Chapter 5 · Born 4.6 billion years ago, shining for 5 billion more
The Sun
A ball of fire that is not on fire
Imagine placing the Earth next to the Sun. The Sun is 1.39 million kilometres across, which is 109 Earths side by side, and about 1.3 million Earths would fit inside it. It holds 99.86 percent of all the mass of the solar system. It is not burning like a log, because there is no oxygen to burn. It is a ball of hot gas, mostly hydrogen (about 73 percent by mass) and helium (about 25 percent). In the picture a tiny blue Earth sits next to the glowing disc so that you can feel the proportion.
The power plant in the core
In the core, the temperature is about 15 million degrees Celsius and the matter is about 150 times denser than water. Here hydrogen nuclei are squeezed so hard that they fuse into helium. Four hydrogen nuclei make one helium nucleus, which weighs about 0.7 percent less. The missing mass becomes energy according to Einstein's equation E = mc². Every second the Sun turns about 4 million tonnes of mass into energy and about 600 million tonnes of hydrogen into helium. The light made in the core needs a very long time to get out. Estimates range from about 10,000 to 170,000 years.
Eight minutes and twenty seconds
Once light leaves the surface, it crosses the empty space at 299,792 km/s. Worked example: the Sun is 149.6 million kilometres away, and 149,600,000 divided by about 300,000 gives roughly 499 seconds, which is 8 minutes and 20 seconds. The sunlight you feel now left the Sun just before. The visible surface is about 5,500 degrees Celsius. Planets farther out get less light: it falls with the square of distance, so Jupiter, 5.2 times farther away, receives about 1 over 27 of the light that Earth does. Pick a planet in the sandbox and compare.
Spots and flares
Dark patches on the surface are sunspots. They are cooler, about 3,500 to 4,500 degrees, and they are places where strong magnetic fields break through. Near them the field can snap and release a flare, a burst of light and particles, sometimes with a coronal mass ejection that sends a cloud of matter into space. Above the surface lies the corona, a thin glow that is hotter, at one to three million degrees. Why it is so hot is still debated. Never look at the Sun directly or through a telescope. Use certified eclipse glasses or project the image.
A heartbeat of 11 years
The number of sunspots rises and falls in a cycle that lasts about 11 years, though it can be 9 to 14. The German amateur astronomer Heinrich Schwabe found it in 1843 after counting spots for 17 years. At the maximum the Sun has many spots and strong flares, and aurora is more frequent. At the minimum it can be nearly blank. Every cycle the Sun's magnetic poles swap places, so the full magnetic cycle is about 22 years. Solar Cycle 25 began in December 2019 and approached its maximum in 2024. Between 1645 and 1715 the Sun had almost no spots, a quiet time called the Maunder Minimum.
A star's long life
The Sun is about 4.6 billion years old, in the middle of its life, and has hydrogen for about 5 billion years more. It is slowly brightening, around 10 percent every billion years, so in roughly one billion years Earth's oceans may begin to evaporate. These are model predictions, not observations. When the core runs out of hydrogen, the Sun will swell into a red giant, perhaps 200 times wider, and whether it swallows the Earth is debated. Then it will shed its outer layers and leave a white dwarf, a hot core about the size of the Earth that will cool for trillions of years.
In numbers
- 109 ×
- the Sun's diameter compared with the Earth's
- 8 min 20 s
- the time sunlight needs to reach the Earth
- ≈ 15 million °C
- temperature in the core of the Sun