At 13.8 billion years old, our universe is trillions of years too young for a single black dwarf to exist. To predict their formation, scientists track the slow, freezing death of white dwarfs.
When a star like our Sun exhausts its nuclear fuel, it blows off its outer layers, leaving behind an incredibly dense, Earth-sized core known as a white dwarf. These stellar remnants are intensely hot, often burning at over 100,000 degrees Kelvin at their surface. However, a white dwarf has a fundamental limitation—it no longer undergoes nuclear fusion. It has no internal mechanism to generate new energy.
Because they possess only residual thermal energy from their active years, white dwarfs follow the basic laws of thermodynamics. They are akin to a red-hot iron ember pulled from a fire. With no internal heat source, the ember will inevitably radiate its heat into the surrounding environment until it matches the ambient temperature. In the vacuum of space, a white dwarf slowly radiates its energy away, cooling and dimming over vast cosmic timescales.
Physicists predict the emergence of black dwarfs not through observation, but through thermal calculations. By factoring in a white dwarf's mass, core composition (typically carbon and oxygen), and specific heat capacity, scientists can determine its cooling rate. These calculations show that the process is agonizingly slow. It takes tens of trillions to over a quadrillion years for a white dwarf to radiate all its heat away and reach the cosmic background temperature, a few degrees above absolute zero. At that point, it emits no significant light or heat, officially becoming a black dwarf.
While a true black dwarf remains a mathematical certainty of the very distant future, observational astronomy backs up the theoretical timeline. Telescopes continually detect white dwarfs at various stages along the cooling track. By plotting the temperatures of thousands of these remnants, astronomers can trace their predictable descent. The oldest and coldest known white dwarfs, some of which have been cooling for over 10 billion years, still maintain surface temperatures of around 3,000 to 4,000 Kelvin. Their steady, measured loss of heat perfectly matches the models that point toward an eventual, darkened end.