The lack of observable stellar parallax for nearby stars like Sirius is primarily due to the limitations of our measurement techniques and the relatively short baselines available for observations from Earth.
Stellar parallax is the apparent shift in the position of a star when observed from different vantage points as the Earth orbits the Sun. It provides a method for measuring the distances to nearby stars. However, the amount of parallax decreases as the distance to the star increases.
For stars like Sirius, which is one of the closest and brightest stars to Earth, the parallax shift is still relatively small. Sirius has a distance of about 8.6 light-years from Earth, and its parallax angle is only about 0.38 arcseconds. To put that into perspective, one arcsecond is 1/3,600th of a degree. The challenge lies in measuring such tiny angles accurately.
Historically, attempts to measure stellar parallax faced limitations due to the technology and precision of instruments. Even with modern telescopes, measuring the parallax of stars beyond a certain distance becomes increasingly challenging.
To overcome this limitation, astronomers have developed space-based observatories like the Hipparcos satellite and its successor, the Gaia mission. These missions have significantly improved our ability to measure stellar distances and parallaxes accurately. Gaia, for instance, has provided highly precise parallax measurements for millions of stars, allowing us to create a more detailed and accurate map of our galaxy.
In summary, while we do observe stellar parallax for many stars, the parallax shifts for stars like Sirius are relatively small and challenging to measure accurately from Earth's surface. Space-based observatories like Gaia have played a crucial role in obtaining precise parallax measurements for a larger number of stars, enabling us to better understand the structure and distances within our galaxy.