Observing the sky flooded with brilliant stars is probably one of the favorite pleasures on clear nights. Of different luminosity, color or in different groupings, enjoying this free landscape is within everyone’s reach. However, when looking up, there is a detail that is not overlooked: the stars are blinking in the sky constantly. Have you ever wondered what this phenomenon is due to? Are the stars really emitting light intermittently, or is it all just an optical effect?
Scatter flicker
To answer this question there are two things we need to take into account: the composition of our atmosphere and why the stars shine. The first is simple, since it is known that the layer that surrounds our planet is a accumulation of gases and suspended particles, which are found at different temperatures, concentrations and pressures depending on the study area. For the second, let’s stay with the fact that the star pretends to be a large nuclear reactor; so it’s a helium thermonuclear fusion inside it the one that causes the release and irradiation of energy, part of it in the form of light.
Therefore, it seems unlikely that the flicker has its origin in this constant and intrinsic reaction to the star, and in turn, everything indicates that a changing and inhomogeneous atmosphere It could be what causes the flicker. And it is that, indeed, it is so: everything is due to an optical effect produced by the atmosphere itself.
Magnetars, the most powerful neutron stars in the Universe
In this way, the energy and light that is generated in the star constantly reaches the Earth’s atmosphere. Now, when the particles that make up the light, the photons, enter the gas layer, they can collide with the scattered particles, completely altering the address that they carry In addition, changes in the magnitudes of pressure and temperature as they go deeper into it, can also bend their trajectory, deviating from linearity. These two effects mean that the deflected photons do not reach the eyes of the observers, giving rise to small periods where the light disappears. These are the famous flickers in the brightness of the stars.
Do the planets and the Sun blink?
The dispersion effect is more observable, and striking, in those astronomical objects that are at a greater distance. That is, how much further away is the star, the amount of light that reaches the atmosphere is less, since there are more possibilities that energy has been lost along the way, in phenomena of dispersion and curvature of the trajectory when coming into contact with other cosmic materials. In nearby objects, the amount of light that arrives is much greater and, although part of it is scattered in the atmosphere, it does not affect the constancy and this flickering is not appreciated. This is what happens, for example, with the Sun: the closest star to planet Earth.
However, in the starry night sky, the planets are also seen as small luminous points easily confused with the rest of the stars. Do they really blink? The answer is negative. And it is that, although the planets do not really emit light, but rather reflect the light that comes from the sun or from stars a little further away, this continues to come from a point of emission so close (the planets within the Solar System itself) that it does not the consequences of this atmospheric dispersion are appreciated. In fact, this can be a good trick to tell them apart at night: planets don’t blinkwhile the stars do.
The importance of color
As if the flickering phenomenon were not curious, to this is added that, if the observer pays attention, the stars have different colors, ranging from more bluish to others more yellow or reddish. In fact, differentiating the color in which a star shines in the sky can provide a lot of information of interest and characteristic of that star, and even help to identify it from the rest.
The color that the star presents will depend on the temperature at which it is found. In this way, those that shine with a tone the bluer they will be the hotter, with a surface temperature of up to 25,000 degrees Kelvin. On the other side, a reddish color will correspond to less hot starswith a temperature of about 3,000 degrees Kelvin on the surface.
Montevideo Planetarium
Color-temperature diagram for stars
In addition, there may also be a relationship with the age of the star. so the stars the younger would be those that we would see in the sky with a more bluish hue, that is, the hottest according to the previous classification. This is because younger stars generally have more quality gas to burn, so they can generate more energy and reach much higher temperatures. For example, the Sun now has a more yellowish tone, which indicates that it is at the midpoint of its life. As it ages, it will get bigger and bigger and cooler, turning redder in color.
However, there may be cases where this relationship between age and temperature is not the only one, since the size may be involved most times. The explanation is brief: larger stars consume their fuel much faster than smaller ones, so a larger star will use its resources faster and turn red sooner, without needing to be very old.
The Cepheids: the stars that really blink
There is, however, a type of star that really changes the intensity of its brightness periodically: it is about the cepheids. This type of stars is capable of varying its temperature as a result of contractions and expansions of its own radius around an average value. In this way, the contraction of the star would increase the temperature of the central regions of the star, managing to increase the number of nuclear reactions on the surface, and thus the energy and luminosity emitted. After expansion, the star cools, decreasing its luminosity together.
This process would occur in a cyclical way in periods of 1 to 50 days, therefore, contrary to the flickering effect due to dispersion, it is not something observable with a simple glance at the sky: it requires continuous monitoring over a certain period of time.
NASA, ESA and HE Bond
Snapshots taken by Hubble in 2002 of changes in the luminosity of Cepheid V838
Although the discovery of the Cepheids is attributed to John Goodricke, in 1784, it was the studies of the American astronomer Henrietta Swan Leavitt those who gave fame to these stars for their importance to science. This researcher discovered up to 2,400 Cepheids, which allowed her to establish a relationship between their luminosity and the periods with which they change brightness: cThe more brightness, the harder your pulsation. This law, known as Leavitt’s law, made it possible to calculate the distance at which different stars and galaxies are from the planet, as well as to observe that these distances increased and to determine the current expansion of the universe.