The Sun

Our nearest star, a glowing orb in the sky, has captivated mankind. Worshipped as a god by the Egyptians and Mayans, the sheer power of the Sun has always been pretty self-evident. With a surface burning at over 5,000 °C, it's no surprise that entire cultures have been built around the Sun.

What is the Sun?

The Sun is made up of 4 main sections (see right), the core, photosphere, chromosphere and corona. The hottest of these is the core which is at around 15 million K, whereas the outer layers are coolest at around 5,500 K.

In the core of the Sun, hydrogen ions (protons) are pressed together at such high heat and temperatures that they begin to fuse together to form helium in a process known as nuclear fusion (see below). This gives off lots of energy in the form of heat and light, though because of the density of the Sun, this light takes a long time to get out as it reflects off of the atoms and ions inside the Sun.

Nuclear fusion

  1. Two protons (H+ ions) fuse to form a 2H nucleus, a positron (e+) and an electron neutrino (ve).
  2. A 2H nucleus and a proton fuse to form a 3He nucleus.
  3. Two 3He nuclei fuse to form an α particle (4He nucleus) and 2 protons.


Occasionally, sudden build ups in the Sun's magnetic field occur, stopping the convection of hot matter and so resulting in cooler regions which (due to their dark appearance) are known as sunspots. The number of sunspots vary according to an 11 year cycle which can be represented in a "butterfly diagram", a plot of sunspot number against date (time).

Within sunspots there are two parts, the umbra and penumbra which can be identified by their differing shade. This is related to their temperature with the darker umbra being about 3000 K - 4000 K, whereas the lighter penumbra is (relatively) not much cooler than the photosphere).

Models of our Solar System

Over the history of mankind, two particular models of the Solar System have been particularly prevalent - the geocentric model, where the Sun and the other planets orbit the Earth, and the heliocentric model, where the Earth and the planets orbit the Sun. The geocentric model was the first to be proposed and was held as the correct theory up until the 1600s and 1700s (primarily due to the Catholic Church's support of the model). The first astronomers to suggest that the heliocentric model was actually the correct model were Nicolaus Copernicus, Galileo Galilei and Tycho Brahe. Copernicus and Galileo were heavily condemned by the Church, though Tycho Brahe was not persecuted. Having now gone into space and taken thorough observations, it is believed that the heliocentric model is correct.

Observing the Sun

Due to its brightness, it is difficult to safely take direct observations of the Sun. Despite this, astronomers have developed simple, yet effective ways of observing the Sun, listed below.

Using a pinhole camera

Projecting onto a screen

Using a H-alpha filter

The light from the Sun can be shone through a small hole and projected onto a screen. Through this method, very little detail can be seen, however, if done well, sunspots can be observed.

The light from the Sun can be shone through a telescope or binoculars and projected onto a screen. Through this method, some detail can be seen, without having to look directly at the Sun. It is important to reduce the amount of light entering the objective lens(es), as to avoid melting the eyepiece(s).

A H-alpha filter shows up particular features of the Sun as it only allows light around 650 nm in wavelength (red light). This shows up prominences and filaments, though does not show the full visible colour range of the Sun.

Important Facts and Figures about the Sun

  • The Sun is, on average,149.6  million km (1 AU) away from the Earth.
  • The diameter of the Sun is1.3914 million km.
  • A full rotation of the Sun takes approximately 25 days at the equator and 36 days at the poles.

James Gooding, 2017

All image credits go to NASA (and respective centers/universities)