It has been said that all the light ever radiated by all the stars that ever existed in the history of observable universe –a bubble 14 billion light-years in radius, which represents how far we have been able to see since its beginning– is still with us, filling the universe with a sea of photons, the cosmic fog known as the extragalactic background light –providing as much illumination as a 60-watt bulb seen from 2.5 miles away.
Fossil Stars of the Cosmos
“The first stars,” reports the University of Texas in a new study, “are hypothesized to have formed about 100 million years after the Big Bang out of universal darkness from the primordial gases of hydrogen, helium, and trace light metals. These gases cooled, collapsed, and ignited into stars up to 1,000 times more massive than our sun. The bigger the star, the faster they burn out. The first stars probably only lived a few million years, a drop in the bucket of the age of the universe, at about 13.8 billion years. They’re unlikely to ever be observed, lost to the mists of time.”
The aim of the new study is to know the origin of elements, such as carbon, oxygen, and calcium that we human beings are made of. “These elements are concentrated through the repetitive matter cycles between the interstellar medium and stars,” said Gen Chiaki, a post-doctoral researcher in the Center for Relativistic Astrophysics at Georgia Tech. “Our bodies and our planet are made of metal and dust, carbon and oxygen, nitrogen, and calcium.”
As the metal-free first stars collapsed and exploded into supernovae, they forged heavier elements such as carbon that seeded the next generation of stars. One type of these second stars is called a carbon-enhanced metal-poor star. They’re like fossils to astrophysicists. Their composition reflects the nucleosynthesis, or fusion, of heavier elements from the first stars.
Search for ‘Giga-Metal-Poor’ Stars
“We can get results from indirect measurements to get the mass distribution of metal-free stars from the elemental abundances of metal-poor stars,” said Chiaki, the lead author of a study that modeled for the first time faint supernovae of metal-free first stars, which yielded carbon-enhanced abundance patterns through the mixing and fallback of the ejected bits.
“We find that these stars have very low iron content compared to the observed carbon-enhanced stars with billionths of the solar abundance of iron. However, we can see the fragmentation of the clouds of gas. This indicates that the low mass stars form in a low iron abundance regime. Such stars have never been observed yet. Our study gives us theoretical insight of the formation of first stars,” Chiaki said about simulations that also showed the carbonaceous grains seeding the fragmentation of the gas cloud produced, leading to formation of low-mass ‘giga-metal-poor’ stars that can survive to the present day and possibly be found in future observations.
Galactic Archaeology — Living Fossils
The investigations are a part of a field called ‘galactic archaeology.’ where the character of long-gone stars can be revealed from their fossilized remains.
“We can’t see the very first generations of stars,” said study co-author John Wise, an associate professor also at the Center for Relativistic Astrophysics, School of Physics, Georgia Tech. “Therefore, it’s important to actually look at these living fossils from the early universe, because they have the fingerprints of the first stars all over them through the chemicals that were produced in the supernova from the first stars.”
Fingerprints of the Nucleosynthesis of Metal-Free Stars
“These old stars have some fingerprints of the nucleosynthesis of metal-free stars. It’s a hint for us to seek the nucleosynthesis mechanism happening in the early universe,” Chiaki said.
“That’s where our simulations come into play to see this happening. After you run the simulation, you can watch a short movie of it to see where the metals come from and how the first stars and their supernovae actually affect these fossils that live until the present day,” Wise said.
The First Star –Population III
The scientists first modeled the formation of their first star, called a Population III or Pop III star, and ran three different simulations that corresponded to its mass at 13.5, 50, and 80 solar masses. The simulations solved for the radiative transfer during its main sequence and then after it dies and goes supernova. The last step was to evolve the collapse of the cloud of molecules spewed out by the supernova that involved a chemical network of 100 reactions and 50 species such as carbon monoxide and water.
The majority of the simulations ran on the Georgia Tech PACE cluster. They were also awarded computer allocations by the National Science Foundation (NSF)-funded Extreme Science and Engineering Discovery Environment (XSEDE). Stampede2 at the Texas Advanced Computing Center (TACC) and Comet at the San Diego Supercomputer Center (SDSC) ran some of the main sequence radiative transfer simulations through XSEDE allocations.
Source: “Seeding the second star — II. CEMP star formation enriched from faint supernovae,” was published in the September 2020 issue of the Monthly Notices of the Royal Astronomical Society.
The Daily Galaxy, Sam Cabot, via University of Texas at Austin Advanced Computing Center
Image at top of page: Adolf Schaller for STScI –artist’s conception of early star formation The first stars are thought to have formed as early as 100 million years after the big bang, when dense regions of hydrogen and helium collapsed under their own gravitational pull. Once the pressure and temperature in the center of the cloud was high enough, hydrogen atoms began to fuse together, releasing energy in the form of light.