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Biography of a Star: Our Sun's Birth, Life, and Death
Depending on the size of the original lump of gas and dust, the process of stellar birth can give
rise to different sorts of stars. A small lump never develops high enough pressures and
temperatures to start nuclear fusion. It is doomed to remain a dark, dismal stellar wanna-be -- a
so-called brown dwarf. A larger lump becomes a large star, so hot and bright that it burns itself out
in a few tens of millions of years. A lump in the middle, not too small and not too large, becomes
a middling star such as the Sun. Which is good: If the Sun had been much smaller, Earth would
have been a dark, dead world; much larger, and Earth would have been broiled.
In its early years, the Sun went through a tempestuous youth, whipping up strong winds that
cleared the solar system of whatever gas had not been incorporated into a planet. But then the Sun
settled down. From studying rocks, fossils, and Antarctic ice, scientists think the Sun has been
brightening over time, but only slightly.
And how much longer will it continue to shine? For an idea of the Sun's life expectancy,
astronomers look to clusters of stars, such as one named Messier 67, which is about the same age
as our Sun. By simulating the life cycles of these stars on a computer, astronomers have
ascertained how long stars live. They predict that the Sun will be able to fuse hydrogen into
helium in its core at about the same rate for another 5 billion years. (What a relief!) If the Sun
were a car, the gas tank would now be half full.
What will happen when the Sun does run out of gas? (Hydrogen gas, that is.) Fortunately, the Sun
will still have reserves of hydrogen in the layers that surround the core. The core will heat up this
shell of hydrogen. When the shell gets hot enough to fuse hydrogen to helium, the release of
energy will carry on there. It is as if the driver of the car poured an extra few gallons into the fuel
tank.
But this trick has a price. The source of energy will no longer be the dense, massive core, but
rather a shell closer to the surface -- and that will make a big (so to speak) difference in the
structure of the Sun. The Sun will puff up until its radius is 30 times greater. It will become a red
giant, similar to the star Arcturus, though much smaller than a supergiant such as Betelgeuse (see
photo on p. 3). A red giant is red because its exterior cooled from 9,000 to 3,000 degrees
Fahrenheit as it expanded; for a star, red means cool. This red-giant stage will last for about 2
billion years.
That Time Bomb in the Middle
The striking but now-outdated video Universe, produced by NASA in the 1970s, shows the
red-giant Sun engulfing the Earth. Though certainly dramatic, this is now thought to be incorrect.
Astronomers have had to scale down their estimates of the size of red giants based on data from
the satellite Hipparcos and from the new optical and infrared interferometers -- networks of
telescopes which can take images of large, nearby stars. Now we think the Sun will not engulf us
when it becomes a red giant.
But that is small comfort. In its retirement from normal core fusion, our previously nurturing star
will care little for its planetary children. It will be pumping out a thousand times more energy,
making Earth a good approximation to hell. To add insult to injury, the solar wind -- a stream of
particles which now gives us fun things such as the aurora borealis -- will become a cyclone that
will make radio communication impossible and perhaps evaporate the atmosphere altogether.
Looking on the bright side, the red-giant Sun may be warm enough to melt the water-rich but
now-frozen moons of Jupiter and Saturn. Humanity, if it is still around, might relocate there.
Meanwhile, what happens to all that helium being produced in the shell? It gently rains onto the
dead, but still toasty, core of the Sun, making the core more massive and more compressed. This
raises the temperature of the core until suddenly -- and I really do mean suddenly, as in seconds -the helium in the core fires up and begins to fuse itself into carbon. Using the fuel-tank analogy,
this is as if the exhaust itself starts to burn.
The end is drawing near. Now the Sun has to rearrange its internal structure all over again, as its
source of energy is once again the central core. The Sun will contract back to a bit larger than its
original radius and will give off 10 times as much energy as what we are used to now. This phase
only lasts another 500 million years, as there are a lot fewer helium nuclei (it took four hydrogen
nuclei to make one helium nucleus, and three heliums to make one carbon) and the energy
production is much less efficient.
As the Sun exhausts the helium in the core, it desperately staves off the inevitable by resorting
again to those reserves in its outer layers. Again the Sun expands. This time, it grows so large that
its outer edge is only weakly gravitationally bound to the core. The Sun barely holds itself
together anymore. This eleventh-hour attempt at life-support is pitifully ineffective; the final
red-giant stage can be maintained for only 100 million years.
At this point, things will really start falling apart. The Sun's outer layers, freed from the
gravitational clutches of the core, will waft away. Over the course of about 10,000 years, these
layers will spread out into space as an enormous sphere of gas lit up by the now-naked hot core.
These layers constitute a "planetary nebula," so called because in a small telescope the gas cloud
looks a bit like the disc of a planet (see photo on p. 3). The hot core is now a "white dwarf," a
stellar cinder. As a white dwarf, the ex-Sun will glow white-hot for a near-eternity.
Alas, there will be no dramatic explosions to entertain our distant descendants: The Sun would
have had to start with at least eight times more mass to die the spectacular death of a supernova.
The Sun, modest in life, is subdued in death. After the planetary nebula fades, there is no nuclear
fusion at all (no extra fuel, no fuel tank, not even the trunk is left), just a lump of hot carbon and
some happy memories. The Sun will be well and truly dead.
The sphere of gas drifts off and eventually is gathered up in a new cloud, and become part of the
next generation of star formation. Perhaps one day, the ashes of the Sun will throw their lot in with
another star to be born, live, die, and, perhaps, give sustenance to other warm little planets.
BETH HUFNAGEL is a postdoctoral researcher at Michigan State University in East Lansing. As
an auditor, she used to ferret out the secrets of corporate finance -- talents now applied to the
evolution of Sun-like stars. Her email address is [email protected]. George Musser contributed
to this article.
Earth
Earth is the third planet from the Sun and the fifth largest:
orbit:
149,600,000 km (1.00 AU) from Sun
diameter: 12,756.3 km
mass:
5.972e24 kg
Planet Earth
Amazing pictures of Earth from space combine useful science and artistic beauty.
Orbit : Nasa Astronauts Photograph the Earth
A beautiful coffee table book. Kids often ask me which is my favorite planet. My answer is
always "Earth". This book shows why.
Earth is the only planet whose English name does not derive from Greek/Roman mythology. The
name derives from Old English and Germanic. There are, of course, hundreds of other names for
the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the
fertile soil (Greek: Gaia, terra mater - Mother Earth).
It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth
is just another planet.
Mir space station and Earth's limb
Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the
twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are
of considerable importance; for example, they are an enormous help in weather prediction and
especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.
The Earth is divided into several layers which have distinct chemical and seismic properties
(depths in km):
0- 40 Crust
40- 400 Upper mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D'' layer
2890-5150 Outer core
5150-6378 Inner core
The crust varies considerably in thickness, it is thinner under the oceans, thicker under the
continents. The inner core and crust are solid; the outer core and mantle layers are plastic or
semi-fluid. The various layers are separated by discontinuities which are evident in seismic data;
the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.
Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a
tiny fraction of the whole (values below x10^24 kilograms):
atmosphere
oceans
crust
mantle
outer core
inner core
= 0.0000051
= 0.0014
= 0.026
= 4.043
= 1.835
= 0.09675
The core is probably composed mostly of iron (or nickel/iron) though it is possible that some
lighter elements may be present, too. Temperatures at the center of the core may be as high as
7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon,
magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly
olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this
only from seismic techniques; samples from the upper mantle arrive at the surface as lava from
volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon
dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by
mass) is:
34.6% Iron
29.5% Oxygen
15.2% Silicon
12.7% Magnesium
2.4% Nickel
1.9% Sulfur
0.05% Titanium
The Earth is the densest major body in the solar system.
The other terrestrial planets probably have similar structures and compositions with some
differences: the Moon has at most a small core; Mercury has an extra large core (relative to its
diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not
have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores.
Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.
Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which
float around independently on top of the hot mantle below. The theory that describes this is known
as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading
occurs when two plates move away from each other and new crust is created by upwelling magma
from below. Subduction occurs when two plates collide and the edge of one dives beneath the
other and ends up being destroyed in the mantle. There is also transverse motion at some plate
boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e.
India/Eurasia). There are (at present) eight major plates:

North American Plate - North America, western North Atlantic and Greenland
Earth's Plate Boundaries delineated by
earthquake epicenters







South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
African Plate - Africa, eastern South Atlantic and western Indian Ocean
Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
Nazca Plate - eastern Pacific Ocean adjacent to South America
Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)
There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates.
Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy
to see the plate boundaries.
The Earth's surface is very young. In the relatively short (by astronomical standards) period of
500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's
surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact
craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6
billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3
billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old.
There is no record of the critical period when life was first getting started.
Space Shuttle view of the Strait of Gibraltar
71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water
can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's
surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for
life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's
temperature relatively stable. Liquid water is also responsible for most of the erosion and
weathering of the Earth's continents, a process unique in the solar system today (though it may
have occurred on Mars in the past).
Earth's atmosphere seen at the limb
The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and
water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere
when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks
and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and
biological processes now maintain a continual flow of carbon dioxide from the atmosphere to
these various "sinks" and back again. The tiny amount of carbon dioxide resident in the
atmosphere at any time is extremely important to the maintenance of the Earth's surface
temperature via the greenhouse effect. The greenhouse effect raises the average surface
temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a
comfortable +14 C); without it the oceans would freeze and life as we know it would be
impossible. (Water vapor is also an important greenhouse gas.)
View from Apollo 11
The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very
reactive gas and under "normal" circumstances would quickly combine with other elements. The
oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life
there would be no free oxygen.
The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per
century. Current research indicates that about 900 million years ago there were 481 18-hour days
in a year.
Earth has a modest magnetic field produced by electric currents in the outer core. The interaction
of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras
(see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move
and even reverse relative to the surface; the geomagnetic north pole is currently located in
northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly
above the south pole of the Earth's field; see this diagram.)
The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen
radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around
the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies
between 13,000 km and 7,600 km in altitude.