galactic clusters, galactic halo, Galilean moons and heliosphere

galaxy clusters

An image taken by the Chandra telescope of the Perseus Cluster, one of the most massive objects in the universe. Its relativistic jets emit radio waves.

Stars that exist within galaxies often gather into clusters – groups of stars that are gravitationally bound to each other. A gravitationally bound system of stars, star clusters, interstellar gas and dust, dark matter and planets, we call a galaxy.

At the same time, on an even larger scale, galaxies also gather into clusters – groups of galaxies interconnected by gravity. If the mass of galaxies is within 106 – tenthirteen solar masses, then a galaxy cluster can have a mass of 10fourteen – tenfifteen solar, and contains from 100 to 1000 galaxies. To divide galaxy clusters into classes depending on their morphology, a Bautz-Morgan classification.

The key feature of galaxy clusters is the intercluster medium, which consists of superheated plasma. This is a gas containing mainly ionized hydrogen and helium, heated to temperatures from 10 to 100 MK, emitting in the X-ray range. Most of the baryonic matter of galactic clusters is concentrated precisely in the intercluster plasma. It is not known exactly what exactly heats up this medium – most likely, a combination of relativistic jets emitted by the centers of the galaxies that make up the cluster, and the interaction of the atoms of the medium with each other during mergers of galactic subclusters. The galaxies themselves are responsible for approximately 1% of the mass of the cluster, the intercluster medium for 9%. The rest of the mass is dark matter.

Our local group of galaxiesof which the Milky Way is a member, is included, along with other galaxies, their groups and clusters, into Virgo Supercluster. Also among the interesting objects of this type, we can mention the “Great Attractor” – the point of attraction of many clusters and superclusters of galaxies, among which is our Virgo supercluster. The center of the Great Attractor, the Norma Cluster (ACO 3627, or Angle) in the constellation Angle, lies at the intersection of two major structures, namely the Centaurus Wall, which includes the Virgo Supercluster, the Centaurus Cluster, and the Norma Cluster itself, and another stretching from the Peacock Cluster to the Parus Supercluster . Apparently, this is a huge supercluster of galaxies with a mass of 105 times the mass of the Milky Way.

Galaxies and their clusters also, in turn, form even larger structures – galactic filaments. And these are already the largest structures known in the Universe. They are also called galactic walls, because their structure often resembles soap suds – the walls of huge bubbles, inside which mainly contains emptiness (or void in terms of astronomy). Such “walls” have a length of 160 to 260 million light years.

Galactic halo

Stars form the basis of galaxies, and they are also the easiest to see when considering a galaxy. However, the galaxy extends much further due to such a component as the halo. It has a roughly spherical shape and consists of a stellar halo, a galactic corona, and a dark matter halo.

The easiest way to see the stars is closer to the center of the galaxy and in the regions surrounding it. However, there are usually stars in the outskirts of the galaxy – and most often these are the oldest of the stars in the galaxy, as well as stars captured during mergers of the main galaxy with its galactic satellites. These stars make up the stellar halo.

The galactic corona is the hot ionized gas, or plasma, surrounding the galaxy. It is believed that it is formed by “galactic fountains” – jets emitted by supernova explosions and their remains.

Dark matter permeates every galaxy and extends far beyond its visible part. By definition, such a halo of dark matter cannot be observed directly, but its existence can be judged by indirect signs – by the gravitational influence on the motion of stars, gas, and by the presence of gravitational lensing. Dark matter halos play a key role in the formation of galaxies – at the first stages of this process, the temperature of baryonic matter, according to calculations, should be too high for gravitationally bound objects to form from it. Therefore, for the formation of galaxies, it is necessary that structures consisting of dark matter first appear and provide additional gravitational influence.

Galilean satellites

Galileo Galilei is one of the greatest scientists. He was born in Italy in the 16th century, studied physics, mechanics, astronomy, philosophy, mathematics, and as a result had a significant impact on the science of his time. One of the first Galileo began to use a telescope to observe celestial bodies.

At the junction of 1609 and 1610, Galileo improved his telescope, having received a 20-fold increase, and saw four celestial bodies located in the sky near Jupiter. At first he mistook them for stars, but in the spring of 1610 he came to the conclusion that these were the satellites of Jupiter. These were the first satellites of the planet of the solar system, after the moon, discovered by man.

However, their names were given by another astronomer, the German Simon Marius, who discovered these satellites simultaneously with Galileo and independently of him. The names of the satellites given by him have survived to the present: Io, Europa, Ganymede, Callisto. They received their names in honor of the mistresses of Zeus – the Greek counterpart of the Roman god Jupiter. Marius also owns the first mention of a nebula in the constellation Andromeda (spiral galaxy M31).

These four moons are the most massive of the giant planet’s moons. In total, 80 satellites of Jupiter are already known today.

Io is the fourth largest satellite in the solar system, slightly larger than the moon, and at the same time the most geologically active object in the entire solar system. It has about 400 active volcanoes and more than 100 mountains, many of which are taller than Everest.

Europa is a satellite slightly smaller than the Moon. It is believed that its surface consists of a thick crust of ice, under which lies a water ocean 100 km deep. Although no evidence has yet been found for this, tidal action on the satellite provides heating, keeping water liquid and making life possible under the ice.

Ganymede is the largest moon in the solar system, and is larger than Mercury (although it has half the mass, since Ganymede is an icy world). It is also the only satellite in the solar system that has magnetosphere. It is believed that between layers of ice at a depth of about 200 km under the surface of Ganymede there is an ocean of salt water.

Callisto is the third largest moon in the solar system, slightly smaller than Mercury and having a third of its mass. By the number of craters on the surface, this satellite is considered the champion of the solar system. The largest crater on its surface with a diameter of 350 km is called “Valhalla“. It is located in a lowland with a diameter of 3000 km. Probably, at a depth of about 300 km under its surface, a water ocean may exist.


The sun, like any star, emits a powerful stream of fast particles into the surrounding space, called the stellar wind. They start their journey from the upper layers of the star’s atmosphere and diverge from it in all directions in the form bubble. At some distance from the sun, the particles slow down to subsonic speeds. The speed of sound in the interplanetary or interstellar medium depends on the density of matter – in the solar system it is about 100 km/s. Then the kinetic energy of the stellar wind is converted into thermal energy, generating X-ray-emitting plasma with a temperature of 106 K. This inner shell of the bubble is called the terminal shock wave. In the solar system, this shell has from 75 to 90 AU. across.

Further, this stellar wind continues to expand, interacting with the interstellar medium until, due to this interaction, its speed drops to zero. This outer shell of the bubble is called the astropause (in the particular case with the Sun, the heliopause).

In addition to the stellar wind bubble, there is an area around the star, within which its magnetic field affects the movement of charged particles. This region is called the star’s magnetosphere. According to modern theories, a star or a planet generates a magnetic field due to the dynamo effect – a rotating conductive substance with the properties of a liquid with convection occurring inside it is able to maintain a magnetic field for quite a long time from an astronomical point of view. It is believed that it is due to this effect that the Sun, the Earth, Mercury and the satellites of Jupiter have magnetic fields (and their corresponding magnetospheres).

Near the star, the magnetic field lines resemble those of a magnetic dipole. The farther from the star, the more these lines begin to distort due to interaction with its own stellar wind.

The upper layers of a star’s atmosphere, the bubble of its stellar wind, and the magnetosphere are collectively called the astrosphere (in the case of the Sun, the heliosphere). Typically, this structure is several light-years across.

The term “heliosphere” was coined by a cosmologist Alexander Dessler in 1967. The heliospheric bubble, constantly fueled by the solar wind, protects the entire solar system from interstellar ionizing radiation. As the solar system moves around the center of the Milky Way and interacts with the interstellar medium, its heliosphere has an elongated shape resembling a comet – on one side it is approximately spherical, and on the other it has a long “heliotail”. In 2012, Voyager 1 and in 2018 Voyager 2 went beyond the heliopause and are currently in interstellar space.

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