Transcript Stars Unit 1-2: Stars

Stars Unit 1-2: Stars
How Far to a Star?
• The closest star to Earth is the Sun.
• The average distance from the Earth to
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approximately
kilometers.
– This distance is designated as 1
Astronomical Unit, or AU.
• The next closest star is Alpha Centauri,
which is 4x1013km away, or 270000AU.
How Far to a Star?
• As you can imagine, if our neighboring
star is almost 300000AU away, this unit
doesn’t work too well for interstellar
distances.
• A second unit is the light-year,
measures the distance that light travels
in one year.
– One light-year is approximately
9500000000000km.
How Far to a Star
• Using light-years, Alpha Centauri can be
expressed as being 4.3 light-years away.
– Sirius is 9 light-years away.
– Vega, the brightest summertime star, is 27 lightyears away.
– Polaris, the North Star, is 680 light-years away.
– The Milky Way Galaxy is about 100,000 lightyears across.
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Physical Properties of Stars
• The physical properties of a star
include:
– Size
– Density
– Mass
– Color &Temperature
– Brightness & Luminosity
Size
• Stars vary in size a great deal.
– The sun has a diameter of approximately
1,380,000km.
– Some stars are smaller than the Earth!
– The largest observed stars have diameters
some 2,000 time the size of the sun!
• That’s approximately 2,760,000,000km.
• For the record, there are just about 11 football
fields in one km.
Density
• Although it seems that stars vary greatly
in size, they vary even more in density!
– Our sun has a density about 1.4 times
greater than water.
– Betelgeuse (don’t say it two more
times!) is one-millionth the density
of the sun.
– Sirius is so dense, that one
teaspoon of it would weigh
more than a ton on earth!
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Mass
• The difference between mass in stars in
significantly less than the difference in
density.
– The sun’s mass is about 300,000 times
that of Earth. Approx 2x1030kg.
– Most stars are fairly close to the mass of
the sun.
Color & Temperature
• The color of a star tells us the star’s
approximate temperature.
– Cooler stars are more red.
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– The sun is a yellow star, so it’s actually on
the cooler side of the color spectrum.
Brightness & Luminosity
• Brightness is the perceived strength of
the star’s light when observed from
earth.
– This is more commonly called apparent
magnitude.
• The actual brightness of a star is called
its luminosity.
Brightness & Luminosity
• Luminosity depends on the stars temperature
and size.
• Apparent magnitude depends on the distance
from the star to the Earth.
• Example:
– A 100W light bulb has a higher luminosity than a
20W flashlight bulb.
– But if that flashlight bulb is right in front of your
eye, it will appear brighter than the 100W bulb far
away.
Brightness & Luminosity
• Both the brightness and luminosity of
the stars is used to categorize them.
• This is the purpose of studying star
magnitudes.
Elements in Stars
• Scientists use spectral analysis to
determine the composition of stars.
– Spectral analysis is a process that uses a
tool called a spectroscope to separate the
different wavelengths of light coming from
a star.
– By observing which wavelengths are
stronger, missing, or weaker, scientists can
determine the composition of the star.
Elements in Stars
• Through spectral analysis, scientists have
found stars are primarly made of the following
elements:
– Hydrogen
– Helium
– Rarely:
•
•
•
•
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Iron
Titanium
Calcium
Sodium
Etc.
Elements in Stars
• Spectral analysis of the sun:
– 70% Hydrogen
– 28% Helium
– 2% heavier elements
• Every star has slightly different spectra, and
every star has slightly different composition.
• Almost every star powers itself by fusing
hydrogen atoms together and making helium.