Meet Gamma Cassiopeiae, the Classic Eruptive Variable

Gamma Cassiopeiae Vitals

Common name	Gamma Cassiopeiae
Other designations	HIP 4427, HR 264, HD 5394
Nicknames	Navi, Tsih (the Whip) in old China
Apparent magnitude	Varies; currently about 2.15 (historically 1.6–3.0)
Distance from Earth	613 light-years
Type	B0.5IVpe
Color	Blue
Mass	17 M☉
Radius	10 R☉
Constellation	Cassiopeia
Right Ascension	0h 56m 42s
Declination	60° 43' 00”
Multiple system?	Yes
Variable star?	Yes
Exoplanet status	None known
Probable fate	Supernova

Physical Characteristics

A variable star is a star that fluctuates in brightness over a period of time, perhaps less than a day, or maybe weeks, months, or years. Variable stars are a large class of objects that amateur and professional astronomers have long chased for both fun and science. Even the Sun is slightly variable, brightening and dimming over a roughly 11-year period by 0.1%. Other stars are more dramatic, including a class of irregularly changing stars known as eruptive variables.

If you’d like to seek out one of these for yourself, there’s a bright and easy-to-find eruptive variable in the northern sky — and you’ve probably seen it many times without knowing it. That star is Gamma Cassiopeiae, the middle point in the W shape of Cassiopeia. Sometimes Gamma Cassiopeiae dims to 3rd-magnitude, while during other periods it shines as brightly as magnitude 1.6, as it did in the 1930s. But those changes aren’t predictable, so what’s going on?

Gamma Cassiopeiae rotates at a high speed — so fast that its shape is distorted, similar to the effects seen on Vega, Regulus, and Altair. But Gamma Cassiopeiae spins much faster, some 400 kilometers per second (900,000 mph), and the resulting colossal forces are strong enough to occasionally dislodge matter from the star. This material then forms a disk of orbiting material. While distance prevents us from directly observing the disk, we can nevertheless determine its existence through spectroscopy. The star’s brightness changes as matter is flung from the star — hence the name eruptive variable.

Gamma Cassiopeiae is classified as a Be star, different from ordinary B-class stars. The lowercase e stands for emission lines from hydrogen, first noted by Father Angelo Secchi in the 19th century. These lines come from the ejected gases.

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Origin / Mythology

In Greek myths, Queen Cassiopeia was the rather vain mother of Andromeda, and some star map illustrations portray the constellation as the queen seated in a W-shape chair, looking at herself in a mirror. In some Chinese mythology, the W is incorporated in a chariot driver constellation, with Gamma Cassiopeiae representing the whip (“Tsih” in Chinese).

However, there is little existing mythology involving the star Gamma Cassiopeiae specifically, and the star oddly lacks a traditional Greek or Arabic name, and doesn’t have a proper name at all. The Gamma Cassiopeiae label we use today is strictly utilitarian, part of Johann Bayer’s star-cataloging efforts of the 1600s.

Occasionally, though, Gamma Cassiopeiae is referred to informally as “Navi.” This recent nickname came out of the Apollo Program of the 1960/70s. Gamma Cassiopeiae was one of the many stars used as to calibrate the gyroscopes aboard Apollo spacecraft. Noticing that the star lacked a traditional name, astronaut Gus Ivan Grissom playfully labeled Gamma Cassiopeiae “Navi” on his star chart — using his own middle name spelled backwards. Later Apollo astronauts continued the tradition, and Navi has since found its way into general use in some instances — such as in the popular astronomy software Stellarium.

How to See Gamma Cassiopeiae

Gamma Cassiopeiae is easy to find from the U.S. From about 35°N latitude, the star is circumpolar and never drops below the horizon. Cassiopeia is probably one of the most recognizable constellations in the northern sky. The familiar W shape is easy to remember, and Cassiopeia rotates around Polaris roughly opposite the Big Dipper.

Cassiopeia sits right in the middle of the Milky Way band, so it’s worth trying to find a dark sky away from artificial lights for viewing, so that you can see the full effect of the constellation backlit by the edge of our galaxy. While you’re there, use the larger point of Cassiopeia’s lopsided W to take a quick hop over to a different galaxy altogether: the majestic Andromeda. (Remember, the Andromeda Galaxy is of course in the constellation Andromeda, and Andromeda is Cassiopeia’s daughter, so they’re next to each other in the sky — makes sense, right?)

Observing Gamma Cassiopeia is also a nice way to investigate the interstellar medium. We tend to think of space as completely empty, but there are plenty of gas and dust particles drifting about, particularly in line with the edge of our galaxy. Gamma Cassiopeiae would actually shine about 0.35 magnitudes brighter if it were in another section of the sky, but its location in line with the plane of the Milky Way places it in a rather dusty region. When you point your telescope at Gamma Cassiopeiae, you’re not only looking at a distant star, you’re looking through untold numbers of space dust particles slowly rotating around the center of our galaxy.

While you observe Gamma Cassiopeiae, try to visualize the immense activity taking place there and in between — and at the same time, be thankful for the relative tranquility of our own Sun.

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Daniel Johnson is a Wisconsin-based freelance writer and professional photographer and the co-author of over a dozen books. He’s a longtime amateur astronomer and fortunate enough to live in a rural region with excellent seeing conditions. You can view some of Dan’s photography (he does a lot of animals!) at www.foxhillphoto.com