stars, galaxies & the universe · stars, galaxies & the universe. composition of stars •...
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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 tell us 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 brighter the
star, the lower the magnitude.
– Sirius, has a magnitude of -1.4.
– The dimmest star that can be seen with a
teloscope 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.
*1 parsec = 19.2 trillion miles
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 longest stage of its life cycle as a
main sequence star.
• Energy is generated in the core as hydrogen
atoms fuse into helium atoms.
Stellar Evolution
• Giants and Super Giants After the main-
sequence stage, a star can enter the Red Giant
stage.
• 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