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The Universe. Chapter 14. The night sky . Sky observations date to ancient civilizations Stonehenge was built in England for observing the sky. The differences between stars and planets. Stars Appear as point sources that produce their own light.

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the universe

The Universe

Chapter 14

the night sky
The night sky

Sky observations date to ancient civilizations

Stonehenge was built in England for observing the sky.

the differences between stars and planets
The differences between stars and planets


Appear as point sources that produce their own light.

They appear to move counter-clock relative to Polaris our north star

Twinkle from atmospheric turbulence

Distance measured in light years (ly): 9.5x1012 km/yr or 6x1012mi/yr.


Visible by reflected light

the celestial sphere
The celestial sphere

Celestial objects are projected onto imaginary sphere around the Earth

Celestial equator, is the Earth’s equator projected into the sky.

Altitude angle and azimuth angle determine location on this celestial sphere. They act like latitude and longitude lines in space.

Objects appear to rotate about north/south poles

birth of a star protostar phase to main sequence star
Birth of a StarProtostar Phase to Main Sequence Star

Gravitational collapse of a gigantic cloud of mainly hydrogen gas and dust produces a Protostar .

When density, temperature and pressure increases until conditions are right for nuclear fusion a star is born.

Temperatures of 10 million K must be reached for hydrogen atoms to be fused to other hydrogen atoms thus causing nuclear fusion.

once fusion occurs and a star is born
Once fusion occurs and a star is born -
  • The gigantic cloud of gas and dust spends billions of years calmly shinning while it fuses hydrogen nuclei in the core.
  • When inward force of gravity equals outward pressure of fusion stars enter the main sequence where it will remain most of its life.
  • Life not observed but theoretically based on knowledge of nuclear reactions.
  • Predicted outcomes seem to agree with observations of stars today.
star model


Very hot

Nuclear fusion

Radiation zone

Diffuses outward over millions of years

Convection zone

Material rising from the interior, cooling, and sinking

Visible “surface” of star

Sun surface temp. ~5,800 K

hertzsprung russell diagram
Hertzsprung-Russell diagram
  • A graph used to plot where stars are in their life cycle.
  • Stars do not just stay in one star stage of the HR diagram
  • They enter different stages as their masses and temperatures change due to fusion.
star types
Star types

Hertzsprung-Russell diagram

Plots absolute magnitude and temperature

Each dot = star

Stars grouping

Main sequence stars

Red giants


White dwarfs

Cepheid variables

lifetime of our star the sun
Lifetime of Our Star, the Sun

Converts 1.4x1017 kg of matter to energy each year

About two thousand seven hundred 6000 lb SUVs!

Born 5 billion years ago, should last another 5 billion years.

Lifetime depends on mass

Our sun’s fate could be a white dwarf because of its size.

Less massive stars have longer lifetimes

More massive stars have shorter lifetimes

brightness of stars
Brightness of stars

Differences in stellar brightness is caused by-

The amount of light produced by star

The size of star

The distance to the star

Absolute magnitude vs Apparent Magnitude

Example- Our Sun

Absolute magnitude = +4.8

Determined by a standard distance

Apparent magnitude = -26.7

How objects looks compared to other objects around it. The more negative the number the brighter the object.

star temperature related to color of star
Star temperature(related to color of star)

Colors appear as: red, yellow, bluish white

Color related to surface temperature:

Red: cooler stars

Blue: hotter stars

Yellow: in between (Sun)

Classification scheme

Based on temperature: hottest to coolest

O, B, A, F, G, K, M

death of massive stars produces supernovas
Death of Massive Stars Produces Supernovas


Star collapses

Elements beyond iron created in explosion and distributed throughout Universe

More mass: more gravitational contraction and heat

Critical temperature: 600 million K for supernova

depending on mass that remains after the supernova
Depending on mass that remains after the Supernova-

Neutron star

Remaining core between 1.4 and 3.0 solar masses

Gravitational pressure fuses protons and electrons into neutrons

Pulsar: rotating, magnetized neutron star

Black hole

Remaining core greater than 3 solar masses

Gravitational collapse overwhelms all known forces

Even light cannot escape the dense, compact object

most stars exist in groups
Most stars exist in groups.

Binary systems

Two gravity bound stars

Most stars are in binary pairs, not ours

Star clusters

Tens to hundreds of thousands or more gravity bound stars

Often share a common origin


Basic unit of the Universe

Billions and billions of gravitationally bound stars

Larger scale still

Clusters of galaxies

Super clusters of galaxies

Billions and billions of galaxies!


Early Astronomers

  • Galileo- used telescope to look at Milky Way and determined it was made up of a multitude of stars in about 1630.
the milky way
The Milky Way

Visible as a band in night sky.

Billions of stars

Spiral structure

other galaxies
Other galaxies

Our nearest neighbors - the Local Group

Dwarf galaxies

~1,000 light years across

Nearest dwarf is disrupted gravitationally by the Milky Way


2 million light years away

Very similar to Milky Way

Classification scheme (Hubble)

Elliptical, spiral, barred and irregular

Possible suggestion of how

God created the Universe is

the Big Bang Theory

the life of a galaxy theorized by scientists
The life of a galaxy theorized by scientists

Big Bang Theory

Universe evolved from an explosive beginning about 13.7 billion years ago.

Raisin bread theory

Supporting evidence

Microwave background radiation

Large scale expansion

Abundances of elements

Diffuse cosmic background radiation

The end:

Expansion forever or the big crunch?