- Appearance: A 0.5°-wide
Apparently smooth and uniform, during eclipses shows an "atmosphere"; Upon closer inspection *and using the right kind of light) it shows sunspots and other
- What is it? From its spectrum
we know it is a star (a mid-sized star, the only one we can see
from up close), and now we know that stars are burning balls
of gas, with heat pressure and gravity keeping it in equilibrium.
- Distance: 1 AU = 150 million
km away [can be found using parallax of planets like Mars], or 8 minutes
of light travel time.
- Size: Actual radius 700,000 km = 100
Earths [ = 10 Jupiters]; [mass = 300,000 Earth masses
= 1000 Jupiters; so the Sun's mean density = 1.4 times that of water.]
- Temperature, Luminosity: On the surface T
= 6000 K (from spectrum), so it is partially ionized; Given the power
it emits, the Sun has existed much longer than could be explained if
it was a regular
- Rotation: Slow and differential (about
27 days at the equator, slower at the poles – it is made
of gas), so the Sun is almost
Exploration (This part is not up to date)
- Solar Missions: Several spacecraft
are studying various aspects of the Sun [Ulysses was launched
in 1990 to study the Sun's polar regions, SOHO (Solar and Heliospheric
Observatory) in 1995 to monitor solar activity, HESSI in 2002
to study the causes of solar flares].
- Solar Environment Missions:
Other spacecraft are studying the corona, solar wind and magnetosphere,
and the interplanetary medium [examples are Wind launched in
1994, ACE (Advanced Composition Explorer) launched in 1997, TRACE
(Transitional Region and Coronal Explorer) launched in 1998].
- Special Missions: Genesis, launched
in 2001, collected solar wind samples and returned them to Earth
in 2004 (although the capsule crashed).
Main Parts of the Sun
- Core: T = 15 M degrees, totally
ionized; Nuclear reactions take place here: H nuclei (protons) → He
nuclei mostly by the p-p chain,
with a huge release of energy.
- Other interior: Radiation zone (ionized
and transparent to radiation, energy carried outward by high energy
photons) and convection
slowly carried outward by the gas, continuously absorbed
and re-emitted at decreasing temperatures).
- Photosphere: The bubbling "surface" we
see from here, because most of the visible light comes from it; 91% H,
9% He, some O, C, N, ...; Granules (1000 km across, 15-20 min in duration)
are evidence for convection.
- Chromosphere: A thin [only 2000 km!],
cooler and reddish layer.
- Corona: Thick, extremely hot and ionized
layer, T > 1
- How do we know? From observations,
theory, and computer models; one important aspect that provides
a lot of information is the ringing of the Sun, its vibrations
(the subject is also called helioseismology).
- Sunspots: They look
like holes even in detailed pictures,
but are in reality slightly cooler places 10,000 km
wide, that last for many days, where gas
flows downward; Due to twisting of magnetic
field lines; We can use them to see the Sun rotate.
- Flares: Bursts of
energy on short time
scales (minutes to hours).
- Prominences: Huge (100,000 km, may last
hours to weeks), they extend way
out; Eventually may give rise to coronal
mass ejections that intensify the 500 km/s solar wind made of protons,
other ions, and subatomic particles, and reach some of the planets
- The solar activity cycle: Sunspots and
associated activity have an 11-year cycle; In 2012 we are near a maximum (even though there are now few sunspots on the surface).
Solar Weather and Effect on Earth
- General effect: Intense
solar activity is potentially dangerous for power and communications
(including cell phone calls – especially near a maximum!);
It also has a less understood effect on climate.
- Mechanism: Coronal mass ejections
can cause magnetic storms when the charged particles reach the Earth.
- Nicer effect: They cause auroras
(Northern and Southern lights).
- Further out: Matter ejected
from the Sun reaches
the edge of the Solar System.
Sun's Environment |
- Major planets: A total
of eight or nine of them; Four relatively "small", "closely
spaced" and mostly rocky planets in the inner Solar System;
Four large, widely spaced, mostly gaseous planets in the outer
Solar System; Pluto, has been called a
planet for over 70 years but is now a "dwarf planet".
- Minor planets: Thousands of
asteroids between the inner and outer Solar System,
from almost 1000 km
across to tiny ones.
- Beyond Neptune: A huge number
of additional, more icy objects of size somewhat similar
to asteroids, in the Kuiper Belt and the Oort Cloud.
- Interstellar environment: The
termination shock, and the heliopause that
some of our spacecraft are starting
to explore. Beyond that is the local
cloud of interstellar matter, that blends into
the galaxy (in which there must be many other stellar
systems similar to ours...).
page by luca bombelli <bombelli at olemiss.edu>,
modified 28 mar 2013