{"id":7579,"date":"2017-05-06T08:03:36","date_gmt":"2017-05-06T08:03:36","guid":{"rendered":"https:\/\/blogs.futura-sciences.com\/feldmann\/?p=7579"},"modified":"2017-05-15T16:08:38","modified_gmt":"2017-05-15T16:08:38","slug":"quest-lechelle-magnitudes-astronomie","status":"publish","type":"post","link":"https:\/\/blogs.futura-sciences.com\/feldmann\/2017\/05\/06\/quest-lechelle-magnitudes-astronomie\/","title":{"rendered":"Qu’est-ce que l’\u00e9chelle des magnitudes en astronomie ?"},"content":{"rendered":"

Tous les astres n’ont pas le m\u00eame \u00e9clat.\u00a0\u00c0 chaque intensit\u00e9 correspond une valeur sur\u00a0une \u00e9chelle appel\u00e9e \u00e9chelle des magnitudes.<\/strong><\/p>\n

Un simple regard en direction du ciel nocturne permet de constater que tous les astres ne brillent pas de la m\u00eame fa\u00e7on : alors que la Pleine Lune<\/a> va nous \u00e9blouir, certaines \u00e9toiles se devinent \u00e0 peine. Bien entendu ces diff\u00e9rences de brillance ne sont qu’apparentes puisqu’elles ne tiennent pas compte de la distance qui nous s\u00e9pare de ces astres (rappelons que la luminosit\u00e9 varie selon une loi en carr\u00e9 inverse : si nous doublons la distance qui nous s\u00e9pare d’une source lumineuse, nous recevrons quatre fois moins de lumi\u00e8re).<\/p>\n

\"\"<\/a><\/p>\n

Au IIe si\u00e8cle av. J.-C., bien avant l’invention de la lunette astronomique<\/a>, l’astronome grec Hipparque fut le premier \u00e0 essayer de classer les \u00e9toiles visibles \u00e0 l’\u0153il nu en fonction de leur \u00e9clat.<\/p>\n

Il d\u00e9cida de r\u00e9partir les \u00e9toiles dans six “grandeurs” (les futures magnitudes), les plus brillantes (comme Sirius<\/a>, Arcturus<\/a>, V\u00e9ga<\/a> et quelques autres) \u00e9tant de premi\u00e8re grandeur et les plus faibles (celles qu’on devinait \u00e0 peine dans un ciel bien noir) de sixi\u00e8me grandeur. La m\u00e9thode de classement d’Hipparque fut reprise et popularis\u00e9e dans “l’Almageste”, la c\u00e9l\u00e8bre encyclop\u00e9die r\u00e9alis\u00e9e par Ptol\u00e9m\u00e9e quatre si\u00e8cles plus tard.<\/p>\n

Tr\u00e8s empirique, ce syst\u00e8me d’estimation des luminosit\u00e9s perdura pendant des si\u00e8cles et \u00e0 l’apparition des lunettes et t\u00e9lescopes<\/a>, les astronomes qui d\u00e9couvrirent dans leurs instruments des astres invisibles \u00e0 l\u2019oeil nu se content\u00e8rent de rajouter progressivement de nouvelles “grandeurs”.<\/p>\n

En 1856, l’astronome britannique Norman Robert Pogson (1829 \u20131891) qui avait remarqu\u00e9 qu’une \u00e9toile de premi\u00e8re “grandeur” est 100 fois plus brillante qu’une \u00e9toile de sixi\u00e8me “grandeur”, proposa une nouvelle classification utilisant une \u00e9chelle logarithmique : d\u00e9sormais le passage d’une magnitude (terme qu’il proposa pour remplacer celui de “grandeur”) \u00e0 la magnitude sup\u00e9rieure correspondrait \u00e0 une baisse de luminosit\u00e9 d’environ 2,512.<\/p>\n

Aujourd’hui l’\u00e9chelle des magnitudes s’\u00e9tend de -26,7 (Soleil<\/a>) \u00e0 30 pour les astres les plus faibles que l’on puisse d\u00e9tecter avec les plus grands t\u00e9lescopes disponibles, en passant par -12,6 pour la Pleine Lune, -4,6 pour V\u00e9nus<\/a> (\u00e9clat maximal), -1,5 pour Sirius, 2 pour l’\u00e9toile polaire<\/a>\u00a0(visible sur cette image) ou encore 3,4 pour la grande galaxie d’Androm\u00e8de<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Tous les astres n’ont pas le m\u00eame \u00e9clat.\u00a0\u00c0 chaque intensit\u00e9 correspond une valeur sur\u00a0une \u00e9chelle appel\u00e9e \u00e9chelle des magnitudes. Un simple regard en direction du ciel nocturne permet de constater que tous les astres ne brillent pas de la m\u00eame fa\u00e7on : alors que la Pleine Lune va nous \u00e9blouir, certaines \u00e9toiles se devinent \u00e0 … Continuer la lecture de Qu’est-ce que l’\u00e9chelle des magnitudes en astronomie ?<\/span> →<\/span><\/a><\/p>\n","protected":false},"author":8,"featured_media":2364,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_links_to":"","_links_to_target":""},"categories":[87],"tags":[2147,802,2146,192,2148,1151],"class_list":["post-7579","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-les-hommes-et-le-ciel","tag-almageste","tag-hipparque","tag-loi-en-carre-inverse","tag-magnitude","tag-norman-robert-pogson","tag-ptolemee"],"yoast_head":"\nQu'est-ce que l'\u00e9chelle des magnitudes en astronomie ?<\/title>\n<meta name=\"description\" content=\"Tous les astres 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