Context: Astronomers have finally found a unique class of stars (Helium stars) they had been looking for, for a decade.
How do stars sustain?
- Stars are giant balls of hot gas – mostly hydrogen, with some helium and small amounts of other elements. Every star has its own life cycle, ranging from a few million to trillions of years, and its properties change as it ages.
- 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. However, the sun does not collapse as the nuclear fusion prevents the Sun from shrinking.
- In the heart of the star, two hydrogen nuclei (protons) ultimately combine to form one helium nucleus, releasing enormous amounts of heat and energy in the form of gamma rays and neutrinos.
- 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.
- 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.
Hydrogen devoid stars (Helium stars):
- When a star no longer fuses material and allows gravity to gain the upper hand, it blows up in an explosion called a supernova when it is massive enough.
- The light from supernovae 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. (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.
- An 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). Many binary systems start out with two main sequence stars. The gravitational attraction of one star can peel away the hydrogen layer off the other, resulting in one helium star and one main sequence companion.
- Helium stars are Hydrogen depleted stars. They have a core of helium and their outer layers are stripped of Hydrogen. They are roughly 8-20 times the mass of our Sun and their surface gravity is about 1,000-times that of the earth. Helium stars are expected to be so hot that they emit more of their energy as ultraviolet radiation, which lies beyond the visible range.
- These Helium 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.