The Universe & Stellar Evolution

I.   How Do Astronomers Know That?

Star Knowledge

Gathering Light

Composition

Temperature

Distance

Mass

Size

Speed

Light from stars is collected and analyzed

Light is gathered by pointing tubes at stars & using a mirror to concentrate their lightt.  The light is then studied.

Finding Composition  (spectra)

Each element has its own pattern of frequencies of light. Starlight -when it has those patterns in it - means those elements are in those stars.

Finding Temperature

• color tells temperature  red hot 3,000C  yellow 5,000C  blue 10,000C
• a gas like an ideal blackbody glows  according to Wien’s Law - the higher the f , the higher the Temp  Stars follow this.

 

Finding Distance - 3 methods

•  Parallax - for close stars -(good to 50 ly; using a satellite 3000 ly)  If a close star appears to move if front of its background when viewed from earth at six month intervals, the amount it seems to move (the angle of parallax) tells its distance from the earth
•  Cepheid variables  -are special stars- work for medium distances
• These strange stars flicker according to their true brightness  (eg. 3 day period ~ -6 mag star)
• We then compare this true brightness with their apparent brightness to us.  How much dimmer it seems means how far away.
•  Brightness - stars of same composition are same real brightness so dimmer stars are farther away.

Finding Mass

Motions of binary stars show their masses -we can see them both and tell their distance apart; then we can measure the time it takes them to revolve.
Any two bodies follow laws of motion  & the laws give the  masses of each.

Finding Size

Compare location on main sequence chart and a  typical size to  real brightness. Find density  by mass/volume

Speed

Doppler effect;  A red shift occurs in the stars color; the greater the shift in frequency, the faster the star is moving away