Why Vega is the most important star in the sky after the Sun

A few months ago, we told you about the stars visible in the so-called summer triangle. Among them is Vega, often described as “the most important star in the sky after the Sun.” But why? What makes Vega so special? What makes it even more interesting is that it overturns centuries of understanding.

Shortly after sunset, you can see Vega to the northwest in the constellation Lyra, at this time. This constellation is one of the 48 that Ptolemy described almost 2,000 years ago. In other cultures it was called the eagle or vulture (in Arabic), the Malleefowl (in Australia), and King Arthur’s harp (in Wales). In any case, this area of the sky has been widely observed throughout history — and likely, also in prehistory — because Vega is the sixth-brightest star in the sky, if the Sun is included in the list. What’s more, Vega was the North Star about 14,000 years ago, and it will take that role again in approximately 12,000 years. Let’s see what they say then! In fact, of the stars that can occupy the polar star position, Vega is the brightest: we now have a not-very-bright substitute.

Vega’s fifth-place ranking in brightness is not the only reason why it is special. It was the first distant star ever photographed, back in 1850. Years later, in 1872, it also became the first star to have its spectrum photographed. Beyond these historical milestones, Vega holds immense value in astrophysics for three main reasons, two of which contradict the third.

First of all, Vega has been used as the standard for measuring the brightness of other stars and galaxies for over a century — this the main reason why it is considered “the most important star in the sky after the Sun.” Establishing something as a reference for a unit of measurement is essential in science, and in life in general. But it is also quite arbitrary. What’s decisive about a standard is that it is constant, easily definable, sufficiently precise and adapted to what is to be measured, and widely accepted, which usually takes time and involves conflicts with tradition and history.

For this reason, defining a unit of measurement as the length of a thumb, or three grains of dried barley placed one after the other, does not seem like a good idea for a place where there is no barley, or where barley grows taller, or where people have very large hands. Nor does it seem a suitable way for measuring pipes or nuts, which have little to do with seeds. But if people have used that unit of measurement for centuries, it is very difficult to change their minds.

To be honest, a definition like that of the meter, based on one ten-millionth of the shortest distance between the North Pole and the Equator via Paris, is not very practical, at least in theory. The weight of a liter of water, on the other hand, does seem much more reproducible. However, the way the French defined the kilogram in this manner can also pose issues depending on who is using it (perhaps that’s why the British prefer their pound). In any case, what we now know as the International System is a clever framework of measurement units, based on multiples of ten, which are far more manageable than systems based on 12 or 60.

Let’s go back to Vega. It all began with a system introduced by the Greek astronomer Hipparchus in the 2nd century BCE, in which he classified around a thousand stars visible to the naked eye (this was 18 centuries before the telescope was invented) into six classes of brightness, or magnitudes. He called the brightest stars magnitude 1 and the faintest magnitude 6. Magnitude itself is a unit of measurement, but it’s a rather unusual one by our standards, because as the magnitude increases, the brightness decreases, and brightness is something that is easier to understand. So the magnitude scale works in reverse to the way we naturally think about physical quantities.

Centuries after Hipparchus’ work, with the help of telescopes — and in an effort to move away from his crude, subjective system — it was determined that a star of magnitude 1 is about 100 times brighter than a star of magnitude 6. From this relationship, we can infer that a star of magnitude 1 is approximately 2.5 times brighter than a star of magnitude 2, 2.5 multiplied by 2.5 times brighter than a star of magnitude 3, 2.5 × 2.5 × 2.5 times brighter than a star of magnitude 4, and skipping a magnitude, it would be 2.5⁵ (2.5 multiplied by itself five times, which equals 97.7, almost 100) times brighter than a star of magnitude 6. This follows a logarithmic scale, which we also use for sound decibels.

The point is that once we moved from a rough definition like Hipparchus’ to a more mathematical system using logarithms, a star (or other celestial body) could be brighter than magnitude 1. This is where Vega comes in. Just over a century ago, it was established that Vega would have a magnitude of 0 (a “class 0,” which Hipparchus did not define, as his system started at 1). From this point, the brightness of all other stars and celestial bodies being discovered was measured relative to Vega. For example, the Sun has a magnitude of -26.74. Vega’s brightness became a standard for astrophysicists, much like the meter or kilogram in measurements, or perhaps more like the inch — at least it was, until just two decades ago

So why did Vega fall out of favor, much like the inch? Both should have been replaced, but traditions and history have a way of prevailing. First, Vega is a star with variable brightness. To continue with our analogy, it’s as if the meter stick kept in Paris changed its length from time to time. And, indeed, this does happen: that very stick, which defined the meter for centuries, changes in size with fluctuations in temperature. This is why we now define the meter based on a more stable reference, such as the distance light travels in a vacuum in a fraction of a second — specifically, 1/299,792,458 of a second. Vega, too, varies in brightness by up to 10%, from its dimmest point to its brightest, likely due to rotational effects. Since we are observing one of its poles, the axis of rotation is aligned closely with our line of sight, unlike the axis of the Moon, which is (more or less) perpendicular to our view.

Vega’s physical peculiarities don’t stop there. Twenty years ago, it was discovered that Vega is surrounded by a disk of dust. Vega is a young star, about 450 million years old — roughly 10 times younger than our Sun. However, it is larger, and among stars, that means it has a shorter lifespan. In fact, both Vega and the Sun are roughly halfway through their lifecycles, which means Vega will burn out long before the Sun. (Who knows where humanity will be when that happens — something I find increasingly worrisome.)

Vega’s dust disk makes it less suitable as the standard star for measuring brightness, more than its variability. Above all because this disk is what dominates the light that comes to us from the star in the infrared spectrum. The star dust that forms this disk must be particles containing silicon and perhaps carbon. The size of these dust grains is a few hundred microns, at most a millimeter; if they were larger, they could not survive, the radiation from the star would drag them away, causing the disk to dissolve.

Vega’s dust disk also has peculiarities that are not well understood. It is very homogeneous and does not seem to have formed planets like the gas giants in our system (Saturn or Jupiter), despite the star’s age. By comparison, the planets in our solar system were formed in a time frame that ranges from a few million years after the Sun began its collapse — even before it began to fuse hydrogen (as is the case of Jupiter) — to a few tens of millions of years in the case of rocky planets, including Earth. Vega is much older, and still maintains a dust disk — likely the result of multiple collisions of planetesimals, which break apart rather than join together to form planets. This unusual feature is currently being studied by the James Webb Space Telescope, continuing the work of previous infrared telescopes. But we will leave that story for another day, because Vega is more fascinating for its disk than as a star, even though it was a benchmark in astronomy for centuries.

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