Why do stars flash red and blue?

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  • Member since
    October, 2007
Why do stars flash red and blue?
Posted by Kodack on Sunday, October 28, 2007 1:14 AM

I keep coming across these dazzling star in the eastern sky.

 

I thought it was the same star each night, and it was, but I've also found other stars that do it if you look for it.

 

They almost seem like strobe lights alternating between red and blue but at such a fast rate you can't spot the colors except when you blink or look quickly away.

 

I am proud to say I've found the stars names thanks to some careful observations on location, and Stellarium.

 

The first star I saw to do it was Betelgeuse, and the most brilliant one doing it at this moment is Sirius.

 

I know the atmosphere makes stars appear to twinkle, but this is almost something different.

 

Please, I want to learn about it.  

Kodack 130mm Astromaster 130EQ 70mm Astromaster 70AZ The gods shine brightly above and weave their dance for no ones pleasure save their own.
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Posted by carmel40 on Sunday, October 28, 2007 6:11 AM

 

 This is called 'scintillation' and you rightly mentioned our atmosphere causing stars to twinkle - both the twinkling effect and the flashing colours are due to our atmosphere.

  • Member since
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Posted by tkerr on Sunday, October 28, 2007 8:38 AM

Usually it is the larger brighter stars such as Sirius that will flash the blue and red color and usually when below 30° above the horizon.. The flickering you see in stars is caused by atmospheric turbulence and is often referred to as scintillation, and, is often a sign of poor "astronomical seeing". The red and blue you see is also caused by the atmosphere and is a phenomenon called "Atmospheric Prismatic Dispersion".. Sirius and Venus are two popular objects where that phenomenon is most often observed..


Whenever light travels through one medium, such as air, water, or different types of glass, it slows down. For each different medium the speed of light is different. When light travels from one medium to another, it bends and changes direction as its speed changes. This is called refraction.

Different (colors) wavelengths of light bend differently, this is the dispersion effect. Longer red wavelengths don't slow down as much and bend less than the others. On the other hand the blue (shorter) wavelengths slow the greatest, and bend the most. .
When we see the light from stars in the night sky, that light is passing through the Earth's atmosphere where it is refracted, and the different wavelengths are dispersed. This is called atmospheric prismatic dispersion
Usually blue on top and red on the bottom.. Lower to the horizon the more noticeable.. Sirius and Venus are always good for observing this effect/phenomenon.

 

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Posted by Red1530 on Sunday, October 28, 2007 1:19 PM
I have seen the other extreme when stars don't twinkle at all.  It is a very cool experience.
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Posted by Stargazer at Large on Sunday, October 28, 2007 5:04 PM

Bright stars can be surprisingly colorful as they rise or set, for reasons described above. As they move toward the zenith, the atmospheric effect is reduced and a star's true color shows more steadily.

Right now, from the northern hemisphere, Vega (in Lyra) is near the zenith in the evening and is distinctly blue to blue-white. 

Capella (in Auriga) is a bright star rising in the east below Comet Holmes. Capella always looks very colorful to me as it rises. East of Capella is Aldebaran (in Taurus), a red giant.

I enjoy looking at these well-known stars and observing their beautiful colors, both their true colors, and the tiny light shows they produce as they rise and set.

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Posted by badfrog42020 on Tuesday, October 30, 2007 12:26 AM

If im not mistakin when the star appears blue it is hot,And when i say hot i mean hotter then the sun,And when they are red/orange they present les heat and is more similar to the sun..Im not completly sure but i think im right.

                                                         Roger B

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Posted by Stargazer at Large on Tuesday, October 30, 2007 8:20 AM

Yes. True star color correlates to temperature with blue being hottest and red being coolest. The sun is yellow-white with a surface temp of 5000 to 6000 degrees Kelvin or roughly 8500 to 10,000 Farenheit. (Technically stars are classified according to spectral type, which is a little more complex than just temperature.)

The temperature/color of a star changes during the life cycle of the star. But the twinkling color changes we see in the night sky are due to an amosphereic effect here on Earth.

The topic is an interesting one and there are many good sites to look at for more detail.

Here is an interesting summary from a professor at Ohio State.

And an article from Sky and Telescope.

 

  • Member since
    October, 2007
Posted by Stargazer at Large on Tuesday, October 30, 2007 8:40 AM

Here is another excellent resource about stars:

Nasa Cosmicopia 

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    October, 2007
Posted by Kodack on Tuesday, October 30, 2007 3:18 PM
 Stargazer at Large wrote:

Yes. True star color correlates to temperature with blue being hottest and red being coolest. The sun is yellow-white with a surface temp of 5000 to 6000 degrees Kelvin or roughly 8500 to 10,000 Farenheit. (Technically stars are classified according to spectral type, which is a little more complex than just temperature.)

The temperature/color of a star changes during the life cycle of the star. But the twinkling color changes we see in the night sky are due to an amosphereic effect here on Earth.

The topic is an interesting one and there are many good sites to look at for more detail.

Here is an interesting summary from a professor at Ohio State.

And an article from Sky and Telescope.

 

 

 

Something I know a little bit about actually. :) One of the factors to consider is what a star's fuel is. We were all taught that stars fuse 2 hydrogen atoms into a single helium like our sun. But what happens when all the hydrogen is gone? The sun (depending on mass and this is a gross over simplification) will begin to fuse the helium atoms together similar to how the hydrogen made the helium.

 

Depending on the mass of the star this cycle continues creating heavier elements until the balance of mass and energy output is disrupted when it runs out of fusionable fuel and the star can't maintain it's stability. You either get a bang or a whimper.

 

Anyway, much the same way that different elements on earth burn with different colors (copper is green etc), it affects the spectral signiture of the star. One of the ways we can tell what the star's internal makeup is.

 

As for the color scinitlations I think I have those figured out as well thanks to your help. If the air refracts the light, then the air acts like a lense. And Lenses introduce false color through chromatic aberations. So I'm betting the false colors are a result of atmospheric lensing.

 

 

Kodack 130mm Astromaster 130EQ 70mm Astromaster 70AZ The gods shine brightly above and weave their dance for no ones pleasure save their own.
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Posted by tkerr on Thursday, November 01, 2007 11:04 AM
 Kodack wrote:

As for the color scinitlations I think I have those figured out as well thanks to your help. If the air refracts the light, then the air acts like a lense. And Lenses introduce false color through chromatic aberations. So I'm betting the false colors are a result of atmospheric lensing. 

 

 

My understanding of atmospheric lensing is what causes us to see objects at a point different than where they actually are. Similar to a mirage, when light rays are bent to produce a displaced image of distant objects or the sky. You can often see this with the rising or setting sun. 

 

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Posted by Kodack on Thursday, November 01, 2007 5:19 PM

tkerr as always your posts are helpful and informational.

 

 

Speaking of lensing do you know if it's possible to view gravitational lensing with an amateur scope? 

Kodack 130mm Astromaster 130EQ 70mm Astromaster 70AZ The gods shine brightly above and weave their dance for no ones pleasure save their own.
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Posted by Anonymous on Friday, November 02, 2007 2:21 PM

I'd like to ask a new question here, was at the back of my mind always.

Why is it that the stars twinkle but the planets don't? What difference does it make, when both the stars and the planets are outside our atmosphere?

My guess is that maybe the star light's intensity is lesser compared to planet light intensity as star light come from far far away. But is it so? Or some other reason?

Thanks.

Kap. Smile [:)]

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Posted by Kodack on Friday, November 02, 2007 3:28 PM

I'm speaking out of pocket, but I assume it is a combination of light intensity as you suggested, and angular size. Smaller, brighter objects like sirius probably elicit a glow when viewed through our atmosphere as some of the light is scattered.

 

The planets have a larger angular size and are therefor less prone to the scattering of our atmosphere.

 

In a nutshell all objects "twinkle" because it's not the object that is twinkling, it's our atmosphere. This of course excludes variable stars, binary stars etc, which will vary in intensity.  

 

the difference is like looking at a laser beam projected from 5 miles away, VS a spotlight projected from a few hundred feet.  

Kodack 130mm Astromaster 130EQ 70mm Astromaster 70AZ The gods shine brightly above and weave their dance for no ones pleasure save their own.
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    May, 2005
Posted by Anonymous on Tuesday, November 13, 2007 3:23 AM
I think the reason stars twinkle and planets do not has something to do with planets reflecting light vs stars being a direct light source.
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Posted by tkerr on Tuesday, November 13, 2007 6:21 AM

 jvhecker wrote:
I think the reason stars twinkle and planets do not has something to do with planets reflecting light vs stars being a direct light source.

I believe that's correct.. However, if the atmosphere is especially turbulant and when the planet is closer to the horizong you can see a little scintilation out of them every now and then..  

 

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