White dwarf star
An artist's impression of debris around a white dwarf star. it is thought that the extreme helium star investigated here formed formed as a result of a double helium white dwarf merger. Nasa, ESA, STScI, and G. Bacon (STScI)

A rare example of an extreme helium star has just been identified by astronomers. It is known as GALEX J184559.8−413827 - or J1845−4138 for short.

Extreme helium stars are very rare and mysterious, made up mostly of helium rather than hydrogen, which is the most common element in the Universe.

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This oddity has long puzzled scientists, but they have also been fascinated by the fact that these stars are supergiants much larger and hotter than the Sun.

Besides helium, these stars also have significant amounts of carbon, nitrogen and hydrogen. The first one ever identified was spotted in 1942 by an astronomer called Daniel Popper.

Over the 30 years that followed, a small number of extreme helium stars were discovered with the help of spectroscopic surveys but astronomers continued arguing about how these stars formed. Identifying them beyond doubt also proved tricky on many occasions.

In the case of J1845−4138, it was in fact first identified in 2011 as another type of star known as a faint helium-rich 'hot subdwarf'.

However, scientists have now shown that it is more hydrogen-deficient than previously thought and as such, it has to be classified as an extreme helium star. It is the first extreme helium star to be discovered in the last 40 years.

The scientists also found that this star shares many similarities with a previously discovered extreme helium star known as V652. This suggests that both stars have had a similar evolution and formed as a result of two helium dwarf stars merging. Thus, the study confirms a potential way that these large enigmatic stars form in the universe.

J1845−4138 is documented in detail in a study now published on arXiv.org.