The Sun: Our Star - The Sun is an ordinary star and shines the same way other stars do.

The Sun: Our Star

The Sun is an ordinary star and shines the same way other stars do.

Announcements

qHomework # 4 is due today!

Assigned Reading
q Units 49 and 51

Today’s Goals
q Today we start section 3 of our Course: STARS.

qWe begin by investigating the closest star to us:
the Sun:
   qThe Sun

q The Sun provides virtually all of the energy
necessary for life on Earth: food, weather, etc.
q Our main experience with the Sun is through the
heat and light it provides (`sunshine ), and visually,
as a yellow/whit-ish sphere in the sky

The Sun’s ID
vRadius: 700,000 km (109 times the Earth)
vMass: 2 x 1030 kg (330,000 times the Earth)
vLuminosity: 4 1026 W

v Average density: 1.4 kg/l (a little over water); however:
   v Central density: 160 kg/l (20 times the density of
   steel)
   v Photospheric density: ~1/3000 that of air (and
   about 1/2,400,000 that of water)

v Surface Temperature: 5,800 K
v Central Temperature: 15,000,000 K

The Sun is hotter towards the
            center:
  the limb is slightly darker

The Solar Constant
n   The S.C tells us the amount of energy the Sun
     produces:
     • It is 1360 Joules per second per square meter…
     • …or about the luminosity (=ENERGY PER SECOND) of 60
       W bulb every square millimeter.

n   A stable change of ~1% in the S.C. would change
     Earth’s temperature by 1-2 degrees
n   Random variation ~0.1% over days or weeks
n   Long-term, cyclical variation 0.018% per year
     (period longer than the 22 years of the sunspots)
n   Small random fluctuations do not affect Earth’s
     climate

What is the Sun?
n   The Sun is a star, a fairly common and
     typical star

n   A star is a ball of gas held in hydrostatic
     equilibrium against its own self-gravity by
     the thermal pressure of the hot gas,
     cooling from center to the surface
n   This energy comes from nuclear fusion
     reactions at the center of the Sun (we will
     expand on this in the next lecture)

What is the Sun made of
 It is made of hot gas; there is no solid material in the
                           sun!
The gas is mostly hydrogen (71%) and helium (~27%),
          plus traces of heavier elements (2%)

   Why does the Sun have such a sharp edge?

Because it is made of dense gas; it takes 100,000 years
  for a photon to travel from the Sun’s center to the
  photosphere (and only 8.3 minutes to come to us!).

               Hot+dense gas = plasma

What is that Yellow-ish Sphere
The bright part normally
seen is called the
photosphere, which is
about 500 km deep.

  It is an almost perfect
black body with a surface
 temperature of 5800 K.

Structure of the Sun

The Surface and Atmosphere of
               the Sun
Although the sun appears to have sharp edge, its
“surface” (Photosphere) actually has a complicated
    structure.
Its atmosphere is complex, too.

      Name        Temperature       Spectrum
"   Photosphere: 5,800 K,        continuum + abs.
    Chromosphere: 4,500-500,000 K,     emiss.+abs.
"   Corona:     1-2 106 K, continuum+emiss.+abs.

"   Chromosphere and Corona are the `Atmosphere

Corona (1-2,000,000 K)
Each square millimeter of the
Photosphere emits the same
Energy as a ~60 Watt light bulb
                                                       spicules
The Sun surface (photosphere)
would cool off very quickly if Energy
were not produced continuously from
below

               (4500-500,000 K)            (5,800 K)
The light we see originates from the thin photosphere

Spectrum of the Photosphere

n   The photosphere: the region where the
     black body radiation at 5,800 Kthat we
     see is made
n   Thin gas: about 3,000 times thinner
     than air (you need to travel about
     7,000 km into the Sun to find a density
     similar to that of air)
n   Its upper layer produces the absorption
     spectrum (Fraunohfer spectrum)

Fraunhofer Spectrum

The Photosphere is NOT Perfect:
    Granulation of the Photosphere

Each Granule is about the size of Texas and lasts for only
5-10 minutes before fading away! It is due to the convection
zone

Granules: convective cells
n   The granules are just densely packed
     convective cells
n   Convective cells are very similar to
     thunderstorms

Sunspots
     Sunspots, about 1000-1500 K
     cooler than the rest of the Sun's
     photosphere, appear as dark
     spots. Size range ~500-5,000 km

     Sunspots come and go with
     time. Big sunspots can live for
     several weeks.

     They are `magnetic hurricanes ,
     regions with magnetic fields
     ~100 times stronger than the rest
     of the Sun.

Summary of Photosphere
n   It is the `surface’ of the Sun, where most of the
     light we observe is coming from
n   It is an almost perfect blackbody with
     temperature 5,800 K
n   The photosphere shows `granularity’; the
     granules are convective cells of cooling gas
n   Sun spots are magnetic disturbances (`magnetic
     hurricanes’) on the surface of the Sun that can
     live up to several weeks; they are regions about
     1,000-1,500 K cooler than the surrounding
     regions (hence they appear to be as dark spots).

The Atmosphere of the Sun

Atmosphere of the Sun:

      Name        Temperature       Spectrum
"   Chromosphere: 4,500-500,000 K,   emiss.+abs.
"   Corona:     1-2 106 K, continuum+emiss.+abs.

Chromosphere
          Reddish in color, which
          "

          is the origin of its name
          (chromos meaning
          ``color'')
          2000-3000 km thick
          "

          Faint relative to the
          "

          photosphere
          From 4,500 up to
          "

          500,000K: hotter than the
          photosphere and much
          less dense.
          Contains thin columns of
          "

          gas called spicules

Solar Prominence
          Composed of hot gas
          trapped in magnetic
          fields extending from
          one sunspot to another.

          They jut from the
          lower chromosphere
          into the corona.

Prominence

UltraViolet (UV) image of the Sun.

It shows that regions of the
Photosphere corresponding to
Sunspots
are more energetic than
elsewhere.

It also shows magnetic
links (prominences) between
Sunspots
extending into the
Chromosphere and Corona

Solar Flare
 A solar flare is a violet outburst that lasts an hour or less. It
 radiates X-ray, ultraviolet, and visible radiation, plus streams
 of high-energy protons and electrons.

Solar Flares originate in/near Sun spots and juts into the
chromosphere.
A large flare can be a billion times more energetic than a large
hydrogen bomb.

Solar flares originate in/near sunspots, and
 are violent, but brief (~1 hour), outbursts of
 energy.

Picture of a Solar Flare

- the outermost layer
Corona   -made up of very diffuse (tenuous) but
         extremely hot gas (T~1 million K)
         - coronal emission is dominated by X-rays
         - carries very little energy
         - `stirred’ by the Sun’s magnetic field

The heating of the corona

                                   The heating of the corona has
                                   puzzled scientists for a long time.

                                   The corona is heated by energy
                                   outflowing from the sun's interior
How can heat go from cooler
regions to hotter ones?            not as heat but as magnetic energy.

It doesn’t! Radiant heat           The gas is heated by the motion
penetrates the corona nearly
undisturbed                        through it of long lines of magnetic
                                   force, which act like whips.
Corona heated by something
else:                              The gas is being whipped .
whipping by magnetic force lines

The heating of the corona
The corona is heated by energy from the sun's interior not as heat
but as magnetic energy. Heated by the whipping motion of lines
of magnetic force

Solar Activity

Solar Wind

The corona’s high temperature gives sufficient kinetic energy to
the atoms that the gas `escapes’, creating the solar wind. The
Sun is loosing mass, at a rate of 1.5E6 tons/s, but is only a small
fraction of Sun’s mass: 10-14 Mo/yr

Auroras: the Northern and
     Southern Lights

Auroras
n   Auroras seen as far south as New
     England
n   Caused by a gust of solar wind (coronal
     mass ejections)
n   These powerful gusts are guided by
     Earth’s magnetic field and can excite
     gases in the upper atmosphere,
     causing the air there to glow

Activity on the Sun
•    Two key points to remember:
    1. nearly all “solar weather” is a result of changes in
       the magnetic fields on the Photosphere.
    2. Magnetic Fields are the result of
       convection in the Sun

The Sun’s Rotation and Magnetic Field:
              Impact on Activity

The Sun is a `ball of gas’ that rotates on its axis; the equatorial
regions rotate faster (25 days) than the north or south (30 days).

Babcock Model

Sunspots are created at the `kinks’ of the magnetic field.
After the maximum of activity (flares, mass ejection, many
sunspots), the field relaxes again, and inverts polarity.

The differential rotation causes magnetic cycles:
appearance of sun spots, and their cycling through a
maximum every ~ 11 years

The Solar Cycle

       Solar activity peaks
       roughly each 11 years.

Survey Question
The bulk of the violent surface activity on the
Sun is due to “seasonal” variations in the
Sun’s _________.

              1) energy output
              2) radius
              3) electric fields
              4) magnetic fields