Some stars become red giants as their nuclear fuel gradually runs out, causing an expansion of their outer envelope. This makes them shine in red and significantly increase their size.
Stars are born from massive clouds of gas and dust called nebulae. Under the influence of gravity, these clouds contract, heat up, and trigger nuclear fusion: this is the main sequence phase, the star's calm period where it burns its hydrogen. Then, when the hydrogen at its core begins to deplete, the star leaves its equilibrium and gradually transforms into a red giant. At this stage, it expands enormously and can become hundreds of times larger than the Sun. Finally, after expelling its outer layers, the star eventually collapses and becomes either a white dwarf, or, if it is really massive, it can explode in a supernova to become a neutron star or even a black hole.
A star spends most of its life burning hydrogen in its core. But when this hydrogen begins to run low, nuclear reactions slow down and the star loses stability. Its core then contracts under gravity and its temperature increases sharply, allowing the outer layers to expand enormously. It is this extreme expansion that transforms the star into a red giant, much larger but cooler on the surface than before. At this stage, the star is no longer primarily fueled by the fusion of hydrogen, but starts burning helium, thus creating new chemical elements.
Red giants are primarily distinguished by two elements: their enormous size and their reddish color. These stars become truly gigantic when they leave the main sequence—some even reach several hundred times the radius of the Sun. Even though they grow tremendously, their surface temperature strangely becomes cooler, hovering around 3000 to 4000 degrees Celsius. That’s why they exhibit this characteristic reddish-orange hue.
Another astonishing fact: despite their enormous volume, they do not necessarily have a very high mass. Their material indeed dilutes impressively, which gives red giants an extremely low density. So much so that if some of them could be placed in the Sun's position, they would easily engulf everything up to the orbit of Mars!
The red giant phase begins when the star finally exhausts all of its usable hydrogen in its core. Without hydrogen at the center, nuclear reactions move to an outer layer around the core. There, hydrogen continues to fuse into helium, but in the form of a burning shell around the inactive core. This shell fusion releases an enormous amount of energy, causing the star to swell to enormous sizes, sometimes a hundred times its original size or even more. During this phase, the core compresses and heats up enough to finally trigger helium fusion, this time producing heavier elements like carbon and oxygen. These new nuclear reactions become essential for maintaining the temporary balance of the star. They push the outer layers outward, thereby delaying the star's end of life for several million additional years. Without these nuclear reactions specific to red giants, average stars like our Sun would collapse much faster, without going through this spectacular stage.
Did you know that the red color of red giants primarily comes from their cooler surface temperature (around 3,000°C) compared to blue or white stars (up to 30,000°C), making their light distinctly reddish to the human eye?
As they become red giants, stars change their type of nuclear reaction: they begin to fuse helium into heavier elements such as carbon and oxygen, thereby altering their internal structure.
The diameter of a red giant can reach up to several hundred times that of the current Sun. For example, Betelgeuse, a famous red giant, has a diameter about 700 times greater than that of the Sun!
A red giant typically loses large amounts of matter into space, thereby contributing to the formation of gas-rich and dusty nebulae that are conducive to the birth of future stars.
Yes, these two categories of stars are different. The red giant is a medium-mass star that has reached the end of its life, like our future Sun. The red supergiant, on the other hand, is a very massive star, much larger and brighter, typically destined to end its life in a supernova.
Red giants appear red because their surface temperature is relatively low compared to other stars, ranging from about 3000 to 4500 °C. This lower temperature promotes the emission of light in the red wavelengths of the visible spectrum.
The duration of the red giant phase varies depending on the initial mass of the star. For a star similar to the Sun, this stage typically lasts several hundred million years. More massive stars go through this phase more quickly, over much shorter periods.
Yes, the Sun will become a red giant in about 5 billion years. At that stage, it will expand to engulf the orbits of Mercury and probably Venus, radically transforming the solar system.
A red giant is a star at the end of its life that has exhausted the hydrogen in its core. Lacking nuclear fuel, its core contracts while its outer layers expand significantly, giving it a reddish color and a considerably larger size.
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