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
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
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
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
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
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
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
• 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
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
(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 sunBecome 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
Life cycle of a low mass star• Nebula• Protostar• Main sequence• Red Giant
– Planetary nebula
• White Dwarf• Black Dwarf
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• Protostar• Main sequence• Red giant or red supergiant• Supernova• Neutron star or black hole
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• 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– SupernovaSupernova 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 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
Lab—HR diagram• 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• Black holes• Nebula• 1 million• 1 thousand• Light years• Parsec• White dwarf • Magnitude• Red giant
• 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 a gamma ray burst animation
Gravity Lensing—or how we know where they are
Image copyright © 1998 by John Chang. http://www.rdrop.com/users/green/school/detect.htm