Two stars stand out in the long straggly northern constellation Lynx, and are so remarkably similar that they will be described together. At magnitude 3.16, Alpha Lyncis tops the brightness list, and is also the only star in the constellation that carries a Greek letter. Most of the other stars that form Lynx use Flamsteed numbers, including 31 Lyncis, which at magnitude 4.26 is fourth in brightness. Oddly, 31 Lyncis is the only star that carries a proper name, Alsciaukat, which comes to us from ancient Arabic lore, and refers to a "thorn."
Properties
The similarity of these stars is amazing, yet like identical twins there are subtle differences as well. α Lyn is brighter because it is closer, 220 light years away, as opposed 390 light years for 31 Lyn. A class K7 III giant, α Lyn is also slightly cooler than the class K4.5 III giant 31 Lyn (3860 K vs 3930 K). When we take infrared radiation into account, their luminosities are nearly identical: 700 suns for α Lyn, 740 for 31 Lyn. From their temperatures and luminosities, α Lyn is 59 solar diameters across, as is 31 Lyn. Direct measure of their angular diameters gives respective values of 65 and 75 solar radii, suggesting that some of the parameters for 31 Lyn are in error. Both contain around two solar masses, and have metal contents that are just a bit lower than the Sun's.
It is difficult to know whether these stars are becoming brighter giants with dead helium cores, or dimming with active helium cores fusing into carbon and oxygen, or brightening with dead carbon cores and preparing to slough off their outer envelopes. Each may in fact be in different stages. Nevertheless, both are around 1.4 billion years old, and finished their core hydrogen fusion about 300 million years ago. Alsciaukat is a variable star, designated BN Lyncis. Its variation is a subtle 0.05 magnitudes, about 5%. The variability is a tipoff that the star is indeed a giant brightening for the second time with a dead carbon core, and will eventually turn into a long-period variable like Mira.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]