Download A Star is

Measuring
Starlight
Objectives
• Describe how astronomers determine the compositions
and temperature of stars.
• Explain why stars appear to move in the sky.
• Describe one way astronomers measure the distances
to stars.
• Explain the difference between absolute magnitude and
apparent magnitude.
Stars
•
A Star is- a large gaseous body which generates energy
through nuclear fusion in its core
•
Stars are made up of different elements, primarily
Hydrogen and Helium
•
Stars are essentially all made of the same
elements!!!
•
Nuclear fusion: the combination of 2 or more atomic nuclei
into a single, heavier atomic nuclei
• The Sun fuses 2 Hydrogen nuclei together to form
Helium
• See Periodic Table behind you
The Amazing Power of Starlight
Light from Stars- Also Known As
“Spectra” allows astronomers
to determine the star’s:
a. Chemical Composition
b. Temperature
c. Luminosity…a fancy word
for Brightness
d. Velocity
e. Rotation rate
f. Radius (Size)
Analyzing Starlight
• Because each element
absorbs different
wavelengths of light, we
can determine the
elements which make up
a star by studying its
spectrum.
• A spectrum is a display of
colors and lines produced
when light passes
through a prism.
• Starlight passing through
a spectrograph produces
such a spectrum ------->
• ***The spectral
difference is due to the
difference in
temperature of the star
• The dark lines in the spectrum reveal the star’s composition
and temperature.
• Every element has a different spectra
• Here are the spectra for some common elements:
• Notice how each element has unique dark bands? These are
called absorption bands.
Too Much Information??
• What you need to know:
• We can determine the
chemical composition and
temperature of a star by
analyzing its spectra.
• We divide the stars up
into seven major spectral
or temperature classes
O, B, A , F, G , K, M
(Oh Be A Fine Girl (Guy)
Kiss Me
O – Hottest Stars
M – Coolest Stars
The Spectral Sequence
Spectral
Class
O
Temperature
Color
Spectral Lines
Example
30,000 to
50,000 K
BlueViolet
Ionized Helium
Minataka
B
11,000 to
30,000 K
7,500 to
11,000 K
5,900 to
7,500 K
5,200 to
5,900 K
BlueWhite
White
Neutral HELIUM,
Hydrogen
Hydrogen (Strong)
Rigel, Spica
YellowWhite
Yellow
Ionized Metals
Procyon
Ionized CALCIUM,
Ionized and Neutral
Metals
The Sun,
Capella
Orange
Neutral Metals
RedOrange
Neutral Metals,
Molecular Bands
Arcturus,
Aldebaran
Betelgeuse,
Antares
A
F
G
K
M
3,900 to
5,200 K
2,500 to
3,900 K
Sirius, Vega
Color and Temperature
Stars appear in
different colors,
from blue (like Rigel)
Orion
Betelgeuse
to red (like Betelgeuse).
These colors tell us
about the star’s
temperature.
Rigel
The Temperatures of Stars
• The spectral difference is due to variations in temperature of
different stars.
• The various temperatures also lead to a difference in colors that we
see:
•
- Hotter stars appear Blue
•
- Cooler Stars appear Red
• Blue stars have average surface temperatures of 35,000˚C.
• Red stars have average surface temperatures of 3,000˚C.
• Yellow stars, such as the sun, have surface temperatures of about
5,500˚C.
Stellar Brightness
• Two factors determine the luminosity (brightness) of a
star:
– Temperature
– Size
• Apparent magnitude: brightness of a star as seen from
the Earth.
• Absolute magnitude: brightness a star would have at a
distance of 32.6 light-years from Earth
Stellar Brightness
• The brighter a star is, the lower the number of its
absolute magnitude, as you can see from the
diagram below.
Compositions of Stars
• Scientists have learned that all stars are made up of the same
elements which compose Earth.
• The most common elements in stars are hydrogen and helium, in
that order.
• Small quantities of carbon, oxygen, and nitrogen are also found in
stars, but stars are primarily composed of….
• HYDROGEN and HELIUM
The Sizes and Masses of Stars
• Stars vary in size and mass, and a star’s brightness
is related to its size
• Big stars appear brighter than smaller stars of the
same temperature
• Our sun is considered a medium-sized star
• Most stars visible from Earth are medium-sized
stars.
• Many stars also have about the same mass as the
sun, however some stars may be more or less
massive.
Stellar Motion
• Apparent Motion of Stars:
– motion visible to the
unaided eye. Apparent
motion is caused by
the movement of
Earth.
• The rotation of Earth
causes the apparent
motion of stars seem as
though the stars are
moving counter-clockwise
around the North Star.
Stellar Motion
• Actual Motion of Stars:
• Stars rotate on an axis.
• Some stars may revolve around another star.
• Stars either move away from or toward our solar system
• Remember Red Shift and Blue Shift
Doppler Effect
• Doppler effect: an
observed change in the
frequency of a wave
• Red and Blue Shifting
occurs due to the Doppler
Effect…in other words:
• The Doppler Effect
causes a change in the
frequency of the light
wave, which is why we
observe a red shift for
those stars moving away
from Earth and blue shift
for those moving toward
Earth
Stellar Motion
Question
Why does Polaris (the North Star) appear to remain
stationary in the night sky?
Polaris is almost exactly above the pole of Earth’s
rotational axis, so it moves only slightly around the pole
during one rotation of Earth.
Relative Motion
• Circumpolar Stars: stars which are always visible
in the night sky. These stars never pass below the
horizon.
• In the Northern Hemisphere, the movement of
these stars makes them appear to circle the North
Star.