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Stars and Galaxies
Chapter 12
Stars
 Definition: a large ball of gas that
emits energy produced by nuclear
reactions in the star’s interior
 Planets, comets, and asteroids shine by
reflecting light from the Sun (which is
an average sized star).
 Distances between stars
 Distance between stars is measured in
Light Years (LY), which equals the
distance that light travels in one year.
 Light travels at 300,000 km/s
 1 LY = 9.5 trillion km
Formation of a Star
 Stars are formed in a NEBULA, which is a large cloud of gas
and dust
 Also called interstellar clouds
 Space between stars is called interstellar space
 Nebula can be so dense that they can block light (interstellar
cloud)
1) Gravitational force causes clumps of matter to form in
nebula
2) As they move closer, they move faster and heat up
3) As the clump contracts, it becomes spherical
4) When mass gets large enough it become a protostar
5) Temperature continues to increase and sphere becomes
disk-like (elliptical)
6) Continues to become denser and hotter and boom!
Nuclear Fusion! A new star is born!
Types of Stars
 By analyzing the light emitted by a star, you can learn about
the star’s motion, temperature, and chemical elements it
contains.
 The color of a star depends on its temperature (just like when
you heat metal):
 Red (3500° K) longest wavelength
 Yellow (5000° K)
 White (7000° K)
 Blue (25000° K) shortest wavelength
 The brightness of stars depends on two things:
 The amount of energy in the star
 The distance the star is from Earth
 Things far away look smaller!
Three sizes of stars can be born in a Stellar Nursery (nebula) – their lifecycle is below:
 Red Giant: sun-sized stars that use up their fuel
Types of Stars
 Hydrogen converts to helium and the core collapses
 As it expands, it cools and becomes a red giant
 White Dwarf: the dense core of a red-giant star
 Red-giant stars lose mass from their surfaces
 Fusion in the core has ceased and gravity causes it to
contract until it is about the size of Earth
 Some become so hot they emit a blue light
 The Sun will become a dwarf star in billions of years
 Supernova: the explosion of a supergiant star
 A supergiant star can explode before it dies
 The debris is still visible as an interstellar cloud
Kuiper Belt
 Similar to the astroid belt
(between Mars and Jupiter)
 20 times as wide and 20-200 times as
massive
 Extending from the orbit of
Neptune to about 50AU from the
sun.
 Only discovered in 1992
 Pluto is the largest known member
of the Kuiper Belt
 This discovery is what helped Pluto
to be reclassified as a “dwarf
planet”
 Definition: massive systems of stars, dust, and gas held
together by gravity are called galaxies
Galaxies
 Some contain billions of stars
 Shapes and Sizes:
 Spiral
 The Milky Way
 Elliptical
 Oval Shape
 Irregular
 No apparent shape
 The Big Bang Theory
 14 billion years ago the universe was the size of a tiny point that was
extremely hot. It began to expand rapidly and after several hundred
thousand years, it cooled, and hydrogen and helium atoms began to
form.
 Our solar system is about 4.5 billion years old