chapter15

chapter15 - Surveying the Stars How do we measure stellar...

Info iconThis preview shows pages 1–22. Sign up to view the full content.

View Full Document Right Arrow Icon
Surveying the Stars
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
How do we measure stellar luminosities?
Background image of page 2
The brightness of a star depends on both distance and luminosity
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Luminosity:  Amount of power a star  radiates  (energy per second =  Watts)  Apparent brightness:   Amount of starlight that  reaches Earth  (energy per second per  square meter)
Background image of page 4
These two stars have about the same luminosity -- which one appears brighter? A l p h a C e n t u r i T S 0% 0% 1. Alpha Centauri 2. The Sun 0 of 5 10
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Luminosity passing  through each sphere is  the same Area of sphere:          4  (radius) π 2 Divide luminosity by  area to get brightness
Background image of page 6
   The relationship between apparent brightness and  luminosity depends on distance:                                   Luminosity      Brightness =                                  4  (distance) π 2     We can determine a star’s luminosity if we can measure  its distance and apparent brightness:       Luminosity  =   4  (distance) π 2  x (Brightness)
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
How would the apparent brightness of Alpha Centauri change if it were three times farther away? I t w o u l d b e n y 1 / 3 a . 6 9 h r i m 0% 0% 0% 0% 1. It would be only 1/3 as bright 2. It would be only 1/6 as bright 3. It would be only 1/9 as bright 4. It would be three times brighter 10 0 of 5
Background image of page 8
So how far are these stars?
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Parallax is the  apparent shift  in position of  a nearby  object against  a background  of more  distant  objects
Background image of page 10
Apparent  positions of  nearest  stars shift  by about an  arcsecond  as Earth  orbits Sun
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Parallax  angle  depends on  distance
Background image of page 12
Parallax is  measured by  comparing  snapshots  taken at  different times  and measuring  the shift in  angle to star
Background image of page 13

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 14
Parallax and Distance p = parallax angle d (in parsecs) = 1 p (in arcseconds) d (in light-years) = 3.26 × 1 p (in arcseconds)
Background image of page 15

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
 Most luminous  stars:       10 6   L Sun Least luminous  stars:       10 -4  L Sun (L Sun  is luminosity  of Sun)
Background image of page 16
The Magnitude Scale m = apparent magnitude , M = absolute magnitude apparent brightness of Star 1 apparent brightness of Star 2 = (100 1/5 ) m 1 - m 2 luminosity of Star 1 luminosity of Star 2 = (100 1/5 ) M 1 - M 2
Background image of page 17

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
How do we measure stellar temperatures?
Background image of page 18
Every object emits  thermal radiation  with a  spectrum that depends on its temperature
Background image of page 19

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
An object of  fixed size grows  more luminous as  its temperature  rises
Background image of page 20
1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average
Background image of page 21

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 22
This is the end of the preview. Sign up to access the rest of the document.

This document was uploaded on 10/26/2011 for the course UW 20 at GWU.

Page1 / 94

chapter15 - Surveying the Stars How do we measure stellar...

This preview shows document pages 1 - 22. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online