RED DWARF: DISCOVERY, CHARACTERISTICS, EVOLUTION, COMPOSITION - ARTE - 2024

A red dwarf is a small, cool star whose mass is between 0.08 and 0.8 times the mass of the Sun. They are the most abundant and longest-lived stars in the universe: up to three-quarters of all known so far. Due to their low luminosity, they are not observable with the naked eye, despite being numerous in the vicinity of the Sun: of 30 nearby stars, 20 are red dwarfs.

The most notable for its proximity to us is Proxima Centauri, in the constellation Centaurus, 4.2 light years away. It was discovered in 1915 by the Scottish astronomer Robert Innes (1861-1933).

Figure 1. The red dwarf Proxima Centauri is part of the Alpha Centauri star system in the constellation Centauri. Source: ESA / Hubble & NASA via Wikimedia Commons.

However, before Proxima Centauri was discovered, the telescope of the French astronomer Joseph de Lalande (1732-1802) had already found the red dwarf Lalande 21185, in the constellation Ursa Major.

The term "red dwarf" is used to refer to various classes of stars, including those with spectral types K and M, as well as brown dwarfs, stars that are not really such, because they never had enough mass to start their reactor internal.

The spectral types correspond to the surface temperature of the star, and its light breaks down into a series of very characteristic lines.

For example, the spectral type K has between 5000 and 3500 K of temperature and corresponds to yellow-orange stars, while the temperature of the type M is less than 3500 K and they are red stars.

Our Sun is spectral type G, yellow in color and has a surface temperature between 5000 and 6000 K. Stars with a certain spectral type have many characteristics in common, the most determining of them being mass. According to the mass of a star, so will its evolution.

Characteristics of red dwarfs

Red dwarfs have certain characteristics that differentiate them. We have already mentioned some at the beginning:

-Little size.

-Low surface temperature.

-Low rate of material combustion.

-Low luminosity.

Mass

Mass, as we have said, is the main attribute that defines the category that a star reaches. Red dwarfs are so abundant because more low-mass stars are formed than massive stars.

But curiously, the time it takes for low-mass stars to form is longer than for very massive stars. These grow much faster because the force of gravity that compacts the matter in the center is greater, the more mass there is.

And we know that a certain amount of critical mass is required for the temperature to be appropriate, in order to initiate fusion reactions. In this way the star begins its adult life.

The Sun took tens of millions of years to form, but a star 5 times larger requires less than a million years, while the most massive ones can begin to shine in hundreds of thousands.

Temperature

The temperature of the surface is, as already mentioned, another important characteristic that defines red dwarfs. It should be less than 5000K, but not less than 2000K, otherwise it is too cool to be a true star.

Stellar objects with a temperature lower than 2000 K cannot have a fusion nucleus and they are aborted stars that never reached critical mass: brown dwarfs.

Deeper analysis of spectral lines can ensure the difference between red dwarf and brown dwarf. For example, evidence of lithium suggests that it is a red dwarf, but if it is methane or ammonia it is probably a brown dwarf.

Spectral types and Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram (HR diagram) is a graph that shows the characteristics and evolution of a star according to its spectral characteristics. This includes the temperature of the surface, which as we have said is a determining factor, as well as its luminosity.

The variables that make up the graph are luminosity on the vertical axis and effective temperature on the horizontal axis. It was created independently in the early 1900s by astronomers Ejnar Hertzsprung and Henry Russell.

Figure 2. HR diagram showing red dwarfs in the main sequence, in the lower right corner. Source: Wikimedia Commons. THAT.

According to their spectrum, the stars are grouped according to the Harvard spectral classification, indicating the temperature of the star in the following sequence of letters:

OBAFGKM

We start with the hottest stars, type O, while the coldest are type M. In the figure, the spectral types are at the bottom of the graph, on the blue colored bar on the left until reaching the red on the right.

Within each type there are variations, since the spectral lines have different intensity, then each type is divided into 10 subcategories, denoted by numbers from 0 to 9. The lower the number, the hotter the star. For example the Sun is type G2 and Proxima Centauri is M6.

The central region of the graph, which runs roughly diagonally, is called the main sequence. Most of the stars are there, but their evolution can lead them to leave and place themselves in other categories, such as a red giant or white dwarf. It all depends on the mass of the star.

The life of red dwarfs always takes place in the main sequence, and as for the spectral type, not all M-class dwarfs are red dwarfs, although most are. But in this class there are also supergiant stars such as Betelgeuse and Antares (top right of the HR diagram).

Evolution

The life of any star begins with the collapse of interstellar matter thanks to the action of gravity. As matter agglutinates, it rotates faster and faster and flattens into a disk, thanks to the conservation of angular momentum. In the center is the protostar, the embryo so to speak of the future star.

As time passes, the temperature and density increase, until a critical mass is reached, in which the fusion reactor begins its activity. This is the energy source of the star in its time to come and requires a core temperature of about 8 million K.

The ignition in the core stabilizes the star, because it compensates for the gravitational force, giving rise to the hydrostatic equilibrium. This requires a mass between 0.01 and 100 times the mass of the Sun. If the mass is greater, overheating would cause a catastrophe that would destroy the protostar.

Figure 3. In a red dwarf, the fusion of hydrogen in the nucleus balances the force of gravity. Source: F. Zapata.

Once the fusion reactor is started and equilibrium is achieved, the stars end up in the main sequence of the HR diagram. Red dwarfs emit energy very slowly, so their hydrogen supply lasts a long time. The way a red dwarf emits energy is through the mechanism of convection.

The energy-producing conversion of hydrogen to helium is carried out in red dwarfs by proton-proton chains, a sequence in which one hydrogen ion fuses with another. Temperature greatly influences the way this fusion takes place.

Once the hydrogen is exhausted, the star's reactor stops working and the slow cooling process begins.

Proton-proton chain

This reaction is very common in stars that have just joined the main sequence, as well as in red dwarfs. It starts like this:

¹ ₁ H + ¹ ₁ H → ² ₁ H + e ⁺ + ν

Where e ⁺ is a positron, identical in everything to the electron, except that its charge is positive and ν is a neutrino, a light and elusive particle. For its part ² ₁ H is deuterium or heavy hydrogen.

Then it happens:

¹ ₁ H + ² ₁ H → ³ ₂ He + γ

In the latter, γ symbolizes a photon. Both reactions occur twice, to result in:

³ ₂ He + ³ ₂ He → ⁴ ₂ He + 2 (¹ ₁ H)

How does the star generate energy by doing this? Well, there is a slight difference in the mass of the reactions, a small loss of mass that is transformed into energy according to Einstein's famous equation:

E = mc ²

As this reaction occurs countless times involving an immense number of particles, the energy that is obtained is enormous. But it is not the only reaction that takes place inside a star, although it is the most frequent in red dwarfs.

Life time of a star

How long a star lives also depends on its mass. The following equation is an estimate of that time:

T = M ^-2.5

Here T is time and M is mass. The use of capital letters is appropriate, due to the time and the enormity of the mass.

A star like the Sun lives for about 10 billion years, but a star 30 times the mass of the Sun lives 30 million years and another even more massive can live for about 2 million years. Either way, it's an eternity for humans.

Red dwarfs live much longer than that, thanks to the parsimony with which they spend their nuclear fuel. In terms of time as we experience it, a red dwarf lasts forever, because the time it takes to deplete the hydrogen from the nucleus exceeds the estimated age of the Universe.

No red dwarfs have died yet, so all that can be speculated about how long they live and what their end will be is due to computer simulations of models created with the information we have about them.

According to these models, scientists predict that when a red dwarf runs out of hydrogen it will transform into a blue dwarf.

No one has ever seen a star of this kind, but as hydrogen wears off, a red dwarf does not expand into a red giant star, as our Sun will one day. It simply increases its radioactivity and with it its surface temperature, turning blue.

Composition of red dwarfs

The composition of the stars is very similar, for the most part they are huge balls of hydrogen and helium. They retain some of the elements that were present in the gas and dust that gave rise to them, so they also contain traces of the elements that the preceding stars helped create.

For this reason, the composition of red dwarfs is similar to that of the Sun, although the spectral lines differ significantly due to temperature. So if a star has weak hydrogen lines, it does not mean that it lacks this element.

In red dwarfs there are traces of other heavier elements, which astronomers call "metals."

In astronomy, this definition does not coincide with what is commonly understood as metal, since it is used here to refer to any element, except hydrogen and helium.

Training

The star formation process is complex and affected by numerous variables. There is much that is still unknown about this process, but it is believed to be the same for all stars, as described in previous segments.

The factor that determines the size and color of a star, associated with its temperature, is the amount of matter that it manages to add thanks to the force of gravity.

A question that worries astronomers and that remains to be elucidated is the fact that red dwarfs contain elements heavier than hydrogen, helium and lithium.

On the one hand, the Big Bang theory predicts that the first stars formed must be composed of only the three lightest elements. However, heavy elements have been detected in red dwarfs.

And if no red dwarfs have died yet, it means that the first red dwarfs that formed must still be out there somewhere, all made up of light elements.

Then the red dwarfs may have formed later, because the presence of heavy elements is required in their creation. Or that there are first-generation red dwarfs, but being so small and with such low luminosity, they have not yet been discovered.

Examples of red dwarfs

Next Centauri

It is 4.2 light years distant and has a mass equivalent to one eighth that of the Sun, but 40 times denser. Proxima has a strong magnetic field, which makes it prone to flare.

Proxima also has at least one known planet: Proxima Centauri b, unveiled in 2016. But it is believed to have been swept away by flares the star frequently emits, so it is unlikely to harbor life, at least not as that we know, since the star's emissions contain X-rays.

Barnard's Star

Figure 4. Comparison of sizes between the Sun, Barnard's star and the planet Jupiter. Source: Wikimedia Commons.

It is a very close red dwarf, 5.9 light years away, whose main characteristic is its great speed, about 90 km / s in the direction of the Sun.

It is visible through telescopes and like Proxima, it is also prone to flares and flares. Recently, a planet was discovered orbiting Barnard's star.

Teegarden Star

This red dwarf of only 8% of the mass of the Sun is in the constellation of Aries and can only be seen with powerful telescopes. It is among the closest stars, at a distance of about 12 light years.

It was discovered in 2002 and in addition to having a remarkable movement of its own, it appears to have planets in the so-called habitable zone.

Wolf 359

It is a variable red dwarf in the constellation of Leo and is almost 8 light years distant from our Sun. Being a variable star, its luminosity increases periodically, although its flares are not as intense as those of Proxima Centauri.

References

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4. Martínez, D. The stellar evolution. Recovered from: Google Books.
5. Taylor, N. Red Dwarfs: The Most Common and Longest-Lived Stars. Recovered from: space.com.
6. Fraknoi, A. The Spectra of Stars (and Brown Dwarfs). Recovered from: phys.libretexts.org.