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

1 Answer

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


  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.

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picture credit astronomy ohio state ed.