Do you know how a star is born? Stars are born inside large clouds of gas and dust. Within these formations, turbulences occur that create massive clumps of matter, which eventually collapse in on themselves. With the collapse, the material begins to be heated and forms a protostar that, one day, will carry out nuclear fusion and then it will be able to be considered a star itself.
Stars are the most widely known astronomical objects. After all, ancient peoples already used the constellations as a reference to follow the passage of time and the seasons. Even today, constellations continue to serve as navigational tools.
In addition to the beauty they bring to night skies far from light pollution, stars represent the most fundamental building blocks of galaxies, aiding in the production and distribution of heavy elements across the universe. By analyzing the age, distribution and composition of a galaxy’s stars, astronomers can understand its history, dynamics and evolution.
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What is a star?
Before understanding how stars are born, it is important to first know what a star is. In popular parlance, we can say that stars are the bright dots that seem to twinkle in the night sky. A scientific explanation is that stars are distant, massive and bright objects, with enough gravitational pressure to carry out the nuclear fusion of the elements in their interior. It is this fusion that makes the star “burn”, generating its brightness.
The smallest object we know is capable of doing this is about 10% of the mass of the Sun — equivalent to 333,000 planets Earth. It is impossible to know exactly how many stars there are out there, but current estimates indicate that the Milky Way alone has at least 300 billion of them, of the most diverse types.
Some types of stars
There are the so-called red dwarfs, which, as the name implies, are small and red, this being the smallest and coldest class of stars. They perform weak hydrogen burns in their interior and emit radiation mainly in the infrared part of the electromagnetic spectrum, being considered the most common in our galaxy. As they are small and not very bright, they are invisible to the naked eye.
Another type are yellow dwarfs, stars of average mass and brightness — the Sun is a yellow dwarf, by the way. These stars emit radiation along the visible light spectrum and are therefore white in color. This is also true of our Sun, which, despite appearing yellow in our daytime sky, is actually white.
But there are also giant stars. This is a category that includes extremely massive stars, and the luminosity of supergiants can range from 10,000 to millions of times that of the Sun. As they emit huge amounts of energy, these stars are bluish. This is because the radiation they give off passes through the entire ultraviolet part of the electromagnetic spectrum, and a tiny bit of that radiation makes its way to the blue end of visible light.
How do stars form?
Stars are born inside large clouds of gas and dust. Over time, gravity gives rise to dense clumps of matter there, and these clumps collapse in on themselves as a result of the gravitational pull they exert.
Computer models show that these collapsing clouds of gas and dust can split into two or three pieces — this would explain why most stars in our galaxy appear in pairs or groups, accompanied by many neighbors.
With the collapse, the material at the center of the cluster begins to heat up, forming a protostar; the rest of the matter can give rise to other objects, but it can also form nothing else, continuing to exist as dust. Little by little, what was a “baby star” enters its “adolescence”, with stellar winds and radiation expelling the layer of gas and dust that was left over.
When this envelope is cleared, we say that the star has entered the T Tauri phase, a brief stage in its evolutionary process. A few more million years later, the temperature of the star’s core will reach about 15 million degrees Celsius, starting the fusion of hydrogen atoms into helium. At this time, the star entered the so-called main sequence, the longest phase of its life.
Fusion releases immense amounts of energy in an opposite process, but similar to that of detonating nuclear bombs. Most of the Milky Way’s stars are on the main sequence, proceeding smoothly with nuclear fusion in their interiors. For example, the Sun is about 4.6 billion years old and is in this phase, and is expected to remain there for a few more billion years.
Stars that are born in groups
Star clusters are formed by groups of stars with a single origin, which are gravitationally bound together for the same period of time.
There are two types of star clusters: open clusters, formed by stars that came from the same molecular cloud; and globular clusters, which group thousands to millions of stars in spherical systems.
They are also home to some of the oldest stars in their galaxies — so much so that most of those present in the most massive clusters formed during the “infancy” of the universe, about 13 billion years ago. They kept their structures, but the stars that form them evolved over time. Therefore, they serve as physical reminders of the earliest stages of star formation.
Observations from the Hubble telescope have shown some discrete but existing differences in the members of the globular clusters. There are chemical variations and even evidence that they harbor multiple generations of stars. Due to the intense gravitational pull, globular clusters are more stable, which allows them to survive for billions of years.
With Hubble, astronomers were able to observe star clusters of different sizes, using spectroscopy (analysis and interpretation of the electromagnetic spectrum of objects) to determine their chemical composition. Thanks to the precision of the observations, scientists have been using the telescope to determine the luminosity and temperature of these stars, refining the knowledge of how they are born and evolve.
How do stars die?
Generally speaking, the bigger the star, the shorter its life, which ends when it runs out of hydrogen in its core. When that happens, nuclear reactions stop happening; consequently, without the energy production process that sustains it, the star’s core begins to collapse on itself, getting higher and higher temperatures.
What happens from there depends on how massive the star in question is. In the case of stars like the Sun, this increasingly hot core will push the outer layers of the star outwards, causing it to inflate like a balloon and become a red giant. This is the fate of our star in a few billion years. When that happens, the rocky planets of the Solar System will be engulfed by the expanded layers of the Sun — including Earth.
In this stage of stellar “death”, there are different destinations. If the star is “average”, like the Sun, it will continue to shed its layers until only the core is exposed, forming a white dwarf, which will gradually cool down. But in the case of stars much more massive than the Sun, the fate is not expansion followed by cooling: stars with at least eight solar masses explode in supernovae when they are at the end of their lives.
And if the dying core at the heart of the supernova is three to five solar masses, the process of collapse continues and it could be that this generates a neutron star, an extremely dense object with immense gravitational pull. If the star’s collapsed core is more than five solar masses, it can collapse completely, giving rise to a black hole.