Transcript Chapter 27 Stars and Galaxies

Chapter 27
Stars and Galaxies
Section 1
Characteristics of Stars
Notes 27-2
Distance to Stars
 Light years:
 Distance that light travels in one year
300,000 km/s = speed of light
9.5 trillion km in one year
Sun in 8 light minutes from Earth
Proxima Centauri: closest star to Earth (other
than the sun) is 4.2 light years away
Sirius (brightest star): 9 ly
Polaris: 700 ly
Distance to Stars
 Parallax: method used to determine the distance
to stars
Because the earth is moving the star appears
to shift in the sky depending on the time of
year
The closer the star is to the earth the greater
the shift
Astronomers use a photo to determine the
shift (photographed twice in a 6 month time)
Then calculate the distance to within 1000
ly
Parallax
Distance to Stars
 Astronomers compare the actual
brightness and apparent brightness
Determines the distance from the earth
 Cepheid (Sef-EE-id) variable star
Star’s brightness varies at a constant
pattern
Brightens and fades in a cycle that
can be used to determine how far
away it is
Astronomers can then determine the
distance to the galaxy it is in
Stellar Magnitudes
 3 billion stars can be seen through telescopes on
the surface
6000 can be seen with the unaided eye
 Over a trillion stars can be seen with the Hubble
Space Telescope
 Apparent Magnitude: brightness as it appears
from Earth
 Absolute Magnitude: brightness as it appears
32.6 ly away
Apparent Magnitude
 The measurement of brightness is assigned a
number on a scale
Brightest stars have lowest numbers
Dimmest stars have highest numbers
 Most powerful telescopes can detect an apparent
magnitude of 29
Apparent Magnitude
 Unaided eye can detect a 6
Called a sixth magnitude star
First magnitude is one of the brightest in the
sky
 If the number is negative it is brighter than the
first magnitude star
Sun (-26.8), Moon (-12.5), Venus (-4.6),
Jupiter (-2.7), Sirius (-1.46)
Absolute Magnitude
 Brightness from 32.6 ly
 Ex: If the sun was 32.6 ly away, it would
be a fifth magnitude star.
Its absolute magnitude = +5
 Most stars are between a -5 and +15
Absolute Magnitude
 Each star has two magnitudes
Two depend on the distance the star is from
Earth
 Stars with a low apparent mag. and higher
absolute mag. appear brighter than if they were
32.6 ly away
Like the Sun
 Stars with a high apparent mag. and low absolute
mag.
Stars are too far away to look bright
Absolute Magnitude
 How far away is a star with apparent mag.
of +7 and absolute mag. of +7?
32.6 light years
HR Diagram
 Graph plotting the surface temperatures
versus the absolute magnitudes of stars
 HR diagram (Hertzsprung-Russell)
Ejnar Hertzsprung
Henry Russell
Both discovered the pattern
 Brightness increases as the surface
temperature increases
HR Diagram
 Main sequence stars
 Found in the band running down through the diagram
 Sun
 Giants:
 Large and cool
 Betelgeuse
 Supergiants:
 Very large and cool
 Antares
 White Dwarfs
 Hot and small
 About the size of Earth