In astrophysics, the masses of stars and galaxies are most commonly expressed in terms of which standard reference mass?

Introduction / Context:
In astrophysics and cosmology, the objects being studied are extremely massive compared to everyday bodies like cars, buildings or even planets. To describe the masses of stars and galaxies conveniently, scientists use suitable reference units instead of kilograms alone. This question checks whether you remember which standard reference mass is typically used for stars and galaxies in astronomy, a basic but important general knowledge fact.

Given Data / Assumptions:

• The context is masses of stars and galaxies, which are very large.
• We want a convenient astronomical unit of mass.
• The options include neutron mass, Earth's mass, solar mass and lunar mass.
• Only one of these is widely used as a standard unit for stellar and galactic masses.

Concept / Approach:
The Sun is an ordinary star in terms of size compared to many others, and its mass is about 1.99 × 10^30 kilograms. Astronomers define one solar mass (symbol often written as M☉) as the mass of the Sun. It becomes a natural unit to compare other stars: a star might have, for example, 0.5 solar masses or 20 solar masses. For galaxies, which contain billions of stars, masses are often given in billions of solar masses. The masses of Earth or the Moon are much smaller, and neutron mass is far too tiny, so they are not convenient reference units for stellar and galactic scales.

Step-by-Step Solution:
Step 1: Recognise that the question concerns very massive objects: stars and galaxies.Step 2: Recall that the Sun is a typical reference point in astronomy.Step 3: Note that one solar mass equals the mass of the Sun and is used as a standard unit.Step 4: Realise that Earth's mass and lunar mass are many orders of magnitude smaller than a typical star.Step 5: Understand that neutron mass is a subatomic quantity and totally unsuitable as a unit for stars.Step 6: Conclude that solar mass is the convenient and commonly used unit for stellar and galactic masses.

Verification / Alternative check:
If you look at astronomy articles or textbooks, you will frequently see statements such as "the star has a mass of 3 M☉" or "the galaxy contains about 10^11 solar masses." You rarely, if ever, see the same information given in Earth masses for stars, because the numbers would be extremely large and inconvenient to read. This consistent practice in the literature confirms that solar mass is the standard unit for expressing masses of stars and galaxies.

Why Other Options Are Wrong:
Neutron mass is far too small and is relevant for subatomic physics, not for cosmic bodies. Earth's mass is used occasionally for exoplanets, not for stars. Lunar mass is used even less frequently and is too small for galaxies. Therefore, none of these serve as the main reference unit for stellar and galactic masses, whereas the solar mass does.

Common Pitfalls:
Students sometimes confuse units used for planets and exoplanets (like Earth masses) with those for stars. Another mistake is to think that the most familiar object, the Earth or Moon, should always be the reference. To avoid this, remember that for planets we may use Earth masses, but for stars and galaxies the natural and widely accepted reference unit is the solar mass.

Final Answer:
The masses of stars and galaxies are usually expressed in terms of the solar mass.