A Surprise in Cygnus

Deneb, the brightest star in the constellation Cygnus and the 19th brightest star in the night sky, has surprised astronomers: it’s not just a variable star — its polarization changes, too!

Full of Surprises

constellation of Cygnus

Location of Deneb, in the constellation Cygnus. Click to enlarge. [International Astronomical Union/Sky & Telescope, CC BY 4.0]

It’s not every day that a discovery involves a recognizable star rather than one with an inscrutable name. Deneb is a blue supergiant star with a mass roughly 20 times that of the Sun. Its brightness varies by about 0.08 magnitude in an irregular way, making it the prototype for the Alpha Cygni class of variable stars. The irregular brightness changes are thought to come from the combination of many different pulsation modes acting simultaneously, including non-radial pulsations in which some parts of the star expand while others contract.

Recently, researchers started a new survey of bright stars in the northern celestial hemisphere to learn more about these stars’ polarization. Polarization refers to the orientation of light waves as they travel through space. If the light waves are oriented in a particular direction, the light is polarized. If instead the waves are oriented randomly, the light is unpolarized. A star’s polarization can reveal critical information about its winds, pulsations, and surroundings.

plots of brightness and polarization of Deneb

Deneb’s normalized brightness (top panel), degree of polarization (middle panel), and angle of polarization (bottom panel). Click to enlarge. [Adapted from Cotton et al. 2024]

Polarization Patrol

Daniel Cotton (Monterey Institute for Research in Astronomy and Western Sydney University) and collaborators presented more than a year of polarization data for Deneb, revealing for the first time that the star’s polarization varies. Deneb’s degree of polarization — how much of its starlight is oriented in an orderly way rather than every which way — is considerable, about 3,000–5,000 parts per million. The variability is large as well, bouncing around by hundreds of parts per million.

Deneb is a prominent star in the northern celestial hemisphere: how did this variability go unnoticed for so long? Like its brightness changes, Deneb’s polarization changes are irregular, changing significantly but unpredictably over the course of several weeks. Because few measurements of the star’s polarization were made until 2022, these changes escaped notice until now.

Reasons for Change

Cotton’s team identified two possible causes for the polarization changes: variable winds and non-radial pulsations. Other supergiant stars are known to have clumpy winds that can scatter and polarize starlight in a way that varies over time. This is likely the most straightforward explanation for Deneb’s polarization variations. In this case, the star’s polarization variations and its brightness variations would not be strongly correlated.

Another possibility is that Deneb’s non-radial pulsations could distort the surface of the star, causing polarization changes that are correlated with the star’s brightness and radial velocity changes. After examining the available data, Cotton’s team concluded that winds are the most likely cause of Deneb’s variable polarization, but non-radial pulsations might play a small role as well.

This marks just the beginning of our understanding of Deneb’s polarization properties — future photometric, polarimetric, and spectral observations of the star are planned, helping to illuminate the star’s behavior further.


“Deneb Is a Large-Amplitude Polarimetric Variable,” Daniel V. Cotton et al 2024 ApJL 967 L43. doi:10.3847/2041-8213/ad4b0f