BW Vulpeculae
An extreme Beta Cephei variable with a unique standstill feature.
BW Vulpeculae, also designated BW Vul, is a variable star located in the northern constellation Vulpecula. It sits near the threshold of naked-eye visibility, with a typical apparent visual magnitude around 6.5. Parallax measurements place it roughly 2,800 light-years away, and it is moving toward Earth at a baseline heliocentric radial velocity of about −6 km/s.
This star is a B-type giant, classified as B2 IIIv, where the "v" denotes variability in its spectral lines. Mass estimates from different researchers range from 11 to 14 solar masses, though Tetzlaff et al. (2011) gives a lower value. Its age is approximately 3.4 million years, and it rotates with a projected rotational velocity of 24 km/s. The star radiates about 515 times the Sun's luminosity from its photosphere, which has an effective temperature of 23,014 K.
The star's variability was first announced in 1937 at the 58th Meeting of the American Astronomical Society by Canadian astronomer Robert Methven Petrie. It is a Beta Cephei variable, fluctuating between magnitudes 6.44 and 6.68 over a 4.8-hour period. For reasons not yet understood, its periodicity undergoes sudden shifts, followed by long intervals of stability. BW Vul is among the most extreme β Cephei stars in both light and radial velocity variation, a trait hypothesized to stem from its relatively high metallicity—an abundance of elements heavier than hydrogen and helium. A distinctive feature of its radial velocity cycle is a "standstill" phase, caused by a shockwave generated when material from a previous cycle falls back onto the star.
- Apparent visual magnitude
- 6.44 to 6.68
- Spectral type
- B2 IIIv
- Distance
- about 2,800 light years
- Mass
- 11 to 14 solar masses (Tetzlaff et al. 2011 gives just)
- Luminosity
- 515 times the Sun's luminosity
- Effective temperature
- 23,014 K
- Period
- 4.8 hours
Lore & Background
The variability of BW Vulpeculae was announced in 1937 at the 58th Meeting of the American Astronomical Society by Canadian astronomer Robert Methven Petrie. It is a Beta Cephei variable that ranges between magnitudes 6.44 and 6.68 over a period of 4.8 hours. For unknown reasons, the periodicity of the star has undergone sudden changes, followed by long periods of stability. BW Vul is one of the most extreme β Cephei stars in terms of variability of light and radial velocity. This is hypothesized as being due to the star's relatively high metallicity, meaning the abundance of elements other than hydrogen and helium. A distinctive feature of its radial velocity cycle is a unique 'standstill' feature, which is caused by a shockwave generated by infall of material from a previous cycle.
Reader's Guide
BW Vulpeculae holds significance as a benchmark among Beta Cephei variables due to its extreme amplitude in both light and radial velocity variations. Its unique 'standstill' feature in the radial velocity cycle provides a direct observational signature of shockwave dynamics in stellar atmospheres, offering insight into the complex interaction between infalling material and the star's pulsations. The star's sudden period changes, followed by long stable intervals, remain unexplained and present a puzzle for stellar astrophysics. The hypothesis linking its extreme behavior to high metallicity suggests that chemical composition may play a key role in driving the most dramatic pulsational instabilities. As a B-type giant with a mass around 11 to 14 solar masses, BW Vul also contributes to understanding the evolution of massive stars in the giant phase. Its variability, discovered by Robert Methven Petrie in 1937, has made it a long-studied object in variable star astronomy, and its continued monitoring helps refine models of non-radial pulsations and shock propagation in hot stars.
Did You Know?
- BW Vulpeculae is one of the most extreme Beta Cephei stars in terms of variability of light and radial velocity.
- Its radial velocity cycle features a unique 'standstill' caused by a shockwave from infalling material.
- The star's periodicity has undergone sudden changes for unknown reasons, followed by long periods of stability.
- It is moving closer to Earth with a heliocentric radial velocity of about −6 km/s.
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