APOD: Orionids in Taurus (2023 Oct 26)

[APOD Robot]

Orionids in Taurus

Explanation: History's first known periodic comet, Comet Halley (1P/Halley), returns to the inner Solar System every 76 years or so. The famous comet made its last appearance to the naked-eye in 1986. But dusty debris from Comet Halley can be seen raining through planet Earth's skies twice a year during two annual meteor showers, the Eta Aquarids in May and the Orionids in October. In fact, an unhurried series of exposures captured these two bright meteors, vaporizing bits of Halley dust, during the early morning hours of October 23 against a starry background along the Taurus molecular cloud. Impacting the atmosphere at about 66 kilometers per second their greenish streaks point back to the shower's radiant just north of Orion's bright star Betelgeuse off the lower left side of the frame. The familiar Pleiades star cluster anchors the dusty celestial scene at the right.

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[Ann]

I always love seeing the Pleiades next to comets or meteors, because it is so hugely interesting to compare their colors! And indeed, the Pleiades are strikingly blue in this APOD, and the meteors are strikingly yellow-greenish.

Orionids in Taurus. Image Credit & Copyright: David Cortner

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But me being me, I noticed some very greenish-looking stars in the lower left of the picture. Before I talk about them, let's look at an attempt to show the true optical colors of the 25 brightest stars in the sky (26 if we include the Sun, which we should):

The optical colors and B-V indices of the 25 brightest stars
in the night sky. Credit: Tragoolchitr Jittasaiyapan

The Sun as seen from ESA/NASA's Solar Orbiter.

I have used B-V color indices to give a "scientific" description of stellar colors. Yes, I can hear you groaning, Chris.

The B-V index measures a star's brightness in the blue (B) channel versus the yellow-green (V) channel. The B-V index of our G2V Sun is 0.656 ± 0.005. Any star with a higher (more positive) B-V index is going to be yellower (or redder) than the Sun. Any star with a lower (less positive, or in some cases negative) B-V index is going to be bluer than the Sun.

The B-V indices of the 25 brightest stars that I have added to Tragoolchitr Jittasaiyapan's image are Johnson B-V indices. They may not agree with other measurements of these stars' B-V indices.

Matching a star's visual appearance with its B-V index is a tricky business. In Tragoolchitr Jittasaiyapan's image, you can see, for example, that Castor in the bottom row looks bluer than Adhara in the same row, even though Adhara is a lot hotter, produces a lot more blue light and has a much more negative B-V index than Castor. Then again, our eyes are typically unable to tell the difference in the saturation of blue color in (not so hot) A-type stars versus (much hotter) B- and O-type stars, although cameras can sometimes do it.

In the picture at left below you can - just barely, but you can - see that one of the blue stars (middle row, to the left) between nebulas IC 405 (right) and IC 410 is bluer than the others. And why not take a look at Tragoolchitr Jittasaiyapan's picture of the same star in the same stellar environment?

Very blue star IQ Aurigae between nebulas IC 404 and IC 410. Credit: rwittich.

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IC 405 and the Leaping Minnow asterism. IQ Aurigae is at center left, looking very blue.

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The very blue star in rwittch's and Tragoolchitr Jittasaiyapan's images is IQ Aur, and its B-V index is -0.17. The star to the right (west) of IQ Aur, 17 Aur, has a B-V index of -0.04. IQ Aur really and truly is much bluer than its "neighbor". Our eyes can't tell the difference, but a good photograph might.

(And yeah... the color balance in rwittich's image, particularly when it comes to the color of the stars, is not the same as the color balance in Tragoolchitr Jittasaiyapan's image. That's the way it is in the world of astrophotography.)


I'm sure you know that stars don't look green. They just don't. And no stars look green or even cyan in either Tragoolchitr Jittasaiyapan's or in rwittch's images.

Which brings us to today's APOD:

As I said, I find the colors of the stars at lower left in David Cortner's image remarkably cyan-looking or even greenish. In my attachment I have identified some of the stars and added their B-V indices. There is no (good) reason for these stars to look greenish.

But there may be one reason after all. The meteors themselves are greenish. Perhaps the person processing the image decided to somewhat enhance the green color of the meteors (in fact, they look yellow-green in the APOD). As a consequence, the stars may have acquired a greenish tint, too. But the Pleiades look blue, no matter what. Good old Pleiades!


Anyway, if we talk about the color of stars, we surely should look at the color of some meteors, too!

Geminids over LAMOST telescope. Image from Royal Museums Greenwich's homepage. Credit: I don't know.

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As you can see, the brightest meteor explodes in brilliant bluish-cyan light. But many of the fainter meteors do look greenish, just like they do in today's APOD. Those that are "medium bright" look faintly pink. (Dilute Barbie, perhaps?).

So Chris, what elements or chemical components cause the green, pink and blue-white colors in meteors?

Ann

P.S. This is my post number 13031. I like the number.

[Chris Peterson]

So Chris, what elements or chemical components cause the green, pink and blue-white colors in meteors?

It's nearly impossible to say. Because our eyes (and cameras) are not spectrometers. Sodium produces yellow. But oxygen produces red and green. So when you see yellow, are you seeing sodium, or are you seeing oxygen? (Red + green = yellow.) There are probably a few common color patterns that we can associate with components in meteors, but usually, disentangling them visually (especially when we throw in the colors our own atmosphere generates) is problematic.

There was a nice image on Spaceweather a few days ago of an intense orange aurora. But there are no elements that radiate at that wavelength in auroras. It was produced by a mixture of red and green emissions from oxygen.

[DavidCortner]

First, it's a pleasure to be back. As to Ann's examination of anomalous star colors, I put their green/cyan cast down to casual (which is to say imperfect) color correction of a strong cast and gradient in the original image. I can barely see the Milky Way from here on a good night, so there's a pretty aggressive histogram stretch going on to bring the molecular cloud out from the light of nearby small towns and more distant, 50 miles or so, Charlotte. Applying local color correction takes care of the worst sins of light pollution, but the astrophysically minded might imagine that stars caught up in that correction have been shoved into heretofore unexplored and fantastic regions of the H-R diagram. Meticulous star masking could have controlled that. Cheers.

[Chris Peterson]

But me being me, I noticed some very greenish-looking stars in the lower left of the picture. Before I talk about them, let's look at an attempt to show the true optical colors of the 25 brightest stars in the sky (26 if we include the Sun, which we should):

Since the last couple of years, when I've been quite rigorous with color calibrating my own astroimages, I've been struck by the visually bimodal color structure of stars. To the eye (well, my eyes anyway) stars fall strongly into two colors, blue and orange (I'm not considering the white ones, which sort of disappear in the mix). If we look closely we can see some variation in the blue and variation in the orange, but there's really no sense of a continuum. Well processed star fields simply look like they contain two stellar populations, blue and orange.

[Ann]

But me being me, I noticed some very greenish-looking stars in the lower left of the picture. Before I talk about them, let's look at an attempt to show the true optical colors of the 25 brightest stars in the sky (26 if we include the Sun, which we should):

Since the last couple of years, when I've been quite rigorous with color calibrating my own astroimages, I've been struck by the visually bimodal color structure of stars. To the eye (well, my eyes anyway) stars fall strongly into two colors, blue and orange (I'm not considering the white ones, which sort of disappear in the mix). If we look closely we can see some variation in the blue and variation in the orange, but there's really no sense of a continuum. Well processed star fields simply look like they contain two stellar populations, blue and orange.

And there is a reason for why most bright stars are either blue or orange. This reason is that stars that are bright enough to stand out are typically either blue or orange. Consider clusters M44 and M41:

M44. Credit: Fried Lauterbach

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M41. Credit: Giuseppe Donatiello

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In M44, all the bright stars are typically either spectral class G9III or about A5. The bright stars are all typically some 90-100 times brighter than the Sun in visual (yellow-green) light. In M41, the bright stars are typically spectral class K0-K3III or very early A or late B. They are typically some ~ 300 timers the luminosity of the the Sun, but with a relatively large variation. The brightest red giant of M41 is some 800 solar luminosities. So, in short, the stars of M41 are younger and brighter than the stars of M44 - some 200 million years old versus some 600 million years old - and the stars of M41 are more brightly colored. But in any case, the stars are spectral class A or B and therefore blue, or spectral class late G or K, and therefore orange.

Even younger clusters are even more colorful. The young Double Cluster of Perseus, only some 12 million years old, is full of early B-type giants and supergiants, which are intrinsically very blue. But there are also several red supergiants, which are deeply orange in color. These stars are thousands of times brighter than the Sun.

The two clusters of the Double Cluster, NGC 869 and NGC 884, are so far away that their blue stars are significantly reddened by dust, but a photographer can choose to bring out their intrinsic unreddened color. Credit: Chris Schur.

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A photographer can also choose to bring out the dramatic color and brightness of the red supergiants of NGC 884. Credit: Prabhuskutti

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There are clusters in which all the bright stars are blue, such as the Pleiades. And there are clusters where we find no blue stars at all, but seemingly only orange ones, such as Trumpler 5. (There may be some blu-ish stars in Trumpler 5, whose colors are completely hidden by dust.)

The Pleiades. Credit: Mohamed Usama/IAU OAE

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Trumpler 5 and red giant HD 26059. Credit: zirl.

And there is M11, and there is NGC 3532:

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NGC 3532. Credit: ESO/G. Beccari

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The list goes on and on. These clusters are dominated bly blue and orange stars, because blue (A-, B- and in very rare cases O-type) stars and orange (K- and M-type) giant stars are the stars that are bright enough to stand out.

Stars like the Sun just fade into the night. Unless you have it at a cosy distance, as we do.

The solar neighborhood within 30 light-years of the Sun. Credit: NASA/JPL-Caltech

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It is hard to believe that the Sun is not a puny, wimpy little thing in a Milky Way dominated by giants. In reality, it is not. Would you believe that some 80-90% of all stars in the Milky Way are fainter than the Sun, because they are either tiny little red or orange dwarf stars or even tinier burnt-out cinders of white dwarf stars?

Ann

[Ann]

I found a nice picture that really shows the green color of many meteors. Can't resist it!

Composite image of Orionids from 2015 to 2022. Credit: Petr Horálek

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Ann

[orin stepanek]

I remember as a boy I was excited about Haily's return; now I don't remember seeing it!

[johnnydeep]

All this color talk sure is pretty, but is there a particular reason why there are two showers caused by Halley (at about 5 months apart)? Are there any other comets whose remnant dust we can see twice a year? My flawed intuition thinks that should be rare.