How They Are Classified
How They Work
Birth of a Star
• Nebula - a cloud of gas and dust in space.
• Some nebulas are where stars are formed or
are the remains of dead or dying stars
• Graph showing the luminosity (objects
radiation) of a star as a function of its surface
• Luminosity is measured in terms of absolute
• Absolute magnitude is the brightness of a star.
• Brightest stars are of 1st magnitude and the
dimmest stars are of 6th magnitude.
• The sun has a magnitude of -26.8 (really, really,
Most Stars are
on the Main
Sun is on here.
Absolute versus Apparent Magnitude
• The brightness that a
star would have at
distance of 32.6 lightyears from Earth.
• The brightness of a
star as seen from the
What Fuels Stars?
• Nuclear Fusion - Hydrogen and Helium
coming together to produce energy and
• The most well known star that affects our
lives every day is the SUN.
• It take approximately 8 minutes and 18
seconds for light to reach earth from the
• Gravity pulls all of the mass inward and
creates an intense pressure.
• The pressure is high enough to force atoms
of hydrogen to come together in nuclear
• Two atoms of hydrogen are combined to
create helium-4 and energy (light)
• In this zone, the energy from the core is carried
outward by photons.
• As one photon is made, it travels about 1
micron (1 millionth of a meter) before being
absorbed by a gas molecule.
• The gas molecule is heated and re-emits
another photon of the same wavelength.
• The cycle repeats itself until a photon reaches
the convective zone.
• Dominated by convection currents that carry the
energy outward to the surface (convection
currents are rising movements of hot gas next to
falling movements of cool gas).
• Carry photons outward to the surface faster than
the radiative transfer that occurs in the core and
• It takes a photon approximately 100,000 to
200,000 years to reach the surface!
The Sun’s Atmosphere
• Corona - extremely hot outermost layer
extending outward from the chromosphere
several million miles or kilometers
• Is the region that can be seen from Earth
• Has an average temperature of 5,800 degrees
• As we pass up through the photosphere, the
temperature drops and does not emit as much
• Therefore, the outer edge of the photosphere
looks dark, an effect called limb darkening that
accounts for the clear crisp edge of the sun's
• Lies above the photosphere.
• The temperature rises across the
• Thought to be heated by convection within
the underlying photosphere.
• Final layer of the sun and extends several
million miles or kilometers outward from the
• Can be seen best during a solar eclipse and in
X-ray images of the sun.
• The corona has bright areas (hot) and dark
areas called coronal holes.
• Coronal holes are relatively cool and are
thought to be areas where particles of the solar
Total Solar Eclipse 2/26/1998 from Maricaibo, Venezuela
Features on the Sun
• Solar Prominences
• Solar Flare
• Dark, cool areas that appear on the photosphere.
• Always appear in pairs.
• Are intense magnetic fields (about 5,000 times
greater than the Earth's magnetic field) that
break through the surface.
• The magnetic field is caused by movements of
gases in the sun's interior
• Sunspots happen in 11-year cycles (it is not
known why they occur in 11-year cycles).
• Clouds of gas that rise
• Appear as arches.
• Can last two to three
months and extend as
far out as 30,000 miles
from sun’s surface.
• Violent explosions from the sun.
• Thought to be caused by sudden changes in the
magnetic field where the magnetic field is
• Accompanied by the release of gas, electrons,
visible light, ultraviolet light and X-rays.
• Produce auroras and can disrupt
communications systems on Earth when the
release of energy caused by these violent
eruptions reaches Earth.
Fate of the Sun
• The sun has been shining for about 4.5 billion
• It has enough hydrogen fuel to "burn" for about
10 billion years.
• The size of the sun is a balance between the
outward pressure made by the release of energy
from nuclear fusion and the inward pull of
• When the core runs out of hydrogen fuel, it will
contract under the weight of gravity and expand
becoming a Red Giant.
Fate of the Sun Continued
• When the sun becomes a Red Giant it’s
expansion will reach beyond Earth’s orbit and
therefore, the earth will be vaporized!!!!!
• At some point the helium will fuse into carbon
and when there is no more helium the core will
expand and cool and become a White Dwarf
• After becoming a White Dwarf the sun will
then turn into a Black Dwarf.
• The entire process will take a few billion years.