-
Activité solaire Mai 2015
58,8
C'est pratiquement le même chiffre que le mois dernier ( 54,4 ).
Pour rappel, nous glissons très tranquillement vers le minimum solaire qui devrait se situer en 2022 ou 2023. Le prochain maximum solaire, lui, devrait se situer en 2027 ( tiens donc... ).
-
Commentaires
6neoSamedi 6 Juin 2015 à 12:33http://www.space.com/29575-pluto-myths-nasa-new-horizons-mission.html?cmpid=NL_SP_weekly_2015-06-05
7 Wild Myths About Pluto
by Mike Wall, Space.com Senior Writer | June 05, 2015 07:50am ET
Credit: ESO/L. Calcada View full size image</td> </tr> </tbody> </table>Faraway Pluto is difficult to study from Earth, so the dwarf planet has remained largely mysterious to scientists and laypeople alike since its discovery in 1930.
But Pluto is about to get its first close-up. On July 14, NASA's New Horizons spacecraft will zoom just 7,800 miles (12,500 kilometers) from the dwarf planet, capturing supersharp images of its frigid surface.
With this highly anticipated unveiling less than six weeks away, now is an opportune time to revisit some of the most common myths and misconceptions about Pluto. Here's a brief rundown. [NASA's New Horizons Pluto Mission in Pictures]
Myth 1: Pluto was named for the Disney character.
A close look at the chronology dashes this myth. The famous Disney dog debuted in 1930, but he was initially named Rover; a cartoon featuring "Pluto the Pup" didn't air until 1931, a year after the celestial object was discovered and named.
"People were repeatedly saying, 'Ah, she named it after Pluto the dog.' It has now been satisfactorily proven that the dog was named after the planet, rather than the other way round. So, one is vindicated," Venetia Phair (née Burney), who suggested the moniker "Pluto" for the newfound ninth planet as a schoolgirl in 1930, told the BBC in 2006. (New Horizons launched in January of that year.)
Myth 2: Pluto is tiny.
Some people think Pluto is small, like a run-of-the-mill asteroid. But the dwarf planet is a robust 1,466 miles (2,360 km) in diameter — about two-thirds as wide as Earth's moon, and three-quarters as wide as Jupiter's ocean-harboring moon Europa. In fact, Pluto's largest moon, Charon, is itself 750 miles (1,207 km) across. (The dwarf planet's four other known satellites are tiny.)
Further, Pluto is considerably bigger than pretty much every other object in the Kuiper Belt, the ring of icy bodies beyond Neptune's orbit. The vast majority of Kuiper Belt objects are the size of comets, just a few kilometers across. Several dozen are at least a few hundred kilometers wide, but only two — Pluto and the dwarf planet Eris — are more than 1,240 miles (2,000 km) across. (Eris and Pluto are about the same size.)
Myth 3: It's dark there all the time.
Pluto orbits more than 3 billion miles (4.8 billion km) from the sun on average, so many people imagine that it's pitch-dark on the dwarf planet's surface 24 hours a day. But that's not the case, said New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado.
"The lighting on Pluto at noon isn't as low as people think; it's like a very grey cloudy day on Earth, or like dusk levels after sunset," Stern told Space.com via email. [Photos of Pluto and Its Moons]
0 of 10 questions completeMyth 4: Pluto was once a moon of Neptune.
This is an old theory that became popular shortly after Pluto's discovery. It was disproven in 1965, when researchers found an orbital resonance — a gravitational sweet spot in which the orbits of two bodies are related by a ratio of two whole numbers — between Pluto and Neptune. This resonance prevents the two objects from ever closely approaching each other, Stern said.
Myth 5: Pluto is an ice world.
Pluto's surface is covered by a number of ices, including frozen nitrogen and frozen methane. But the density of Pluto as a whole is twice that of water ice, showing that the dwarf planet's mass is made up of about two-thirds rock and just one-third ice. Therefore, it's more accurate to say that Pluto is a rocky body with an icy shell.
Myth 6: Pluto is airless.
Researchers discovered in the 1980s that Pluto has an atmosphere, which is composed mainly of nitrogen, just like Earth's atmosphere. But Pluto's air, which also contains carbon monoxide and methane, is much thinner than Earth's, and it extends significantly farther out into space.
For example, Earth's atmosphere reaches 370 miles (600 km) or so beyond the planet's surface — about 10 percent of Earth's radius. By contrast, the outer limit of Pluto's wispy, variable atmosphere lies about seven Pluto radii from the dwarf planet's surface. The volume of Pluto's atmosphere is thus more than 350 times that of the dwarf planet itself, according to New Horizons co-investigator Michael Summers.
Myth 7: Pluto's orbit is one of a kind.
Pluto's orbit is quite elliptical, taking the dwarf planet as close as 2.75 billion miles (4.43 billion km) to the sun, and as far away as 4.5 billion miles (7.31 billion km) from our star. The dwarf planet's orbit is also inclined by 17 degrees relative to the ecliptic, the plane of Earth's path around the sun.
Pluto's orbital parameters are thus quite different from those of the eight "official" planets (a group that included Pluto until 2006, when the International Astronomical Union demoted the object to "dwarf planet" status), which tend to lie roughly in the same plane and move around the sun in more or less circular paths.
But some other Kuiper Belt denizens, such as the dwarf planets Eris and Haumea, have even more elliptical or inclined orbits, as do a number of alien planets.
Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
5neoSamedi 6 Juin 2015 à 12:32http://www.space.com/29571-why-pluto-is-a-planet-and-eris-is-too.html?cmpid=NL_SP_weekly_2015-06-05
Why Pluto Is a Planet, and Eris Is Too (Op-Ed)
Tim DeBenedictis, Simulation Curriculum | June 04, 2015 03:39pm ET<table style="width: 1px; border-spacing: 0;" border="0"> <tbody> <tr> <td></td> </tr> <tr> <td style="border: 1px solid lightgray; padding: 10px; font-size: 12px; line-height: 16px;"> This artist's view shows Pluto center, and its large moon Charon, as well as two of its four smaller moons (at top left and right). Pluto's four tiny moons are Nix, Hydra, Kerberos and Styx.
Credit: David A. Aguilar (CfA) View full size image</td> </tr> </tbody> </table>Tim DeBenedictis is the lead developer of the SkySafari line of iOS and Android apps at Simulation Curriculum, the makers of Starry Night, SkySafari and the free Pluto Safari app. DeBenedictis has been writing astronomy software since high school, and graduated from MIT in 1993 with a degree in earth, atmospheric and planetary science. Passionate about space, DeBenedictis is self-taught in mechanical and electrical engineering, and has launched his own private microsatellite into space. He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
The International Astronomical Union (IAU) got it wrong. Our solar system has 10 planets.
As NASA's New Horizons spacecraft glides its way to the cold outer reaches of our solar system to take the first-ever up-close look at Pluto, the time is right to revise the International Astronomical Union (IAU)'s 2006 definition of a planet, which resulted in Pluto's "demotion" from planet to ambiguous dwarf-planet status.
Pluto's place
For those unfamiliar with the issues that led to that highly controversial decision, here's a quick recap: It started with Pluto itself, discovered on Feb. 18, 1930, by Clyde Tombaugh, a young American astronomer working at Lowell Observatory in Flagstaff, Arizona. Pluto turned out to be rather unlike the other eight large objects orbiting the sun. Pluto is much smaller than Mercury, and only two-thirds the size of Earth's moon. Its orbit is tilted and eccentric, crossing Neptune's. No other planet acted like this. In 2000, astronomers found other objects orbiting the sun in the deep outer solar system, with qualities very much like Pluto's. They were given names like Sedna , Quaoar, Ixion, Varuna, Makemake and Haumea . Many were close (but not quite equal) to Pluto in size. All of them had tilted, eccentric orbits; quite a few of those orbits crossed Neptune's.
The tipping point came in 2005. California Institute of Technology astronomer Mike Brown, along with Chad Trujillo of Gemini Observatory and David Rabinowitz of Yale University, discovered a new massive body in the solar system. This new body, which astronomers latter dubbed Eris, was particularly noteworthy: Not only did it possess a moon, but at the time, it was estimated to be larger than Pluto. Subsequent observations revealed that Eris and Pluto are nearly identical in size, though Pluto is likely a few kilometers larger. Initially, Brown had named the newly discovered body Xena (after the protagonist of the eponymous TV show, with a sneaky Planet X reference). Although the name Xena didn't stick, the IAU later officially — and aptly — christened it Eris after the Greek goddess of chaos and discord.
So, it seemed quite clear that if Pluto was our solar system's ninth planet, then Eris should be its 10th. And if Eris and Pluto were planets, why shouldn't Makemake and Haumea be considered planets as well? And what if there were even bigger objects out there to be discovered? Why shouldn't the solar system have 15 planets, or 40? (Can you imagine the mnemonic device that would be required to remember 40 planets in the solar system?!)
For all who were in support of granting planet status to these objects, an equally adamant camp insisted that none of these objects, including Pluto, deserved to be called planets, and that our solar system contained only eight objects worthy of planet status. Neptune would be the last and final.
A worsening problem
With the intention of solving the debate once and for all, members of the IAU met in 2006. They spent days debating how to establish unambiguous definitions for the objects in our solar system. In the end, Resolution 5A was born:
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our solar system, except satellites, be defined into three distinct categories in the following way:
(1) A planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
(2) A dwarf planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.
These are, to put it bluntly, terrible definitions. Despite its goal of providing unambiguous definitions, Resolution 5A actually contains the kind of ambiguity that most scientific organizations would protest. It adds confusion, resolves little and makes nobody happy.
So how did that become the official definition of a planet? A very strange vote. If you think low voter turnout is limited to politics, consider this: Only about 4 percent of IAU members were present for the vote on Resolution 5A. But it was travel schedules, not apathy, that caused this abysmal turnout. You see, the vote took place on the last day of the IAU meeting, when many people had to leave to catch flights back home — 424 astronomers were present, even though IAU membership in 2006 was just more than 10,000. As a result, Pluto lost the status it had enjoyed for more than 80 years and became a dwarf planet overnight. [Pluto Demoted: No Longer a Planet in Highly Controversial Definition]
The term "dwarf planet" itself causes confusion. You often hear people say Pluto is still a planet but that it just happens to be a dwarf planet now. But despite the name, the IAU does not consider a dwarf planet a planet — unlike a dwarf star, which is still a star, or a dwarf galaxy, which is still a galaxy. So much for eliminating ambiguity! But just the conflicting use of the word planet isn't the most unclear part of the resolution.
The New Horizons spacecraft will fly by Pluto on July 14, 2015.
Credit: Made with the Pluto Safari app for iOS and Android.Picking apart Resolution 5A
Let's dissect resolution 5A "Definition of 'planet'", one of six IAU Resolutions that were passed at the Closing Ceremony of the General Assembly in 2006.
*Nearly round shape. There is an element of something good here. We all intuitively feel that a planet should be round, or nearly so. But what is "nearly"? How lumpy and bumpy must an object be to no longer qualify as a planet or a dwarf planet? How smooth must the "ball" be? The Earth, which we all agree is a planet, is nearly round on some scales, but on others, it's not. If you're standing in the bottom of the Grand Canyon, the Earth isn't even close to nearly round.
*Cleared the neighborhood. I've tried to wrap my head around this phrase for years, tried to convince myself that it makes sense — but I just can't swallow it. The IAU is trying to express that, in addition to being round, a planet should be the dominant gravitational force in its local region of the solar system. That's not an unreasonable position. Certainly the Earth and Jupiter are the dominant objects in their local regions. Neptune surely is, too. Even though Pluto's orbit crosses Neptune's, Neptune forces Pluto into something called a 3:2 resonance (for every three times Neptune goes around the sun, Pluto goes around twice), preventing collision. But have any of these planets actually "cleared the neighborhood" around their orbits? No. Pluto is still clearly in Neptune's "neighborhood." For that matter, Jupiter has two well-known groups of asteroids, the Trojans, which lead and follow Jupiter along in its orbit. For that matter, the Earth hasn't quite "cleared the neighborhood" around its orbit, either, to which anyone who saw the near-Earth asteroids that entered Earth's atmosphere near Chelyabinsk, Russia, on Feb. 15, 2013, or Tunguska, Siberia, on June 30, 1908, can attest. So are Earth, Jupiter and Neptune the dominant gravitational objects in their local neighborhoods? Yes, clearly. Have they cleared their neighborhoods? No. Not by a long shot.
Other scientists have weighed in on the matter. Alan Stern, principal investigator for the New Horizons mission to Pluto, made it clear he disagrees with the IAU resolution. "Any definition that allows a planet in one location but not another is unworkable. Take Earth. Move it to Pluto's orbit, and it will be instantly disqualified as a planet," Stern said.
The biggest problem with the IAU's planet definition is that it replaces an already-ambiguous concept ("What is a planet?") with three more ambiguous concepts, ("nearly round," "cleared" and "neighborhood"). Indeed, the only definitive part of the IAU resolution on which everyone can agree is the first part: (1) A planet is in orbit around the sun. It's why the moon is not a planet. My 6-year-old niece intuitively understands this. It's the only part of the IAU definition I would keep.
The way out
Let's look at some other kinds of definitions that are clear and unambiguous.
*International boundaries. It's well understood that the portion of North America north of 49 degrees, between the Canadian provinces of British Columbia, Alberta, Saskatchewan and Manitoba, and the U.S. states of Washington, Idaho, Montana, North Dakota and Minnesota, is called Canada, and the portion below that latitude is called the United States. There's no physical demarcation — no river, no mountain range — along the 49th parallel. There's no subtle change in vegetation or geological structure. But there is a hard, sharp, clearly defined, well-understood boundary that unambiguously answers the question, "What is Canada?" It's the country north of the 49th parallel. It passes the 6-year-old-niece test.
*Constellation boundaries. Back in 1888, the IAU defined an intricate set of boundary lines in the sky, precisely outlining the groups of stars that were commonly referred to as constellations. It also declared that there would be 88 of these constellations. Lines were chosen carefully, to respect traditional choices about which star might lie in which constellation, and in the end, the definitions were clear. There is no ambiguity about which particular point in the sky falls within which constellation. And, you can tell precisely when a moving celestial object (like Pluto) might cross from one constellation to another. It's an arbitrarily agreed upon but well-defined system of definitions that has served the astronomical community well for more than 100 years.
*The Karman line that defines the edge of space. Where does the Earth's atmosphere end and outer space begin? Clearly, there is no physical boundary. There is no bubble holding the atmosphere that one must pierce on one's way to the International Space Station. The air just gets thinner and thinner until you can ignore it. But in reality, air molecules continue to exist, albeit in smaller numbers, out to an altitude of many thousands of kilometers and beyond — indeed, some of these air molecules may have made it as far as Pluto by now! However, since the early days of manned spaceflight, a near-universally accepted definition is that space begins at an altitude of 100 kilometers (62 miles). In fact, this definition is accepted by the Fédération Aéronautique Internationale (FAI), an international standard-setting body for aeronautics and astronautics. Pilots who've flown higher than 100 km have officially earned the title of astronaut. It's another arbitrary, but widely accepted convention that is clear, unambiguous and easily passes the 6-year-old comprehension test.
What the IAU should have done in 2006, and could easily do moving forward, is to crystallize the definition of the word "planet" as unambiguously as it defined the boundaries of the constellations in 1888. Yes, that definition would have been arbitrary, and yes, the actual physical objects themselves would gradually transition from larger to smaller, and don't care in the least what we choose to call them. But the IAU could have chosen a definition that resolves the debate in a far more satisfying manner than it actually did.
The orbits of some of the trans-Neptunian objects.
Credit: Made with the Pluto Safari app for iOS and AndroidThe 1,000-km rule
So, what would be a better definition for the objects in the solar system?
(1) A "planet" [1] is a celestial body that (a) is in orbit around the sun, and (b) has a maximum surface radius greater than 1,000 km (620 miles).
(2) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.
"But that's completely unscientific," you may say. "Why 1,000 km? Why not 1,200, or 750?"
I submit that the precise definition of a planet as an object with a radius of at least 1,000 km is no less scientific than the definition of a kilometer as being a unit of distance equal to 1,000 m, or a degree being 1/360 of a circle.
And there are other reasons why the 1,000-km definition is more scientific than it might seem at first. But let's put that aside momentarily. Instead, let's see what would have happened if the IAU had adopted this definition.
Here is a list of the largest known objects orbiting the sun, and their radii in kilometers:
Object Radii (km)
Jupiter: 69,911 km (43,441 miles)
Saturn: 58,232 km (36,184 miles)
Uranus: 25,362 (15,759 miles)
Neptune: 24,622 (15,299 miles)
Earth: 6,378 (3,963 miles)
Venus: 6,052 (3,761 miles)
Mars: 3,390 (2,106 miles)
Mercury: 2,440 (1,516 miles)
Pluto: 1,184 (736 miles)
Eris: 1,163 (723 miles)
Makemake: 715 (444 miles)
Haumea: 620 (385 miles)
Quaoar: 555 (345 miles)
Sedna: 498 (309 miles)
Ceres: 475 (295 miles)
Orcus: 458 (285 miles)By the 1,000-km definition, all eight classical planets would remain planets. So would Pluto. And we'd add Eris. The solar system would have exactly 10 planets, a number that is deeply satisfying to two-handed, five-fingered humans who've been practicing base-10 mathematics for thousands of years. The "Plutophile" camp, fond of keeping Pluto's planetary status for historical reasons, would retain its dignity. And elevating Eris to a first-class planet would be an honorable nod to the cutting-edge astronomers whose work led to a need for this new definition in the first place.
If you're a topical expert — researcher, business leader, author or innovator — and would like to contribute an op-ed piece, email us here.
Credit: SPACE.comAnd finally, the 1,000-km rule, like any good arbitrary rule, actually does a pretty good job of respecting the underlying physical phenomena that it purports to define. Planets are made of physical materials like rock, metal, gas and ice. They may come in different proportions, but those materials all respect the same physical laws. When you put together a lump of rock, metal or ice, in any proportion, certain things start to happen as that lump approaches 1,000 km in radius. The materials will pull together under the force of their own gravity. Solid rock will start to deform. Ice, even frozen hard as granite at the edge of the solar system, will slowly flow. There is no known substance that can resist the force of its own gravity when made into a lump with a 1,000-km radius. Any object, made of any substance, of approximately that size, will eventually flow under action of its own gravity into a shape that is "nearly round" when viewed from far away. The 1,000-km radius just happens to describe something that naturally takes place for objects of a certain size and results in what we all intuitively want a planet to look like.
And for space enthusiasts, there's one more benefit to my proposed definition. While we're all looking forward to the New Horizons Pluto flyby, there's also a certain sadness to knowing that, after Pluto, there will be no more planets in our solar system to explore. If our solar system has 10 planets, that's no longer true. As far away and difficult as it was to reach Pluto, it will be even more difficult to reach Eris. It's another frontier, another first and another project to fund. That prospect alone should give the scientific community a reason to rethink and resolve Resolution 5A.
As the legendary astronomer Sir Patrick Moore said, "You can call it whatever you like. It's there!" Pluto is, and will always remain Pluto.
With the free Pluto Safari app for iOS and app for Android, users can simulate the July 14 flyby of Pluto, get regular mission news updates and learn the history of Pluto. Follow Simulation Curriculum on Twitter @SkySafariAstro, Facebook and Instagram. Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com.
4neoSamedi 6 Juin 2015 à 12:31http://www.space.com/29576-nasa-mars-spacecraft-radio-blackout.html?cmpid=NL_SP_weekly_2015-06-05
NASA Mars Spacecraft Enter Communications Blackout Sunday
by Mike Wall, Space.com Senior Writer | June 05, 2015 07:50am ET<table style="width: 1px; border-spacing: 0;" border="0"> <tbody> <tr> <td></td> </tr> <tr> <td style="border: 1px solid lightgray; padding: 10px; font-size: 12px; line-height: 16px;"> This diagram depicts the relative positions of Mars, Earth and the sun during a period that occurs approximately every 26 months, when Mars passes almost directly behind the sun from the perspective of Earth.
Credit: NASA/JPL-Caltech View full size image</td> </tr> </tbody> </table>An alignment of Mars, Earth and the sun will force NASA's fleet of Red Planet spacecraft to fend for themselves for two weeks beginning on Sunday (June 7).
From June 7 through June 21, Mars will be behind the sun from Earth's perspective. This celestial geometry, known as a Mars solar conjunction, makes radio communications between the two planets difficult — and potentially dangerous, as choppy or garbled instructions could actually harm spacecraft or hamper their missions, NASA officials said.
As a result, engineers won't send commands to NASA's three active Mars orbiters, or to the agency's two rovers, Opportunity and Curiosity, during this two-week stretch. Commanding will also be reduced on the days leading up to and following conjunction. [How NASA Deals with a Mars Solar Conjunction (Video)]
But that doesn't mean Opportunity, Curiosity, Mars Odyssey, the Mars Reconnaissance Orbiter (MRO) and the Maven spacecraft will get a two-week vacation.
"Spacecraft will continue making some science observations during the conjunction period, though rovers will not do any driving or arm movements," NASA officials wrote in a statement Wednesday (June 3).
Mars solar conjunctions occur every 26 months, so NASA has dealt with them before. In fact, this will be the seventh conjunction that Mars Odyssey has experienced, the sixth for Opportunity and the fifth for MRO. (Odyssey arrived at the Red Planet in 2001, while Opportunity and MRO got there in 2004 and 2006, respectively.)
The Curiosity rover touched down in August 2012 and is therefore a conjunction veteran as well.
"Our overall approach is based on what we did for the solar conjunction two years ago, which worked well," Nagin Cox of NASA's Jet Propulsion Laboratory in Pasadena, California, who is leading conjunction planning for Curiosity, said in the same statement. "It is really helpful to have been through this before."
Curiosity and Opportunity will send some data up to the orbiters during the conjunction period, NASA officials said. MRO and Mars Odyssey will continue to relay information to Earth from June 7 through June 21, but they will also store the data on board for re-transmission after conjunction. Maven (whose name is short for Mars Atmosphere and Volatile Evolution) won't transmit any information back home until conjunction is over.
Curiosity will store its own information on board as well. But Opportunity, which has been experiencing memory problems lately, will rely on the orbiters to safeguard the science data it gathers during conjunction.
Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
More from Space.com2neoVendredi 5 Juin 2015 à 18:57BON W.E. à toutes et tous !
Cadeau pour toi Damien:
http://www.nasa.gov/multimedia/imagegallery/iotd.html?id=357414
zieute aussi les autres ,
il y a de la matière ...
@+
-,)
1neoVendredi 5 Juin 2015 à 15:52Slt ,
de passage très rapidement avant les orages,
ensuite ,
je coupe ...
http://www.franceseisme.fr/seisme.php?IdSei=543
Evènement sismique 47km à l'O de Dieppe (dépt. 76, le 04/06/15 à 21h24 h. loc, ML=4.2) selon CEA-LDG
Date (en temps universel) : 04/06/2015
Heure (en temps universel) : 19h24
Magnitude : 4.2
Coordonnées :Latitude : 49.9466°N
Longitude : 0.4262°E
Témoigner sur ce séisme
Pourquoi?Carte d'intensités internet issue de 2 témoignages (Date de création : 05/06/2015 13:37 T.U. )
Le temps sur cette page est donné en temps universel (TU) ; pour obtenir le temps local français
- Métropole ajouter + 1h l'hiver, + 2h l'été
- Guadeloupe et Martinique - 4h / Guyane : - 3h / Réunion : + 4h / Mayotte +3h
Ajouter un commentaire
http://www.sciencesetavenir.fr/fondamental/20150603.OBS0109/la-voie-lactee-un-poids-lourd-de-210-milliards-de-fois-la-masse-du-soleil.html?cm_mmc=EMV-_-SEA-_-20150607_NLSEAACTU-_-la-voie-lactee-un-poids-lourd-de-210-milliards-de-fois-la-masse-du-soleil#xtor=EPR-6-[ActuSciences17h]-20150607
La Voie lactée, un poids lourd de 210 milliards de fois la masse du Soleil
Voir tous ses articles
Publié le 07-06-2015 à 14h00
La première estimation précise de la masse de notre galaxie a été obtenue par une équipe américaine.
<form class="down" action="http://emv.nouvelobs.com/inscription_emv.php" method="post"><label for="email">Recevoir les alertes</label><input id="email" name="mail" type="email" value="Votre adresse e-mail" /></form>
CALCUL. L’équivalent de 210 milliards de fois la masse du Soleil, voilà la quantité de matière rassemblée dans un rayon de 60.000 années-lumière autour du centre galactique, soit le rayon du disque de notre galaxie… C’est ainsi que les astronomes expriment la masse des objets célestes, en nombre de fois la masse de notre étoile. Sachant que cette dernière pèse 1,989.1030 kg, alors un rapide calcul met la masse de la galaxie à 420.1039 kg, soit 4,2.1041 kg, autrement dit 42.000 milliards de milliards de milliards de milliards de kilogrammes. Voici donc la valeur la plus précise obtenue à ce jour par une équipe du département d’Astronomie de l’université de Columbia (Etats-Unis) grâce à une nouvelle méthode publiée dans la revue américaine The Astrophysical journal.
Repérer la vitesse des étoiles périphériques
Pour avancer cette estimation, les astrophysiciens n’ont pas chômé : la difficulté vient du fait qu’il est impossible de peser la Voie Lactée directement, alors que notre système solaire en fait partie… Il faut donc employer quelques astuces : repérer la vitesse des étoiles périphériques par exemple. En effet, la vitesse d’une étoile en rotation est fonction de sa distance au centre galactique et de la valeur de l’attraction gravitationnelle à cet endroit. Or, ce dernier paramètre est relié à la masse du disque… et plus l’étoile est au bord du disque galactique plus sa vitesse nous informe sur la masse de l’intégralité du disque. Pendant longtemps, les évaluations basées sur cette méthode donnaient une valeur entachée d’une grosse erreur… qui pouvaient atteindre jusqu’à quatre fois la valeur.
Le coup de pouce de Palomar 5
Les astronomes de Columbia ont utilisé les données d’un des relevés astronomiques les plus ambitieux, le Sloan Digital Sky Survey qui balaie depuis dix ans le ciel de l’hémisphère nord pour dresser des catalogues d’étoiles. Ils ont aussi bénéficié du supercalculateur de l’université de Columbia, le Yeti. Ils se sont intéressés aux "courants stellaires", ces trainées de gaz et d’étoiles qui gravitent au-delà du disque galactique. Un de ces courants baptisé Palomar 5 est particulièrement connu, car découvert dans les années 1950 par l'astrophysicien américain d’origine allemande Walter Baade. Les étoiles de Palomar 5 appartenaient à un amas d’étoiles qui s’est délité au cours du temps. Grâce à la mesure de leur vitesse, l’équipe a déterminé une fourchette de valeurs pour la masse de la galaxie.
Observer la forme des trainées d'étoiles
Puis ils ont affiné leur méthode en observant la forme de cette trainée d’étoiles : des sillons denses et régulièrement espacés, qui ne pouvaient pas être le fruit du hasard. Ils ont ensuite modélisé à l’aide du super-calculateur Yeti plusieurs millions de formes de trainées d’étoiles en fonction des valeurs probables de masse de la galaxie pour finir par comparer la trainée observée avec les millions de modèles de trainées calculés par Yeti. Une fois repéré la bonne forme de sillon, ils ont regardé, dans les modèles, la masse galactique censée fournir ce sillon. Bingo ! Le travail fastidieux a bien payé : c’est ainsi qu’ils ont pu déterminer la masse de la Voie Lactée à 20% près ! Un record de précision pour un tel astre…