What are Stars? A Short Introduction.


One of the most beautiful spectacles that exists in our world is the view of the starry sky on a dark moonless night. But what are stars?

Stars are huge plasma balloons “floating” in outer space. They are made mostly of hydrogen and helium. The substance in them is held by forces of mutual attraction. They are heated to such a high temperature that they can radiate light. This is the reason are able to observe them. In fact, they are so hot that any substance, even the hardest metal, found on them in the form of an electrically charged gas – plasma.

Do stars shine on their own?

Inside their core, the temperature is much higher than on the surface. In the stellar core, it can reach 10 million degrees and above. At such temperatures, thermonuclear reactions occur in the conversion of some chemical elements to others.

For example, hydrogen, of which almost all stars are composed, turns into helium in their core. It is thermonuclear reactions that serve as their main source of energy. Thanks to these reactions, stars are able to shine for many millions of years.

Stars and galaxies

Astronomers estimate there are about 1 billion trillion stars in the universe! In accordance with the laws of nature, they gathered in huge stellar islands, which astronomers called galaxies. We live in one of these galaxies, whose name is the Milky Way.

All stars visible in the sky with the naked eye or through a small telescope belong to the Milky Way. We can also observe other galaxies with a telescope, but they all look like dim misty specks of light.

The Sun is the closest star to us. It does not stand out against the background of millions of other stars that can be seen through a telescope. The Sun is not the brightest, but not the faintest of them. It is not the hottest, but not the coldest, not the most massive, but also not the lightest. We can say that the Sun is a middle-aged star. And only to us the role of the Sun seems extremely important, because it gives us heat and light. Life on Earth is possible because of the Sun.

Dimensions, mass and luminosity of stars

The size and weight of even small stars is huge. For example, the Sun is about 109 times larger than the Earth in diameter and 330,000 times more massive than our planet. To fill the volume that the Sun occupies in space, we would need more than a million planets the size of the Earth (source: NASA Solar Exploration).

There are stars much larger than the Sun, such as the star Sirius, the brightest star in the night sky. Sirius is 2 times more massive than the Sun and 1.7 times its diameter. It also emits 25 times more light than our daylight star.

Another example is the star of Spica , who leads the constellation Virgo. Its mass is 11 times larger than the Sun, and its luminosity is 13,000 times higher. It is hardly possible to even imagine the sizzlingly powerful radiation of this star.

Red Dwarfs

Most of the stars in the universe are still smaller than the Sun. They are lighter and shine much weaker than our star. The most common are red dwarfs, as they emit mainly red light. A typical red dwarf is about 2-3 times lighter than the Sun, 4 or even 5 times smaller in diameter and 100 times dimmer than our star.

There are between 100 – 400 billion stars in our galaxy. Of these, at least 500 billion will be red dwarfs. But, unfortunately, all the red dwarfs are so dim that not one of them is visible in the sky with the naked eye. To observe them, you need a telescope or at least binoculars.

In addition to the red dwarfs, which make up the majority of all the stars in the Universe, those similar to the Sun, as well as binary stars such as Sirius and Spica, there is also a small fraction of unusual ones whose characteristics – size, luminosity or density – are very different.

White Dwarfs

One of these White Dwarfs is the satellite of Sirius. Many stars do not reside alone, like our Sun, but in pairs. These are binary stars. In the same way as the Earth and other planets of the Solar System move in orbits around the Sun under the influence of its gravity, so a satellite star can orbit around the main one.

In fact, the planets, together with the Sun, revolve around a common center of mass. The same thing happens with the binary star components – they both rotate around a common center of mass.

In 1862, American telescope-maker and astronomer Alvan Graham Clark first observed Sirius’ satellite. It was named Sirius B. However, it turned out that its surface is as hot as the surface of Sirius. At that time, astronomers already knew that the more light a star emits, the hotter it is. Consequently, from each square meter of the surface of the Sirius satellite, as much light was emitted as from a square meter of Sirius himself. Why was the satellite so dim?

Because the surface area of ​​Sirius B was much smaller than the surface area of ​​Sirius A. It turned out that the size of the satellite is equal to the size of Earth. At the same time, its mass turned out to be equal to the mass of the Sun. Simple calculations show that every cubic centimeter of Sirius B contains 1 ton of substance.

Red supergiants

There are also stars of huge sizes and luminosities in the Universe. For example, Betelgeuse, is 900 times larger than the Sun in diameter and emits 60,000 times more light than our daylight. Another star, VY Canis Majoris, is 1,420 times larger than the Sun in diameter. If we place this star in the place of the Sun, then the surface of the star would be between the orbits of Jupiter and Saturn, and all the planets from Mercury to Jupiter (including Earth) would be inside the star.

A distinctive feature of supergiants is that for all their colossal sizes they contain only 5, 10 or 20 times more matter than the Sun. This means that the density of such luminaries is very low. For example, the average density of VY Canis Majoris is 100,000 times lower than the density of room air.

Both white dwarfs and supergiants are not born that way, but become in the course of evolution. Usually after the hydrogen in their cores is converted into helium.

The hidden mass of the universe

More recently, astronomers believed that the stars contained almost all the matter in the universe. But in recent decades, it turned out that the lion’s share of the mass of the Universe is mysterious dark matter and even more mysterious dark energy. Thus, stars account for only about 2% of all matter (and planets, comets and asteroids even less). But it is precisely these 2% that we are able to observe, since it is they who emit light.

Nevertheless, astronomers keep making new discoveries. Only recently a team of Chinese astrophysicists discovered the fastest rotating star in the Milky Way.

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