Solar Astronomy

The Sun

Our nearest star — and the most dangerous object to observe without proper equipment

At a distance of just 93 million miles, the Sun is the only star we can study in intimate detail. Every other star is a point of light no matter how large a telescope you point at it. The Sun, by contrast, reveals a churning, dynamic surface with storms larger than Earth, arcs of plasma leaping hundreds of thousands of miles, and a corona that somehow reaches millions of degrees despite being far from the energy source. Solar astronomy is one of the richest fields available to amateur astronomers — if you observe safely.

Safety first — always. Never look at the Sun through any optical instrument without a certified solar filter rated for direct solar viewing (ISO 12312-2). Welding glass, exposed film, CDs, and stacked neutral-density filters are not safe. Instant, permanent blindness is possible in a fraction of a second.

Layers of the Sun

The Sun is a ball of plasma 865,000 miles across — about 109 Earths side by side. Its interior is divided into distinct zones, each with a different energy transport mechanism. What we observe from Earth are the outermost layers.

Photosphere

The visible surface of the Sun is the photosphere, a thin layer about 300 miles deep with a temperature of roughly 10,000°F (5,500°C). It has a granular texture caused by convection cells — columns of rising hot gas about the size of Texas. These granules live for only about 10 minutes before sinking back down and being replaced.

Sunspots appear dark against the photosphere because they are cooler (around 7,000°F) — still blindingly bright in absolute terms, but dim compared to the surrounding surface. They are regions where intense magnetic fields suppress convection.

Chromosphere

Just above the photosphere lies the chromosphere ("sphere of color"), about 1,200 miles thick. Normally invisible against the brightness of the photosphere, it flashes into view as a thin red rim during the first and last seconds of a total solar eclipse. The red color comes from hydrogen-alpha emission at 656 nm — the same wavelength used in dedicated H-alpha solar telescopes.

Spicules — jets of gas about 300 miles wide and 3,000 miles tall — constantly erupt through the chromosphere, giving it a grass-like texture in H-alpha images.

Corona

The outermost layer, the corona, extends millions of miles into space and reaches temperatures of 1–3 million degrees — paradoxically far hotter than the photosphere below. The mechanism for this heating is still an active area of research. The corona is visible to the naked eye only during a total solar eclipse, when it appears as a pearly white halo with streamers extending outward.

Solar Features

Sunspots

Sunspots are the most accessible solar feature for amateur observers. In white light through a proper solar filter, they appear as dark blemishes with two distinct zones: the very dark central umbra and the lighter surrounding penumbra. Large sunspot groups can be larger than the planet Jupiter and are occasionally visible to the naked eye (with a proper filter).

The Sun follows an approximately 11-year activity cycle between solar minimum (few or no spots) and solar maximum (many large, complex sunspot groups). We are currently in Solar Cycle 25, which reached maximum around 2025.

Prominences and Filaments

Prominences are arcs and curtains of plasma that extend from the chromosphere into the corona, following magnetic field lines. Seen at the solar limb (edge), they appear as bright pink or red structures. When seen projected against the solar disk rather than the limb, the same structures appear dark and are called filaments.

Some prominences are stable for weeks or months. Others — called eruptive prominences — can blast material into space in a matter of hours.

Solar Flares and CMEs

Solar flares are intense bursts of radiation caused by the sudden release of magnetic energy near sunspot groups. A major flare can briefly outshine the entire disk in certain wavelengths. They are usually associated with sunspot groups with complex magnetic configurations.

Coronal mass ejections (CMEs) are massive clouds of magnetized plasma ejected from the Sun. When a CME is directed at Earth and arrives 1–3 days later, it can trigger geomagnetic storms — and spectacular auroral displays visible at mid-latitudes, including Oregon.

Observing the Sun

White Light Observation

The most affordable entry into solar observing is a white-light solar filter — a film or glass filter that fits over the front of your telescope. These show sunspots clearly and reveal the photospheric granulation in good seeing conditions. Many observers also note faculae (bright regions near the limb) and the slight darkening of the Sun's edge compared to its center (limb darkening).

Dedicated solar filters are available from Thousand Oaks Optical, Baader Planetarium, and others for $30–$80 depending on aperture.

Hydrogen-Alpha Observation

A dedicated H-alpha telescope (such as the Lunt or Coronado product lines) uses an extremely narrow bandpass filter to isolate the 656 nm hydrogen-alpha emission line. This reveals the chromosphere in stunning detail: prominences, filaments, flares in progress, and the intricate texture of active regions. The view through an H-alpha scope is unlike anything else in astronomy — a living, constantly changing surface.

Entry-level H-alpha scopes start around $500; serious instruments run $1,000–$3,000+. Several HVA members own H-alpha equipment and bring it to star parties and outreach events.

Solar Projection

A traditional method requiring no special filters: project the Sun's image through an eyepiece onto a white card held behind the telescope. This works well for showing sunspots to groups without everyone needing to look through an eyepiece — ideal for outreach events. Use only refractors or Newtonians for this method; Schmidt-Cassegrains and other designs with internal mirrors or lenses can be damaged by the concentrated heat.

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