Transcript Stars - Moodle

Stars
Chapter 30
Pages 775-
Star
• A ball of gases that
gives off a tremendous
amount of
electromagnetic
energy
• Notice that I did not say
light
• Stars emit all
wavelengths of the
electromagnetic
spectrum
Analyzing Starlight
• Astronomers analyze stars by looking at
the light they emit
• They use a spectrograph
• A spectrograph separates light into
different colors or wavelengths
• Stars produce a display of colors and
lines called a spectrum
Three types of spectra
• Emission or bright
line
• Absorption or
dark line—dark line
shows composition
and temperature
• Continuous
Sources of Spectra
Star classification
• If we look at a star
with a spectroscope
we see dark lines
called absorption
spectra
• These lines indicate
the star’s chemical
composition
The most common element in
stars is hydrogen
Each chemical element has a
characteristic spectra in a given
range of temperatures
The colors and lines indicate the
elements that make up a star
Chemical Composition Lab
• Use the transparency and the paper copy to
identify the elements found in the “Spectra of
Unknown Composition”
• Slide the transparency up and line the
unknown spectra just above the Calcium lines
– Are all lines present?
– Then calcium is in the star
• List results in two columns on your 3x5 card
– Elements Present and Elements Not Present
Color and Stars
Think of a candle
• Hottest part is blue
• Red is the cool part
at the top
Examples
• Temperature?
• Composition?
Apparent Motion of Stars
• Time lapse photography
show a circular pattern
revolving around the
star Polaris
• We know Polaris as the
North Star
• It is located directly
above the North Pole so
as the Earth spins the
stars appear to move
Doppler Effect
• The apparent shift in
the wavelength of
light emitted by a
light source moving
toward or away
from a viewer
Which is closest?
• See how the lines
shift farther to the
red end of the
spectrum as a star is
farther away
• The lines are from
Ca, H, Na, and Mg
parallax
The apparent shift in a
stars location when
viewed from
different locations in
orbit
The closer the star is,
the more it appears
to move
Another view of parallax
Absolute vs. Apparent Magnitude
• Absolute—how bright a star actually is
• Apparent—how bright it appears from
Earth
http://www.astronomynotes.com/starprop/s4.htm
Stellar Evolution
Section 2
Page 781
The Hertzsprung Russell diagram –
HR Diagram
– 90% of stars are
found on a diagonal
line called the main
sequence
– Hotter on the left
cooler on the right
– Brighter at top
dimmer at bottom
Another View of the HR Diagram
Birth of Stars
• Nebula—interstellar clouds of gas
and dust
• Drawn together by gravitational
attraction called accretion
• Begins to spin as it condenses
• This huge ball of gas is called a
protostar
Birth of a Star Part 1
Birth of a star
• This ball continues to be drawn
together by gravitational attraction
• When temperatures inside reach about
10million Kelvins nuclear fusion begins
• This produces a huge amount of radiant
and thermal energy
• Ignition of nuclear fuel marks the
change from protostar to star
A Red Giant you know--Betelgeuse
Main sequence star
• Most of a star’s life is spent as a
main sequence star
• Its size, temperature, and color are
relatively stable
• During this time it burns up its
supply of hydrogen
• What happens next depends on the
star’s size
Red Giant
• When the sun has burned all its
hydrogen, the helium core will
contract because of gravity
• The rise in temperature will ignite
the helium in the core and the
core expands to become a red giant
• Our sun will reach this stage in about
5 billion years
Famous Red Giants
• Antares
The fate of stars differs according
to their size
(Low mass stars)
• Small stars will shrink and throw off the
outer layers in rings becoming a
planetary nebula
• The shrunken core that remains is a
white dwarf
– There is no nuclear fusion, it is only
glowing hot
– If it is a binary star it may form a nova,
or,
• When the core cools it becomes a black
dwarf
Medium Mass stars—like the sun
Become RED GIANTS
• Starts to burn
helium-contracts
because of gravity
• This raises the
temperature and the
star expands
• Our sun will reach
this stage about
5 Billion years from
now
White Dwarf
• What remains after the star swells and
ejects the outer layers
• What remains no longer burns fuel
• It slowly cools
• It is now a stellar remnant
• It can change color as it cools
Black Dwarf
• A cold, dark lump of matter that
remains when a star has burned out
and cooled off
Life cycle of a low mass star
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Nebula
Protostar
Main sequence
Red Giant
– Planetary nebula
• White Dwarf
• Black Dwarf
Binary star interaction
High Mass Stars have a different
fate
They burn faster
And
Die differently
High mass stars
• After the main sequence, stars with a mass
much greater than the sun can burn and
create larger and larger elements
• When it gets to iron, it takes too much
energy to create other elements so it
collapses
• This causes a supernova, this is when heavier
elements are made
• After a supernova a high mass star may
become a
• neutron star or a
• black hole if it is VERY massive
Life Cycle of a High Mass Star
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Protostar
Main sequence
Red giant or red supergiant
Supernova
Neutron star or black hole
Black Holes
Singularity—
density is infinite
So how does the life cycle of a high mass
star differ from a small mass star?
• High mass burn quicker and brighter and
burn out faster
• They also have a different fate
Groups of Stars
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Main sequence—diagonal line
Red Giants– bright and cool
Supergiants—brighter and cooler
White Dwarfs—dim and hot
Spectral Class
• OBAFGKM
• Oh Be A Fine Girl Kiss Me
Revisit the HR diagram
• Where are stars most of their lives?
• Where are they when they begin to die?
• What are they after they use up their
fuel?
How is the Life cycle of a high mass
different????
• Starts the same but burns faster and
ends differently
• Either
– Supernova then
• Neutron star
• Black hole
Large Mass Stars
have a different fate
• They end in a supernova
• Generate the elements of life
• The inner part implodes to form a super
dense neutron star
The Hertzsprung
–HR Diagram
• 90% of stars are
found on a
diagonal line
called the main
sequence
• Hotter on the left
cooler on the
right
• Brighter at top
dimmer at bottom
Russell diagram
Lab—HR diagram
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Study lists and answer 21.1 &21.2
Temp on horizontal
Absolute magnitude on vertical
Note graph lines are not equal
Chart nearest stars with a (+) sign
Chart brightest stars with a
Show stars on both as circled +
Use an * to show the sun on the diagram
Word bank for paragraph
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Black holes
Nebula
1 million
1 thousand
Light years
Parsec
White dwarf
Magnitude
Red giant
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Black dwarf
Fusion
Fission
Temperature
Massive
Neutron star
Condense
Closer
Super nova
Pulsars
• emit low frequency
radio transmissions
– A few emit X-rays or
visible light
– The picture to the left is
gamma ray
burst animation
a
Extremely Massive stars
• Extremely massive
stars that undergo
gravitational
collapse may
become a
Event horizon—point of no return
Gravity Lensing—
or how we know where they are
Image copyright © 1998 by John Chang.
http://www.rdrop.com/users/green/school/detect.htm
NASA Life Cycle of Stars Review
Lab—HR diagram
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Study lists and answer 21.1 &21.2
Temp on horizontal
Absolute magnitude on vertical
Note graph lines are not equal
Chart nearest stars with a (+) sign
Chart brightest stars with a
Show stars on both as circled +
Use an * to show the sun on the diagram