HR 4686 UMI (HR 4686 Ursae Minoris). Eleven naked eye stars (defined as sixth magnitude or brighter, that is, brighter than 6.5) can be found in a six-degree-wide box centered on Polaris, which dominates the set, six in Ursa Minor (including HR 4686, the name from the Bright Star Catalogue), the rest in Cepheus. At sixth magnitude, HR 4686 is the third faintest (beaten out Lambda UMi and HR 286, the latter tucked up against Polaris).
pole Polaris (the jewel in a small semicircle of faint telescopic stars called the "Engagement Ring") is centered in a six-degree-wide field of view that shows a variety of other "polar stars." HR 4686 is just to the right of the top center edge. Lamdba Ursae Minoris is the reddish star up and to the right of Polaris, while Yildun (Delta UMi) is the brighter of the two stars at the upper right corner. Roughly between Lambda and Polaris lies the North Celestial Pole, around which they all seem to revolve.

See the full-resolution image and more on polar stars in the Polar Project.
The star's relative faintness comes from a combination of relatively low luminosity (for naked eye stars) and its goodly distance of 152 light years. This class F (F2) star almost reminds us of our own Sun. A hydrogen-fusing dwarf, HR 4686 shines with the light of 5.2 Suns from a surface of 7000 Kelvin, not much hotter than that of our own star, from which we derive a radius and mass both 1.5 times solar. But there the relative similarity ends. The slightly higher mass severely truncates the star's dwarf lifetime at 2.7 billion years, less than a third that of the Sun. With an age of 400 million years, "youthful" HR 4686 is still some 15 percent of the way to beginning its death process. The star is distinguished in pair of other ways. Spinning with a minimum equatorial velocity of 68 kilometers per second, 34 times faster than the Sun, its rotation period must be less than 1.1 days. (It is more massive than the divide at class F5 where stars begin to spin faster). It is also a relatively rare "Gamma Doradus star" that varies subtly with multiple periods (0.7 and 4.9 days known), such stars similar to, but just slightly less massive than, the more common Delta Scuti stars. The star's oddest claim to any kind of notoriety is its central position at the bend of the stem of a mostly telescopic "constellation" called "The English Rose," invented by the famed scientist Robert Hooke (1635- 1703),
English Rose HR 4686 is the bend in the stem of the English Rose, a "constellation" fully visible only through the telescope that was invented by Robert Hooke and presented in his "Discourse on Earthquakes." While the other stars are not visible in the above photo, the charming Rose can be seen down and just to the right of Polaris in the deep image of Ursa Minor. Can you find it?

Information and drawing from Martin Beech, "In Search of the English Rose, Robert Hooke's Lost Constellation," in the Journal of the Royal Astronomical Society of Canada, October 2004, p. 183.
a contemporary of Isaac Newton, thus showing that even the faintest of the sky's naked eye stars can carry their own charm.
Written by Jim Kaler. Return to STARS.