1. THE SUN: AWESOME AND ACTIVE
Boston University PHOTON
Outreach Presentation

2. The Sun
An average-size yellow star, just like billions of others in the universe
Center of the solar system, Earth orbits around it
Formed about 4.6 billion years ago
The sun has a mass of 2x1030 kg or 330,000 Earths, that’s 99.8% of the mass in the solar system.

3. The Sun’s Energy & Earth
The Sun is the source of all of the energy on Earth.
The amount of energy that reaches Earth from the Sun in one second is enough energy to meet Massachusetts’ current energy needs for 32,655,294 years!!!*
About half the energy that hits the Earth is absorbed and used to heat the planet.
You can feel this energy in the form of heat on your skin on a sunny day.
A lightbulb provides energy in the form of light just as the Sun does. You can feel this energy on your skin as heat. A Watt is a measurement of energy per time.
If a 75 Watt lightbulb feels warm… the Sun produces 2 quadrillion times that amount of energy! Although only ~50% of that energy reaches the Earth’s crust.
*figure based on data from California Energy Commision:

4. Where does the Sun’s Energy Come From?
Energy is produced in the Sun’s core through a process known as nuclear fusion.
Nuclear fusion is when nuclei of one kind of element combine to make a new nucleus in that requires less energy to hold it together. This difference in energy is spit out as a result of the reaction.
The Sun’s nuclear fusion occurs when four hydrogen nuclei combine to make helium and release energy.
This type of reaction can’t happen just anywhere, it can only happen at incredibly high temperatures and pressures like those found in stars, somewhere around 23 million degrees!!
100 billion megatons is roughly 10 billion nuclear weapons and billion “Little Boys” (the Hiroshima bomb) +
Energy
1 Helium nucleus
4 Hydrogen nuclei

5. How much energy does the Sun make?
We know how much energy the Sun emits by measuring the amount of energy that reaches Earth.
The Sun consumes 660 million tons of hydrogen releasing 100 billion megatons of energy per second! That’s the same energy as 20 billion Hydrogen bombs every second!
Based on the amount of hydrogen the Sun consumes per second and the mass of the Sun, we estimate its lifetime to be 50 billion years.
Nuclear Fusion Hydrogen Bomb—50 megatons

6. Diameter of the Sun = Diameter of 56 Earths!
What’s inside the sun?
Layers of the Sun
1. Core
the core of the sun alone is the size of 13 Earths!
2. Radiative Zone
energy from the core radiates through this part of the Sun
3. Convective Layer
convection cells move energy through this part of the Sun
4. Photosphere
This is the part of the sun we see
5. Chromosphere
a part of the Sun’s atmosphere
6. Corona
This is a bright halo around the Sun
Diameter of the Sun = Diameter of 56 Earths! 6

7. Energy Escapes the Sun
Fusion occurs in the Sun’s core, the hottest part.
The energy released by the fusion in the Sun’s core is in the form of light.
It only takes 8 minutes for light from the surface of the Sun to reach Earth
But it takes 170,000 for the light energy from the core to reach the Sun’s surface because it doesn’t go in a straight line
When energy leaves the core, it moves out into the radiative zone. In the radiative zone, the energy passed from the core gets transferred from atom to atom by photons. Photons are packets of energy that we perceive as light when they fall into our eye. The radiative zone is 55% of the sun’s radius or 236,000 miles deep. Since light travels at 190,000 mps, it should take light 1.2 second to cross the zone. But, the photons don’t move in a straight line in this zone. When a photon goes into an atom, it is absorbed and when that atom releases a photon, there’s no telling the direction in which the photon moves. The new photon could go back where the old photon came from! So we get an unpredictable pattern when we trace the paths of photons in the radiative zone. If we wan, we can find out on average how fast the photons move through the zone. We know that it takes the energy 170,000 years to go from core to convection zone, so the average velocity follows from rate is distance divided by time and we get V=1.4 miles per year. That’s slow! This is why energy in the sun spends most of its time in the radiative zone. Also, the temperature falls by 75% in this zone going from 8 million K to 2 million K. 7 7

8. The Changing Sun The surface of the Sun is constantly changing
The Sun is shooting particles off it all the time called the Solar Wind
There is an 11-year “Solar Cycle” when the Sun goes from being very active (solar maximum) to not not very active (solar minimum

9. How is the Sun active?
The solar cycle is characterized by the rising and falling of solar activity
The solar maximum is the period of greatest solar activity
During solar maximum the Sun is more active meaning there are more sunspots and solar flares

10. Sunspots
What?
‘Dark’ spots on the Sun's surface that are cooler than the surrounding areas
Temperature of:
Sunspot= 3800 K
Surrounding area= 5800 K
Why?
The strong magnetic field in that area stops the area from being able to transfer energy so the temperature isn’t as high

11. More about Sunspots
Sunspots can be about the size of Earth or even bigger!!!
They come in pairs
They are still really hot, just not as hot as the rest of the Sun’s surface
Sunspots move across Sun’s surface as it rotates

12. Magnetic Activity on the Sun
Sunspots are caused by strong magnetic fields
Magnetic “ropes” break through the Sun’s surface
At each end of the rope is a sunspot
It is the Sun’s magnetic activity that causes solar storms and solar flares

13. How do we know all of this about the Sun?
Telescopes
You can look at the Sun through a telescope with special filters.
You can also use a solar telescope that projects the image of the Sun onto a piece of paper instead of into your eye.
Satellites
There are also several spacecraft that
are observing the Sun: SOHO, TRACE, STEREO
These satellites look at the Sun in all
different kinds of light (ultraviolet, visible,
infrared, X-ray) to see everything that is
going on
Many cameras on telescopes need to use
a caronagraph, where a disk is used to block
out the Sun’s surface so we can see all the dim
stuff that the Sun’s brightness would drown out