# How far are other galaxies from earth?

Dec 26, 2015

Between $25 \times {10}^{3}$ light years (nearest) and $13.3 \times {10}^{9}$ light years (farthest)

#### Explanation:

The nearest would be a satellite galaxy of the Milky Way.

There seem to be two possible candidates:

(1) Canis Major Dwarf Galaxy / Canis Major Overdensity.

This is a disputed dwarf irregular galaxy about $25$kly (thousand light years) from Earth.

(2) Saggitarius Dwarf Spheroidal/Elliptical Galaxy about $70$kly from Earth.

The nearest major galaxy is the Andromeda Galaxy M31, situated about $2.5$Mly (million light years) from Earth. At about $220$kly in diameter, it is definitely larger than the Milky Way galaxy.

The third largest spiral galaxy in the Local Group is the Triangulum Galaxy at a distance estimated to be somewhere between $2.38$Mly and $3.07$Mly, with a diameter of about $60$kly.

The most distant known galaxy is MACS0647-JD at a distance of $13.3$Bly (billion light years), meaning that we see it as it was about $420$ million years after the Big Bang.

Given that a light year is about $5.8785 \times {10}^{12}$ miles, that means that our nearest neighbouring galaxy is about $1.5 \times {10}^{17}$ miles (Canis Major Dwarf Galaxy) or $4 \times {10}^{17}$ miles (Saggitarius Dwarf Spheroidal Galaxy) away and the most distant known galaxy is $7.8 \times {10}^{22}$ miles away.

Actually the (comoving) distance to the farthest galaxy is considerably more than this due to the expansion of the universe, probably closer to $44.8 \times {10}^{9}$ ly or $2.6 \times {10}^{23}$ miles.