Chapter 27
Stars and Galaxies
Section 2 and 3
27.2 Stellar Evolution
A star begins in a nebula, which is a cloud
of gas and dust
 Gravity causes the gas and dust to
compact.
 The shrinking, spinning region begins to
flatten into a disk with a central
concentration called a protostar

27.2 Stellar Evolution
The second and longest stage in the life of
a star is the main-sequence stage
 Main-sequence stars do not expand
because the force of gravity pulls the
matter inward.
 Some white dwarfs will just cool and die,
they are then called black dwarfs

27.2 Stellar Evolution
Some white dwarfs do not just cool and
die.
 They will have one or more large
explosions and become a nova
 A supernova is a star that has such a
tremendous explosion that it blows itself
apart.
 Before a supernova all of the stars
elements will be fused into iron

27.2 Stellar Evolution
After an explosion, the core of a
supernova may contract into a very small
but incredibly dense ball of neutrons,
called a neutron star
 Some neutron stars emit two beams of
radiation that looks like a lighthouse, these
are called pulsars

27.2 Stellar Evolution
Some massive stars produce leftovers that
are too massive to become neutron stars.
 The contraction crushes the dense core of
the star and become a black hole

27.3 Star Groups
Fixed patterns of stars are called
constellations.
 Astronomers label stars within each
constellation according to apparent
magnitude using greek letters.
 The brightest is labeled alpha, then beta
and so on.

27.3 Star Groups
Galaxies are large scale groups of stars
are bound together by gravitational
attraction.
 A typical galaxy is about 100,000 lightyears across and contains 100 billion stars
 There are 3 types of galaxies

27.3 Star Groups
A spiral galaxy has a center of bright stars
and flat arms that spiral around.
 An elliptical galaxy is oval in shape and
has a very bright center with no arms
around it.
 An irregular galaxy has no particular
shape and they tend to be smaller and
fainter.

27.3 Star Groups

Our galaxy is a spiral galaxy and is called
the Milky Way Galaxy