Our universe is full of quirky critters. Face it, given 13.8 billion years to work with mother nature has had plenty of time to conjure up interesting stuff. Enter helium stars.
While hydrogen is the most abundant element and stars usually spend most of their lives fusing it into helium, then helium to carbon etc., sometimes funny things happen.
Lower mass stars like our Sun evolve. They start by fusing hydrogen, then billions of years later helium when their hydrogen runs low. But hydrogen is still present, existing in the star’s outer layers, showing up in spectrographic images. Helium stars possess much less hydrogen than evolved stars. Their mass at birth appears to help explain.
Helium stars are class O or B stars, born with high mass, typically more than 15 times that of our Sun. Their birth is even disruptive within their molecular cloud nursery, blowing apart newly forming small stars. They burn through their hydrogen with a vengeance, living hard and dying young in millions of years, not billions like our Sun. Type O and B stars are destined to end up as black holes or neutron stars. During their rapid evolution, type O and B stars run low on hydrogen too and start fusing helium. Some of them have extra strong stellar wind and blow off their outer hydrogen layers, ending up with strong helium spectral lines and weak hydrogen lines. These are called helium stars. They will evolve the same as other type O and B stars, BOOM! – black hole or neutron star.
Helium stars don’t appear exotic beyond their classification but there is a group of helium stars that are oddities, rare, and maybe exotic. They are called Extreme Helium Stars.
Extreme Helium Stars and helium stars are members of an eclectic group called hydrogen-deficient stars. This broader group includes the likes of Wolf-Rayet stars, carbon stars, and WC spectral type white dwarfs. From giant to small, hyperactive to sublime. We are focusing on extreme helium stars here.
Also known as PV Telescopii Variables, extreme helium stars are rare, only 21 known to date. So far, they are all super-giant, low-mass stars (exotic) with little to no hydrogen (helium to hydrogen ratio of 10,000:1). Their surface temperatures can vary from 9,000 K to a scorching 35,000 K. HD 124448, was the first extreme helium star discovered, in 1942 by Daniel M. Popper at the McDonald Observatory right here in Texas. PV Telescopii was the second extreme helium star discovered, in 1952.
The origin of Extreme helium stars is currently under study with the most convincing model being two white dwarfs merging: One a helium white dwarf, the other a carbon-oxygen white dwarf. Once merged they can evolve into extreme helium stars.
Their fate? Like with all stars, mass decides.
What’s in the Sky?
May 3; 45 minutes after sunset; west-northwest: See Venus hugging the horizon with Mercury and the Pleiades about 10 degrees above.