The temperature of distant luminous bodies such as stars can be determined by using which type of thermometer or device?

Introduction / Context:
Measuring the temperature of very hot and distant luminous objects, such as stars or the surface of the sun, cannot be done with ordinary contact thermometers. Instead, we use instruments that infer temperature from the radiation emitted by those bodies. One such instrument is a pyrometer, which is designed to measure high temperatures from a distance by analysing thermal radiation. This question tests your knowledge of which device is appropriate for such purposes.

Given Data / Assumptions:
• The objects considered are distant and luminous, for example stars. • It is not practical or possible to touch them with a physical thermometer. • We require a non contact method based on radiation.

Concept / Approach:
Hot bodies emit electromagnetic radiation, and the spectral distribution and intensity of this radiation depend on the temperature of the body, as described by black body radiation laws. A pyrometer is an instrument that estimates temperature from this emitted radiation without making physical contact. Astronomers and physicists use similar principles when they apply Wien displacement law and Stefan Boltzmann law to starlight to estimate surface temperatures. Ordinary thermometers like mercury thermometers and clinical thermometers work only over limited temperature ranges and require direct contact, making them unsuitable for distant luminous bodies.

Step-by-Step Solution:
Step 1: Recognise that for distant luminous bodies, only the radiation reaching us can be used for measurement. Step 2: Recall that pyrometers are specifically designed to measure temperature based on thermal radiation, especially at high temperatures. Step 3: Note that mercury and gas thermometers must be in contact with the object and have limited temperature ranges. Step 4: Conclude that the correct instrument for determining the temperature of distant luminous bodies is a pyrometer.

Verification / Alternative check:
In many textbooks, the temperature of the sun surface is stated to be about 5800 kelvin, found using analysis of its spectrum and radiation laws. This type of non contact measurement falls under the general category of pyrometric methods. Industrial radiation pyrometers are similarly used to measure furnace and molten metal temperatures without inserting a physical probe. These examples confirm that the term pyrometer is associated with radiation based temperature measurement.

Why Other Options Are Wrong:
Option a (Mercury thermometer): This requires contact with the object and operates over a relatively small temperature range, making it unsuitable for stars. Option b (Gas thermometer): Although very accurate for laboratory use, it still requires direct contact and controlled conditions. Option d (Colour thermometer): This phrase is sometimes used informally but is not the standard instrument name used in physics and engineering. Option e (Clinical thermometer): This is designed specifically for human body temperature and cannot measure extremely high or distant temperatures.

Common Pitfalls:
Some learners may be tempted by terms that mention colour or light because stars appear coloured. However, the formal and widely accepted instrument class for radiation based temperature measurement is pyrometers. Another confusion is to think about the general category of thermometers and forget that contact methods are impossible at astronomical distances. Always remember that distant high temperature measurements rely on radiation and therefore instruments like pyrometers are appropriate.

Final Answer:
The temperature of distant luminous bodies can be determined using a Pyrometer.