StarsChapter 30Pages 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 EvolutionSection 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 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

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• Nebula• Protostar• Main sequence• Red Giant

– Planetary nebula

• White Dwarf• Black Dwarf

Binary star interaction

High Mass Stars have a different fate

They burn fasterAnd

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• Protostar• Main sequence• Red giant or red supergiant• Supernova• Neutron star or black hole

Black HolesSingularity—

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• 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

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• 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