Constellations
 A group of stars that appear to form a pattern in the
sky.
 Only appear together from Earth, actually at dif.
Distances.
 Far distance of stars make any movement not apparent
in a constellation for thousands of years.
 Asterism- small grouping of stars. Ex. Big Dipper part
of constellation Ursa Major (Big Bear). Can find
Polaris (North Star) using Big Dipper.
Star Movement/Sky Appearance
 Movement of stars in sky
caused by Earth’s
rotation & revolution.
 Earth turns West to East
so sky appears to move
from East to West to us
on Earth, that’s why the
sun, moon, & stars rise
in the East & set in the
West.
 Sky directly above poles
seems stationary as
Earth turns on it’s axis,
so Polaris appears fixed
in the sky while all other
stars seem to move
counter-clockwise
around it. These stars are
Circumpolar Stars.
Earth’s Movement & Constellations
 Constellation positions in the sky change with the
seasons. Ex. Big Dipper: Fall-near North horizon,
Spring- overhead.
 Some constellations can only be seen during certain
seasons. Ex. Lyra in Summer & Orion in Winter.
 These changes are due to Earth’s position as it orbits
the Sun.
Apparent Magnitude
 A measure of how bright a star appears to be to an
observer on Earth.
 Magnitude is measured on a scale from 1 (brightest)- 6
(faintest).
 The lower a stars magnitude #, the brighter the star.
3 units of measurements
1. AU (Astronomical Unit)
 Distance from the Sun to the Earth.
 Equal to 1.5 x 108 Km or 150 million Km.
2. Light-Year
 Measures the distance that a ray of light travels in one
year.
 In one year, light travels 9.5 x 1012 Km or 9.5 trillion Km.
 Ex: Proxima Centauri is 4.2 light years from Earth.
3. Parsec (Parallax Second)
 Measures distances between celestial objects (nearest
stars).
 One parsec = 3.258 light-years or 3.086 x 1013 Km.
 Parallax- change in objects direction due to change in
observers position. Occurs because Earth orbits the sun.
 Using parsec measurement accounts for the shift by
knowing the angel between the 2 points.
• Each star is different, but they are mostly
composed of H, He & some heavier elements.
•A stars spectrum is used to determine it’s
composition.
•Each star has a unique spectrum.
Mass, Size & Temp. of Stars
 Vary more in size than in mass.
 Mass= total amount of material in a body. Determined
by gravitational influence on other bodies.
 Stellar (star) masses are expressed as multiples of the
sun or One Solar Mass.
Temperature & Color of Stars
 Range of color a star emits depends on it’s surface




temp.
To us on earth, stars look mostly white with a bit of
color.
Cooler stars= reddish, Hotter stars= Bluish-white.
Ex: Iron when heated, 1st turns red, changes to orange,
to yellow to white. Very Hot objects glow Bluish.
The color/temp. of a star helps astronomers determine
the elements present.
Luminosity & Absolute Magnitude
Luminosity
Absolute Magnitude
 Actual brightness of a star.
 True measure of how bright a
 Depends on size & temp.
Ex:
 If 2 stars had the same
surface temp, but 1 was
bigger, the bigger is more
luminous.
 If 2 stars were same size, but
dif. temps, the hotter is more
luminous
star is.
What’s the difference between
apparent & absolute
magnitude?
Stars are born from great clouds of gas & dust. They mature,
grow old, & die. When they die, they produce new clouds of dust,
from which new stars may arise, along w/planets to orbit them.
The more massive a star, the shorter its life will be.
What is the H-R diagram?
 Hertzsprung-Russell diagram – a diagram that plots
the stars surface temperature against their absolute
magnitude.
Groups of the H-R
Diagram:
•
•
Groups represent stages in the
life cycles of the stars.
90% of stars fall in the Main
Sequence band & range in
size & temp. these are the stars
that are fusing H to He.
•
Giant Stars are above main
sequence & are more luminous
& larger.
•
Supergiants are even more
luminous & large.
•
White Dwarfs are near the
end of their lives, were once
red giants but lost atmosphere
& now only glowing stellar
core.
•Where in the graph are
the bright, cool stars?
•Which axis represents
temperature?
•What area would blue
stars be located?
Birth of a Star
 A star begins its life in a cloud of gas & dust called a
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
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
Nebula.
A nebula condenses when an outside force
(shockwave) acts upon it.
Parts of the nebula then condense due to gravity & the
temp. increases.
Large enough nebulas will begin to glow (Protostars)
Protostars get hotter & brighter eventually fusion
occurs & a star is born.
How would
the amounts
of H & He in
the cores of 2
identical stars
compare if
one star was
older than the
other?
Death of a star like the Sun
(main sequence star)
 Balance of fusion & gravity occurs until H is used up & the
core shrinks & releases new heat that expands the star into
a Giant.
 The star begins to die when temps rise & He (helium) fuses
to heavier elements (C-O core)
 Gas layers are blown away leaving C-O core (white dwarf)
w/halo glow= Planetary Nebula.
 Eventually gases dissipate into space & white dwarf is left.
Death of a Massive Star
 Same start as before, but fusion occurs until Fe (iron)
nuclei form & star swells more than 100 times the
diameter of our sun, becoming a Supergiant.
 Fe cores absorb energy & suddenly collapse sending
shockwaves & outer layers out in a huge burst of light=
Supernova and produces new elements.
 Left behind is either a Black Hole (if original star was
15 x or more massive than our sun) or a Neutron Star.
Remnants of Massive Stars
 After a massive star “goes supernova,” it leaves behind
its core.
 Neutron Star: super dense, all e- are pushed into the
nuclei they orbit, result is a dense mass of neutrons
that are trillions times more dense than the sun. At
first, it spins & emits pulsing beams of radio waves out
(Pulsar).
If star was at least 15 x more massive than the sun:
 Black Hole: Intense gravitational field that not even
light can escape. Detected by strong sources of x-ray
(given off by atoms being ripped apart by gravity)
Life Stages of Stars: From Birth to Death
Self-Quiz
1.
2.
3.
4.
5.
What determines which life-cycle course a star
takes?
How does cloud-like nebula form protostars?
What process marks the transition from protostar to
star?
What remains balanced during the long period when
a star’s H is fusing into He?
At what point is this balance lost?
What Are Galaxies?
 Systems that contain millions or billions of stars.
 Look like hazy patches of light in the sky.
 Estimated to be 50-100 billion galaxies in observable
universe.
 Most are millions of light years apart.
To what galaxy do we belong?
 Milky Way Galaxy
 It’s a spiral galaxy (pinwheel-shaped)
 Diameter of about 100,000 light-years
 Our sun is about 26,000 light-years from it’s center
Types of galaxies
 Spiral: pinwheel-shaped; nuclei of stars bulging at
center & wound spiral arms.
 Elliptical: nearly spherical to oval-shaped; stars
concentrated at their centers, little interstellar gas &
dust for new stars to form so few to no young stars.
 Irregular: irregular in shape; much smaller & fainter
than other 2 galaxy types, stars spread unevenly.
Spiral Galaxy
Elliptical Galaxy
Irregular Galaxy
Spiral, Elliptical, & Irregular
Active Galaxies
 Emit more energy than their stars can give off.
 Some emit radiation, or change largely in brightness in
short periods of time.
 Possible powered by a supermassive black hole centers
that jet out hot gas in opposite directions.
 Quasars: very distant, extremely luminous celestial
object believed to be a type nuclei in an active galaxy.