tidal heating, protoplanetary disk, radiation belt, open star cluster

Tidal heating

Jupiter’s moon Io

Tidal heating is a consequence of the rotation of a planet around a star or a satellite around a planet. The rotational energy is dissipated as heat, heating the surface oceans and/or the interior of the celestial body. In an elliptical orbit, tidal forces act on the body more strongly when it is closer to the periapsis, so the body’s tidal deformations change as it travels along the orbit. These changes in shape lead to the appearance of internal friction, which heats up the body.

Thus, the gravitational energy of the body is converted into internal energy, and the original elliptical orbit in the system of two bodies gradually turns into a circular one (this effect is called tidal circularization). However, if the system is more complex, other bodies prevent the orbit from turning into a circular one, thereby fueling the constant heating.

Jupiter’s moon Io is the most volcanic body in the solar system. It receives the heat necessary for this precisely due to tidal heating. Europa and Ganymede keep Io’s orbit from becoming circular. The same mechanism melts the lower layers of ice on Europa, but it heats up less because it deforms more slowly – its orbit radius and rotation frequency are less than those of Io, and in addition, the forces of tidal heating decrease with increasing distance according to the cubic law.

protoplanetary disk

The main parts of the protoplanetary disk: in the center is a protostar, around it there is a central section of the disk, the planets of which contain only minerals and metals. It is surrounded by a soot line, beyond which carbon components also enter the planets. Then comes the freezing line, beyond which volatile molecules such as water, ammonia and methane can enter the planet.

According to modern concepts, accumulations of gas and dust gather into dense groups under the influence of gravity, and then, as a result of rotation, gradually turn into disks. This stage takes about 100,000 years.

A young star ignites in the center of the disk (young stars up to 10 million years old that have not yet reached the main sequence are classified as T Taurus or to the stars Herbig (Ae/Be)), and planets are gradually formed from the matter surrounding it. In this case, the protoplanetary disk can simultaneously be an accretion disk for a star, which can absorb matter from its inner part. This process can continue up to 10 million years.

If there are other stars close to the disk, the radiation of which ionizes it, then it is called a protoplanetary ionizable disk, or proplyd.

Gradually, under the influence of a combination of forces (stellar wind, attraction to the star, attraction to each other and elasticity of matter), lumps of matter begin to form inside the disk, which grow and pass into the state planetesimals – the embryos of the planets. An example is asteroid (21) Lutetia, which has a dense core under a thick kilometer-long layer of dust. Each gravitational cataclysm, which does not lead to a significant destruction of the planetesimal, rearranges its material depending on its density and flowability.

As a result, the disk is mostly cleared of matter, with the exception of a few asteroid belts, and matter located too far from the central star, and in its place a planetary system appears, in which all bodies are plus or minus in the same plane of the ecliptic. Some satellites of the planets are formed from the same disk, some are captured in a “ready” form when flying past the planet. The Moon, for example, is thought to have formed after a Mars-sized protoplanet collided with proto-Earth about 30 million years after the formation of the solar system.

There is a hypothesis, supported by computer simulations, that the complex organic molecules necessary for the existence of life are also formed in the protoplanetary disk from dust particles – at least this could happen in our solar system.

radiation belt

After the launch of the world’s first artificial Earth satellite Sputnik 1 and the second Sputnik 2 by the Soviet Union in 1957, the Americans launched their first satellite, Explorer 1, in early 1958. The satellite had several scientific sensors, but no equipment to record the received data. After that, in the spring of that year, they launched an improved Explorer 3, which already had a data recording apparatus.

Thanks to the analysis of the collected data, the American astrophysicist James Van Allen was able to confirm the hypothesis put forward earlier. Due to the presence of a pronounced magnetic field at the Earth, it has zones, or belts of energetic charged particles, for the most part originating from the solar wind, captured by the planet’s magnetosphere. In Western literature, these belts are often referred to as Van Allen belts.

These belts extend from 640 to 58,000 km from the Earth’s surface. The magnetic field, which captures energy electrons and protons with the help of belts, protects our atmosphere from destruction. The downside of their presence is the threat to artificial satellites and other spacecraft passing through these belts. The devices have to be protected from harmful radiation – otherwise the solar panels, integrated circuits and sensors are damaged. Astronauts also have to be protected from radiation.

In 2011, using the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) instrument installed on the Russian Resurs-DK satellite, the hypothesis of the presence of antimatter in belts was confirmed – in in particular, antiprotons. Their layer is formed by the interaction of the upper atmosphere with cosmic radiation.

Russian physicist Valentin Vladimirovich Danilov proposed a bold project (Eng. High Voltage Orbiting Long Tether, HiVOLT, “High-Voltage Orbital Tether”), which could make it possible to clear the radiation belts of energy particles, making them safe for spacecraft and astronauts. It is not known if there could be any negative consequences from cleaning the belts like this.

open star cluster

Pleiades Cluster

Humanity has noted the natural grouping of stars in the sky since the beginning of looking at the stars. Since ancient times, such a cluster as the Pleiades (they are also the Seven Sisters or Stozhary), located in the constellation Taurus, has been known. Before the invention of the telescope, astronomers saw some clusters simply as patches of light.

For example, in the classic work “Almagest”, or “The Great Mathematical Construction on Astronomy in 13 Books” by Claudius Ptolemy from 140 AD, which includes the full range of astronomical knowledge of Greece and the Middle East of that time, the Manger cluster in the constellation Cancer is mentioned, the double cluster in Perseus, the Coma Berenices cluster, and the cluster in the constellation Scorpio, which was later called the Ptolemy cluster, or M7 in Messier’s catalog.

But only Galileo, with the help of his telescope at the beginning of the 17th century, was able to discern that the objects described by Ptolemy were groups of several stars. Inspired by the successes of Galileo, the Italian astronomer Giovanni Battista Hodierna was the first to discover previously unknown clusters using a telescope.

By the end of the 18th century, astronomers began to guess that these groups of stars were physically interconnected, and not just side by side in the sky when viewed from our planet. After the publication of Messier’s extensive catalog, some scientists began to investigate what were seen as “nebulae” and concluded that some of them are simply groups of closely spaced stars. In the New General Catalog of Nebulae and Clusters of Stars (NGC) published at the end of the 19th century, hundreds of different star clusters were already listed. And thanks to more accurate measurements, it was shown that the stars in clusters move as a single whole.

With new telescopes, it became clear that star clusters are divided into two types. Globular star clusters contain a large number of densely packed stars that are gravitationally bound and rotate about a common center of mass. They usually live in the galactic halo.

Open star clusters usually contain several thousand stars. They all formed at about the same time from the same molecular cloud. Open clusters are located in the galactic disk, rotate around the center of the galaxy, and the stars in them are not strongly bound by gravity, so they often change their relative position due to interaction with other stars. They exist for about hundreds of millions of years and appear in spiral and irregular galaxies, where active star formation processes are still going on.

Since the stars of open clusters have a similar age and chemical composition, it can be convenient to study the evolution of stars using their example, and their properties (distance, age, metallicity, velocity, interstellar extinction) are easier to determine than for individual stars.

In our Galaxy, more than 1100 star clusters are already known, but in reality there may be 10 times more. In spiral galaxies, open clusters are usually found in the arms – where there is the most matter and where new stars are most likely to appear. Open clusters are usually found in the plane of the galaxy – our Milky Way is about 180 light-years high and 50,000 in diameter.

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