Transcript Chapter 27 Stars and Galaxies

Chapter 27
Stars and Galaxies
Section 1
Characteristics of Stars
Notes 27-2
Distance to Stars
Light years: used to measure distance to
stars and galaxies
– 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
Homework
Sticky Notes
Key Terms Ch. 27
Due: Wednesday, Dec. 9