Star clusters appear the same from Earth throughout the year becausethe stars within them are gravitationally bound and move together, making their relative positions stable over short periods.While individual stars do have proper motion, the movement of stars within a cluster is relatively slow and negligible over a human timescale, causing the overall cluster pattern to remain consistent.
Here's a more detailed explanation:
Gravity holds them together:
Stars in a cluster are bound together by gravity, meaning they are physically associated and tend to move together.
Slow movement:
While stars do move across the sky (called proper motion), the speeds are often slow, and the distances between them are vast, so their relative positions within the cluster change very little over a year or even decades.
Constellations as references:
Constellations, which are patterns formed by stars, are also relatively stable, and because many stars within a constellation may also be part of a star cluster, the overall pattern of the constellation appears the same year after year.
Earth's orbit:
The Earth's movement around the Sun does cause a slight change in the apparent position of stars over the course of a year, but this effect is much smaller than the movement of stars within a cluster.
