* Distance: Stars are generally much closer to us than galaxies. Even the closest galaxies are millions of light-years away, while the nearest stars are just a few light-years distant. This closer proximity means that the parallax angle, the apparent shift of a star against the background due to Earth's orbit, is larger and easier to measure for stars.
* Size: Galaxies are vast collections of stars, gas, and dust, spanning thousands of light-years. This large size means that their individual components, including stars, appear incredibly small from Earth. This makes measuring the parallax angle for individual stars within a galaxy nearly impossible.
* Internal Motion: Galaxies are not static. Stars within a galaxy move relative to one another, making it difficult to isolate the parallax shift caused by Earth's orbit.
In summary:
Parallax measurements rely on the detectable shift of an object against a distant background due to Earth's orbital motion. This shift is directly proportional to the object's distance. Because stars are much closer and smaller than galaxies, their parallax angles are larger and easier to measure, making this technique much more effective for determining stellar distances.
Let me know if you have any more questions about parallax or astronomical distances!
