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

Characteristics of Stars

a. Colorb. Temperaturec. Mass

We use these characteristics to understand and examine stars

a. 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 lightBLUE

• 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

• 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

9.5 x 10 to the 12th powerOr

9.5 trillion kilometersOr

186000 miles per second

Measuring distance to stars – it’s difficultThe 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 backgroundshifting angles are comparedthe 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

1. How big2. How hot3. How far away

Absolute Magnitude – how bright a star actually is

Variable Stars – some stars fluctuate in brightness

1. Cepheid – gets brighter in a variable pattern

2. 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

1.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

1. Contraction lasting 1 million years

2. Collapse causes the core to heat more than the outer layer– Causes gas to increase

it’s motion3. When the core reaches

10 million K, nuclear fusion of hydrogen begins

4. 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

• Rare1. Death is triggered when

nuclear fuel is consumed2. Star collapses 3. Implodes4. 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

1. Gamma Rays2. X-Rays3. Ultraviolet Rays4. Visible Light5. Infrared Radiation6. Microwaves7. 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

1. Wavelengths 2. 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