The Sun – Our Star Our sun is considered an “average” star and is one of the 100 BILLION stars that make up the Milky Way galaxy. But by no MEANS does that make our star insignificant! Everything - from the fossil fuels we burn to the foods we eat – is ultimately derived from solar energy.

General Properties Average sized star 109 times Earth’s diameter 333,000 times Earth’s mass Consists entirely of gas (density = 1.4 g/cm3; compared to Earth at 5.5 g/cm3) Central temperature = 15 million 0K (27 million 0F) Surface temperature = 5800 0K (10000 0F) Ultra-violet image

Structure of the Sun Because the sun is made of gas, no sharp boundaries exist between its various layers. Keeping this in mind, we can divide the sun into four parts: solar interior photosphere (the visible surface) chromosphere (atmospheric layer) corona (atmospheric layer)

Solar Interior

Layers of the Solar Atmosphere X-ray Visible Sun Spot Regions Photosphere Corona Chromosphere Coronal activity, seen in visible light

Solar Interior: Nuclear Fusion • Nuclear fusion is the way that the sun produces energy. This reaction smashes hydrogen atoms together to form helium, releasing a tremendous amount of energy. • During nuclear fusion, energy is released because some matter is actually converted to energy.

Nuclear Fusion Makes no sense without caption in book Nuclear Fusion Creating New Elements Makes no sense without caption in book

The Photosphere Region of the sun that radiates energy into space. This is the part we see! Depth ≈ 500 km Temperature ≈ 5800 oK (10,000 oF) 10 Day Solar Animation

Energy Transport in the Photosphere Energy generated in the sun’s center must be transported outward. In the photosphere, this happens through Convection. This convection creates a grainy texture called granules that are the size of Texas! Cool gas sinking down Bubbles last for ≈ 10 – 20 min. Bubbles of hot gas rising up ≈ 1000 km

Granulation Granules … is the visible consequence of convection

Cooler regions of the photosphere (T ≈ 4240 K). Sun Spots Cooler regions of the photosphere (T ≈ 4240 K). Only appear dark against the bright sun. Would still be brighter than the full moon when placed on the night sky!

Visible X-ray Sun Spots -occur in the photosphere; were originally discovered by Galileo in 1610 -we know the sun’s rotation is 27 days by observing the movement of the sunspots! -they are created by the Sun’s magnetic field They’re magnetism is 1000x’s stronger than the magnetic field of the sun! Sunspots

Chromosphere • The chromosphere is the next layer of the solar atmosphere found directly above the photosphere. Temperature is about 20,000 °C (36,000 °F) • This burning gas is a few thousand kilometers thick. • Its top contains numerous spicules, which are narrow jets of rising material. They are typically thousands of kilometers high with a 10,000 kilometer maximum height and last 5-15 min.

photosphere chromosphere

Solar Activity, seen in soft X-rays

Prominences • Prominences are huge cloudlike structures consisting of gases from the chromosphere. • Prominences are trapped by magnetic fields that extend from regions of intense solar activity.

Solar Flares Solar flares are tremendous explosions on the surface of the sun. They normally last about an hour and appear as a sudden brightening of the region above a sunspot. Solar flares release enormous amounts of energy, much of it in the form of ultraviolet, radio, and X-ray radiation. This intensifies solar wind YouTube: Solar Flares

Corona • The corona is the outer, weak layer of the solar atmosphere. • The temperature at the top of the corona exceeds 1 million °C (1.8 million °F), which makes it the hottest layer. • The streams of protons and electrons that boil from the corona make up the Solar wind.

Coronal Mass Ejection

Aurora Borealis Solar flares can intensify the auroras. Youtube: Secrets of a Dynamic Sun Auroras are bright displays of light caused by solar radiation interacting with the upper atmosphere near Earth’s poles. Solar flares can intensify the auroras.

YouTube: From Space Station YouTube: Over Norway

The Sun’s Magnetic Dynamo The sun rotates faster at the equator than near the poles. Why? …because it’s a huge ball of gas!!! This differential rotation might be responsible for magnetic activity of the sun.

Life Cycle of the Sun • It is thought that a star the size of the sun can exist in its present stable state for 10 billion years. As the sun is already 4.5 billion years old, it is “middle-aged.”

Death of a Star

Star size comparison video