are physical models already that relate star spin rate (measured from blurring of emission lines?) to oblateness but to my naive view it looks like direct oblateness measurements may be much less available. Perhaps this could be a way of identifying populations of very fast spinning stars?

Maybe I should email a query to the Gaia people - give them something else to do with their data that doesnt appear to be on the current TTD list?

Gaia is not a large telescope (and it does not do very high resolution spectroscopy).

Spinning Gaia sees many many multiple blurred images of each individual star.

By statistically combining those many multiple blurred images
Gaia can very accurately pinpoint the central position of each star
but it was never designed to actually resolve those multiple blurred images.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby neufer » Thu Jun 29, 2017 11:34 am

http://www.solstation.com/x-objects/achernar.htm wrote:
<<In July 2003, a team of astronomers (including Armando Domiciano de Souza, Lyu Abe, Farrokh Vakili, Pierre Kervella, Slobodan Jankov, Emmanuel DiFolco, and Francesco Paresce) announced that Achernar is much more oblate than originally expected, with an equatorial radius that is more than 50 percent larger than its polar one -- a surprisingly high axial ratio of 1.56 ± 0.05 (ESO press release; and Domiciano de Souza et al, 2003). According to the ESO team, the angular size of Achernar's elliptical profile is 0.00253 ± 0.00006 arcsec (major axis) and 0.00162 ± 0.00001 arcsec (minor axis), respectively. At its measured distance, the star's equatorial radius is estimated to be 12.0 ± 0.4 and Solar radii, while the upper value of its polar radius was calculated to be 7.7 ± 0.2 times the Sol's -- or 8.4 and 5.4 million kilometers (5.2 and 3.4 million miles), respectively. The ESO's estimates were upper values that are dependent on the actual inclination of the star's polar axis to their line-of-sight from Earth, and so they may well be slightly smaller. On the other hand, the high degree of flattening measured for Achernar cannot be reproduced by common models of stellar interiors unless certain phenomena are assumed, including meridional circulation on the surface ("north-south streams") and non-uniform rotation at different depths inside the star.

A side effect of the extreme oblateness would be a high rate of mass loss from the surface which is enhanced by the rapid rotation through the centrifugal effect. Ejecting mass at a rate thousands of times greater than Sol, Achernar's high spin velocity of 225 to 300 kilometers per second has helped to turn it into a "Be" (B-emission) star, which has an expanding circumstellar envelope (CSE) of gas circulating around its equator with "episodic Balmer lined in emissions" when its CSE is enlarged by mass ejections (Domiciano de Souza et al, 2003; and Oegerle and Polidan, 1984). As a very young high-mass star, Achernar is a fast rotator with a rotational period of hours and a substantial magnetic field around one kG.

According to Professor Jim Kaler's Stars page on Achernar, the star's high spin velocity of at least 250 kilometers per second contributes to its status as a "Be" (B-emission) star that has a belt of emitting gas circulating in its equator, causing the star to lose mass at a rate thousands of times greater than Sol's. Achernar is also a member of a peculiar class of Lambda Eridani-type stars that show small but very regular periodic light variations (with a period of 1.26 days) that may be caused by actual complex pulsations or by rotation and dark "starspots" (Balona et al, 1987). Although Achernar is a massive star, it is still young enough to be fusing hydrogen into helium in its core and may be small enough to evolve off the sequence as a massive white dwarf like Sirius B.>>
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby BDanielMayfield » Thu Jun 29, 2017 12:19 pm

JohnD wrote:The trouble with high resolution pictures us that they remove the mind's ability to visualise - is that paradoelia?
No doubt, in a more fuzzy picture of the Trifid, you can say, yes, tri-fid (not triffid, SF carniverous plant) split in three.
Now all I can see is - er, um, how can I put this? - a sphincter.
Just as well, the name **** h*l* Nebula would never have been adopted.

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Dr. John, a proctologist, perhaps? :lol2:

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

Postby geckzilla » Thu Jun 29, 2017 3:34 pm


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

Postby neufer » Thu Jun 29, 2017 3:50 pm

geckzilla wrote:
I was reading about this amazing star [Acherar] 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.

10,000º F may be a warm orange... but not 10,000K.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby geckzilla » Thu Jun 29, 2017 4:08 pm

neufer wrote:
geckzilla wrote:
I was reading about this amazing star [Acherar] 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.

10,000º F may be a warm orange... but not 10,000K.

Yeah, I realized this while I was in the shower earlier and was just throwing some clothes on to try to fix it. You beat me to it. Thanks. Not sure how I managed that.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby neufer » Thu Jun 29, 2017 5:00 pm

geckzilla wrote:
Yeah, I realized this while I was in the shower earlier and was just throwing some clothes on to try to fix it.
You beat me to it. Thanks. Not sure how I managed that.

Ah...what were we talking about again? (My mind just got totally distracted.)
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby Newtownian » Fri Jun 30, 2017 4:58 am

neufer wrote:
Newtownian wrote:
It may not be that the oblateness of many stars is sufficient to be measured. But if a useful relationship could be derived then spectra from space telescopes (Gaia?) might provide a means of estimating the oblateness of fast spinning stars (would Gaia be able to provide measurements of oblateness for nearby stars by virtue of its resolving power?).

I presume there are physical models already that relate star spin rate (measured from blurring of emission lines?) to oblateness but to my naive view it looks like direct oblateness measurements may be much less available. Perhaps this could be a way of identifying populations of very fast spinning stars?

Maybe I should email a query to the Gaia people - give them something else to do with their data that doesnt appear to be on the current TTD list?

Gaia is not a large telescope (and it does not do very high resolution spectroscopy).

Spinning Gaia sees many many multiple blurred images of each individual star.

By statistically combining those many multiple blurred images
Gaia can very accurately pinpoint the central position of each star
but it was never designed to actually resolve those multiple blurred images.


Thanks for the interesting background on Achernar. As to your response to my response perhaps you could comment on the following:

1. Gaia I agree is not a telescope in the old fashion sense of a Galiliean tube. But saying it is not a telescope in the modern sense seems strange - at bit like saying a radio telescope is not a telescope because it receives a complex signal that needs to be electronically processed to obtain a picture like photograph. To use the definition in Wiki:

"an optical instrument that aids in the observation of remote objects by collecting electromagnetic radiation"

Gaia certainly fits this description at least. e.g. http://sci.esa.int/gaia/59023-gaia-s-sn ... er-galaxy/ Its way of assembling images is 'unconventional'. But then most of the spectacular 'telescope' pictures we get these days are fitered ensembles and artefacts to varying degrees.

Perhaps you could clarify what the definition of 'telescope' is - say by a link to generally accepted authority say the IAU noting that 'Gaia' itself is also refere to the space observatory.

2. Gaia may not do high resolution spectroscopy across the spectrum, but:

" The Radial-Velocity Spectrometer (RVS) is used to determine the velocity of celestial objects along the line of sight by acquiring high-resolution spectra in the spectral band 847–874 nm (field lines of calcium ion) for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s (V=11.5) and 30 km/s (V=17.5). The measurements of radial velocities are important to correct for perspective acceleration which is induced by the motion along the line of sight." The RVS reveals the velocity of the star along the line of sight of Gaia by measuring the Doppler shift of absorption lines in a high-resolution spectrum." (Wiki)

And it does respectable 'synthetic' spectroscopy with a resolution of 10 nm give or take "The passbands are derived by the convolution of the response curves of the optics and the QE curves of the CCDs" - Jordi, C., Gebran, M., Carrasco, J., de Bruijne, J., Voss, H., Fabricius, C., . . . Mora, A. (2010). Gaia broad band photometry. Astronomy & Astrophysics, 523, A48. which I wonder might be sufficient to look at black body radiation changes.

3. Gaia complements high resolution spectroscopy enormously - Gilmore, G., Randich, S., Asplund, M., Binney, J., Bonifacio, P., Drew, J., . . . Micela, G. (2012). The Gaia-ESO public spectroscopic survey. The Messenger, 147, 25-31. section "Why not just wait for Gaia".

To my untutored self this suggests:

a. Gaia should detect lots of slow v. fast rotating stars based on spectroscopy and the broadenning of the Calcium lines, identify stars which are comparable (e.g. similar metalicity and emission peak) but differ in their rotation, and concurrently provide spectra from which you could provide a first cut on how spectra change the faster a star rotates using an enormous sample of comparable stars small and large, which could then be combined with more detailed data such as that from Gaia-ESO.

b. Gaia does not provide high resolution spectra in the strict single instrument sense. But these references suggest it will be central to transforming stellar spectroscopy way beyond its current state by providing statistical power that seems to come when you combine different measurement methods.

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

Postby Ann » Fri Jun 30, 2017 5:23 am

geckzilla wrote:

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.

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

Postby neufer » Fri Jun 30, 2017 10:22 am

Ann wrote:
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.)
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby Chris Peterson » Fri Jun 30, 2017 1:56 pm

neufer wrote:
Ann wrote:
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.

Hipparcos didn't collect Johnson B or V data. The B-V data published in the Hipparcos catalog either uses ground-based values or transformed BT-VT values, which will not typically match Johnson B-V values all that well. You can collect Johnson B-V data in space that correlates very well with the same measurements made from the ground.

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

That's true, but they don't need to. 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.
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby neufer » Fri Jun 30, 2017 3:26 pm

Chris Peterson wrote:
neufer wrote:
(Ground based systems can't observe UV and 20,000K peaks in the UV.)

That's true, but they don't need to. 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).

And Gaia seems well designed to measure radial (calcium line) Doppler velocities (including rotational Doppler spreading) from G, K and M stars but it's not clear what it can do for rapidly rotating O, B, and A stars with weak calcium lines.>>
https://en.wikipedia.org/wiki/Gaia_(spacecraft) wrote:
<<The Gaia payload [includes]:

The photometric instrument (BP/RP) allows the acquisition of luminosity measurements of stars over the 320–1000 nm spectral band, over the same magnitude 5.7–20. The blue and red photometers (BP/RP) are used to determine stellar properties such as temperature, mass, age and elemental composition. Multi-colour photometry is provided by two low-resolution fused-silica prisms dispersing all the light entering the field of view in the along-scan direction prior to detection. The Blue Photometer (BP) operates in the wavelength range 330–680 nm; the Red Photometer (RP) covers the wavelength range 640–1050 nm.

The Radial-Velocity Spectrometer (RVS) is used to determine the velocity of celestial objects along the line of sight by acquiring high-resolution spectra in the spectral band 847–874 nm (field lines of calcium ion) for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s (V=11.5) and 30 km/s (V=17.5). The measurements of radial velocities are important to correct for perspective acceleration which is induced by the motion along the line of sight." The RVS reveals the velocity of the star along the line of sight of Gaia by measuring the Doppler shift of absorption lines in a high-resolution spectrum.>>
https://en.wikipedia.org/wiki/Calcium_triplet wrote:
<<The infrared Ca II triplet, commonly known as the Calcium triplet, is a triplet of three ionised calcium spectral lines at the wavelength of 8498 Å, 8542 Å and 8662 Å. The triplet has a strong emission, and is most prominently observed in the absorption of spectral type G, K and M stars.>>
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Re: APOD: Composite Messier 20 and 21 (2017 Jun 28)

Postby Chris Peterson » Fri Jun 30, 2017 4:11 pm

neufer wrote:
Chris Peterson wrote:
neufer wrote:(Ground based systems can't observe UV and 20,000K peaks in the UV.)

That's true, but they don't need to. 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).
Chris

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