What is a Mira Variable?
Mira variables are a type of pulsating variable star that exhibit periodic changes in brightness over time. They belong to the class of long-period variables, with pulsation periods typically ranging from several hundred days to over a thousand days. The name “Mira” is derived from the Latin word for “wonderful” or “admirable,” reflecting the awe-inspiring nature of these stars.
Mira variables are red giants that have exhausted the hydrogen fuel in their cores and are now in the later stages of their evolution. As they pulsate, their outer layers expand and contract, causing fluctuations in their luminosity. These stars are also known for their strong stellar winds, which can create intricate structures such as shells and arcs of gas and dust around the star.
Characteristics of Mira Variables
One of the defining characteristics of Mira variables is their large amplitude of variability. These stars can vary in brightness by several magnitudes over their pulsation cycle, making them easily observable even with small telescopes. The period of variability for Mira variables is typically stable over long periods of time, although some stars may exhibit irregularities in their pulsation patterns.
Mira variables are also known for their red color, which is a result of their cool surface temperatures. These stars have low effective temperatures compared to other types of stars, with spectral types ranging from M5 to S5. Their spectra show strong absorption lines of molecules such as titanium oxide and carbon compounds, indicating the presence of cool, molecular-rich atmospheres.
Formation and Evolution of Mira Variables
Mira variables are thought to have evolved from stars with initial masses between 1 and 8 solar masses. As these stars exhaust their core hydrogen fuel, they begin to expand and cool, eventually becoming red giants. The pulsations in Mira variables are believed to be driven by a combination of helium burning in a shell around the core and the effects of radiation pressure on the outer layers of the star.
During their evolution, Mira variables will eventually shed their outer layers of gas and dust, creating a planetary nebula around the remnant core of the star. The core will then evolve into a white dwarf, while the ejected material enriches the interstellar medium with heavy elements produced in the star’s interior.
Importance of Studying Mira Variables
Studying Mira variables provides valuable insights into the late stages of stellar evolution and the processes that govern the formation of planetary nebulae. By observing the pulsations of these stars, astronomers can measure their distances, luminosities, and other physical properties, which can help constrain models of stellar evolution and nucleosynthesis.
Mira variables also play a crucial role in the chemical enrichment of the galaxy, as the material ejected from these stars contains elements such as carbon, nitrogen, and oxygen that are essential for the formation of planets and life. By studying the abundances of these elements in Mira variables and their surrounding environments, astronomers can better understand the origins of the chemical elements in the universe.
Observing Mira Variables
Observing Mira variables is a popular pastime for amateur astronomers, as these stars are bright and easily visible in small telescopes. By monitoring the changes in brightness of a Mira variable over time, observers can contribute valuable data to the study of these stars and help refine their pulsation periods and other properties.
Professional astronomers also study Mira variables using a variety of observational techniques, including photometry, spectroscopy, and interferometry. These observations can reveal detailed information about the physical processes occurring in these stars, such as the dynamics of their atmospheres, the structure of their winds, and the properties of their circumstellar material.
Variability of Mira Variables
The variability of Mira variables is not always regular or predictable. Some stars may exhibit sudden changes in their pulsation periods or amplitudes, known as “mode switching,” which can complicate efforts to model and understand these stars. Other Mira variables may show irregularities in their light curves, with unexpected dips or peaks in brightness that are not easily explained.
Despite these challenges, the study of Mira variables continues to provide valuable insights into the late stages of stellar evolution and the processes that shape the universe. By combining observations from ground-based telescopes, space-based observatories, and theoretical models, astronomers can unravel the mysteries of these wonderful and admirable stars.