Black holes and the life cycle of matter
Black holes are among the strangest objects in the universe. Together with stellar evolution, they show how matter cycles through the cosmos.
Evidence for black holes
Black holes cannot be seen directly, but their effects can. In X-ray binaries such as Cygnus X-1, gas from a companion star swirls into an unseen object of over 10 solar masses, heating to millions of degrees and emitting X-rays. In 2015, LIGO detected gravitational waves from two black holes, about 36 and 29 solar masses, merging.
Supermassive black holes
Most large galaxies have supermassive black holes at their centres, with millions to billions of solar masses. Andrea Ghez and Reinhard Genzel tracked stars orbiting the Milky Way's centre for decades, proving the presence of a black hole of about 4 million solar masses called Sagittarius A*. They shared the 2020 Nobel Prize in Physics with Roger Penrose.
Imaging black holes
In 2019, the Event Horizon Telescope released the first image of a black hole's shadow, in the galaxy M87, about 6.5 billion solar masses. In 2022, it released an image of Sagittarius A*. The images show a bright ring of hot gas around a dark central region, matching general relativity's predictions.
Near a black hole
Approaching a black hole, tidal forces stretch objects, called spaghettification. To a distant observer, clocks near the event horizon appear to run slower and light is strongly redshifted. Black holes are not cosmic vacuum cleaners: at a distance, their gravity is the same as any object of the same mass.
Hawking radiation
In 1974, Stephen Hawking predicted that quantum effects cause black holes to emit faint radiation and slowly lose mass, eventually evaporating. For stellar-mass black holes, this radiation is far too weak to detect and evaporation would take far longer than the current age of the universe.
The cycle of matter
Stars form from gas clouds, fuse light elements into heavier ones, and return enriched material to space through winds, planetary nebulae, and supernovae. This material mixes into new clouds, forming new generations of stars and planets. The Sun is a later-generation star; its planets, including Earth, contain elements made by earlier stars.
Variable stars
Some stars pulsate, changing brightness regularly. Cepheid variables have a precise relationship between pulsation period and luminosity, discovered by Henrietta Swan Leavitt in 1912. By measuring a Cepheid's period, astronomers know its true luminosity and can calculate its distance, a crucial tool for measuring distances to other galaxies.
Worked example
Sagittarius A* has about 4 million solar masses. Its Schwarzschild radius is roughly 4 million x 3 km = 12 million km, about one-twelfth of the Earth-Sun distance. The Event Horizon Telescope needed resolution equivalent to seeing a doughnut on the Moon from Earth to image its shadow.
Key takeaways
Black holes are detected through X-ray binaries such as Cygnus X-1 and gravitational waves from mergers. Supermassive black holes sit in galaxy centres; Sagittarius A* has about 4 million solar masses, proven by Ghez and Genzel. The Event Horizon Telescope imaged M87's black hole in 2019 and Sagittarius A* in 2022. Tidal forces cause spaghettification; at a distance, black holes act like any mass. Hawking radiation slowly evaporates them. Matter cycles through generations of stars. Cepheids measure cosmic distances.
Flashcard
X-ray binary with a black hole
Check your understanding
If the Sun were suddenly replaced by a black hole of the same mass, what would happen to Earth's orbit?