CASSIOPEIA: AN INTRODUCTION

Updated: Sep 13, 2021

Woah! Sorry everyone, took a bit of a hiatus there this month. Over the past few weeks, I've been helping to put together the final concert for Good Question A Cappella! The entire video is up on our YouTube page now, so I’d love it if you guys check all of our hard work. After that was done, I went down to Williamstown to celebrate the graduation of the Class of 2021! Congratulations to each and every one of them on their incredible accomplishment. I wish them all the best in this exciting new chapter of their lives, and I’m so glad I got to see so many of them at the ceremony. Now that I am back, I’m ready to talk about our next constellation: Cassiopeia!

I must admit, as much as I love the constellations and stories around the Big and Little Dipper, it is refreshing to move on to something new. We packed a LOT into those last two constellations, from bears to wagons, gods to world trees, and asterisms to pole stars. By comparison, Cassiopeia is a far simpler constellation to tackle. There are still so many rich stories and lore to cover, but all of them will center around the constellation prominent “W” pattern and the five stars that form it. Overall, far less confusing!

Figure 2: Finding Cassiopeia with the Big Dipper and Polaris

Cassiopeia is another core constellation in the northern celestial hemisphere, and it is circumpolar north of the 40th parallel north. Once you know what you’re looking for, it is difficult to miss the bright “W” pattern after a quick scan of the northern sky. However, a bright full moon or a starry night free of light pollution can actually drown out the constellation. If you’re having trouble making it out, there is a way to find it using asterisms we’ve already covered! Imagine Polaris as the hub of a wheel and the Big Dipper riding the wheel’s tire. In this image, Cassiopeia is riding near the opposite side of the tire! For much of the northern hemisphere, the two dance around Polaris all year long, and this makes finding Cassiopeia an easy task. First, locate the Big Dipper and find Polaris using the pointer stars. Next, go back to the Big Dipper and choose one of the three stars that make up its handle. A line from any of these stars drawn back through Polaris will lead you directly to Cassiopeia! (1)

Five bright stars make up the points of Cassiopeia’s “W” pattern. Each has an interesting name with origins spanning the vast history of astronomy. As such, I’d like to go into more detail here than I have in the past with stellar nomenclature. From left to right when the “W” is right-side-up, the stars are: Segin, Rukbah, Navi, Schedar, and Caph, or Epsilon Cassiopeia, Delta Cassiopeia, Gamma Cassiopeia, Alpha Cassiopeia, and Beta Cassiopeia, respectively. The first set of names are the stars’ official names, while the second set is their Bayer designations, a system developed by German astronomer Johann Bayer (1572 - 1625) (2). Such designations consist of the star’s constellation preceded by a Greek letter. Generally, the Greek letters rank the stars within the constellation by their brightness from the Earth, or their apparent magnitude, with Alpha being the brightest. This, however, was by no means consistent. Even within Cassiopeia, Gamma Cassiopeia, a variable star similar to Polaris Aa, occasionally outshines Alpha Cassiopeia (3).

Figure 3: Gus Grissom, Ed White, and Roger Chaffee of Apollo 1

Speaking of Gamma Cassiopeia, the story behind its official name Navi is particularly interesting, albeit having a tragic ending. As NASA was kickstarting its Apollo program, the astronauts learned the positions and names of 36 prominent stars for navigation. As a private joke, Apollo 1 Command Pilot Virgil Ivan “Gus” Grissom renamed three of the stars on the charts for each member of his crew. Navi is his own middle name “Ivan” spelled backward. The other two are Dnecos, which is “Second” spelled backward for Senior Pilot Edward White II, and Regor, which is “Roger” spelled backward for Pilot Roger Chaffee. These changes went largely unnoticed by their superiors at NASA. Tragically, on January 27, 1967, a fire erupted in the command module during a launch rehearsal test. All three astronauts were killed in the fire. In reviewing their star charts after the accident, NASA discovered the new star names. The names were made official in memory of the sacrifice made by the Apollo 1 crew in the advancement of human space exploration. (4)

Figure 4: Perseus and Cassiopeia illustrations from copy of Al-Sufi's "Book of Fixed Stars"

The nomenclature of the other stars of Cassiopeia (except for Segin, whose origins have been lost in translation) are all derived from Arabic names. Rukbah comes from “Ar-Rukbah” for “the knee,” Schedar comes from “As-Sadr” for “the breast,” and Caph comes from “Al-Kaff” for “the palm” (5). You’ve probably noticed that most of the stars I’ve mentioned in these posts carry names of Arabic origins. This is no coincidence. The rise of the Abbasid Caliphate (750 - 1258) and the Buyid Dynasty (934 - 1062) saw the city of Baghdad at the forefront of the Islamic Golden Age and as a major hub for the study of astronomy. Astronomers based out of the city, such as Abd al-Rahman al-Sufi (903 - 986) (6), translated older Greek, Persian, and Indian studies of the night sky into Arabic and, using advanced scientific instruments and better measurement techniques, greatly improved the accuracy of the placement of the stars and the movements of the planets. A more accurate understanding of the night sky was particularly important in Islam at the time, as it meant more accurate timekeeping. This was instrumental in determining the correct times for prayer, the sun's rising and setting, and the proper start of a new month. These star charts set a golden standard across the world, and as such the vast majority of names designated to the stars by Muslim astronomers are used to this day. (7)

Figure 5: Internal structures of a Red Giant star

Sharp-eyed stargazers may have noticed that Schedar, or Alpha Cassiopeia, shines with a more reddish-orange hue than most other visible stars in the night sky. This is not your eyes playing a trick on you. Schedar is a Red Giant star, a great example of the very literal names astronomers love to use (8). Red Giants are a late stage of stellar evolution for stars with masses ranging from approximately half to eight times the mass of our Sun. Schedar itself is about four times the mass of our Sun (3). Such stars spend most of their lives as Main Sequence stars, where hydrogen is fused into helium in their cores. These reactions produce an outward pressure to counteract the ever-present effects of gravity. Without this equilibrium, the outer layers of the star would collapse on themselves. After billions of years, however, the core exhausts its hydrogen fuel. Gravity begins to overtake the internal gas pressure and the now-helium core contracts. This contraction, in turn, results in a rapid release of heat, and the unburned hydrogen at the edge of the core reaches the proper temperature and density for fusion. This is called “shell-burning,” and this new source of outward pressure causes to outer layers of the star to expand. The expansion of the outer layers decreases the star’s effective temperature, shifting its emission to the cooler, and redder, end of the visible spectrum. Schedar’s current radius is 42 times the radius of our Sun, and its effective temperate is 4530 K (3), compared to the Sun’s 5780 K (9). (10)

All in all, these stars only spend about 1% of their lives as Red Giants before the core becomes sufficiently hot and dense enough to actually fuse helium into carbon (10)(11). Over time, increasing layers of shell-burning will eject the star’s outer layers in a Planetary Nebula, leaving an Earth-sized compact remnant known as a White Dwarf at the center (12). This fate is not unique to Schedar. Our own Sun is about halfway through its tenure as a Main Sequence star, and in some five billion years it, too, will exhaust its supply of hydrogen. Its expansion into a Red Giant will swallow up Mercury, Venus, and perhaps the Earth itself. Even if the Earth is spared, it will become a dry barren wasteland, too hot for liquid water to exist. The so-called habitable, or Goldilocks, zone, will expand outwards to the gas giants of Jupiter and Saturn. At that point, my personal favorite celestial body, Saturn’s atmosphere-rich moon Titan, may become the next viable home for life in the solar system. (13)

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CASSIOPEIA: AN INTRODUCTION

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