Solar System and the
Universe
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Temperature
Mass
Common Characteristics
of Stars
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Most appear white to our eyes. (Not really their
color)
Most stars have a predominant color that is
dependent upon their surface temperature.
 The hotter the star, the more blue light it emits.
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Cooler stars emit more red light.
The predominant “color” can also be outside of the
visible range of wavelengths, for very hot (>
20,000K) or very cool (<4500K) stars. If different
colors are emitted with each about the same
intensity, the star will appear white; this can occur
for stars whose surface temperature is moderate.
Temperature of a Star
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Mass
The mass of a star determines how the star
changes, including its rate of change and
ultimate demise.
Stellar masses are compared to that of the Sun,
rather than using kilograms. The Sun is 1 MSun
(“one solar mass”).
The mass of the star will influence most of its
other properties, including diameter,
temperature, and lifespan.
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A star’s mass determines the strength of its
gravitational attraction
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Influences the temperature and density at the core of the
star.
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Most massive stars exist for the shortest amount of
time, while the low-mass stars can last hundreds or
even thousands of times longer.
Our Sun is expected to have a main sequence lifespan
(when it is fusing hydrogen into helium) of 10 billion
years.
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Mass
Will impact the rate at which hydrogen is fused into helium
thus determining the star’s lifespan.
A star with a mass of 15 MSun has a main sequence
lifespan of only 15 million years, whereas a star with 0.5
MSun has a main sequence lifespan of 200 billion years.
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The star’s mass will also determine how it will
“die.”
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Mass
Planetary nebula and white dwarf (in the case of
low mass stars), or as a supernova which leaves
behind a neutron star or black hole (the most
massive stars).
Humans have not been able to observe stellar
evolution directly, of course: rather, computer
and theoretical models are supported by
observations of thousands of individual stars.
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A star’s diameter is also determined by its mass
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More massive stars, during their main sequence
period, have larger diameters.
Luminosity: determined by diameter and
temperature together
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Total amount of energy emitted by the star every
second.
The luminosity of stars ranges from about 0.0001
to more than 100,000 solar units (or LSun).
We cannot measure luminosity directly – how
bright the star appears to us depends upon its
distance as well as its luminosity.
Diameter & Luminosity
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Before Main Sequence
Main Sequence
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Hydrostatic equilibrium
Red Giant
White Dwarf
Possibly Black Dwarf
Lifespan of a Star
Nuclear Fusion and
Energy
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Telescopes observe light and it is through this
light that scientists gather all their information
about the universe beyond Earth.
Space-based observatories (ex. Hubble Space
Telescope). Provided an unimpeded view of the
universe without Earth’s atmosphere acting as
a filter. Indeed, some light frequencies, such as
infrared, x-ray, and gamma ray, are only
observed from space (or high in Earth’s
atmosphere) because these frequencies do not
penetrate to Earth’s surface.
Technology & the
Universe
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Radio telescopes: detect radio waves (not
sound), which are a low frequency form of
light. Black holes and neutron stars create such
intense magnetic field lines and radio
telescopes have been instrumental in our
understanding of these phenomena.
Technology & Universe
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Stars similar in mass to the Sun convert hydrogen
into helium in their centers during the mainsequence phase, but eventually there is not enough
hydrogen left in the center for fusion to provide the
necessary radiation pressure to balance gravity.
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The center of the star contracts until it is hot enough
for helium to be fused into carbon. The hydrogen in
a shell continues to convert to helium, but the outer
layers of the star have to expand to conserve energy
Turns into a red giant until the carbon converts into
heavier elements, energy dies and becomes a white
dwarf, then possibly a black dwarf.
Star Evolution
Universe Expansion