Stars A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures

Massive luminous balls of plasma held together by gravity Stars Massive luminous balls of plasma held together by gravity

Characteristics of Stars Color Temperature Mass We use these characteristics to understand and examine stars

Color Color is a clue to a star’s temperature. Blue stars are hotter/ newer/closer Red stars are cooler/older/more distant

b. Temperature Very hot stars emit short-wavelength light BLUE Cooler stars emit long-wavelength light RED

c. Mass - A unified body of matter with no specific shape Mass is how much “stuff” is squeezed into a space

Japan subway car without much “stuff” inside = Less Mass

Crowded Japan subway car with lots of “stuff” = More mass Lots of “stuff” squeezed into a space, place or thing

Binary Stars – pairs of stars pulled toward each other by gravity Many stars orbit each other More than 50% of stars occur in pairs or multiples. Binary stars are used to determine the star property most difficult to calculate – It’s mass

If the sizes of the orbits are known Then the stars mass can be determined

Hertzsprung-Russell Diagram Shows the relationship between the absolute magnitude and the temperature of stars 90 % main-sequence stars Giants Supergiants White dwarfs

Light Year – the distance light travels in 1 year 9.5 x 10 to the 12th power Or 9.5 trillion kilometers 186000 miles per second Distances to stars are so large that units like miles or kilometers are too hard to use The numbers get really, really confusing and big

Measuring distance to stars – it’s difficult The most basic way to measure distance to stars is parallax.

Parallax – the slight shifting of a nearby star due to the orbital motion of Earth uses photographs of stars to compare to distant stars in the background shifting angles are compared the angles compared are very small Close stars = large parallax angles Distant stars = smaller parallax angles

Apparent Magnitude - a stars brightness as it appears from Earth 3 factors control the brightness from Earth How big How hot How far away

Absolute Magnitude – how bright a star actually is

Variable Stars – some stars fluctuate in brightness Cepheid – gets brighter in a variable pattern Nova – sudden brightening of a star due to a flare up

Interstellar Matter - between existing stars is “the vacuum of space” Except for Nebulae…

Nebulae – clouds of dust, gas, and thinly scattered matter Stars and planets form from this interstellar matter Nebulae begin to contract Gravity squeezes particles in the nebula towards the center Nebula shrinks Gravitational energy is converted into heat energy

Protostar Stage – a developing star not hot enough to begin nuclear fusion Contraction lasting 1 million years Collapse causes the core to heat more than the outer layer Causes gas to increase it’s motion When the core reaches 10 million K, nuclear fusion of hydrogen begins A star is born

Main-Sequence Star – a star balanced between 2 forces, gravity and gas pressure Gravity(external force) Gas (internal force) Hydrogen fusion lasts a few billion years 90% of an average stars life is in this hydrogen burning stage When a star’s hydrogen fuel in the core is depleted, it evolved rapidly and dies. Some stars delay death by burning heavier elements and become giants

Gravity (external force) Gas (internal force)

Red Giant Stage Inner Core consumes all hydrogen fuel energy and begins to contract Helium core is left behind Core contracts and heat is radiated outward. This energy heats the outer layer and causes expansion Results in giant body size of star 100 to 1000 X size of it’s original main-sequence size

Death and Burnout of Stars All stars run out of fuel and collapse because of gravity

Death of Low Mass Stars Small Cool Red Consume hydrogen fuel slowly Not hot enough to fuse helium Remain on the main sequence for up to 100 billion years Collapse into White Dwarfs

Death of Medium Mass Stars Masses similar to our Sun Evolve into Giants consume hydrogen and helium at fast rate Collapse into White Dwarfs During collapse from Red Giant into White Dwarf they cast off their outer shell and leave a cloud of gas called planetary nebulae

Death of Massive Stars Massive stars have short lives End star life in brilliant explosions called supernova Rare Death is triggered when nuclear fuel is consumed Star collapses Implodes Sends shock wave out from the stars interior, this destroys the star blasting the shell into space None have been observed since the invention of the telescope

Nucleosynthesis – the process that produces chemical elements inside stars Occurs in dying stars Stars produce all naturally occurring elements beyond helium in the periodic table Mass of the star determines the highest atomic number of the elements it can produce More massive stars produce heavier elements

Study of Light Information about the universe is obtained from the study of the light emitted from stars and other bodies in space

Electromagnetic Radiation Gamma Rays X-Rays Ultraviolet Rays Visible Light Infrared Radiation Microwaves Radio waves All energy travels through the vacuum of space at the speed of light

Electromagnetic Spectrum – arrangement of waves according to their wavelengths and frequencies Photons – a stream of particles

When the spectrum of a star is studied, the spectral lines act as “fingerprints”. Spectral lines identify the elements present in a star and tell of the stars chemical composition