KX Andromedae

Lore & Background

The primary component is a Be star with a spectral classification B3pe as of 2017, though historical records show it has varied from B1 to B7. The secondary star is difficult to detect in the spectrum but has been given a K1III spectral type; it is likely an asymptotic giant branch star that fills its Roche lobe, transferring mass to the primary. The system is only about 25 million years old, and the pair complete a circular orbit every 38.919 days at an inclination of 50°.

KX Andromedae is surrounded by bipolar jets of ejected material that extend 19 light-years out from it in each direction, with an angular size of 20 arcminutes. These faint jets were discovered in hydrogen-alpha emission in 2024 by astronomer Stefan Ziegenbalg. Subsequent studies in other emission lines revealed a more complex structure: the jets were detected only in singly ionized sulfur and hydrogen, while an outer shell is also visible in doubly ionized oxygen. This suggests the presence of different ionization mechanisms. The shell may be visible as a result of shock ionization of the surrounding interstellar medium, whereas the gas within the jets may be ionized by the B3pe component, which likely does not emit sufficient energy to doubly ionize oxygen.

Reader's Guide

KX Andromedae is significant as a spectroscopic binary system that exhibits bipolar jets, a relatively rare phenomenon among Be stars. The discovery of these jets in 2024 by Stefan Ziegenbalg, using hydrogen-alpha emission, and the subsequent detection of a more complex structure in singly ionized sulfur and hydrogen, along with an outer shell in doubly ionized oxygen, highlights the system's role in studying ionization mechanisms in stellar outflows. The differing ionization states—where the jets are ionized by the B3pe primary but not enough to doubly ionize oxygen, while the shell is likely shock-ionized by the interstellar medium—provide a natural laboratory for understanding how massive stars interact with their environment. The system's age of 25 million years and the mass transfer from the secondary, an asymptotic giant branch star filling its Roche lobe, further contextualize its evolutionary stage. The jets extend 19 light-years in each direction, making KX Andromedae a notable example of large-scale bipolar outflows in a binary system, as featured in the Astronomy Picture of the Day on 5 December 2025.

Did You Know?

• The primary star's spectral classification has historically varied from B1 to B7.
• The bipolar jets extend 19 light-years in each direction and were discovered in 2024.
• The outer shell is visible in doubly ionized oxygen, likely due to shock ionization.