When It Comes to Stars... Size Matters!

One of the really awesome and mind-blowing aspects of astronomy is the sheer immense scale of the distances between planets, stars, galaxies and galaxy clusters.  Our everyday terrestrial notions of scale, size, and distance must be discarded, even if we just consider a transit between the Earth and Mars. 

Kilometres first fall as units of measurement, then astronomical units (AU)(one AU is the distance between the Earth and Sun) -- when we start to consider interstellar distances we have to  look at light years as units of measurement (the distance that light travels in one year).

If distances become truly ‘astronomical’, then it comes as no surprise that likewise sizes and masses follow suit.  We all think that the Sun is massive, and it is, with a radius of 695,990km, this is 109 times that of the Earth.  

With a mass of 1.989x10³⁰ kg, the Sun has the equivalent of 333,000 Earth masses, and yet it is still just a run-of-the-mill yellow dwarf class G2 star.  As the diagram above shows, although there are many considerably smaller than the Sun (very common red dwarf stars) such as our nearest neighbour Proxima Centauri, there are also stars very much more massive.

The largest and most luminous star known is VY Canis Majoris, a red hypergiant located in the constellation Canis Major.  At between 1,800 and 2,100 solar radii (approximately 2,750,000,000km across), it is a single star nearly 5,000 light years away from the Earth, and quite probably the largest star in our galaxy. 

To gain some perspective of its size, if the Earth were to be represented by a sphere one centimetre in diameter, the Sun would be represented as a sphere with a diameter of 109 centimetres, at a distance of 117 meters. At these scales, VY Canis Majoris would have a diameter of approximately two kilometres!

Of course, this is all very interesting information, and will certainly entertain your friends, but a star’s size is intrinsically involved in determining attributes such as its luminosity, colour, temperature and lifespan.  Put simply, when it comes to stars, size really does matter!

Generally speaking, the larger a star the greater its mass, and hence the more its gravity. High mass stars with stronger gravity have greater pressure in their cores, greater pressure leads to higher temperatures and these lead to much faster nuclear fusion reactions, whereby the star’s hydrogen fuel is converted into helium,

with the release of massive amounts of energy.  This energy creates a radiation pressure, and while gravity tries to contract the star, this radiation pressure simultaneously tries to expand it -- the result is a stable hydrostatic equilibrium which can last for millions, if not billions of years.

However, once a star runs out of hydrogen fuel and starts to fuse helium into even heavier elements, this equilibrium cannot continue, and it won’t be long before the star is no longer what could be regarded as a normal stellar main sequence object.  Because high mass stars burn their fuel much, much quicker due to the greater core pressure caused by gravity, they live relatively short lives -- they live fast and die young as supernovae -- they are the James Dean of the stellar zoo.

A star such as Rigel, in the constellation of Orion, a hot blue supergiant with a diameter sixty times that of the Sun, has a mass of seventeen times that of our star, and hence 40,000 times its luminosity. Under its massive core pressure, its nuclear fusion reactions will race away, it will quickly run out of fuel, and hence it will live for only 20 or 30 million years. 

Our Sun on the other hand has enough hydrogen fuel to burn at its leisurely pace for ten billion years or more --  small red dwarfs with lower pressure and lower temperatures will undergo nuclear fusion for much longer.  With smaller mass and less gravity, Proxima Centauri for example will live for at least 20 to 30 billion years.

An interesting consequence of a star’s size and temperature is its brightness.  Generally speaking, a larger mass star main sequence star, having a higher temperature will be bluer in colour, while a smaller, cooler star will be redder -- the inverse of the colour conventions used on our devices warning of hot or cold temperatures!

So the next time you gaze at brilliant blue white Rigel, white Sirus, or yellow Arcturus with your telescope or binoculars, you’re looking at stars in decreasing masses and sizes.

And remember -- when it comes to stars, size really does matter!

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