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Stars
The Electromagnetic Spectrum
• What are electromagnetic waves?
• Stars, including our sun, emit many types of
radiation, which travel to Earth in the form of
waves.
• There are also some man-made waves in
Earth’s atmosphere, such as TV waves. These
waves are called electromagnetic waves
Electromagnetic Waves
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Cosmic Rays
Gamma (γ) Rays
X-Rays
Ultraviolet (UV) Radiation
Visible Light
Infrared Radiation
Microwaves
TV Waves
Radio Waves
What is the electromagnetic spectrum?
Wavelength – the distance between 2
consecutive crests (tops) of a wave
Frequency – the number of crests that
pass any given point during a period of
time
What is the difference between the
various types of electromagnetic
waves?
• The only difference between the various types
of waves is the amount of energy the waves
carry: The shorter the wavelength, the higher
the energy
• http://earthguide.ucsd.edu/eoc/special_topic
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• https://www.youtube.com/watch?v=bjOGNV
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What can we determine about a star
from its visible light radiation?
• We can use a star’s visible light spectrum to tell what
that star is made of.
• Prisms can be used to separate the colors of white light.
The various wavelengths making up white light bend at
different angles when they pass through the prism.
They separate from each other and the colors of the
visible light spectrum become visible.
• The visible light spectrum is the series of colored bands
broken up and arranged in the order of their
wavelengths. The visible light spectrum makes up a
small portion of the electromagnetic spectrum.
• The following are the seven main colors of the
visible light spectrum in order from longest to
shortest wavelength:
• Red, Orange, Yellow, Green, Blue, Indigo, and
Violet (ROYGBIV)
• Stars, like all matter in the universe, are composed
of chemical elements. Each chemical element
absorbs certain wavelengths of light, producing a
certain pattern of dark lines. We can take a
photograph of the visible light spectrum emitted
from a star. The pattern of dark lines that show up
in the picture of the spectrum of a star can be used
to identify which elements are in that star.
• Spectroscope:Instrument used to determine
which elements are present in a star by
examining the dark spectral lines and
comparing them to the spectral lines of known
elements.
Doppler Shift
• How does the movement of an object in
space affect how we see the light from that
object on Earth?
• Light from moving objects in the solar system
will appear to have different wavelengths than
light from stationary objects, depending on
the relative motion of the source and the
observer.
• There are two types of wavelength changes,
depending on the movement of the object
emitting light:
• 1) Red Shift: If the source of light is moving
away from you, then the wavelength of the light
is stretched out and the light is shifted toward the
red end of the spectrum.
• 2) Blue Shift: If the source of light is moving
toward you, then the wavelength of the light is
compressed and the light is shifted toward the
blue end of the spectrum.
What is a “Doppler” shift?
• Red shift and blue shift are together referred
to as Doppler shifts and are named after
Christian Doppler, the Austrian scientist who
discovered them in 1842.
https://www.youtube.com/watch?v=z0EaoilzgGE
https://www.youtube.com/watch?v=a3RfULw7aAY
The diagram below and to the right shows a
galactic star at the bottom left with its spectrum
on the bottom right.
• Notice the dark absorption lines
in the spectrum.
• The other three spectra show the
spectrum of a nearby galaxy, a
medium distance galaxy, and a
distant galaxy. Notice how the
pattern of absorption lines shifts
to the red end of the spectrum as
the galaxies become more
distant. The numbers above and
below the spectra are the
measured wavelengths in
nanometers.
Classification of Stars
• 3 Ways to Classify Stars
1.)Temperature (Kelvin (K) = Degrees Celsius +
273)
• Coolest Stars: Temperature = 2000 Kelvin
• Hottest Stars: Temperature = 40,000 Kelvin
Another way to describe temperature
of stars is Spectral Class
• Spectral Class O stars are the hottest; Spectral
Class M stars are the coolest.
• Mnemonic device to remember the order of
the spectral classes from hottest to coolest:
Oh Be A Fine Guy, Kiss Me or Oh Be A Fine
Girl, Kiss Me
• However, temperature doesn’t tell a star’s whole
story and is therefore not the best way to classify
them.
• For example, Betelguese and Proxima Centauri
have the same surface temperature (they are
both Spectral Class M stars). However,
Betelguese is millions of times brighter (it has a
greater or more negative absolute magnitude).
Apparent and Absolute Magnitude
• Recall from earlier that apparent and absolute
magnitude are ways of classifying stars by
their brightness or luminosity.
H-R Diagrams
• Just prior to World War I (1911 – 1913), two
astronomers working separately had the same
idea. They both determined a way to classify
stars by both their temperature and absolute
magnitude at the same time.
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• These two scientists were Ejnar Hertzsprung
(from Denmark) and Henry Norris Russell (from
the U.S.). They developed the HertzsprungRussell (H-R) Diagram
Based on their temperature and absolute
magnitude, stars fall into one of 4 categories on
the H-R Diagram:
• Main Sequence Stars
• About 90% of all stars in the universe are main sequence stars.
• The main sequence can be found on a narrow band running from
the upper left corner (hot, bright stars) to the lower right corner
(cool, dim stars) on the H-R diagram.
• The distinguishing characteristic of main sequence stars is that
they start off as hot and bright and go until they are cool and dim.
• Our sun is a Main Sequence star. It is found in the middle of the
main sequence diagonal.
• By comparing the temperature of other Main Sequence stars to
the temperature of our sun, we can make certain assumptions
about their brightness and size relative to our sun:
• Example  All Main Sequence stars HOTTER
than our sun are also BRIGHTER than our sun and
LARGER than our sun, such as Sirius.
• Example  All Main Sequence stars COOLER
than our sun are also DIMMER than our sun and
smaller than our sun, such as Proxima Centauri
• Note: The above statements are true ONLY for
Main Sequence stars and do not apply to the
remaining 10% of stars not located on the Main
Sequence of the H-R Diagram