Powstawanie galaktyk i ewolucja gwiazd

Before you start, do the following.

• Remind the stages of the formation of the universe in the Big Bang theory.

The current state of knowledge does not allow for the formulation of an unambiguous theory explaining the formation of galaxies. Researchers assume that all galaxies were formed from a gas that uniformly filled the entire universe at the beginning of its existence. It consisted mainly of hydrogenhydrogenhydrogen and heliumheliumhelium, with a small admixture of light elements.

Over time, heterogeneity in the distribution of matter appeared, and its temperature decreased. In clouds with higher density of matter, so‑called protogalaxies, the first stars were created, referred to as the stars of the third population. They were large, massive stars, hundreds of times larger than our Sun. Their life time was relatively short, ended with a violent explosion, which resulted in further light elements remaining in the area of protogalaxies. The first generation of stars, according to modern hypotheses, was created around 200‑300 million years after the Big Bang.

The gravitational force resulting from the enormous density of the cloud caused its collapsecollapsecollapse and temperature droptemperature droptemperature drop. The movement of matter around the rotation axis caused the cloud to flatten and produce a characteristic disc and further attract gas and dustdustdust. Inside the disk, new stars were formed, and on the outskirts of the old cloud a halo of gas, dust and dark matter and old stars remained.

It is assumed that two factors decided whether spiral or elliptical galaxygalaxygalaxy was formed from the protogalaxyprotogalaxyprotogalaxy.

• Angular momentum - a protogalactic cloud with larger angular momentum moved faster and formed a spiral galaxy. In turn, the elliptical galaxy formed from a slower moving cloud.

• The temperature drop - very dense protogalaxies cooled faster, and all the matter contained in them was used to create stars. Hence, there is no disk in the elliptical galaxies. On the other hand, in less dense clouds the temperature dropped slower, which allowed to create a disk with gas and dustdustdust.

Stars due to their age are divided into so‑called stellarstellarstellar populations.

• Population III stars - these are the oldest stars mentioned above. They are not observed, but there are hypotheses that suggest that some of them may have survived to our times. These stars should have a very poor composition with elements other than heliumheliumhelium and hydrogenhydrogenhydrogen.

• Population II stars - are old stars from times when the concentration of light elements in protogalactic clouds was very low. They are found mainly in the centres of galaxies and in the surrounding halo, and form globular clusters. They move in orbits at different angles to the plane of the galaxygalaxygalaxy.

• Population I stars - these are relatively young stars. In their composition there are much more metallic elements, from 2 to 4%. They are observed in the areas of galactic disks, in the arms of galaxies and in open clusters. Stars of I population usually move in regular, elliptical orbits around the centre of the galaxy and in its plane.

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Na rysunku przedstawiony jest kształt naszej Galaktyki w postaci dysku opisanego disc (population I). Wokół niego punktami zaznaczone są gwiazdy II populacji, opisane globular clusters (population II), znajdujące się w zgrubieniu centralnym, opisanym nuclear bulge (population II) oraz wokół Galaktyki w obszarze tzw. halo opisanym halo (population II). Na dysku punktami zaznaczone są gwiazdy I populacji.

The universe is constantly evolving, and the process of creating stars is not over.

Some stars end their lives, and others just come into being.

It is assumed that the stars are created in the clouds of interstellar matter. The gas, consisting of hydrogenhydrogenhydrogen and heliumheliumhelium and a small admixture of other elements at the level of 1%, undergoescondensation. Such condensation may be caused by an electromagnetic wave coming from the explosion of another star. The newly created cloud begins to collapse due to gravity, and such collapsecollapsecollapse is accompanied by an increase in temperature. The gas‑dust cloud rotates around its own axis and forms a disc with a central spherical part called the protostar. The process of protostar creation can last for hundreds of thousands of years.

When the temperature inside the protostar reaches a sufficiently high value, it allows thermonuclear processes to take place. The energy released in these processes inhibits the gravitational collapsecollapsecollapse of the star, because the pressure inside the star resulting from the energy flow from the centre to the outer parts of the star is enough to prevent it from collapsing under its own weight.

The radiation emitted by it causes the star to glow.

When the nuclear reactions occur slower and slower due to the lack of elements for synthesis, the star begins to collapsecollapsecollapse. The dying star expands in a giant or supergiant phase. The star eventually explodes and becomes a planetary nebulanebulanebula or supernova. Finally, it turns into a white dwarf, a neutron star or becomes a black hole. The final fate of the star depends on its initial mass.

Remember

• The current state of knowledge does not allow for the formulation of an unambiguous theory explaining the formation of galaxies.

Exercises

Glossary

Keywords

collapsecollapsecollapse

galaxygalaxygalaxy

nebulanebulanebula

protogalaxyprotogalaxyprotogalaxy

temperature droptemperature droptemperature drop