Plenty of issues out within the Universe may cause a supernova, from the gravitational collapse of a large star, to the collision of white dwarfs. However many of the supernovae we observe are in different galaxies, too distant for us to see the main points of the method. So, as an alternative, we categorize supernovae by noticed traits equivalent to the sunshine curves of how they brighten and fade and the varieties of components recognized of their spectra. Whereas this provides us some concept of the underlying trigger, there are nonetheless issues we don’t solely perceive. That is significantly true for one specific sort of supernova often known as Sort Ia.
You’ve gotten possible heard of Sort Ia supernovae as a result of they’re central to our understanding of cosmology. They’ve an essential attribute of getting a uniform most brightness. This implies we will observe their obvious brightness, examine it to their precise brightness, and calculate their distance. Because of this, they’re also known as normal candles, they usually had been the primary method we realized that the Universe isn’t just increasing; it’s accelerating below the affect of darkish power.
From the spectra of those supernovae, we will see that the preliminary brightness is powered by the radioactive decay of nickel-56, whereas a lot of the later brightness comes from the decay of cobalt-56. We additionally see the presence of ionized silicon close to peak brightness, which no different kind of supernova has. This tells us that Sort Ia supernova usually are not brought on by the core collapse of a star, however somewhat some sort of thermal runaway impact.
The most well-liked mannequin for Sort Ia supernovae is that they’re brought on by the collapse of a white dwarf. When a white dwarf is a part of a detailed binary with an getting old pink big, the white dwarf can seize a few of the companion’s outer layer. Over time, the white dwarf captures sufficient mass that it crosses the Chandresekhar restrict, which triggers the supernova. Because the Chandrasekhar restrict is all the time at 1.4 photo voltaic plenty, this may clarify why Sort Ia supernovae all the time have the identical most brightness.
However as we’ve noticed ever extra supernovae, we’ve realized that Sort Ia supernovae don’t all the time have the identical most brightness. There are some which might be significantly brighter, with weaker silicon traces of their spectra and stronger iron traces. There are some which might be a lot dimmer than regular, with sturdy titanium absorption traces. This doesn’t forestall their use as normal candles since we will determine them by the spectra and regulate our brightness calculations accordingly, nevertheless it does recommend that the one progenitor mannequin is incomplete.
One chance is that some Sort Ia supernovae are brought on by white dwarf collisions. Given the calculated variety of binary white dwarf techniques, collisions can’t account for all supernovae of this sort, however stellar collisions are recognized to happen, they usually wouldn’t be sure by the Chandresekhar restrict, thus permitting for supernovae which might be brighter or dimmer than regular. It’s additionally potential that some Sort Ia supernovae are brought on by accretion from a detailed companion, however the ensuing supernova doesn’t destroy the white dwarf, which may clarify the dimmer subtypes of those supernovae.
Proper now, there are many potentialities, and we merely don’t have sufficient knowledge to pinpoint causes. However the excellent news is that with new observatories and sky surveys equivalent to Rubin Observatory coming on-line quickly, we’ll collect a wealth of observational knowledge, significantly from supernovae that happen inside our personal galaxy. This may present us with the data we have to lastly resolve this longstanding astronomical downside.
Reference: Ruiter, Ashley J., and Ivo R. Seitenzahl. “Sort Ia supernova progenitors: a recent view of a long-standing puzzle.” arXiv preprint arXiv:2412.01766 (2024).
