Stars

What are stars?

Stars are giant spheres of superhot gas

made up mostly of hydrogen and helium.

Stars get so hot by burning hydrogen into

helium in a process called nuclear fusion.

Nuclear fusion is what makes them so hot

and bright.

Constellations

A constellation is a group of stars that

appear to form a pattern in the sky.

The stars in a constellation often have

no relationship to each other in space.

Properties of Stars

Astronomers classify stars by their

color, SIZE, and brightness.

Other properties include chemical

composition and mass.

A Star’s Color and Temperature

A star’s color indicates the temperature of its’ surface.

The HOTTEST stars appear BLUE

The COOLER stars appear RED

The spectrum of color in a star is from blues to greens to yellows to reds.

Properties of Stars

Mass- The single most important property that

determines other properties of the star.

Luminosity-(Brightness)-the total amount of light

energy that a star gives off into space.

Temperature- surface temperature, closely

related to the luminosity and color of the star.

Spectral Type- closely related to the surface

temperature.

Size-together with temperature determine the

luminosity.

Brightness of Stars

The brightness of a star viewed from Earth is dependent on a few things.

Color intensity

Distance

Apparent Brightness- is the brightness of a star as it appears from Earth.

Absolute Brightness-How bright the star really is-not dependent on distance.

Composition of Stars

Each star has its own color spectrum.

Most stars have a chemical make-up that is similar to the Sun, with hydrogen and helium making up most of the star’s mass.

Stars all start out the same way:

Stars start out in a giant cloud of interstellar dust,

hydrogen, helium gas, and plasma called

nebulae (nebula is singular).

Life Cycle continued

Sometimes part of the cloud shrinks

because of gravity.

As more and more dust gathers and

gravity gets stronger, the accumulation of

dust and gravity will create a protostar.

Once the center gets hot enough, nuclear

fusion will begin and a young star is born!

Main Sequence Star

Nuclear fusion keeps the star from collapsing.

A star will continue to burn energy and glow for billions of years.

This is the “state” of the star for the majority of its life and is called the “main sequence.”

During this time there is a balance (equilibrium) between gravity wanting to shrink the star and heat wanting to make it grow bigger.

The star will remain this way until it runs out of hydrogen.

Life Cycle to the END…

Average Star vs. Massive Star Death…

Classifying Stars

Two scientists, Ejnar Hertzsprung and Henry Russell made

discovery in the early 1900’s that in general, stars with

higher temperatures also have brighter absolute

magnitudes.

They developed a graph to show the relationship.

They placed temperature across the bottom and

absolute magnitude (increasing brightness) up one side.

It is known as the H-R diagram.

Most stars fit into a diagnol band that runs from the

upper left to the lower right of the graph. This band is

called the “main sequence.”

H-R Diagram

Neil DeGrasse Tyson explain a Black

Hole

https://www.youtube.com/watch?v=h1iJXOUMJpg#t=175