BETELGEUSE: CHARACTERISTICS, FORMATION, STRUCTURE AND COMPOSITION - ARTE - 2025

Betelgeuse is the alpha star of the constellation Orion, which is why it is also called alpha Orionis. It is a star of the red supergiant type, the largest volume stars, but not necessarily the most massive.

Despite being the alpha star of Orion, Betelgeuse at first glance is not the brightest in the constellation, since Rigel -beta Orionis- is the one that stands out the most. However, in the infrared and near red spectrum, Betelgeuse is the brightest, a fact that is directly related to its surface temperature.

Figure 1. Constellation Orion and four of its main stars, including Betelgeuse. Source: Pixabay.

This star was surely observed from ancient times by the first humans, due to its great luminosity. In order of brightness, it is usually the tenth brightest in the night sky and, as we have said, the second in brightness in the constellation Orion.

Chinese astronomers in the 1st century described Betelgeuse as a yellow star. But other observers like Ptolemy referred to it as orange or reddish. Much later, during the 19th century, John Herschel observed that its brightness is variable.

What happens is that all stars evolve, that is why their color changes over time, as it expels gas and dust from the most superficial layers. This also alters its luminosity.

General characteristics

Betelgeuse is the characteristic example of a red supergiant star, which are characterized by having a spectral type K or M and luminosity type I.

They are low temperature stars; In the case of Betelgeuse, it is calculated that it is around 3000 K. Temperature and color are related, for example a piece of hot iron is red hot, but if the temperature increases it turns white.

Despite being only 8 million years old, Betelgeuse has rapidly evolved out of the main sequence, because its nuclear fuel has been depleted and swelled to its present dimensions.

These giant stars also have a variable luminosity. In recent years its brightness has diminished, which has worried the scientific community, although it has recently been recovering.

Here are its main characteristics:

- Distance: Between 500 to 780 light years.

- Mass: Between 17 to 25 solar masses.

- Radius: Between 890 to 960 solar radii.

- Brightness: Between 90,000 to 150,000 solar brightness.

- State of evolution: red supergiant.

- Apparent magnitude: +0.5 (visible) -3.0 (infrared J band) -4.05 (infrared K band).

- Age: Between 8 and 10 million years.

- Radial speed: +21.0 km / s

Betelgeuse belongs to the spectral class M, which means that the temperature of its photosphere is relatively low. It is classified as type M1-2 Ia-ab.

In the Yerkes diagram of spectral classification, the suffix Ia-ab means that it is a supergiant of intermediate luminosity. The luminous spectrum of Betelgeuse is used as a reference for the classification of other stars.

Betelgeuse's diameter is estimated to be between 860 to 910 million kilometers and it was the first star whose diameter was measured by interferometry. This diameter is comparable to that of Jupiter's orbit, however it is not the largest of the red supergiants.

Despite its large size, it is only 10 to 20 times more massive than our Sun. But its mass is large enough for its stellar evolution to be rapid, since the lifetime of a star is the inverse of the square of its mass.

Formation and evolution

Betelgeuse, like all stars, began as a huge cloud of hydrogen gas, helium and cosmic dust with other chemical elements, which condensed around a central point and increased its mass density.

There is evidence that this is the case in forming star clusters, usually located within nebulae composed of cold, sparse interstellar matter.

Figure 2. IC396 nebula with numerous stars in the formation stage. The image was taken in infrared, since the visible spectrum is absorbed by the nebula. Source: NASA / Spitzer.

The formation of a star, its life and its death, is an eternal combat between:

• The gravitational attraction, which tends to condense all matter at one point and
• The individual kinetic energy of each particle, which together exerts the pressure necessary to escape and expand from the point of attraction.

As the original cloud shrinks toward the center, a protostar forms that begins to emit radiation.

The gravitational attraction causes the atomic nuclei to acquire kinetic energy, but when stopped in the densest center of the protostar, they emit electromagnetic radiation and thus begin to shine.

When the point is reached where the hydrogen nuclei are so tightly packed and acquire enough kinetic energy to overcome electrostatic repulsion, the strong attractive force begins to act. Then the fusion of the nuclei occurs.

In nuclear fusion of hydrogen nuclei, helium and neutron nuclei are formed, with huge amounts of kinetic energy and electromagnetic radiation. This is due to the loss of mass in the nuclear reaction.

This is the mechanism that counteracts the gravitational compression of a star, through kinetic pressure and radiation pressure. As long as the star is in this equilibrium, it is said to be in the main sequence.

The red giant stage

The process described above does not last forever, at least for very massive stars, since as hydrogen is converted to helium, the fuel is depleted.

In this way, the pressure that counteracts the gravitational collapse is decreasing and therefore the core of the star compacts, at the same time that the outer layer expands and part of the particles, the most energetic, escape into space, forming a dust cloud surrounding the star.

When this happens the state of red giant has been reached and this is the case of Betelgeuse.

Figure 3. Betelgeuse, a red supergiant the size of 800 suns to 130 parsecs in the constellation Orion, shows its stellar disk. (Source: HST).

In stellar evolution, the mass of the star defines the time of life and death.

A supergiant like Betelgeuse has a short life span, passing through the main sequence very quickly, while the less massive red dwarfs shine modestly for millions of years.

Betelgeuse is estimated to be 10 million years old and is considered to be in the final stages of its evolutionary cycle. It is thought that in 100,000 years or so, its life cycle will end with a large supernova explosion.

Structure and composition

Betelgeuse has a dense core surrounded by a mantle and an atmosphere, which is 4.5 times the diameter of Earth's orbit. But in 2011 it was discovered that the star is surrounded by a vast nebula of material originating from itself.

The nebula that surrounds Betelgeuse extends 60 billion kilometers from the surface of the star, this is 400 times the orbital radius of the Earth.

In their final stages, the red giants expel material into surrounding space, an enormous amount in a relatively short time. Betelgeuse is estimated to shed the equivalent of the Sun's mass in just 10,000 years. This is just an instant in stellar time.

Below is an image of the star and its nebula, obtained with the VLT telescope located at Cerro Paranal, Antofagasta, Chile by ESO (European Organization for Astronomical Research in the Southern Hemisphere).

In the figure, the central red circle is properly the star Betelgeuse, with a diameter of four and a half times the Earth's orbit. Then the black disk corresponds to a very bright area that was masked to allow us to see the nebula that surrounds the star, which, as has been said, extends up to 400 times the Earth's orbital radius.

This image was taken in the infrared range and colored so that the different regions can be visible. Blue corresponds to the shortest wavelengths and red to the longest.

Figure 4. The small red circle in the center is the star Betelgeuse, the black circle is the masking of an extremely bright area. Around the black circle you can see the nebula composed of the material ejected by the star. (Source: ESO-VLT)

The elements present in Betelgeuse

Like all stars, Betelgeuse is composed mainly of hydrogen and helium. However, as it is a star in its final phases, inside it begins to synthesize other heavier elements from the periodic table.

Observations of the nebula surrounding Betelgeuse, made up of material thrown by the star, indicate the presence of silica dust and alumina. This material is what makes up most of the rocky planets, like Earth.

This tells us that millions of stars similar to Betelgeuse existed in the past, providing the material that made up the rocky planets in our solar system, including Earth.

Betelgeuse attenuation

In recent times Betelgeuse is news in the international press, since at the beginning of October 2019 its light began to dim appreciably, in just a few months.

For example, for January 2020 its brightness decreased by a factor of 2.5. However, by February 22, 2020, it stopped dimming and began to regain its brightness.

This refers to the visible spectrum, however in the infrared spectrum its brightness has remained fairly stable over the last 50 years, which leads astronomers to think that it is not a light variation like the one that occurs in the stages leading up to a supernova explosion.

On the contrary, it is about the absorption and dispersion of the visible band of the electromagnetic spectrum, due to the dust cloud that the star itself has expelled.

This dust cloud is transparent to infrared, but not to the visible spectrum. Apparently the thick dust cloud surrounding the star is rapidly moving away from it, so the shoulder of Orion, the mythological hunter, will surely remain in the sky for much longer.

References

1. Astronoo. Betelgeuse. Recovered from: astronoo.com.
2. Pasachoff, J. 2007. The Cosmos: Astronomy in the New Millenium. Third Edition. Thomson-Brooks / Cole.
3. Seeds, M. 2011. Foundations of Astronomy. Seventh Edition. Cengage Learning.
4. Open window. Mass-luminosity relationship. Recovered from: media4.obspm.fr
5. Wikipedia. Betelgeuse. Recovered from: es.wikipedia.com
6. Wikipedia. Orion OB1 stellar association. Recovered from: es.wikipedia.com