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Learning objectives

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 ( [link] ). 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.

Earth and the sun.

An image that compares the size of Earth to the Sun. A close-up of the solar surface shows a loop of hot erupted gas. Next to the loop is a small dot labeled “Earth for size comparison”. At the bottom right is an inset showing the entire 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 [link] . 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.

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 × 10 8 km
Minimum distance from Earth   1.471 × 10 8 km
Mass Orbit of Earth 333,400 Earth masses (1.99 × 10 30 kg)
Mean angular diameter Direct measure 31´59´´.3
Diameter of photosphere Angular size and distance 109.3 × Earth diameter (1.39 × 10 6 km)
Mean density Mass/volume 1.41 g/cm 3
(1400 kg/m 3 )
Gravitational acceleration at photosphere (surface gravity) GM/R 2 27.9 × Earth surface gravity = 273 m/s 2
Solar constant Instrument sensitive to radiation at all wavelengths 1370 W/m 2
Luminosity Solar constant × area of spherical surface 1 AU in radius 3.8 × 10 26 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 [link] . 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.

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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