By Robert Hazen, Ph.D., George Mason University
Astronomers have a large and an ever-growing array of land- and space-based telescopes that now peruse the heavens in all the different wavelengths, from radio waves to gamma rays. As astronomers peer into the heavens, they find that almost everything they see are stars; countless billions of stars in the process of being born, living out their lives, and dying. Observing the Sun’s structure helps astronomers understand stars better.
Hertzsprung-Russell Diagram
Stars can be characterized by their position on a Hertzsprung-Russell diagram, a diagram that plots the star’s temperature versus its total energy output. The star’s temperature is measured by its color. Red stars are relatively cool, with surface temperatures that may be no more than about 3,000 degrees, whereas the hottest stars, the blue-white stars, may have surface temperatures of tens of thousands of degrees.
The Hertzsprung-Russell diagram also plots, on the vertical scale, the total energy output of the star, and this is measured by a combination of the star’s brightness—how bright it appears to us—and our knowledge of its distance. There are ten-orders-of-magnitude difference between the brightest stars, the most intense energy-producing stars, and the dimmest stars, those with the lowest energy output.
Learn more about how stars like the Sun die, and what is left behind.
Main Sequence
One major discovery of the Hertzsprung-Russell diagram is that most stars, including our own Sun, lie on a broad band, a diagonal band which is called the main sequence. The main sequence includes all the stars that primarily burn hydrogen. We see, from the Hertzsprung-Russell diagram, that the Sun is a rather ordinary star. It lies right near the middle of the Hertzsprung-Russell diagram. There are many stars that are hotter, and many stars that are cooler. There are many stars that produce more energy, but many stars that produce a lot less energy than the Sun.
Think about the life of the Sun. Our star can’t burn forever. The mass of hydrogen in the Sun is admittedly immense, but day-by-day that mass is diminished. Someday, the Sun’s energy has to run out. Studies of the Sun, combined with observations of the life and death of many, many other distant stars, reveal the likely ultimate fate of our own solar system.
Because of its closeness to us, the Sun is by far the most thoroughly studied star.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Layers of the Sun
In cross-section, we know the Sun has several different layers. Almost all the energy produced in the Sun by fusion takes place in the core. Hydrogen burning in the core takes place in the central ten percent of the Sun’s volume, and that’s the region where the temperature and pressure is highest. Energy has to be transferred from the core through the thick layers, leading out to the surface, and this occurs primarily by radiation in the central part of the Sun.
That radiation moves from plasma, to plasma, to plasma, throughout the Sun, and it takes a long time; it could take hundreds of thousands of years for the Sun’s radiant energy to go from the core to the outer layers. In the outer 200,000 kilometers, the outer mantle of the Sun, you have energy transferred by convection. In this case, huge swirls of plasma convect in giant cells, and this is a more efficient way, a more rapid way of transferring energy from the inside of the Sun to the outside; but the total energy-transfer process can take hundreds of thousands of years.
Learn more about nuclear fission.
The Photosphere
When we look at the Sun, what we see is the photosphere. That’s only the outer 150 kilometers or so of a transparent atmosphere of the Sun. This is the zone from which the Sun’s energy radiates out into space, coming to us at 186,000 miles per second.
In fact, it only takes eight minutes or so for the Sun’s energy to reach us from the surface, but it’s taken hundreds of thousands of years to come from the center to the surface of the Sun.
The surface of the Sun is a violent place. It has magnetic storms and eruptions of plasma, and we can see those surface markings with solar telescopes. There are other layers of the Sun as well. During a total eclipse of the Sun, we can see the Sun’s hot, glowing atmosphere, which may extend more than a million miles into space.
The Solar Wind
In addition to light from the Sun, we learn about that star from its solar wind. There’s a constant stream of hydrogen and helium ions; these are positively charged atoms that have electrons that have been stripped off of them, and they stream outward from the Sun, propelled against the magnetic field of the Sun. This wind can be measured in outer space; you can also measure it at the Moon’s surface. You can actually collect particles, hydrogen and helium atoms, that have come to us from the Sun in the solar wind.
One important effect of the solar wind is that these particles interact with the Earth’s magnetic field. The intensity of the solar wind varies; at its peak, it causes the Northern Lights. The solar wind is composed of ions; these are charged particles, and these charged particles can often disrupt satellite communication and other kinds of radio communication.
Our intimate knowledge of the Sun, combined with the telescope observations of countless distant stars, allows us to model the life cycles of all the different kinds of stars.
Common Questions about the Sun and Its Structure and Layers
Because of its closeness to us, the Sun is by far the most thoroughly studied star. Not only can we see it easily, but there are several solar observatories that are dedicated entirely to studying the Sun and its properties.
The photosphere is the outer 150 kilometers or so of a transparent atmosphere of the Sun. This is the zone from which the Sun’s energy radiates out into space.
There’s a constant stream of hydrogen and helium ions streaming outward from the Sun, propelled against the magnetic field of the Sun. This is called the Solar Wind.
