Previous Year Question
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?
Correct Answer: A
Detailed Solution
Pair 1 is incorrectly matched. Cepheids are not clouds of gas and dust. They are a special class of pulsating variable stars. They expand and contract at regular intervals, causing their brightness to increase and decrease periodically. There is a definite relationship between a Cepheid’s pulsation period and its intrinsic luminosity. Astronomers therefore use Cepheids as standard candles for measuring astronomical distances.
Pair 2 is incorrectly matched. Nebulae are not periodically brightening and dimming stars. They are giant clouds of gas and dust in space. Some nebulae are regions where new stars form, while others consist of material expelled by dying stars or remnants produced by supernova explosions.
Thus, the descriptions in the first two pairs have effectively been interchanged—Cepheids are periodically varying stars, whereas nebulae are clouds of gas and dust.
Pair 3 is correctly matched. Pulsars are rapidly rotating neutron stars possessing extremely strong magnetic fields. When a massive star exhausts its nuclear fuel, its core may collapse gravitationally and form a neutron star following a supernova explosion. If beams of radiation emitted by the rotating neutron star sweep across Earth, they are observed as regular pulses; hence, the object is called a pulsar.