Cassiopeia and Cosmic Distances

                If you can spot the Big Dipper in the night sky, then you can also find the constellation Cassiopeia. Bearing the name of a mythological queen who tried to kill her daughter, this asterism is composed of five stars that have comparable levels of brightness as seen from our home, planet Earth. We will discover however, that these five stars are separated by incredible cosmic distances, making their appearance from Earth nothing more than an illusion of perspective.

                Cassiopeia is visible year-round in the northern hemisphere and is easy to locate using this simple trick your doctor won’t tell you about! Presumably because your doctor isn’t an astronomer! Begin with the two stars that make up the front part of the Big Dipper. These are the stars Merak and Dubhe as discussed in this article. By tracing a line from Merak to Dubhe, and continuing on, the first bright star you pass is the north star, Polaris. Polaris is part of the constellation Ursa Minor and is very close to the point around which the whole night sky appears to rotate. By returning to the Big Dipper and tracing a line from the star Alioth through to the other side of Polaris, you encounter a distinctive ‘W’ shaped constellation. This is the constellation of Cassiopeia, the queen of Aethiopia.

Figure 1 - Queen Cassiopeia and King Cepheus thank Perseus for saving Andromeda.

The ancient Greek myth of Cassiopeia does not paint her as a particularly good queen or mother. According to Apollodorus, Cassiopeia incurred the wrath of Poseidon on her entire kingdom after bragging that she was more beautiful than the sea god’s daughters, the Nereids. In response, Poseidon sent a terrible sea monster to terrorize Aethiopia. This troubled Cassiopeia and her husband, King Cepheus, so the two of them went to consult an oracle. The oracle, in their infinite wisdom, advocated for infanticide, telling the royal couple to chain their innocent daughter Andromeda to some rocks by the sea as seen in figure 1. Luckily for Andromeda, as the sea monster approached, the legendary hero Perseus was on his way home from his quest to slay Medusa and he came across the sacrificial scene. Falling in love at first sight with Andromeda, Perseus leaped into action, slaying Poseidon’s sea monster using his sword and Medusa’s disembodied head. Queen Cassiopeia and King Cepheus were thrilled that Perseus had rescued their daughter from the peril that they placed her in, and Andromeda and Perseus were soon married. After Perseus killed Andromeda’s previous fiancé and all his friends in self defence, they all lived happily ever after. Except Poseidon was never appeased for the initial offense so he chained Cassiopeia to a throne in the heavens, creating the constellation we see in the night sky.

From Earth, Cassiopeia has a distinctive ‘W’ shape, composed of five bright stars. The stars that make up this constellation are in Greek alphabetical order: Alpha Cassiopeiae (α), Beta Cassiopeiae (β), Gamma Cassiopeiae (γ), Delta Cassiopeiae (δ), and finally Epsilon Cassiopeiae (ε). Don’t name your children the way astronomers name stars. One might assume that these stars are nearby one another in the universe due to their proximity in the night sky, but that’s far from the case.

Figure 2 - The stars that make up Cassiopeia.

α Cassiopeiae is the largest of the five stars, with a radius of 45 times that of our Sun. This colossal star dominates its four-star system as its dimmer dwarf stars orbit around it. This massive star has blown through much of its hydrogen fuel during its cosmically modest 200-million-year lifespan and is on its last legs, now fusing helium into heavier elements. α Cassiopeiae also is referred to as Schedar, from Arabic meaning ‘the breast’. Many of the stars in the constellation Cassiopeia have names in relation to a part of the queen’s body. Get your mind out of the gutter.

On the far-right side of the ‘W’ asterism, and closest to Earth, is the star β Cassiopeiae. This star also bears the name Caph, meaning ‘hand’ in Arabic. β Cassiopeiae is a lonely old dying star. It once was a brilliantly luminous star, but now that it’s more than one billion years old, it has used most of its fuel and is beginning to cool and expand into a red giant. Eventually, β Cassiopeiae will shed its outer layer, leaving only the small inert core behind to cool as a white dwarf star. However, just because β Cassiopeiae is old doesn’t mean that it’s sedentary! This star spins about its axis once every day, causing the middle of the star to bulge outward into an odd sideways egg shape. Parkour!

Moving down the queen’s body, we come to δ Cassiopeiae, also called Ruchbah, meaning ‘knee’. This subgiant star has also exhausted its hydrogen fuel and is beginning to expand similarly to β Cassiopeiae. Due to its larger mass and larger gravitational forces, δ Cassiopeiae has burned through its fuel twice as fast as β Cassiopeiae. It approaches the end of its hydrogen burning life after only 600 million years compared to β Cassiopeiae’s 1.2 billion years. δ Cassiopeiae’s brightness has been observed to fluctuate every two Earth years, which suggests that δ Cassiopeiae may actually be part of a binary star system, where two stars orbit each other. The dip in brightness that astronomers observe every two years corresponds to one of the stars passing behind the other. As the two stars in the Ruchbah system orbit back and forth, I like to imagine that Cassiopeia is doing the Bee’s Knees dance move.

ε Cassiopeiae is also called Segin. Know ones knows why! This massive star burns with the luminosity of 3000 Suns, but because of its great distance from Earth, ε Cassiopeiae is the dimmest of the five stars discussed from this constellation. This incredible luminosity comes at a cost however, as this star will never reach old age. ε Cassiopeiae is only about 15 million years old and will only live to be about 100 million years old, a fraction the lifespan of the previous stars. Then it will die in a brilliant explosion called a supernova.

γ Cassiopeiae makes ε Cassiopeiae look puny. If you were struggling to imagine what the brightness of 3000 Suns would look like, I invite you to imagine γ Cassiopeiae which is brighter than 30,000 Suns. Someone get me my Ray-Bans! γ Cassiopeiae has a small Sun sized companion and a disk of debris. This blazing star spins so fast that it occasionally throws its luminous surface material into space increasing the brightness of the star for a time. This variability in brightness allows γ Cassiopeiae to occasionally beat out α Cassiopeiae for the title of brightest star as seen from Earth. That’s an impressive feat keeping in mind that γ Cassiopeiae is at a severe disadvantage, being twice as far from Earth! γ Cassiopeiae has an informal name Navi, after the middle name of the astronaut Virgil Ivan 'Gus' Grissom spelt backwards. Gus is a tragic hero in the history of human spaceflight as the commander of the Apollo 1 mission. During a pre-launch test Gus and his crew died after their command capsule caught fire, and they were unable to be rescued. The Apollo program was named by their suggestion and in their honour. When I gaze at γ Cassiopeiae, in the middle of its constellation, I give thanks to those three brave astronauts who selflessly gave their lives for the scientific development of all humankind.

Figure 3 - The stars of Cassiopeia as seen from distant side view to scale. Compare with Earth perspective in Figure 2.

Unlike the Ursa Major moving group, the stars in the constellation Cassiopeia do not move through the universe together. As seen in figure 3, they aren’t anywhere near each other relative to their distances to Earth. The distinctive ‘W’ shape of Cassiopeia made by five comparably bright stars in the night sky is merely an illusion of perspective from Earth’s relative position. Another fun way to illustrate the vast distances between these stars is to consider how long ago the light we now see was produced by each star. In our day to day lives, we don’t often think about how long it takes light to travel from what we’re looking at to our eyes. However, it takes about 3 seconds for light to travel from the Moon to the Earth, or about 8 minutes for light to travel from the Sun to the Earth. On a cosmic scale, the speed limit of light (300,000 km per second) is increasingly significant! Astronomers often use the light-year (ly), the unit of distance a beam of light travels in a year, to discuss the large distances between stars. The light we currently see from β Cassiopeiae, closest of the five to Earth, has taken 55 years to travel to Earth. As of 2024, the light we see today from β Cassiopeiae was emitted in 1969; the year humans first walked on the Moon. δ Cassiopeiae emitted the light we see now 99 years ago, in 1925, when the Chrysler automobile manufacturing corporation was founded. 230 years ago, when the light we now see from α Cassiopeiae was emitted, George Washington was President. 410 years ago, when the light we see from ε Cassiopeia was emitted, Shakespeare's Macbeth was being performed at the Globe theatre but had yet to be officially published. For the most distant, super bright γ Cassiopeiae, its light was emitted 550 years ago in 1474, the same year Queen Isabella I of Castille began her reign, before going on to finance Cristopher Columbus’ voyages.

The constellation Cassiopeia is one of my favourites due to how easy it is to find in the night sky year-round. However, a deeper understanding of Cassiopeia inspires a certain terrifying awe. The constellations our stargazing ancestors have admired for millennia are not always true cosmic structures. In this case, the stars we see as one object are completely unassociated with each other. These points of light are so far away from us, and from one another, that all of one’s worldly references of distance are reduced to nothing as one confronts this cosmic scale. Cassiopeia reveals that the universe is a beautifully large place, but no larger than the human capacity for wonder.

Written by Evan Nelles Henderson, September 2024