CHAPTER 28
STARS AND GALAXIES
28.1 A CLOSER LOOK AT LIGHT
 Light is a form of electromagnetic radiation,
which is energy that travels in waves.
 Waves of energy travel at 300,000 km/sec (speed of
light
 Ex: radio waves and x-rays
 Electromagnetic radiation waves are arranged into a
continuum call the electromagnetic spectrum.
 Wide range of wavelengths
 Long wavelengths with low frequencies at one end, short
wavelengths with high frequencies at the other end
 Wavelength measured crest to crest/trough to trough
 Frequency the number of that crests of the same
wavelength that pass a point in one second.
 Scientists study the visible light portion of the
electromagnetic spectrum.
 Spectra of a stars allow for astronomers to learn
about the star’s elements and motion.
 Spectra studied using a spectroscope
 Three types of visible spectra
 Continuous spectrum: unbroken band of colors,
emitting all colors of visible light
 Emission spectrum: unevenly
space of lines of different colors,
emitting light of only some
Wavelengths
 Absorption spectrum: dark lines that
cross a continuous spectrum.
 Doppler Effect - change in the wavelength of
sound due to motion between the object and
the receiver.
 Doppler effect applies to lights as well as sound.
 Shift of the emission spectra can indicate if the object
is moving towards or away from Earth
 Shift towards red end of spectrum, object moving
away Earth – Redshift
 Shift towards blue end of spectrum, object moving
towards Earth - Blueshift
 Doppler effect determined that the universe is
expanding
TELESCOPES
 Optical Telescopes – gather far more light than
an unaided eye and magnify imagers
 Reflecting
 Uses one lens at back to gather and focus light
 Image reflected on to a small mirror and then the eye
piece
 Refracting
 Uses two lenses
 Lens at the front gathers light
 Eyepiece magnifies image
 Radio Telescopes = big satellite dishes
 Use to detect energy waves at frequencies lower
than visible light
 Other
 Usually satellites in space
 Gamma ray
 Background radiation
 X-ray
 Hubble (infrared)
28.2 Stars and Their Characteristics
 Observation of stars has been going on for over
5000 years
 The grouping of stars are called constellations
 Constellations
 only appear together as viewed from Earth; from a
different angle they do not look like the constellation
 Constellations (continued)
 Constellations will change
shape over thousands of years
due to the universe expanding
 Move across the sky from east
to west (though Earth rotates
west to east)
 North Star – Current is Polaris
 Sits directly over the North pole
 Does not move to the naked
eye
 Very powerful tool for
navigation
 Due to Precession, Polaris will
not always be the “North Star”
 The Constellations that dominate the night sky change
from month to month. This is the result of the Earth’s
change in position as it orbits the Sun.
 Distances to stars and other objects in space
 Astronomical Unit (AU) - the distance from Earth to the
Sun (150 million kilometers)
 Light year - the distance light travels in one year (9.5
trillion kilometers)
 It is a distance measurement
 Example - 4.2 light-years means that the light we see has been
traveling for 4.2 years before we can see it (4.2 X 9.5 trillion km)
 Parallax - change in an object’s direction due to a change
in the observer’s position
 Parsec short for “parallax second” equal to 3.258 lightyears.
Parallax
The further the object
from the viewer, the less
the parallax shift.
 Stars
 No two stars have the same proportions of
elements
 Elements
 Hydrogen ~69%
 Helium ~29%
 Heavier elements ~2%
 light that radiates is dependent on composition
and temperature, this differs in every star
 Star spectrum is its fingerprint
 Mass, Size and Temperature
 Stars vary greatly in masses, size and temperature
 Cannot observe directly so we are estimating what
the mass might be
 Gravitational effect on bodies around the star help with
estimating its mass
 Star mass is expressed as multiples of the mass of our
Sun (which has a stellar mass of 1)
 Size varies more than mass
 Smallest stars are smaller than Earth
 Largest have diameters more than 2000 times that of
our Sun
 Stars differ even more in density
 Betelguese is about one ten-millionth of our Sun
 One star is so dense that one teaspoon would weight
more than a ton on Earth
Star size comparison
 Temperature of stars vary
 Range of color emitted is dependent on the surface
temperature
 Cool stars are red
 Ex. Betelguese with a surface temperature of 3000oC
 Mid-temperatures are yellow
 Ex. The Sun with a surface temperature of 5500oC
 Hot stars are blue
 Ex. Sirus
 Harvard Spectral Classification Scheme - group
stars by temperature and color
 Luminosity = brightness of a star
 Dependent on size and temperature of the star
 If two stars are the same size the hotter star
would be more luminous
 Apparent magnitude - how bright a star
appears
 Does not factor in distance
 Absolute magnitude - how bright the star
would be if all stars were the same distance
from Earth (10 parsec)
 Variable stars -show a variation in brightness
 Cepheid variables are yellow supergiant stars with a
cycle of brightness ranging from 1- 50 days.
 Most have a cycle of 5 days.
 Nonpulsating star change in brightness due to
fact that it is more than one star.
28.3 LIFE CYCLE OF STARS
 Hertzsprung-Russell (H-R) diagram – shows
luminosity, temperature, and stages in the life
cycle of the stars
 Main Sequence – 90% of stars run in a band
from upper left to bottom right of diagram
 Giants & Supergiants – more luminous, found
above main sequence
 White dwarfs – near the end of their lives,
below main sequence, glowing stellar core
Hertzsprung-Russell Diagram
 http://www.spacetelescope.org/videos/heic1017b/
Life cycle of a star like our Sun
 A star begins its life in a cloud of gas ( mostly
Hydrogen) and dust called a nebula
 nebula condenses, becomes denser, temp. increases 
becomes a protostar fusion begins and star is “born”
 Hydrogen in core continues to fuse into helium
 When hydrogen “runs out”, fusion occurs
outside the core and the star expands (giant)
 Gas layers are blown away and the carbonoxygen core is left (a white dwarf)
Life cycle of a Massive Star
 Begins like our sun
 Instead of a carbon-oxygen core forming, an
iron nuclei forms, and the star expands to
100x the size of our sun (supergiant)
 Iron nuclei absorbs energy and collapses
(supernova)
 Massive star remnants become a Neutron Star or
Black Hole
http://www.teachertube.com/viewVideo.php?video_id=73345 (about 9 minutes)
Galaxies and the Universe
 Universe –everything that exists, 10 billion-20
billion years old
 Galaxy – group of stars held together by gravity
 3 types:
 We live in the Milky Way Galaxy ( a spiral galaxy)
 Elliptical Galaxy- concentrated, spherical shape
 Irregular Galaxy – smaller, fainter, spread unevenly
3 types of galaxies
SPIRAL GALAXY (MILKY WAY)
ELLIPTICAL GALAXY
IRREGULAR GALAXY
Origin of the Universe
• Big Bang Model – explains the history of the
universe from a fraction of a second AFTER it
came into being up to present time
– Evidence supporting it = distance between galaxies is
increasing (universe is expanding)
– Edwin Hubble found redshifts in the spectra of the galaxies
– Cosmic background radiation found with radio telescopes
– http://www.youtube.com/watch?v=HEheh1BH34Q