Free Astronomy Magazine March-April 2018

27 MARCH-APRIL 2018 SOLAR SYSTEM These features place them at the top of the list of the brightest stars in the universe. The very high temperature generates very powerful stellar winds (1000-2000 km/s) which, during the short life of WRs, sweep away their outer layers, leaving uncovered the red-hot innermost layers where helium is abundant. The material blown away by the stellar wind forms a giant bubble (up to tens of light-years long), which over time is enriched by the elements just pro- duced in the outer stellar layers, including 26 Al. Researchers have calculated that the impetuous WR stellar winds can expel up to half of the stellar mass over a period of hundreds of thousands of years. By processing observations made by various teams of researchers, Dwarkadas and his colleagues calculated that the average amount of 26 Al transferred to the shell of the bubble from a WR is equal to the mass of 3 Earths. They also determined the me- dium that carries the radioisotopes in that external environment. If the atoms of 26 Al darted freely through the bubble, they would not be effectively captured by the shell material. For this to happen, the atoms must ‘cling’ to something more voluminous. We have known for decades that WRs can have infrared emissions attributable to the G raphic simula- tion of the evolution of a bubble of dust and gases around a WR of 40 solar masses, in succes- sive periods, start- ing from the up- per left quadrant (t = 1.27 million years from the be- ginning of the stellar activity). Blue represents the wind-driven bubble, and ochre is the region ion- ised by the star, with the thick shell further out. [V. Dwarkadas et al.] another scenario that leads to the same re- sult but that can also explain the anomaly of the two radioisotopes? Maybe so. In an article recently published in The Astrophys- ical Journal , a team of researchers from the uni- versities of Chicago and Clemson led by Vikram Dwarkadas has put for- ward an alternative hy- pothesis, which no longer sees the Solar System born from a supernova’s shock wave crashing into an interstellar cloud, but from the gravitational collapse of part of a bub- ble of material ejected by a Wolf-Rayet star (WR). This type of star is very rare, gigantic (40-50 times more massive than the Sun) and has very high surface tempera- tures, from 30,000°C to 200,000°C. his animation shows how stellar winds can, over millions of years, produce bubbles inside the envelope of material that surrounds a giant star. A team from the University of Chicago claims that the Solar System could have formed within one of these bubbles. [V. Dwarkadas and D. Rosenberg]