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

Mar 9, 2016

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

#### Explanation:

1. It is about $40 \setminus \quad p c$ for ground based observations.
2. Hipparcos : In 1989 ESA launched Hipparcos (HIgh Precision PARallax COllection Satellite) which could measure parallaxes as small as $1 \setminus \quad$ milli-arc seconds which translate to a distance of $1000 \setminus \quad p c = 1 \setminus \quad k p c$
3. GAIA: In 2013 ESA launched the GAIA satellite, a successor to Hipparcos which can measure parallaxes as small as $10 \setminus \quad$ micro arc-seconds which translate to a distance of ${10}^{5} \setminus \quad p c = 100 \setminus \quad k P c$
4. SIM: NASA had its own Space Interferometry Mission (SIM) which planned to measure parallaxes as small as $4 \setminus \quad$ micro src-seconds which translates to a distance of $250 \setminus \quad k p c$. 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 $50 \setminus \quad k p c$ and $64 \setminus \quad k p c$ come inside the resolution of GAIA. But the Andromeda galaxy our nearest neighbour after these dwarf galaxies lie at a distance of $778 \setminus \quad k p c$ 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.