To determine the distance to a type Ia supernova, astronomers need to measure the apparent magnitude at peak brightness. Type Ia supernovae are standard bulbs that can be observed at large distances, and measuring redshifts of the host galaxies can also help in distance determination. Option D is correct.
After determining a supernova is a type Ia, astronomers need to measure the apparent magnitude at peak brightness to determine the distance to the supernova.
Astronomers have found that type Ia supernovae are standard bulbs that can be observed at very large distances. By measuring the apparent magnitude at peak brightness, astronomers can estimate the distance of the supernova.
Large telescopes can be used to obtain the spectra needed to measure the redshifts of the host galaxies, which further helps in determining the distance to the supernova.
Hence, D. is the correct option.
--The given question is incomplete, the complete question is
"After determining a supernova is a type Ia, what other measurement needs to be made so that astronomers can determine the distance to the supernova? A) the absolute magnitude just before it exploded. B) the absolute magnitude immediately after it exploded. C) the absolute magnitude at peak brightness. D) the apparent magnitude at peak brightness. E) the apparent magnitude just before it exploded."--