Size comparison of galaxies and the universe
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Galaxy Size Comparison: Typical Galaxy Sizes and Their Variation
Galaxies come in a wide range of sizes, but most fall within a few key ranges. In the local universe, typical galaxy half-light radii (a common measure of size) are a few kiloparsecs (kpc), with massive galaxies often having radii of 4–5 kpc or more, while less massive galaxies are smaller, sometimes below 1 kpc in size 67. At higher redshifts (earlier in the universe), galaxies tend to be even more compact, with many star-forming galaxies at z > 2 having sizes around 2 kpc or less 356. Some of the faintest and earliest galaxies observed, especially those magnified by gravitational lensing, can be extremely small—down to just 10–30 parsecs (pc), which is similar in size to large star clusters in our own galaxy .
Size Evolution of Galaxies Over Cosmic Time
Galaxies have not always been the same size. Observations and simulations show that galaxies generally grow larger over time. Early in the universe, galaxies were more compact, and their sizes have increased as they evolved, merged, and formed new stars 3568. This growth is especially pronounced for massive galaxies, which can increase their sizes by factors of several from high redshift (z ~ 2) to the present day 56. The growth can occur through processes like star formation in outer regions, mergers with other galaxies, and the outward migration of stars formed at earlier times 58.
Galaxy Size–Mass Relation
There is a strong relationship between a galaxy’s stellar mass and its size: more massive galaxies are generally larger 5679. This relation holds across different types of galaxies and over a wide range of cosmic time, though the exact slope and normalization of the relation can change with redshift. Star-forming galaxies tend to be larger than quiescent (non-star-forming) galaxies at the same mass, especially at higher redshifts 568. The scatter in the size–mass relation is relatively small, especially when using certain measures of size that account for surface density 910.
Galaxies vs. the Universe: Scale Comparison
While galaxies are vast on human scales, they are tiny compared to the universe as a whole. For context, a typical large galaxy like the Milky Way has a diameter of about 30 kpc (roughly 100,000 light-years), while the observable universe is about 28 billion parsecs (or 93 billion light-years) across. This means that even the largest galaxies are minuscule compared to the scale of the universe—by a factor of billions.
Conclusion
Galaxies range in size from tens of parsecs for the smallest, faintest early galaxies, up to tens of kiloparsecs for the largest galaxies today. Over cosmic time, galaxies have grown larger, and their sizes are closely linked to their mass and star formation history. Despite their impressive size, galaxies are tiny compared to the vastness of the universe itself.
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