Recent Distance Estimate
At least four distinct techniques have been used to measure distances to the Andromeda Galaxy.
In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 megalight-years (790 ± 18 kpc) was derived.
Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.
In 2005 Ignasi Ribas (CSIC, Institute for Space Studies of Catalonia (IEEC)) and colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236, has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure the absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 megalight-years (770 ± 43 kpc) and the whole Andromeda Galaxy at about 2.5 Mly (770 kpc). This new value is in excellent agreement with the previous, independent Cepheid-based distance value.
M31 is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (780 ± 25 kpc).
Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly (780 ± 18 kpc). Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly (43,000 ± 920 pc). Applying trigonometry (arctangent), that figures to extending at an apparent 3.18° angle in the sky.
Read more about this topic: Andromeda Galaxy, General
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