Final answer:
The second-most abundant element in the universe after hydrogen is helium, due to its production in stars from hydrogen fusion. Helium is rare in Earth's atmosphere because it escapes into space more easily than heavier elements.
Step-by-step explanation:
Behind hydrogen, the most abundant element in the universe is helium. Studies of stellar spectra have shown that hydrogen makes up about three-quarters of the mass of most stars, while helium accounts for almost a quarter of a star's mass. This large presence of helium is due to the processes occurring within stars, where hydrogen atoms fuse to form helium in nuclear reactions that emit energy.
Helium, being lighter than other elements like nitrogen and oxygen, can achieve speeds above Earth's escape velocity of about 11 km/s. Over billions of years, more helium has escaped Earth's atmosphere compared to heavier molecules. Consequently, despite being the second-most abundant element in the universe, helium is quite rare in our atmosphere.