A black hole has an event horizon: signals from inside cannot reach distant observers. Formation creates that region; feeding and mergers can enlarge an existing black hole. Different environments can produce different histories.
Strongly supported pathway
Stellar collapse
A sufficiently massive stellar core loses the support needed to resist gravity. Depending on the star’s evolution, mass loss and the collapse outcome, a black hole can remain. Some collapses are accompanied by an explosion; “every black hole comes from a visible supernova” is too strong.
Draw: Draw a massive star, a contracting core, and a compact remnant. Keep the exploding envelope optional.
Still to resolve: The precise remnant depends on more than a star’s birth mass. Rotation, composition, binary interactions and mass loss matter.
NASA — Black holes ↗Well-motivated models; early populations uncertain
First-star remnants: light seeds
Some of the first stars could leave black-hole remnants. Those seeds must then acquire enough matter, and sometimes merge, to explain very massive early objects. The challenge is whether their environments can feed them efficiently for long enough.
Draw: Draw a first star, a small seed, and several distinct growth stages.
Still to resolve: A massive early black hole does not by itself rule out every light-seed model; growth assumptions must be tested too.
Haemmerlé and colleagues — First stars and black holes (2020 review) ↗Active hypothesis with suggestive observations
Direct collapse: heavy seeds
In suitable early environments, a large gas reservoir may avoid breaking into many ordinary stars and instead produce a much more massive precursor and black-hole seed. “Direct” does not require every model to skip a supermassive-star stage.
Draw: Draw a gas cloud concentrating into one central object; use a dashed branch for the proposed intermediate stage.
Still to resolve: UHZ1 observations favor a heavy starting seed under the analysis assumptions. They are not a movie of a gas cloud turning into a black hole.
Chandra — UHZ1 and heavy seeds (2023) ↗Plausible growth channels; contributions debated
Dense clusters and repeated mergers
Dense stellar environments can bring objects together. Models explore repeated stellar collisions and mergers of existing black holes as routes toward larger objects. Whether remnants remain in their cluster and continue growing is an important constraint.
Draw: Draw several objects approaching and a sequence of merger remnants, not one giant suction funnel.
Still to resolve: Relative channel contributions depend on the environment and population; an individual merger need not reveal the entire ancestry.
NASA/Hubble — Early black-hole formation pathways ↗Direct gravitational-wave evidence
Black-hole mergers
Two existing black holes can orbit, inspiral and merge. GW150914 established a binary black-hole merger signal. The final black hole is not simply the sum of the starting masses: energy is carried away by gravitational waves.
Draw: Draw two horizons, an inspiral, one remnant and outward wave marks.
Still to resolve: This establishes merger growth, not the original formation route of the two components.
LIGO — GW150914 evidence ↗Hypothetical; no established population
Primordial black holes
Some early-universe models allow unusually dense regions to collapse before stars exist. Such black holes have been proposed as seeds and as contributors to dark matter. Their abundance and masses are constrained by multiple observations.
Draw: Draw an early-universe density patch; label the entire pathway “hypothesis.”
Still to resolve: A possible fit to one observation is not a confirmed primordial detection. Review constraints as well as arguments in favor.
Carr & Kühnel — Primordial black holes (2022 lecture review) ↗