Selected Pulsating Stars

The idea that stars pulsate first appeared in 1879. Since then, astronomers have discovered and described many types of pulsating stars. For example, stars similar to the Sun but already in the final stages of their lives are called red giants. These are cool stars that, as they expand, engulf nearby planets and simultaneously lose mass due to strong stellar winds. Many red giants pulsate, causing regular changes in their brightness – such stars are called “long-period variables” (LPVs). Another intriguing type of pulsating stars are the so-called BLAPs – hot pulsating stars. Thanks to observations within the OGLE project, many mysteries connected with variable stars have been clarified.

Researchers from the Astronomical Observatory of the University of Warsaw solved a nearly century-old puzzle concerning the so-called Long Secondary Periods (LSPs) in variable red giants (LPVs). About 30% of bright red giants exhibit additional variability with periods several times longer than their pulsations. Explaining this process had long been difficult – hypotheses included non-radial oscillations, convection, dust ejections, or dark spots, but none could account for all observations. Using data collected by the OGLE project, scientists examined a sample of 16,000 LSP stars. Crucial information came from NASA’s WISE telescope, which observes the sky in infrared light.

By comparing OGLE’s optical data with WISE’s infrared data, scientists discovered an additional brightness minimum visible only in the infrared. This was explained by the presence of an object orbiting close to the red giant’s surface, surrounded by a dust cloud that obscures the star once per orbit, causing the primary minimum. Dust heated to about 1000–1500 K emits infrared radiation, and its occultation by the star produces a secondary minimum in that spectral range. The companions of red giants shrouded in these dusty clouds are most often brown dwarfs – bodies with masses between those of planets and stars. They were probably once planets orbiting at larger distances but increased their mass by accreting material lost by the star through stellar winds. This discovery opens new possibilities for studying the evolution and distribution of planetary systems in our own and other galaxies.

Thanks to the OGLE project’s regular monitoring of hundreds of millions of stars in the Milky Way, thousands of new variable objects have been discovered, including previously unknown types. Among them are mysterious hot pulsating stars. Their surface temperatures are about five times higher than that of the Sun. These objects have been named BLAPs (Blue Large-Amplitude Pulsators). Their remarkable feature is that they change their brightness by 40% in just about half an hour, despite being nearly the size of the Sun. The internal structure and origin of these stars remain unknown nearly a decade after their discovery. To date, almost two hundred BLAPs have been identified, 90% of which were found thanks to OGLE data.