The Core: The Sun's core has a tremendously high temperature and pressure. The temperature is roughly 15 million °C. At this temperature, nuclear fusion occurs, turning four hydrogen nuclei into a single helium nucleus plus a LOT of energy. This "hydrogen burning" releases gamma rays (high-energy photons) and neutrinos (particles with no charge and almost no mass).

The Radiative Zone (or radiation zone): The next layer out from the core is this zone which emits radiation. This radiation diffuses outwards. The temperature ranges from 15 million °C to one million °C. It may take photons of radiation millions of years to pass through the radiative zone, as they gradually make their way outwards.

The Convective Zone: In this next layer, photons continue to make their way outwards via convection (towards lower temperature and pressure). The temperature ranges from one million °C to 6,000 °C.

The Photosphere: This is the lower atmosphere of the Sun and the part that we see (since it emits light at visible wavelengths). This layer is about 300 miles (500km) thick. The temperature is about 5,500 °C.

The Chromosphere: This reddish layer is an area of rising temperatures. The temperature ranges from 6,000 °C (at lower altitudes) to 50,000 °C (at higher altitudes). This layer is a few thousand miles (or kilometers) thick. It appears red because hydrogen atoms are in an excite state and emit radiation near the red part of the visible spectrum.

The Chromosphere is visible during solar eclipses (when the moon blocks the Photosphere).

The Corona: This is the outer layer of the Sun's atmosphere. The corona extends for millions of miles and the temperatures are tremendous, reaching one million °C.

Holes in the corona occur where the Sun's magnetic field loops out into space. These coronal holes may be the source of the solar wind, a stream of energetic particles that permeate the Solar System.

The Sun rotates around an axis which is roughly perpendicular to the plane of the ecliptic; the Sun's rotational axis is tilted by 7.25° from perpendicular to the ecliptic. It rotates in the counterclockwise direction (when viewed from the north), the same direction that the planets rotate (and orbit around the Sun).

The Sun's rotation period varies with latitude on the Sun since it is made of gas. Equatorial regions rotate faster than polar regions. The equatorial regions (latitude = 0 degrees) rotate in about 25.6 days. The regions at 60 degrees latitude rotate in about 30.9 days. Polar regions rotate in about 36 days.

The Sun's diameter is 864,938 miles (1,391,980 km). This is almost 10 times larger than the planet Jupiter and about 109 times as big as the Earth. The volume of the Sun is 1,299,400 times bigger than the volume of the Earth; about 1,300,000 Earths could fit inside the Sun

As compared to other stars, however, the Sun is about average; red giants like Betelgeuse are about 700 times bigger than our Sun (and roughly 50 times as massive). Betelgeuse is also about 14,000 times brighter than the Sun. Red supergiants dwarf the Sun.

The Sun's apparent size in our sky is 32 arc minutes (it is the biggest object in the sky - 1 arc minute larger than the moon).

THE MASS OF THE SUN
The Sun's mass is roughly 1.99 x 10³⁰ kg. This is about 333,000 times the mass of the Earth. The Sun contains 99.8% of all of the mass of the Solar System.

The mass of the sun is decreasing over time, as fusion reactions convert hydrogen into helium, releasing huge amounts of energy in the process.

Sunspots are relatively cool, dark patches on the sun's surface. They come in many shapes and sizes; they often appear in groups. These spots are much bigger than the Earth; they can be over 10 times the diameter of the Earths.

Individual sunspots only last for one to two weeks, but the number of sunspots follows an 11 year cycle. The current sunspot cycle will peak in the middle of 2000. Sunspots are visible from Earth.

The sunspot cycle was discovered by S. Heinrich Schwabe in 1843 (he started his observations in 1826).

WARNING: do NOT look at the sun; it can damage your eyes permanently!

UMBRA:
The umbra is the inner, dark, cool (3700 K = 6600 °F = 3400 °C) region of a sun spot. The umbra of a sunspot can be up to 12,000 miles (20,000 km) wide. In the umbra, the Sun's magnetic field is very strong.

PENUMBRA:
The penumbra is the outer, relatively light region of a sun spot. It is shaped like an annulus (a ring) surrounding the darker, cooler umbra.

PORES:
A pore is a small sunspot that doesn't have a penumbra. Pores are up to about 1,500 miles (2,500 km) across and are lighter than a sunspot's umbra.

GRANULATION:
Granulation is solar granules together with intergranular lanes (dark, cool areas between granules where solar material is descending into the surface). Granulation covers the visible surface (the photosphere) of the Sun.

GRANULES:
Granules are regions of the sun where hot solar material comes to the solar surface. Granules are about 600 miles (1,000 km) across and only exist for about 5 to 10 minutes before they fade away. It is almost as though the surface of the Sun is bubbling like a pot of boiling water.

What Causes Sunspots?
Sunspots occur where the sun's magnetic field loops up out of the solar surface and cool it slightly, making that section less bright. These disturbances in the sun's magnetic field make the sunspot about 2700°F (1500°C) cooler than the surrounding area.

SOLAR FLARES
A solar flare is a magnetic storm on the Sun which appears to be a very bright spot and a gaseous surface eruption. Solar flares release huge amounts of high-energy particles and gases and are tremendously hot (from 3.6 million to 24 million °F). They are ejected thousands of miles from the surface of the Sun.

Solar flares were first observed by in 1859 by Lord Richard C. Carrington. He wrote that as he was watching the sun with a telescope, he saw "two patches of intensely bright and white light" near a huge group of sunspots. Just a few seconds later, the flare has disappeared.

It has been recently discovered that solar flares can cause sunquakes. Sunquakes are violent seismic events on the Sun. When a sunquake occurs, energy is released in seismic waves on the relatively fluid surface of the Sun. These waves radiate in concentric circles from the epicenter of the sunquake. These seismic waves seem to be compression waves (perhaps like "P" waves generated by earthquakes). Sunquakes would rate about 11.3 on the Richter scale. These huge quakes release about 40,000 times more energy than the 1906 San Francisco earthquake. Sunquakes were first observed by Alexander G. Kosovichev (Stanford University) and Valentina V. Zharkova (Glasgow Univ., UK).

SOLAR WIND
The solar wind is a continuous stream of ions (electrically charged particles) that are given off by magnetic anomalies on the Sun. The solar wind is emitted where the Sun's magnetic field loops out into space instead of looping back into the Sun. These magnetic anomalies in the Sun's corona are called coronal holes. In X-ray photographs of the Sun, coronal holes are black areas. Coronal holes can last for months or years.

It takes the solar wind about 4.5 days to reach Earth; it has a velocity of about 250 miles/sec (400 km/sec). Since the particles are emitted from the Sun as the Sun rotates, the solar wind blows in a pinwheel pattern through the solar system. The solar wind affects the entire Solar System, including buffeting comets' tails away from the Sun, causing auroras on Earth (and some other planets), the disruption of electronic communications on Earth, pushing spacecraft around, etc.

SOLAR PROMINENCE
A solar prominence (also known as a filament) is an arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space. Prominences are held above the Sun's surface by strong magnetic fields and can last for many months. At some time in their existence, most prominences will erupt, spewing enormous amounts of solar material into space.

CORONAL MASS EJECTION
Coronal mass ejections (abbreviated CME's) are huge, balloon-shaped plasma bursts that come from the Sun. As these bursts of solar wind rise above the Sun's corona, they move along the Sun's magnetic field lines and increase in temperature up to tens of millions of degrees. These bursts release up to 220 billion pounds (100 billion kg) of plasma. CME's can disrupt Earth's satellites. CME's usually happen independently, but are sometimes associated with solar flares.

A Globule of Gas:
The Sun, like other stars, was formed in a nebula, an interstellar cloud of dust and gas (mostly hydrogen). These stellar nurseries are abundant in the arms of spiral galaxies (like our galaxy, the Milky Way).

In the stellar nursery, dense parts of the clouds undergo gravitational collapse and compress to form a rotating gas globule.

The globule is cooled by emitting radio waves and infrared radiation. It is compressed by gravitational forces and also by shock waves of pressure from supernova or the hot gas released from nearby bright stars. These forces cause the roughly-spherical globule to collapse and rotate. The process of collapse takes from between 10,000 to 1,000,000 years.

A Central Core and a Protoplanetary Disk:
As the collapse proceeds, the temperature and pressure within the globule increases, as the atoms are in closer proximity. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces (a radial force on spinning objects) that causes the globule to have a central core and a surrounding flattened disk of dust (called a protoplanetary disk or accretion disk). The central core becomes the star; the protoplanetary disk may eventually coalesce into orbiting planets, asteroids, etc.

Protostar:
The contracting cloud heats up due to friction and forms a glowing protostar; this stage lasts for roughly 50 million years. If there is enough material in the protostar, the gravitational collapse and the heating continue.

A Newborn Star:
When a temperature of about 27,000,000°F is reached, nuclear fusion begins at the core of the Sun. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy (radiation) production prevents further contraction of the Sun.

Young stars often emit jets of intense radiation that heat the surrounding matter to the point at which it glows brightly. These narrowly-focused jets can be trillions of miles long and can travel at 500,000 miles per hour. These jets may be focused by the star's magnetic field.

Later, the Sun stabilizes and becomes a yellow dwarf, a main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the Sun begins to die.

 

THE DEATH OF THE SUN

 

The Sun is about 4.5 billion years old. it has used up about half of its nuclear fuel (hydrogen). In about 5 billion years from now, the sun will begin to die.

As the Sun grows old, it will expand. As the core runs out of hydrogen and then helium, the core will contact and the outer layers will expand, cool, and become less bright. It will become a red giant star.

After this phase, the outer layers of the Sun will continue to expand. As this happens, the core will contract; the helium atoms in the core will fuse together, forming carbon atoms and releasing energy. The core will then be stable since the carbon atoms are not further compressible.

The Pistol nebula: a planetary nebula in Sagittarius.

The Egg nebula: a planetary nebula that formed a few hundred years ago.

Then the outer layers of the Sun drift off into space, forming a planetary nebula (a planetary nebula has nothing to do with planets), exposing the core.

Most of its mass will go to the nebula. The remaining Sun will cool and shrink; it will eventually be only a few thousand miles in diameter!

A White dwarf star: (circled) in the globular cluster M4.

The star is now a white dwarf, a stable star with no nuclear fuel. It radiates its left-over heat for billions of years. When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star (perhaps replete with diamonds, highly compressed carbon).

 

Sun Statistics
Mass (kg)	1.989e+30
Mass (Earth = 1)	332,830
Equatorial radius (km)	695,000
Equatorial radius (Earth = 1)	108.97
Mean density (gm/cm^3)	1.410
Rotational period (days)	25-36*
Escape velocity (km/sec)	618.02
Luminosity (ergs/sec)	3.827e33
Magnitude (Vo)	-26.8
Mean surface temperature	6,000°C
Age (billion years)	4.5
Chemistry * The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days.   Hydrogen Helium Oxygen Carbon Nitrogen Neon Iron Silicon Magnesium Sulfur All others	92.1% 7.8% 0.061% 0.030% 0.0084% 0.0076% 0.0037% 0.0031% 0.0024% 0.0015% 0.0015%