Have you ever noticed the different colors of the stars?

Have you ever noticed the different colors of the stars? In the sky we can find thousands of stars shining, although each one does so with different intensity depending on its size, its 'age' or its distance from us. 
Have you ever noticed the different colors of the stars?

But if we look closely or observe them through the telescope we will see that, in addition, the stars can have different colors or shades, from red to blue. So, we find more blue stars or redder stars. This is the case of the bright star Antares, whose name means precisely “the rival of Mars”, since it competes with the intense color of the red planet.
Why the stars have different colors?

The color of the stars basically depends on the surface temperature. So, although it seems a bit contradictory, the blue stars are the hottest; and the red ones, the coldest (or, rather, the least hot). 

If we remember the light spectrum, which almost all of us have studied in the small school, we will easily understand this apparent contradiction. According to the electromagnetic spectrum, ultraviolet light is much more intense than infrared light. 

Thus, the bluish color implies more intense, more energetic radiations and, therefore, corresponds to higher temperatures. In the same way it happens on a daily basis. 

If we look closely at the flame of a lighter, we will see how the area with the highest temperature (that is, the one closest to the lighter) is bluish in color, while the rest of the flame is reddish in color.

Something similar occurs in astronomy, where the colors of the stars vary depending on their temperature and age. Thus, in the sky we find white-bluish stars or orange or reddish stars. 

For example, the blue Bellatrix star has a temperature above 25,000 degrees Kelvin. While reddish stars like Betelgeuse barely reach 2,000 K.

Classification of the stars according to their color
In astronomy, stars are classified into 7 different categories according to their color and size. These categories are represented by letters and are further subdivided into numbers. 

For example, young stars (smaller and hotter) have a bluish color and are classified as O-type stars. On the other hand, older stars (larger and colder) are classified as M-type stars.

Our Sun is a star of medium size and yellowish color. Its surface temperature is about 5000-6000 degrees Kelvin and is considered a star of category G2. As you get older, the sun will increase in size and cool down as it becomes increasingly reddish. But for that there are still several billion years left!

The color of the stars indicates their age
In addition, the color of the stars gives us an idea of ​​their age. Thus the younger stars have a more bluish hue and the older, more reddish stars. This is because the younger a star is, the more energy it generates and the higher the temperature. 

On the contrary, when the stars age, they generate less energy and their temperature decreases to more reddish colors. However, this relationship between their age and their temperature is not universal since it depends on the size of the stars. 

If a star is very large, it will consume its fuel more quickly and will turn reddish in a shorter time. In contrast, smaller stars have a longer "life" and will take longer to abandon their blue color.

Sometimes we see stars very close to each other in very contrasting colors. This is the case of the star  Albireo , in the constellation of the Swan. At first glance, Albireo looks like a simple star. 

But with a telescope or binoculars we will see that it is a double star of very different colors. The brightest star is yellow (Albireo A) and its bluish companion (Albireo B). It is undoubtedly one of the most beautiful and easy to observe doubles.

Sirius and atmospheric turbulence
There are many people who come to our astronomy courses  and have asked us about the peculiar brightness of Sirius. This star is one of the brightest stars in the Northern Hemisphere and easily visible in winter. 

When Sirius is very close to the horizon, it  seems to shine in all colors, as if it were party lights. This phenomenon is not much produced by the star, but by something much closer: our atmosphere. 

The different layers of air at different temperatures of our atmosphere cause the light of the stars not to follow a straight path, but that it is refracted again and again as it passes through our atmosphere. This is what astronomy fans know as atmospheric turbulenceand makes the stars "flicker".

Sirius, star
Surely you've ever noticed that frantic swagger of the stars, that "flickering" or "flickering"constant. In addition, you will have noticed that this flickering turns out to be more intense as we look closer to the horizon. 

This is because the closer to the horizon a star is, the more amount of atmosphere its light has to travel to reach us and, therefore, the more it is affected by atmospheric turbulence. 

Well, in the case of Sirius, being very bright the effect is more striking. So, on less stable nights and when it is close to the horizon, this turbulence makes the star seem not to be still and we see as if it shines with different shades. 

A natural and everyday effect that has nothing to do with the stars and also affects the quality of observations and astrophotographs. In this other article we show you a video in which the classification of the stars is explained very clearly according to their temperature and luminosity.

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