Transcript Chapter 27 Stars and Galaxies

Stars and Galaxies
BIG Idea:
►The
life cycle of every star is
determined by its mass,
luminosity, magnitude,
temperature, and composition.
►Much
of our information about
our galaxy and the universe
comes from ground-based
observations
Hubble Space Telescope:
► In
orbit since 1990.
► Takes extremely
sharp images from
space.
► Hubble observations
have led to numerous
scientific
breakthroughs,
including rate of
expansion of the
universe.
I. Characteristics of Stars
A.
Composition and
Temperature
►
What are stars
made of?
► mostly
hydrogen
(H)…about 73% of
a star’s mass,
► approximately
25% helium (He),
► and the other
elements in small
amounts: oxygen
(O), carbon (C),
nitrogen (N),
silicon (Si),
magnesium (Mg),
neon (Ne), iron
(Fe), sulfur (S)
Spectral Types:
►Spectroscope:
instrument used
to determine chemical
composition by separating
light into different colors
(wavelengths).
Which stars are the hottest
stars?
•
Blue stars are the hottest
•
Red stars are the coolest
B. Motion and Distance to the
Stars
► Doppler
Effect: shift in wavelength of
light source moving toward or away
from an observer.
 Blue Shift: shorter wavelengths,
stars moving towards Earth
 Red Shift: longer wavelengths, stars
moving away from Earth
Distances between stars and Earth
are measured in light-years.
Light
Year = distance a light
wave travels in one year
(one
light year = 236,750,151
times around the Earth)
C. Stellar Magnitude
1. Apparent Magnitude: how bright a star
appears from Earth (depends on light emitted
and distance from Earth).
LOWER NUMBER = BRIGHTER STAR
2. Absolute Magnitude: true
brightness of a star.
 How bright a star would appear if
seen from the same distance (32.6
light years)
 Most stars fall between -5 and
+15
 Our sun is +5: middle of the range
3. Luminosity: energy output from
the surface of a star per second;
measured in watts.
► An
Astronomer must know both the
star’s apparent magnitude and how far
away the star is.
► The brightness depends on both a
star’s luminosity and distance from
Earth.
D. Classification of Stars
► H-R
Diagram – shows relationship between
absolute magnitude and surface temperature of
star
 the brighter the star, the hotter it is
►Stars
have a finite lifetime and
evolve over time
►The mass of a star controls its
evolution, length of lifetime,
and ultimate fate
►As stars evolve, their positions
on the Hertzsprung-Russell
diagram move…
II. Stellar Evolution “theory”
► 1.
Nebula – stars
start out as clouds
of gas and dust
 70 % hydrogen
 28 % helium
 2% heavier
elements
►Nebula
in the
Constellation
Orion
►Star
Formation
► Gravity
pulls particles together,
forming a sphere
► As density increases, gravitational
attraction increases
► Gravitational forces cause denser
regions of nebula to shrink
► As regions become smaller, they spin
more rapidly
►Think
of an
ice skater…
…as he/she
pulls his or her
arms in closer,
what happens
to the rate of
the spin?
Protostar – flattened disc of matter
with a central concentration (caused
by shrinking, spinning region)
►Pressure
and density build within the
protostar’s center, causing
temperature to rise
►Gas is so hot it becomes plasma (a
fourth state of matter)
►Temperature continues to increase
until it reaches 10,000,000ºC
►At this temperature nuclear fusion
begins
►A star is born !
2. Main Sequence Stars –
► The
second and
longest stage in the
life of a star
► Most
stars fall within
the main sequence
band
► Our
sun is a Main
Sequence Star
►During
this main sequence stage,
energy is generated in the core of the
star as hydrogen atoms fuse to
become helium atoms
►Fusion releases huge amounts of
radiant energy
3. Giants – very large cool bright star
► Hydrogen
starts to run
out and the star
expands greatly.
► Super Giants are very
big Giants
► Our sun is 5 billion
years old and has only
converted 5 % of its
hydrogen to helium.
A Giant is 10x bigger than
the Sun and a Supergiant is
100x bigger
4. White Dwarf – final stage of a star
►
Planetary
Nebula – expanding
shell of gases shed by
a dying star
►Gravity
causes the last of the matter in
the star to collapse inward
►What remains is a hot, dense core of
matter…a WHITE DWARF
►White dwarfs shine for billions of years
before they cool completely
►As white dwarfs cool they become
fainter and fainter…
►When
they no longer emit energy, they
become a black dwarf, a dead star
Black dwarfs probably do not yet
exist…
WHY?
Novas – explosions that occur as a
white dwarf cools
► Supernova
– star
that has such a
tremendous
explosion that it
blows itself apart.
Neutron Stars – small but incredibly dense
ball of neutrons, formed from the collapsed
core of a supernova.
► One
teaspoon of
material from a
neutron star would
weigh 100 million tons
on Earth.
Black Holes – hole left by the collapse of
a supernova.
► The
gravity of a black hole is so great that
not even light can escape from it.
III. Star Groups
A. Constellations: patterns of stars
in the sky
 there are 88 different patterns of
stars recognized.
►
►
►
B. Galaxies
►A
galaxy is a large group of stars
bound by gravity.
 typically 100,000 light-years
wide
 contain billions of stars
Types of Galaxies
1. Spiral: central mass (nucleus) of
bright stars with flat arms that spiral
around it
 arms contain millions of young
stars, gas, and dust.
Top View
Side View
2. Elliptical: nearly spherical with very
bright centers; no spiral arms
No young stars, dust, or gas
3. Irregular: no particular shape.
 small and faint, with little
gravitational attraction to organize it
into a shape
 may also be unorganized due to the
collision with another galaxy
What type of galaxy do we
live in?
The Milky Way
-Spiral galaxy in which
our solar system is
located
-100,000 light-years
wide
- The sun is 30,000 lightyears from the center
- The sun revolves
around the center at 250
km/sec
- It takes 200 million
years to make one
revolution.
IV. FORMATION OF THE
UNIVERSE
► The
Big Bang Theory: most widely
accepted theory explaining the formation of
the universe.
 All matter and energy in the universe was
once concentrated in a very small, very
hot, very dense volume
 12-15 billion years ago, the “big bang”
occurred
 Matter and energy were propelled
outward in all directions
 The universe began to E X P A N D…
 As matter and energy moved outward,
the force of gravity had an effect
 Matter began to condense, forming the
galaxies
 The galaxies continued to move outward,
as they continue to do today…
V. The Sun
► There
is nothing special about the Sun…
 It’s just close enough to Earth to give us
light and warmth
 The Sun is similar to most other stars in
our galaxy:
►A large ball of gas made mostly of
hydrogen and helium held together
by gravity.
► The Sun is 300,000 times larger than Earth
A. Structure of the Sun
1. Core
 The very hot (15,000,000ºC)
gaseous center
 This is where nuclear fusion occurs
 FUSION is how the Sun makes its
energy!
Hydrogen
Helium
Hydrogen
2. The Inner Zones
a. Radiative Zone
►Zone surrounding the core
►Heat energy moves by radiation
b. Convective Zone
►Around the radiative zone
►Heat energy moves by convection
3. The Sun’s Atmosphere
(uppermost region of solar
gases)
 a. Photosphere:
“light sphere”
►visible
surface of the
Sun
►what we see
b. Chromosphere: “color sphere”
thin
layer
seems to glow with reddish light
► c.
Corona: “crown”
 outermost layer of Sun’s atmosphere
 cannot be seen with naked eye,
except during TOTAL solar eclipse
 huge cloud of gas that keeps the
atomic particles from the surface
from escaping into space
►Solar
winds
 the corona does not have a
defined edge
 gas flows outward from the corona
at high speeds, forming solar
winds
 electrically charged atomic
particles stream into space
through holes in the corona
 flow to distant parts of solar
system
B. Solar Activity
1. Sunspots: cooled regions within the
photosphere
►Appear darker than the areas around
them
2. Prominences:
 disturbances in solar atmosphere
 great clouds of glowing gases
 huge arches that reach high above Sun’s
surface
3. Solar Flares: sudden violent eruptions of
electrically charged atomic particles.
4. Auroras
(aka “Northern Lights” or
“Southern Lights”):
► Magnetic
storms in the Earth’s upper atmosphere
► Solar winds attracted to Earth’s magnetic poles by
the Earth’s magnetosphere (space around Earth
that contains a magnetic field)
► Electrically charged particles strike gas molecules
in the upper atmosphere
► Green, red, blue, or violet sheets of light are
produced
► Visible about 5 times per year, usually in polar
regions
►
VI. Our Solar System
► How
many stars do we have in our
Solar System?
► What
bodies make up our Solar
System?
Formation of the Solar System
► Solar
system = the Sun and the planets and
other bodies that travel around the Sun
► Solar
Nebular Theory: our best current
idea for the origin of the solar system…
► Big
Bang spread matter throughout universe
► Some matter gathered into clouds of dust
and gas
► Cloud of gas and dust that eventually
developed into our solar system is called the
‘SOLAR NEBULA’
► Solar nebula was larger than our solar
system is now
► Heat and pressure from force of gravity
caused center of solar nebula to become
hotter and denser
► While
the Sun was forming in the center of
the solar nebula, planets were forming in
the outer regions
► Planetesimals (small bodies of matter
within the solar nebula) joined together
through collisions and the force of gravity to
form larger bodies called protoplanets
► Protoplanets then condensed into existing
planets and moons…
►The
four protoplanets closest to the
Sun became:
►Mercury
►Venus
►Earth
►Mars
►The
next four protoplanets became:
►Jupiter
►Saturn
►Uranus
►Neptune