The Sun: Our Star Explained

The Sun is the star at the heart of our solar system — a 4.6-billion-year-old sphere of incandescent plasma whose gravity holds every planet, moon, asteroid and comet in orbit, and whose light makes life on Earth possible. As an astrophotographer who has chased the night sky since 2008 and who now runs a remote imaging rig at Deepsky Chile, I spend most of my time photographing faint deep-sky objects after dark. But the one object I always tell beginners to respect more than any other is the daytime star: the Sun. It is the most rewarding target in the sky and, handled carelessly, the most dangerous.

This guide covers everything you actually need to know — the Sun’s internal structure, sunspots and the roughly 11-year solar cycle (we are right at the maximum of Solar Cycle 25 in 2026), solar eclipses, and, above all, how to observe and photograph the Sun without destroying your eyesight or your gear.

Why is safe solar observing the first thing you must learn?

Before a single fact about the Sun, the safety rule comes first because the consequences are immediate and irreversible. Never look at the Sun through a telescope, binoculars, finderscope, or camera viewfinder that does not have a certified solar filter fitted over the front (the aperture end). A telescope concentrates sunlight to a focus that can burn paper and melt plastic; aimed at your retina, it causes painless, permanent blindness in a fraction of a second. There are no pain receptors in the retina, so you will not feel the damage happening.

I cannot overstate this. In nearly two decades in this hobby, the only injuries I have heard of among astronomers came from solar mistakes — a forgotten finderscope cap, a child swinging an unfiltered scope toward the Sun, a cheap eyepiece “sun filter” that cracked under heat. Treat the Sun as the one target that punishes a single lapse.

The three safe ways to observe the Sun

• White-light solar filter: a certified glass or film filter (such as Baader AstroSolar film, ISO 12312-2 compliant) mounted securely over the front aperture of your telescope or binoculars. It blocks more than 99.99% of incoming light. Never use an eyepiece-end “sun filter” — heat builds up at that focus and they shatter.
• Dedicated solar telescope: a purpose-built Hydrogen-alpha (Hα) scope (for example, a Coronado or Lunt) with built-in safe filtration designed only for the Sun.
• Projection: project the Sun’s image through a small refractor or pinhole onto a white card. You never look through the optic — you look at the projected image on the card.

What does NOT work

• Sunglasses, smoked glass, exposed photographic film, CDs, Mylar food wrap, or stacking multiple pairs of eclipse glasses.
• Eyepiece-mounted solar filters (the dangerous old design).
• Welder’s glass below shade 14 (only shade 14 is safe for naked-eye glances, and never through optics).
• Pointing any unfiltered camera, phone, or DSLR through a telescope at the Sun — the sensor and your eye are both at risk.

Always cap or remove your finderscope before solar sessions, and supervise children constantly. For eclipse viewing, use only ISO 12312-2 certified eclipse glasses, and inspect them for scratches first. NASA maintains a clear, authoritative safety page worth reading before your first session at NASA’s Sun science portal.

What is the Sun, exactly?

The Sun is a G-type main-sequence star (a “yellow dwarf,” though it actually appears white from space). It accounts for about 99.8% of all the mass in the solar system — everything else, every planet and moon combined, is rounding error by comparison. It is an enormous ball of hot plasma held together by its own gravity and powered by nuclear fusion in its core.

The Sun formed roughly 4.6 billion years ago from the gravitational collapse of part of a giant molecular cloud. It is a little under halfway through its main-sequence lifetime and will continue fusing hydrogen for another 5 billion years or so before swelling into a red giant and finally settling down as a white dwarf.

Key Sun facts at a glance

That last point — differential rotation — is one reason the Sun has such a complex, ever-changing magnetic field, which in turn drives sunspots and the solar cycle we’ll come to shortly.

What are the layers of the Sun?

The Sun has no solid surface. Instead it is organized into distinct layers, from the fusion furnace at its center out to the wispy corona that stretches millions of kilometers into space. Energy generated in the core takes an astonishingly long time — tens of thousands of years — to fight its way out to the surface.

The interior: core, radiative zone, convective zone

• Core: The central ~25% of the Sun’s radius, where temperatures hit ~15 million °C and pressure is crushing. Here hydrogen nuclei fuse into helium, converting mass into energy via the proton-proton chain. This is the engine that powers the entire star.
• Radiative zone: Surrounding the core, energy travels outward as photons that are absorbed and re-emitted countless times. A single packet of energy can take tens of thousands of years to cross this region.
• Convective zone: In the outer third, plasma physically boils — hot material rises, cools, and sinks in giant convection cells. This churning is visible at the surface as a granular, bubbling texture.

The atmosphere: photosphere, chromosphere, corona

• Photosphere: The visible “surface,” about 5,500 °C, where sunlight escapes into space. This is where you see sunspots and granulation through a white-light filter.
• Chromosphere: A thin reddish layer above the photosphere, best seen in Hydrogen-alpha light, where prominences and filaments live.
• Corona: The Sun’s outer atmosphere, mysteriously hotter than the surface — up to 2 million °C — and visible to the naked eye only during a total solar eclipse, when the Moon blocks the photosphere’s glare. The European Space Agency’s solar missions, described at ESA’s Solar Orbiter pages, are dedicated to understanding why the corona is so hot.

To put the layers in context with the other bodies the Sun governs, it helps to step back and see the whole system — our guide to the solar system maps how the Sun’s gravity shapes everything that orbits it.

What are sunspots and how do they form?

Sunspots are the single most rewarding feature to observe in white light, and they are the visible fingerprints of the Sun’s magnetism. A sunspot is a region of the photosphere where intense magnetic fields suppress the convection that normally carries heat upward. Because less heat reaches the surface there, the region is cooler — around 3,500 °C versus the surrounding 5,500 °C — so it looks dark by contrast. In reality a sunspot is blindingly bright; it only appears dark against the hotter background.

Sunspots often appear in groups and can be larger than Earth. They have two parts: the dark central umbra and the lighter, filamentary penumbra around it. Through my refractor with a white-light filter, even a modest sunspot group shows this structure beautifully, and tracking the same group day to day as it rotates across the disk is one of the most satisfying projects in amateur astronomy.

What causes them

The Sun’s differential rotation — faster at the equator than the poles — winds up and tangles its internal magnetic field over years. Where bundles of magnetic field lines pierce the surface, they choke off convection and create sunspots. The number of sunspots rises and falls in a roughly 11-year rhythm, which brings us to the solar cycle.

What is the solar cycle, and where are we in 2026?

The solar cycle is the roughly 11-year rise and fall in the Sun’s magnetic activity, measured most simply by counting sunspots. At solar minimum the disk can go days without a single spot; at solar maximum it can be peppered with large active regions, and flares and coronal mass ejections become frequent. At the end of each cycle the Sun’s magnetic field flips polarity entirely.

We are currently in Solar Cycle 25. NASA and NOAA announced in October 2024 that the Sun had entered its solar maximum period, and this cycle has been notably stronger than originally forecast — sunspot counts reached a 23-year high, and the Sun unleashed an X9.0 flare, its most powerful of the cycle, on 3 October 2024. Through 2025 and into 2026 we remain near the peak, with the declining phase expected to set in gradually thereafter.

What this means for you in 2026: this is one of the best times in a decade to observe and photograph the Sun. Sunspots are plentiful, prominences leap off the limb, and aurora-producing geomagnetic storms are frequent. If you have ever wanted to start solar imaging, do it now while activity is high — the next maximum won’t arrive until the mid-2030s. For activity forecasts I check NOAA’s Space Weather Prediction Center, and you can read the cycle background at Britannica’s solar cycle entry.

What happens during a solar eclipse?

A solar eclipse occurs when the Moon passes directly between the Earth and the Sun, casting its shadow on our planet. By a remarkable cosmic coincidence, the Moon and the Sun appear almost exactly the same size in our sky, which makes total solar eclipses possible.

• Total eclipse: The Moon completely covers the photosphere, revealing the ghostly corona. For the brief minutes of totality — and only during totality — it is safe to look with the naked eye.
• Partial eclipse: The Moon covers only part of the disk. You must use eclipse glasses or filtered optics the entire time.
• Annular eclipse: The Moon is near apogee and appears slightly too small to cover the Sun, leaving a bright “ring of fire.” This is never safe to view without protection.

The single most common eclipse injury comes from people removing eclipse glasses a few seconds too early or too late around totality. If you are not certain you are in the path of totality, keep your filters on the entire time. To understand the geometry of these alignments, our overview of the Moon and its phases explains why the Moon’s orbit makes eclipses both possible and rare.

How do you photograph the Sun safely?

Solar imaging splits into two distinct disciplines, and they produce completely different photographs. I’ve done both, and they each have their own learning curve and gear.

White-light imaging

This captures the photosphere — sunspots and granulation. You fit a certified white-light filter (glass or Baader AstroSolar film) over the front of any telescope, then attach a camera. Key tips from my own sessions:

• Use a high-frame-rate planetary camera and shoot short video clips, then stack the sharpest frames (lucky imaging) to beat atmospheric turbulence.
• Image when the Sun is high in the sky to minimize the thick, turbulent air near the horizon.
• Keep exposures short — the filter cuts the light, but the Sun is still extremely bright.
• Double-check the filter is seated and undamaged before the scope ever points sunward.

Hydrogen-alpha (Hα) imaging

This requires a dedicated Hα solar telescope, which isolates a single deep-red wavelength emitted by the chromosphere. The reward is dramatic: writhing prominences arcing off the limb, dark filaments snaking across the disk, and bright active regions around sunspots. Hα scopes are more expensive and have a tuning “etalon” you adjust for contrast, but nothing in white light compares to a big prominence in Hα.

Whichever path you take, the framing and focal-length choices matter. The Sun is about half a degree across, the same as the full Moon, so before buying a camera-and-scope combination it is worth running the numbers through our telescope field-of-view calculator to confirm the full disk will fit your sensor at your chosen focal length.

How does the Sun compare to other stars and to the planets?

By the standards of the galaxy the Sun is fairly ordinary — a middle-aged, middle-sized star. There are stars far larger and far smaller, far hotter and far cooler. But it is exactly stable and long-lived enough to have given life on Earth billions of uninterrupted years, which is anything but ordinary for the beings who depend on it.

Its gravity defines the entire neighborhood. The innermost world, scorched Mercury, races around it in just 88 days, while distant ice giants take well over a century. The Sun is the reference point for everything else — if you want the broader tour, our planets guide walks through each world the Sun holds in orbit, and our meteor showers calendar covers the cometary debris that the Sun’s heat sheds into glowing trails in our skies.

Practical tips for your first solar session

If you’re setting up to observe the Sun for the first time, here is the workflow I follow every time, refined over years of sessions:

1. Cap or remove the finderscope before you go outside. This is the most common cause of accidents.
2. Fit the certified front filter and check it is secure and unscratched. Tape it if there’s any wobble.
3. Aim using the scope’s shadow — never sight along the tube. Minimize the tube’s shadow on the ground to center the Sun.
4. Start at low magnification, find the disk, then zoom in on sunspot groups.
5. Observe in short sessions and keep bystanders, especially children, supervised at all times.

Do all of that and the Sun becomes the most accessible serious target in astronomy — it’s up in daylight, it changes day to day, and during the Cycle 25 maximum of 2026 it is putting on a genuine show.

Frequently asked questions

Is it ever safe to look at the Sun through a telescope?

Only with a certified solar filter fitted over the front aperture, a dedicated solar telescope, or by projecting the image onto a card. Never look through any unfiltered telescope, binoculars, finder, or camera at the Sun — even for an instant. The focused light causes painless, permanent retinal burns and blindness with no warning.

How big and how hot is the Sun?

The Sun is about 1.4 million kilometers (865,000 miles) across — roughly 109 Earths wide — with a mass around 333,000 times Earth’s. Its visible surface is about 5,500 °C, its core reaches roughly 15 million °C, and its outer corona is hotter still at up to 2 million °C.

What is the solar cycle and are we at maximum in 2026?

The solar cycle is the roughly 11-year rise and fall in the Sun’s magnetic activity, tracked by sunspot numbers. NASA and NOAA confirmed the Sun reached the maximum of Solar Cycle 25 in October 2024, and through 2025–2026 we remain near that peak, making it an excellent time to observe sunspots, prominences, and aurorae.

Why do sunspots look dark?

Sunspots are regions where strong magnetic fields block the convection that carries heat to the surface, so they are cooler — about 3,500 °C versus the surrounding 5,500 °C. They appear dark only by contrast; in absolute terms a sunspot is still extremely bright.

What’s the difference between white-light and Hydrogen-alpha solar imaging?

White-light imaging uses a front-aperture filter to show the photosphere — sunspots and surface granulation. Hydrogen-alpha imaging uses a dedicated solar scope tuned to a single red wavelength to reveal the chromosphere, including prominences and filaments. Both are safe when done with proper certified equipment; they simply show different layers of the Sun.