Our Solar System is navigating through a particularly empty region of space. Surrounded by a local bubble, as astronomers call it. And today, they tell us how this bubble was formed. Following the explosion of several supernovae.
For decades, the astronomers know that it exists around our solar system, like a gigantic bubble. A “local bubble” which still measures some 1,000 light years of diameter. Today, some researchers from Harvard University and STScI (Space Telescope Science Institute) report finding that all youngest stars around us are located precisely on the surface of this bubble. All the stars younger ones, but also several star-forming regions and a few clouds molecules — these are dense regions in which stars can successfully form — well known. From these observations, these astronomers tell us the story of this strange bubble.
It all started nearly 14.5 million years ago. With a fairly intense star formation phase. Then, with thesupernova explosion of fifteen massive stars. A series of explosions which began to push the gas interstellar and dust outwards. Leaving us in a low density region. And further giving rise to a bubble-like structure, the surface of which has just the right characteristics for star formation.
According to astronomers, this local bubble still continues to expand today. To one speed almost 6.5 kilometers per second. It’s something like 23,400 kilometers per hour. A high speed on our scale. But speaking of an astronomical phenomenon, researchers believe that our local bubble “lost its punch”. And practically reached a plateau.
Other bubbles in the Milky Way?
To rewrite the history of this astonishing bubble, astronomers relied on numerous tools. Supernova models, stellar movements — according to data from the european space mission Gaia — or even 3D maps of the matter which precisely surrounds our local bubble.
The researchers point out that it is a happy coincidence that our Sun is now almost at the center of this bubble. The result of his journey through the Milky Way. He led it there about 5 million years ago now. A proof, according to astronomers that this type of structure must exist elsewhere in our Galaxy. Because if these giant bubbles were rare, it would be statistically unlikely that we would find ourselves in the heart of one of them.
Our Milky Way would thus appear a bit like a cheese with holes. Holes – the bubbles – dug by dying stars, supernovae, and on the edges of which one can observe nascent stars. It now only remains for researchers to map a few of these bubbles. To establish their shape and size as well as their location. And why not, their modes of interaction. A new way to understand what role end-of-life stars play in star formation, but also in the evolution of galaxies.
Who created the local Bubble, where is the Sun? Supernovae…
Highlighted during the 1970s, the local Bubble within which our Sun and its planets are currently moving would have been created by the successive explosion of multiple supernovae, a fire of cosmic fireworks that occurred about 10 million years ago. However, another interpretation is possible: charge exchanges of the solar wind with neutral gases from the interstellar medium. To decide between them, a team of researchers has developed a specific instrument. Result: the supernova hypothesis is strengthened.
Article of Xavier Demeersmann published on 09/09/2014
As we mentioned recently, according to a study that made the front page of Nature, our galaxy the Milky Way, in which we live, belongs to a supercontinent galaxy named by its discoverers Laniakea (“Immense celestial horizons” in the Hawaiian language). So much for the very large scale painting. At more modest dimensions, our Sun currently evolves inside one of the lobes of what the astrophysicists call since its highlighting in the 1970s and 1980s, the “local bubble”.
With an estimated length of 300 light-years, its shape resembles that of a peanut or an hourglass. Its density is very low (0.001 atom per cubic centimeter) and the temperature of the gases is particularly high in all directions, as shown by the first observations in X-rays.
Plethora of supernovae
To explain these cavities in the interstellar medium, the main scenario proposed is that of a series of supernovae. Remember that these incredibly powerful star explosions (theenergy deployed can surpass that developed by the Sun during a few million years) are not rare in the Milky Way. Astronomers estimate that in our galaxy, there is one every fifty years on average. If many escape us (dating back to 1604, the Kepler supernova was the last in our galaxy to have been observed), it is mainly because they are obscured by clouds of stars and dust from the galactic disc.
However, it was very different in our neighborhood 10 million years ago. Indeed, it appears that several members of a massive star cluster literally popped out like popcorn, leaving behind huge residual bubbles that continue to expand even today! This must have happened at a reasonable distance from Earth, because our biosphere does not seem to have suffered from possible shock waves, showing no stigma for this period.
Supernovae seem to have created the Bubble
The hypothesis of a local Bubble dug by the explosion of stars is not unanimous in the scientific community. As F. Scott Porter, one of the authors of the article published this summer in Nature (issue of July 27, 2014), “Over the past decade, researchers have challenged the interpretation of supernovae, suggesting that most if not all of this soft X-ray radiation is the result of charge exchange”. A charge swap? For them, these are surges of electrically charged particles emitted by our star (the solar wind) on neutral gas beaches, because the glows observed in the X-ray range produce the same effect as ancient supernova remnants.
Also, to decide between the two proposals and shed light on this subject, the researcher joined forces with Massimiliano Galeazzi (University of Miami) and his team to develop a detector sensitive to this wave length, able to distinguish the two signatures. Launched at an altitude of around 275 km on December 12, 2012, their baby named DXL (Diffuse X-ray emission from the Local Galaxy) spent only five minutes in space before returning to Earth, loaded with valuable data on charge exchanges in the Solar System. Finally, after several months of research, it appears that around 40% are of solar origin. Ancient supernovae would therefore be responsible for everything else.
“This is an important discovery” concludes Professor Galeazzi, because it touches on “our understanding of the region near the Sun and can therefore be used as a foundation for future models of the structure of our galaxy”. Together with new measuring instruments, DXL should make its next jump into space in December 2015.
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