Stars

We explain what stars are, how they originate and their composition. Also, what are their characteristics and how are they classified.

What are stars?

Stars are massive astronomical objects that emit light and energy. The gravitational force inside stars generates a temperature and pressure so high that they allow the hydrogen atoms that make them up to fuse to form helium, releasing a large amount of energy in the process. The best-known star is the Sun, which provides light and heat to the Earth and the entire solar system.

Humanity has observed the stars since the beginning of civilization, and has tried to see hidden messages or signs from their gods in them. Furthermore, its organization and arrangement in the sky served to make the first calendars, for cartography and for navigation.

It is estimated that in the Milky Way, the galaxy in which the Earth is located, there are approximately 100,000 million stars. There are at least 100 billion galaxies in the observable universe, so the total number of stars is very difficult to calculate.

Star characteristics

The stars have the following characteristics:

• They emit light and heat.
• They are composed of gases such as hydrogen and helium.
• They are grouped into galaxies.
• There are more than 100 billion in the Milky Way.
• They are formed from nebulae.
• The best known is the Sun.

Origin of stars

Stars form from large clouds of gas and dust called nebulae.. When nebulae rich in hydrogen and helium begin to contract due to external forces or disturbances in their structure, they fragment into smaller parts, each with its own concentration of gas and dust.

In the center of each fragment, a protostellar disk forms, that is, a flat structure rotating around a dense core. Matter from the protostellar disk accumulates in the central core as it continues to contract, and the temperature and pressure inside it increase significantly.

When the temperature and pressure reach high enough levels, nuclear fusion begins in the core of the protostellar disk. This process converts hydrogen into helium, which releases a large amount of energy in the form of light and heat. In this way, the birth of a star occurs.

Life cycle of stars

Stars have an extremely long life cycle, lasting billions of years. This cycle goes through the following stages:

1. star formation. Stars emerge from nebulae that begin to contract due to their own gravity. Hydrogen begins to fuse, contraction stops, and helium production begins.
2. Main sequence. The young star begins to fuse all of its hydrogen, building up helium inside until its main fuel, hydrogen, runs out.
3. Main post sequence. Its outer layers expand and cool, and the inner layers contract. The star increases in size, turns reddish due to cooling, and begins to expel large amounts of matter due to the fusion of helium.
4. Final stage. When the helium runs out, carbon fusion begins in the star’s core. By depleting its helium supply, it undergoes a new phase of expansion. The outer layers of the red giant are expelled into outer space in the form of a gaseous envelope, and a hot, dense star called a white dwarf remains in the central core. Although it is small and very hot, it no longer produces nuclear fusion, so it eventually cools down over time.

For more massive stars, the final stage may be different. After becoming a red giant and depleting its helium, a massive star can continue to fuse heavier elements in its core and form layers of different elements until it reaches iron. When you try to fuse iron, you get no net energy and instead absorb it, so you can become a black hole.

Star rating

There are various criteria to classify stars:

• According to its life cycle. It takes into account where the stars are in their lives. From youngest to oldest, they are classified as protostars, main sequence stars, red giants, white dwarfs, black dwarfs or neutron stars (also called black holes).
• Depending on its luminosity and temperature. Their level of brightness and intensity, and their temperature are taken into account. From smallest to largest, they are classified as white dwarfs, subdwarfs, dwarfs (like the Sun), subgiants, giants, luminous giants, supergiants, luminous supergiants or hypergiants.
• According to the color of your light. The type of light in its spectrum is taken into account. They are classified as type O (violet), type B (blue), type A (blue-white), type F (yellowish white), type G (yellow, like the Sun), type K (yellow-orange), type M (red-orange). ).

Composition of the stars

Stars are mostly made of hydrogen (74%) and helium (24%)plus a small percentage of other elements such as iron, oxygen or carbon (2%).

These values ​​are approximate and may vary depending on the methodology used for the measurements and the characteristics of the star. More massive stars tend to have a higher proportion of helium and other heavier elements compared to hydrogen, as they have consumed more hydrogen through nuclear fusion. On the other hand, smaller stars, such as red dwarfs, may have a higher proportion of this element.

Information about the composition of stars is obtained through spectroscopy, a technique that analyzes the light they emit to identify their different elements.

The galaxies

Galaxies are collections of stars, planets, gases, cosmic dust and dark matter bound together by gravitational forces.. These arrays can contain billions of stars.

Our galaxy, the Milky Way, has about 100 billion stars and a diameter of approximately 100,000 light years. However, there are galaxies larger and smaller than ours. There are believed to be at least two trillion galaxies in the observable universe similar to the Milky Way.

Galaxies can be classified according to their shape:

• Elliptical galaxies. They have an ellipse-shaped internal structure, without arms or spiral structures.
• Spiral galaxies. They have a central core from which arms emerge that form a spiral. In the core, there are a large number of stars. A subtype of spiral galaxy is the barred spiral galaxy, which has only two arms, just like the Milky Way.
• Lenticular galaxies. It is estimated that they were spiral galaxies that lost their interstellar matter and their arms, so they only preserve the ellipse-shaped nucleus. They are galaxies that are in a transition state between an elliptical galaxy and a spiral galaxy.
• Irregular galaxies. They are those that do not have a defined configuration.

Age and size of stars

Galaxies are made up of stars of different sizes and ages.. Star HD 140283, popularly known as “Methuselah”, is the oldest star on record. At 13.7 billion years old, it is almost as old as the universe. In contrast, some young stars, such as T Tauri, are in the early stages of their life and have not yet reached their main sequence.

The star EBLM J0555-57Ab is located in the constellation Pisces. It is barely larger than Saturn, making it the smallest star observed so far. Its small size makes it interesting to astronomers, as it offers valuable information about the stellar properties necessary for a star to initiate nuclear fusion.

The star UY Scuti is the largest known. The diameter of this red hypergiant star is estimated to be 1,700 times larger than that of the Sun.

Stargazing

The observation of the stars was key in the development of many sciences and techniquessuch as maritime navigation, terrestrial measurement or, in modern ages, astronomical physics.

Furthermore, since ancient times, humans have observed the stars with a mixture of fascination and mystery. In the endless field of its bright lights, you have tried to see figures, signs and messages from your gods, or clues to the future.

Thus, based on stellar observation, the zodiac has been invented, a celestial wheel in which twelve constellations represent key figures of the Greco-Roman imaginary, whose location at the time of birth would shape the personality and destiny of individuals.

References

• Acosta, MB (2023). Types of stars. EcologyGreen. https://www.ecologiaverde.com/
• AstroMía. (sf) Star rating. https://www.astromia.com/
• Milo, A. (2023) What is a star and how these luminous colossi are formed in the cosmos. National Geographic in Spanish. https://www.ngenespanol.com/