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Ciel Extrême #38 À Lire (Pdf De 12.0Mo) 1 ÉDITORIAL Observations en OIII au prisme-objectif Des nouvelles de la reprise de CE. A ce Cyril BAZIN jour, une seule personne s'est manifestée, à condition que la gestion de CE se fasse à trois et qu'elle puisse partager avec une autre personne la mise en page. Elle s'occuperait de la mise en page « brute », celle où l’on dispose les articles dans la revue. On cherche donc une personne pour la peaufiner et imprimer le prototype, et une troisième pour l'envoi de la revue et la gestion des abonnements. Bien sûr ce n’est qu’une suggestion et on peut s’organiser autrement. Pour ceux qui le souhaitent, j’ai rédigé un texte reprenant point par point la gestion de CE. Bon ciel, Nébuleuse Rosette, TN ø400mm, 71x+OIII, En couverture : M16 (Ser), Bruno Salque (89), TN transparence 7/10, humidité, du 4 au 5/02/2005, ø300mm, F=1200mm, x67, filtre OIII, 20/07/2004, Orgemont, sud de l'Essonne (91). Bien visible. Mont Lachens (83), alt.1715m, transparence bonne (Voie Lactée belle et structurée, plus pâle au sud dû a la pollution de lumineuse de la Côte d'Azur), turbulence forte. "Elle est assez brillante avec le Speers-WALER 18 mm et le OIII. On devine même une zone sombre qui n'est en fait que la base des "piliers"." Image de logo © David Malin, AAO. -------------------------------------------------------- VdB 38, TN ø400mm, 71x+OIII, transparence 7/10, humidité, du 4 au 5/02/2005, Orgemont, sud de l'Essonne (91). A peine visible en VI, invisible sans filtre. NGC 2392 ; TN ø205mm F/6 muni d'un prisme de 5° et 220x220mm, G=107x, T=3, S=3, 11 au 12/04/2005, Richardville (91) 2 Virginis au Periastre Pour les puristes, les données de l’orbite γ réelle sont les suivantes : excentricité 0.89, demi grand axe 3.68 arcsecondes, période Bertrand LAVILLE 168.9 ans. Le couple a fait l’objet de près [email protected] de 1500 mesures, ce qui est exceptionnel, et nous a permis de déterminer une orbite qui s’est progressivement affinée, la dernière émanant de Soderhjelm en 1999, et estimée de grade 2, dans une échelle Certaines années sont bénies des dieux de 5, préliminaire, à 1, définitive. Au cours astronomiques : par exemple, 1994, lor- des 98% précédemment cités, l’écarte- sque la comète Shoemaker-Levy 9 s’est ment angulaire des composantes varie écrasé sur Jupiter, ou 1997, lorsque celle entre 1 et 6 secondes d’arc, mais le corol- découverte par Hale et Bopp a brillé de laire de l’excentricité est un passage au tous ses feux, ou encore 1987, lorsque la périastre extrêmement serré. Selon l’orbite supernova du Nuage de Magellan explosa. de Soderhjelm, le périastre se produit en Pour les amateurs d’étoiles doubles, c’est 2005, avec un rhô de 0.35", mais cette 2005 qui sera l’année du siècle. C’est en imprécision, quoique faible, est maximum effet cette année qu’est en train de se pro- autour de cette période puisque la loi des duire l’événement le plus spectaculaire, et aires implique une accélération con- de loin, des XXème et XXIème siècles sidérable de la vitesse de l’étoile orbitante, concernant les étoiles doubles, je veux et donc une évolution extrêmement rapide parler du passage de Gamma Virginis à de l’angle de position, c’est-à-dire de son périastron. l’angle formé par la droite joignant les deux C’est en effet la seule fois qu’un amateur, étoiles avec le nord. Par contre, la valeur équipé simplement d’un télescope de de la période est précise et fiable, car elle 150mm, voire de 100mm, peut mettre en émane des 1500 mesures faites sur les évidence la rotation d’un couple en seule- 98% dont nous parlions tout à l’heure. ment quelques mois. L’HISTOIRE LA THEORIE Heureusement, nous avons les comptes Gamma Virginis est une double brillante et rendus d’observations de John Herschel, parfaitement équilibrée, dont les deux W.R. Dawes et de l’Amiral Smyth, au composantes sont de magnitude 3.48 et cours de la période 1830 à 1840. La 3.53V. Ces deux étoiles tournent l’une symétrie de leur mesures, entre 1831-35 autour de l’autre en un peu moins de 170 et 1837-40, conduit à fixer la date du ans, suivant une orbite réelle très allongée périastre à 1836.32, à estimer le rhô à 0.3" et un grand axe apparent suffisamment maxi et probablement 0.25", ainsi qu’une conséquent pour que nous puissions les variation de l’angle de position d’au moins séparer très facilement pendant 98% de 120° au cours des 12 mois à cheval sur leur révolution. cette date. 3 Le diamètre des instruments utilisés à fin avril 2005, l’angle de position avait près cette époque, 95 mm à 150 mm, permit de de 9° d’avance sur celui d’il y a 169 ans. suivre les deux composantes jusqu’à C’est un peu comme s’il y avait une con- environ 2 mois du périastre, mais celui ci centration « d’énergie noire accélérante » ne fut pas observé en tant que tel. Si dans les parages de l’étoile orbitée. D’où maintenant, on fait un copier collé de vient cette énergie noire ? Elle n’est l’évolution de l’orbite résultant des me- probablement pas aussi mystérieuse que sures faites au cours des 2 ou 3 dernières ça. Il suffit de se reporter non pas 170 ans, années, en essayant de la superposer à mais 250 ans en arrière, lorsque Laplace celle de 1836, on est amener à fixer le déduisit l’existence de Neptune des per- périastre de 2005 à la mi-Mai, +/- 3 se- turbations qu’il constatait dans l’orbite maines, tout en constatant qu’il est impos- d’Uranus : il est vraisemblable que cette sible de faire coïncider exactement les accélération provient de la présence d’un deux portions d’orbites au fur et à mesure troisième corps, invisible, qui interfère en que l’on s’en approche. Car voilà, il y a 2005 différemment de 1836. La suite des souvent un hic entre prédictions et me- mesures dans les prochains mois et an- sures : patatras ! Celles des derniers mois, nées permettra peut être d‘en savoir plus entre décembre 2004 et avril 2005 ont mis sur ce mystérieux comparse. en évidence un phénomène à première Voilà pour ce qui est du roman policier de vue « surnaturel » : Gamma Virginis ne Gamma Virginis, du moins pour le mo- respecte pas la loi des aires ! L’étoile ment. orbitante accélère et accélère encore, et à 4 Mais revenons à la partie observations, Chaque commentaire est précédé des celles que nous pouvons faire. valeurs de rhô et de l’AP à la date de l’observation. Vous disposez ainsi d’une LES OBSERVATIONS sorte d’échelle graduée du rapprochement, Ce couple représente en effet une chance et il vous suffira de vous reporter à exceptionnelle pour vérifier, ou étalonner l’observation symétrique de celle que vous nos instruments, en nous fournissant un ferez éventuellement dans les mois et test de séparation en vraie grandeur, années futures pour connaître à l’avance s’agrandissant progressivement mais dans l’écartement et l’angle de position que un délai extraordinairement court à vous vous proposerez de mettre en évi- l’échelle céleste, suivant des valeurs pré- dence. De la date à laquelle votre obser- cisément connues par symétrie entre vation sera réussie, vous en déduirez l’avant et l’après périastron. Cerise sur le aisément le pouvoir séparateur de votre gâteau, les composantes sont parfaite- instrument. Notez, en passant, que du- ment équilibrées et de magnitude large- plicité ne veut pas dire séparation, car il y ment suffisante pour être adaptée aux plus a toute une gradation entre le moment où petits instruments. Ce sont, en quelque l’on s’aperçoit que l’étoile n’est pas ronde, sorte, les conditions théoriques parfaites n’est pas « normale », et le moment où pour atteindre le pouvoir séparateur de elle devient deux étoiles distinctes. Il s’agit votre instrument, quel que soit son là de ce que l’on appelle communément diamètre, qui vont s’offrir à vous au cours « l’observation visuelle des étoiles doubles des quelques dizaines de mois à venir. serrées » ; nous y reviendrons tout à Voici les observations que j’ai pu faire du l’heure. couple depuis quelques années, ainsi que celles qui ont été réalisées, notamment Observation A : B. Laville, 09/03/1998, ces derniers mois, par le Hanwell Commu- 22h47HL, Marseille (13). SC ø203mm, nity Observatory. J’ai rajouté 4 croquis F/10 (Meade LX200), 285x (Meade SWA correspondant à des observations vir- 18mm + barlow),H=16°, T=2, P=4-5, S=3- tuelles de ce qu’aurait vu un observateur à 4/285x ; rhô=1.75", AP=268° des dates constituant des chaînons man- Couple brillant, assez serré, légèrement quant dans le rapprochement progressif déséquilibré, les deux blanches. des deux composantes. Pour qu’il n’y ait Observation B : B. Laville, 12/03/1999, pas de confusion entre observations 02h49HL, Marseille (13). SC ø254mm, réelles et virtuelles, j’ai représenté les F/10 (Meade LX200), 185x (Meade SWA observations réelles en blanc sur fond noir, 13.8mm), H=45°, T=5, P=2, S=4/185x, en conformité avec ce que l’œil a vu, et les rhô=1.59", AP=263° ; Couple très brillant, observations virtuelles en négatif, étoiles bien séparé mais image turbulente, équili- noires sur fond du ciel blanc.
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