Day 6 – How Many Galaxies Are Out There?

Cover: Astronomer Gerard Bodifee with wife TV Presenter Lucette Verboven

Imagine if our world had a diary and on the interesting days, someone pulled out a big book and wrote about it.

the bookDay 6.  By this stage in the life of the Universe (11,500,000,000 years ago) there exists something of the order of 170,000,000,000 (170 billion) galaxies each containing between tens of millions and billions of stars.

The name ‘galaxy’ is from the Greek word for milk which is how they described the whiteish appearance of the band of light we know as our home galaxy. Each of the 170 billion galaxies is indeed a group of stars, but much more than that. They almost certainly hold billions of planets, star systems, star clusters and in most cases, a massive black hole near the centre. They also contain a lot of material in the form of vast clouds of gas and dust particles that are mostly the remains of stars that have exploded at the end of their lives.
It has become clear that every galaxy has more mass (the ‘weight’ of something on earth) than we can measure by adding up what we can see. (A cannon ball in space may be weightless but it would still make a mess if it hit you because it still has the same mass.) This non-reflective material is predictably called ‘dark matter’ which incorporates ‘dark energy’. As Einstein showed, matter and energy are different forms of the same thing.

Galaxies come in a range of sizes and types from the dwarf galaxies with only 10 million stars to the big boys that can have one hundred trillion (that is, a hundred million million). 100,000,000,000,000. That’s a lot of zeros and it’s hard to imagine a number so large, especially when we are talking about very hot objects the size of our Sun, which itself is a million times bigger than the earth.
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