Читать книгу Exploring the Solar System - Peter Bond - Страница 38

The Sun is one of more than 100 billion stars in the barred spiral galaxy that we call the Galaxy, or, more commonly, the Milky Way (Figure 2.1). By mapping the distribution of stars in the sky, astronomers have found that it is located in the Orion Arm, a small, partial arm located between the Sagittarius and Perseus arms of our Galaxy.

The Sun lies in the main disk of the Galaxy, about 28,000 light years from its center. This is quite a desirable location for a planetary system that supports life, far from the excessive gravitational disturbances and intense radiation that exist nearer the densely populated center. As the Galaxy rotates, it takes the Sun about 225 million years to complete one circuit, traveling at a velocity of 220 km/s. This amounts to about 20 trips over the course of its 4.5 billion year lifetime.

The Sun is the central star of our Solar System and by far the largest single object in the System. With a diameter of approximately 1.4 million km, it is more than 100 times wider than Earth. 1.3 million Earths would fit inside the Sun. Indeed, all of the planets combined could easily be swallowed by its enormous sphere. However, its size is modest compared with giant stars such as Betelgeuse in Orion, whose vast bulk would extend all the way to the orbit of Jupiter if it swapped places with the Sun.

The Sun's rotation was first detected by observing the motion of sunspots in the photosphere (Figure 2.2). Evidence for its gaseous composition is provided by different rotation rates at different latitudes – on solid bodies, such as rocky planets, all parts of the surface have the same period of rotation. At the equator, the Sun spins on its axis once every 25 days, but this increases to more than 30 days at the poles. The cause of this differential rotation is an important area of current research.

Figure 2.1 The Milky Way is a spiral galaxy with a bar‐shaped mass of stars at the center. The Sun lies near a small, partial arm called the Orion Arm, or Orion Spur, about 28,000 light years from the center – a little over halfway towards the outer edge.

(ESO)

Figure 2.2 Since the Sun is a ball of gas/plasma, it does not rotate rigidly like the rocky planets. The rate of rotation generally decreases toward the poles. The Sun's equatorial regions rotate once every 25 days, compared with more than 30 days in the polar regions. This differential rotation was first detected by observing the motions of sunspots. The Sun's rotation axis is tilted by about 7.25 degrees from the axis of the Earth's orbit, so we see more of the Sun's north pole each September and more of its south pole in March.

(NASA)

Table 2.1 The Sun

Diameter	1,392,000 km = 109 Earth diameters
Mass	1.988,500 x 1024 kg = 333,000 Earth masses
Volume	1.412 x 1033 cm3 = 1.3 million Earths
Density	1.408 g/cm3 (water = 1)
Temperature at the surface	5,772 K
Temperature in the corona	> 2,000,000 K
Temperature at the center	15,500,000 K
Luminosity (energy output)	= 3.846 × 1033 erg/s
Absolute magnitude	4.79
Apparent magnitude	–26.78
Rotation time at equator	25 days
Rotation time at 60° latitude	29 days
Photosphere composition	hydrogen (90.965%), helium (8.889%)

The Sun's rotation axis is tilted about 7.25° to the pole of the ecliptic, so we see more of the solar north pole each September and more of its south pole in March. Its gaseous composition and low density result in a mass “only” 333,000 times that of Earth.

Figure 2.3 A SOHO image of a huge prominence rising from the solar disk, with Earth shown to scale. 1.3 million Earths would fit inside the Sun.

(ESA/NASA)

The temperature of the Sun's visible surface (the photosphere) is about 5,500°C, but about 2,000 km above this region is the much hotter corona – the Sun's outer atmosphere – which is usually only visible during a total solar eclipse. The temperature of the gas in the corona exceeds 2 million degrees Celsius. However, even this figure is dwarfed by solar flares, sudden releases of energy whose temperature typically reaches 10 or 20 million degrees Celsius, occasionally soaring to 100 million degrees.