stars, galaxies, and the universe section 1 ... · composition of stars astronomers use an...
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Stars, Galaxies, and the Universe
Section 1 – Characteristics of Stars
Color of Stars How do astronomers learn
about stars when the stars
are too far away to visit?
Astronomers study starlight
Think of the flame of a Bunsen burner or a campfire Which color of flame is the hottest?
The blue flame is the hottest
The yellow flame is the coolest
Astronomers can use this same idea to judge how hot a star is burning
Composition of Stars Stars are made up of different elements in the form of
gases
White light is made up of all the colors of the spectrum
Composition of Stars Astronomers use an instrument called a spectrograph
to break a star’s light into a spectrum (just like a prism breaks white light into the colors of the rainbow)
Each star gives off its own unique spectrum which is indicative of its composition and temperature
Classifying Stars Stars are classified by how hot they are
Temperature differences between stars result in color differences you can see
Table 27-1 on Page 547
How hot are blue stars? Yellow? Red?
How hot is our Sun?
Classifying Stars Stars are also classified based on
the brightness.
Positive numbers represent dimmer stars
Negative numbers represent brighter stars
Brightness of Stars The brightness of a light or star is called apparent
magnitude
If you look at street lights, the ones further away look less bright and the ones closer look brighter. Apparent magnitude is dependent on distance
Brightness of Stars Astronomers use a star’s apparent magnitude and its
distance from Earth to calculate its absolute magnitude
Absolute magnitude is the actual brightness of a star
Our sun has an absolute magnitude of +4.8, which is ordinary for a star. But because the sun is so close to Earth it has an apparent magnitude of -26.8
Distance to Stars Remember, stars are very very far
away. So far that we have to measure the distance to them in light-years
The North Star (Polaris) is 431 light-years away or 4,080,000,000,000,000 km
How do astronomers measure the distance to stars?
Distance to Stars Stars near earth seem to move, while more-distant
stars seem to stay in one place
It’s like looking out into a field when your driving in a car. The trees way in the background seem to stay in place while the fence next to the ditch goes by you very quickly
Distance to Stars A star’s apparent shift
in position is called parallax.
This shift can be seen only through telescopes
Astronomers can use parallax and simple trigonometry to find the actual distance to stars
Motion of Stars Because of Earth’s rotation, the sun and stars appear to
move across the sky.
At night, the sky appears to be rotating above us due to the rotation of the Earth on its axis
Because Polaris is directly above the North Pole this star does not appear to move while all other stars rotate around Polaris
Motion of Stars Things in space are not standing
still. The stars are all moving in space.
Because stars are so distant their actual motion is hard to see
The constellations we see in the sky would have looked different hundreds of thousands of years ago, and they will look different in the future
Stars, Galaxies, and the Universe
Section 2 – Stellar Evolution
Stars Life Cycle A star begins as a ball of gas and dust called a nebula.
Gravity pulls the gas and dust together into a sphere.
As it becomes more dense, it gets hotter and the hydrogen changes to helium in a process called nuclear fusion.
Types of Stars Stars can be classified by their size, mass, brightness,
color, temperature, spectrum, and age.
Types of stars include:
Main-sequence stars, giants, supergiants, and white dwarf stars
Main-Sequence Stars After a star forms, it enters the second and longest
state of its life cycle known as the main sequence
Energy is generated in the core of the star as hydrogen atoms fuse into helium atoms
This process releases huge amounts of energy
The star will stay the same size as long as there is enough hydrogen
Giants and Supergiants Third stage of a stars life cycle
A red giant is a star that expands and cools once it uses all of its hydrogen
Red giants can be 10 times bigger than the sun
Supergiants can be at least 100 times bigger than the sun
White Dwarfs The final stage(s) of a star’s
life cycle
A white dwarf is a small hot star that is the leftover center of an old star
No more hydrogen left
Can shine for billions of years before they cool completely
H-R Diagram Hertzsprung-Russell Diagram
It is a graph that shows the relationship between a star’s surface temperature and its absolute magnitude
When Starts get Old Most stars go from main-sequence, to red giants, to
white dwarfs
Some stars that are more massive than the sun may explode with extreme intensities
Supernovas Massive stars use their
hydrogen faster, and therefore become much more hot
A supernova is a gigantic explosion in which a massive star collapses and sheds its outer layers into space
Neutron Star Following a supernova, the center of the
collapsed star contracts to form a new star
The particles inside the star’s core are forced together to form neutrons
A star that has collapsed under gravity to the point at which all the star’s particles are neutrons is called a neutron star
If a neutron star is spinning it is called a pulsar
Black Holes A black hole forms when the
center of a collapsed star is so massive that the force of gravity crushes it into an extremely dense object
This black hole is so massive and dense that even light cannot escape its gravitational pull
Stars, Galaxies, and the Universe
Section 3 – Galaxies
Types of Galaxies Large groups of stars, dust, and gas are called galaxies
Our galaxy is called the Milky Way
Galaxies can contain more than a TRILLION stars!!!
Galaxies are primarily classified by their shape
Spiral Galaxies Spiral galaxies have a bulge at the center and spiral
arms
The spiral arms are made up of gas, dust, and “new” stars
The Milky Way is a spiral galaxy
28,000 light years from center
In the Orion arm
Elliptical Galaxies 1/3 of all galaxies are simply massive blobs of stars
Many look like spheres, others are more stretched out
These are called elliptical galaxies
Irregular Galaxies Edwin Hubble first classified galaxies and he had a
group of leftovers
Irregular galaxies are essentially all the “leftover” galaxies that don’t fit into the other categories
Contents of Galaxies Galaxies are composed of billions of stars and
planetary systems
Some of these stars form large, features, such as gas clouds and star clusters
Gas Clouds The Latin word for “cloud” is
nebula
A nebula is a large cloud of gas and dust
Some nebulas glow, others absorb light and hide stars
Spiral galaxies usually contain nebulas
Star Clusters Globular clusters are groups of older stars
A globular cluster is a group of stars that looks like a ball
There can be up to 1 million stars in a globular cluster
Origin of Galaxies Scientists investigate the early universe by observing
objects that are extremely far away in space
Because it takes time for light to travel through space, looking through a telescope is like looking back in time
Origin of Galaxies Some of the most distant objects are quasars
Quasars are star like sources of light that are extremely far away
Quasars are one of the most powerful energy sources in the universe
They may be caused by massive black holes in the cores of some galaxies
Many astronomers believe that quasars are the most distant objects yet detected in the universe. Quasars give off
enormous amounts of energy - they can be a trillion times brighter than the Sun! Quasars are believed to produce their energy from massive black holes in the center of the galaxies in which the quasars are located. Because quasars are so bright, they drown out the light from all the other stars in the same galaxy.
Stars, Galaxies, and the Universe
Section 4 – Formation of the Universe
Universal Expansion Cosmology is the study of the origin, structure, and
future of the universe
Remember, the universe is expanding
Astronomers know this because of the red-shift effect
The Big Bang Theory If the universe if
expanding…what does this tell us if we go back in time?
As we go back in time the universe would appear to be contracting, until it comes together at a single point
The theory that the universe began with a tremendous explosion is called the Big Bang Theory
The Big Bang Theory The Big Bang
occurred approximately 13.7 billion years ago
The contents of the universe were compressed under extreme pressure, temperature, and density in a very tiny spot
Big Bang Theory In 1964 two scientists accidentally found radiation
(radio waves) coming from all directions in space
The call this “noise” cosmic background radiation and it is believed to be left over from the big bang
How Old is the Universe? Measure the distance from Earth to various galaxies
Extrapolate back in time
Calculate the ages of the oldest nearby stars
The universe obviously has to be at least as old as the oldest star
A Forever Expanding Universe The universe will continue to expand for probably
billions of years, maybe forever
Scientists think that if there is enough matter that eventually gravity could pull all the matter of the universe together back to a single point (the reverse of the big bang)