AuroracastUKLearn
The Sun
The star everything here runs on, and the only one you can watch changing day to day.
Seeing it from the UK
This is the one object on this site that can blind you, and it can do it faster than you can look away. Never point anything at the Sun without a proper solar filter fitted on the FRONT of it, and never use eyepiece filters or improvised ones. With a certified filter and a small telescope, the larger spot groups are easy from any garden in the country, no dark sky required and no waiting for night. The picture here was taken from a back garden in Leicester at half past seven in the morning. The one thing worth knowing about timing is that the middle of the day is not the best of it: by early afternoon the ground has warmed and the rising heat blurs the detail, so late morning is steadier.
The Sun is a star, and an ordinary one. It is 1.39 million kilometres across, which is about 109 Earths side by side, and it holds more than 99 per cent of all the mass in the solar system. Everything else, every planet and moon and comet, is made of what was left over.
It is 150 million kilometres away, so its light takes eight minutes and twenty seconds to reach us. The surface you can photograph sits at around 5,500 degrees, and the core where the energy is actually made is nearer 15 million. It has been running for about 4.6 billion years and is a little under halfway through.
It does not turn as a solid body. The equator goes round in about 25 days and the poles take nearer 35, because it is gas rather than rock. That difference is what winds its magnetic field up, and the wound up field is what produces everything on this page, the spots, the flares, and eventually the aurora.
Sunspots, and what the picture is showing you
The dark patches are sunspots, and the four ringed groups in the photograph were all there on the morning of 21 August 2026. They are places where the Sun’s magnetic field has become so concentrated that it stops the hot gas below from circulating up to the surface. Less heat arrives, so that patch runs cooler than everything around it.
They are not actually dark. A sunspot is around 3,500 degrees against 5,500 for the surface beside it, and if you could cut one out and hang it in the night sky it would be far brighter than the full Moon. It only looks black because your eye is judging it against something hotter still. Look closely at a large one and it has two parts, a properly dark core and a lighter fringe around it where the field is weaker.
Size is the thing that surprises people. A single ordinary spot is comfortably bigger than the Earth, and a large group can run for a hundred thousand kilometres end to end. In the photograph the biggest group, AR 4513, held twelve separate spots.
Every group gets a number from the American forecasters at NOAA when it appears, which is what AR stands for, active region. They are also given a class describing how tangled the magnetic field is. AR 4513 in this photograph is a delta class group, the most tangled sort, and those are the ones that produce the largest flares. Two days earlier the same group had been a plain, quiet pair of spots, and it grew into that in forty eight hours.
Because the Sun turns roughly once a month, a group crosses the face over about two weeks and then goes round the back. If it survives, it comes round again. That is why photographing the Sun regularly shows you something a single picture never can: the same group returning, bigger or smaller than it was.
This is also where the aurora starts. The flares and the great clouds of gas that get thrown out towards us come from active regions, and delta class groups produce the strongest of them. So a photograph like this one is the first link in the chain that ends with a forecast on this site.
The numbers
- What it is
- A star, ordinary as stars go
- Width
- 1.39 million kilometres, about 109 Earths
- Distance
- 150 million kilometres, 8 minutes 20 seconds at light speed
- Surface temperature
- About 5,500 degrees
- Core temperature
- About 15 million degrees
- One rotation
- 25 days at the equator, nearer 35 at the poles
- Age
- About 4.6 billion years
- Activity cycle
- Roughly 11 years. The current one peaked in 2024
To find out whether it is up tonight, and when it gets properly dark where you are, see Sky Tonight.