Thinking in terms of dimensions such as length, size and volume often makes our ideas restricted to the everyday events of our daily lives. When we think of distances, for example, we associate going from our home to the bakery, from our work to the market, from one city to another and, at most, we think of the distance between states, countries, continents, etc.
Astronomical dimensions are impressive. Still thinking of local scales, our home, planet Earth, has a diameter of 12,742 km on the Equator. It doesn’t sound like much, I know, but it’s about 11 times smaller than the diameter of the Solar System’s largest planet, Jupiter. In turn, a thousand planets like Jupiter would fit easily inside the Sun.
About a thousand planets like Jupiter would fit inside the Sun.Source: Mark A. Garlick
150 million kilometers away from us, the Sun occupies about 99.8% of the entire mass of the Solar System. However, it is not a grandiose star in the universe. On the contrary, the Sun is considered a typical yellow dwarf star.
It doesn’t stand out in size or brightness when compared to the rest of the stars in the Milky Way. And not even our galaxy, with its 100,000 light-years across and more than 100 billion stars, ranks high. In fact, the Milky Way is also a typical spiral galaxy, with no specific special features that place it at the top of any podium.
Artistic representation of the Milky Way.Source: Universe Today
To talk about cosmic titans, we talk about galaxy clusters. Galaxy clusters are the largest gravitationally bound structures in the universe and, according to the Standard Cosmological Model (which describes the behavior and properties of the universe as we know it), they are also the newest structures to form.
Observed since at least the 18th century — when it was still unknown whether galaxies were inside or outside the Milky Way — by astronomers such as William Herschel (1738-1822) and Charles Messier (1730-1817), galaxy clusters are sets of hundreds to thousands of galaxies also made up of an extremely hot gas (possible to be detected only with X-ray observations) and unknown dark matter.
Although it cannot be directly observed, dark matter can have its distribution inferred through specific techniques, such as gravitational lensing.
Abell 1689 galaxy cluster. Galaxies in the visible spectrum (left), hot gas observed in X-rays (centre) and dark matter distribution inferred by gravitational lensing (right).Source: NASA/Hubble
Of the entire mass of a cluster of galaxies, about 80% corresponds to the mass of dark matter. Of the part referring to ordinary matter (called baryonic matter, the one that makes up me, you, the puppies, the planets and everything else that is made up of ordinary atoms and particles), about 15% is in the form of hot gas.
Only 5% of the mass of a cluster of galaxies actually corresponds to galaxies. This is equivalent to saying that, of the largest structures in the universe, we are completely unaware of about 80% of their nature, restricting ourselves to trying to understand only what we can see through light.
These structures comprise regions in space that are typically 6.5 million light years in size. This means that if a person wanted to travel at the speed of light through a single cluster of galaxies in a spacecraft, it would take no less than 6.5 million years to get from one end to the other. Does it seem like a short time? On Earth, 6.5 million years ago, the homo sapiens he didn’t even dream of existing, and the first human species were still rehearsing the separation of chimpanzees and bonobos.
Abell S1063 galaxy cluster.Source: NASA, ESA, Jennifer Lotz (STScI)
In order to understand the general behavior of the Cosmos, it is important to know what its matter content is. Because they are extremely large and massive, galaxy clusters are the best representative structures for understanding how much mass there is in the universe.
This makes these systems large cosmic windows, extremely important structures to shed light on the history of the universe, making it possible to read its past, understand its present and predict its future.
Nicolas Oliveira, columnist of TechWorld, has a degree in Physics and a Master in Astrophysics. He is a doctoral candidate at the National Observatory, where he researches orphan stars in galaxy clusters. He has experience in Teaching Physics and Astronomy, with research in Extragalactic Astrophysics and Cosmology. It acts as a scientific communicator and disseminator, seeking the popularization and democratization of science.