The first stars in the universe formed from primordial hydrogen, helium, and a smattering of lithium. These materials were created in the cosmos’ first few minutes, whereas the stars formed hundreds of millions of years later. Those suns, which were likely 100 to 200 solar masses, fused heavier elements, called metals, in their cores.
After a few million years, they died in fantastic explosions that converted their cores to black holes and ejected the rest of their material into the cosmos to create new stars. That gas included heavy elements created as the explosions’ energies heated the stars’ outer material.
Astronomers have found a handful of these second-generation suns, but a new study in the February 27 issue of Nature describes the newest find: the purest star ever seen. This star, SMSS J031300.36– 670839.3 (or SM0313 for short), has some 12 million times less iron than our Sun.
Stefan Keller of Mount Stromlo Observatory in Australia and colleagues could place only an upper limit on the iron abundance in the star, and that value is still 30 times less than the previous most iron-poor sun.
The researchers determined abundances of just a few elements in this star: calcium, magnesium, carbon, lithium, and hydrogen. They then computationally modeled the life cycles (and deaths) of stars of different masses to determine what elements they spewed during their deaths.
Keller’s team compared the theoretical compositions to that of SM0313 to figure out what size first-generation star would have produced the newly observed sun: a 60-solar-mass star died in a low-energy supernova, leaving behind a black hole and the lighter elements observed in SM0313. The astronomers suggest that the universe’s first stars weren’t only behemoths hundreds of times the mass of our Sun, which exploded as extremely energetic supernovae; low-energy supernovae (and their lower-mass progenitor stars) seeded the cosmos, too.
Fast Facts: NuSTAR reveals lopsided stellar explosion When NASA launched the Nuclear Spectroscopic Telescope Array (NuSTAR) in June 2012, the agency had a specific goal of using its X-ray vision to better understand how massive stars explode. With a recent study published in the February 20 issue of Nature, astronomers are doing just that.
By mapping radioactive material in a supernova remnant for the first time, NuSTAR scientists have been able to peer into the inner workings of the dying star that produced Cassiopeia A. The Cassiopeia A supernova remnant was created when a star with more than eight times the Sun’s mass reached the end of its life as a core-collapse supernova; the leftover debris of the event is what’s visible to astronomers 334 years later. [/one_half]
Using NuSTAR’s ability to detect high-energy X-rays invisible to other telescopes, scientists studied Cassiopeia A to record the locations of the radioactive isotope titanium-44, which was produced when the massive star’s core collapsed.
What they found were concentrated clumps of titanium instead of a uniform distribution. “Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would look like a uniform ball expanding out with great power,” says Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology in Pasadena.
“Our new results show how the explosion’s heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating.”