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Stars, Galaxies & The Universe

Composition of Stars

• Made of different elements in gaseous state

• The gases in the atmosphere of a star absorb

different wavelengths of light depending on

which elements make up the gases.

• The light from a star tell us which

elements make up that star.

The Colors of Light

• A prism breaks white light into a rainbow of

colors called a spectrum.

• A spectrograph breaks a star’s light into a

spectrum.

• The spectrum of a star depends on

which elements are present.

Making An ID

• Emission lines are lines made when certain

wavelengths of light (colors) are given off by hot

gasses.

• Each element produces a unique set of lines,

which allows them to help identify elements in a

star.

Classifying Stars

• Differences in Temperature Stars are now

classified by how hot they are.

• Differences in Brightness The brighter the

star, the lower the magnitude.

– Sirius, has a magnitude of -1.4.

– The dimmest star that can be seen with a

teloscope has a magnitude of 29.

Magnitude of Big Dipper Stars

How Bright Is That Star?

• Apparent Magnitude The amount of light

received on Earth from a star.

• Absolute Magnitude The actual brightness of

a star.

*1 parsec = 19.2 trillion miles

Distance to the Stars

• Astronomers use light-years to measure

the distances from Earth to the stars.

• A light year is the distance that light

travels in a year. (9.5 trillion km)– Radius of our solar system: 5.5 light hours

– Nearest Star (alpha centauri) 4.3 light years

– Radius of Milky Way: 50,000 light years

Distance to the Stars

• Parallax is the apparent shift in the position of

an object when viewed from different locations.

• Measuring parallax enables scientists to

calculate the distance between a star and the

Earth.

Motions of Stars

• The Apparent Motion of Stars The stars

appear to move. This is due to Earth’s rotation.

Motions of Stars

• The Actual Motion of Stars Each star is

moving in space. Their actual movements

are difficult to see.

http://www.yorku.ca/ns1745b/bigdipper.mov

Stellar Evolution: Life Cycle of a Star

Stellar Evolution

• The Beginning A star enters the first stage of

its life cycle as a ball of gas and dust (nebula).

• Gravity pulls the gas and dust together, and

hydrogen changes to helium during nuclear

fusion.

Orion Nebula

Stellar Evolution

• Main-Sequence Stars After a star forms, it

enters the longest stage of its life cycle as a

main sequence star.

• Energy is generated in the core as hydrogen

atoms fuse into helium atoms.

Stellar Evolution

• Giants and Super Giants After the main-

sequence stage, a star can enter the Red Giant

stage.

• A red giant is a large, reddish star formed from

the heating core and expanding size.

• The core continues to heat and the star

expands even more to form a

super giant.

Stellar Evolution

• White Dwarf As the core of a supergiant uses

up its helium supply, the outer layers escape into

space and the remaining core is white hot, thus

called a white dwarf.

Stellar Evolution

• A supernova is a gigantic explosion in which a

massive blue star collapses.

• A small star that has collapsed under gravity so

all of its particles are neutrons is called a

neutron star.

• If a neutron star is spinning, it is called a pulsar.

Stellar Evolution

• Black Holes Sometimes the leftovers of a

supernova are so massive that they collapse to

form a black hole.

• A black hole is an object that is so massive

that even light cannot escape its gravity.

Stellar Evolution: Life Cycle of a Star

A Tool for Studying Stars

• The H-R Diagram the Hertzprung-Russell

diagram is a graph that shows the relationship

between a star’s surface temperature and

absolute magnitude.

• Reading the H-R Diagram The diagonal

pattern on the H-R diagram where most stars lie

is called the main sequence.

H-R Diagram