Consider the following pairs : Objects in space - Description 1. Cepheids - Giant clouds of dust and gas in space 2. Nebulae - Stars which brighten and dim periodically 3. Pulsars - Neutron stars that are formed when massive stars run out of fuel and collapse How many of the above pairs are correctly matched?

1. Pair 1: Cepheids are a type of variable star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. They are not giant clouds of dust and gas; that description fits Nebulae. Thus, Pair 1 is incorrectly matched. 2. Pair 2: Nebulae are vast interstellar clouds of dust, hydrogen, helium and other ionized gases. Stars which brighten and dim periodically are known as variable stars, with Cepheids being a prominent example. Thus, Pair 2 is incorrectly matched. 3. Pair 3: Pulsars are highly magnetized, rotating neutron stars that emit beams of electromagnetic radiation. Neutron stars are indeed the collapsed cores of massive stars that have run out of fuel and undergone a supernova collapse. Thus, Pair 3 is correctly matched. 4. Only one pair (Pair 3) is correctly matched.