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A star is simply a thermonuclear explosion in very, very slow motion. But it is towards the end of their lives that stars reach an almost always impressive maturity, and some a more than spectacular death, releasing billions of tons of matter in a swan song that reaches the ends of the Universe. A star like ours burns hydrogen in the core, converting it into helium which, being heavier than the hydrogen in the rest of the core, accumulates in the center.

When the percentage of hydrogen in the core drops below a certain concentration, the star has been poisoned by helium (that’s a term astronomers use, really) and with no hydrogen left to use as fuel, it begins to compress under weight. of its outer layers. From here begins the process of birth of a red giant, and they become truly gigantic. For example, the closest red giant star to our System, Gamma Crucis, Gacrux (which literally means Southern Cross, like the constellation it names and of which it is the third brightest). Although it is only 30% more massive than the Sun, it has grown to more than 80 times its radius. Or Alpha Tauri, Aldebaran, the brightest star in the constellation of the Bull, thirteenth in luminosity in the celestial vault and the first red giant that always comes to mind. Aldebaran is far from even having two solar masses, but it is 420 times larger than the star we see every day (although not very bright orange, it must be said).

Our Sun, being a main sequence star of a certain mass, is assured of a future as a red giant, eventually. Many measurements have been made by different teams of solar physicists – specialists in the Sun – who today agree that in about 5 or 6 billion years, the star will see most of its hydrogen consumed to burn, and will start to become a full-fledged red giant. With some calm, since it is estimated that the process lasts about 500 or 600 million years, which is not long in astronomical terms either; by the time this happens the Sun’s mass will have increased to about 260 times its current size, and it will be more than 2,600 times more luminous. Of course, our planet and everything that inhabits it at that time will be consumed in what will be part of the Sun’s corona, Venus and Mercury will go with us. Nothing stands in the way of a star.

From here, a red giant has two paths each depending on its mass, and the critical number here is 2.5 solar masses. For the Sun, once the hydrogen fusion phase is complete, it will begin to fire enormous amounts of matter into the universe in the form of pulsations. Until it cools down and shrinks enough to become a white dwarf. A more massive star, however, has a longer and brighter future depending on its size; red giants have a limit of 8-9 solar masses according to the most recent consensus. A star of this magnitude will go through a process of billions of years in which it will change its size a few times, gradually expanding and compressing, until it ignites the helium resulting from the fusion of hydrogen, which will serve as fuel for at least a billion of years to continue enjoying its status as a red giant, growing larger and larger until its outer layer abandons it by sheer lack of gravity, leaving behind a super dense metallic core the size of the Earth, floating in the Universe and cooling itself, forever.

Ramon Martinez Leyva


a pale blue dot

He is a Computer Systems Engineer. His areas of knowledge are technologies, science and the environment.

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