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Neutral atoms formed a few hundred thousand years after the Big Bang. The very first stars started to ionize these atoms again, but it took hundreds of millions of years of star formation and galaxies for this process, called reionization, to be completed.Table of the time of the reionization of hydrogen (HERA)
Forming stars seems to be the easiest thing to do in the universe. Get some mass together, give it time to gravitate and watch it crumble into small, dense tufts. If you have enough together in the right conditions, stars will result. That's how you train the stars today, and that's how we formed stars throughout our cosmic history, dating back to the earliest, about 50 to 100 million years after the Big Bang.
But even with the first burning stars, which melt hydrogen into heavier elements and emit enormous amounts of light, the Universe absorbs too well and blocks that light. The reason? All atoms in the universe are neutral and they are simply too numerous for starlight to enter. It took hundreds of millions of years to the Universe to let the light through. It's a vital part of our cosmic story that hardly anyone realizes.
Schematic of the history of the universe, highlighting the reionization. Before stars or galaxies formed, the Universe was full of neutral atoms blocking light. While most of the Universe reionizes only 550 million years later, with the first great waves occurring around 250 million years ago, some lucky stars can form 50 to 100 million years after Big Bang. good tools, we can reveal the first galaxies.S. G. Djorgovski et al., Caltech Digital Media Center
The universe is always illuminated by the microwave cosmic background: the remaining radiation of the Big Bang itself. Less than half a million years after the Big Bang, neutral atoms formed and this radiation simply flowed freely in the sea of atoms. But this is only due to the fact that cosmic radiation had a much lower energy than neutral atoms (mainly hydrogen) are able to absorb.
If the radiation were more energetic, the atoms would not only absorb it, but re-diffuse it in all directions, where it would then be absorbed by additional atoms. This is only because the radiation is so low in energy – mostly infrared light – that it can freely traverse the space.
This four-panel view shows the central region of the Milky Way in four different wavelengths of light, with the longest (submillimetric) wavelengths at the top, passing through the far-infrared (2nd and 3rd) and ending with a view in visible light. of the Milky Way. Note that the dust bands and stars in the foreground obscure the center in visible light, but not in the infrared.ESO / ATLASGAL consortium / NASA / GLIMPSE consortium / VVV / ESA survey / Planck / D. Minniti / S. Guisard Acknowledgments: Ignacio Toledo, Martin Kornmesser
We even see it in our own galaxy: the galactic center can not be seen in visible light. Dust and gas block it, but the infrared light becomes clear. This explains why cosmic microwave background is not absorbed, but Starlight does.
Fortunately, the stars we form can be massive and warm, the most massive being much brighter and warmer than our own sun. The first stars can be tens, hundreds, even thousands of times more massive than our own Sun, which means that they can reach surface temperatures of several thousand degrees and a brightness that is millions of times brighter than our Sun. These mastodons are the biggest threat to neutral atoms scattered throughout the Universe.
An artist's design of what the Universe might look like when it will form stars for the first time. As they shine and fuse, electromagnetic and gravitational radiation will be emitted. The surrounding neutral atoms are ionized, but as long as there are more neutral atoms around them, the light does not penetrate through an arbitrary distance.NASA / ESA / ESO / Wolfram Freudling et al. (STECF)
The key is that, for stars above a certain temperature, they will emit a fraction of their light in the ultraviolet part of the spectrum: energy enough to ionize a neutral atom. To obtain a hydrogen atom at its lowest level of energy, it takes a photon of 13.6 eV (or more) to ionize it, that very few photons emitted by most stars own. But the hotter and more massive your star, the more it produces ionizing photons. Because these are the shortest stars, it's only a few million years after forming a new wave of stars that you get an excessive amount of ionizing photons.
The first stars and galaxies in the universe will be surrounded by neutral atoms of hydrogen (mainly) hydrogen, which absorb starlight. The large masses and high temperatures of these early stars help to ionize the universe, but more is needed than what these early generations of stars can provide.Nicole Rager Fuller / National Science Foundation
If all the atoms of the universe were ionized, the depths of the star-free space would be clearly traversed by light, which means that we could see the distant universe without a problem. But even if a small percentage of atoms remained neutral, this stellar light would be efficiently absorbed, which made it extremely difficult to detect everything that was happening at the time of the first stars and galaxies.
Sufficient star formation must therefore occur to flood the Universe with a sufficient number of ultraviolet photons to ionize enough neutral matter so that starlight can travel unhindered. This requires a large amount of star formation and requires this to happen fast enough so that ionized protons and electrons do not end up and recombine again.
A huge star formation region in the UGCA 281 galaxy, such as Hubble imagines in the visible and ultraviolet, as part of the LEGUS study. Blue light is the light of warm, young stars reflected from the background, a neutral gas, while the brightest areas indicate the greatest emission of ultraviolet rays. The red parts, however, testify to ionized hydrogen gas, which emits a characteristic red glow when electrons combine with free protons.NASA, ESA and the LEGUS team
The first stars make a small sprain, but the first clusters of stars are small and short. The universe will remain largely neutral with them alone. The second generation of stars, formed as a result of the death of the first generation, does not do much better.
The problem is that these newly formed stars form clusters of up to a few million solar masses. While a modern galaxy like our Milky Way could have a mass of about one trillion solar masses, filled with hundreds of billions of stars, the old star clusters do not have a star. contain about 0.001%. During the few hundred million years of our universe, they are barely enough to reduce the neutral matter in space.
Stars form in a wide variety of sizes, colors, and masses, including many bright blue ones that are tens, even hundreds of times more massive than the Sun. This is demonstrated here in open star group NGC 3766, in the constellation Centaurus. Star clusters can form much faster than early Universe galaxies, but by merging, they can make their way into galaxies.ESO
But this begins to change as star clusters merge to form the first galaxies. When large masses of gas, stars and other materials merge, they unleash a tremendous explosion of star formation, illuminating the Universe like never before. Over time, many phenomena occur at the same time:
- regions with the largest collections of material attract even more early stars and groups of stars,
- regions that have not yet formed stars can begin,
- and the regions where the first galaxies are made attract other young galaxies,
which serves to increase the overall rate of star formation.
If we were to map the Universe at that time, what we would see, is that the rate of star formation increases at a relatively constant rate during the first billion years. of the existence of the Universe. In some favorable regions, enough matter will ionize early enough that we can see through the universe before most regions are reionized; in others, it can take up to two or three billion years for the last neutral matter to go away.
If you were to map the neutral matter of the Universe from the very beginning of the Big Bang, you'd discover that it's starting to switch to ionized matter in clumps, but you'll also find that it took hundreds millions of years to disappear for the most part. This occurs unevenly and preferentially along the locations of the densest parts of the cosmic network.
After a certain distance, or a redshift (z) of 6, the Universe always contains a neutral gas, which blocks and absorbs light. These galactic spectra show the effect of a zero drop in the left of the big bump (Lyman series) for all galaxies that have exceeded a certain red shift, but not for those at a lower shift. This physical effect is called the Gunn-Peterson Hollow and will block the brightest light produced by the first stars and galaxies.X. Fan et al, Astron.J.132: 117-136, (2006)
On average, it takes 550 million years from the creation of the Big Bang for the Universe to reionise and become transparent in the light of the stars. We see it by observing ultra-distant quasars, which continue to display the characteristics of absorption that only neutral matter causes. In the same spirit, however, there are some directions where the question is re-ionized much earlier, which indicates that the formation of the structure is unequal and lets us hope to find early galaxies even before the limit of 550 million years .
In fact, the oldest galaxy discovered by Hubble, GN-z11, already comes from a time before that: barely 407 million years after the Big Bang.
This is only because this distant galaxy, GN-z11, is located in a region where the intergalactic medium is mainly reionized that Hubble can reveal to us at the present time. To see further, we need a better observatory, optimized for this type of detection, than Hubble.NASA, ESA and A. Feild (STScI)
There are still no clusters of galaxies in the Universe and the first galaxies, which formed largely between 200 and 250 million years after the Big Bang, do not will not be revealed in visible light. But through the eyes of an infrared observatory, where the light has a sufficient wavelength to not be absorbed by these neutral atoms, this stellar light could shine after all.
So it is no coincidence that the James Webb Space Telescope was designed to examine the spectrum in the near and medium infrared, up to a wavelength of 30 microns: about 50 times the longest wavelength. light that human eyes can see.
By exploring more and more of the universe, we can look further into space, which equates to further in time. The James Webb Space Telescope will take us directly into depths that our current observation facilities can not match, Webb's infrared eyes revealing the ultra-distant light that Hubble can not hope to see.NASA / JWST and HST teams
The light created at the beginning of the era of stars and galaxies plays a role. Ultraviolet light acts to ionize the surrounding material, allowing visible light to gradually increase as the ionization fraction increases. Visible light is diffused in all directions until the reionization is sufficiently advanced to allow our best current telescopes to see it. But the infrared light, also created by the stars, crosses even the neutral matter, which allows our telescopes of the years 2020 to find them again.
When the light of the stars crosses the sea of neutral atoms, even before the end of the reionization, it allows us to detect the oldest objects we have ever seen. When the James Webb Space Telescope is launched, it will be the first thing we look for. The farthest limits of the universe are within our reach. We just have to search and find out what is really there.
To learn more about the nature of the universe when:
- How was it when the Universe inflated?
- How was it when the Big Bang started?
- How was it when the Universe was at the hottest?
- How was it when the Universe created more matter than antimatter?
- How was it when the Higgs gave mass to the universe?
- How was this the first time we made protons and neutrons?
- How was it when we lost the last of our antimatter?
- How was it when the Universe created its first elements?
- How was it when the Universe created the atoms?
- How was it when there were no stars in the universe?
- How was it when the first stars started to illuminate the universe?
- How was it when the first stars died?
- How was it when the Universe created its second generation of stars?
- How was it when the Universe created the very first galaxies?
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Neutral atoms formed a few hundred thousand years after the Big Bang. The very first stars started to ionize these atoms again, but it took hundreds of millions of years of star formation and galaxies for this process, called reionization, to be completed.Table of the time of the reionization of hydrogen (HERA)
Forming stars seems to be the easiest thing to do in the universe. Get some mass together, give it time to gravitate and watch it crumble into small, dense tufts. If you have enough together in the right conditions, stars will result. That's how you train the stars today, and that's how we formed stars throughout our cosmic history, dating back to the earliest, about 50 to 100 million years after the Big Bang.
But even with the first burning stars, which melt hydrogen into heavier elements and emit enormous amounts of light, the Universe absorbs too well and blocks that light. The reason? All atoms in the universe are neutral and they are simply too numerous for starlight to enter. It took hundreds of millions of years to the Universe to let the light through. It's a vital part of our cosmic story that hardly anyone realizes.
Schematic of the history of the universe, highlighting the reionization. Before stars or galaxies formed, the Universe was full of neutral atoms blocking light. While most of the Universe reionizes only 550 million years later, with the first great waves occurring around 250 million years ago, some lucky stars can form 50 to 100 million years after Big Bang. good tools, we can reveal the first galaxies.S. G. Djorgovski et al., Caltech Digital Media Center
The universe is always illuminated by the microwave cosmic background: the remaining radiation of the Big Bang itself. Less than half a million years after the Big Bang, neutral atoms formed and this radiation simply flowed freely in the sea of atoms. But this is only due to the fact that cosmic radiation had a much lower energy than neutral atoms (mainly hydrogen) are able to absorb.
If the radiation were more energetic, the atoms would not only absorb it, but re-diffuse it in all directions, where it would then be absorbed by additional atoms. This is only because the radiation is so low in energy – mostly infrared light – that it can freely traverse the space.
This four-panel view shows the central region of the Milky Way in four different wavelengths of light, with the longest (submillimetric) wavelengths at the top, passing through the far-infrared (2nd and 3rd) and ending with a view in visible light. of the Milky Way. Note that the dust bands and stars in the foreground obscure the center in visible light, but not in the infrared.ESO / ATLASGAL consortium / NASA / GLIMPSE consortium / VVV / ESA survey / Planck / D. Minniti / S. Guisard Acknowledgments: Ignacio Toledo, Martin Kornmesser
We even see it in our own galaxy: the galactic center can not be seen in visible light. Dust and gas block it, but the infrared light becomes clear. This explains why cosmic microwave background is not absorbed, but Starlight does.
Fortunately, the stars we form can be massive and warm, the most massive being much brighter and warmer than our own sun. The first stars can be tens, hundreds, even thousands of times more massive than our own Sun, which means that they can reach surface temperatures of several thousand degrees and a brightness that is millions of times brighter than our Sun. These mastodons are the biggest threat to neutral atoms scattered throughout the Universe.
An artist's design of what the Universe might look like when it will form stars for the first time. As they shine and fuse, electromagnetic and gravitational radiation will be emitted. The surrounding neutral atoms are ionized, but as long as there are more neutral atoms around them, the light does not penetrate through an arbitrary distance.NASA / ESA / ESO / Wolfram Freudling et al. (STECF)
The key is that, for stars above a certain temperature, they will emit a fraction of their light in the ultraviolet part of the spectrum: energy enough to ionize a neutral atom. To obtain a hydrogen atom at its lowest level of energy, it takes a photon of 13.6 eV (or more) to ionize it, that very few photons emitted by most stars own. But the hotter and more massive your star, the more it produces ionizing photons. Because these are the shortest stars, it's only a few million years after forming a new wave of stars that you get an excessive amount of ionizing photons.
The first stars and galaxies in the universe will be surrounded by neutral atoms of hydrogen (mainly) hydrogen, which absorb starlight. The large masses and high temperatures of these early stars help to ionize the universe, but more is needed than what these early generations of stars can provide.Nicole Rager Fuller / National Science Foundation
If all the atoms of the universe were ionized, the depths of the star-free space would be clearly traversed by light, which means that we could see the distant universe without a problem. But even if a small percentage of atoms remained neutral, this stellar light would be efficiently absorbed, which made it extremely difficult to detect everything that was happening at the time of the first stars and galaxies.
Sufficient star formation must therefore occur to flood the Universe with a sufficient number of ultraviolet photons to ionize enough neutral matter so that starlight can travel unhindered. This requires a large amount of star formation and requires this to happen fast enough so that ionized protons and electrons do not end up and recombine again.
A huge star formation region in the UGCA 281 galaxy, such as Hubble imagines in the visible and ultraviolet, as part of the LEGUS study. Blue light is the light of warm, young stars reflected from the background, a neutral gas, while the brightest areas indicate the greatest emission of ultraviolet rays. The red parts, however, testify to ionized hydrogen gas, which emits a characteristic red glow when electrons combine with free protons.NASA, ESA and the LEGUS team
The first stars make a small sprain, but the first clusters of stars are small and short. The universe will remain largely neutral with them alone. The second generation of stars, formed as a result of the death of the first generation, does not do much better.
The problem is that these newly formed stars form clusters of up to a few million solar masses. While a modern galaxy like our Milky Way could have a mass of about one trillion solar masses, filled with hundreds of billions of stars, the old star clusters do not have a star. contain about 0.001%. During the few hundred million years of our universe, they are barely enough to reduce the neutral matter in space.
Stars form in a wide variety of sizes, colors, and masses, including many bright blue ones that are tens, even hundreds of times more massive than the Sun. This is demonstrated here in open star group NGC 3766, in the constellation Centaurus. Star clusters can form much faster than early Universe galaxies, but by merging, they can make their way into galaxies.ESO
But this begins to change as star clusters merge to form the first galaxies. When large masses of gas, stars and other materials merge, they unleash a tremendous explosion of star formation, illuminating the Universe like never before. Over time, many phenomena occur at the same time:
- regions with the largest collections of material attract even more early stars and groups of stars,
- regions that have not yet formed stars can begin,
- and the regions where the first galaxies are made attract other young galaxies,
which serves to increase the overall rate of star formation.
If we were to map the Universe at that time, what we would see, is that the rate of star formation increases at a relatively constant rate during the first billion years. of the existence of the Universe. In some favorable regions, enough matter will ionize early enough that we can see through the universe before most regions are reionized; in others, it can take up to two or three billion years for the last neutral matter to go away.
If you were to map the neutral matter of the Universe from the very beginning of the Big Bang, you'd discover that it's starting to switch to ionized matter in clumps, but you'll also find that it took hundreds millions of years to disappear for the most part. This occurs unevenly and preferentially along the locations of the densest parts of the cosmic network.
After a certain distance, or a redshift (z) of 6, the Universe always contains a neutral gas, which blocks and absorbs light. These galactic spectra show the effect of a zero drop in the left of the big bump (Lyman series) for all galaxies that have exceeded a certain red shift, but not for those at a lower shift. This physical effect is called the Gunn-Peterson Hollow and will block the brightest light produced by the first stars and galaxies.X. Fan et al, Astron.J.132: 117-136, (2006)
On average, it takes 550 million years from the creation of the Big Bang for the Universe to reionise and become transparent in the light of the stars. We see it by observing ultra-distant quasars, which continue to display the characteristics of absorption that only neutral matter causes. In the same spirit, however, there are some directions where the question is re-ionized much earlier, which indicates that the formation of the structure is unequal and lets us hope to find early galaxies even before the limit of 550 million years .
In fact, the oldest galaxy discovered by Hubble, GN-z11, already comes from a time before that: barely 407 million years after the Big Bang.
This is only because this distant galaxy, GN-z11, is located in a region where the intergalactic medium is mainly reionized that Hubble can reveal to us at the present time. To see further, we need a better observatory, optimized for this type of detection, than Hubble.NASA, ESA and A. Feild (STScI)
There are still no clusters of galaxies in the Universe and the first galaxies, which formed largely between 200 and 250 million years after the Big Bang, do not will not be revealed in visible light. But through the eyes of an infrared observatory, where the light has a sufficient wavelength to not be absorbed by these neutral atoms, this stellar light could shine after all.
So it is no coincidence that the James Webb Space Telescope was designed to examine the spectrum in the near and medium infrared, up to a wavelength of 30 microns: about 50 times the longest wavelength. light that human eyes can see.
By exploring more and more of the universe, we can look further into space, which equates to further in time. The James Webb Space Telescope will take us directly into depths that our current observation facilities can not match, Webb's infrared eyes revealing the ultra-distant light that Hubble can not hope to see.NASA / JWST and HST teams
The light created at the beginning of the era of stars and galaxies plays a role. Ultraviolet light acts to ionize the surrounding material, allowing visible light to gradually increase as the ionization fraction increases. Visible light is diffused in all directions until the reionization is sufficiently advanced to allow our best current telescopes to see it. But the infrared light, also created by the stars, crosses even the neutral matter, giving our 2020 telescopes a chance to find them.
When the light of the stars crosses the sea of neutral atoms, even before the end of the reionization, it allows us to detect the oldest objects we have ever seen. When the James Webb Space Telescope is launched, it will be the first thing we look for. The farthest limits of the universe are within our reach. We just have to search and find out what is really there.
Pour en savoir plus sur la nature de l'univers quand:
