Stars & Galaxies
Chapter 21
Stars & Their
Characteristics
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Constellation – Group of stars that appears
to form a pattern. 88 recognized by
science.
Ursa Major – “Big Bear”, “Big Dipper”
Ursa Minor - “Little Bear”, “Little Dipper”
Cassiopeia – Lopsided “M”, Queen’s Chair
Circumpolar Constellations – Constellations
that appear to move around the North Star.
Stars & Their
Characteristics
Polaris – “North Star”
 Apparent movement of circumpolars
caused by rotation on axis.
 Seasonal Constellations –
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– Summer Triangle
– Orion
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Constellations lost in sunlight at
different times of the year.
Stars & Their
Characteristics
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Summer Triangle – 3 Stars
– VEGA in the constellation Lyra (The Harp)
– Altair in the constellation Aquila (The Eagle)
– Deneb in the constellation Cygnus (The Swan)
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Deneb at the top of the “Northern Cross”.
Orion – “The Hunter” Winter Constellation
– BETELGEUSE & RIGEL
– Belt stars help locate two other constellations.
Stars & Their
Characteristics
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Left of belt is SIRIUS of the constellation
Canis Major.
– SIRIUS-Brightest star in the nighttime sky.
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Right of belt is Taurus the Bull, Pleides.
Distances to Stars
– Astronomical Unit (AU)- Average distance from
our sun to the Earth.
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150 Million Kilometers
Alpha Centauri next nearest star to us. 300,000 x as
far from Earth as we are from the sun.
Stars & Their
Characteristics
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Stars – Most are different in density.
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Less different by mass.
Color of star depends on surface temperature.
Hot stars are Blue, Cooler stars are Red.
Made mainly of Hydrogen and Helium.
Our sun is 70 % Hydrogen/ 28 % Helium.
Each star has its own spectrum. (Fingerprint)
Stars & Their
Characteristics
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Brightness –
– Apparent Magnitude – How bright a star
appears from the Earth’s surface.
Kinds of Stars
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Red Giants - ALDEBARAN, ARCTURUS
Blue-white- hot, SIRIUS, VEGA
SUPERGiants- More luminous, greater in
size. Include blue-white RIGEL, Whiteyellow CANOPUS, red SuperGiants
ANTARES & BETELGUESE.
REDSUPERGIANTS- Largest of all stars
Kinds of Stars
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Dwarf stars – Absolute magnitude of less
than 1.
Most either red, orange, or yellow.
White dwarfs – very faint, small, dense.
Variable stars – Shine with different
brightness over several day periods.
Pulsating Stars- Change brightness, expand
& contract. Example is Cepheid Variables.
Kinds of Stars
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Eclipsing Binaries- When one star rotates
around another brighter star. The stars look
like 1 star that changes brightness.
Pulsars- 1967 Jocelyn Bell-Burnell. Crab
Nebula discovered. Both radio waves and
light came from a star in this nebula. Pulse
due to rapid rotation of star.
Formation of Stars
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Nebulae – Clouds of dust or gas in space.
Nebula dust seen by 2 methods:
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1. Light from nearby star is reflected off dust.
Example is a DIFFUSE NEBULA
Brightest known is in constellation ORION.
2. Nebulae clouds contact each other and glow.
These are called PROTOSTARS.
– Dark Nebulae- Away from stars. “HorseHead”
Formation of Stars
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Gas and Dust in space are called Nebulae.
Stable State – Stars whose diameter and
radiation stay the same for millions of years.
Our sun is in a stable state.
When fusion doesn’t balance gravity’s force,
core contracts(gets hotter) and outer layers
expand.
Star is bigger, brighter, more surface area.
Formation of Stars
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Dwarfs – Core temperature and pressure
not enough to support outer layer(s) weight.
Star collapses and becomes very dense.
Occasional flare ups called “NOVAS”
NOVA – “new star”
Our sun – Stable now, will become Red
Giant, fade to White Dwarf.
Formation of Stars
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SUPERNOVA – 1054 Chinese observed
one. It’s remaining dust cloud “Crab
Nebula” found in Taurus the Bull.
Best known modern SuperNova appeared in
1987. (Page 387 text) Figure 21.10
NEUTRON STAR – Mass that remains after
an exploded SuperNova.
Formation of Stars
BLACKHOLES - Object of space with
high gravitational pull. Light cannot
escape.
 Atoms are ripped apart before entering
a Blackhole. These atoms emit XRays that are detected by radio
telescopes.
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Galaxies and the
Universe
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Most galaxies are millions of light years
away.
Our sun is one star of 100 billion in the
MILKY WAY Galaxy.
Milky Way – 140,000 LY long, 20,000 LY
thick. Sun is 23,000 LY from its center.
Local Group – Small cluster of 17 galaxies.
Galaxies and the
Universe
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Nearest galaxy is the ANDROMEDA Galaxy
at 2 million LYs away.
Galaxy Types:
Spiral – Bright center, usually two arms that
hold majority of its stars. ( ¾ of galaxies)
Elliptical – spherical shape, most stars in the
center. No arms, gas, or dust.
Irregular- No set shape, least common.
Galaxies and the
Universe
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QUASARS – Discovered in 1961
– Very far away
– Calculated to be the most luminous
objects in space.
– Larger & more massive than stars.
– High radiation emission.
Science Background
Polaris is a cepheid variable with a
period of just 4 days. Its magnitude is
only 2.5 to 2.6. Polaris is a double
star. Its companion has a magnitude
of 8.8.
 Scientists compare a Cepheid’s
absolute and apparent magnitudes to
determine its distance from Earth.
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Science Background
Proxima Centauri – Closest star to
Earth. Its magnitude of 11 is no visible
to unaided eye.
 Alpha Centauri – Closest VISIBLE star
to Earth with the unaided eye.
 Both Centauri stars can only be seen
at locations within 30 degrees of the
south pole.
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Ch 21Vocabulary Words
Absolute magnitude - The brightness of a star
compared to other stars if they were compared at the
same distance.
Constellation – A group of stars that appear to
form a pattern.
Apparent magnitude – The brightness of a star as it
appears from Earth.
Dwarf Stars – Stars that have gone to the unstable
state. The inner core collapses and the star is very
dense.
Astronomical Unit (AU) – The average distance
between the sun and the Earth. 150 million kilometers.
Eclipsing Binary – Two stars that are in rotation
around each other. One is brighter than the other.
Looks like 1 star.
Big Bang Hypothesis – Theory of how the universe
first started.
Galaxies – Collections of billions of stars. Three
types include spiral,elliptical, and irregular. Local
Group
Luminosity- The actual brightness of a star as it
appears from just outside of the Earth’s atmosphere.
Light-year – The distance light travels in one year.
9. 5 trillion kilometers. (300,000 kilometers per
second)
Nebulae – A collection of space dust and gas. This is
the ingredients for starting a new planet.
Protostar – When two nebulae clouds rub together
and cause a glow.
Neutron star – The mass that remains after a star
goes supernova.
Pulsar – Stars that emitt radio waves that are in
time with the visible light waves that it also emitts.
“Crab Nebulae”
Nova – “New Star”
Quasars – The most luminous objects in space.
Very large. Very, very far away. Bigger than super
giant stars.
Black holes – Invisible objects in space that have a
gravitational pull so strong that no EM energy can
escape.
Red Giants – Highly luminous stars that are
considered to be “warm” compared to other stars.
Aldebaran Arcturus
Cepheid Variables – Yellow super giant stars that
Super Giants – Hundreds of times brighter than