In practice, large distances to stars in astronomy are most commonly expressed in which unit among the following?

Introduction / Context:
Astronomers must measure extremely large distances to stars and galaxies, far beyond the range where everyday units like kilometres are convenient. To express these enormous scales, special units are used. This question asks which unit is most commonly used for the distance to stars, reinforcing the idea that astronomy uses its own natural distance scales.

Given Data / Assumptions:

• We are concerned with distances to stars, not to planets within our solar system.
• The options include several invented sounding units, but only one is standard.
• A light year is a well known astronomical unit.
• We assume standard definitions from astronomy and physics.

Concept / Approach:
The most commonly used unit for distances to nearby stars is the light year, which is defined as the distance light travels in vacuum in one year. While astronomers also use parsecs and astronomical units, a light year is widely used in popular science communication and in many basic physics questions. Terms like galactic unit, stellar mile or cosmic kilometre are not standard scientific units for star distances and are not used in serious astronomical measurements.

Step-by-Step Solution:
Step 1: Recall that the distance to the nearest star, Proxima Centauri, is about 4.2 light years.Step 2: Recognise that distances to stars are usually given in light years or parsecs, not in kilometres or miles because numerical values would be too large.Step 3: Note that the term light year is specifically designed as a distance unit from the product of the speed of light and one year of time.Step 4: Understand that the other units listed such as galactic unit or cosmic kilometre are not standard names used in astronomy textbooks.Step 5: Match the correct and recognisable unit, light year, to the requirement of the question.Step 6: Conclude that distances of stars are measured in light years in this context.

Verification / Alternative check:
Open any introductory astronomy or astrophysics text and you will find phrases like the star is thirty light years away or the galaxy is many thousand light years across. Such texts rarely use kilometres for star distances because the numbers would be unwieldy. Also, standard scientific unit lists do not mention stellar mile or cosmic kilometre as accepted units, which confirms that light year is the correct choice here.

Why Other Options Are Wrong:
Galactic unit is not a widely defined or accepted term; while an astronomical unit (AU) exists for distances within the solar system, a galactic unit is not standard for stars. Stellar mile and cosmic kilometre sound like invented descriptive phrases rather than rigorous units and are not used in serious astronomical measurement. Therefore, these options are incorrect.

Common Pitfalls:
Some students may confuse astronomical unit (AU), used for distances between the Earth and the Sun, with units for star distances. Others may be attracted to impressive sounding names like galactic unit. To avoid mistakes, remember that light year and parsec are the standard for stars, while AU is for planetary scales within the solar system.

Final Answer:
Distances of stars are most commonly expressed in light years.