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Constellation of the Month: Cepheus, the King

Five of the constellations near our north pole (opposite Ursa Minor) are all related to the myth of the Boast of Cassiopeia. This story has been told here previously, since the constellations of Cassiopeia, Perseus, Andromeda and Cetus are all part of this myth. Cepheus, the king of Aethiopia (not modern Ethiopia), was married to Cassiopeia, who bore him a daughter named Andromeda. Cassiopeia was very boastful, and as her daughter grew into womanhood, Cassiopeia proclaimed that Andromeda was the most beautiful woman in the world. The Nerieds heard this and complained to Poseidon, god of the sea, who sent Cetus the Sea Monster to ravage the coast of Aethiopia. To save his people, Cepheus consulted an oracle who told him to offer up Andromeda as a sacrifice.

Click the image for a larger map

Looking up above Polaris in our November sky is one of the five constellations from the myth of "The Boast of Cassiopeia" — our constellation of the month, Cepheus. Just to its right is the constellation named for Cepheus' wife, Cassiopeia. Above Cassiopeia is Andromeda, her daughter, and to Andromeda's right is Cepheus's son-in-law, Perseus. Cetus, the sea monster in this myth, is two constellations above Andromeda, always ready to pounce on the morsel denied him by Perseus.

Andromeda was chained to the costal rocks. Cetus saw this morsel and headed toward her. But the hero Perseus, flying by on his winged shoes, spotted Andromeda and was taken by her beauty. He went to Cepheus and made a deal that if he saved her, he would get Andromeda's hand in marriage and a kingdom of his own. King Cepheus agreed, and Perseus went off to slay Cetus. Finding a weak point between the scales on Cetus' belly, he thrust with his sword until he hit Cetus's evil heart, killing him. True to his word, Cepheus gave Andromeda's hand in marriage to Perseus and they led a long and joyous life. Their first-born son, Peres, gave rise to the Persians. Upon their death, Athena placed the couple among the stars — where Cetus awaited, to chase them forever around the sky. Cepheus and Cassiopeia were placed among the stars by Poseidon. So this one myth provides us with five different constellations.

Among the stars of Cepheus is Delta Cephei, which John Goodricke discovered to be a variable in 1784. This star has used up most of its hydrogen and started burning helium as well. A pulsating star, Delta Cephei gets larger and smaller in a regular rhythm, so it gets brighter and then fainter every 5.4 days, varying in brightness from magnitude 3.5 to 4.3.

The importance of pulsating stars was discovered by American astronomer Henrietta Leavitt, who was examining variable stars in the Small Magellanic Cloud, a nearby galaxy. She found a group of them whose period was directly related to their brightness. Since the stars were all the same distance away in the Small Magellanic Cloud, their brightness relative to each other as we observe them is the same as their actual brightness differences. Leavitt discovered that the brighter they were, the longer they would take to pulse. Once she found this period-luminosity relationship in 1912, she could use it to determine the absolute brightness of any of these pulsing stars by measuring its period. She could then measure the observed brightness, which would tell her the distance to the variable. The only rub was that to calibrate this distance scale, Leavitt needed to find one of these pulsing stars whose distance could be measured in some other way.

This is where Delta Cephei comes in. At 890 light-years away, Delta Cephei is close enough to directly determine its distance by measuring its position relative to the background stars from either side of the Earth's orbit around the Sun. So now, with the measured distance to Delta Cephei, its pulsing period and its brightness, the distance to any "Cepheid variable" can be determined solely from the period and its measured brightness. This has allowed astronomers to determine the distance to many a galaxy where they were able to find and measure a Cepheid variable among the stars of that galaxy.

Years latter, Edwin Hubble would use this distance scale to determine a new scale that was based not on measuring an individual Cepheid variable star in a galaxy, but the red shift of all the light coming from that galaxy. Using the Cepheids in the closer galaxies, he could plot the distance to each galaxy and its red shift. Hubble used this to determine how far away the more distant galaxies were, even though he could not see the Cepheid variables in them. His work led to the realization that the universe was actually expanding, giving birth to the Big Bang Theory and modern cosmology.


The Planets for November 2007

November is the month we bid farewell to Jupiter as it goes behind the Sun. The King of the Planets is still visible low in the southwest after sunset, and it sets less than two hours later. Still in Ophiuchus, Jupiter is magnitude -1.9 at midmonth, and is only 32.9 seconds-of-arc.

Mars is a prime target for telescopic observing, 12.3 seconds-of-arc across and glowing like a ruby at magnitude -0.7. Heading toward opposition in December, Mars is in Gemini all month. At the beginning of the month it is heading east. By midmonth, it has stopped at what is called the stationary point, and then turns westward in retrograde motion.

Watch the Sky
(all times MDT/MST)

Nov. 1, 3:18 p.m.
Last Quarter Moon

Nov. 3, 4:53 a.m.
Regulus reappears from
behind the Moon

Nov 4, 2 a.m.
Daylight Savings Time ends

Nov. 8, morning
Mercury greatest
elongation west of Sun

Nov. 9, 4:03 a.m.
New Moon

Nov. 15, 9 a.m.
Mars stationary

Nov. 17, 3:32 p.m.
First Quarter Moon

Nov. 19, early morning
Leonid Meteor Shower

Nov. 24, 7:30 a.m.
Full Moon

Saturn comes up around 1 a.m. The Ringed Planet is still in Leo and will not be moving much during the month. Saturn is magnitude 0.7, while its rings still have the southern face showing and are tipped only 7.1 degrees to our line-of-sight. They are now 39.4 seconds-of-arc across, while the ball of the planet is 17.3 seconds-of-arc across. The Earth is getting a little closer to Saturn, so it will continue to grow in size for the next few months.

Around 3 a.m., Venus rises in the east. It starts the month in Leo, but after only two days enters Virgo, where it remains the rest of the month. At midmonth, Venus will be magnitude -4.2 and 17.7 seconds-of-arc across. The Goddess of Love is now in the gibbous phase, 65 percent illuminated at midmonth.

Mercury makes an appearance in the morning sky at the beginning of the month. Mercury is low in the east starting around 5 a.m. On Nov. 8, Mercury will be at its best, reaching 11 degrees in altitude while the Sun is still six degrees below the horizon. Mercury will be magnitude -0.5, and will measure 6.8 seconds-of-arc across its 57 percent sunlit disc. After that, Mercury will turn and start back toward the Sun as it gets smaller and becomes fuller. Early in December it will pass south of the Sun as it goes around the Sun's far side.

The Leonid meteor shower appears on the morning of Nov.19. These dust particles from the tail of Comet Tempel-Tuttle will appear to come from the direction of the constellation Leo. Unlike the Leonids at the turn of this century, this year's shower is not expected to provide more than about one meteor every four minutes. So bundle up warmly and snuggle up in your chaise lounge lawn chair for this shower and "keep watching the sky"!


An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.


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