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)