During my recent all-nighter for the Perseid meteor shower, my mind turned to John Russell Hind and Herschel’s Garnet Star.
First, look for the constellation Cepheus, which is low in the northeastern sky just after dark. He looks a bit like a squarish head wearing a pointed dunce cap.
Hidden in Cepheus is a true astronomical treasure, albeit a faint one. Herschel’s Garnet Star is visible, sometimes barely so, to the unaided eye under dark, rural skies like the ones we get in southern Ohio.
To find the Garnet Star, look above and halfway between the top two stars of the constellation. Imagine an upside-down head wearing a garnet necklace.
As its name suggests, the Garnet Star is red. In fact, it’s the reddest star visible to the unaided eye, Betelgeuse notwithstanding. A small telescope or binoculars reveal its ruddiness most spectacularly.
The star looks increasingly orange in larger telescopes. It appears unpleasantly yellow-orange in observatory-sized telescopes like the ones at Perkins or the John Glenn Astronomy Park.
Since the late 19th century, astronomers have studied the Garnet Star with every possible scientific instrument. However, the star remains a testament to the power of simple visual observation.
Because of its naked-eye visibility, stargazers have been able to see it since ancient times. However, it took one of history’s finest observers to distinguish it as significant. In 1783, William Herschel noted its “very fine deep garnet colour.”
Most stars don’t show their colors very well to human vision. Herschel recognized the extraordinary rarity and beauty of the Garnet Star.
The matter lay dormant until 1848 when another eagle-eyed observer, English astronomer John Russell Hind, noticed that its brightness varied inconsistently.
For centuries, amateur astronomy has included a vigorous subset of telescopists who provide data on variable stars to their professional brethren. Variable-star observers make careful visual estimates of a star’s changing brightness by comparing it to the fixed brightness of nearby stars.
As a result, we have an almost continuous record of the Garnet Star’s variations in brightness dating back to 1881.
At first, such variability data seemed unimportant. However, astronomers slowly realized that the star’s behavior is crucial to understanding the end of life of stars much more massive than our home star, the sun.
Color and variability tell us much about a star. The Garnet Star is a red hypergiant, an enormous star that has reached a premature old age similar to the more famous Betelgeuse in the constellation Orion.
Despite a wealth of data, astronomers have had difficulty finding a pattern in its variability. Apparently, the star goes from bright to faint and back again every 850 days, 4,400 days, or both.
The upshot seems to be that the star’s behavior is far more chaotic than many other old stars. The Garnet Star is flailing about because it is in its death throes.
Stars cool down dramatically when they reach their decrepitude. The Garnet Star’s surface temperature is a tepid 3,700 degrees Kelvin, 6,200 degrees on the Fahrenheit scale.
Our middle-aged star, the sun, measures about 10,000 degrees Fahrenheit on its surface. That fact doesn’t stop the Garnet Star from shining 283,000 times brighter than the sun.
Hypergiants puff up when they reach their decrepitude. The Garnet Star is well over 1,000 the sun’s diameter, large enough that if we removed the sun from our solar system and replaced it with the Garnet Star, Earth would be deep inside the star. It would engulf Jupiter and make it almost to Saturn.
It will not remain that way for long. In their youth and middle age, stars are hydrogen bombs, fusing hydrogen into helium in a sustained explosion that, in the case of our sun, can last 10 billion years.
Stars like the Garnet Star expend their hydrogen quickly, within millions, not billions, of years. Having sped through a very abbreviated life, it now fuses its helium into carbon and its carbon into iron.
Eventually, the heavy elements in the star smother its thermonuclear conflagration. In a short time, perhaps within the next million years, the Garnet Star’s thermonuclear fusion will cease.
The star’s core will then collapse into a black hole. The resulting shock wave will blow away the star’s outer shell in a supernova explosion that will light up Earth’s distant sky.
Tom Burns is the former director of the Perkins Observatory in Delaware.
