At the dark hearts of galaxies like the Milky Way lie supermassive black holes, with millions or even billions of times the sun’s mass. Some of those supermassive black holes are what scientists call active galactic nuclei (AGN), which spew out copious amounts of radiation like X-rays and radio waves. AGN handles the twin jets of ionized gas you see shooting away in pictures of many galaxies.
Nothing lasts forever; black holes all have a shutdown day. A black hole’s death spiral looks like this;
TWO BLACK HOLES SWALLOWING A NEUTRON STAR:
A neutron star-black hole system
Neutron stars, and black holes are among the most extreme objects in the universe. They are the fossil relics of massive dead stars. When a star that is over eight times as massive as the sun runs out of fuel, it undergoes a spectacular explosion called a supernova. What remains can be a neutron star or a black hole.
Neutron stars are typically between 1.5 and 2 times as massive as the sun but are so dense that they pack all their mass into an object the size of a city. At this density, atoms can no longer sustain their structure and dissolve into a stream of free quarks and gluons: the building blocks of protons and neutrons.
There is no upper limit to how massive a black hole can be, but all black holes have two things in common: a point of no return at their surface called an event horizon, from which not even light can escape; and a point at their center called a singularity, at which the laws of physics either go away, or we run out of understanding.
When a black hole and a neutron star hook up, waves of gravity ripple out; these three solar masses start emitting gravitational waves during the merger. Black hole-neutron star mergers outnumber black hole-black hole mergers when neutron stars merge with black holes, tens or hundreds of thousands of such collisions across the universe per year.
Dark radiation: Dark energy develops in time; a bath of dark radiation dominates its dynamic component.
In space, the repulsive forces of dark energy are much larger than the attractive forces of gravity. Dark radiation (dark electromagnetism) is a type of radiation that mediates the interactions of dark matter. Just as photons mediate electromagnetic interactions between particles in the Standard Model (called baryonic matter in cosmology), they proposed dark radiation to mediate interactions between dark matter particles. Similar to dark matter particles, the hypothetical dark radiation does not interact with Standard Model particles.
Misfit stars in that odd mass middle ground also have an exit strategy. The divide between stars that will and won’t explode. Also, have an exit strategy.
Electron-misfit stars in the mass middle ground sit right on the precipice of exploding. Stars with more than about 10 times the sun’s mass go supernova after nuclear fusion reactions within the core cease, and the star can no longer support itself against gravity. The core collapses inward and then rebounds, causing the star’s outer layers to explode outward. With less than about eight solar masses, smaller stars can resist collapsing instead of forming a dense object called a white dwarf. But between about eight and 10 solar masses, there’s a poorly understood middle-ground for stars. For some stars that fall in that range, scientists have long suspected that electron-capture supernovas should occur. During this type of explosion, neon and magnesium nuclei within a star’s core capture electrons. In this reaction, an electron vanishes as a proton converts to a neutron, and the nucleus morphs into another element. That electron capture spells bad news for the star in its war against gravity because those electrons are helping the star fight collapse.
When electrons are packed closely together, they move faster. Those zippy electrons exert a pressure that opposes the inward pull of gravity. But if reactions within a star chip away at the number of electrons, that support weakens. Suppose the star’s core gives way — boom — that sets off an electron-capture supernova.
