Astronomie et astrophysique 101 : Géante rouge

Une étoile brillante est entourée d’une coquille de gaz ténue dans cette image inhabituelle du télescope spatial Hubble de la NASA/ESA. U Camelopardalis, ou U Cam en abrégé, est une étoile en fin de vie. Comme il commence à manquer de carburant, il devient instable. Tous les quelques milliers d’années, il crache une coquille de gaz presque sphérique alors qu’une couche d’hélium autour de son noyau commence à fusionner. Le gaz éjecté lors de la dernière éruption de l’étoile est clairement visible sur cette image sous la forme d’une légère bulle de gaz entourant l’étoile. Crédit : ESA/Hubble, NASA et H. Olofsson (Onsala Space Observatory)

Qu’est-ce qu’une géante rouge ?

Une géante rouge se forme après qu’une étoile a manqué d’hydrogène pour la fusion nucléaire et a commencé le processus de mort.

Une étoile maintient sa stabilité grâce à un équilibre subtil entre sa propre gravité, qui la rassemble, et la pression extérieure des processus de fusion thermonucléaire en cours qui se déroulent en son cœur.

Cependant, une fois que le noyau d’une étoile est à court d’hydrogène, cet état d’équilibre est perdu et le noyau commence à s’effondrer. Ni le noyau ne s’effondre, la coquille de[{” attribute=””>plasma surrounding the core becomes hot enough to begin fusing hydrogen itself. As fusion in this shell begins, the extra heat causes the outer layers of the star to greatly expand, and the surface extends up to several hundred times beyond the former size of the star. As the energy at the star’s surface becomes far more dissipated, it causes the star’s bloated surface to cool, turning from white or yellow to red. A red giant is formed.

This process can take hundreds of millions of years to transpire. It only applies to intermediate-mass stars (with a mass between 80% and 800% of the Sun’s mass), which then go on to form planetary nebulae. When a more massive star runs out of hydrogen at its core, it forms a red supergiant instead, and then goes on to explode as a supernova.

Une géante rouge se forme après qu’une étoile a manqué d’hydrogène pour la fusion nucléaire et a commencé le processus de mort. Le crédit:[{” attribute=””>NASA & ESA

Red giant stars are used by Hubble to calculate the distances to different galaxies. Astronomers are able to determine how far away galaxies are by comparing the brightness of the galaxies’ red giant stars with nearby red giants, whose distances have been measured by other methods. This is possible because red giants are reliable milepost markers — they all reach the same peak brightness in their late evolution — and so they can be used as a “standard candle” to calculate distance. Hubble’s outstanding sharpness and sensitivity allow it to find red giants in the stellar halos of the galaxies.

Hubble has observed U Camelopardalis, which coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse every few thousand years. The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail due to Hubble’s sensitivity. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable, the telescope has observed that the shell of gas expelled from this red giant is almost perfectly spherical.

Word Bank Red Giant

Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory)

The Hubble Space Telescope has also captured the red giant R Sculptoris, which has shown us what is likely to happen to the Sun in a few billion years from now. It has also helped astronomers to understand how the elements we are made up of are distributed throughout the Universe. All stars with initial masses up to about eight times that of the Sun will eventually become red giants in the later stages of their lives. They start to cool down and lose a large amount of their mass in a steady, dense wind that streams outwards from the star. With this constant loss of material, red giants like R Sculptoris provide a good portion of the raw materials — dust and gas — used for the formation of new generations of stars and planets.

Hubble has also shown us the beautiful and colorful remnants of former red giants. These include NGC 2371, NGC 2022, and NGC 5307.

Hubble’s 29th-anniversary image was of the hourglass-shaped Southern Crab Nebula, which was formed by the interaction between a red giant and a white dwarf. The red giant is shedding its outer layers in the last phase of its life before it too lives out its final years as a white dwarf.

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