As we move through the universe, due to the great distances we travel and the limiting speed of light propagation, we often say that to look into the distance is to look back in time. The recent discovery by the Hubble telescope of the most distant star ever observed, Earendel, could suggest that due to the problematic spacetime in astrophysics, Earendel is also the oldest star we have. found. Nothing could be further from the truth. Earendel is far away and its light takes a long time to reach us, but it is not an old star.
If we want to talk about ancient stars, we could, for example, look at one of the oldest stars we know of with a definite age: HD 140283, nicknamed Methuselah. HD 140283 is quite close to us, at a distance of 190 light years, and it is almost as old as the universe itself, at 13.7 billion years. Methuselah has been well known for over 100 years because it has a very large proper motion – the change in its position in the night sky over a period of time. This means that as long as he is in our neighborhood, he is just visiting. It comes from the area of our galaxy known as the stellar halo and travels at a speed of over 620,000 miles per hour. By tracking its movement in the past, we were able to determine that it originated in an early dwarf galaxy that was torn apart by our galaxy’s gravitational field at least 12 billion years ago. By measuring its chemical composition, we know that it does not come from our own environment because it is deficient in heavy elements. That is to say, it was formed in an environment made up of old materials in which the chemical elements had not yet been transformed by the different generations of stars.
And that brings us to the problem raised at the start of efforts to determine the age of celestial objects. In astrophysics there are things we can and there are things we can’t measure is a relatively simple way, bearing in mind of course that we’re talking about the cosmos and objects that at best are so far apart that we have to use special units of measurement. The mass of an astronomical object, for example, falls into the first category and is easy to determine, as are its composition and chemical composition. Nor is it too painful, ignoring the hours of sleep one loses looking through the telescope, to calculate the speed at which a star is moving, the presence of stars that accompany and magnetic fields. However, determining the age of a star is considerably more complicated.
It is perhaps surprising that the only star whose age we know precisely is the Sun. We can only calculate its exact age with some certainty because we have access to the material it is made of. The process involves taking a sample of the solar system in as pristine a state as possible, usually in the form of a meteorite, and measuring the amount of long-lived radioactive isotopes it contains. The problem is that we don’t have material from another star to study in the lab, so how can we determine the age of the others?
It depends. When a star comes from an ancient population, it has few metals because it was born without them. The universe did not have time to form them in these stars and disperse them in the void in the form of stellar winds. By breaking down the light that reaches us in the form of spectra, if we are able to measure the chemical fingerprints of uranium, and in particular that of another element of the periodic table called thorium, we can determine its age. But this measurement is very difficult to make because little thorium light reaches us and it mixes with the marks of the other chemical elements.
Another way to calculate the age of stars is to study their movement and judge their orbits in the past, which is made possible when they belong to young groups of stars that move together. We can also study how the speed at which they spin decreases, since they are like spinning tops that slow down over time. We also sometimes calculate their lithium content, which tells us about their age. Another technique that I have personally always found fascinating is asteroseismology, which measures oscillation modes in stars and is similar to what scientists do through earthquake measurements to infer the structure of interior of the Earth. Asteroseismology is very useful when dealing with ancient stars because their vibrational modes, which we call lower order, pass through the center of the star and give us an idea of its density, which translates reasonably directly by measuring his age.
However, in general, we cannot accurately determine the age of single stars. We have techniques that we can work with, but we cannot measure the age of these stars as such. We can guess it, infer it, but we cannot absolutely determine it in isolated stars, except in the case of the Sun. If we think about it, it’s the same with humans. Purely by sight, it is difficult to guess the age of a child, if we do not have several other children nearby as a point of reference. For this reason, one of the most used methods is to observe how they age together in star clusters, since these are born in groups and time does not pass in the same way for all of them.
Moreover, the age of a star is uncertain partly because the moment when it is born is poorly defined. In the same way it is very clear for a mammal, if we extrapolate our daily experience to the world of the skies, we encounter vicissitudes difficult to resolve from an operational point of view. From a theoretical point of view, the problem is simple: a star is born when its structure reaches what is called hydrostatic equilibrium. This happens when the tensions balance out and the structure neither expands nor contracts because the pressure exerted by the energy generated inside the star is counterbalanced by the pressure exerted inward by the gravitational force. This precise moment is forbidden to us, we cannot see it, we only have access to the structure in equilibrium thousands of years later, when the star has freed itself from an envelope which forms at following its own collapse. We have tried to work with other definitions: when we begin to see the photosphere, the luminous surface that delimits a star, from the moment a star reaches what is called age zero in the main sequence, that is- that is to say when it burns hydrogen in its core, in thermonuclear terms, but from there the star already has an age and it would be necessary to use negative times.
The fact is that no definition of when a star is born is perfect, and no technique works for all stars, although they provide us with complementary information. Time, by definition, has an elusive quality.