In astronomy, we study everything that exists in space, outside the Earth. And we study the Earth as well, as seen from space, to make a comparison with the other planets that circle our Sun. While all these objects fall in the domain of astronomy, the distances between us and the objects of our study vary enormously. The moon is a mere light-second away, the distance that light travels in one second. To reach the Sun with the speed of light takes about eight minutes, and the distance to Saturn is more than a light-hour. Yet even the nearest star is far, far more distant, separated from us by several light-years.
Our Milky Way Galaxy has a diameter of more than a hundred thousand light years, and the edge of the visible Universe lies at a distance of roughly ten billion light years. In other words, the furthest galaxies that we can see with our largest telescopes are further away from us than our Moon is by a factor that is equal to the number of seconds in ten billion years, about a factor 300,000,000,000,000,000.
With this enormous range of distances, it would seem only natural to guess that there is little we can say about the most remote stars and galaxies. Given that we can observe the Moon and planets and our own Sun with incredible detail, we would expect to know far more about them than about far more distant objects.
In fact, almost the opposite is true. In many cases in astronomy, we know far less about what is close by than about what is far away.
Let us start with a comparison of the Earth and the Sun. We stand on the Earth. We can drill into it. We can measure seismic waves traveling through the Earth. And yet, we have a far better picture of the detailed conditions and composition of the Sun. The reason is that the interior of the Sun is extremely hot, with most of it well over a million degrees. At these temperatures, all forms of matter are broken down into their subatomic constituents, atomic nuclei and electrons, in a state that we call a plasma.
In contrast, the temperature in the interior of the Earth is far lower. Most of the interior is solid, with a core that is partially liquid. In the solid state, matter can take on an enormous variety of shapes and conditions. As a comparison, take a drop of water and a snowflake. A drop of water is a drop of water, but a snowflake can come in seemingly endless varieties. In general, the higher the temperature, the simpler the state of matter. As a result, we can compute with a high degree of confidence what the interior state of the Sun is, while there is far more uncertainty about the interior state of the Earth.
Now let us take a second example. The Sun is a star, a rather average type of star, of which there are billions in our Galaxy alone. Most of the matter in the sun is hydrogen, about three quarter of its mass. Of the remaining one quarter, the bulk is helium, with only two percent of the mass made up of other chemical elements. Hydrogen and helium were formed in the Big Bang, while the other elements were produced in earlier generations of stars. After these earlier stars died, some of their matter was mixed with the rest of the interstellar gas in our Galaxies, from which later the Sun was born.
Even though all the other chemical elements constitute only two percent of the Sun, they play an important role in the Sun's structure because they influence important properties such as the opacity of the interior and the rate of nuclear reactions. The Sun would be much simpler to model if it were only made out of hydrogen and helium. In fact, the very first stars there were formed in the Universe only contained hydrogen and helium. And those are exactly the stars that now seem furthest away from us. The further we look in space, the further we look back in time, so the very first stars are at the largest distance from us.
This means that we can say more about the stars at the edge of the visible Universe than we can say about all stars at intermediate distances. Isn't that paradoxical?!
I could continue with many more examples. To mention just one: the nearest black hole is far more distant, fortunately, than the Sun, yet we can describe its structure much more accurately, with a single relatively simple mathematical equation.
And more complex than any star is the wiring of a human brain. Of what is the closest to us, closer than our own eyes, we still know a lot less than we know of the distant stars.