Le laboratoire des étoiles

Les chercheurs du Laboratoire d’Astrophysique de Marseille scrutent l’espace à la recherche de petites galaxies fossiles couleur azur. Les naines bleues compactes sont les laboratoires de la naissance des étoiles.

Observer la naissance d’une étoile. Comme un film, comme celui de votre enfance qui prend la poussière dans ce placard chez vos parents. Si l’on pouvait échographier les galaxies, cela arrangerait bien Georges Comte, astrophysicien émérite du Laboratoire d’Astrophysique de Marseille (LAM), « mais ce serait moins amusant, » sourit-il. Pour savoir comment une étoile se forme, il faut trouver des galaxies où les étoiles sont très jeunes, massives et lumineuses. « Cela leur donne une jolie couleur bleue, » explique le chercheur, « on les nomme les naines bleues compactes. »

This sprinkle of cosmic glitter is a blue compact dwarf galaxy known as Markarian 209. Galaxies of this type are blue-hued, compact in size, gas-rich, and low in heavy elements. They are often used by astronomers to study star formation, as their conditions are similar to those thought to exist in the early Universe. Markarian 209 in particular has been studied extensively. It is filled with diffuse gas and peppered with star-forming regions towards its core. This image captures it undergoing a particularly dramatic burst of star formation, visible as the lighter blue cloudy region towards the top right of the galaxy. This clump is filled with very young and hot newborn stars. This galaxy was initially thought to be a young galaxy undergoing its very first episode of star formation, but later research showed that Markarian 209 is actually very old, with an almost continuous history of forming new stars. It is thought to have never had a dormant period — a period during which no stars were formed — lasting longer than 100 million years. The dominant population of stars in Markarian 209 is still quite young, in stellar terms, with ages of under 3 million years. For comparison, the Sun is some 4.6 billion years old, and is roughly halfway through its expected lifespan. The observations used to make this image were taken using Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys, and span the ultraviolet, visible, and infrared parts of the spectrum. A scattering of other bright galaxies can be seen across the frame, including the bright golden oval that could, due to a trick of perspective, be mistaken as part of Markarian 209 but is in fact a background galaxy. A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Nick Rose. Links:  Nick Rose’s Hidden Treasures entry on Flickr
La galaxie naine bleue compacte Markarian 209 capturée par le télescope Hubble.

 

Lorsqu’une étoile d’une galaxie naine bleue compacte nous éclaire, elle nous dévoile en même temps sa composition. A l’œil, elle parait bleue à cause de sa forte température et de sa vigoureuse luminosité. « Les galaxies naines bleues compactes sont notre laboratoire, » raconte Georges Comte, « c’est comme observer un service de maternité à l’hôpital…mais à des millions d’années lumières de nous ! » La lumière bleue des étoiles apporte aux scientifiques les informations qui vont permettre de retracer et standardiser leur évolution. À partir de cela, il est établi des modèles, de plus en plus précis, de la naissance et de la petite enfance des étoiles.

The bright streak of glowing gas and stars in this NASA/ESA Hubble Space Telescope image is known as PGC 51017, or SBSG 1415+437. It is type of galaxy known as a blue compact dwarf. This particular dwarf is well studied and has an interesting star formation history. Astronomers initially thought that SBS 1415+437 was a very young galaxy currently undergoing its very first burst of star formation, but more recent studies have suggested that the galaxy is in fact a little older, containing stars over 1.3 billion years old. Starbursts are an area of ongoing research for astronomers — short-lived and intense periods of star formation, during which huge amounts of gas within a galaxy are hungrily used up to form newborn stars. They have been seen in gas-rich disc galaxies, and in some lower-mass dwarfs. However, it is still unclear whether all dwarf galaxies experience starbursts as part of their evolution. It is possible that dwarf galaxies undergo a star formation cycle, with bursts occurring repeatedly over time. SBS 1415+437 is an interesting target for another reason. Dwarf galaxies like this are thought to have formed early in the Universe, producing some of the very first stars before merging together to create more massive galaxies. Dwarf galaxies which contain very few of the heavier elements formed from having several generations of stars, like SBS 1415+437, remain some of the best places to study star-forming processes similar to those thought to occur in the early Universe. However, it seems that our nearby patch of the Universe may not contain any galaxies that are currently undergoing their first burst of star formation. A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Nick Rose.
Le nuage de lumière bleue au centre de cette image du télescope Hubble est connu sous le nom de PGC 51017, ou SBSG 1415+437, une autre galaxie naine bleue compacte.

 

Si les galaxies naines bleues compactes portent leur nom en raison des amas d’étoiles naissantes qui les composent, elles n’en sont pas pour autant de jeunes galaxies. « Il y a parmi ces étoiles bleues, quelques étoiles gigantesques et rouges, prêtes à finir leur vie,  » raconte le chercheur. « Celles-ci montrent que ces galaxies naines sont très âgées ». Véritables pouponnières à étoiles professionnelles qui explosent dans tous les sens, les naines bleues compactes sont les galaxies les moins évoluées chimiquement dans l’univers. Elles pourraient, selon certains scientifiques, être des fossiles vivants du temps des premières galaxies. « Elles constituent en tout cas d’excellentes zones dans l’univers local pour étudier la formation d’étoiles et l’évolution des galaxies…Tout ça, grâce à une jolie lumière bleue, » conclut l’astrophysicien.

Renaud Levantidis

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