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?

Defines 'nebula' as a cloud of gas and dust and lists the stages of star formation (nebula → protostar → star), distinguishing clouds (nebulae) from stars.

A student could use this rule to ask whether Cepheids are listed among stages of star formation (they are not nebulae) and instead check if Cepheids appear in lists of stellar stages or types.

States that 'stars are localised lumps of gas within a nebula', implying discrete star objects are distinct from the larger gas/dust clouds that form them.

One could extend this by checking whether Cepheids are described as 'localised lumps' (stars) or as extended gas clouds (nebulae) in astronomical references.

Describes 'planetary nebula' as the outer layers of gas and dust ejected from a dying star — an example of gaseous shells distinct from stars themselves.

Use this example to contrast: if Cepheids were giant gas shells they would be described like nebulae (extended gaseous structures) rather than as objects undergoing stellar evolution like stars.

Explains brown dwarfs form from collapsing clouds of gas and dust but become compact objects when collapse halts — showing a pattern: clouds → collapse → compact object (star/brown dwarf), not that clouds and stars are the same.

A student could apply this pattern to test whether Cepheids are compact collapsed objects (implying stars) or uncollapsed extended clouds.

Describes gravitational collapse of a nebula forming a protosun (star), illustrating how large clouds become centralized stellar objects rather than remaining as clouds.

A student might use this to reason that objects named and studied for periodic behaviour (like Cepheids) are more likely to be individual stars formed by collapse than extended nebulae.