Week 3: Stellar Evolution, Defining Characteristics and Typical Luminosity changes of Type Ia Supernovae

Hello and welcome to another installment of our journey! Last week we covered how Hubble’s constant was found — through distance calculations — and how we use that constant to find out more about our universe, such as expansion rate and age.  

This week, our adventure will take us from the birth of stars to their spectacular demise, where we learn about the history behind supernovae. Then, we explore some defining characteristics of Type Ia supernovae and how these characteristics lead towards our eventual distance calculation. And finally, I’ll explain my first steps to obtaining a distance out of raw images. 

Stellar Evolution and Supernovae:

Typically, main-sequence stars fuse hydrogen until only helium is available, which causes the nuclear fusion reaction within the star to lessen, thereby decreasing pressure. The star swells, turning into a red giant, until gravity squishes the star into a state where helium can be fused. The star continues this process with heavier and heavier elements (carbon, oxygen), and then begins to shed its outer layers as its inner temperature increases. When temperatures exceed 30,000 K, the emitted ultraviolet photons number greatly enough to ionize the ejected atmosphere, creating a planetary nebula. This leaves the degenerated star as a white dwarf, surrounded by a glowing gas cloud.

Type Ia supernovae occur in binary star systems, where one star — a white dwarf — orbits a larger star, slowly accreting mass from its companion, which can be anything from a much larger star to an even smaller white dwarf. This mass accretion eventually reaches a limit, known as the Chandrasekhar limit (around 1.44 solar masses), where the outwards electron degeneracy pressure becomes lesser than the force of gravity. The star collapses, which releases energy as a supernova. This process is marked by a specific signature in the electromagnetic spectrum and a uniformity in luminosity, making type Ia supernovae invaluable as “standard candles” for measuring cosmic distances, as they all grow unstable near the same mass point (and thus, release similar amounts of energy).

A Closer Look at the Explosion Mechanism:

However, the explosion which occurs when the white dwarf reaches 1.44 solar masses is more complex than simply gravity > electron degeneracy pressure. As the white dwarf approaches the Chandrasekhar limit, the pressure and temperature at its core rise dramatically, setting the stage for a runaway nuclear fusion reaction.

The heart of a white dwarf is composed primarily of carbon and oxygen. When the core temperature reaches 600 million K, carbon fusion ignites in a fraction of a second, followed by oxygen fusion. This sudden onset of fusion unleashes an immense amount of energy.

Initially, the fusion front moves outward in a subsonic combustion wave known as deflagration. However, high temperatures and turbulence can trigger a faster, supersonic wave known as detonation. This transition is crucial, as it accelerates the energy release. Rayleigh-Taylor instability, where lighter, hot material pushes through the denser star material, plays a pivotal role in mixing up the star (as expanding gas in the core accelerates into denser outer-shell gas) and increasing the energy of the supernova.

The Big Picture:

What matters here is that since the mass of the star is directly proportional to energy release, and when one remembers that energy release is proportional to absolute luminosity (brightness), all white dwarf supernovae have the same absolute luminosities, from which distance can be calculated (after one measures the apparent luminosity and performs some calculations).

Looking Ahead:

Now that we have most of the background information ready, next week will be full of data and raw images from telescopes in New Mexico. Thanks for reading my blog, and stay tuned for next week’s issue!