In astronomy, which of the following is commonly used as a unit for measuring large astronomical distances between stars and galaxies?

Introduction / Context:
Distances in astronomy are so enormous that ordinary units like kilometres quickly become inconvenient. Astronomers therefore use special distance units that are better suited to interstellar and intergalactic scales. One of the most widely known of these is the light year. This question asks you to identify which option represents a unit of large astronomical distance, not a unit of wavelength, magnetic flux or illumination.

Given Data / Assumptions:

• We are interested in large distances in space, such as between stars or galaxies.
• Options include light year, angstrom, weber and lux.
• Each of these is a real physical unit, but they measure different types of quantities.
• We assume standard scientific definitions of these units.

Concept / Approach:
A light year is defined as the distance that light travels in vacuum in one year. Since light travels at about 3 * 10^8 metres per second, the distance covered in a year is roughly 9.46 * 10^15 metres. This huge distance makes the light year a convenient unit for describing how far away stars and galaxies are. In contrast, an angstrom is a very small unit of length used mainly for atomic scale distances, the weber is a unit of magnetic flux and the lux is a unit of illumination or illuminance.

Step-by-Step Solution:
Step 1: Recall that a light year is the distance travelled by light in one year in vacuum. Step 2: Estimate that this distance is around 9.46 * 10^15 metres, suitable for interstellar scales. Step 3: Identify that astronomers commonly quote distances to nearby stars in light years. Step 4: Recognise that an angstrom is 10^-10 metre, far too small for galaxy scale distances. Step 5: Note that a weber measures magnetic flux and lux measures light falling on a surface, not distance. Step 6: Conclude that among the given options, only the light year is used as a unit of large astronomical distance.

Verification / Alternative check:
If you look up the distance to well known stars such as Proxima Centauri, you will find values like 4.2 light years. Distances to galaxies are often given in millions of light years. This consistent use of light years in astronomical references confirms that it is a distance unit. By contrast, angstrom units are used when discussing spectral lines and atomic sizes, not galaxy separations. Textbooks on electromagnetism use webers to measure magnetic flux, while lighting engineers use lux to describe how bright a surface is.

Why Other Options Are Wrong:
Angstrom: This is a unit of length equal to 10^-10 metre, used for atomic and molecular scales, not for astronomical distances.

Weber: This is the SI unit of magnetic flux and is unrelated to distance measurement.

Lux: This is the SI unit of illuminance, describing how much light falls on a unit area, not a measure of distance.

Common Pitfalls:
Sometimes students see the word light in lux and assume it must be related to light travel like a light year. In reality, lux quantifies how bright a surface appears, not how far away something is. Another confusion is equating any obscure sounding unit with astronomy. To avoid this, remember the simple definition: a light year is literally the distance light travels in one year, which directly ties it to distance in space.

Final Answer:
The commonly used unit of large astronomical distance among the options is the light year.