Stars and Their Characteristics

28.2

Constellations

• Constellation- groups of stars that appear to form patterns– 88 constellations can be seen from n. and s.

hemispheres– So far away that only after thousands of years

might the motions be observed– Big Dipper- asterism (small-star grouping)

• Part of Ursa Major- Great Bear

Stars

• Polaris- north star– Doesn’t appear to move

• Circumpolar stars- seem to move around Polaris because Earth rotates

• Position of Earth

Distance to Stars

• Light year- distance light travels in a year– Used instead of

kilometers/astronomical units

• Parallax- basic way to measure star distance

• The nearest stars have the largest parallax angles, while those of distant stars are too small to measure

Elements in Stars

• sphere of mostly hydrogen and helium gases– small percentage may be heavier elements

(oxygen, carbon, nitrogen, etc..)

• no two stars contain exactly the same elements in the same proportions– wavelengths depend on both composition and

temperature

Mass, Size, and Temperature of Stars

• mass can be determined by the inertial properties of the body or by its gravitational effect on the bodies around it– more mass = more gravitational effect

• stars vary more in size than they do in mass; and even more in density

• range of colors a star emits depends on its surface temperature– Blue= hottest; red = coolest

Luminosity and Absolute Magnitude• Luminosity (apparent

magnitude)- brightness of a star at its current distance from Earth– depends on its size and

temperature– Bigger stars tend to be

brighter– Bluer stars tend to be

brighter• absolute magnitude-

measure of how bright the star would be if all stars were at the same distance from Earth– The more negative the

number, the brighter the star

Object mv Mv

Sun -26.8 4.83

Sirius -1.47 1.41

Vega 0.04 0.5

Betelgeuse 0.41 -5.6

Polaris 1.99 -3

Variable Stars

• variable stars are those stars that show regular variation of brightness – pulsating stars change brightness as they expand and

contract– Cepheid variable- yellow supergiants whose cycles

of brightness range from about 1 day to 50 days• the slower the cycle, the greater the luminosity of the star• can calculate the distances to galaxies in which they can

identify Cepheid stars

• non-pulsating star can change brightness because is part of a binary star system– results in eclipses

H-R Diagram

• Hertzsprung-Russell Diagram• diagram plots the luminosity of stars against

their surface temperatures• most stars (90%) are in a band that runs from

the upper left (high, high) to the lower right (low, low)– main sequence stars– main sequence stars vary in surface temperature and

absolute magnitudes– commonality: actively fusing hydrogen into helium

H-R Diagram cont…

• giant stars- great luminosity and diameter; 10-100x greater than sun

• supergiants- higher luminosity; diameter > 100x than sun

• white dwarfs- stars near end of life– once red giants that lost atmosphere

HR Diagram

Stellar Evolution

Birth of a Star• begins as a nebula- cloud of dust and gas (99%

hydrogen)• nebula may condense when an outside force

acts upon it• particles move closer together under gravity• increase density = increase temperature• if nebula glows, called protostar• center will become hotter until fusion takes place

and a star is born

Nebula and Protostar

Stellar Evolution

Death of a Star (size of the sun)• remain same size (main sequence) for billions of years

because energy produced through fusion equals gravitational pull

• hydrogen is used up so gravity takes over, which then produces heat by contraction

• entire star expands (Red Giant)• core temperature rises enough for helium to fuse into

heavier elements, producing a carbon-oxygen core• surface gases are blown away, leaving core (white

dwarf)• planetary nebula- glowing halo of gases

– fades as gases dissipate into space leaving white dwarf behind– once all fuel is used up, the core will no longer glow, becomes a

black dwarf

Main Sequence and Red Giant

White Dwarf, Planetary Nebula, and Black Dwarf

Stellar EvolutionDeath of a Massive Star• fusion process will continue until iron nuclei are formed

– absorbs energy, so iron core quickly collapses• supernova- explosion that produces a brilliant burst of

light– produces many elements: copper, uranium, silver, and lead

Remnants of Massive Stars• neutron star-dense mass of neutrons formed through

gravity• pulsar- a distant neutron star that emits rapid pulses of

light and radio waves instead of steady radiation• black hole- remnant of a star at least 15 times as

massive than the sun– gravitational force is so strong that light cannot escape

Supernova and Neutron Star

Pulsars and Black Holes