Size Matters… in Astrophysical Terms

Little Big Earth and the Universe

Let’s talk about size…  😉  How big are the objects floating in our Universe and how big can they get?  Starting with a “big” object, our very own Moon…  Embark on a tour of space…  A tour of our Universe…

The Moon is 3,500 km in diameter.  That’s bigger than the ninth largest planet of our Solar System – Pluto is only 2,400 km.

Planets and their Approximate Sizes

The following are approximate diameters of some of the most familiar astronomical objects of the Solar System:

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“Big” Sun and Beyond

The Sun is 109 times the diameter of Earth.


We use a new unit here the solar radius, 1 R = 6.955 \times 105 km = 695,500 km.

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The largest known star, VY Canis Majoris red hypergiant 1420 ± 120 R.

This star has a diameter of 2,800,000,000 kilometres.

Such a size is impossible to imagine…

A passenger plane flying at 900 km per hour over the surface of such a big object would take 1,100 years to circle around it once!

Not the Centre of the Universe…

An artist's impression of our very own barred spiral Galaxy - the Milky Way - with a red tag pointing to its outer edge, labelled "You Are Here".

Compared to it, Earth is tiny.  The Sun’s bigger.  And yet, it is merely a dot among several hundred billion stars that form our Galaxy.

And there are a hundred billion galaxies in the vast expanses of the Universe.

We are NOT the centre of the Universe.  Far from it!  We’re not even the centre of the Galaxy…