Is a Milky Way-Sized Star Possible?

No, it would be impossible for a star to be as large as the Milky Way or any other galaxy. This is because, fundamentally, a star is a vastly different entity from a galaxy. Let’s delve deeper into why this is the case.

What is a Galaxy?

According to Merriam-Webster, a galaxy is defined as 'any of the very large groups of stars and associated matter that are found throughout the universe.' A galaxy is essentially a massive collection of stars, gas, dust, and dark matter held together by gravity.

Stars vs. Galaxies

Stars are comprised of incredibly hot plasma and do not occupy nearly as much physical space as galaxies do. In fact, if you compared a single star to the tiny proportion of stars in a galaxy, the star would be minuscule in comparison. The vast majority of a galaxy's mass and space is not actually taken up by the stars, but rather by the empty space between them.

Moreover, there are physical limits on how large a star can grow due to fundamental astrophysical principles such as hydrostatic equilibrium. Hydrostatic equilibrium describes the balance between the inward force of gravity and the outward force of radiative pressure within a star. For a star to maintain this equilibrium, the forces must be perfectly balanced. If the star were to somehow grow as large as a galaxy, the imbalance between these forces would be catastrophic, leading to one of two extreme outcomes:

Implosion or Explosion

1. Implosion:

If gravity wins the battle, the intense self-gravity of the star would cause it to collapse almost instantly. Under such conditions, the star could form a black hole with a size comparable to that of the galaxy's core.

2. Explosion:

Alternatively, if radiative pressure prevails, the star would become exceptionally luminous, generating so much energy that it would fuel a pair-instability process. Pair-instability occurs when the energy output from the core of the star reaches such levels that it creates electron-positron pairs from the star's mass. This depletion would cause a rapid decline in thermal pressure, leading to a partial core collapse and a runaway thermonuclear reaction. The result of this would be a supernova, often referred to as a Pair-Instability Supernova (PISN).

Theoretical studies and current astrophysical models suggest that such a massive star—one billions of times larger than our Sun—could never form in the present universe. The constraints on star size are heavily influenced by the universe's current conditions and the available supplies of fuel (hydrogen and helium).

A Historical Perspective

Even if a star of this magnitude could have existed in the early universe, its lifespan would have been extremely short. The intense energy output and extreme conditions would result in either a quick collapse into a black hole or a spectacular explosion, both of which would not allow the star to persist as a viable celestial object.

Conclusion

In summary, while the concept of a star as large as a galaxy is fascinating, it is entirely theoretical and cannot be achieved in the current physical laws of the universe. A star and a galaxy are vastly different entities, and the principles governing their formation and maintenance keep them from merging.