Glossary term: Estrela gigante

Description: Estrelas gigantes, abreviadamente gigantes, são estrelas excepcionalmente grandes e luminosas, em comparação com outras estrelas da mesma cor. As estrelas não nascem como gigantes; em vez disso, elas se tornam gigantes de forma intermitente em vários estágios de sua evolução.

Para a maioria das estrelas que não são gigantes, há uma relação direta entre sua cor e sua luminosidade (ou seja, a energia que emitem ao longo do tempo). Essas estrelas são chamadas de "estrelas de sequência principal" e suas emissões de energia são alimentadas pela fusão nuclear de hidrogênio em hélio em seus núcleos. Quando o suprimento de hidrogênio no núcleo se esgota, a fusão de hidrogênio continua em uma camada esférica ao redor do núcleo e a estrela se expande, tornando-se muito mais luminosa e avermelhada. Por fim, a temperatura no centro pode ser alta o suficiente para que a fusão nuclear de elementos mais pesados que o hidrogênio se torne possível, fornecendo uma fonte de energia adicional para a estrela. No decorrer dessa conversão, essas estrelas se expandem para um tamanho muito maior, esfriam, ficando mais avermelhadas e, em geral, tornam-se muito mais luminosas — elas se tornam o que chamamos de estrelas gigantes vermelhas. O Sol, por exemplo, será centenas de vezes maior e mais brilhante, além de muito mais frio, quando se expandir e se tornar uma estrela gigante vermelha. Para estrelas de massas diferentes, fases adicionais de evolução, geralmente de curta duração, fazem com que essas estrelas se tornem gigantes azuis, ou supergigantes vermelhas ou azuis ainda mais luminosas.

As gigantes são classificadas na classe de luminosidade III, mais brilhantes que as subgigantes (classe IV), mas mais fracas que as gigantes brilhantes (classe II) e as supergigantes (classe I).

Em geral, as estrelas gigantes são raras. Isso se deve à duração relativamente curta da fase gigante (para uma estrela como o Sol, algumas centenas de milhões de anos contra dez bilhões na sequência principal). Mas, devido à sua alta luminosidade, elas estão significativamente super-representadas entre as estrelas que são visíveis a olho nu no céu noturno.

Exemplos de gigantes vermelhas incluem Arcturus, na constelação do Boieiro, e Mira, na Baleia.

Related Terms:

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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher
The translation of this term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Diagrams

A line of stars goes from cool faint stars to hot bright stars. Some stars lie above or below this line

Hertzsprung-Russell diagram

Caption: This diagram shows the temperature and luminosity of different stars. The size of each point represents the star’s radius and its colour is the colour the human eye would see. The stars range in colour from a washed-out blue to a washed-out reddish-orange. No star has a pure colour like red, green or blue as stars’ spectra include light from lots of different colours. However the reddest stars are commonly referred to as red and the bluest stars as blue. The sample of stars used to make this diagram was chosen to show a wide range of stars of different types so the relative number of each type of star is not representative of how commonly each type is found. From the top left to bottom right there is a long line of stars burning hydrogen in their cores. This is called the main sequence. On this line, one sees the stars Mintaka, Achenar, Sirius A, the Sun and Proxima Centauri. The objects around Proxima Centauri at the lower right end of the main sequence are known as red dwarfs. To the lower right of the red dwarfs are Teide 1 and Kelu-1 A. These two objects are brown dwarfs, objects too low in mass to have cores hot enough to fuse hydrogen for a sustained period of time. As they do not burn hydrogen, brown dwarfs are not considered main sequence stars. The name brown dwarf is unrelated to their colour. Above the main sequence, we find subgiants, giants and supergiants. These are stars that have finished burning hydrogen in their core and have evolved into larger objects. A star’s brightness depends on its temperature and size so giant stars are brighter than stars with a smaller radius but the same temperature. In time these objects will move towards the end of their lives and undergo either a planetary nebula phase or become supernovae. Stars which end their lives with a planetary nebula phase become a type of stellar remnant called a white dwarf. Such objects are much smaller than stars of the same temperature and thus are fainter and are found significantly below the main sequence. Stars which end their lives as supernovae become either black holes or neutron stars. These are not shown on this plot.
Credit: IAU OAE/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

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