07/10/2017 – Ephemeris – Deneb, a super bright star

Ephemeris for Monday, July 10th. Today the Sun will be up for 15 hours and 20 minutes, setting at 9:27, and it will rise tomorrow at 6:08. The Moon, 1 day past full, will rise at 10:24 this evening.

This evening when it gets dark the bright star Deneb in Cygnus the swan will be high in the east-northeast. Deneb is the dimmest star of the summer triangle. Of the other stars of the triangle, Vega is higher in the east, while Altair is lower in the southeast. While Deneb’s apparent magnitude, or brightness as seen from Earth, makes it the dimmest of the three bright stars, Deneb’s vast distance of possibly 1,400 light years* makes it over 50 times the distance of Vega. If brought as close as Vega, Deneb would be as bright at least as the quarter moon. It is possibly as bright as 200 thousand Suns; and a huge star, possibly as large in diameter as the orbit of the Earth. For all this it is only 20 to 25 times the mass of the sun.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

* Deneb’s distance is not well known.  Over the years with the Ephemeris program and this blog I’ve given various distances to the star.  Having no companion and being beyond the distance where trigonometric parallax is used, at least from the ground, makes distance measurement difficult.  Hipparcos  satellite measurements give a distance about twice as great.  Estimates of the true brightness of a star are based on its known distance.  An error in distance by a factor of two gives an error in terms of intrinsic luminosity by a factor of 4, due to the inverse square law of brightness with distance.

Addendum

Deneb with the other stars and constellations in the Summer Triangle. Created using Stellarium.

One of my old photographs of Deneb and the North American Nebula digitized from a slide. Deneb’s ultraviolet emissions cause the North American and unnamed here Pelican Nebulae to glow.  The red color is due to hydrogen.