The Double Degenerate Dance: A Close(ish) Look at a Special Supernova
Type 1a Supernovas are as fascinating as they are mysterious. New evidence points to a more likely origin for them involving two white dwarf stars colliding.
This news was quite a surprise to me. Before I get into it, let’s lay some groundwork to set the cosmic scene.
Supernovas are exploding stars. That was easy right? There are however different types of these gargantuan explosions. My favorite and probably the one you think of immediately is called a core-collapse supernova. Fusion stops at the core of a giant star ending a battle that was raging for millions upon millions of years. This battle pitted gravity pushing inwards and the nuclear fires pushing light and heat outwards. When fusion at the core stopped, gravity yells “Yipeee” and begins a mad dash inwards in an attempt to squish everything into a black hole or neutron star. That outward rush of radiation has a final laugh however. A percentage of the mass of the stars rebounds back outwards into the universe and our eyes and telescopes in the form of light and neutrinos. This of course creates the outpouring of light we see in the night-sky that can outshine the entire host galaxy the explosion resides in.
This is not what this news-item is all about though. It’s about a second type of supernova called a 1a. This has somewhat more modest beginnings but its details are much more controversial. All astronomers generally agree that Supernova 1a explosions involve a (carbon and oxygen rich) white dwarf star. This is a dead(ish) stellar remnant that’s about the size of the earth yet has a mass comparable to our sun’s. This is an incredibly dense substance referred to as degenerate matter. In its prior life it was a low to medium mass star that never had what it takes to become a neutron star or black hole. It can explode however quite magnificently if it can somehow accrete enough mass onto itself to exceed
1.44 times the mass of our sun. This input of extra mass cause the carbon and oxygen to fuse into heavier elements raising the temperature to billions of degrees but without the ability to expand itself (as regular stars do) to tame these extraordinary temperatures. This quickly becomes overwhelming causing the entire star to explode at perhaps 6% of the speed of light (40 million miles per hour).
How this mass increases is what is controversial. Some models envision the white dwarf siphoning off mass from various types of partner stars like a red giant. Other models postulate it’s caused by the merger of two white dwarf stars in what must be one helluva head-on collision. It is perhaps most likely that both scenarios can occur.
What actually happens may be a little clearer now that astronomers have had the closest look in decades at a 1a supernova earlier this year in a nearby galaxy. Astronomers are weird because when they say “nearby’ in this context they mean 11 million light years away. This happened this past January in a galaxy called M82 or The Cigar Galaxy. It was seen using a huge network of radio telescopes throughout Europe and the UK plus the seven-telescope collective known as e-MERLIN and run by The University of Manchester’s Jodrell Bank Observatory.
Even when this was reported on last January after it was determined to be a 1a, the default assumption by the discoverers was that the explosion was caused by stellar siphoning:
“Spectra collected by astronomers at other observatories around the world suggest that it is a Type Ia supernova, caused by a white dwarf star pulling matter off a larger neighbouring star until it becomes unstable and explodes.”
After careful analysis however, the researchers conclude that, for this specific case at least, the supernova was caused not by the slow stealing of matter from another star but the merger of 2 white dwarf stars.
“The non-detection of radio and x-ray emission form SNe 1a have added to a growing consensus that a large fraction of SNe 1a may not be the result of SD (Single Degenerate or siphoning) scenarios”
This doesn’t mean all 1a Supernovas are caused by white dwarf collisions but perhaps this scenario is even more common than we thought. This is important to learn because supernovas such as these are common throughout the universe and greatly influence the evolution of galaxies and the composition of the cosmos as they spew out the heavy elements that seed the next generation of solar systems just as previous generations seeded the calcium in your bones.
P.S. Don’t forget that 1a Supernovas are also important because of their homogeneity. This makes them great for use as “standard candles” to calculate extreme stellar distances. This also allowed us to use them in the 1990s to determine that the expansion of the universe is not slowing down but in fact speeding up!! (hello Nobel Prize)
Image Credit: GSFC/D. Berry.