Final answer:
Type Ia supernovae act as standard bulbs because of their predictable peak brightness and uniform light curves, allowing astronomers to measure the distances to galaxies by comparing the apparent and absolute luminosities. These measurements have been pivotal in understanding the expansion rate and acceleration of the universe.
Step-by-step explanation:
Type Ia supernovae are incredibly powerful and bright stellar explosions that serve as one of the best standard bulbs in astronomy. These supernovae are the result of a white dwarf in a binary system accreting enough material from its companion star to exceed the Chandrasekhar limit and exploding. The resulting explosion has a peak luminosity of about 4.5 × 10⁹ Lsun, which is so bright that it can outshine the galaxies that host them, making these supernovae visible over very large distances.
The key reason Type Ia supernovae can be used to determine a galaxy's distance lies in their consistency. These supernovae have a predictable peak brightness and a standardized light curve, which astronomers can use to determine how far away the supernova is. Since luminosity decreases with distance, and we know the actual luminosity of the Type Ia supernovae, we can calculate the distance of a galaxy by observing the apparent brightness of the supernova it contains. This technique has been critical in measuring the distances to faraway galaxies, and in understanding the expansion of the universe, including the surprising discovery that the expansion is accelerating, rather than decelerating as previously assumed.