The Solar System

CHAPTER 8 | THE SUN 147

Visual-wavelength image

Visual image

Visual image

Twisted streamers in the corona

suggest magnetic fields.

The corona extends

far from the disk.

Ultraviolet

Two nearly simultaneous images

show sunspots in the photosphere

and excited regions in the

chromosphere above the sunspots.

Sun hidden

behind mask

Sun hidden

behind mask

Background stars
■ Figure 8-5

Images of the photosphere, chro-
mosphere, and corona show the
relationships among the layers of
the sun’s atmosphere. The visual-
wavelength image shows the sun
in white light—that is, as you
would see it with your eyes. (SOHO/
ESA/NASA)

Th e corona is exceedingly hot gas. In fact, as you learned
earlier, the gas in the corona and upper chromosphere is so hot
it can emit X-rays. Nevertheless, the coronal gas is not very
bright. Its density is very low, only 10^6 atoms/cm^3 in its lower
regions. Th at is about 10^12 times less dense than the air you
breathe. In its outer layers the corona contains only 1 to 10
atoms/cm^3 , fewer than in the best vacuum in laboratories on
Earth. Because of its low density, the hot gas does not emit much
radiation.
Astronomers have wondered for years how the corona and
chromosphere can be so hot. Heat fl ows from hot regions to cool
regions, never from cool to hot. So how can the heat from the
photosphere, with a temperature of only 5800 K, fl ow out into
the much hotter chromosphere and corona? Observations made
by the SOHO satellite have mapped a magnetic carpet of
looped magnetic fi elds extending up through the photosphere
(■ Figure 8-6). Remember that the gas of the chromosphere and
corona has a very low density, so it can’t resist movement of the
magnetic fi elds. Turbulence below the photosphere seems to fl ick
the magnetic loops back and forth and whip the gas about, heat-
ing the gas. Furthermore, observations with the Hinode space-
craft have revealed magnetic waves generated by turbulence
below the photosphere traveling up into the chromosphere and

corona and heating the gas. Because the gas in the corona has

such a low density, it doesn’t take much energy fl owing outward

as agitation in the magnetic fi elds to heat the corona to high

temperature.

Ionized, low-density gas cannot cross magnetic fi elds, so

where the sun’s fi eld loops back to the surface, the gas is trapped.

However, some of the magnetic fi elds lead outward into space,

and there the gas fl ows away from the sun in a breeze called the

solar wind. Like an extension of the corona, the low-density

gases of the solar wind blow past Earth at 300 to 800 km/s with

gusts as high as 1000 km/s. Earth is bathed in the corona’s hot

breath.

Because of the solar wind, the sun is slowly losing mass, but

this is only a minor loss for an object as massive as the sun. Th e

sun loses about 10^7 tons per second, but that is only 10−14 of a

solar mass per year. Later in life, the sun, like many other stars,

will lose mass rapidly in a more powerful wind. You will see in

future chapters how this aff ects the evolution of stars.

Do other stars have chromospheres, coronae, and stellar

winds like the sun? Stars are so far away they never look like more

than points of light, but ultraviolet and X-ray observations sug-

gest that the answer is yes. Th e spectra of many stars contain

emission lines in the far-ultraviolet that could have formed only