Przybylski's Star

In 1961 the Polish-Australian astronomer Antoni Przybylski took the spectrum of an unremarkable-looking eighth-magnitude star in the southern constellation Centaurus. A star’s spectrum is a barcode of its chemistry, and this one made no sense. Iron and nickel, normally prominent, were weak. Instead the spectrum was crowded with the lines of rare-earth elements, the lanthanides, in amounts far beyond any normal star. The element holmium was identified in it before it was ever seen in the Sun.

A chemical puzzle

Przybylski’s Star, catalogued as HD 101065, is about 356 light-years away. Its surface is about 6 130 K (5 860 °C) and it is about twice as wide as the Sun. In most respects it is an ordinary star. Its chemistry is anything but.

It belongs to a group called chemically peculiar Ap stars, which have strong magnetic fields and surfaces with unusual mixtures of elements. It is also the prototype of the rapidly oscillating Ap stars: its brightness pulses every 12.15 minutes. It turns extremely slowly, taking at least about 188 years to rotate once.

Radioactive elements?

The most remarkable claims concern elements that should not exist in a star at all. In 2008 Gopka and colleagues reported lines of short-lived radioactive actinides, from actinium to einsteinium, in its spectrum. Some of these decay in days or years. If they are really there, something must be making them now. Technetium and promethium, which also have no stable forms, were reported as well.

These identifications are disputed. Stellar spectra are crowded, and many weak lines can be mistaken for others. A 2023 study by Andrievsky and colleagues did not confirm technetium or promethium.

Possible explanations

In magnetic Ap stars, some elements are thought to rise to the surface and others to sink, sorted by radiation pressure and the magnetic field, which can concentrate rare elements in the outer layers. More exotic ideas include particles accelerated in the star’s magnetic field creating new elements by bombardment, or a hidden neutron star companion feeding it unusual material. No neutron star has been found, and none of the explanations accounts for everything that has been claimed.