Final answer:
To observe how a hot upper atmosphere of a star changes, spectroscopy is used. The composition of a distant star can also be determined using spectroscopy. The velocity of a galaxy moving away from Earth is measured through timing.
Step-by-step explanation:
1. To observe how a hot upper atmosphere of a star changes, you would need to use spectroscopy. Spectroscopy allows us to analyze the star's spectrum and look for changes in the absorption or emission lines, which can provide insights into the temperature and composition of the star's upper atmosphere.
2. To determine the composition of a distant star, you would also need to use spectroscopy. By studying the star's spectrum, we can identify the specific elements present in the star and their relative abundances.
3. To measure how fast a galaxy is moving away from Earth, you would use timing. This typically involves measuring the redshift of the galaxy's light, which is caused by its motion away from us. By analyzing the timing of the observed redshift, we can estimate the galaxy's velocity relative to Earth.