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Beyond Our Solar System –
The Universe in a Nutshell!
Chapter 25
Star Temperature
• Color is a clue to a
star’s temperature
• Very hot (30,000 K)
stars emit their light in
the blue spectrum, red
stars are much cooler,
stars with
temperatures between
5000 and 6000 K
appear yellow
Binary Stars
• Binary Stars –
pairs of stars,
pulled together by
gravity, that orbit
each other
• Binary stars are
used to determine
the star property
most difficult to
calculate – its mass
• The nearest stars have large parallax angles, while
those of distant stars are too small to calculate
• Light-Year – unit used to express stellar distance,
the distance light travels in one year (~9.5 trillion
• Our closest star (besides the sun), Proxima
Centauri, is about 4.5 light-years away from the sun
Stellar Brightness
• Apparent Magnitude – a star’s
brightness as it appears to Earth
• Absolute Magnitude – how bright a
star actually is
A Hertzsprung-Russell diagram shows the
relationship between the absolute magnitude
and temperature of stars
Nebulae – clouds of dust and gases in
Reflection Nebula in
Dark Nebula – Horsehead
Emission Nebula –
The Birth of a Star
•The birthplaces are dark, cool interstellar
clouds (nebulae)
•The initial contraction of the nebula can be
triggered by the shock wave from an explosion
of a nearby star
The Orion Nebula in
normal color and infrared
Protostars in the Horsehead
Nebula are circled
• Protostar – a
developing star not yet
hot enough to engage
in nuclear fusion
• When the core of a
protostar has reached
about 10 million K,
pressure within is so
great that nuclear
fusion of hydrogen
begins, and a star is
• Main-Sequence
Stage – From the
moment of birth
until the star’s
• The more massive
a main-sequence
star, the shorter its
life span
• A yellow star, like
our sun, can
remain in the
main-sequence for
approximately 10
billion years
Red-Giant Stage
• The Sun will spend less than 1 billion years
as a Red-Giant
Globular Star Cluster, some of the oldest stars in the universe
White Dwarf – remains of low and medium
mass stars, extremely small stars with
densities greater than anything on Earth
Life Cycle of a Sun-like Star
Neutron Stars – remnants of supernova
events, stars that are smaller and more
massive than white dwarfs
Black Hole – A massive star that has
collapsed to such a small volume that its
gravity prevents the escape of everything,
including light
Stellar Evolution
The Milky Way Galaxy
• Galaxies – groups of stars, dust, and gases held
together by gravity
• The Milky Way is a large spiral galaxy whose disk is
about 100,000 light-years wide and about 10,000
light-years thick at the nucleus
• It has at least three distinct spiral arms, the sun lies
about 2/3 of the way from the center on one of
these arms and orbits the nucleus about every 200
million years
• There may be more than 100 billion stars in our
galaxy alone
Structure of the Milky Way
Spiral Galaxies – like our galaxy, these
galaxies have multiple arms that sweep
out from a central nucleus
Elliptical Galaxies –
do not have spiral
arms, makes up
~60% of known
galaxies, can range
from round to oval
Irregular Galaxies – consist mostly of
younger stars, appear as clouds of stars
Galaxy Cluster – a system of galaxies
containing from several to thousands of
member galaxies
The Expanding Universe
• Hubble’s Law – galaxies are retracting from
us at a speed that is proportional to their
• The red shifts from distant galaxies indicate
that the universe is expanding
• Big Bang Theory – The universe began as a
violent explosion from which the universe continues
to expand, evolve, and cool
• The big bang theory states that at one time, the
entire universe was confined to a dense, hot,
supermassive ball. Then, about 13.7 billion years
ago, a violent explosion occurred, hurling this
material in all directions