Please choose the correct answer to the question: The North America Nebula is named for its similarity in shape to the North American continent. It was discovered within the Milky Way by astronomer William Herschel in 1790. What was Mr. Herschel most likely observing?

Answer1: the explosion following the collapse of a star Answer2: a dense cluster of stars of varying magnitudes Answer3: meteorites as Earth passed through the tail of a comet Answer4: a cloud of hydrogen gas and dust where stars are born

Answer format: answer1/answer2/answer3/answer4
Answer: answer4