by Harrison Blake-Goszyk
Harrison Blake-Goszyk is an Astrophysics Ph.D. student at Vanderbilt University studying extreme mass-ratio inspirals (EMRIs), their dynamics, and the multimessenger signals they produce from the centers of galaxies. He received his B.S. in Astronomy and Physics with minors in Math and Creative Writing from the Ohio State University in 2024, and was a recipient of the Astronaut Scholarship in 2023. He is a member of the LISA Consortium and Establishing Multi-messenger astronomy Inclusion Training (EMIT) program. In his free time, he likes writing science fiction, reading, and longswordfighting.
Title: How Massive Single Stars End Their Life
Authors: A. Heger, C.L. Fryer, S.E. Woosley, N. Langer, D.H. Hartmann
First Author’s Institution: Department of Astronomy and Astrophysics, Enrico Fermi Institute, University of Chicago
Status: Published in The Astrophysical Journal [open access]
What happens when stars die? Many stars orbit in pairs with others, but the single ones give us unperturbed insight into the end of a star’s life. The deaths of the biggest stars are directly dependent on the masses of their hydrogen envelope, helium core, and the amount of metals (everything that is neither hydrogen nor helium) lying around inside them. This astrobite and the paper it’s based on focus on these mammoth stars, detailing the ends of their lives and what we can learn from them.
The Biggest Bangs — Massive Star Supernovae
Before we start, what counts as a massive star? Stars use the power of gravity to fuse elements like hydrogen together into heavier ones. Fusion produces the light that escapes the star’s surface and enough force to push back on gravity and prevent the star from collapsing in on itself. However, when stars are too massive, they eventually run into a problem: iron. Fusing two iron atoms together takes more energy than it produces. This means that the pushing force keeping gravity in check vanishes. As soon as too many atoms fuse into iron, the outward pushing force withers away and the star collapses in on itself. The star’s material, crushing down at nearly the speed of light, reflects back off the iron core and leads to the stunning explosions we know as supernovae.
Figure 1: Graph of categories of supernovae explosions, organized by mass and metallicity. Note that some of the smaller lines overlap with Figure 2 to show what kinds of explosions can cause what remnants.
The team behind this paper investigated the type of supernovae process a star would go through at the end of their lives depending on their initial mass and initial metallicity (percent of stuff in a star that is neither hydrogen nor helium). The two biggest corners of the graph they made (depicted above) are SN IIp and SN Ib/c. Let’s break down what these names mean!
Firstly, type I and II supernovae differ because there are no hydrogen lines in the spectra of a type I supernovae, since it either eats up all their hydrogen before dying or ejects it all into space before the explosion. Type II burn and die at a more relaxed pace, leaving more hydrogen behind at the end of their lives. Going a layer further in depth, if stars eject their outer layers, or ‘envelope’, they can deprive themselves of more than just hydrogen. Type c supernovae show no hydrogen AND helium in their spectra, while type b only show no hydrogen. After that, we must describe the behavior of the spectra after the explosion. If it gets steadily dimmer over time, it is called a type L supernovae (L for linear). If it plateaus, it is a type p supernovae.
The orange patch in the bottom right represents Pair-Instability Supernovae. This type is triggered when high-energy gamma rays hitting atomic nuclei belch out tons of pairs of free electrons and positrons in a process called pair production. Instead of bouncing back off the iron core, pair production induces a chain reaction that entirely obliterates the core of the star, leaving no remnant behind. The region right next to it represents Pulsational Pair Instability Supernovae, which goes through a similar process. However, instead of completely destroying the star, repeated pulses of pair production launch mass up and away, eventually leaving a black hole in their wake.
The last category of explosion on this graph is no explosion at all. The white regions represent Direct Collapse Black Holes (DCBHs). No fanfare, no fireworks, no nothing. The stars are so massive and metal-poor that all their mass simply rushes to the center and stays there, creating a black hole that retains all the mass of the star it came from.
Crushing, Collapsing, Combusting — Massive Star Supernovae Remnants
Figure 2: Graph of categories of supernovae remnants, organized by mass and metallicity. Note that some of the smaller lines overlap with Figure 1 to show what kinds of explosions can cause what remnants (especially the white portion, which accounts for Pair-Instability Supernovae and have no remnant at all).
Like the supernovae type plot before, we can describe supernovae remnants by plotting the initial mass and metallicities against each other, falling into three distinct categories. The first kind, indicated in green, is called a neutron star. They are formed during the end stages of a supernova explosion when gravity crushes the core down so compactly that most or all of the protons and electrons inside it fuse into neutrons. Neutron stars are the second densest objects in the universe behind black holes, so dense that a neutron star the mass of the Sun is only as wide as a city! The maximum mass a neutron star can have sits between 2 and 2.2 solar masses, known as the Tolman-Oppenheimer-Volkoff limit (yes, THAT Oppenheimer). If any more mass is added on, especially if some of the ejecta falls back down (the red checkered region on the graph), it will collapse further until all the mass collects at a single point of infinite density: a black hole.
Both of black holes and neutron stars live lives of their own after the deaths of the stars they come from. However, there is one other outcome of a supernova explosion: no remnant of all. The pair-instability supernovae discussed above occupy the white section of this graph, leaving nothing behind after pair production obliterates everything in its wake. This means the three outcomes of the death of a massive star are crushing down into a neutron star, collapsing into a black hole, or combusting into a pair-instability explosion which leaves nothing behind.
Conclusions
This paper shaped how astronomers understand the death throes of massive stars. The categorization of stellar remnants and supernova type by initial mass, metallicity, and the mechanisms driving the events after the initial explosion became a cornerstone of the stellar evolution models in use today. It also gave critical input into the process of simulating supernovae, providing a simple framework to connect the physics of massive stars to the supernovae they produce. Through these simulations, astronomers were able to map the rapid fusion of a supernova’s atoms into heavier things, like the uranium in our reactors or the gold in our jewelry. All of this post-explosion material lives on in the gas floating around a galaxy that then collects together and forms new massive stars, starting the process all over again.
From this material, smaller stars like our Sun also form, live, and die. Therefore, as the saying goes, we are made of star stuff.
Astrobite edited by Ryan White
Featured image credit: Chandra X-ray Observatory (NASA) & Very Large Array (NSF)
