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 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:

Outer WorldDiameter (km)
Earth (You are Here!)13,000


“Big” Sun and Beyond

The Sun, our star, 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.


Sirius A (brightest star in the sky)1.711
Pollux (giant orange star)8.8
Arcturus (red giant star)25.4
Aldebaran (red giant star)44.2
Rigel (blue supergiant star)78.9
Pistol Star (blue hypergiant star)306
Antares A (red supergiant star)883
Mu Cephei (red supergiant star)650-1,420


And 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…