Download Chapter 28 – Stars and Galaxies

Chapter 28 – Stars and Galaxies
I.
A Closer Look at Light
A. What is light?
1. Light is a form of electromagnetic radiation (EM)
2. Other types of EM listed from longest to shortest
wavelengths
a. Radio
b. Microwave
c. Infrared
d. Visible light
e. Ultraviolet
f. x-rays
g. Gamma rays
 This is known as the EM spectrum
3. All EM energy travels in waves and at the speed of light
4. Can travel through empty space (vacuum)
B. Spectroscope
1. Visible white light is actually made up of light of various
colors each with a different wavelength
2. The various color can be observed
a. In a rainbow
b. Passing light through a prism
c. Or an astronomers instrument known as a spectroscope
3. The colors ROY G. BIV are aligned from longest to
shortest (violet)
4. Longer wavelengths refract less
C. Types of spectra from spectroscopes
1. Continuous spectrum – unbroken band of colors which
are emitted by
a. Glowing solids (filament)
b. Glowing liquids (molten iron)
c. Hot compressed gases (inside stars)
2. Emission spectrum – lines of different colors
a. Produced by glowing thin gases
i
Each element has its own spectra therefore
scientists can identify the gas
3. Absorption spectrum – a continuous spectrum crossed by
dark lines
a. Elements in the thin gas that surround a star absorb
the same wavelength they would emit
b. The stars absorption spectrum indicates the
composition of the stars outer layer
 The sun radiates a continuous spectrum, however,
the gases in the atmosphere absorb some
wavelengths. By analyzing the absorbed bands,
scientists can figure out the composition of the
sun’s outer layer.
c. Absorption spectrum can also determine a planets
atmosphere
D. Doppler Effect
1. By observing how the spectral lines are shifting, scientists
can tell how a star is moving compared to the earth
a. Move to the red end, “red shift,” the star is moving
away
b. Move to the blue end, “blue shift,” star is getting
closer
c. By using spectrums from the lab and the stars, they
can determine how fast they are moving
II.
Stars and Their Characteristics
A. Constellations – name given to groups of stars
1. There are 88 of them
2. Big dipper – best known asterism (small star grouping)
is actually part of a larger grouping known as Ursa
Major
3. We can use the dipper to find other constellations
4. The stars appear to move in two ways
a. Nightly variation – due to earth’s rotation
b. Yearly due to earth’s revolution
 Orion – winter constellation
 Lyra – summer constellation
B. Apparent magnitude
1. Is the brightness of the star as seen from earth
2. The lower the number, the brighter it is
 Sun is –26.7
 Faintest is +6
C. Distance to stars
1. Astronomical Unit (AU) distance from earth to sun – 150
million km
2. Proxima Centauri – next nearest star is 260,000AU
3. Therefore, astronomers use light-years – a unit of
measure that light travels in one year.
a. Light travels about 300,000km/sec  1 year =
9.5×1012
b. Proxima Centauri is about 4.2 light years away
D. Elements in Stars
1. Stars are mostly of super-hot gases – mostly H & He
E. Mass, Size and Temperature of Stars
1. Mass is something that can not be observed directly. It
can only be calculated based on other observations
2. Stellar mass is expressed as multiples of the sun’s mass
 Betelgeuse’s mass – 20 solar masses
F. Temperature and Color
1. Blue stars are hot
2. Red stars are cool
G. Luminosity
1. The actual brightness of the star is luminosity
2. If two stars have the same surface temperature, the larger
star would be more luminous
3. If the same size, hotter one would be brighter
4. Types of magnitude
a. Absolute – as if all stars were same distance from earth
b. Apparent – as they appear in the nighttime sky
H. Variable Stars
1. Some stars show regular variation of brightness over
cycles that last from days to years
2. Cepheid Variables – yellow supergiants whose cycles
range from 1 to 50 days
a. If a cepheid is located in another galaxy,
astronomers can find the distance to these galaxies
by comparing absolute and apparent magnitudes
3. Other stars change in brightness because they revolve
around another star. This is known as a ‘binary star
system.’
III. Life Cycles of Stars
A. Hertzsprung-Russel Diagram
1 A diagram to help explain a stars life
2 Most stars fall into 4 distinct groups
a. Main sequence – 90% of stars
 Our sun
b. Giants – 10-100x bigger than our sun – more luminous
c. Supergiants – more than 100x bigger than our sun
d. White dwarves – stars near the end of their lives
B. Birth of a Star
1 Stars begin their life as a nebula
a. Huge cloud of gas
C. Death of a Star
1 2 paths
a. nebula  protostar  main sequence  red giant 
planetary nebula  white dwarf
b. nebula  protostar  massive star  red supergiant 
supernova  blackhole or neutron star
c. our sun will swell into a red giant, then its outer
layers will get blown away and only an earth-sized
fiery hot carbon-oxygen core will remain (white
dwarf)
D. Remnants of Massive Stars
1 Massive star goes supernova, the core that is left behind is
so massive that the electrons of elements are pulled into
the nucleus forming a neutron star.
2 Some neutron stars spin rapidly, burst of radio waves,
these are known as pulsars
3 Black hole – so dense that their gravity will not even let
light escape
IV. Galaxies and the Universe
A. What are galaxies
1 A system of millions of stars that appear as a single
star in our sky
2 There is between 50-100 billion galaxies
3 There are millions of light years between galaxies
4 Sun belongs to the Milky Way Galaxy which is a spiral
galaxy
5 Milky Way belongs to the Local Group of about 30
galaxies
B. Types of galaxies
1 Spiral – like the Milky Way
2 Elliptical – spherical to lens shaped
3 Irregular – much smaller and fainter with no shape
C. Active Galaxies
1 Galaxies that emit more energy than their combined stars
are said to be active
2 Currently it is thought that super massive black holes
are at the center of these galaxies