C HAPTER 25 Beyond Our Solar System

P ROPERTIES OF STARS We know about stars by studying the electromagnetic energy that they give off (all objects emit and absorb radiation) Color & Temperature Stars give off the max amt. of radiation possible at its temperature Color is a clue to a star’s temperature Very hot stars with temp. above 30,000 K emit energy in short wavelength light – appear blue Cooler stars emit their energy as longer wavelength red light Stars with temperatures between K appear yellow

S TELLAR B RIGHTNESS Magnitude – a measure of a stars brightness Apparent Magnitude – how bright a star appears from Earth Depends on 3 Factors: 1) How big it is 2) How hot it is 3) How far away it is Absolute Magnitude – how bright a star actually is B/c a star close to us could have the same brightness as a star farther away, we use a standard distance to calculate absolute magnitude – 32.6 light-years

H ERTZSPRUNG -R USSELL D IAGRAM A diagram that shows the relationship between the absolute magnitude and temperature of stars

H-R D IAGRAM Surface Temperature is shown on the horizontal axis of the diagram Temperature decreases as you go to the right

H-R D IAGRAM 90% stars in main-sequence The hottest main-sequence stars are the brightest The coolest main-sequence stars are the dimmest Brightness of the main- sequence stars also related to their mass The hottest blue stars are much more massive than the sun, and the coolest red stars are 1/10 as massive Main-sequence stars appear in decreasing order from hotter, more massive blue stars to cooler, less massive red stars Main sequence stars have similar internal structures & functions

G IANTS & D WARFS To the upper right corner, there are very bright stars called red giants They are cool stars with large surface areas White dwarfs – dim, hot stars plotted in the lower left corner Much fainter than main-sequence stars of the same temp. Not all are white

L IFE OF A S TAR

B IRTH OF A S TAR Stars begin their lives as parts of nebulas What is a nebula? The nebulas in our galaxy (the Milky Way) are made up 92 % Hydrogen, 7% Helium, and less than 1% other elements For some reason, the gas and dust starts to contract and shrink up. As it shrinks, gravitational energy -> Heat Energy

P ROTOSTAR S TAGE Protostar is the earliest stage of a star’s life Contraction time can last a million years or so until the temperature is hot enough to radiate energy in the form of red light The large red object is called a protostar When the protostar has gotten even hotter in its core, nuclear fusion of hydrogen begins (Hydrogen atoms fuse to form Helium) When nuclear fusion begins, a star is born. (starts to glow)

P ROTOSTAR S TAGE When the balance between pressure and gravity is reached, the star becomes a stable, main-sequence star Gravity is trying to squeeze the star smaller Pressure is trying to expand the star bigger

M AIN -S EQUENCE S TAGE The 2 nd stage of the life cycle – 90% of its life Hydrogen fusion continues for a few billion years – when it has used all its hydrogen, it dies Stars age a different rates Hot, big, blue stars = few million years Smaller main-sequence stars = hundreds of billions of years Yellow stars (like the sun) = about 10 billion years

R ED -G IANT S TAGE When the core first runs out of Hydrogen and only Helium remains, hydrogen is still fusing in the star’s outer shell No source of energy since no fusion at the core w/out Energy, pressure & gravity get off balance -> core starts to contract As it contracts the temp. rises until it starts to expand into a giant body much bigger than its main sequence size.

R ED G IANT S TAGE If it gets so hot, why is it red? As it gets bigger, the surface cools down These stars are very bright and use up fuel quickly (the sun will be a giant less than a billion years)

B URNOUT & D EATH All stars, regardless of their size, eventually run out of fuel and collapse due to gravity When a star runs out of fuel, is core no longer releases energy A star can then become one of 3 things: 1) A white dwarf 2) A neutron star 3) A black hole

L OW M ASS S TARS Use their fuel slowly Live very long Interior never reaches high enough temperatures to have fusion, so Hydrogen is only fuel Never become red giants Main sequence until they use their fuel and become a white dwarf

M EDIUM M ASS S TARS Fuse Hydrogen and Helium fuel at a fast rate until collapsing into white dwarfs During their collapse, they let go of an outer layer, creating a round cloud of gas – planetary nebula

M ASSIVE S TARS Short life span End is an explosion called a supernova Supernova = a star becomes millions of times brighter and bursts inward, destroying the star Rare None recorded since telescopes were invented

S TELLAR REMNANTS All stars consume their fuel and collapse to become: A white dwarf Neutron star Black hole Our sun began as a nebula, will spend most of its life as a main sequence star, and then become a red giant, a planetary nebula, a white dwarf, and finally, a black dwarf.

White Dwarfs – remains of low and medium mass stars Small stars Can be very heavy for their size (they are dense) – for some materials, a spoonful could weigh several tons Neutron Stars – the remnants of supernova events Not large enough to create a black hole Smaller and more massive than white dwarfs Black Hole – dense objects with gravity so strong that not even light can escape Even smaller and more dense objects than neutron stars

C LASSWORK Page 714 In your Book Turn in for a classwork grade 1. What is a protostar? 2. At what point is a star born? 3. What causes a star to die? 4. Describe the life cycle of the sun?