The Sun: Light and Color Temperature

Jordan Butler

Student Projects, Color

Introduction

The Sun is more important in our daily lives (and our existence on the earth) than most of us ever consider. Every living thing depends upon the Sun for its survival. Astronomers have been studying the Sun for years and still this star is largely a mystery. Through tedious reasearch, astronomers have come to understand, among other things, how the sun produces light and the temperatures of the various layers of the Sun.

Did you know...

The Sun has been burning for 4.6 billion years.

The Sun's energy output is 386 billion billion megawatts.

The Sun is the largest object in our Solar System, containing more than 99.8% of the total mass of the Solar System.

The Sun is the source of all energy that sustains life on out planet.

Light from the Sun

Sunbeams we see today began their journey from the center of the sun before the last Ice Age. These sunbeams spent hundreds of thousands of years trying to get out of the photosphere before making the 8 minute, 93 million mile trip to Earth.

 

Sun's Electromagnetic Actvity

An enormous amount of enery is emitted by the Sun and dispersed throughout space in all directions. The Sun emits more than just wavelengths from the electromagnetic spectrum. Electromagnetic energy comprises all types of energy that travel through space at the speed of light. Solar radiation constitutes only a portion of the spectrum, both visible and invisible. What we see as sunlight refers to wavelengths detectable by human eye (400-700 nm).

Band Wavelength (nm) Atmospheric Effects
Gamma ray <0.03 Completely absorbed by the upper atmosphere
X-Ray 0.03-3 Completely absorbed by the upper atmosphere

Ultraviolet (UV)

UV (B)

UV (A)

3-300

300-400

 

Completely absorbed by oxygen, nitrogen, and ozone in the upper atmosphere

Transmitted through the atmosphere, with moderate scattering is severe

 

Visible 400-700 Transmitted through the atmosphere, with moderate scattering of the shorter waves

Reflected Infrared

Thermal Infrared

700-3000

3000-14000

Mostly reflected radiation

Absorbtion at specific wavelengths by carbon dioxide, ozone, and water vapor

Examine the Sun at different wavelengths (Link not working 12-28-04)

What we call visible light can be put into two categories: white light and filtered light.
White light pictures without any filter show how the sun appears to the naked eye when all of the colors of the visible spectrum are collected by the camera. Pictures can be taken of the sun using different filters to show the different wavelengths emitted.
A filtered picture can also show features of the sun that are harder to see when in unfiltered light.
Two filters commonly used:

1. H-alpha filter: allows camera to see only red light (656.3 nm)
2. Calcium K filter: allows camera to see only blue light (393.4 nm)

Images of Sun through different filters


The following false-color image shows the Sun at UV wavelengths.

The picture above was taken with a camera that sees light with wavelengths between about 0.3 and 4.5 nanometers.

Photons

Photons are particles that make up these sunbeams and are the building blocks of electromagnetic radiation. The intensity of a beam of light depends on the number of photons emitted per second. Photons have no invariant mass but a definite and finite energy. A photon can be created or destroyed. One way a photon can be created is by emission from an electron as it changes energy states or orbitals; when a photon gives up all of its energy, it disappears.
The potential energy lost when the electron jumps energy levels results in energy released in the form of a photon:  a single "unit" of light. 

  (The arrow represents a photon)

Different colors of light are thought of as photons of different energies. A photon's energy is described in units of energy called electron volts (eV). Photons in the realm of visible light are absorbed and disappear in giving their energies to the outer most electrons that are held in place by energies of only a few electron volts.

Nuclear Fusion

Photons are produced in the center of the sun through the process of nuclear fusion, thus producing light. The center of the sun is not a solid but actually a very hot gas (composed of mostly H and some He). The reaction occuring inside the sun, known as nuclear fusion, is the is the “burning” of hydrogen to make helium.
4 H + 2 e --> He + 2 neutrinos + 6 photons


Neutrinos leave the sun’s core, not reacting with matter. The solar core is so dense, a photon produced by nucelar fusion in the Sun's core can't go a fraction of a milimeter before banging into a subatomic particle. After an extended time in the solar core, the photon eventually leaves the sun as puny radiation we call visible light

Color Temperature/The Sun as a Black Body Radiator

Sun behaves as a black body radiator. A black body radiator is a theoretical standardized material that the color temperature model is based on. The energy a black body radiator emits is generated by heating the material. Photons produced in the sun (a balck body radiator) through nuclear fusion heat the gases in the solar core. The color of a black body radiator is determined by the energy distribution of the visible light it emits when heated to a given temperature.

The surface temperature of sun is about 6000 K. So a passenger in a space shuttle would see a bluish-white light from the sun. But, the light must pass through the Earth's atmosphere which filters, diffuses and reflects the light altering its correlated color temperature, giving us blue skies and red sunsets. Color temperature of outdoor daylight ranges from 2,000 K to 30,000 K. How can daylight reach 30,000K?
As the lower frequency components of the light are absorbed in the atmosphere, the bluer components predominate, shifting the correlated color temperature upward.


Sunlight passes through the atmosphere in direction parallel to earth’s surface at sunrise and sunset. So, the light passes through a relatively thick layer of atmosphere, causing refraction. This emphasizes the redder components and results in lower color temperature and reddish color.

SOHO (Solar and Heliospheric Observatory)

So where did all of the information we know about the Sun come from? Much of what is known about the Sun has been discovered by new technology developed in the last 25 years. SOHO launched in 1995, was designed to study, among other things, the internal structure of the sun. SOHO has provided an unprecedented depth of information and the most detailed and precise measurements of the temperature in the solar interior.

EIT (Extreme ultraviolet Imaging Telescope)

EIT is a multilayer telescope on SOHO designed to look at coronal structures. EIT takes images of the solar atmosphere at several wavelengths and therefore shows solar material at different temperatures. The hotter the temperature, the higher you are looking in the solar atmosphere.

SOHO EIT images

More EIT images

SOHO location (Site loads slowly!)

 

Conclusions

The Sun is more important than most of us recognize and is still largely a mystery. Astronomers are still searching for answers to many of the questions they have about the Sun. Through more reasearch and advances in technology, new insights into the complexities of the sun can be made.

 

Other Links & References

Solar Energy

SOHO info

"The sun: living with a stormy star" National Geographic July 2004

Acknowledgements

We will miss you Dr. B!