Cepheus

(SEE-fee-us or SEF-fee-us)

The King of ancient Ethiopia

 

Cepheus is the northernmost of the constellations that tell the story of Princess Andromeda and her hero, Perseus.

 

Visibility at 8 PM (9 Daylight Saving):  Close to the Celestial North Pole, Cepheus is always visible in the clear night sky.  It is at its highest, just above Polaris, the North Star, in October.

 

What to look for:  When you connect the stars of Cepheus, you get an asterism that looks much like a child's drawing of a house, a box with a triangle on top.  Except for second magnitude Alderamin (Alpha Cephei), all of the stars of Cepheus are of the third magnitude or fainter.

 

Cepheus stands in the north between Cassiopeia, his Queen, and Ursa Minor, the Little Dipper.  Andromeda, the Princess,  lies farther to the south.

 

 

Cepheus

 

Mythology:  Cepheus, a fourth generation descendant of Zeus (Jupiter in Latin), King of the Olympian Gods, and the nymph Io, was the king of ancient Ethiopia, a kingdom some story tellers claim reached clear from modern Ethiopia across parts of two continents to include modern Egypt, Jordan, and Israel.  Perhaps they stretched this country to such an unlikely extent just so that the Princess Andromeda, heroine of one of the most widely known Greek myths, could be non-African.  (Earlier myth makers seem to have had no such hang-ups!)

 

The story of how Andromeda was offered as a sacrifice to save Ethiopia from the Sea Monster called Cetus, and how she was rescued by Perseus, a hero from far-away Greece, is told on her own page.

 

A deeper look:  The star Delta Cephei is the prototype of a class of stars that has become a linchpin of cosmology, the Cepheid Variables.

 

These are stars that are dying.  They've used up their available supply of hydrogen, the nuclear "fuel" that powers a star for most of its life, and have reached a point of instability as they squeeze energy from the helium that hydrogen has been turned into, and from the still heavier elements that are produced from that.  If the star's mass is within certain limits, the instability takes the form of regular pulsations.  The star swells rapidly from too much energy production, causing the star to cool off and lower its energy production, and to shrink until pressures and temperatures in the core go up, and the whole cycle starts over.  The resulting periodic cycle of rapid brightening and slower dimming is easily recognized and distinguished from the patterns of other types of variable stars.

 

In 1912, Henrietta Swan Leavitt of the Harvard College Observatory discovered that cepheid variables in the Large and Small Magellanic Clouds showed an interesting pattern:  Within each cloud, the brighter the star, the longer its period of pulsation.  While the Clouds of Magellan were not yet recognized as galaxies, tiny satellites of our own galaxy, the Milky Way, Leavitt did know that they were systems of stars at a large distance compared to most Milky Way stars.

 

 

Henrietta Swan Leavitt (1868-1921)

 

Reasoning that the nearest stars of one of these clouds is not much closer than the farthest ones, and that the stars of each cloud could thus all be regarded as being equally distant, she concluded the period of any cepheid told us how bright the star actually was as seen from a standard distance.  If you knew how bright a star actually is, its "absolute" magnitude, and how bright it actually looks from Earth, its "apparent" magnitude, you could calculate the actual distance of the star from Earth.

 

Unfortunately, you need to know the actual distance of one such star before you can calculate the distance to any other.  And none of the cepheids were close enough to Earth to directly measure by methods known in Leavitt's time.  But by using statistical methods involving the observed motions of stars within our galaxy, astronomers could get a (somewhat fuzzy) estimate of the distance to an "average" cepheid.

 

In 1924, Edwin Hubble reported the discovery of cepheid variable stars in M31, the object then known as the Great Nebula in Andromeda.  By 1929, Hubble was able to estimate the distance to those cepheids based on the then latest (from 1925) estimate of the distance to an average Milky Way cepheid.  Hubble's estimate placed the Andromeda "Nebula" at 700,000 light years away, making it not just a nebula, but an entire galaxy far beyond our own Milky Way.  The Andromeda Galaxy turns out to be one of hundreds of  billions of galaxies that make up a much vaster universe than had been accepted before Hubble.

 

Over the decades since the 1920s, more observations have placed the "average cepheid" much farther away, and most sources give the distance to the Andromeda Galaxy as between 2 and 2.5 million light years.  Most recently, the Hipparcos satellite has been able to measure distances to some actual cepheids.  Delta Cephei itself, for example, is about 980 light years away, and the distance to M31 may be as great as 2.9 million light years away!

 

Levitt's cepheids are still the best "standard candles" for measuring distances to galaxies.  Using them and the Hubble Space Telescope, we are able to find the distances to galaxies a hundred times as far as M31, and to calibrate new "standard candles" visible to even greater distances,.