V Sagittae
The only super soft X-ray non-magnetic cataclysmic variable known.
V Sagittae (V Sge) is a cataclysmic variable star in the constellation Sagitta, and it stands out as the only non-magnetic cataclysmic variable known to be a super soft X-ray source. Its variability was first spotted by Lidiya Tseraskaya in 1902, and it received its variable star designation in Annie Jump Cannon's 1907 *Second catalogue of variable stars*.
Two competing models try to explain V Sagittae’s behavior. One proposes a white dwarf pulling material from a companion star through an accretion disk; the other suggests two hot stars in a nearly contact binary. Both have been challenged. In 2022, Smak argued that the primary resembles a Wolf-Rayet star, and that the white dwarf model fails to account for orbital period changes and mass loss from the primary, favoring a Wolf-Rayet star paired with a main sequence star. However, a 2025 study by Hakala, Charles, and Rodríguez-Gil countered that the hot binary model cannot explain the system’s variability, strong Hα emission, or the presence of stationary, double-peaked narrow emission lines, while the white dwarf model accounts for nearly all these features.
Material from the donor star is accreting onto the primary at an exponentially increasing rate, driving a powerful stellar wind. The accretion rate—and thus the system’s luminosity—doubles roughly every 89 ± 11 years. It is predicted that V Sagittae will erupt as a nova sometime between 2067 and 2099, briefly becoming one of the brightest stars in the sky.
- Discoverer
- Lidiya Tseraskaya
- Discovery year
- 1902
- Variable star designation catalog
- Annie Jump Cannon's 1907 work Second catalogue of variable stars
- Doubling time for accretion rate
- 89±11 years
- Predicted nova eruption window
- 2067–2099
Lore & Background
Lidiya Tseraskaya discovered the variability of V Sagittae in 1902. It appeared with its variable star designation in Annie Jump Cannon's 1907 work Second catalogue of variable stars. Two models have been proposed to explain its properties: one describes a white dwarf accreting mass from a companion via an accretion disk, while the other posits two hot stars forming a contact binary. Both models have been disputed. Smak (2022) noted that the primary component shows similarities with Wolf-Rayet stars and that the white dwarf model does not explain many aspects, including orbital period variations and mass loss, supporting a configuration with a Wolf-Rayet star and a main sequence star. However, a 2025 study by Hakala, Charles and Rodríguez-Gil found that the 'hot binary' model fails to explain multiple properties such as variability, strong Hα emission, and stationary double-peaked narrow emission lines, while the white dwarf model explains nearly all of them. Material from the donor is accreting onto the primary at an exponentially increasing rate, generating a huge stellar wind. The doubling time for the accretion rate and system luminosity is about 89±11 years. It is predicted that the system will erupt as a nova sometime between 2067 and 2099, at which point it will become one of the brightest stars in the sky.
Reader's Guide
V Sagittae holds a unique place among cataclysmic variables as the only known super soft X-ray source that is non-magnetic. Its disputed nature—whether it is a white dwarf accreting via a disk or a contact binary of two hot stars—has driven ongoing research. Smak (2022) argued for a Wolf-Rayet star and main sequence companion, while a 2025 study by Hakala, Charles and Rodríguez-Gil countered that the white dwarf model explains nearly all observed properties. The system's exponentially increasing accretion rate, with a doubling time of about 89±11 years, leads to a predicted nova eruption between 2067 and 2099, when it will become one of the brightest stars in the sky. This prediction makes V Sagittae a target for long-term monitoring and a key test case for models of binary evolution and nova outbursts. Its discovery by Lidiya Tseraskaya in 1902 and inclusion in Annie Jump Cannon's 1907 catalog highlight its historical significance in variable star astronomy.
Did You Know?
- V Sagittae is the only super soft X-ray source non-magnetic cataclysmic variable found so far.
- Lidiya Tseraskaya discovered its variability in 1902.
- The system's accretion rate doubles approximately every 89±11 years.
- It is predicted to erupt as a nova between 2067 and 2099, becoming one of the brightest stars in the sky.
Frequently Asked Questions
What is V Sagittae?
V Sagittae is a cataclysmic variable star located in the constellation Sagitta. It holds a unique distinction as the only non-magnetic cataclysmic variable ever confirmed to emit super soft X-rays.
Who first noticed V Sagittae's variability and when was it officially catalogued?
Russian astronomer Lidiya Tseraskaya detected the star's changing brightness back in 1902. It later received its formal variable star designation when Annie Jump Cannon included it in her 1907 Second catalogue of variable stars.
What makes V Sagittae stand out from every other cataclysmic variable?
It is the sole non-magnetic cataclysmic variable known to produce super soft X-ray emission, a property no other member of that class shares. This singular trait has made it a key object for probing the physics of accretion onto white dwarfs.
What are the two competing models for V Sagittae's behavior?
One picture has a white dwarf stripping material from a companion star through an accretion disk; the other envisions two hot stars locked in a nearly contact binary. Neither model has been fully accepted, as both face significant observational challenges.
When is V Sagittae expected to go nova?
Given the measured doubling time of its accretion rate—roughly 89 years, with an uncertainty of about 11—astronomers project a nova eruption sometime in the 2067-to-2099 window. That span marks the period when accumulated fuel is expected to reach the threshold for a thermonuclear runaway.
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