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Stars, Galaxies
& The Universe
Composition of Stars
• Made of different elements in gaseous state
• The gases in the atmosphere of a star absorb
different wavelengths of light depending on
which elements make up the gases.
• The light from a star indicates which
elements make up that star.
The Colors of Light
• A prism breaks white light into a rainbow of
colors called a spectrum.
• A spectrograph breaks a star’s light into a
spectrum.
• The spectrum of a star depends on
which elements are present.
Making An ID
• Emission lines are lines made when certain
wavelengths of light (colors) are given off by hot
gasses.
• Each element produces a unique set of lines,
which allows them to help identify elements in a
star.
Classifying Stars
• Differences in Temperature Stars are now
classified by how hot they are.
• Differences in Brightness The brightest star,
Sirius, has a magnitude of -1.4. The dimmest
star that can be seen with a microscope has a
magnitude of 29.
Magnitude of Big Dipper Stars
How Bright Is That Star?
• Apparent Magnitude The amount of light
received on Earth from a star.
• Absolute Magnitude The actual brightness of
a star.
Distance to the Stars
• Astronomers use light-years to measure
the distances from Earth to the stars.
• A light year is the distance that light
travels in a year. (9.5 trillion km)
– Radius of our solar system: 5.5 light hours
– Nearest Star (alpha centauri) 4.3 light years
– Radius of Milky Way: 50,000 light years
Distance to the Stars
• Parallax is the apparent shift in the position of
an object when viewed from different locations.
Measuring parallax enables scientists to
calculate the distance between a star and the
Earth.
Motions of Stars
• The Apparent Motion of Stars The stars
appear to move. This is due to Earth’s rotation.
Motions of Stars
• The Actual Motion of Stars Each star is
moving in space. Their actual movements
are difficult to see.
http://www.yorku.ca/ns1745b/bigdipper.mov
Stellar Evolution: Life Cycle of a Star
Stellar Evolution
• The Beginning A star enters the first stage of
its life cycle as a ball of gas and dust (nebula).
• Gravity pulls the gas and dust together, and
hydrogen changes to helium during nuclear
fusion.
Orion Nebula
Stellar Evolution
• Main-Sequence Stars After a star forms, it
enters the second and longest stage of its life
cycle known as the main sequence. Energy is
generated in the core as hydrogen atoms fuse
into helium atoms.
Stellar Evolution
• Giants and Super Giants After the mainsequence stage, a star can enter the third stage
of its life cycle. A red giant is a large, reddish
star formed from the heating core and
expanding size.
• The core continues to heat and the star
expands even more to form a
super giant.
Stellar Evolution
• White Dwarf As the core of a supergiant uses
up its helium supply, the outer layers escape into
space and the remaining core is white hot, thus
called a white dwarf.
Stellar Evolution
• A supernova is a gigantic explosion in which a
massive blue star collapses.
• A small star that has collapsed under gravity so
all of its particles are neutrons is called a
neutron star.
• If a neutron star is spinning, it is called a pulsar.
Stellar Evolution
• Black Holes Sometimes the leftovers of a
supernova are so massive that they collapse to
form a black hole.
• A black hole is an object that is so massive
that even light cannot escape its gravity.
Stellar Evolution: Life Cycle of a Star
A Tool for Studying Stars
• The H-R Diagram the Hertzprung-Russell
diagram is a graph that shows the relationship
between a star’s surface temperature and
absolute magnitude.
• Reading the H-R Diagram The diagonal
pattern on the H-R diagram where most stars lie
is called the main sequence.
H-R Diagram
Galaxies
• A galaxy is a collection of stars, dust, and gas
held together by gravity.
Types of Galaxies
• Spiral Galaxies have a bulge at the center and
spiral arms made up of stars and dust
• The Milky Way Our spiral galaxy, which contains
about 200 billion stars and many nebulas
Types of Galaxies
• Elliptical Galaxies Most common type of
galaxy; large 3-dimensional football shaped
galaxy holding older and dimmer stars.
• Irregular Galaxies Galaxies that do not fit into
any other class are called irregular galaxies.
Contents of Galaxies
• Gas Clouds A large clouds of gas and dust in
interstellar space is called a nebula.
• Star Clusters
A globular cluster is a tight group of stars that
looks like a ball and contains up to 1 million stars.
An open cluster is a group of stars that
are close together relative to other stars.
Gas Clouds and Star Clusters
Origin of Galaxies
• Looking at distant galaxies reveals what early
galaxies looked like.
• Quasars A very bright, star-like object that
generates lots of energy. These may be the
core of young galaxies that are in the process of
forming.
The Universe
• The universe is possibly limitless and contains
everything.
• Cosmology is the study of the origin, properties,
processes, and evolution of the universe.
Formation of the Universe
• Universal Expansion The universe, like the
rising raisin bread dough, is expanding. Think of
the raisins in the dough as galaxies. As the
universe expands, the galaxies move farther
apart.
• Doppler Shift As waves
(sound, light, etc.) move away,
the lengths of waves increase
– Approaching stars move toward blue
– Departing stars move toward red
The Big Bang Theory
• The theory that the universe began with a
tremendous explosion, causing the expansion
we observe
• Cosmic Background Radiation In 1964, two
scientists accidentally found radiation
coming from all directions in space
possibly left over from the big bang.
A Forever Expanding Universe
• Expansion or destruction of the universe
depends on the amount of matter it
contains.
• Lots of matter would cause gravity to stop
the expansion, and start collapsing.
• We think that there isn’t enough matter, so
the universe would continue to expand
forever and become cold and dark as
stars die.
Structure of the Universe