Deneb and Mu, farthest stars of a winter’s night

Deneb in the Northern Cross and Mu in Cepheus are the most distant, easily visible stars you can see with the naked eye. Deneb’s a snap to see. To find Mu, stick out your arm and look a little more than one vertically-held fist above and to the right of Deneb to Alpha Cephei. Mu is “two fingers” to the left of Alpha and shines at 4th magnitude. Created with Stellarium

Yesterday we looked at the obscure yet up-close-and-personal Wolf 359. Today I thought it would be fun to touch the other extreme and visit with two of the most distant stars visible with the naked eye. That jaunt takes us to a pair of stellar supergiants – Deneb in the Northern Cross and Mu Cephei (SEF-ee-eye) in the constellation Cepheus the King. Happily, both are near one other in the northwestern sky during early evening hours in February.

Deneb will be familiar to many of you as the star at the head of the Northern Cross also known as Cygnus the Swan. To our best knowledge, Deneb lies between 1,550 and 2,600 light years from Earth. The wide range of values has to do with different methods for estimating its distance, the first based on satellite observations and the other on computer modeling and direct measurement of star’s size.

Size comparison of the sun and the supergiant star Deneb.

We know it shines at least 54,400 times brighter than our sun with a diameter 114 times larger. Size-wise, the sun is to Deneb as Earth is to the sun.

Despite the vast gulf separating us, Deneb still shines at first magnitude. But what fun we’d have if it were moved to a distance of 7.7 light years, the same as terribly faint Wolf 359. From there Deneb would be a commanding sight at magnitude -11, glowing as brightly as the gibbous moon.

I came across Mu Cephei, better known as the Garnet Star, when looking up the most distant star visible with the naked eye. In a previous blog post, I’d written that V762 Cassiopeia, a variable star in Cassiopeia, held the record at 16,308 light years. While that distance is a best estimate, V762 lies at the naked eye limit of 6th magnitude. Most of us will probably never see it from our light-polluted towns and cities.

Both Mu and Deneb are classified as supergiant stars because of their stupendous girth and luminosity, but while Deneb is young, hot and white, Mu is older, cooler and red. As supergiants age, they puff out their outer layers, cool and redden. Betelgeuse in Orion’s shoulder is another example of a red supergiant. Both stars are unstable and routinely shrink and expand, which causes their brightness to vary over periods of months and years.

The sun is no match for Mu Cephei’s vast starscape. Illustration and photo: Bob King

With a diameter 1,650 times greater than the sun, Mu would dazzle our eyes off if it weren’t so far away. Distance estimates vary widely but 2,400 light years is often cited. If we could haul Mu to our solar system and put it in place of the sun, its surface would reach all the way to Saturn’s orbit.

To the eye, Mu is a dim and undistinguished 4th magnitude star, but binoculars show its orange-red color clearly. Astronomer William Herschel, who discovered the planet Uranus, described its color as “a very fine deep garnet”, hence its nickname the Garnet Star.

With the moon now out of the early evening sky, go out the next clear night and face northwest at nightfall to see this pair of bloated beauties. They’re the most distant stars within easy naked eye range. If only we could haul them closer to Earth, we’d better appreciate their true splendor.

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