History of Electromagnetic Radiation

Energy Escapes in the form of Electromagnetic Radiation

Incredibly, the sun only makes gamma radiation, yet x-rays, ultraviolet, visible, infrared, microwave, and radio waves are all emitted from the sun. The interior of the sun is so dense, that the gamma rays cannot escape without colliding with atomic particles and losing a small portion of their energy. Traveling at 300,000 km/sec, a gamma ray typically spends 30,000 years colliding with atomic particles and re-emitting energy at a slightly lower energy level, until it finally escapes from the sun and heads to earth. The gamma ray now has lost so much of its initial energy that it appears as visible light, still traveling at 300,000 km/sec, but now only with energy between 400 and 700 nm. This is the light of the sun which we see as "visible light." Gamma radiation which may escape more quickly will leave the sun with higher energy levels as ultraviolet or x-radiation, while those gamma rays which take longer to escape after surviving more collisions will escape as infrared, microwave or even radio waves.

The sun ONLY makes gamma radiation, but due to time and chance collisions, all forms of radiation are released. To be sure, it is more complicated than this, but I am trying my best to keep the course at a high school introductory level. I even find complete understanding of the energy interactions with electrons on the escape path to be exceedingly challenging. I am getting better at understanding this actual events, so if you want more knowledge or a place to go to learn more, contact me :)

Absorption and Emission Spectra

From here, we enter to one of my favorite parts of the course ... the history of the spectral analysis of the Sun. Isaac Newton is credited with the explanation for the splitting of light into the color spectrum by a prism during the 1600's. In the 1700's, a young Austrian orphan named Joseph Fraunhofer (left) found himself alone and homeless when his orphanage collapsed. Most of the children were killed in the tragedy, and Joseph suddenly had nowhere to live. A wealthy man found the young teenager and took pity on him, offering a small bag of coins to get residence and a connection to a glass-making shop. Joseph quickly demonstrated his talents for glass-making and moved from apprentice to master status, becoming one of Europe's finest craftsmen. One of Joseph's specialties was etching. He would scrape the glass without breaking it, resulting in a mixture of clear and more opaque contrast within a piece of glass. One evening, Joseph noticed that the sunlight passing through his etched glass was broken into the corresponding color spectrum described by Newton. As he examined the "rainbow" more closely, he noticed little dark lines, barely perceptible, throughout the rainbow. Not knowing what they were or their cause, he set up a little experiment to learn more about these lines. First, he made a detailed map of the dark line locations, serving as a baseline for comparison to other glowing objects. Then, by allowing moonlight to pass through one of his etched glass pieces, Fraunhofer found dark lines in the rainbow from moonlight in the exact same locations as in the solar rainbow. When he passed telescopic light from Jupiter and Venus through the etched glass, the Jupiter and Venus rainbows had the same dark lines in the same locations as the solar and lunar rainbows. BUT, when he passed starlight from Sirius through his telescope and etched glass, he found a rainbow with dark lines, but in different positions than the solar rainbow. Fraunhofer discerned two things that evening. Moonlight, as well as light from Venus, Jupiter, and other planets are mere reflections of light from the Sun. The Moon and planets do not make their light, but reflect light from the Sun. Meanwhile, others stars must also be making their own light, but each star seems to make somewhat different forms of light.

Today, we know that these dark lines are called "absorption lines," and Fraunhofer's etched glass is called a diffraction grating. When an electron in any atom is excited up to a higher energy orbital, it absorbs a photon of a specific energy --- that energy described by the difference between the two orbitals. The Sun's outermost layers and atmosphere are made of a large variety of atoms. If enough electrons of those atoms absorb photons at a given energy, an absorption line is formed. Given enough energy, electrons can jump more than one level at a time. Fraunhofer was looking at the absorption lines created by a light source shining through a gas, with the gas atom electrons absorbing light energy from behind it. We call this spectrum an Absorption Spectrum. The particular absorption spectrum for Hydrogen is found below. The two main lines are in the red at 656.3 nm (H alpha) and in the blue at 486.1 nm (H beta). We also know now that the many other lines in the solar absorption spectrum are due to the presence of different ions of the same element and different elements burning behind the gas envelope of the Sun.

Fraunhofer did little with his absorption line work, returning to the more lucrative efforts from making glass in pretty forms. In the mid-1800's Robert Bunsen was tinkering with chemistry experiments in Germany when he made an interesting discovery. Yes, Bunsen is the same person who invented the Bunsen Burner that you all enjoy so much in Chem labs, and are repeatedly warned to never use in the presence of Ether, nor to roast tiny colored marshmallows in. Bunsen was igniting elemental gases in vacuum tubes and looking at their colors. This proved to be the beginning of "Neon Lights," for different gases glowed with different colors. Bunsen one day looked at the glowing gas through a diffraction grating and whoa! was he surprised. He did not see a nice continuous spectrum of the rainbow as expected, but instead saw individual bright lines. Every element that he burned and then looked at with his diffraction grating generated a different set of bright lines. Bunsen discovered that these bright line spectra were descriptive and unique to each element.

Today we know that when an excited electron drops to a lower energy orbital, a photon is released. The result is an emission line (seen below), and the particular emission spectrum for Hydrogen is shown below. Sending an electric current through the vacuum tube filled with the trapped elemental gas causes the electrons to jump to a higher energy level, and upon returning to the resting state, a photon is released.

Each kind of atom has a different kind of ladder with a different number of energy jumps available in different places. As a result, each kind of atom or ion has a different absorption or emission spectrum. The dark lines of an absorption spectrum can be laid atop the bright line spectrum of an element and with fit perfectly. It is therefore possible to take bright line spectra from different elements and their ionic species and discern what lies within the Sun. From this work, we know that the Sun has over 70 different elements burning as gas, including one newly discovered element never seen before on Earth. This elements unique spectrum was given the name "Helium" since it was first found in the Sun, and then later on Earth.

Below is the most detailed spectrum of the Sun as imaged from the National Optical Astronomy Observatory (NOAO) in Chile, showing as many of the absorption lines as I can imagine are there. What is so incredible to me is that scientists can sift through such a complex array of lines and discern which elements and compounds are present. I think the detail in this image is worth keeping the size of the picture as big as it is.

Okay, so what have we learned here:

  1. Every star makes its own light.

  2. Every star has a different pattern of absorption lines

  3. The absorption lines can tell us what elements are inside a given star

Is it possible that stars can be grouped according to their absorption spectra? This was the goal of Henry Draper and is the subject of the next page on the history of the Electromagnetic Spectrum. Please move forward to HR Diagram for the next part of this lesson, or you can return to the Sun Introduction, or to the Syllabus.

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