Will we ever glimpse the universe's first stars?
来源：未知 作者：慕容炎 时间：2019-03-07 02:19:01
By Rachel Courtland (Image: NASA/ESA/G Illingworth/UCSC/R Bouwens/Leiden U/HUDF09 team) The race to see the universe’s most distant objects continues. Astronomers reported today the discovery of the farthest galaxy seen to date. Its light, which was captured by the Hubble Space Telescope, was emitted when the universe was less than 500 million years old, making it the oldest galaxy yet observed. If history is any guide, the galaxy, dubbed UDFj-39546284, is unlikely to hold that title for long. So how far back in time can ultimately we go? New Scientist takes a look at the prospects for seeing the very first galaxies and stars. What is the oldest light that reaches Earth? The very oldest light we can see comes from the cosmic microwave background, a haze of radiation that was emitted when the universe was less than 400,000 years old. After this light was released, the universe entered the cosmic dark ages. It likely took at least 50 million years before the first stars lit up the night sky, and the first galaxies probably formed roughly 200 million years later. Why are astronomers interested in these ancient objects? Studying the first stars and galaxies could help shed light on an era called ‘reionisation‘, which occurred within the first billion years after the big bang. During that time, neutral hydrogen atoms in the space between stars were broken into protons and electrons, making the universe transparent to ultraviolet light. Radiation from the first stars and galaxies, and possibly dark matter, is thought to have caused the reionisation, but exactly how the process occurred is still unclear. Studying early galaxies could also reveal how the colossal black holes inside galaxies, which may have grown up faster than the galaxies themselves, reached maturity. Can we see the first stars and galaxies with current telescopes? A deep image of a small patch of the night sky, made with the Hubble Space Telescope, was needed to find the new galaxy. But current telescopes are unlikely to probe much farther. That’s because the light from ancient stars and galaxies is stretched to longer, and redder, wavelengths by the expansion of space. To properly see these objects, astronomers need a sensitive telescope that can detect light farther into the infrared part of the spectrum. The best bet for seeing stars and galaxies much beyond the current record-holder is NASA’s James Webb Space Telescope (JWST), which could launch as early as 2015. Could JWST see the very first stars? It depends on how big the first stars, dubbed Population III stars, became and how late they formed. The first stars coalesced from a pure mix of hydrogen, helium, and lithium forged in the big bang, but it is not clear how massive they were. Interactions with dark matter may have allowed these first stars to grow into bloated behemoths dubbed dark stars. “One thing that really hasn’t converged yet is the typical mass of these Population III stars,” says Erik Zackrisson of Stockholm University in Sweden. “We don’t know whether it’s 10 or 100 times the mass of the sun. If the universe can form dark stars, it could be 1000 solar masses.” Since the mass of the star has a strong influence on its brightness and its longevity, “it is an important factor in trying to figure out whether [the first stars] will be observable or not”, he says. Individual Population III stars are likely to be too dim for even JWST to detect, Zackrisson says. He says a better bet would be to look for galaxies that are mostly made of Population III stars or to look for the bright flashes created when those stars explode. Will JWST see the first galaxies? Maybe. “JWST has been marketed as the ‘first light’ machine, but if you look at the predictions in the literature, they are pretty bleak,” Zackrisson says. “The earliest galaxies may be too distant and dim to see with JWST.” Galaxies that formed hundreds of millions of years after the first ones, in parts of the universe where the density of matter is relatively low, may offer the best chance of spotting stars like those that formed soon after the big bang. That is because in low-density regions, primordial gas clouds would take longer to collapse into stars, so they could be forming their first stars hundreds of millions of years after the first Population III stars were born. Zackrisson reckons that clusters of galaxies relatively close to Earth could aid in the search for these late-blooming galaxies, since the clusters’ gravity could bend and magnify the distant galaxies’ light. More on these topics: