How Are Stars Formed? (Part1)

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19 Jan 2024

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Similar to human life, stars are born, live and die.

Stars are the brightest and most magnificent objects that guide us in our efforts to understand and make sense of the universe. We know this because most of the data we have so far about the universe comes from stars and their characteristics. Therefore, there is no doubt that the most important components in the universe are stars.
Luckily, there are quite a few stars we can examine. So exactly how many stars are there in the universe? In Cosmos, one of the most famous television programs of the 1980s, American astronomer Carl Sagan made the following claim; “There are more stars in the universe than grains of sand on all the beaches in the world.” As a result of the research, this claim of Carl Sagan turned out to be true. The number of stars in the universe is 1 number with 22 zeros.

So what do we know about stars? Did they exist in the first seconds of the universe, or did they form much later than the Big Bang? What do the elements inside tell us? Does every star turn into a black hole when it dies? In this series of articles, we will start from the basic components of stars, look at their life cycle and death, different star types and classification conditions. Sit back, you are about to embark on a journey into the magnificent and mysterious structure of the brightest objects in the universe.

A Star Contains Plenty of Hydrogen and Helium

If it has ever occurred to you that the elements found in stars may be similar to the elements that make up the earth, you are not alone. Because in 1923, this was the common opinion of astronomers. The reason for this was the analysis result of Fraunhofer Lines. These lines are dark lines in stellar spectra caused by elements in the stellar atmospheres absorbing light. When scientists examined these lines, they saw strong lines that correspond to elements such as oxygen, carbon, magnesium, sodium, hydrogen, iron and helium found on Earth. So was this assumption really true? British astronomer Cecilia Payne would give the answer to this question.

Cecilia Payne was a professor of astronomy at Harvard University in 1956 and the first female professor at Harvard. (Source:Scientific Women)

Payne, a graduate student, decided to analyze stellar spectral photographs after coming to Harvard College Observatory. He also wanted to study the connection between stellar spectra and temperatures. Additionally, the pattern of absorption lines seemed to vary across different classes of stars, so he wanted to find out what chemical differences there were in these classes of stars. Indeed, Payne proved that there is a connection between the spectral classes of stars and their surface temperatures. The diversity of absorption lines between stellar spectra was not due to the abundance of chemical elements, but was due to the amount of ionization. Payne then used his knowledge of atomic physics to determine the ratios of 18 elements in the star's spectrum. The result he saw was quite different. He saw that helium and hydrogen were found in much greater amounts in stars than on Earth. In fact, almost the entire composition of the star consisted of these two elements.

If we exclude dark matter, hydrogen and helium make up 98% of the matter in the Universe. (Source:Clear Science)

Payne's discoveries were revolutionary. There were three reasons for this:

He found that many stars are chemically similar.
He demonstrated how to determine the temperature of a star based on its spectrum.
He showed that the dominant elements in the universe were hydrogen and helium, a result that was a key step towards the Big Bang theory.

As can be seen, the components of the stars do not have similar properties to the components of the earth. The most accurate and definitive judgment that can be made for stars is that they consist largely of hydrogen and helium. When stars form, approximately 70% of their mass is hydrogen, 28% is helium, and the rest is heavy elements.

The Formation Process of Stars

The formation of stars is perhaps one of the most magical and mysterious moments in the universe. As physicist Heinz Ragels said in 1985: “Star birth is a veiled and secret event.” So how is a star born?

Star birth begins in huge clouds of gas and dust spread over a large area in the universe. These clouds consist largely of hydrogen and are also called molecular clouds.
Over time, an event occurs that disturbs this huge cloud. This could be a shock wave from a nearby supernova or the gravitational field of a passing comet. This event affects the cloud and the cloud begins to rotate and compress.
As the atoms in the cloud begin to rotate, they get closer to each other, and this approach creates frictional heat. Temperature and density increase. With the resulting pressure, atoms want to reach the center of the mass, that is, the nucleus.
Now the intensity is at its peak. As the gas is compressed into a smaller and smaller volume, the pressure and temperature gradually increase. As the object continues to collapse, the pressure force tries to resist this collapse and, unlike other forces, applies a force towards the outside of the core.
Eventually, the pressure force against the gravitational force reaches a balance and the object does not continue to collapse. We call this balance hydrostatic balance.
If you noticed, I said 'object' because what is formed cannot be called a star yet. In order for the object to become a star, it must convert the hydrogen in its core into helium. That is, the temperature in its core must be high enough to initiate nuclear fusion of hydrogen.
The object fuses hydrogen with deuterium, which we call heavy hydrogen, in its nucleus, and when deuterium continues to fuse, helium is formed. With this event, a star is born.
The process of creating new elements that occurs in the nucleus is called stellar nucleosynthesis. The fusion of hydrogen into helium is the key point for a star because the birth of the star begins with nuclear fusion. With this fusion, the object has now become a main sequence star.