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    Astronomy Lecture Number 14

    Physics 202

    Intro to Astronomy:  Lecture #14

    Prof. Dale E. Gary NJIT Our Star The Sun The Sun is a Star

    All of the thousands of stars you can see at night with the naked-eye, and all of the millions you can see faintly shining in the Milky Way, are suns, similar to ours.  Our galaxy contains 100 billion suns, and most are ordinary stars very similar to our Sun.  So what we learn from studying the Sun also applies to all of these ordinary stars.  Since nearly every other star can be seen only as a mere point of light, we can see how important it is to have one star close enough to resolve and study in detail.

    We went into quite some detail talking about the birth of the solar system from a collapsing cloud, examining the phenomena taking place in the outer reaches of the solar nebula, but we glossed over what was happening in the central part that ultimately became the Sun.  We will revisit these ideas again when we talk about star birth in general, but here are the main events that formed the Sun from the center of the cloud:

    As the outer solar nebula became a disk from which the planets formed, the central part formed a spherical object, where collisions between particles initiated .  The text calls this --it is when the inner pull of gravity is balanced by the outward pressure of the hot, dense cloud.  From this moment on, the inner part remained in exact gravitational equilibrium, as it does today.

    This inner object, made of the same stuff as the rest of the solar nebula (73% hydrogen gas, 26% helium gas, and 1% heavier elements), was very hot, heated by the release of gravitational potential energy into kinetic energy, which in turn became heat (remember that heat is just randomized motions).

    In order to shrink further, the object had to cool, which it did by emitting infrared light.

    As the object continued to shrink, the very inner part became opaque to infrared light and heated up more and more until the central part reached about 15 million K.

    At this moment, the temperature was high enough to start , which converts hydrogen to helium.  .  Before that it is called a .

    The tremendous energy release halted the contraction of the Sun, and started a period of strong solar wind, which swept the solar system clean of gas and dust.  This marked the end of the formation of the planets, except for the coalescing of the larger pieces during the era of bombardment.

    All stars form in basically the same way, and so all stars have the same basic structure.  The power output of the Sun, called the solar luminosity, is constant at 3.8 x 1026 watts.  This is an incredible amount of power--even at the distance of the Earth we receive 1370 watts/m2, meaning that we could power about 14 lightbulbs of 100 W each, for every square meter of area.

    The Structure of the Sun

    Let's take a look at the structure of the Sun starting from the outside, from Earth, and moving inward toward its center.  Here are the regions of the Sun that we will visit:

    Name Temperature

    Distance from center


    Solar Wind Variable >100 AU 400-800 km/s

    Corona 3 million K 0.15 AU (30 Rsun) Highly dynamic loops

    Chromosphere 10,000 K 2000 km above surface Filaments, Plage

    Photosphere 6,000 K Surface (1 Rsun) Sunspots, Granulation

    Convection Zone 100,000 to several million K 0.7 to 1 Rsun Upward and downward motions

    Radiation Zone 10 million K 0.2 to 0.7 Rsun Energy carried by photons

    Core 15 million K 0 to 0.2 Rsun Region of nuclear fusion

    Solar Wind: Even at the distance of the Earth we can be said to be inside the Sun.  Particles and magnetic fields from the Sun make up the solar wind, which blows past the Earth at speeds from 400-800 km/s (about 1 million mph!).

    Corona: The source of the solar wind is the solar corona, which is the tenuous outer atmosphere of the Sun.  The is very hot (several million K), and so it shines in X-rays and ultraviolet light, but also can be seen in visible light during an eclipse (it is too faint to see otherwise).  The corona is highly dynamic, and looks different every time you look at it.

    TRACE image of coronal loops in the UV

    Chromosphere: This is a relatively thin layer that shows as pink during a solar eclipse, hence the name chromosphere, which means color sphere.  Some of the strongest spectral lines from the Sun come from the chromosphere, and we can take pictures using the light in one of these lines.  NJIT runs the Big Bear Solar Observatory, where we can see the latest images in the hydrogen alpha line.  Here we see a number of features of the solar atmosphere, including filaments (dark clouds of hydrogen gas suspended above the surface) and prominences (same as filaments, but sticking out above the limb of the Sun) , sunspots (dark spots), and plage (bright areas around sunspots).

    Photosphere: This is the visible surface of the Sun.  It is called the photosphere because this is the point of the last scattering of photons before they escape directly into space.  It has a remarkably uniform temperature of about 6000 K, except in small spots called that come and go with time.  The sunspots are regions of high magnetic field, which inhibit the flow of heat from the hotter surroundings, so they are cooler (about 4000 K).  This is why they appear dark.  In fact, they only appear dark because they are seen against the brighter, hotter surroundings.  If you could see them in isolation, they would glow brightly, and would be reddish in color.  In fact, many stars have surfaces about 4000 K or less, and they emit plenty of light.  If you look closely at the photosphere away from sunspots, you can see a fine convection pattern called . Click here for a movie.

    Source : web.njit.edu

    The Structure and Composition of the Sun

    The Structure and Composition of the Sun


    By the end of this section, you will be able to:

    Explain how the composition of the Sun differs from that of Earth

    Describe the various layers of the Sun and their functions

    Explain what happens in the different parts of the Sun’s atmosphere

    The Sun, like all stars, is an enormous ball of extremely hot, largely ionized gas, shining under its own power. And we do mean enormous. The Sun could fit 109 Earths side-by-side across its diameter, and it has enough volume (takes up enough space) to hold about 1.3 million Earths.

    The Sun does not have a solid surface or continents like Earth, nor does it have a solid core (Figure 1). However, it does have a lot of structure and can be discussed as a series of layers, not unlike an onion. In this section, we describe the huge changes that occur in the Sun’s extensive interior and atmosphere, and the dynamic and violent eruptions that occur daily in its outer layers.

    Figure 1. Earth and the Sun: Here, Earth is shown to scale with part of the Sun and a giant loop of hot gas erupting from its surface. The inset shows the entire Sun, smaller. (credit: modification of work by SOHO/EIT/ESA)

    Some of the basic characteristics of the Sun are listed in Table 1. Although some of the terms in that table may be unfamiliar to you right now, you will get to know them as you read further.

    Table 1. Characteristics of the Sun

    Characteristic How Found Value

    Mean distance Radar reflection from planets 1 AU (149,597,892 km)

    Maximum distance from Earth 1.521 × 108 km

    Minimum distance from Earth 1.471 × 108 km

    Mass Orbit of Earth 333,400 Earth masses (1.99 × 1030 kg)

    Mean angular diameter Direct measure 31´59´´.3

    Diameter of photosphere Angular size and distance 109.3 × Earth diameter (1.39 × 106 km)

    Mean density Mass/volume 1.41 g/cm3

    (1400 kg/m3)

    Gravitational acceleration at photosphere (surface gravity) GM/R2 27.9 × Earth surface gravity = 273 m/s2

    Solar constant Instrument sensitive to radiation at all wavelengths 1370 W/m2

    Luminosity Solar constant × area of spherical surface 1 AU in radius 3.8 × 1026 W

    Spectral class Spectrum G2V

    Effective temperature Derived from luminosity and radius of the Sun 5800 K

    Rotation period at equator Sunspots and Doppler shift in spectra taken at the edge of the Sun 24 days 16 hours

    Inclination of equator to ecliptic Motions of sunspots 7°10´.5

    Composition of the Sun’s Atmosphere

    Let’s begin by asking what the solar atmosphere is made of. As explained in Radiation and Spectra, we can use a star’s absorption line spectrum to determine what elements are present. It turns out that the Sun contains the same elements as Earth but not in the same proportions. About 73% of the Sun’s mass is hydrogen, and another 25% is helium. All the other chemical elements (including those we know and love in our own bodies, such as carbon, oxygen, and nitrogen) make up only 2% of our star. The 10 most abundant gases in the Sun’s visible surface layer are listed in Table 1. Examine that table and notice that the composition of the Sun’s outer layer is very different from Earth’s crust, where we live. (In our planet’s crust, the three most abundant elements are oxygen, silicon, and aluminum.) Although not like our planet’s, the makeup of the Sun is quite typical of stars in general.

    Table 1. The Abundance of Elements in the Sun

    Element Percentage by Number of Atoms Percentage By Mass

    Hydrogen 92.0 73.4 Helium 7.8 25.0 Carbon 0.02 0.20 Nitrogen 0.008 0.09 Oxygen 0.06 0.80 Neon 0.01 0.16

    Magnesium 0.003 0.06

    Silicon 0.004 0.09 Sulfur 0.002 0.05 Iron 0.003 0.14

    Figure 2. Cecilia Payne-Gaposchkin (1900–1979): Her 1925 doctoral thesis laid the foundations for understanding the composition of the Sun and the stars. Yet, being a woman, she was not given a formal appointment at Harvard, where she worked, until 1938 and was not appointed a professor until 1956. (credit: Smithsonian Institution)

    The fact that our Sun and the stars all have similar compositions and are made up of mostly hydrogen and helium was first shown in a brilliant thesis in 1925 by Cecilia Payne-Gaposchkin, the first woman to get a PhD in astronomy in the United States (Figure 2). However, the idea that the simplest light gases—hydrogen and helium—were the most abundant elements in stars was so unexpected and so shocking that she assumed her analysis of the data must be wrong. At the time, she wrote, “The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real.” Even scientists sometimes find it hard to accept new ideas that do not agree with what everyone “knows” to be right.

    Before Payne-Gaposchkin’s work, everyone assumed that the composition of the Sun and stars would be much like that of Earth. It was 3 years after her thesis that other studies proved beyond a doubt that the enormous abundance of hydrogen and helium in the Sun is indeed real. (And, as we will see, the composition of the Sun and the stars is much more typical of the makeup of the universe than the odd concentration of heavier elements that characterizes our planet.)

    Source : courses.lumenlearning.com

    Convection Zone of the Sun Overview & Process

    Learn about the convection zone of the sun and understand what happens to energy in the sun's convection zone. Discover how hot is the center of...

    Science Courses / Course / Chapter

    Convection Zone of the Sun Overview & Process

    Marian Fuchs, Artem Cheprasov

    Learn about the convection zone of the sun and understand what happens to energy in the sun's convection zone. Discover how hot is the center of the sun. Updated: 03/05/2022

    Table of Contents

    How is Energy Transported in the Sun?

    Radiative Zone of The Sun

    Convection Zone of The Sun

    Lesson Summary Create an account

    How is Energy Transported in the Sun?

    The sun is the star that holds our solar system together. The gravitational pull of the sun controls the orbits and locations of the planets, including Earth. The sun appears significantly larger and brighter to us because the sun is much closer than any other star in the sky. Made up of different layers, the sun is diverse in temperature, movement of photons, and other factors.

    How hot is the center of the sun? The center of the sun, the core, burns at approximately 27 million degrees Fahrenheit (15 million degrees Celsius). The core is by far the hottest layer of the sun, while the surface of the sun, the photosphere, is 10,500°F (58,000°C).

    The energy of the sun travels from the core outward. Nuclear fusion leads to the production of an immense amount of heat and energy. The energy is then carried towards the surface and beyond by photons.

    The innermost core of the sun is surrounded by two layers, making up the internal structure of the sun. These two layers are the radiative zone and the convection zone of the sun. In this lesson, we will focus on these two key layers.

    Zones of the sun including the core, radiative zone, and convection zone

    Quiz Course 6.1K views

    Radiative Zone of The Sun

    In the sun's center sits the core, this is where the sun's energy comes from, through the process of nuclear fusion. So, what happens in the core of the sun for that energy to be created? How does the sun get its energy? In the sun's core, tremendous pressure is applied to hydrogen atoms that fuse, resulting in the creation of helium. This is what leads to the immense amount of energy and heat being released in the core.

    The radiative zone is the zone closest to the core, but has a dramatic change in temperature. The zone sits at about 12 million degrees Fahrenheit (7 million degrees Celsius) closest to the core, and drops to approximately 4 million degrees Fahrenheit (2 million degrees Celsius) at its farthest edge. This zone is important in the transport of photons traveling from the sun's core. Photons are very small particles that travel at the speed of light and are made up of electromagnetic radiation and waves. The photons enter the radiative zone through radiative diffusion, which is where the zone gets its name. The photons will end up bouncing around for thousands of years before moving outward to the convection zone. Due to the fact that energy is transported through gamma-ray (light) photons, it is considered electromagnetic radiation. This bouncing is caused because the radiative zone is so dense that the photons are absorbed by other particles and then retransmitted. The photons travel an indirect route, which slows down their travel time to the convection zone.

    Convection Zone of The Sun

    As the layer surrounding the radiative zone, the convection zone of the sun is the outermost layer of the sun's interior zones. It is 120,000 miles (200,000km) thick and its outer edge sits at the sun's outer visible edge. The convection layer of the sun is 4 million degrees Fahrenheit (2 million degrees Celsius) and energy moves slightly differently here than in the radiative zone.

    So, what happens to the energy in the sun's convection zone? In the convection zone, certain heavier ions, like oxygen, iron, calcium, nitrogen, and carbon, are able to keep electrons due to the "cooler" temperature. Due to this, the ions give the zone a more cloudy or opaque appearance and block most radiation from the radiative zone. It also causes the heat to be trapped in the convection zone. The heating of the plasma (gas) occurs at the deeper area of the convection zone next to the radiative zone, while the outer edge is cooler.

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    Frequently Asked Questions

    What is the temperature of the convection zone of the Sun?

    The convection layer of the sun is 4 million degrees Fahrenheit (2 million degrees Celsius) at its base. This cooler temperature allows heavier ions to hold onto electrons.

    What happens in the convection zone of the Sun?

    In the convection zone, photons are transferred to the photosphere through convection currents. The gases are heated near the radiative zone and are lighter than the gases near the cooler photosphere. The heated gas rises and the cooler gases sink, creating convection currents.

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