Mass Gap

When a very massive star has used up its fuel, its core collapses. What remains is either an extremely dense stellar remnant known as a neutron star, or a black hole—an object with gravity so strong that not even light can escape. When researchers list the weights of all these remnants, something stands out: certain weight ranges hardly ever occur. Such a conspicuously empty range is called a mass gap. So the term does not describe a gap in space, but a gap in the statistics. The question behind it is: are objects really missing there, or do we simply fail to find them?

Why empty spots in the statistics reveal physics

A mass gap is valuable for research because it makes a prediction testable. Theories about the death of stars predict fairly precisely which end products can form. If a certain weight is impossible according to theory, then such objects should not be measurable either. If one is found anyway, the theory has to be revised.

Particularly discussed is the gap between about 2.5 and 5 solar masses. A solar mass is the weight of our Sun used as a unit of comparison. A neutron star can hardly be heavier than about 2.5 solar masses, because beyond that it collapses further under its own gravity. At the same time, for a long time hardly any black holes below 5 solar masses were observed. In between lay a strangely empty band.

A second mass gap lies much higher, roughly between 65 and 120 solar masses. Very massive stars in this range completely destroy themselves in the explosion. They then leave behind no remnant at all that could become a black hole. Objects found in this range are therefore a sign that they formed through the merger of smaller black holes.

How the weights in space are determined in the first place

You cannot put a black hole on a scale. Instead, you use its effect on its surroundings. If an ordinary star orbits an invisible object, the weight can be calculated from the orbital period and the size of the orbit. This is the same principle used to determine the mass of the Sun from Earth’s orbit.

Today, most new data come from gravitational waves. These are tiny ripples in space that arise when two heavy objects orbit each other and merge. Detectors such as LIGO in the USA and Virgo in Italy measure these ripples using kilometers-long laser installations. From the pattern of the signal, researchers can read off how heavy the two objects were.

A common misconception is that a mass gap is a proven law of nature. Initially it is only an observational situation, and observations have blind spots. Lightweight black holes, for example, are harder to detect than heavy ones because their signal is weaker. Such an effect can create the appearance of a gap that does not actually exist. That is why every measurement in the gap region is checked very carefully.

The gap in headlines and research data

The term regularly appears in reports about new gravitational-wave discoveries. Typical phrasings are “object discovered in the mass gap” or “mystery of the missing black holes.” In 2020, the LIGO-Virgo team reported an object of about 2.6 solar masses whose nature remained unclear. It was either the heaviest known neutron star or the lightest known black hole.

The term is also used in particle physics, where it has a different meaning. There it refers to the minimum amount of energy a particle must have within a given theory. Anyone encountering the term in a text should therefore briefly check which field is meant. In astronomy news, it almost always refers to the gap among stellar remnants.

The topic is practically relevant because every new observation sharpens the models of stellar death. These models, in turn, feed into the simulations used to calculate the evolution of entire galaxies. With every observing run of the detectors, the statistics grow. Whether the mass gap will ultimately persist or slowly fill in remains an open question.