Question

Why can’t we determine the distances to galaxies using the geometric method of trigonometric parallax ?

Answer 1

There is a limit in using parallax method to find stellar distance.

Explanation:

1. It is about `40 \quad pc` for ground based observations.
2. Hipparcos : In 1989 ESA launched Hipparcos (HIgh Precision PARallax COllection Satellite) which could measure parallaxes as small as `1 \quad` milli-arc seconds which translate to a distance of `1000\quad pc = 1 \quad kpc`
3. GAIA: In 2013 ESA launched the GAIA satellite, a successor to Hipparcos which can measure parallaxes as small as `10\quad` micro arc-seconds which translate to a distance of `10^5\quad pc = 100 \quad kPc`
4. SIM: NASA had its own Space Interferometry Mission (SIM) which planned to measure parallaxes as small as `4\quad` micro src-seconds which translates to a distance of `250\quad kpc`. But the mission had been finally cancelled.

The LMC and SMC dwarf galaxy systems (Large Magellenic Cloud and Small Magellenic Cloud) at distances of approximately `50 \quad kpc` and `64\quad kpc` come inside the resolution of GAIA. But the Andromeda galaxy our nearest neighbour after these dwarf galaxies lie at a distance of `778\quad kpc` which lies well outside GAIA's capacity to probe.

When distance increase the Angle become very small and is difficult to measure..In such case a Cepheid variable is used as standard candle to measure distances.

picture credit astronomy ohio state ed.