STARS: CHARACTERISTICS, HOW THEY ARE FORMED, LIFE CYCLE, STRUCTURE - SCIENCE - 2024

A star is an astronomical object composed of gas, mainly hydrogen and helium, and kept in equilibrium thanks to the force of gravity, which tends to compress it, and the pressure of the gas, which expands it.

In this process, a star produces huge amounts of energy from its core, in which there is a fusion reactor that synthesizes helium and other elements from hydrogen.

Figure 1. The Pleiades, in the constellation of Taurus, visible during the northern winter, constitute a cluster of about 3,000 stars 400 light years away. Source: Wikimedia Commons.

In these fusion reactions, the mass is not fully conserved, but a small portion is converted to energy. And since the mass of a star is enormous, even when it is one of the smallest, so is the amount of energy it gives off per second.

Characteristics of the stars

The main characteristics of a star are:

- Mass: highly variable, ranging from a small fraction of the Sun's mass to supermassive stars, with masses several times the solar mass.

- Temperature: it is also a variable quantity. In the photosphere, which is the luminous surface of the star, the temperature is in the range of 50000-3000 K. While at its center it reaches millions of Kelvin.

- Color: closely related to temperature and mass. The hotter a star is, the bluer its color and conversely, the colder it is, the more it tends towards red.

- Luminosity: depends on the power radiated by the star, which is not usually uniform. The hottest and largest stars are the most luminous.

- Magnitude: it is the apparent brightness they have when seen from Earth.

- Movement: the stars have relative movements with respect to their field, as well as rotational movement.

- Age: stars can be as old as the universe - about 13.8 billion years - and as young as 1 billion years old.

How do stars form?

The Sun, one of the millions of stars in the Milky Way.

Stars are formed from the gravitational collapse of huge clouds of cosmic gas and dust, whose density constantly fluctuates. The primordial material in these clouds is molecular hydrogen and helium, and also traces of all known elements on Earth.

The movement of the particles that make up this huge amount of mass spread out in space is random. But every now and then the density increases slightly at one point, causing compression.

The pressure of the gas tends to undo this compression, but the gravitational force, the one that pulls the molecules together, is a little higher, because the particles are closer together and thus counteracts this effect.

Furthermore, gravity is responsible for increasing the mass even more. And as this happens, the temperature increases gradually.

Now imagine this condensation process on a large scale and with all the time available. The force of gravity is radial and the cloud of matter thus formed will have a spherical symmetry. It is called a protostar.

In addition, this cloud of matter is not static, but rotates rapidly as the material contracts.

Over time, a core will form at very high temperature and enormous pressure, which will become the star's fusion reactor. A critical mass is needed for this, but when it happens, the star reaches equilibrium and thus begins, so to speak, its adult life.

The mass and subsequent evolution of stars

The type of reactions that can occur in the nucleus will depend on the mass that it initially has, and with it the subsequent evolution of the star.

For masses less than 0.08 times the mass of the Sun - 2 x 10 30 kg approximately - the star will not form, since the nucleus will not ignite. The object thus formed will gradually cool down and the condensation will slow down, giving rise to a brown dwarf.

On the other hand, if the protostar is too massive, it will also not achieve the necessary balance to become a star, so it will collapse violently.

The theory of star formation by gravitational collapse is due to the English astronomer and cosmologist James Jeans (1877-1946), who also proposed the theory of the steady state of the universe. Today this theory, which holds that matter is created continuously, has been discarded in favor of the Big Bang theory.

Life cycle of stars

As explained above, stars are formed by the condensation process of a nebula made of gas and cosmic dust.

This process takes time. It is estimated that it happens between 10 and 15 million years, while the star acquires its final stability. Once the pressure of the expansive gas and the force of compressive gravity balance, the star enters what is called the main sequence.

According to its mass, the star is located on one of the lines of the Hertzsprung-Russell diagram or HR diagram for short. This is a graph that shows the different lines of stellar evolution, all of them dictated by the mass of the star.

In this graph, stars are ranked according to their luminosity based on their effective temperature, as shown below:

Figure 2. HR diagram, independently created by astronomers Ejnar Hertzsprung and Henry Russell around 1910. Source: Wikimedia Commons. THAT.

Stellar evolution lines

The main sequence is the roughly diagonal region that runs through the center of the diagram. There, at some point, the newly formed stars enter, according to their mass.

The hottest, brightest, and most massive stars are at the top and to the left, while the coolest and smallest stars are at the bottom right.

Mass is the parameter that governs stellar evolution, as has been said several times. Indeed, very massive stars use up their fuel quickly, while small, cool stars, such as red dwarfs, manage it more slowly.

Figure 3. Comparison of sizes between planets (1 and 2) and stars (3,4,5 and 6). Source: Wikimedia Commons. Dave Jarvis (https://dave.autonoma.ca/).

For a human being, red dwarfs are practically eternal, no known red dwarfs have died yet.

Adjacent to the main sequence are the stars that, due to their evolution, have moved to other lines. Thus above are the giant and supergiant stars, and below the white dwarfs.

Spectral types

What comes to us from distant stars is their light, and from its analysis a great deal of information is obtained about the nature of the star. At the bottom of the HR diagram is a series of letters denoting the most common spectral types:

OBAFGKM

The stars with the highest temperature are O and the coldest are class M. In turn, each of these categories is divided into ten different subtypes, differentiating them by a number from 0 to 9. For example, F5, an intermediate star between F0 and G0.

Morgan Keenan's classification adds the luminosity of the star to the spectral type, with Roman numerals from I to V. In this way, our Sun is a G2V-type star. It should be noted that given the great variability of stars, there are other classifications for them.

Each spectral class has an apparent color, according to the HR diagram in the figure. It is the approximate color that an observer without instruments or at most binoculars would see on a very dark and clear night.

Here is a brief description of its characteristics according to the classical spectral types:

Type O

They are blue stars with violet hues. They are found in the upper left corner of the HR diagram, that is, they are large and bright, as well as high surface temperatures, between 40,000 and 20,000 K.

Examples of this type of star are Alnitak A, from the belt of the constellation Orion, visible during the northern winter nights, and Sigma-Orionis in the same constellation.

Figure 4. The three stars of Orion's Belt. From left to right Alnitak, Alnilam and Mintaka. In addition, next to Alnitak, the Flame and Horsehead nebulae. Source: Wikimedia Commons.

Type B

They are easy to see with the naked eye. Its color is white-blue, with surface temperatures between 10,000 -7000 K. Sirius A, a binary star in the constellation Canis Major is a type A star, as is Deneb, the brightest star in the Swan.

Type F

They look white tending to yellow, the surface temperature is even lower than those of the previous type: between 7000 and 6000 K. The polar star Polaris, from the constellation Ursa Minor belongs to this category, as well as Canopus, the brightest star of the constellation Carina, visible far to the south of the northern hemisphere, during the northern winter.

Type G

They are yellow and their temperatures are between 6000 and 4800 K. Our Sun falls into this category.

K type

In principle, it is not easy to find out the internal structure of a star, since most of them are very distant objects.

Thanks to the study of the Sun, the closest star, we know that most stars are made up of gaseous layers with spherical symmetry, in the center of which there is a nucleus where fusion takes place. This occupies more or less 15% of the total volume of the star.

Surrounding the core there is a layer like a mantle or envelope and finally there is the atmosphere of the star, whose surface is considered its outer limit. The nature of these layers changes with time and the evolution followed by the star.

In some cases, at a point where hydrogen, its main nuclear fuel, runs out, the star swells and then expels its outermost layers into space, forming what is known as a planetary nebula, in the center of which the bare core remains., hereinafter known as a white dwarf.

It is precisely in the envelope of the star, where the transport of energy from the core to the outer layers takes place.

Figure 5. The layers of the Sun, the most studied star of all. Source: Wikimedia Commons.

Types of stars

In the section devoted to spectral types, the types of stars that are currently known have been mentioned very generally. This in terms of the characteristics discovered through the analysis of its light.

But throughout their evolution, most of the stars travel on the main sequence and also leave it, locating in other branches. Only red dwarf stars remain in the main sequence their entire lives.

There are other types of stars that are frequently mentioned, which we briefly describe:

Dwarf stars

It is a term used to describe very different types of stars, which on the other hand have their small size in common. Some stars are formed with very low mass, but others that were born with much higher mass instead become dwarfs during their lifetime.

In fact, dwarf stars are the most abundant kind of star in the universe, so it is worth dwelling a bit on their characteristics:

Brown dwarfs

They are protostars whose mass was not enough to start the nuclear reactor that propels a star to the main sequence. They can be considered to be halfway between a gas giant planet like Jupiter and a red dwarf star.

Since they lack a stable energy source, they are destined to cool slowly. An example of a brown dwarf is Luhman 16 in the constellation Vela. But this does not prevent the planets from orbiting them, since several have been discovered so far.

Red dwarfs

Figure 6. Comparative size between the Sun, the red dwarf Gliese 229A, the brown dwarfs Teide 1 and Gliese 229 B and the planet Jupiter. Source: NASA via Wikimedia Commons.

Their mass is small, less than that of the Sun, but their life passes in the main sequence because they carefully spend their fuel. For this reason they are also colder, but they are the most abundant type of star and also the longest of all.

White dwarfs

It is the remnant of a star that left the main sequence when the fuel in its core ran out, swelling until it became a red giant. After this, the star sheds its outer layers, reducing its size and leaving only the core, which is the white dwarf.

The white dwarf stage is only one phase in the evolution of all stars that are neither red dwarfs nor blue giants. The latter, being so massive, tend to end their life in colossal explosions called nova or supernova.

The star IK Pegasi is an example of a white dwarf, a fate that may await our Sun many millions of years from now.

Blue dwarfs

They are hypothetical stars, that is, their existence has not yet been proven. But it is believed that red dwarfs eventually transform into blue dwarfs when they run out of fuel.

Black dwarfs

They are ancient white dwarfs that have completely cooled down and no longer emit light.

Yellow and orange dwarfs

Stars with a mass comparable to or less than that of the Sun, but larger in size and temperature than red dwarfs, are sometimes called this way.

Neutron stars

This is the last stage in the life of a supergiant star, when it has already used up its nuclear fuel and suffers a supernova explosion. Due to the explosion, the core of the remnant star becomes incredibly compact, to the point that electrons and protons fuse to become neutrons.

A neutron star is so, but so dense, that it can contain up to twice the solar mass in a sphere about 10 km in diameter. Since its radius has decreased so much, conservation of angular momentum demands a higher speed of rotation.

Due to their size, they are detected by the intense radiation they emit in the form of a beam that rotates rapidly next to the star, forming what is known as a pulsar.

Examples of stars

Although stars have characteristics in common, as with living things, the variability is enormous. As has been seen, there are giant and supergiant stars, dwarfs, neutrons, variables, of great mass, of enormous size, closer and more distant:

-The brightest star in the night sky is Sirius, in the constellation Canis Major.

Figure 7. Sirius, in the constellation Canis Major, about 8 light years away, is the brightest star in the night sky. Source: Pixabay.

-Próxima Centauri is the closest star to the Sun.

-Being the brightest star does not mean being the most luminous, because distance counts for a lot. The most luminous star known is also the most massive: R136a1 belonging to the Large Magellanic Cloud.

-The mass of R136a1 is 265 times the mass of the Sun.

-The star with the greatest mass is not always the largest. The largest star to date is UY Scuti in the constellation Shield. Its radius is about 1708 times larger than the radius of the Sun (the radius of the Sun is 6.96 x 108 meters).

-The fastest star so far had been US 708, which moves at 1200 km / s, but recently another star was discovered that surpasses it: S5-HVS1 of the constellation Crane, with a speed of 1700 km / s. The culprit is believed to be the supermassive black hole Sagittarius A, in the center of the Milky Way.

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