The sky — astronomy

The Sun: Our Star

The Sun is a star: a huge, hot ball of plasma, mostly hydrogen and helium, that shines through nuclear fusion in its core. It holds almost all the mass of the Solar System, and its light and gravity shape our days, our seasons and the sky we see.

The Sun as a star

Yes, the Sun is a star. It looks so different from the stars of the night sky simply because it is so much closer: its light reaches us in a little over 8 minutes, while light from the next nearest stars takes years.

Astronomers classify the Sun as a main-sequence star (a star fusing hydrogen into helium in its core) of spectral type G. It is about 4.6 billion years old, roughly halfway through that stage of its life. Its diameter is about 1.39 million kilometres, around 109 times that of Earth, and it contains about 99.86% of the mass of the whole Solar System.

It is made mostly of hydrogen and helium: by mass about three quarters hydrogen and one quarter helium, with small amounts of heavier elements. It is not solid or liquid but plasma (a gas of electrically charged particles), so it has no surface in the way Earth does.

The Sun has layers. At the centre is the core, at around 15 million degrees. Above it lie the radiative zone and the convection zone, and the visible “disc” is the photosphere, at about 5,500 °C. Higher up are the chromosphere and the outer atmosphere, the corona, which is far hotter, at millions of degrees. Why this is so is still an active topic in solar physics.

How the Sun makes energy

The Sun shines because of nuclear fusion. In the core, pressure and temperature are high enough for hydrogen nuclei to merge, ultimately forming helium. The helium produced has slightly less mass than the particles that went into it, and the difference is released as energy, as described by Einstein’s equation E = mc².

That energy works its way outward through the Sun’s layers, which by accepted estimates takes many thousands of years. Once it reaches the photosphere it escapes into space as light and heat, and crosses to Earth in a little over 8 minutes. Our average distance from the Sun is about 150 million kilometres, close to one astronomical unit, which the IAU (International Astronomical Union) defines as exactly 149,597,870,700 metres.

The Sun has enough hydrogen for roughly another 5 billion years. After that, current models say it will swell into a red giant. There is no cause for worry; that lies in the very distant future.

Sunspots and the solar cycle

Sunspots are dark patches on the photosphere. They look dark because they are cooler than their surroundings, not because they are cold. The cause is strong magnetic fields that hinder the flow of heat. Sunspots can be larger than Earth, and they have been observed for centuries; by tracking them, astronomers found that the Sun rotates, about once in 25 days at the equator and more slowly near the poles.

The number of sunspots rises and falls over a period of roughly 11 years. This is the solar cycle, running from a minimum with few or no spots to a maximum with many, and back again. Around maximum, more energetic events become common: flares (sudden bursts of energy) and coronal mass ejections (clouds of plasma thrown into space). When such clouds reach Earth, they can cause geomagnetic storms and auroras. These phenomena, known as space weather, are monitored by scientific agencies such as NOAA and by NASA and ESA missions, including SOHO, Solar Orbiter and Parker Solar Probe.

The Sun’s apparent motion across the sky

The daily rising in the east and setting in the west comes from Earth’s rotation, not from the Sun circling us. The Sun’s disc spans about half a degree of sky.

Over the year the Sun shifts against the distant stars, because Earth orbits it. From Earth this looks like a slow drift of about one degree per day along a line called the ecliptic. Earth’s axis is tilted by about 23.4° to its orbital plane, so the Sun’s height above the horizon changes through the year. This gives us the seasons, as well as the solstices and equinoxes.

When the Moon passes exactly in front of the Sun’s disc, we see a solar eclipse.

Astrology is a cultural tradition that also gives the Sun a symbolic role; see the Sun in astrology, and for the difference between the two approaches, astronomy versus astrology.

How to observe the Sun safely

Looking at the Sun without protection can permanently damage your eyes, and pain does not always warn you in time. Keep to these rules:

• Use solar viewing glasses that meet the ISO 12312-2 standard. Ordinary sunglasses are not enough.
• Never look at the Sun through binoculars, a telescope or a camera lens unless a proper solar filter is fixed over the front of the instrument (the end facing the Sun).
• Avoid homemade filters and damaged glasses; inspect your equipment before every use.
• A safe alternative is projection, such as a pinhole camera, where you look at an image of the Sun on a screen rather than at the Sun itself.
• For observing with optics, look for a dedicated solar telescope or an approved filter and follow the manufacturer’s instructions.

Above all, remember that safety matters more than the view. More observing tips are in How to watch the sky.

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