The Sun• Composed of hydrogen and helium.• Very dense core, all gaseous because of the

very high temperatures.• The Sun is the largest object in terms of size

and mass in the solar system.• Is more than 99% of all mass in the solar

system. (330,000 times more massive than Earth!)

• The Sun’s mass allows it to control the movements of planets in our solar system.

• Energy is created by fusion in the core of the Sun.

Size comparison of the Sun

Energy formation in the Sun• Extremely high temperatures and pressures in the core

allows fusion to occur.• Fusion is the combining of lightweight nuclei into

heavier nuclei.• 4 hydrogen nuclei combine and form 1 helium and 2

hydrogen nuclei.• It appears that mass is being lost…but the “lost” mass is

actually converted into ENERGY. (E=mc2)• The Sun is slowly converting its hydrogen to helium,

and giving off energy in the process. Currently the Sun is about ½ way through this process and has another 5 billion years of hydrogen remaining.

Solar Zones

How does energy leave the core?

• Through 2 zones.

1. Radiative zone – the first zone out from the core and extending about 86% of the way to the photosphere. Energy is transferred particle to particle by radiation.

2. Convection zone – this zone is above the radiative zone, and energy is transferred by moving gases through the process of convection.

Layers of the Sun’s Atmosphere

• Photosphere – the lowest layer of the Sun’s atmosphere, and the visible surface we see. It is about 400 km thick.

• Chromosphere – is the layer above the photosphere, is about 2500 km thick. (Can be visible during an eclipse when the photosphere is blocked.)

• Corona – the top layer of the Sun’s atmosphere, extends several million km’s into space! (also can be seen during an eclipse)

All major layers of the Sun

Solar Wind• Gas flowing outward from the corona creates the

“solar wind.”• This “wind” contains ions (charged particles) that

rapidly move away from the Sun.• The solar wind strikes each planet of our solar

system.• On Earth the charged particles are deflected by the

Earth’s magnetic field…and are pushed into two huge rings, called the Van Allen belts (they are part of the magnetic field)

• The particles can collide with gasses in our atmosphere, creating a display of light called an aurora.

Deflection of Solar Wind

Aurora

Solar Activity• Sunspots – small dark areas on the surface

of the Sun, as a result of a region of cooler gases. Sunspots come and go in cycles, and on average last about 11.2 years in any one place. The number of sunspots on the Sun at one time also changes, and there is an apparent cycle of maximum and minimum numbers of sunspots. (LAB)

• Solar flares – violent eruptions of particles and radiation from the surface of the Sun.

• Prominence – arc of gas that is ejected from the chromosphere.

Prominence

Spectra of Visible Light

• Most of the radiation reaching the surface of the Earth is visible light.

• Learn the visible spectrum: ROYGBIV

Measuring the Stars

Constellations – several bright stars grouped together. (88 constellations)

Examples: Ursa Major, Orion, Hercules

Although these grouping of stars appear close, it is an optical illusion and they are very distant. Our eyes cannot distinguish how far or close these distant stars are.

Measuring the Stars

• Star clusters are groups of stars that are considered “closer” to each other and have some gravitational effect on each other.

Binaries – two stars that are gravitationally bound to each other around a common center of mass. (Binary star) ex: Sirius These stars appear as one because they are relatively close.

Measuring the Stars

• Methods for measuring distance in space:1. Light-year (ly) – the distance light will

travel in one year. ______________2. Parsec (pc) – 3.26 ly

Parallax – precise distances to stars can be made by observing the apparent shift in nearby stars over the course of ½ year on Earth. (eye example) …the closer the star the larger the shift.

Basic Properties of Stars

• Magnitude – how bright a star appears.• Apparent Magnitude – a refined definition

of magnitude, in which each unit is 2.512 times brighter than the one before.

• Absolute Magnitude – does not actually measure brightness, takes into account distance to the star.

• Luminosity – measure of energy output.• Temperature (classification)

Hertzsprung-Russell Diagram(H-R diagram)