1. Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
The Sun
Is a star
Made of gases
Is our primary source of energy
A star in the Milky Way galaxy
Galaxy- group of billions of stars.
70% hydrogen and 28% helium
Light (radiation)
The Nine Planets website has all of this excellent information:
The Sun contains 99.8% of all of the mass in our Solar System.
It is a yellow, medium-temperature star– 9,981°F (5527°C) on the outside and 28 million°F (15 million°C) in the center.
Image at
Lunar and Planetary Institute

2. Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
How Big is the Sun?
About 110 times wider than Earth Or
1.3 million times bigger than Earth
Only part of the sun that can’t be
seen is the atmosphere.
Our Sun is 865 thousand miles (1,390,000 km) wide. Even a medium-sized sunspot is as big at the Earth!
Photo from
Atmosphere- layer of gases that
surrounds the earth.
Photo from
Lunar and Planetary Institute

3. How does our Sun compare to other Stars?
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
How does our Sun compare to other Stars?
Active stars range in size from supergiants to dwarfs
Stars range from very bright (supergiants) to very dim (dwarfs)
Stars range from very hot blue on the outside (O class) to cool red on the outside (M class)
Our Sun is a dwarf—medium mass
Our Sun is a medium-bright dwarf
Our Sun is in-between--yellow
Supergiants can be hundreds to thousands of times bigger than our Sun, and possibly up to 100 times more massive than our Sun. The smallest stars are about 1/100 of the Sun’s mass.
Temperature ranges for the outside of a star (spectral classification): O (blue), B (blue-white), A (white), F (white-yellow), G (yellow), K (orange), M (red)
Very large stars can be very hot on the outside (O class) or cool on the outside (all the way down to M class)—all large stars are extremely hot in their cores.
All small active stars (excludes remaining cores of old stars) are cooler on the outside and in their cores, compared to the most massive stars.
Lunar and Planetary Institute

4. So is our Sun an average star?
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
So is our Sun an average star?
No—most stars are smaller and cooler than our Sun BUT
Most of the bright stars we see are bigger and hotter
For more details and a graph, go to Nick Strobel’s Astronomy Notes:
Lunar and Planetary Institute

6. Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
More on Sunspots
Our Sun has an activity cycle of 11 years
Sunspots- dark areas on the
sun’s surface.
Basic information and image at
Sunspot cycle and detailed data for activities at
Images at and
Lunar and Planetary Institute

7. Sun as a Source of Energy
Notes for HCDE Workshop on Sun and Seasons Feb. 4, 2009
Sun as a Source of Energy
Plants need light for photosynthesis
Without its heat, the only inhabitable areas on Earth would be near volcanic vents
Images from and
Lunar and Planetary Institute