Why Vega is the most important star in the sky after the Sun | Cosmic Void

A few months ago we told you about the stars that can be seen in the so-called summer triangle. Among them is Vega, known as “the most important star in the sky after the Sun.” Because? What’s so special about Vega? What makes it more interesting is that it destroys everything that has been built on it over the centuries.

Shortly after sunset these days, the star Vega can be seen in the northwest in the constellation Lyra. This constellation is one of 48 that Ptolemy described almost 2,000 years ago. In other cultures it was given the name eagle or vulture (among the Arabs), Australian pheasant (for residents of those regions) or King Arthur’s harp (in Wales). In any case, this area of ​​the sky has been observed quite often throughout history – and certainly in prehistoric times as well – because Vega is the sixth brightest star in the sky if the Sun is included in the list. Moreover, Vega was North Star about 14,000 years ago, and it will be again in about 12,000, tell me then! In fact, of the stars capable of occupying a polar position, Vega is the brightest: we now live with a not very bright replacement.

Finishing fifth in the brightness rankings isn’t the only thing that makes the Vega special; It was the first distant star to be photographed back in 1850. Years later, in 1872, it was also the first star whose spectrum was photographed. But beyond historical reasons, Vega is key to astrophysics. There are three main reasons, although two of them are completely opposite to the first.

First, and the main reason to claim that it is “the most important star in the sky after the Sun”: Vega has been the standard for measuring the brightness of other stars and galaxies for more than a century. Establishing something as a standard unit of measurement is important in science and in life in general. But it’s also quite arbitrary. The crucial point about a standard is that it is constant, easy to define, fairly accurate and adaptable to what you want to measure, and that it is accepted by a wide range of users, which usually takes time and is inconsistent with a standard. traditions and history.

In this sense, defining the unit of measurement as the length of a thumb or three grains of dry barley placed one after the other does not seem to be a good idea for a place where there is no barley or where barley grows. , or where there are people with very large hands. It is also not suitable for measuring pipes or nuts, which has nothing to do with seeds. But if people have been using this unit of measurement for centuries, it is very difficult to change your mind.

Frankly, a definition of a metro based on one ten-millionth of the shortest distance between the North Pole and the Equator, passing through Paris, is not entirely acceptable, at least not a priori. The weight of a liter of water seems to be more repeatable. But the fact that the French defined the kilogram this way can also be an issue depending on who it’s intended for (and perhaps that’s why the English like their pound better). In any case, the truth is that what is known as the International System is a structure of intelligent units of measurement based on multiples of ten that are very easy to use, more so than systems based on the numbers 12 or 60.

Let’s return to Vega. It all begins with a system introduced by the Greek Hipparchus in the 2nd century BC, in which he classified about a thousand stars visible to the naked eye (eighteen centuries before the advent of the telescope) into six classes of brightness or magnitude. He called the brightest magnitude 1, the faintest 6. Magnitude itself is a unit of measurement, but quite rare for what we are used to, because if magnitude increases, brightness decreases, and brightness is what easier to understand. Thus, the quantities are the opposite of the most intuitive physical quantities.

Centuries after Hipparchus’s work, using telescopes – and with the goal of leaving behind this dense and subjective system – it was demonstrated that a star of the first magnitude was about 100 times brighter than a star of the sixth magnitude. I see, I can go on and say that a first magnitude star is about 2.5 times brighter than a 2nd magnitude star, 2.5 times and 2.5 times brighter than a 3rd magnitude star, B 2.5˟2, 5˟2.5 times brighter than 4th magnitude stars, and missing one magnitude it would be 2.5⁵ (2.5 times times itself 5 times; that is, 97.7: almost 100) times brighter than 6th magnitude stars. logarithmic scale, which we also use in decibels of sound.

The point is that once we move from an eye-meter definition like Hipparchus’s to something more mathematical, with logarithms, a star (or other star) can be brighter than 1 magnitude. And this is where Vega comes to the rescue. A little over a century ago, it was determined that Vega would have a magnitude of 0 (a class of 0 that Hipparchus did not define; it started at 1). And the brightness of all other stars and bodies that were discovered was measured from it. The Sun, for example, has a magnitude of -26.74. Vega’s brightness is like a meter, a kilogram, or perhaps more like an inch to astrophysicists, or at least it was just two decades ago.

And why did Vega go inch crazy? Both must fall from grace, but tradition and history rule. First of all, Vega is a star of variable brightness. Returning to our analogy, it is as if a meter stick stored in Paris changed its length from time to time. And indeed it does: this measure that has defined the meter for centuries changes size depending on cold and heat, so now we define the meter relative to a more stable reference point, such as the distance traveled by light in a vacuum. a fraction of a second is equal to 1/299792458. Vega varies up to 10% from the brightest point to the brightest, probably due to the effects of rotation and the fact that we are observing one of its poles, that is, the rotation axis is close to our vision, it is not perpendicular to it as (more or less ) axis of rotation of the Moon.

Vega’s physical quirks don’t end there. 20 years ago, it was discovered that Vega is surrounded by a disk of dust. Vega is a young star, about 450 million years old, about ten times younger than our Sun. But it is larger, and among the stars, it means it lives less. In fact, Vega and the Sun are about halfway through their lives, which means Vega will disappear long before the Sun (who knows where humanity will be then, I’m increasingly worried).

Vega’s dusty disk makes it less worthy of being a standard star for measuring brightness than variability. First of all, because it is this disk that dominates the light coming to us from this star in the infrared. The stardust that forms this disk must be composed of particles containing silicon and possibly carbon. The size of these dust particles is several hundred microns, a maximum of a millimeter; If they were larger, they would not be able to survive; they would be carried away by the radiation of the star itself, dissolving the disk.

Vega’s dust disk also has features that are not fully understood. It is very homogeneous and does not seem to have formed planets like the gas giants of our system (Saturn or Jupiter), despite the age of the star. For comparison: the planets of the Sun were formed over a period of several million years after the collapse of the Sun began, even before it began to synthesize hydrogen (in the case of Jupiter), to several tens of millions of years in the case of rocky planets, including the Earth. Vega is many years older than these values ​​and still retains a dust disk, undoubtedly the result of numerous collisions of planetesimals that break up rather than coalesce into planets. This feature is now being analyzed by the James Webb Space Telescope, continuing research carried out by all the infrared telescopes we have built. But we’ll leave that story for another day, because Vega is more interesting as an album even than as a star, although he has been a standard for centuries.

Well, this was my 100th article in Cosmic Void. I hope you enjoyed this astronomical journey, which will soon be five years old in early 2025.

Cosmic Void This is a section in which our knowledge of the Universe is presented qualitatively and quantitatively. Its purpose is to explain the importance of understanding space not only from a scientific point of view, but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the Universe was and remains largely empty, with less than one atom per cubic meter, despite the fact that our environment, paradoxically, contains quintillions of atoms per meter. cubic, which invites us to think about our existence and the presence of life in the Universe. The section has been compiled. Pablo G. Perez Gonzalezresearcher at the Astrobiology Center, and Eva VillaverDeputy Director of the Institute of Astrophysics of the Canary Islands.