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

Answer: A. Only ONE pair is correctly matched (pair 3 Pulsars). The first two descriptions are SWAPPED. (1) Cepheids - Giant clouds of dust and gas: WRONG. Cepheids are VARIABLE STARS (specifically pulsating variables) that brighten and dim periodically. Their period-luminosity relation, discovered by Henrietta Leavitt, is used to measure cosmic distances. The description given fits NEBULAE, not Cepheids. (2) Nebulae - Stars which brighten and dim periodically: WRONG. Nebulae are GIANT CLOUDS OF DUST AND GAS in interstellar space (emission, reflection, dark and planetary nebulae). The description given fits CEPHEID VARIABLES, not nebulae. (3) Pulsars - Neutron stars that are formed when massive stars run out of fuel and collapse: CORRECT. Pulsars are rapidly rotating, highly magnetised NEUTRON STARS that emit beams of electromagnetic radiation observed as pulses. They are remnants of supernovae from massive stars whose cores collapsed. So only pair 3 is correctly matched. Source: NCERT Class 11 Geography Ch 1 (Universe and Solar System) / NASA Astronomy references.