Download Chapter 28 – Stars and Galaxies

Chapter 28 – Stars and
Galaxies
 Page 610
 Why is this galaxy so bright?
 What shape is this galaxy?
 How do we obtain images like this?
Chapter 28.1
A Closer Look at Light
What is light?
 Light is a form of electromagnetic
radiation (EM)
 Other types of EM listed from longest
to shortest wavelengths
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Radio
Microwave
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays
 This
is known as the EM spectrum
 All EM energy travels in waves and at
the speed of light
 Can travel through empty space
(vacuum)
Spectroscope
 Visible white light is actually made up
of light of various colors each with a
different wavelength
 The various color can be observed
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In a rainbow
Passing light through a prism
Or an astronomers instrument known as a
spectroscope
 The colors ROY G. BIV are aligned
from longest to shortest (violet)
 Longer wavelengths refract less
Chapter 28.2
Types of spectra from
spectroscopes
 Continuous spectrum – unbroken
band of colors which are emitted by
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Glowing solids (filament)
Glowing liquids (molten iron)
Hot compressed gases (inside stars)
 Emission spectrum – lines of different
colors
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Produced by glowing thin gases
 Each
element has its own spectra therefore
scientists can identify the gas
 Absorption spectrum – a continuous
spectrum crossed by dark lines
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Elements in the thin gas that surround a
star absorb the same wavelength they
would emit
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.
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Absorption spectrum can also
determine a planets atmosphere
Doppler Effect
 By observing how the spectral lines
are shifting, scientists can tell how a
star is moving compared to the earth
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Move to the red end, “red shift,” the star is
moving away
Move to the blue end, “blue shift,” star is
getting closer
By using spectrums from the lab and the
stars, they can determine how fast they are
moving
Chapter 28.3
Stars and Their
Characteristics
Constellations – name given
to groups of stars
 There are 88 of them
 Big dipper – best known asterism
(small star grouping) is actually part
of a larger grouping known as Ursa
Major
 We can use the dipper to find other
constellations
 The stars appear to move in two ways
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Nightly variation – due to earth’s
rotation
Yearly due to earth’s revolution
– winter constellation
 Lyra – summer constellation
 Orion
Apparent magnitude
 Is the brightness of the star as seen
from earth
 The lower the number, the brighter it
is
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Sun is –26.7
Faintest is +6
Distance to stars
 Astronomical Unit (AU) distance from earth to sun –
150 million km
 Proxima Centauri – next nearest star is 260,000AU
 Therefore, astronomers use light-years – a unit of
measure that light travels in one year.
 Light travels about 300,000km/sec  1 year =
9.5×1012
 Proxima Centauri is about 4.2 light years away
Elements in Stars
 Stars are mostly of super-hot gases –
mostly H & He
Mass, Size and Temperature
of Stars
 Mass is something that can not be
observed directly. It can only be
calculated based on other
observations
 Stellar mass is expressed as
multiples of the sun’s mass
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Betelgeuse’s mass – 20 solar masses
Temperature and Color
 Blue stars are hot
 Red stars are cool
Luminosity
 The actual brightness of the star is luminosity
 If two stars have the same surface temperature, the
larger star would be more luminous
 If the same size, hotter one would be brighter
 Types of magnitude
 Absolute – as if all stars were same distance
from earth
 Apparent – as they appear in the nighttime sky
Variable Stars
 Some stars show regular variation of brightness over
cycles that last from days to years
 Cepheid Variables – yellow supergiants whose cycles
range from 1 to 50 days
 If a Cepheid is located in another galaxy,
astronomers can find the distance to these
galaxies by comparing absolute and apparent
magnitudes
 Other stars change in brightness because they
revolve around another star. This is known as a
‘binary star system.’
Chapter 28.4
Life Cycles of Stars
Hertzsprung-Russel
Diagram
 A diagram to help explain a stars life
 Most stars fall into 4 distinct groups
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Main sequence – 90% of stars
 Our
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sun
Giants – 10-100x bigger than our sun –
more luminous
Supergiants – more than 100x bigger
than our sun
White dwarves – stars near the end of
their lives
Birth of a Star
 Stars begin their life as a nebula
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Huge cloud of gas
Death of a Star
 2 paths
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Nebula  protostar  main sequence 
red giant  planetary nebula  white dwarf
Nebula  protostar  massive star  red
supergiant  supernova  blackhole or
neutron star
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)
Remnants of Massive Stars
 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.
 Some neutron stars spin rapidly, burst
of radio waves, these are known as
pulsars
 Black hole – so dense that their gravity
will not even let light escape
Chapter 28.5
Galaxies and the Universe
What are galaxies
 A system of millions of stars that appear
as a single star in our sky
 There is between 50-100 billion galaxies
 There are millions of light years between
galaxies
 Sun belongs to the Milky Way Galaxy
which is a spiral galaxy
 Milky Way belongs to the Local Group of
about 30 galaxies
Types of galaxies
 Spiral – like the Milky Way
 Elliptical – spherical to lens shaped
 Irregular – much smaller and fainter
with no shape
Active Galaxies
 Galaxies that emit more energy than
their combined stars are said to be
active
 Currently it is thought that super
massive black holes are at the center
of these galaxies