Short answers, pulled from the story.
Astrophysics applies the methods and principles of physics and chemistry to study what astronomical objects are made of, rather than simply recording their positions or motions. As James Keeler, one of the discipline's founders, put it, astrophysics seeks to ascertain the nature of the heavenly bodies, not their positions in space, which is the domain of celestial mechanics.
Norman Lockyer identified helium in 1868 by detecting a yellow line in the solar spectrum that could not be matched to any element known at the time. Working with chemist Edward Frankland, he named the new element helium after Helios, the Greek personification of the Sun.
Cecilia Helena Payne discovered that hydrogen and helium are the principal components of stars, not the heavy elements that dominate Earth's composition. She wrote this dissertation at Radcliffe College in 1925, but her readers persuaded her to soften the conclusion before publication; later research confirmed her finding.
The Harvard computers were a team of women, notably Williamina Fleming, Antonia Maury, and Annie Jump Cannon, who classified stellar spectra under the direction of Edward C. Pickering at Harvard College Observatory from 1885 onward. By 1890 they had cataloged over ten thousand stars, and by 1924 Cannon had expanded the catalog to more than a quarter of a million stars across nine volumes using the Harvard Classification Scheme, which was adopted for worldwide use in 1922.
The Astrophysical Journal was founded in 1895 by George Ellery Hale and James E. Keeler, together with ten associate editors from Europe and the United States. It was created to bridge the gap between existing journals in astronomy and physics.
Modern astrophysics uses instruments across the full electromagnetic spectrum, including large radio telescopes for radio astronomy, optical telescopes paired with charge-coupled devices or spectroscopes, and space-based observatories such as the Chandra X-ray Observatory and the Compton Gamma Ray Observatory for high-energy radiation. Ground-based imaging air Cherenkov telescopes like H.E.S.S. and the MAGIC telescope, along with gravitational wave observatories and neutrino observatories, extend observations beyond the electromagnetic spectrum.