I'm pretty sure that it was fire
Poe’s story is an allegory that contains multiple layers of meaning. The Red Death is a disease that plagues the city and it is a character that signifies death. The Red Death is a rapidly spreading disease that is feared and dreaded by all. The prince makes arrangements to escape the widespread disease by retreating to his palace. There, he invites guests to a masquerade ball. The guests, similar to the prince, do not feel threatened by the terrifying disease:
The external world could take care of itself. In the meantime it was folly to grieve, or to think. The prince had provided all the appliances of pleasure. There were buffoons, there were improvisatori, there were ballet-dancers, there were musicians, there was Beauty, there was wine. All these and security were within. Without was the "Red Death."
The guests assume at first that the Red Death is another costumed guest. But after the Red Death strikes the prince, the guests realize that they can’t escape death, regardless of their wealth and power. Their confident sense of immunity is quickly exposed as an illusion:
And now was acknowledged the presence of the Red Death. He had come like a thief in the night. And one by one dropped the revellers in the blood-bedewed halls of their revel, and died each in the despairing posture of his fall. And the life of the ebony clock went out with that of the last of the gay. And the flames of the tripods expired. And Darkness and Decay and the Red Death held illimitable dominion over all.
Answer:
astronomy is the correct answer
