The Sun contains about 1030 kg of
hydrogen, which is converted into helium and into energy by nuclear fusion at a
rate of 6x1011 kg of hydrogen
atoms a second. (year = 3x107
sec, so 5 billion years is 5x109 times 3x107 = 1.5x1017
sec. That times 6x1011 H
atoms per sec is 9 x 1028.
Yep, about 1030 .) How
many atoms are in 1030 kg of hydrogen? Use Avogadro’s number, 1 g/mol for hydrogen,
and get 6.022x1056 . In the
universe, about 12.5x1021 stars, so there are about 7.5x1078 H
atoms in stars. There are also H
atoms in interstellar space… lets say as many as there are in stars. That just gives another factor of 2: 15x1078 for the whole universe.
The earth orbits the sun with a period of one year
and rotates on its own axis with a period one day. The earth’s axis of rotation is tilted 23.5°
with respect to its orbital plane, which is what causes the seasons to occur: During summer days, the sun is directly
overhead, and during winter it is lower in the sky, making days shorter and the
temperature cooler. During winter in the
Northern hemisphere, the earth is actually closer to the sun by about 3 million
miles (page 432), so it is not distance from the sun that causes the seasons. (93 million miles average distance.)
One half of earth, but not the same half, is always in the
dark and one half is in sunlight. For
the part of it in sunlight, the amount of energy being received from the sun is
per square meter is 1400 joules per second, or 1400 watts, or 1.4 kW. (multiply by the area of half the earth,
about 1.27x1012 , get 1.78 x
1017 watts)
The celestial sphere is an imaginary sphere with
earth at its center that the stars are attached to. It has coordinates similar to latitude and
longitude on earth. The stars have fixed
places on the celestial sphere. The sun
follows its own circular path on the sphere.
This path is called the ecliptic.
In reality (since the earth goes around the sun and not vice-versa) the
plane of earth’s orbit is what defines the ecliptic. The constellations of the zodiac are located
along the ecliptic. There’s also the celestial equator, which is the projection
of earth’s equator onto the celestial sphere.
Equinoxes are the points of intersection of the celestial equator and
the ecliptic. See figs on pages 500 and
501.
Most stars are binary star systems, some are visible
as such through telescopes. 88
constellations. A major one in the sky
at about 10 pm is Leo, with Saturn currently in it. (This was late November of 2008.) The constellations of the zodiac are located along the ecliptic, and the planets ("wanderers") travel nearly along the ecliptic because their orbits around the sun are approximately in the same plane as Earth's orbit.
Life cycle of stars:
Stars are formed by gravity, in places where there is a higher-than average mass-density of gas and dust. Gravity slowly but ineluctably pulls the mostly-hydrogen gas
and dust closer, collapsing it until the density and pressure gets to be large
enough that nuclear fusion starts (protostar phase).
Low-mass and high-mass stars can be formed. Low-mass stars like the sun burn their
hydrogen fuel and become red giants, then planetary nebulae, then white
dwarf. The hydrogen
core of a star is squeezed down as H is used up, but the pressure of its own gravity causes it to
become hotter, releasing enough energy in radiation and in kinetic energy of
gas particles that the hydrostatic equilibrium is upset and the star starts
expanding. Goes into red giant phase.
As red giant, the core can be hot enough to fuse helium into
carbon, and other fusion reactions can also occur, creating elements up to
iron. Nucleosynthesis is what this is
called. Outer layers are blown off at
some point, creating planetary nebulae, then the remaining core is a white
dwarf. White dwarfs have used all their nuclear fuel (including helium-to-carbon fusion) and basically are just cooling off, getting dimmer and dimmer. The white dwarf can still accumulate
more matter because of its gravity and eventually explode as a nova, or it can absorb a lot of matter quickly from
a nearby star (remember most stars have a companion to which they are gravitationally bound = binary system) and explode as a Type I supernova.
For stars greater than 1.4 solar masses, the life-cycle
occurs more rapidly, and the end result is more dramatic. These stars become massive red supergiants
that undergo Type II supernova explosions, creating elements heavier
than iron [update 2021: neutron star collisions are now believed to be the source of heavy elements] and huge amounts of radiation, including visible
light. Then the remaining core of the
star becomes a neutron star (possibly a pulsar) or a black hole (bottom
of page 511).
Galaxies, not stars themselves, are the fundamental
components in the structure and evolution of the universe. Their motion is like the motion of dots
painted on a round balloon that is being blown up—they are moving apart, so the
universe is said to be expanding. The
nature of the expansion says 1) that there is no center to the expansion, or
equivalently any point can be considered the center, 2) from any point (galaxy)
the farther away another point (galaxy) is the faster it is moving away
(Hubble’s law -- recessional speed proportional to distance), and 3) there must
have been an origin of the expansion (the Big Bang) when all matter was at a
single starting point of the expansion.
Recent evidence says the rate of expansion is increasing, due to some
kind of repulsion force (called dark energy*).
Another not-as-recent discovery is that galaxies (including our Milky Way) don’t
have enough matter to explain their rotation rates, so that dark matter* is
postulated as an unknown form of matter that can account for the rotational
speed. Normal matter is detectable by
visible light, radio waves, x-rays or other electromagnetic radiation.
Nearest Galaxies, where ly = light-year:
42,000 ly Canis Major Dwarf galaxy (2003), 50,000 ly Sagittarius Dwarf
(1994), then the naked-eye visible (in southern hemisphere) Large and Small
Magellanic clouds at about 180,000 ly, and Andromeda, M31, (northern hemisphere) at 2.2 million ly. By comparison, the nearest star to the sun is about 4 ly in the Alpha Centauri star system. Also, the diameter of our Milky Way spiral galaxy is about 100,000 ly and contains 100 - 400 billion stars, and we are about 27,000 ly from the galactic center.
(Get gram, kg weights. Check on large binocular telescope.
Mention LHC, Stardate, Earth & Sky.)
