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
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 magnitudemeasure 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
Hertzsprung-Russell Diagram
Diagram that plots the luminosity of stars against their surface temperatures
H-R Diagram
• Most stars (90%) are
in a band that runs
from the upper left
(high, high) to the
lower right (low, low)
– Called main sequence
stars
• 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
Stellar Evolution
Birth of a Star (any size)
• 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 Evolution
Death 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