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Colorful Stars Galore Inside Globular Star Cluster Omega Centauri.
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The main factor responsible for color of
stars is the surface temperature of Star. The other factors responsible for
color of stars are following:
- Human Vision
- Interstellar Reddening and Extinction
- Metallicity in the star
The surface temperature of a star
The color of a star is basically decided by the surface temperature of the star. Any electromagnetic radiation is generated when an atom comes from excited state to lower energy state. The excitation of atom depends on the temperature; the atom will be excited to higher energy state when the temperature is higher. So the energy and frequency of emitted radiation will depends upon the temperature causing the excitation.
The cooler star with less temperature emits
electromagnetic radiation of low energy which corresponds to larger wavelength
for example red or infrared radiation. Similarly the hotter star with high temperature
emits electromagnetic radiation of high energy which corresponds to smaller
wavelength for example blue, violet or ultra violet (UV) radiations.
The temperature of a star depends on the
size of star. The smaller stars have low temperature due to low gravity and
pressure. The massive stars have larger gravity and pressure, the burning rate
of fuel is also larger which corresponds to higher temperature. Now we can
infer that the smaller stars will be reddish and bigger stars will be bluish in
color.
Classes of Stars
Color
|
Class
|
solar masses
|
solar diameters
|
Temperature
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bluest
|
O
|
20 - 100
|
12 - 25
|
40,000
|
bluish
|
B
|
4 - 20
|
4 - 12
|
18,000
|
blue-white
|
A
|
2 - 4
|
1.5 - 4
|
10,000
|
white
|
F
|
1.05 - 2
|
1.1 - 1.5
|
7,000
|
yellow-white
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G
|
0.8 - 1.05
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0.85 - 1.1
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5,500
|
orange
|
K
|
0.5 - 0.8
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0.6 - 0.85
|
4,000
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red
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M
|
0.08 - 0.5
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0.1 - 0.6
|
3,000
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You will be surprised to know that the star
which appears blue or red in color is not only emitting blue or red radiation
but also radiation of other colors. The stars do not emit single wavelength
radiation; the emission has range of wavelength. The intensity of one
wavelength is highest and intensity is lower for higher as well as lower
wavelength. The wavelength of maximum intensity corresponds to the surface
temperature of star. The radiation from star is similar to the blackbody
radiation. So this peak of intensity shifts towards higher wavelength as the
temperature decreases and peak shifts towards lower wavelength as the
temperature decreases.
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Some people have questioned, “Why do the
stars never appear green?” Since the stars emit radiation in a range of
wavelength, so the appearance of star is the average of all the radiations. All
the colors add together to give white color, so the star which have maximum
intensity peak at green color will appear white due to equal contribution from
red and blue region.
Human Vision
Human eyes are extremely sensitive
detectors for visible radiation. Only few photons are required to trigger the
stimulation in the eyes. Then why do we not see the color of stars readily? The
reason for are following:
- Cones and Rods: Our eyes are made up of two types of photoreceptors rods and cones. The rods are present in the peripheral region of retina and cones are present in exactly center of retina in front of eye lens. The rods are more abundant (120 million) and sensitive than cones (7 million). The rods cannot see colors and they only sense grey scale. They are nearly unresponsive to red colors. The cones are of three types red, green and blue. Since the cones are less sensitive they require more photon of correct frequency to trigger them. That’s the main reason we can’t see colors in the night.
- Stars as point sources: The
stars are present at very huge distance from us, so they appear point source.
The human eyes are insensitive to color of point sources. This effect is called
small field tritanopia. This effect can be easily understood from the
photographs of stars in colored film. The color is hard to distinguish when the
stars are focused but if the camera is defocused so that the stars' light is
spread into discs their colors become more apparent as is seen in the photo
below by David Malin of the constellation Orion. Here the untracked camera has
been increasingly defocused over time so that instead of narrow star trails the
starlight is spread out.
Our eyes can detect color of extended objects not point object. Credit: Anglo-Australian Observatory - Defocused star trails showing the colour of prominent stars in Orion. Compare the colours of the M-class Betelgeuse with the B-class Rigel. Notice the distinctive pinkish colour of the emission nebula M42.
- Pupil size: In the nights, the intensity of light is less so our pupils dilate to allow the more photons come into the eyes. This resulting large aperture of eye lens degrades the quality of image which is called chromatic aberration. Due to this we see colored haloes around the objects.
- Dark adaptation – the Purkinje effect: Our photoreceptors become more sensitive in faint light over time. Cones adapt in about 7 minutes whilst rods take about half an hour to reach maximum sensitivity. Cones still remain far less sensitive than rods but as rods are much more sensitive to blue light than red light we perceive dim light as bluer than it actually is. This effect is called the Purkinje effect. One implication for visual astronomy is the tendency to underestimate the brightness of red stars.
Interstellar Reddening and Extinction:
The interstellar space in not empty, it
contains neutral gas and ionized plasma located in the plane of galaxy. The
cosmic dust contains small grains like silicates, carbon, iron, frozen water
and ammonia ice having size in the range 0.1 to 0.01 micron (μm). Although the
cosmic dust is just 1 % of the mass of interstellar medium but it absorbs and
scatters the radiation coming from stars. This decreases the intensity of the
light coming from the stars. The decrease in intensity is called extinction.
The stars present at the farther distance suffer more extinction than the
closer stars.
The extinction is not same for all
wavelengths because scattering depends on the size of the particles of the
medium. The wavelength which is comparable to the size of particle get
scattered more effectively. If the wavelength of radiation is larger than the
size of particles then the radiation passes the medium without being scattered.
So the smaller wavelength gets scatter more than the larger wavelength. The
blue light get scattered more effectively than red light so the final radiation
have more intensity in red wavelength then blue wavelength. Thus the stars
appear more reddish than they actually are. This is called interstellar reddening.
Metallicity
The Red giants like Betelgeuse are not
actually red instead they are orange in color. There is a group of stars which
appears deep red like19 or TX Piscium. These are the carbon stars because they
have abundance of carbon molecule such as C2, CH and CN in their outer layers
which absorb most of photon in the blue and violet region of spectrum. Thus the
light coming from the stars appears deep red in color. They are collectively
referred to as type C (Carbon) stars.
Conclusion:
The color of a star is
mainly decided by the surface temperature of the star but other factors also
affect color of the star. Human eye is more sensitive to blue light than red in
the night which gives bluish appearance to the stars. The interstellar medium decreases
the intensity of blue light more than the red light which causes the reddening
of light coming from the stars. The presence of carbon molecules gives the deep
red colors to many stars.
If you want to know
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