Multiple soles –

The universe is a huge set of suns, and ours is just one more.

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But there is one more question: if the Earth moves and the stars are placed at arbitrary distances, why do the most distant ones not show a parallax with respect to the closest ones?

An obvious explanation was given to this question, which was immediately admitted: because the nearest stars were so far away that their parallaxes were too small to be detected with the instruments of 1800. Even Copernicus gave this same answer when they criticized the absence of parallax in against his heliocentric theory.

If the stars moved at the speed of the planets it would be possible to estimate the distance at which the stars move.

The fastest self-movement is Barnard’s Star, which was discovered by the American astronomer Edward Emerson Barnard in 1916. Its movement is 10.3 seconds of arc per year, insignificant, since the circumference is divided into 360 degrees , each degree in 60 minutes and each minute in 60 seconds. A second of arc is 1 / 1,296,000 of the sky. The Moon has a diameter of 31 arc minutes, so a second would also be 1/1860 of the diameter of the Moon. Jupiter is a simple point of light in the sky, but its diameter, at first glance, is 30-50 seconds of arc, depending on how far it is from Earth.

Comparación de tamaños

Therefore, if we say that the Barnard star moves 10.3 seconds of arc a year, it is like saying that in a year it travels more or less 1/180 of the diameter of the Moon or ¼ of the diameter of Jupiter. In spite of everything, this own movement is so fast if we compare it with the others, that this star is sometimes called “Barnard’s fugitive star”. The normal thing is that the own movements are of 1 second of arc per year, and sometimes less.

Comparación de tamaños

Assuming that Barnard’s star moved perpendicular to our line of sight with the same velocity as the Earth around the sun, that is, at 28.8 kilometers per second, in a year he would have traveled 940,000,000 kilometers. For this distance to correspond to 10.3 seconds of arc, the star would have to be sixteen billion kilometers from Earth. Then, it would show a parallax of only 1 second of arc. Assuming that it moved at a speed greater than that of the Earth, as it actually happens, we could say that its distance is still greater and its parallax, therefore, still smaller.

To observe in the sky the smallest ellipse, of 1 second of arc or less, of a star was very difficult for the astronomers. It is that if we saw a coin of 20 cents of euro to six kilometers of distance. An own movement of 1 second of arc a year is not difficult to observe, but it is that they accumulate from year to year. After a century, a star that moves 1 second of arc per year will have moved almost 2 minutes of arc in the sky, and this is perfectly visible with a telescope. On the other hand, parallactic movements go from one side to the other without accumulating with the passage of time.

The stars are tens of billions of kilometers from us and, still, we see them. At that distance, an object with a brightness as huge as the Sun would appear as a small dot of light.

And, on the contrary, any star that is the same distance as the Sun, would have a much greater brightness.

In other words, the Sun is a star that only differs from the others in that we see it from a distance of millions of kilometers instead of billions of kilometers, as it happens with other stars.

The universe is a huge set of suns, and ours is just one more.

If the Sirius star were as bright as the Sun and if the light that reaches us is smaller, it is only due to the enormous distance that separates us from it. Sirius has a magnitude of -1.6 and the Sun of -26.9, this means that the Sun is 25.3 times brighter than Sirius. Each magnitude represents an increase in brightness of 2,512, so the brightness of the Sun is 13,200,000,000 times that of Sirius.

The brightness of a luminous object varies in inverse proportion to the square of the distance, which means that if we move the object away from the previous one, the brightness will decrease by ¼; if we move it away five times the previous one, its brightness will be reduced by 1/25.

If Sirius shines 1 / 13,200,000,000 times less than the Sun, it is because it is 115,000 times farther away, since 115,000 X 115,000 equals 13,200,000,000. If the Sun is 150,000,000 kilometers away from us, Sirius must be about 16 billion kilometers away.

This billions of kilometers does not tell us anything. Let’s use other units . Recall that one light-hour is 1,080,000,000 kilometers. At 300,000 kilometers per second, the light travels in a year 9.440.000.000.000 of kilometers, but we can round in ten trillions of kilometers.

So, Sirius is 16 billion kilometers or two light-years from Earth. And since Sirius is one of the closest stars, we will have to measure all stellar distances in light years. A ray of light takes to get from the Sun to Earth eight minutes, and between the Sun and Pluto, five and a half hours.