A Type Ia supernova is a powerful explosion that occurs in a binary star system, where one star is a white dwarf. When the white dwarf accumulates enough material from its companion star, it reaches a critical mass and undergoes a thermonuclear explosion, resulting in a bright and energetic event. These supernovae are important for astronomers because they have a consistent peak brightness, making them useful as standard candles for measuring cosmic distances. This property has helped scientists understand the expansion of the universe and contributed to the discovery of dark energy, a mysterious force driving that expansion.