SW Virginis is a Mira-type variable star located in the constellation Virgo. It is a pulsating red giant star known for its significant variations in brightness over a relatively long period.
Characteristics
- Type: SW Virginis is classified as a Mira variable, named after the prototype star Mira (Omicron Ceti). Mira variables are distinguished by their very large amplitude pulsations (typically varying by more than 2.5 magnitudes) and long periods, which are caused by radial pulsations in the star's outer layers. These stars are in a late stage of stellar evolution, having expanded to become cool, luminous red giants.
- Constellation: Virgo
- Period: The star undergoes a cycle of brightness variation approximately every 328.7 days. This relatively long period is characteristic of Mira variables.
- Magnitude: Its apparent visual magnitude typically ranges from about 9.4 at maximum brightness to around 17.5 at minimum brightness. This makes it a challenging object to observe visually at its faintest, often requiring aperture of 8 inches or more, but it is observable with smaller amateur telescopes when near its maximum brightness.
- Spectral Type: Its spectral type varies significantly throughout its pulsation cycle, typically ranging from M5e at maximum light to M9e at minimum light. The "e" denotes the presence of emission lines in its spectrum, which are common in Mira variables and indicate active mass loss from the star.
- Distance: Estimates of its distance vary, but it is generally considered to be several thousand light-years from Earth (e.g., approximately 3,600 light-years or 1,100 parsecs based on Gaia DR2 data, though these can have associated uncertainties).
Physical Properties
Like other Mira variables, SW Virginis is an evolved star that has left the main sequence. It is in a late stage of stellar evolution on the asymptotic giant branch (AGB), having expanded significantly and cooled to become a red giant. The pulsations are driven by the ionization and recombination of hydrogen and helium in its atmosphere, which creates an opacity mechanism that traps and releases energy, causing the star to periodically expand and contract. This process leads to the observed large changes in luminosity and surface temperature. Mira variables are also known for experiencing significant mass loss through stellar winds.
Observation and Significance
Due to its significant brightness variations, SW Virginis is a prime target for both amateur and professional astronomers studying stellar pulsations, late-stage stellar evolution, and the mechanisms of mass loss in red giants. Monitoring its light curve provides valuable data for understanding the complex physics governing these dying stars. Its changes in brightness can be followed by visual estimates, CCD photometry, or photographic methods.