Astronomers find the small, hot helium stars they had been looking for Premium

Astronomers discover elusive hydrogen-stripped helium stars in binary systems, supporting astrophysical theories of binary-star evolution and predicting future kilonovae.

Astronomers have finally found a unique class of stars they had been looking for for a decade. While these stars – hot and relatively small orbs covered in helium – were predicted to be fairly common, astronomers hadn’t been able to find them.

The discovery provides proof that the underlying theories are legitimate. Since stars are cosmic engines, it also opens new doors to understanding their diverse influences on the universe, from the creation of heavy elements to the release of gravitational waves.

Led by assistant professor Maria Drout of the University of Toronto, the research team found these stars after spending seven years labouring with a simple working hypothesis, extensively cataloguing thousands of stars, and testing different assumptions.

Look at the Sun for a moment (while shielding your eyes) and ask yourself why it doesn’t collapse onto itself. Newton’s law of gravity says all objects with mass attract each other. This should mean the Sun’s outer and inner layers should be attracted to each other, so the star should continuously fall inwards, and eventually simply collapse. Yet we see the Sun rising every day, as it did yesterday and will tomorrow.

This is because nuclear fusion prevents the Sun from shrinking. In the heart of the star, chemical reactions merge the nuclei of two light elements – such as hydrogen or helium – to form a heavier nucleus, such as of silicon, in the process releasing an enormous amount of heat. This energy endows all particles in the star with random motion, or pressure, which then fights against the pull of gravity and maintains the star in a state of equilibrium.

That is, the fusion energy pushes the star out while gravity pulls it in, and they hold the balance for billions of years. Such stars are said to be in the main sequence. (When scientists plotted various stars’ colours against their brightness, they found that many of them occupied a prominent band in the middle of the graph, and named it ‘main sequence’.)

When a star no longer fuses material and allows gravity to gain the upper hand, it blows up in an explosion called a supernova if it’s heavy enough. Scientists have extensively studied the light from many supernovae. The light contains signatures of the various elements it has passed through near the dying star’s surface. Scientists have observed that some supernovae have shown no signs of hydrogen. Yet they also knew that hydrogen, the lightest of elements, makes up the outer layers of main-sequence stars.