Supernova Debris Leaves Clues About Ancient Stars

The explosion of Type Ia (one A) supernovae in binary star systems could tell secrets of the ancient past, revealing the makeup of long-dead stars. When white dwarfs — dense corpses of smaller stars about the size of the Earth — are in binary systems, material from one star can fall onto the surface of the white dwarf, collecting above the surface of the stellar companion. When enough gas collects on the star, these stars can erupt into a massive explosion, called a Type Ia supernova.

These Type Ia supernovae (the plural of “supernova”) all erupt at nearly-identical energies, and have long been used as a “standard candle” by astronomers seeking to measure distances to distant galaxies. These events flare up for a few months before fading away. In recent years, these eruptions have been shown to have some variation, but measurements of the energy from these supernovae can still be calibrated, allowing astronomers to calculate distances to far-flung galaxies.

An artist’s conception of material from a star falling onto a smaller companion in a binary star system. Image credit: NASA

“We call Type Ia supernovae ‘standardizable candles.’ If you look at a candle at a distance, it will look dimmer than when it’s up close… We use them all the time in cosmology. So, it’s important to understand where they come from and characterize the white dwarfs that generate these explosions,” Evan Kirby, assistant professor of astronomy at CalTech, states.

It All Seemed So Simple…

At the start of the 20th Century, an astrophysicist named Subrahmanyan Chandrasekhar determined that once enough material fell on a white dwarf to exceed 1.4 times the mass of the Sun, the body would erupt as a supernova. This limit became known as the Chandrasekhar mass, a rule of cosmology taught in introductory astronomy courses.

The formation of a Type Ia supernova in a binary system. Image credit: NASA, ESA and A. Feild (STScI)

However, although the Chandrasekhar mass explains why white dwarfs over this limit explode as Type Ia supernovae, it does not explain why some stars with lower masses also, sometimes, explode in a massive eruption.

When Caltech astronomers studied ancient supernovae, they found that Type Ia events took place on stars possessing less than the critical Chandrasekhar mass. This finding…