Deneb - Distance and Physical Properties

Distance and Physical Properties

Deneb's absolute magnitude is currently estimated as −8.4, placing it among the most luminous stars known, with an estimated luminosity nearly 200,000 times that of our Sun. This is towards the upper end of various published values over the last few decades.

Deneb's exact distance from the Earth is still rather uncertain. The currently accepted distance of around 2,600 light-years (and the associated physical data shown in the starbox) is derived by a variety of methods, including spectral luminosity classes, atmospheric modelling, stellar evolution models, assumed membership of the Cyg OB7 association, and direct measurements of angular diameter. The original rather inaccurate Hipparcos parallax measurement was not inconsistent with this distance, but the more recent re-analysis gives a much larger parallax and a distance barely half the widely accepted value. One 2008 calculation using the Hipparcos data puts the most likely distance at 1,550 light-years, with an uncertainty of only around 10%, although parallax measurements of asymmetric, pulsating stars embedded within shells is known to be unreliable. The controversy over whether the direct Hipparcos measurements can be ignored in favour of a wide range of indirect stellar models and interstellar distance scales is similar to the better known situation with the Pleiades. The Gaia satellite should provide distance measurements at least two orders of magnitude more reliable than Hipparcos and resolve many such questions, although it will not measure Deneb itself.

Even assuming the lowest estimates of distance and luminosity, Deneb is the brightest and most distant of the stars with apparent magnitude brighter than 1.5, and the most distant (by a factor of almost 2) of the 30 brightest stars. Based on its temperature and luminosity, and also on direct measurements of its tiny angular diameter (a mere 0.002 second of arc), Deneb appears to have a diameter of 100-200 times that of the Sun; if placed at the center of our Solar System, Deneb would extend halfway out to the orbit of the Earth. It is one of the largest white stars known.

Deneb is a bluish-white star of spectral type A2Ia, with a surface temperature of 8,500 kelvin. Since 1943, its spectrum has served as one of the stable anchor points by which other stars are classified. It is the prototype of a class of variable stars known as Alpha Cygni variables. Its surface undergoes non-radial fluctuations which cause its brightness and spectral type to change slightly.

Deneb's mass is estimated at 20 solar masses. As a blue-white supergiant, its high mass and temperature mean that it will have a short lifespan and will probably go supernova within a few million years. It has already stopped fusing hydrogen in its core. It was probably an O class star during its main-sequence lifetime and is now probably expanding into a red supergiant. As it expands, it will go through the F, G, K and M spectral types.

Deneb's solar wind causes it to lose mass at a rate of 0.8 millionth of a solar mass per year, a hundred thousand times the flow rate from the Sun.