Article written by: Simon Jeffery
September 2018 will see over 50 astronomers from around the world gather at the Armagh Observatory and Planetarium to discuss the latest news about hydrogen-deficient stars. These stars have lost nearly all the hydrogen from which they were made, to leave only nuclear ash. Astronomers want to learn how these rare and short-lived remnants formed, and what drives their spectacular changes in brightness.
Most of the lights we see in the night sky are stars … objects like the Sun which generate their own light and shine by themselves. Stars come in all sorts, from ten thousand times fainter to a million times brighter than our Sun. Some shine for tens of billions of years, and some burn themselves out in a few thousands. But all are made of the same stuff that was made in the Big Bang: nine atoms of hydrogen to one atom of helium, the two simplest atoms in the Universe.
Therefore, it is very peculiar when we find a star that contains NO hydrogen … even on its surface. And very rare. Only 2 stars out of roughly 5000 visible to the naked eye are “hydrogen-deficient”, or H-def for short.
Light curve for R Coronae Borealis from 1990 to 2017. Attribution: By Lithopsian – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=61545818. Link to Copyright notice: CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0/
One of these is R Coronae Borealis (RCrB), discovered in 1795 to show huge changes in brightness. From time to time it gets over a thousand times fainter in just a few days, and then recovers in subsequent weeks. Dramatically, RCrB has been uncharacteristically faint since summer 2007, and is only now recovering its usual brightness, just visible in the ‘Northern Crown’ on a clear dark night. Check out more HERE
Fortunately, astronomical surveys have discovered hundreds more H-def stars (Astronotes 2018, February 1). Some were discovered from light variations, and some from their spectra, the individual light rainbows which reveal what a star’s surface is made of. Hydrogen is easily recognised in normal stars. Its atomic spectrum dominates hot stars, and its molecular forms are prominent in cool stars. Its absence is a clear marker that something is unusual.
Rendition of gravitational waves emitted by the death spiral of two compact stars. Credit: NASA
Since 2014, the Gaia spacecraft has measured the distances of hundreds of H-def stars. In the last six months astronomers have obtained critical new data on brightnesses, masses and location in the Galaxy. These clues will help to explain why there is little or NO hydrogen on these stars. This problem is made even more difficult because there are several types of hydrogen-deficient star. Some types are probably connected, but there are clearly several evolution channels . All involve the nuclear burning of helium into carbon.
One channel involves the collision of two white dwarfs, dead stars locked in a death spiral as gravitational wave energy drains orbital energy from the system. After collision, the now heavier star ignites fresh helium in a ‘shell of fire’, enabling it to shine brightly for a few tens of thousands of years. This channel seems to explain the RCrB stars. It is also thought to be likely channel for creating Type Ia supernovae, the most luminous explosions known.
Another channel involves a single white dwarf, where rekindled helium-burning makes a last gasp attempt to keep the star shining. In 1995, Sakurai’s object (V4334 Sgr) suddenly brightened from an insignificant hot white dwarf to a very cool red supergiant, more than ten thousand times brighter.
Remnant of supernova 1006, a Type Ia supernova probably caused by the merger of two white dwarfs. Credit: NASA
Meanwhile, very massive stars shine so brightly that their surfaces are stripped away by the combined effects of a light-driven wind and high-speed rotation (Astronotes 2018, June 5). Left behind is a much lighter but still massive star devoid of hydrogen. The wind still stripping the surface is characterised by the signatures of helium and nitrogen or carbon, the spectrum of a ‘Wolf-Rayet’ star.
A giant bubble blown by the massive Wolf-Rayet star HD 50896, the pink star in the centre of the image. Credit: ESA, J. Toala & M. Guerrero (IAA-CSIC), Y.-H. Chu & R. Gruendl (UIUC), S. Arthur (CRyA–UNAM), R. Smith (NOAO/CTIO), S. Snowden (NASA/GSFC) and G. Ramos-Larios (IAM)
These are just three of many types of H-def star that astronomers will discuss in 2018 September. Others include the very compact AM Canum Venaticorum binary stars (Astronotes 2018, June 27), extreme helium stars, and various classes of supernovae, white dwarfs and hot subdwarfs (look out for future Astronotes!).
The conference in Armagh, ‘Hydrogen-Deficient Stars 2018’ (HDEF2018) will be the fourth major international conference on H-def stars. The first was held in Mysore, in India, in 1985. Two more were held in 1996 and 2007, in Bamberg and Tuebingen, in Germany.
Since Simon Jeffery, Jorick Vink and Gavin Ramsay came to work at the Armagh Observatory and Planetarium, Armagh has played a world-leading role in the field of H-def stars. Major interests include the low-mass extreme helium stars and the massive Wolf-Rayet stars, the exotic ‘heavy-metal subdwarfs’, and the very compact AM Canum Venaticorum binaries. For this reason, Armagh was chosen for HDEF2018.
At HDEF2018, over 50 delegates from 16 nations will give talks explaining their recent research. These will include discoveries of new H-def stars, detailed measurements of their brightness and distance, and explanations of what they have seen. They will present posters, and argue about their results. Which theories work and which don’t? What new observations should we make? And they will return home with new ideas to explore, better equipped to solve the puzzles of the H-def stars. Watch this space for further news.
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