When we study any of the bodies of the solar system we have no choice but to do it from the outside . It is not only that reaching them is a challenge, but getting inside them is completely impossible, or at least with the technology we currently have. Even on Earth itself, where we have access to all the resources and machinery we need, the deepest we’ve ever drilled is exactly 40,000 feet below the surface of northwestern Russia , in the Kola Hole. That is to say, we have barely managed to penetrate a little more than twelve kilometers, of the 6,380 kilometers of radius that our planet has. And yet we know quite well the interior of our planet and its structure.
If we try to study the interior of other bodies in the solar system, things get complicated. In some cases simply because of logistics. We will not be able to have the same machinery on Mars as on Earth and that will limit our study. In fact, the deepest hole ever dug on Mars is 7 centimeters deep . When the Rosalind Franklin rover (currently on hiatus due to the war in Ukraine) lands on the red planet, if all goes according to plan, it will dig a hole 1.7 meters deep to collect samples that will return to Earth. A tremendous achievement, but nothing compared to what has been achieved here on Earth or what is necessary to actually reach the interior of the planet.
On other planets, digging directly is impossible, as there is no solid surface to drill through . The gas giants of the solar system do not have a rocky surface on which we can land our rovers. Or, if they do, it is tens of thousands of kilometers deep , under pressures thousands of times what we experience on the Earth’s surface. And the Sun of course is even worse. Not only does it not have a solid surface to land on, but we can’t even get a probe to penetrate its outer layers, since the temperatures reached there would be capable of melting even the strongest materials we’ve managed to develop.
So how do we go about studying the interior of the Sun? How have we been able to deduce the temperature that it must have inside , the conditions that make nuclear fusion possible or how the balance between the intense gravity that tries to shrink everything and the extremely high pressures that try to make it explode materializes? Basically, with a combination of indirect measures and theoretical models that have been refined using those measures. All of this has led to what is known as the Standard Solar Model . This model consists of a series of assumptions about the internal structure of the Sun (size, composition, density, temperature, etc. of the different layers that form it) that serve to make predictions that are compared with experimental observations.
These observations are of course limited to the very last layer of the Sun , known as the photosphere . This is a layer barely 500 kilometers thick (remember that the radius of the Sun is almost 700,000 kilometers ). This photosphere would be equivalent to the earth’s crust and it is the one that emits the light that we receive on Earth . However, tens of kilometers below its outermost part, it already becomes completely opaque to any radiation , making it impossible for us to see beyond it.
In the 1960s it was discovered that the Sun’s surface vibrates in a pattern of cells . These vibrations are the result of pressure waves that travel through the interior of the star and that when they reach the photosphere are reflected back inside. By analyzing the surface we can learn about the interior, as we do on Earth with seismic waves , by studying how they are reflected and how they travel through the interior of the planet or star. Since the different layers of the Sun will have different densities , these pressure waves will travel at different speeds through its interior. This will also cause the wave to deviate when it reaches the interface between two layers , as happens with light when it passes, for example, from air to water, which deviates and forms distorted images. By therefore studying the propagation of the waves and where and when they reach the surface, a fairly complete model of the interior of the Sun can be created.
These waves are studied by observing the vibrations of the solar surface, measuring the displacement of its spectral lines as a result of the Doppler effect . That is, by measuring how the emitted light varies depending on whether a particular piece of the surface is expanding or shrinking. We also have several observatories (both space-based, such as SOHO or the Parker probe, and ground-based, such as the GONG network) constantly monitoring the Sun, measuring its temperature, density, rotation, etc.
The Standard Solar Model has achieved incredible results, being able to predict and emulate the behavior that we observe in the Sun. This model also predicts that the density of the Sun varies greatly between the core and the photosphere . In the core, densities of hundreds of tons per cubic meter would be reached, twenty times more dense than iron, while in the photosphere densities would be less than one gram per cubic meter , about ten thousand times less dense than air in the Earth. land surface. It also allows us to predict the temperature of the different regions of the Sun, which varies from 5,500 ºC at its surface to almost fifteen million degrees at its core . This core temperature is also necessary for the nuclear fusion processes that keep the star active to take place. Also measurements of the number of neutrinos produced during this nuclear fusion are in good agreement with the predictions of the Standard Solar Model.
E. Chaisson, S. McMillan, 1993, Astronomy Today, Prentice Hall
J. Christensen-Dalsgaard, 2003, Helioseismology, Reviews of Modern Physics. 74 (4): 1073–1129, doi:10.1103/RevModPhys.74.1073