Zosma

Zosma is the star that could be called the butt of the lion. The head of Leo the lion takes the form of a backwards question mark. Alpha Leonis, better known as Regulus, is at the bottom of the question mark and it would be a front paw. Beta Leonis is called Denebola and it shows the tail of the lion. Denebola forms a right triangle with the stars Chertan (the back paw) and Zosma (the, er, ahem, top of the tail). A nice map of Leo is at http://www.astro.uiuc.edu/~kaler/sow/leo-t.html.

While most star names have Arabic origins, Zosma comes from the Greek and it could be translated as “girdle”. Not very masculine, methinks. There is, of course, an Arabic name for this star, Duhr, which could be translated as “hip” - a better fit.

Zosma is an A4 star. Many of the stars that can be seen with the naked eye are A stars, there are more than 75 such stars within 100 light years. It is 23 times as luminous as the sun though its radius is just about double that of our star. It is 58 light years away. Zosma has the somewhat unusual characteristic of being a very fast rotator while still being a main sequence star. It completes a full rotation in half a day which means it spins 90 times faster than the sun.

Zosma is part of a stream of stars called the Ursa Major Stream. This is a set of stars that all appear to be moving in the same direction and at the same speed. Sirius is another star in this stream.

Zosma has been studied for quite a while so we know things about it including its apparent age. It is approximately 700 million years old and stars of its type run out of hydrogen for fusing after a billion years. So even though Zosma is much younger than the sun, it is really over the hill and I know just how it feels. Zosma will eventually become an orange giant star and spend its retirement years as a white dwarf.

Despite the relative closeness and intense study made of Zosma, it is not clear if it is a variable star or not. If it is, it is of a type called Delta Scuti. These stars do not vary in magnitude very much. They also have multiple pulsation periods and each of these periods are superimposed on each other resulting in beats and dips in the measured magnitude. The typical apparent magnitude of Zosma is 2.56 and the absolute magnitude is 1.29.

Zosma is a triple star system (the companions are at magnitude 9 and 13) and it does not have a dusty disk around it which would have indicated the possibility of planetary formation. By comparison Denebola appears to be a solitary star and it does have a dusty disk. Perhaps it is the stellar companions around a star that tend to sweep up the leftovers after stellar formation.

Leo is of interest these days because Saturn is hanging out in this area, not far from Regulus. On March 8, Zosma transits just 10 minutes after midnight.

Whenever I want to look up information on a particular star I start with Google and search on “Kaler starname” using the common or Bayer name for starname and don’t use the quote marks.

OBAFGKM

Stars are classified based on their surface temperatures. The original intent was to classify them alphabetically, A, B, C and so on. But upon further review (as they say in the NFL) it was determined that the spectrum of the O stars (the one thought to be at the end) was really showing those stars to be the hottest stars. Similarly, A and B were transposed and many other designations were confused because of binary stars and other complications. So the resulting order of the stars became OBAFGKM. To see example spectra go to http://apod.nasa.gov/apod/ap010530.html. This order is often memorized with a mnemonic device such as “Oh, be a fine girl, kiss me.” Feel free to alter the phrase to match your preference. Further modifications to the classification scheme occurred with dimmer, cooler stars were found. So L stars are cooler than M stars and T stars are even cooler. In addition, some people use R, N and S classifications although I believe these are mostly subclasses of M stars. To see more about mnemonic devices used to memorize the classification order, go to http://astro2.byu.edu/~sdb/Mnemonic.html.

Bayer Star Names

The brightest stars in a constellation are given designations based on their relative brightness within the constellation. The brightest star is called Alpha, the next is Beta and so on through the Greek alphabet. The Latin possessive (aka genitive) form of the constellation is used with the Greek letter. Thus the first four brightest stars in Leo are called Alpha Leonis, Beta Leonis, Gamma Leonis and Delta Leonis.

These star names were proposed by Johannes Bayer in the early 17th century. These names are obviously limited by the 24 characters in the Greek alphabet. For more information on this and other star naming schemes see http://www.astro.uiuc.edu/~kaler/sow/starname.html.