APOD: Star Colors in Orion (2011 Feb 11)

APOD Robot · Post by **APOD Robot** » Fri Feb 11, 2011 5:06 am

Star Colors in Orion

Explanation: What determines a star's color? Its temperature. Red stars are cool, with surface temperatures of around 3,000 kelvins (K), while blue stars are hotter and can have temperatures over 30,000 K. Our own lovely "yellow" Sun's temperature is a comforting 6,000 K. Differences in star colors are particularly easy to see in this intriguing composite view of the constellation Orion, made while experimenting with a star trail step-focus technique. In it, a series of 35 consecutive exposures were combined to produce trails of stars moving left to right through the frame, while changing focus in steps. Beginning and ending with the camera out of focus produced a sharply focused exposure near the middle of the series and blurs the star trails into a bow tie shape. For the brighter stars, blurring produces more saturated colors in the images. At the upper left, Orion's cool red supergiant Betelgeuse stands out from the other, hotter, bluish stars composing the body of the constellation. Not a star at all, the Orion Nebula contributes a pinkish tint below center. Also remarkable in the field, the fainter step focus trail of cool, deep red carbon star W Orionis is near the center right edge, its red hue enhanced by a carbon-rich composition.

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Chris Peterson · Post by **Chris Peterson** » Fri Feb 11, 2011 6:08 am

This trick works with the human eye, too (but for different reasons). If you have difficulty seeing star colors, try blurring your vision just a hair. It will spread the light out over more receptors on your retina, and can make the color much more apparent. Don't expect the color to be as saturated as in an image, though.

nstahl · Post by **nstahl** » Fri Feb 11, 2011 9:43 am

For those wanting more variety, this certainly hits the spot, (so to speak). And it's a technique an amateur with a flexible camera, firm tripod and steady hand could try.

Good choice.

NickHall · Post by **NickHall** » Fri Feb 11, 2011 11:29 am

Looks like some of the stars have asymmetric colours with front / back focus.
Red on the left and green on the right.
Can anyone explain that ?

neufer · Post by **neufer** » Fri Feb 11, 2011 1:31 pm

Our own "yellow" Sun's step-focus bow tie.
Note the diffraction spikes coming off Betelgeuse.

Our own "yellow" Sun's step-focus bow tie.

http://en.wikipedia.org/wiki/Chevrolet wrote:
<<Chevrolet was founded by racecar driver Louis Chevrolet and ousted GM founder William C. Durant on November 8, 1911. As head of Buick Motor Company, prior to founding GM, Durant had hired Chevrolet to drive Buicks in promotional races. Chevrolet first used its "Bowtie emblem" logo in 1913. It is said to have been designed from wallpaper Durant once saw in a French hotel. More recent research by historian Ken Kaufmann presents a compelling case that the logo is based upon a logo for "Coalettes". Others claim that the design was a stylized Swiss cross, in honor of the homeland of Chevrolet's parents. Chevrolet was profitable enough to allow Durant to buy a majority of shares in GM five years later.>>

NoelC · Post by **NoelC** » Fri Feb 11, 2011 2:54 pm

NickHall wrote:Looks like some of the stars have asymmetric colours with front / back focus.
Red on the left and green on the right.
Can anyone explain that ?

Lens imperfection. Not all the colors of visible light are focused on the same plane (implying not at the same time/position in the APOD image in question). Though lens designers try to achieve the best compromises in the design to make visually pleasing images, economics and the laws of physics limit what they can do.

In my own observations, sometimes a lens that's good for daytime photography won't be particularly good for astrophotography, and vice versa.

-Noel

djbeirne · Post by **djbeirne** » Fri Feb 11, 2011 2:55 pm

I noticed that around the outer edge of the disk the color of each star appears to change - for most of the blue stars the outer edge appears almost green. What causes this to take place?

Motty · Post by **Motty** » Fri Feb 11, 2011 3:06 pm

Not a star at all, the Orion Nebula contributes a pinkish tint below center.

I didn't see this effect. Is there any way someone can put this image up with the relevant phenomenon circled?

Chris Peterson · Post by **Chris Peterson** » Fri Feb 11, 2011 4:16 pm

Motty wrote:
Not a star at all, the Orion Nebula contributes a pinkish tint below center.
I didn't see this effect. Is there any way someone can put this image up with the relevant phenomenon circled?

: OrionColorsjpg.jpg (42.79 KiB) Viewed 3044 times

owlice · Post by **owlice** » Fri Feb 11, 2011 5:04 pm

May be easier to see here:

: Click to view larger image

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Ann · Post by **Ann** » Fri Feb 11, 2011 8:28 pm

I get a huge kick out of seeing star colors. My favorite star color hands down is blue. Seeing blue color in stars isn't easy, partly because I don't think human color vision is particularly sensitive to blue color when the light is faint, as starlight certainly is. We are much better at detecting yellow and green color than blue or red when the light is faint. Because of that, we are never going to see the faint red color of emission nebulae when we look at these nebulae through a telescope. But because our eyes are sensitive to green, we may spot a green tinge in the otherwise apparently grey Orion Nebula (which looks grey to our eyes, that is).

But blue color in stars is also hard to spot. Nevertheless, it is really there in some stars. One of my "bluest" experiences is observing Lambda Orionis. There are many fainter, whitish stars next to O-type Lambda Orionis, and the color contrast between the white "neighbouring stars" and the blue O star was quite stunning.

Lambda Orionis is the brightest, bluest star here.

As for W Orionis, my experience is that it isn't as red as it looks in today's APOD. It was just mildly orangish to me when I observed it. The reddest star I've ever seen is V Aquilae.

: Click to view full size image

V Aquilae. It looked almost redder than this when I saw it through a telescope.

If you want some color contrast in the sky, don't look at much talked about but overrated Albireo. Try 30 and 31 Cygni instead. 30 Cygni is a whitish A-type star, and very nearby 31 Cygni consists of a quite orangish primary and a quite blue secondary. You can see all these three stars through a 14 inch telescope.

: Click to view full size image

30 and 31 Cygni, or Omicron Cygni. If you are interested in star colors at all, don't miss this colorful trio!

Ann

Motty · Post by **Motty** » Fri Feb 11, 2011 8:32 pm

To Chris Peterson:

Thank you. That was what I needed.

NoelC · Post by **NoelC** » Fri Feb 11, 2011 10:19 pm

Ann wrote:I get a huge kick out of seeing star colors.

Me too (and by that I mean VISUAL star colors

- none of those narrowband pink ones

).

I agree - in a telescope with a reasonably large aperture, since stars are point light sources, you can really see star colors quite brightly. I found Rigel to be a beautiful blue and Betelgeuse to be a quite striking orange. My collaborator Greg Parker and I have been capturing a set of new "single bright star" images lately.

In developing software to help astronomers work with their images I've found that even in astroimages where there looks to be very little star color, some color can almost always be found right around the edges of the stars, where they transition from overexposed to the blackness of space.

-Noel

orin stepanek · Post by **orin stepanek** » Fri Feb 11, 2011 10:35 pm

What do you know?---So the sun is kinda pink!!

http://casa.colorado.edu/~ajsh/colour/Tspectrum.html
A couple of days ago I read that the sun was a green star; though I forgot where I read that.

I also heard that it was a white star and a yellow star; So which is correct?

NoelC · Post by **NoelC** » Fri Feb 11, 2011 11:48 pm

We kind of define "white" as sunlight.

Astronomers consider it a "yellow" star, however. Astronomers are like that, always wanting to change the rules. I'm still not over Pluto, frankly.

-Noel

orin stepanek · Post by **orin stepanek** » Sat Feb 12, 2011 12:03 am

NoelC wrote:We kind of define "white" as sunlight.

Astronomers consider it a "yellow" star, however. Astronomers are like that, always wanting to change the rules. I'm still not over Pluto, frankly.

-Noel

To me Pluto will always be a Planet. And in Illinois on March 13th!

http://news.nationalgeographic.com/news ... inois.html

nstahl · Post by **nstahl** » Sat Feb 12, 2011 1:26 am

Ann wrote:I get a huge kick out of seeing star colors. My favorite star color hands down is blue.Seeing blue color in stars isn't easy, partly because I don't think human color vision is particularly sensitive to blue color when the light is faint

I'm guessing that's because we've had a real shortage of blue predators of humans. But on the other hand if there'd been blue predators one might have gotten one of your ancestors - or mine. So I'm content with our blue problem. I won't get too blue over blue.

Chris Peterson · Post by **Chris Peterson** » Sat Feb 12, 2011 2:21 am

nstahl wrote:
Ann wrote:I get a huge kick out of seeing star colors. My favorite star color hands down is blue.Seeing blue color in stars isn't easy, partly because I don't think human color vision is particularly sensitive to blue color when the light is faint
I'm guessing that's because we've had a real shortage of blue predators of humans. But on the other hand if there'd been blue predators one might have gotten one of your ancestors - or mine. So I'm content with our blue problem. I won't get too blue over blue.

Actually, our responsivity to blue light isn't dramatically lower than it is to green-yellow (red is much lower). What is different with blue is that the density of blue cones is much less than that of green or red cones. How well we see color is determined in part by how many receptors get triggered. When you're looking at a star, that's not many, which is why blurring your eyes can help see colors- especially blue.

I don't know how many blue predators might have existed in the past, but at least we can probably assume there weren't many small ones!

Ann · Post by **Ann** » Sat Feb 12, 2011 10:28 pm

Have to add this tidbit about the strangest star color I've ever seen - it was almost pink!

No, not really. But it certainly wasn't what I had expected. The star in question was Mu Cephei, the famous Garnet Star, and although I don't claim to know what color garnet "really" is, I expected something really red. Well, that's not what I got at all! The color of Mu Cephei was like copper! Very pale copper!

The stars in this Christmas wreath could almost have been Mu Cephei!

If I were to observe Mu Cephei again, would it look coppery to me again? I strongly doubt it. I suspect it had something to do with the atmosphere that night or with the telescope and the eyepiece I was using. And indeed, it may have had something to do with me, that very night. But the sight of that coppery light against the blackness of the sky was strange and memorable.

Ann

rstevenson · Post by **rstevenson** » Sat Feb 12, 2011 11:27 pm

... I don't claim to know what color garnet "really" is ...

I'm not surprised Ann. Garnet seems to be your worst nightmare as far as colour consistency goes. From The Modern Jeweller web site...

You will find a vibrant orange garnet ring, lime green garnet ear rings, pink garnet in affordable jewelry, and even shades of violet.

: Different colours of garnet gem stones.; Garnet.jpg (28.92 KiB) Viewed 2831 times

Rob

Beyond · Post by **Beyond** » Mon Feb 14, 2011 2:57 am

orin wrote: A couple of days ago i read that the sun was a green star;

Orin, maybe whomever wrote the article was referring to Sol's age and not the color. A young star could be referred to as green(unripe) as compared to older(ripe)and rotten(SuperNova)where it falls apart. ........ Don't buy that? I don't blame you if you don't, it does sound a little 'fruity' doesn't it

NoelC · Post by **NoelC** » Mon Feb 14, 2011 7:10 am

Ann wrote:Have to add this tidbit about the strangest star color I've ever seen - it was almost pink!

Ann, did you imagine at the time that there was so much other, beautiful stuff nearby?

-Noel

Fisher woman · Post by **Fisher woman** » Sat Mar 05, 2011 5:06 am

Tonight, 11 PM, from the center of the American continent, I see a green-tint star south of Orion. I have not noticed before that it was green. It would take a powerful lens to take a picture.
Has it's temp changed?

Chris Peterson · Post by **Chris Peterson** » Sat Mar 05, 2011 3:40 pm

Fisher woman wrote:Tonight, 11 PM, from the center of the American continent, I see a green-tint star south of Orion. I have not noticed before that it was green. It would take a powerful lens to take a picture.
Has it's temp changed?

There are no green stars- the physics of thermal emission sources and the physiology of the eye preclude them.

Perhaps you were seeing Sirius, which is the bright star south of Orion. For American observers, it is often low in the sky, which means that it can be quite distorted by the atmosphere. Sometimes it twinkles different colors (due to dispersion), including green.

NoelC · Post by **NoelC** » Sat Mar 05, 2011 11:53 pm

Chris Peterson wrote:There are no green stars- the physics of thermal emission sources and the physiology of the eye preclude them.

The physiology of the eye in no way precludes seeing a green star, if such a thing could exist (which of course it can't).

I don't mean to be argumentative, but you yourself said later in the post that Sirius might twinkle green.

Do you think it should be impossible to see a green colored point light source the brightness of a star? One can clearly see reddish and bluish stars, even with the unaided eye. Did I read what you wrote wrongly?

-Noel

Starship Asterisk*

APOD: Star Colors in Orion (2011 Feb 11)

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