The nebular theory, first proposed by German philosopher Immanuel Kant (1724-1804), says that our Sun and its planets formed together from a rotating cloud of interstellar gas and dust called the solar nebula about 5 billion years ago.

The solar nebula condensed into the newly forming Sun encircled by a rotating disk of gas and dust out of which the planets, moons, and other solar system objects formed (Figure 4.4). The Sun has more than 99 percent of the mass of the solar system and provides the gravitational force that keeps the planets circling it. Its surface gravity is practically 28 times Earth's.






Figure 4.4. Solar nebula theory. (a) A rotating nebula condensed to our Sun surrounded by a contracting disk where (b) the planetary system was born.

More than 70 chemical elements have been identified in the Sun's spectrum. The Sun's outer layers likely have the same chemical composition as the Sun had at birth: about 71 percent hydrogen, 27 percent helium, and 2 percent other elements by weight. The Sun's core probably has subsequently changed to about 38 percent helium in nuclear fusion reactions.

Why do astronomers expect to find many other stars that have planets circling around them?_

Answer: The nebular theory says that the planets circling the Sun were born together with their star. Since the Sun is a typical star, it seems likely that many other stars were also born together with a family of planets.

Our picture of the structure of the Sun (and other stars) comes from direct observations of its outer layers plus indirect theoretical calculations of the behavior of gases deep inside that we cannot see.

The three outer layers are called the Sun's atmosphere.

The photosphere, from the Greek "light ball," is the visible surface of the Sun. The photosphere is a hot, 500-km (300-mile), opaque gas layer about 5800 K (10,000° F) from which energy is radiated into space. The limb is the apparent edge of the Sun's disk. It looks darker than the center, an effect called limb darkening, because light from the limb comes from higher, cooler regions of the photosphere.

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