LECTURE № 4. Galaxies. "Corpses" of stars

1. Galaxies. Variety of galaxies

It is not known when man first looked at the sky: many millennia have passed since those distant times. It is only known for certain that man has always revered the sky, and also respected and feared him. It is very easy to explain this attitude: there, in the sky, the gods lived. The man tried to appease them, to propitiate them. If the gods are kind, then they will send rain, and if it rains, there will be a harvest for next year. If the gods get angry, they will send thunder, lightning and drought to the earth. Therefore, in those distant times, no one dared to anger their gods; Every nation had its own gods.

But what are stars? The stars are the souls of the dead who watch and follow us. So it was believed in ancient times.

How could people then know that the stars are far from being souls, but a celestial body with an incredibly high temperature. Centuries later, people began to notice some incomprehensible "hazes", foggy spots in the sky. With the help of telescopes, the human eye has seen that inside these nebulae are entire clusters of stars. Such clusters of stars are called galaxies.

Back in the XNUMXth century V. Herschel, a famous English astronomer and optician who discovered the planet Uranus, explored double stars and the structure of the Milky Way, built several of the largest telescopes of his time, discovered several thousand foggy spots (which were called nebulae). V. Herschel cataloged the nebulae discovered by him. During the study and observation of these nebulae, it was found that many of them have a spiral structure.

In the science of astronomy, all galaxies are divided into three large groups. This classification is based on the appearance of galaxies.

Three groups (classes) of galaxies:

1) spiral galaxies;

2) irregular galaxies;

3) elliptical galaxies.

Consider these types of galaxies.

spiral galaxies. Their branches consist of hot stars, supergiants; they emit radio waves. Approximately ten percent of the mass of such a galaxy is the mass of neutral hydrogen. The main difference between spiral galaxies is that they rotate at breakneck speed.

Wrong galaxies. What makes them different? Let's dive into history first. In the XVI century. Ferdinand Magellan made his famous trips around the world, which helped to "destroy" many "white spots" on the geographical map of our planet. Travelers in the southern hemisphere of the sky noticed and for an extended period of time watched two small stellar clouds. Later, these clouds began to be named after the most famous traveler: the Large and Small Magellanic Clouds. In fact, these are not clouds at all, but real galaxies that belong to the irregular group.

These galaxies are different in that:

1) they have a shapeless appearance;

2) their stellar composition is the same as that of the branches of spiral galaxies, with one exception: the irregular ones do not have a nucleus;

3) irregular galaxies are very, very rare.

Elliptical galaxies. These galaxies are much more common than spiral and irregular galaxies. Let's call salient features of elliptical galaxies:

1) they can be mistaken for globular clusters of stars, if you do not take into account that the galaxy is larger than them;

2) they rotate very slowly, and, consequently, they are slightly flattened. This is their main difference from spiral galaxies (which rotate very quickly and, as a result, are very similar to a spindle);

3) elliptical galaxies contain neither giant stars nor nebulae.

2. "Corpses" of stars: white dwarfs, neutron stars, black holes

White dwarfs

Stars, like many things in our Universe, are not eternal, their life expectancy is tens of millions of years, however, this is incomparable with the years of the existence of the Universe.

At the end of its life, the star becomes a white dwarf. The "death" of a celestial body occurs after it has exhausted the entire supply of its sources of thermonuclear energy. Moreover, not all stars become white dwarfs, but only stars of medium and low mass.

White dwarfs do not glow on their own, since they do not have any nuclear processes inside them. But still, the "corpses" of the stars glow. Why? The glow of white dwarfs is explained very simply: the glow occurs due to slow cooling. The scientist, Indian physicist Raman Chandrasekara calculated the mass of a white dwarf, which he cannot exceed. According to Chandrasekara, the mass of this celestial body is approximately 1,4 solar masses.

neutron stars

Neutron star - this, one might say, is also the “corpse” of a star, but a much larger star, which has a mass exceeding ten times the mass of our Sun. The radius of a neutron star is approximately one and a half to two times the mass of the Sun, and its radius is approximately 10 - 20 km... In very rare cases, the mass of a neutron star can exceed the mass of the Sun, but no more than three times (this is the so-called "limit Oppenheimer - Volkov." We will find out below what will happen to such a “small” star.

In 1930's Walter Baade и Fritz Zwicky put forward the theory that as a result of a supernova explosion (supernovae are stars that "suddenly flare up and die like new stars. However, at the maximum luminosity they are thousands of times brighter than new stars"), an ultradense neutron star is formed. This theory was confirmed almost thirty years later, when a pulsar was discovered in the Crab Nebula, that is, a neutron star that rotates at an incredibly high speed.

Black holes

The concept of "black hole" was introduced in 1968 by an American physicist John Wheeler. With this concept, he designated neutron stars, which, as a result of the action of gravity, have contracted to such an extent that light simply cannot overcome their attraction. It was said above that the mass of a neutron star is approximately one and a half to two times greater than the mass of the Sun, but sometimes its mass can be three or more times greater than the mass of the Sun. So, these “exceptional” neutron stars become black holes.

Gravitational radius is the radius to which a neutron star must collapse to become a black hole. If the star was very large, then this radius is only a few tens of kilometers.

Author: Filin S.P.

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