MATERIAL PARA O GRUPO DO SOL
Autor: Mertalurgica Kato | Publicado: 2013-11-03T13:01:00-08:00

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O Sol, nossa fonte de luz e de
vida, é a estrela mais próxima de nós e a que melhor conhecemos. Basicamente, é
uma enorme esfera de gás incandescente, em cujo núcleo acontece a geração de
energia através de reações
termo-nucleares. O estudo do Sol serve de base para o conhecimento das
outras estrelas, que de tão distantes aparecem para nós como meros pontos de
luz.




Apesar de parecer tão grande e
brilhante (seu brilho aparente é 200&nbsp;bilhões de vezes maior do que o de
Sírius, a estrela mais brilhante do céu noturno), na verdade o Sol é uma
estrela bastante comum. Suas principais características são:




 

  
Massa

  
M =
  1,989 x 1030 kg

 

  
Raio

  
R = 695
  500 km = 109 RTerra

 

  
Densidade
  média

  

   
   
    
    
    
    
    
    
    
    
    
    
    
    
   
   
   
  
   
  = 1409
  kg/m3 

 

  
Densidade
  central

  

   
  = 160
  000 kg/m3

 

  
Distância

  
1&nbsp;UA
  = 149 600 000 km 

 

  
Luminosidade

  
L=3,9×1026
  watts=3,9×1033 ergs/s

 

  
Temperatura
  efetiva

  
Tef
  = 5785 K

 

  
Temperatura
  central

  
Tc
  = 15 000 000 K 

 

  
Magnitude
  absoluta bolométrica

  
Mbol
  = 4,72

 

  
Magnitude
  absoluta visual

  
MV
  = 4,79

 

  
Tipo
  espectral e classe de luminosidade

  
G2 V

 

  
Índices
  de cor

  
B-V=0,62

 

  


  
U-B=0,10

 

  
Composição
  química principal (No)

  
Hidrogênio
  = 91,2 %

 

  


  
Hélio =
  8,7%

 

  


  
Oxigênio
  = 0,078 %

 

  


  
Carbono
  = 0,043 %

 

  
Período
  rotacional no equador

  
25,67 d

 

  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;na
  latitude 75°

  
33,40 d

 





Algumas das características
listadas acima são obtidas mais ou menos diretamente. Por exemplo, a distância
do Sol, chamada Unidade Astronômica, é medida por ondas de radar direcionadas
a um planeta em uma posição favorável de sua órbita (por exemplo Vênus, quando
Terra e Vênus estão do mesmo lado do Sol e alinhados com ele). O tamanho
do Sol é obtido a partir de seu tamanho angular e da sua distância. A massa
do Sol pode ser medida a partir do
movimento orbital da Terra (ou de qualquer outro planeta) usando a terceira lei
de Kepler. Sabendo
então sua massa e seu raio temos a densidade média do Sol. 




Outras características são
determinadas a partir de modelos. Por exemplo, a equação de equilíbrio hidrostático, permite determinar a pressão
e a temperatura no centro do Sol, supondo que elas têm que ser extremamente
altas para suportar o peso das camadas mais externas. 




A primeira determinação
quantitativa da composição química da atmosfera solar foi obtida em 1929 por
Henry Norris Russel (1877-1957), publicada no Astrophysical Journal, 70,
11, baseada em estimativas a olho das intensidades das linhas no espectro
solar. 




O modelo representado na figura
mostra as principais regiões do Sol. A fotosfera, com cerca de 330 km de
espessura e temperatura de 5785 K, é a camada visível do Sol.
A palavra vem do grego: photo = luz. Logo abaixo da fotosfera se
localiza a zona convectiva, se estendendo por cerca de 15% do raio
solar. Na zona convectiva o
transporte de energia é pelo movimento das parcelas de gás (transporte
mecânico). Abaixo dessa camada está a zona radiativa, onde a energia
flui por radiação, isto é, não há movimento das parcelas de gás, só transporte
de fótons. O núcleo, com temperatura de cerca de 15 milhões de graus
Kelvin, é a região onde a energia é produzida, por reações termo-nucleares.
A cromosfera é a camada da atmosfera solar logo acima da fotosfera e tem
baixa densidade. A palavra vem do grego: cromo = cor. Ela tem
cor avermelhada e é visível durante os eclipses solares, logo antes e após a
totalidade. Estende-se por 10&nbsp;mil&nbsp;km acima da fotosfera e a
temperatura cresce da base para o topo, tendo um valor médio de
15&nbsp;mil&nbsp;K. Ainda acima da cromosfera se encontra a coroa,
também visível durante os eclipses totais. A coroa se estende
por cerca de dois raios solares e temdensidade ainda mais baixa que a
cromosfera.




&nbsp;A fotosfera do Sol tem a aparência da superfície
de um líquido em ebulição, cheia de bolhas, ou grânulos. Este fenômeno é
chamado de granulação fotosférica. Os grânulos têm em torno de 5000 km de diâmetro e duram cerca de 10&nbsp;min cada. Eles marcam os topos
das colunas convectivas de gás quente, que se forma na zona convectiva,
logo abaixo da fotosfera. Nestas colunas, o gás quente das
camadas interiores sobe e deposita a energia térmica nas camadas superiores. Ao
perder o calor, aumO fenômeno fotosférico mais notável é o das manchas
solares, regiões irregulares que aparecem mais escuras do que a fotosfera
circundante e que muitas vezes podem ser observadas mesmo a olho
nu, embora olhar
diretamente para o Sol só não é perigoso quando ele está no horizonte. As
manchas foram registradas na China já no ano 28 a.C., mas seu estudo científico
começou com o uso do telescópio, sendo observadas (por projeção da imagem do
Sol) por Galileo,
Thomas Harriot (1560-1621) já em 1610, por Johannes (1587-1616) e David Fabricius
(1564-1617) e por Christoph Scheiner (1575-1650) em 1611. São constituídas de
duas partes: a umbra, parte central mais escura, com temperaturas em
torno de 3800 K, e a penumbra, região um pouco mais clara e com
estrutura radial em torno da umbra. As manchas são mais frias porque o campo
magnético local impede a convecção e, portanto, que o calor das partes mais
internas suba à fotosfera. As manchas solares tendem a se formar em grupos e
estão associadas a intensos campos magnéticos no Sol. enta de densidade e desce
para as camadas mais internas.&nbsp;

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 <![endif]-->



A
cromosfera 




Espículas,
produzidas pelo campo magnético e energia mecânica na fotosfera, aquecem a
cromosfera. Elas são aproximadamente cilíndricas, com cerca de 700 km de
extensão e 7000 km de altura, e duram entre 5 e 15 minutos. 




A cromosfera do Sol normalmente
não é visível, porque sua radiação é muito mais fraca do que a da fotosfera.
Ela pode ser observada, no entanto, durante os eclipses, quando a Lua esconde o
disco da fotosfera.&nbsp;





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A Coroa 




Foto do
Sol obtida pela estação espacial Skylab da NASA em 19 de dezembro de 1973, com
um dos mais espectacular flares solares já gravados. A proeminência abrange
mais de 588&nbsp;000 km. Os pólos solares apresentam pouca super-granulação e
um tom mais escuro do que o centro do disco.

 
 
  
  
  
  
  
  
  
  
  
  
  
  
 
 
 

 





A
cromosfera gradualmente se funde na coroa, a camada mais externa e mais
rarefeita da atmosfera do Sol. A coroa também é melhor observada durante
eclipses, pois apesar de ter um brilho equivalente ao da lua cheia, ela fica
obscurecida quando a fotosfera é visível. 




O espectro da coroa mostra linhas
muito brilhantes que, até 1940, não eram conhecidas. Atualmente sabemos que
elas são produzidas por átomos de ferro, níquel, neônio e cálcio altamente
ionizados e não por algum elemento estranho, como anteriormente foi pensado. O
fato de existirem esses elementos várias vezes ionizados na coroa implica que
sua temperatura deve ser muito alta, pois é necessária muita energia para
arrancar muitos elétrons de um átomo. A coroa deve ter uma temperatura em torno
de 1 milhão de graus Kelvin.&nbsp;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 

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Imagem obtida pelo
satélite Transition Region and Coronal Explorer (TRACE), da NASA, mostrando que o gás nos
arcos se move balisticamente e é aquecido a partir da base dos mesmos. 










A elevação da temperatura na coroa deve ter origem
no mesmo processo físico que aquece a cromosfera: transporte de energia por
correntes elétricas induzidas por campos magnéticos variáveis. 




Da coroa emana o vento solar, um fluxo
contínuo de partículas emitidas da coroa que acarretam uma
perda de massa por parte do sol em torno de 
 
 
  
  
  
  
  
  
  
  
  
  
  
  
 
 
 

 
por ano. O vento solar que atinge a Terra (aproximadamente 7
prótons/cm3 viajando a cerca de 400 km/s) é capturado pelo campo
magnético da Terra, formando o cinturão de Van Allen, na magnetosfera terrestre. 




s ejeções coronais de massas são bolhas de gás quente
(plasma), de cerca de 1 a 10 bilhões de toneladas, aquecidas pelos campos magnéticos do Sol. Os campos magnéticos do Sol se enrolam devido ao movimento turbulento
de convecção mas também devido à rotação diferencial, que faz com que o equador
solar complete uma volta em 25 dias, enquanto que as regiões próximas aos pólos
completam uma volta em 36 dias. A desconexão do campo magnético solar pode
ocorrer em alguns minutos e tem uma energia equivalente a milhares de bombas
atômicas. 




O vento solar, composto de partículas carregadas
desprendidas da coroa solar, viaja a aproximadente 250 a 1000&nbsp;km/s, provocando
as auroras, normalmente entre 60 e 80° de latitude. Entretanto as auroras podem
ocorrer também em baixas latitudes, como por exemplo a observada em 1909 em Singapura, no equador geomagnético. 




As auroras foram observadas na antiguidade pelos
gregos e chineses, mas somente em 1896 o físico norueguês Kristian Birkeland
(1867-1917) deduziu que fluxos de elétrons provenientes do Sol eram canalizados
pelo campo geomagnético aos pólos e, quando colidiam com a alta atmosfera,
estimulavam os átomos de oxigênio e nitrogênio. As auroras são causadas pela
interação de partículas de alta energia, principalmente elétrons, com os átomos
neutros da alta atmosfera da Terra. Estas partículas de alta energia podem excitar,
através de colisões, os elétrons de valença que estão ligados aos átomos
neutros. Estes elétrons excitados então se desexcitam, retornando ao estado
inicial, de mais baixa energia. Aos se desexcitar, eles emitem um fóton, isto é
luz. A combinação destes fótons, emitidos por muitos átomos, resulta na aurora
que vemos. As auroras acontecem a alturas acima de 60&nbsp;km, têm correntes
acima de 100&nbsp;000&nbsp;volts e geram energia acima de 1 milhão de
megawatts. 




A energia do Sol 




Tão logo foi conhecida a distância do Sol, em 1673,
por Jean Richer (1630-1696) e Giovanni Domenico Cassini (1625-1712) que
determinaram a distância (paralaxe) de Marte e com esta estimaram a
unidade astronômica como 140 milhões de km (cerca de 150 milhões de km é o
valor atual), foi possível determinar a sua luminosidade, que é a potência que
ele produz. As medidas mostram que cada metro quadrado na Terra recebe do Sol
uma potência (energia/segundo) de cerca de 1400 watts [James Watt (1736-1819)],
ou seja, a potência de 14 lâmpadas de 100 watts/m2.
O valor mais preciso da constante solar é 1367,5 W/m2, e varia 0,3%
durante o ciclo solar de 11 anos. Multiplicando-se essa potência recebida na
Terra pela área da esfera compreendida pela órbita da Terra em torno do Sol,
determina-se a luminosidade do Sol em 3,9×1026 watts = 3,9×1033
ergs/s.