All stars on the Main Sequence fuse hydrogen in their core (center) using the steps you learned about in the previous lesson. The energy produced is rapidly converted to high energy light - gamma rays. These gamma rays must now make their way to the surface of the star and that isn't easy. A star's interior is so dense that even light has a hard time getting out!
Surrounding a star's core is an area called the radiative zone. This zone is dense with ions and energy but there isn't enough heat or pressure to cause nuclear fusion. Instead, the matter in the radiative zone just gets in the way.
Each photon (packet of light) produced in the core moves into
the radiative zone where it is absorbed, re-emitted, deflected
and generally gets bounced around a lot. The movement of each
photon through the radiative zone produces a pattern called a
"random walk" which means that the photon bounces around
aimlessly but eventually moves upwards away from the core and
towards the surface. With each collision, the photon loses and
redistributes its energy so it becomes "cooler" and
less energetic. The reason this is called the radiative zone is
because the energy (photons) transmitted by this process of collision,
absorption and re-emission is said to be moving by "radiative
means".
This random walk through a star's radiative zone can take around
200,000 years! That's right. Light, the fastest thing in the universe, takes thousands of years to get through a star's radiative zone! That's because the random walk is not at all straight. Imagine you had a very fast car but no map, compass or directions, and you had to get to another house far away. It would take you a long time to get there because you would be searching randomly and traveling in a "random walk". Your speed helps you get there but directions would be useful too. Random walks have no direction. | ![]() |
![]() | Eventually the photon works through the radiative zone and enters the convection zone. Things here are very different. The matter in the convection zone is less dense - but still pretty dense by most standards. Here the energy's path gets organised into columns of moving materials. The photons, along with the materials in the convection zone, quickly move towards the star's surface along the columns. These columns are called "convection cells" and they are similar to the movement of water in a heated sauce pan. The heat from below warms the bottom of each convection cell and that causes the materials there to rise towards the surface. When the material reaches the surface (or pretty close) it loses its energy to the "outside". The material is now cooler so it sinks back to the bottom. This creates circular (actually loop-shaped) movements of materials that act to move the energy from bottom to top. Convection is the movement of matter along with the energy, as in a convection cell. |
The energy from the convection zone "bubbles" to the star's surface.
That is the surface layer of the Sun and it's only 300 kilometers thick, far too thin to be seen in the diagram I have drawn. The Sun is so close that we can actually see details of the convection cells as they poke through the photosphere but, of course, you should NEVER stare at the Sun. [It takes special equipment to observe the Sun.]
The photosphere is the star's "surface" and the
only part we can actually see. The photosphere gives the star its color (due
to its temperature as you learned last month in your lesson about star colors and temperatures). The photosphere provides us with information about
what the star is made of and clues as to what is going on inside the star.
You can stare at the photospheres of any other star OTHER THAN
the Sun and enjoy the view without fear. Even Sirius
is safe to stare at.
OK. But how does the light from a star tell us what it's made of?
Good question. You already know that a star's color indicates its temperature, but it might surprise you to know that hidden within the color is a pattern of lines that tells us what atoms, ions and molecules are at the star's surface. As with most astronomy, this information has been pieced together using a knowledge of physics, experiments in the laboratory and careful observation of our closest star, the Sun.
For centuries scientists have known that light is bent, or refracted, as it passes from one clear medium (air, for example) to another (glass or water, for example). By using specially formed pieces of glass called lenses this effect can be used to magnify an object. This knowledge lead to the production of light microscopes and refraction telescopes. Refraction telescopes were the first kind of telescopes. (You learned all about this in your lesson on optical devices.) The ability of the craftsmen to make good lenses allowed great advancements in all areas of science.
Scientists also learned that sunlight (or "white" light, as it is sometimes called)
is made of a mixture of colors which can be separated by refracting
the light through a prism.
A prism is a triangular piece of glass (not at all like a lens) and its shape splits the sunlight into a band of colors called a spectrum. Strictly speaking, the word "spectrum" can be used for the distribution of anything arranged in order of increasing or decreasing magnitude. This spectrum is produced by the orderly arrangement of light in decreasing (or increasing) wavelength (or energy). | ![]() |
[I mentioned prisms in your lesson on optical devices but in those instruments the prisms are used differently. In scopes and binoculars, prisms are used to invert an image. The prisms I am now talking about do NOT have the property of "total internal refection" that is used in those instruments, and the geometry is also very different. Try not to let yourself get confused about that. The point is - we can use the physics of prisms differently.]
In the case of sunlight's "color spectrum" the colors are produced by the distribution of the wavelengths of the individual packets (photons) of light. The wavelength of light produces the light's color. (More correctly - we have evolved color sensors that detect the different wavelengths of light and our brain interprets them as color.)
We measure the wavelength as the distance between two identical points in the wave, such as from crest to crest. For visible light these distances are tiny and measured in angstroms. One angstrom is a ten billionth of a meter, so an angstrom is very small. A human hair is roughly 500,000 angstroms in diameter! Red light has a wavelength of around 7000 angstroms. Blue light has a wavelength of around 4500 angstroms. [These are rough estimates. There is no agreed wavelength for each color and different books give slightly different values.] The shortest visible wavelengths, blue and violet, are refracted the most during their passage through the prism, while the longest wavelengths, red and orange, are refracted the least. | ![]() |
In 1666 the great English scientist, Sir Isaac Newton, published
his detailed work on the math and physics of light, lenses, prisms
and the spectrum. At that time he thought sunlight produced a
continuous spectrum, without interruptions. In 1802 another Englishman
named Wollaston, using a much higher quality prism than Newton,
discovered that the Sun's spectrum was interrupted by dark lines!
In 1814 the German physicist, Josef Fraunhofer, examined these bands in detail and noted that their positions in the spectrum never changed regardless of the cloudiness or angle of the Sun (although the total brightness of the spectrum changed). | ![]() |
He mapped the position of 324 dark bands in the Sun's spectrum but he had no idea what they were.
Modern day chemistry and physics have advanced to the point where we can explain the Fraunhofer lines using an important technique called quantum mechanics, but we don't need that level of detail to understand the lines. In 1859 two German scientists, Gustav Kirchhoff and Robert Bunsen, explained that the Fraunhofer lines were produced by materials near the surface of the Sun.
I don't understand how that could be possible!
Let's go through this one step at a time using modern science to help us understand it.
Bunsen showed that certain elements, when heated to high temperatures
(in his "Bunsen burner"), gave off specific colors.
Using a prism, Bunsen found that each element produced an emission spectrum, a series of very specific wavelengths of light. For example, when sodium gas is heated it produces a yellow light. By passing the light through a prism, scientists discovered that the yellow color is made of two specific bands of yellowish light - one band has a wavelength of 5896 angstroms and the other has a wavelength of 5890 angstroms. | ![]() |
Modern day chemistry (quantum mechanics) explains that these emissions occur because "excited" electrons, heated by the flame, jump from one atomic shell to another and release photons each time they jump back down to a lower shell. The wavelength (color of the photon) produced, depends upon the energy lost as they jump down. These energy jumps are described as "quantum", meaning they come only in specific units. You can go down stairs one step at a time or even two steps at a time but you cannot go down stairs half a step at a time. The same is true of electrons moving among atomic shells. Each element has specific energies (distances) between atomic shells (stairs) so each element gives off specific wavelengths of light when heated in a flame. Therefore, each element has a specific emission spectrum "fingerprint".
This technique allows chemists to identify what elements are present in any sample by heating it and observing the emission spectrum produced.
If an atom is heated too much it starts to lose electrons and becomes an ion. Each atom (element) becomes an ion at high enough temperatures. [The temperature at which an atom becomes an ion depends on each element's atomic structure and is beyond the scope of this course.] Ions, like atoms, have specific energy levels available for electrons jumping between shells. Some emission bands are produced by atoms and other bands are produced by ions. By measuring the position of the band we know not only the element but also if it is ionized (has gained or lost electrons).
So what? You can't collect star stuff and heat it in a burner!
You're right.
Kirchhoff took Bunsen's ideas and arranged his thinking to look
at it another way. He asked himself, "What would happen
if you shined a complete spectrum of light through a gas of a
heated element?"
Well, the electrons would jump UP the shells (up the stairs) as they absorbed a photon of the correct wavelength. You cannot jump up stairs by fractions of a stair so the jumps up must be like the jumps down - quantum - in discrete units. Indeed, if you shine a complete spectrum of light through hot sodium gas all of the spectrum passes cleanly through it except two wavelengths - 5890 angstroms and 5896 angstroms - the same wavelengths that are emitted. This is an absorption spectrum, the opposite of an emission spectrum, and it represents the photons removed from the continuous spectrum by the atoms and ions in its path. | ![]() |
Other scientists went on to show that the energy absorbed as the electrons are pushed up to a higher shell is emitted later, at the same specific wavelengths of the absorbed photons. But only a small number of photons are released in the same direction as the unabsorbed photons. Most are sent in another direction so the absorbed wavelength does not pass through completely unaffected. The overall effect is that the continuous spectrum is interrupted by dark bands (actually dim bands against a bright spectrum) and each band represents the wavelength of an absorbed photon.
The Sun produces a complete spectrum from its photosphere
but above the photosphere lies an "atmosphere" of low
pressure gasses called a chromosphere.
As the continuous spectrum passes through the chromosphere, photons of the correct wavelength (energy) are absorbed by specific atoms and ions. Most of this energy is re-emitted in a different direction so the final spectrum we see is an absorption spectrum of the Sun full of Fraunhofer lines. | ![]() |
Astronomers analyzing the Fraunhofer lines have found there is a great deal of absorption at wavelengths of 6563, 4861 and 4102 angstroms. These correspond to hydrogen's spectrum. Careful analysis of the amount of light absorbed at these wavelengths allows astronomers to determine that 71% of the chromosphere is made of hydrogen. Further analysis has shown that all the elements can be found in the Sun and we even know the proportions of each by carefully measuring the amount of darkness in the Fraunhofer lines.
Stars undergo a mixing effect due to the convection cells so that whatever is in their chromosphere is (probably) representative of the elements found much deeper inside the star.
Spectroscopy, the use of a spectrum to analyse something, is a powerful technique. It is used throughout astronomy, chemistry and physics. We will be using it to explain many phenomena in the rest of this course so be sure you understand what I have just explained.
Let me give you an example of how spectroscopy has influenced science.
The British astronomer Lockyer did a great deal of work with Fraunhofer lines and in 1868 he found a single band (near the sodium lines) which did not correspond to any known substance. He was so confident in his analysis and observations that he boldly proclaimed to have discovered a new element that was not found on Earth! He called it "helium" in honor of the Greek Sun god "Helios". In 1894 a British Chemist (William Ramsey) discovered helium exists on Earth. He isolated it from the break down of uranium and found traces of helium in the Earth's atmosphere. When he heated helium it emitted a yellowish light that was at exactly the wavelength that Lockyer had predicted 26 years before!
Hmm, that's interesting. But can we use spectroscopy to study stars? They're so dim - compared to the Sun.
Well, we can use telescopes to collect more light (photons) from a star and pass it through a prism. Very long exposures will produce an absorption spectrum of a star. Modern instruments (such as charged coupled devices, diffraction gratings, etc.) now allow us to collect very high quality spectrums from very dim stars and even distant galaxies! The science of spectroscopy now extends across the entire spectrum so we can collect valuable information at invisible wavelengths and, with the help of space technology, we can even collect wavelengths that will not pass through the Earth's atmosphere. Today, spectroscopy is, in my opinion, the most important technique in astronomy!
By carefully studying the emission and absorption spectrums of
atoms and ions at various temperatures in the laboratory, astronomers
are able to predict the temperature of the chromosphere (surface) of stars
including the Sun. Indeed, the absorption lines produced
by stars is a far better gauge of their temperature than the overall
color. And it tells astronomers the composition of the star too! As you might guess, most of the spectrum is dominated by hydrogen but there are other lines that tell us about the star.
O-type stars are the hottest (normal) stars and their spectrum
shows a great deal of ionized helium lines.
B-type stars are cooler with absorption lines for normal (not
ionized, just "neutral") helium.
A-type stars produce an absorption spectrum showing normal (not
ionized, just "neutral")
hydrogen lines.
F-type stars show absorption lines caused by several metal ions
and very weak hydrogen lines.
G-type stars (like our Sun) produce an absorption spectrum
showing many lines from ionized calcium.
K-type stars are cooler and their absorption spectrums show lots of neutral metallic lines and some bands caused by absorption
of certain molecules. Note that molecules, and the bonds that
hold them together, can only occur at fairly low temperatures,
so these stars are pretty cool. They are the orange-red ones.
M-type stars are the coolest stars and their absorption spectrums
have lots of molecular bands.
Spectroscopy allows astronomers to learn the temperature and composition
of any star without ever visiting it. Astronomy is the science of getting the most information from the least amount of data! After all, a point of light is all an astronomer has to work with! Thanks to spectroscopy we know a great deal about the stars and the universe. We will come back to this important technique again and again throughout this course so please make an effort to understand spectroscopy.
That's our lessons for this month. This has been an "astrophysics month" for you and I appreciate that some off this information might have frightened you. Please don't be discouraged. There was a lot of detail in this month's lessons but (you may be happy to know) I won't be going into the math and physics so deeply in the future. However, make an effort to understand what I have explained here this month.
See you next month.
Wishing you "Clear Skies".
Jamie (Dr Love)