Type I supernovas have absorption lines that show they do not have hydrogen in them. Type Ia supernovas are very bright for a short amount of time. Then they get less bright very quickly. Type Ia supernovas happen when a white dwarf star is orbiting a big star. Sometimes, the white dwarf star sucks matter off of the big star. When the white dwarf gets to be about 1.4 times the mass of the sun, it collapses. This makes lots of energy and light, which is why supernovas are very bright. Type 1a have mostly the same brightness. This allows them to be used as a secondary standard candle to measure the distance to their host galaxies.