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.
Push v. pull
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’.)
Peeling a star
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.
The only explanation is that the outer layers of some stars are stripped away before the supernova explosion. How might this happen?
One particularly interesting mechanism happens if the star is part of a binary system – i.e. as one of two stars that are orbiting each other. (Most stars heavier than the Sun are in such binaries.) The gravitational attraction of one star can peel away the hydrogen layer off the other, leaving an exposed surface of helium. This would leave a small, hot helium star with strong surface gravity.
Most stars with more than eight solar masses are expected to be part of binary systems, so we should reasonably expect numerous binary-stripped helium stars in the universe. Yet so far, astronomers have found exactly one binary system in the mass range of eight to 25 solar masses (prior to stripping), and have been looking for the others.
The bright outliers
Dr. Drout & co. have now reported a population of these stars. The team’s findings were published in the journal Science.
Many binary systems start out with two main sequence stars. One of them rips the hydrogen layer off the other, resulting in one helium star and one main sequence companion.
The team realised the companion would outshine the stripped star in the frequencies of light to which both human eyes as well as optical telescopes are sensitive.
Different frequencies roughly correspond to different colours of light. This is why the hotter an object is, the more energy it has, the higher the frequency of emitted light, and the bluer it glows. Helium stars are expected to be so hot that they emit more of their energy as ultraviolet radiation, which lies beyond the visible range. So the research catalogued around half a million stars in two nearby galaxies – the Large and the Small Magellanic Clouds – using data from a telescope that could ‘see’ ultraviolet light.
Most of the stars the team found were main sequence stars, but there were some bright outliers. The team then studied the speeds of these stars. If a star were isolated, it would be seen moving either towards or away from the earth at a constant rate. But some of the outliers had different speeds over time – hinting at the presence of a companion that restricted their free movement.
Fill in the light’s blanks
This way, the team finally had a shortlist of 25 stars for the final test: their optical spectra. A spectrum is a profile of the various energies (or frequencies) at which an object emits radiation. A star’s spectrum is like a fingerprint of the elements in its outer layers. Depending on which elements are present, different frequencies of the light passing through them are absorbed by atoms, leaving the remainder to pass through space to the earth. Here, scientists can play ‘fill in the blanks’ with the light to estimate the composition of the star’s outer layers.
When the researchers examined the spectra of the 25 stars, they found several with a strong presence of helium and an absence of hydrogen (Class 1). Some possessed hydrogen as well (Class 2 and Class 3). Based on this, the team made two conclusions: 1) Class 1 stars have no hydrogen and are rich in helium; 2) their companions are either low-mass not-as-bright main sequence stars or compact objects (neutron stars or blackholes) that don’t emit ultraviolet light strongly.
All is well in astrophysics
To further understand Class 1, the team answered the following question: given the strength of gravity and the temperature on the surface of a helium star, how would the spectrum appear to us?
They modelled different possible answers to this question using computers and found the surface temperature of Class 1 stars to be roughly 20-times that of the Sun and the surface gravity about 1,000-times that of the earth. So these were hot, strongly gravitating, helium-rich, and hydrogen-depleted stars that are also part of binary systems.
In this way, the team discovered the long-awaited intermediate-mass hydrogen-stripped helium stars in binaries after seven years. The work supports the predictions of existing astrophysical theories.
The group has also said these stars will end their lives as hydrogen-poor supernovae that leave behind ultra-dense balls called neutron stars. And these neutron stars may ultimately smash into each other in powerful kilonova explosions, releasing gravitational waves.
The author is a fourth year PhD scholar in the Department of Physics at Ashoka University, Sonepat.
