Yellow dwarfs are a class of Main Sequence star that includes the Sun (Sol). These stars are yellow stars with
spectral class G.
Sol: The Sun is a yellow dwarf star (spectral class G2 V) with a mass of 1.9891 x 10^30 kg (about 2
thousand million billion billion tonnes) and a diameter of 1 392 000 kilometres and a luminosity of 3.83 x 10^26
watts. This high luminosity means that the Sun emits about 30 billion times as much energy as the total
electrical energy produced by all the Earth's power generators! Thus, stars are immensely powerful!
This energy is produced by nuclear fusion. Nuclear fusion builds heavier atoms from lighter ones, as opposed
to nuclear fission which splits atoms. Nuclear power plants on Earth use nuclear fission, because although
nuclear fusion is much more efficient (it produces less waste and releases much more energy) Earth has not
yet developed nuclear fusion technology to the degree needed for useful power generation.
About 70% of the mass of the Sun is hydrogen, 28% helium and 2% of heavier elements (including carbon,
oxygen, nitrogen, metals and other elements). This is not hydrogen and helium in the normal sense, however,
because the hydrogen and helium are ionised (electrically charged) to form a plasma.
The generation of energy takes place within the Sun's core, which is consuming fuel (and losing mass) at the
rate of 4 million tonnes per second. The core is about 400 000 kilometres in diameter and has a temperature of
about 15 million Kelvin (about 15 million Celsius). Although only containing about 2% of the Sun's volume, this
core contains about 60% of the Sun's mass and is therefore very dense. The density of the Sun increases
toward the core. It is often said that the Sun is a ball of hot gas, this is loosely true, though a plasma would be a
better description than a gas, and the density in the core is so great that it is a plasma or gas that is denser
than ordinary solids. Under these extreme conditions of high temperature and pressure matter behaves in
unfamiliar ways and terms like 'gas' lose their conventional meaning.
The visible surface of the Sun is called the photosphere, since it is here that light escapes from the Sun. The
photosphere has a temperature of 6000 to 4000 Kelvins, which is very hot, but much less hot than the Sun's
Why are stars hot?
Most people would say that stars are hot because of the nuclear reaction that take place inside them. However,
this is an error. Nuclear reactions occur because the star is hot! Stars are hot because they form from vast
clouds of gas in space. These clouds, or nebulae, are initially dark and cool and may contain say about a
thousand times the Sun's mass. What heat there is in this gas generates infrared radiation which tends to push
the cloud apart, causing it to expand, whilst the gravitational attraction between particles in the gas, tends to
pull the cloud inwards, causing it to contract. If gravity wins, then the cloud contracts (if the cloud is too hot or
not dense enough then it expands). As it contracts the cloud fragments and many of these fragments may
continue to condense into protostars. As the clouds contract to form protostars, gravitational potential energy is
converted into heat and so the protostar gets hotter and hotter, smaller and smaller, denser and denser. There
comes a point, however, when the core temperature becomes hot enough (around 2 million kelvin) to initiate
nuclear fusion. The energy released from nuclear fusion generates a pressure that halts further contraction of
the gas (plasma) and a star is born! Thus, the star is hot because the nebula it was made from contracted,
turning gravitational energy into heat, but nuclear fusion stops the star from contracting further (which actually
stops it heating up any more!). When the pressure balances the force of gravity, forming a stable body, we say
that the star is in hydrostatic equilibrium.
Stars do not live forever
Since the Sun is losing about 4 million tonnes of mass each second,as spent fuel, clearly the Sun cannot be
immortal. It is possible to combine observations of stars with the known laws of physics and make predictions of
how long stars live for. (There are ways to test these prediction too, since there should be fewer short-lived
stars around). The Sun has a lifespan of about 10 billion years (10 thousand million years) but is already about
half way through its life (at about 4.5 billion years old) and so the Sun is middle-aged. When it gets low on
fuel, the Sun will expand into a red giant and eventually it will blast off its outer layers, forming a vast planetary
nebula and leaving a hot white dwarf remnant, which will slowly fade to a cold black dwarf.
From the ashes a phoenix will arise
When stars die they blast off lots of their material far into space. Since stars are factories that build the
heavier elements from hydrogen and helium, this material is enriched in heavier elements, including the carbon
that makes up the bodies of living things on Earth. The material from many dead stars form nebulae in space,
and if these nebulae become dense enough and cool enough then they will contract and produce a new
generation of stars.
For a long time it was unknown how stars manufacture so much carbon, which is so vital to life, since their
temperatures appeared to cool. However, it just so happens that there is a 'freak' carbon nucleus resonance
with just the right energy to allow the process to easily occur and so stars make plenty of carbon. With each
generation of stars, more and more carbon and other heavy elements are produced. Some of this material
becomes incorporated into planets and into living things. You are made from star dust!
Of course, stars produce more than just the raw materials for planets and life, they provide life with much of the
energy that it needs. Most of the energy that powers life on Earth comes from the Sun. It is curious to note that
long before there were enough heavy elements to make living things, stars were busy churning away and
making these elements. Stars are the engines of the Universe.
Above: not every star within the same spectral class (see Main Sequence Stars) is exactly the same
temperature and colour. Yellow stars are spectral class G, but within this class we have some slightly cooler
and more orange stars and some hotter yellow-white stars. Each class is further divided into ten subclasses,
indicated by a number between 0 and 9. The Sun is a class G2 star, similar perhaps the star on the right
above, with a photospheric temperature of about 5800 Kelvins. The star on the left is cooler and slightly orange
and may be a G7 star with a photospheric temperature of about 5000 K (a G2 star is hotter than a G7 star).
The star in the main picture is yellow-white and may be G0 class star with a temperature of over 6000 K. Thus
the spectral classes form a continuum, as G9 is similar in some ways to K0 and G9 is similar to F9.
Stars also have a luminosity classification, with a larger Roman numeral indicating a less luminous class, as
Ia bright supergiants
II bright giants
So the Sun is a class G2 V star - a relatively hot yellow dwarf. Luminosity (the total energy output by the star)
depends both upon its temperature (given by G2 for the Sun) and its size (given by V for the Sun).