Page 90 - Intro to Space Sciences Spacecraft Applications

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Spacecraft Environment
tion of previous generations of stars along with the primordial elements of
the creation of the universe. It is theorized that some sort of shock wave,
perhaps from a nearby stellar explosion (the fate of all stars) or a galactic
wave front associated with the movement of the spiral arms of our galaxy,
disturbed the cloud of gas, causing it to condense in places. These areas of
increased density and gravitational attraction pulled in nearby molecules,
and the gas cloud began to spiral inward forming a disk-like shape.
Some local clumps of material formed away from the main core and,
through accretion (collision and fusing) with other clumps, eventually
coalesced into what are now the planets and moons. The ring of asteroids
around the sun between the orbits of Mars and Jupiter are thought to be a
region where this process did not continue long enough to form another
planet. The planets all basically lie in the plane of the sun's equator,
known as the ecliptic, and travel in the same direction around the sun.
As the mass at the center of this activity grew larger, pressure and tem-
perature began to rise as the elements packed in tighter and tighter until
conditions reached a critical point and the core burst into spontaneous
nuclear fusion. Many more processes occurred in the 4.6 billion years or
so it has taken the solar system to stabilize into what we see today, but at
that moment a new star was born.
Our sun is a typical star in a typical galaxy containing an estimated 200
billion stars in a universe of perhaps more than a billion galaxies. The sun is
located about 30,000 light-years (1 1.y. = the distance light travels in one
solar year = 9.5 x lo1* km) from the center of our spiral galaxy which is
about 100,OOO 1.y. across. The sun has an orbital period of about 200 million
years and an orbital velocity of about 250 km/sec around the Milky Way.

Structure of the Sun

The sun has a mass (M,) of 2 x 1030 kg composed of about 78% hydro-
gen, 20% helium, and 2% heavier elements by weight. These materials
exist mainly in the form of a plasma, a homogeneous mixture of ionized
elements and their dissociated electrons. Solar density varies from around
lo5 kg/m3 at the core (five times that of uranium) to lo4 kg/m3 at what
we define as its surface (the photosphere). Solar radius (R) at the photo-
sphere is around 696,000 km. For comparison, the density of water is
1,000 kg/m3, the mass of the earth (Me) is 6 x kg, and the radius of
the earth (%) is 6,378 km. The sun has a rotation rate which differs with
solar latitudes-approximately 25 days per rotation at the equator and up