They鈥檙e the stars that refuse to die. Astronomers have detected four stars hurtling through the galaxy that are full of ashes, likely because they are survivors of enormous cosmic explosions. Two of them look set to escape the Milky Way altogether.
The first of these cosmic runaways, called LP 40-365, was discovered in 2017. Roberto Raddi at Friedrich Alexander University Erlangen-N眉rnberg in Germany and his colleagues have used data from the Gaia space observatory and the Sloan Digital Sky Survey to spot three more.
All four of these white dwarf聽stars seem to be survivors of a strange kind of supernova discovered in 2013, called type Iax. They are much dimmer than type Ia supernovae, whose consistent brightness allows us to use them as 鈥渟tandard candles鈥� to measure astronomical distances.
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Both types of explosions involve stars in a pair, where the white dwarf steals material from its partner until it gets too big and blows up. Type Ia supernovae destroy the exploding star completely, but it seems that type Iax blasts leave their stars intact while giving them an explosive kick that sends them hurtling through the galaxy.
The three stars found by Raddi鈥檚 team all have strange compositions.聽Stars聽are generally made of mostly hydrogen and helium, but these stars show no sign of those elements, instead favouring heavier ones like neon, oxygen and magnesium. The heavier elements seem to be a sort of nuclear ash left over from the explosion, Raddi says.
鈥淚t鈥檚 like they tried to go supernova and didn鈥檛 quite make it,鈥� says Ashley Pagnotta at the College of Charleston in South Carolina. 鈥淭hey came through the flames and out the other side.鈥�
These strange stars might help us understand not only the diversity of supernovae, but also聽the more regular Ia explosions we use as standard candles. 鈥淚t often turns out that studying the weirdos gives you some kind of information about the 鈥榥ormal鈥� population,鈥� says Pagnotta. 鈥淏y not following the rules, they help you define what the rules are.鈥�
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