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Chapter 21
STARS
Characteristics of Stars
• Groups of stars that form patterns in the
sky are called constellations
– Example: Ursa Major (Big Bear), Ursa Minor
(Little Bear), and Orion
• The last two stars in Ursa Major’s “dipper”
are called the “Pointer Stars” and are be
used to find Polaris (North Star)
• Polaris is located directly above the North
Pole (90º N), and is only visible in the
northern hemisphere (above the Equator)
Circumpolar Constellations
• Because of the Earth’s rotation, the
constellations appear to move
• If the constellations 1) appear to move
around Polaris and 2) can be seen at all
times of year and 3) can be seen at all
times of night, they are called
circumpolar constellations
– The constellations Ursa Major and Ursa
Minor are both circumpolar constellations
• Using time exposure photography, the
apparent motion of the stars around
Polaris can be recorded as circular trails
VIF
The apparent motion of
stars is due to the
Earth’s daily rotation on
its axis.
The stars don’t move –
WE DO!!!!
Here is a time-lapse photo of circumpolar star movement…
Star trail
photos!!!!!
• The positions of the constellations
as viewed from Earth changes
from season to season
• This is caused by the revolution
of the Earth and the change in
Earth’s position in its orbit around
the sun
– Example: Orion the Hunter is a
winter constellation
Ex – when the Earth is in this position (Nov 21),
the bright sun during the day blocks our view of
all of the constellations toward the lower right
side of the diagram
Physical Properties of Stars
• Stars differ in size, density, mass,
composition, and color
• The color of a star is determined by it
surface temperature (ESRT’s P. 15 top)
– The hotter the star, the bluer the
color. The cooler the star, the redder
the color. (yeah, yeah, I know, it’s
backwards….)
– The sun is an AVERAGE SIZE, medium,
yellow star
Physical Properties of Stars
• Most stars are made up of mostly
hydrogen and helium (approx. 98%)
• The remaining 2% may be other
elements
• A spectral analysis (remember Ch.
20) of the star can tell us what
elements a star is made of, since the
radiated spectrum depends on a star’s
composition and temperature
• Some stars may appear to be
brighter than others
• The star’s brightness may be
described in three ways
1. APPARENT MAGNITUDE
2. LUMINOSITY
3. ABSOLUTE MAGNITUDE
(See the H-R Diagram in the ESRT’s P.15)
Apparent Magnitude
• How bright a star appears (apparent)
to us on Earth
• The farther a star is from Earth
(increasing distance), the dimmer it will
look even though it may actually be a
very bright star
– Because of this, apparent magnitude does
not tell the true brightness of a star
Luminosity
• The actual (true) brightness of
the star
• Depends on the size and
temperature of the star
• Hotter stars are more luminous
(brighter) than cooler stars
• If the temperatures are the
same, a larger star will be more
luminous
Absolute Magnitude
• The luminosity of the stars if
they all brought to the same
distance from Earth
– aka – picture all the stars lined
up the same distance from
Earth, then compare their
brightness
• This is the most useful when
comparing the brightness of the
stars
The H-R Diagram
Distances to the
Stars
• The sun is the closest star to Earth
• It is approx. 150,000,000 km (93,000,000
miles) from the Earth
– This distance is called an astronomical unit (AU)
• The next closest star to Earth, after the
sun, is Proxima Centauri
– It is 300,000 times farther away from Earth
than the sun. Because of the great distances in
space, larger units of measure must be used
• The light-year is the distance that light
travels in one year
• Since light can travel 300,000 km/sec
(186,000 miles/sec), light travels 9.5
trillion km/year!!!
– Proxima Centuri is 4.3 light-years from Earth!
So…
One Astronomical Unit (AU) =
150,000,000 km
And, one light year (LY)=
9.5 trillion km
(9,500,000,000,000 km)
Okay… let’s calculate the
distances from Earth to each
planet in Astronomical Units
(AU)
Remember – 1 AU = 150,000,000 km
Just divide the distance from the
Sun in km by 150,000,000 km.
Example: Jupiter = 778,300,000 km
150,000,000 km
Jupiter is 5.19 AU from the Sun
Star Origin,
Formation
&
Evolution
• large clouds of dust and gas in space
are the basic materials needed for star
formation
• the majority of this gas is hydrogen
• some outside force causes the cloud of
gas and dust to be pushed together
• as the gas and dust get closer, friction
between the particles causes the
temperature to increase
• the attraction of gravity between the
particles causes them to continue to
move together, and density also
increases
• friction increases and temperature
increases until the center becomes so
hot that nuclear fusion takes place
• hydrogen atoms are forced together
to form helium atoms, and a
tremendous amount of energy is
released
• In a nuclear reactor like Indian Point,
nuclear fission takes place
– This is when radioactive atoms are split
apart to release energy
OK, so stars form from
hydrogen gas and dust, but
where does that gas & dust
come from????
SUPERNOVAS
• One of the most energetic explosive events
occur at the end of a star's lifetime, when
its nuclear fuel is exhausted and it is no
longer supported by the release of nuclear
energy
• If the star is particularly massive, then its
core will collapse and in so doing will release
a huge amount of energy
• This will cause a blast wave that ejects the
star's gas envelope into interstellar space
Etna Carinae
Supernova remnant
Supernova rings
SUPERNOVA 1987 – right image is the star that became the
left image after going supernova – shone brighter than most
NEBULAE
• Clouds of dust & gas (supernova
remnants?)
• 2 Main Types:
– Diffuse Nebula – nearby star
illuminates the gas/dust cloud
– Dark Nebula – Dark patch against
more-distant stars (dust/gas is blocking
the light from stars behind it)
Here are some images of nebulae,
courtesy of our friend Hubble…
ORION NEBULA
VEIL NEBULA
HELIX NEBULA
KEYHOLE NEBULA
CAT’S EYE NEBULA
CARINA NEBULA
8000 LY FROM EARTH – 200 LY ACROSS
ESKIMO NEBULA
5000 LY FROM EARTH - 10,000 YRS OLD
EAGLE NEBULA
7,000 LY AWAY
FROM EARTH
STINGRAY NEBULA
YOUNGEST KNOWN NEBULA – 130 SOLAR SYSTEMS
ACROSS – 18,000 LY AWAY FROM EARTH
CRAB NEBULA
HORSEHEAD NEBULA – PART OF THE ORION
NEBULA
SNAKE NEBULA – a Black Nebula!!!
A STAR IS BORN…
I WANT MY
MOMMY!!
LIFE CYCLE OF STARS
• VIF!!!! - A star’s life cycle is
determined by its MASS
– The larger the star, the faster it burns
out!
• A star’s MASS is determined by the
MATTER available in the nebula of
formation
LIFE CYCLE OF STARS
SUN-LIKE
STARS
STELLAR
NURSERY
RED GIANT
(UP TO 1.5 X
MASS OF OUR
SUN)
MASSIVE
STARS
RED
SUPERGIANT
PLANETARY
NEBULA
(NOVA)
WHITE
DWARF
SUPERNOVA
BLACK
DWARF
NEUTRON
STAR
(1.5 – 3 X
OUR SUN)
STARS
FORM IN A
NEBULA OF
GAS &
DUST
SUPERMASSIVE
STARS
> 3 X OUR SUN
RED
SUPERGIANT
SUPERNOVA
BLACKHOLE
DEATH OF A SUN-LIKE STAR
SUN-LIKE
STAR
LONGEST,
MOST
STABLE
PERIOD OF A
STAR’S LIFE –
CONVERTS
HYDROGEN
TO HELIUM,
RADIATING
HEAT &
LIGHT
RED GIANT
NUCLEAR
FUEL
DEPLETES,
CORE
CONTRACTS,
SHELL
EXPANDS
NEBULA
OUTER
LAYERS
DRIFT OFF
INTO SPACE
IN SPHERELIKE
PATTERN
WHITE
DWARF
STAR
COOLS ARE
SHRINKS
BECOMING
ONLY A
FEW
THOUSAND
MILES
ACROSS!
NO
NUCLEAR
REACTION
BLACK
DWARF
STAR
LOSES
ALL HEAT
TO SPACE
AND
BECOMES
COLD
AND
DARK
CARBON
BALL
GIANTS/SUPERGIANTS
• the brightest & largest kind of star
• luminosities of 10,000 to 100,000
• radii of 20 to several hundred solar
radii (they are about the size of
Jupiter's orbit!!!!)
• two types are red supergiants
(Betelgeuse and Antares) and blue
supergiants (Rigel)
Betelgeuse a red supergiant, with about 20
times the mass and 800 times the radius
of the Sun, so huge that it could easily
contain the orbits of Mercury, Venus,
Earth, Mars & Jupiter. It will probably
explode as a supernova at some point
within the next 100,000 years. Even at its
relatively remote distance, it normally
ranks as the tenth brightest star in the
sky.
Rigel, a blue supergiant, has a
diameter of about 100 million
kilometers, some seventy times that
of the Sun. Within a few million
years, it will probably evolve to
become a red supergiant like its
neighbor in Orion (though not in
physical space), Betelgeuse.
Dwarf Stars
• A term used, oddly enough, to
describe any star that is of normal
size for its mass
• The Sun, for example, is classified as
a yellow dwarf
• In general, dwarf stars lie on the
main sequence and are in the process
of converting hydrogen to helium by
nuclear fusion in their cores
White Dwarfs
• A medium sized star that has
exhausted most or all of its nuclear
fuel and has collapsed to a very small
size
• Typically part of a planetary nebula
• Eventually cools into a BLACK dwarf
(lump of carbon)
– This takes BILLIONS of years!
– This is the fate of OUR SUN!
Neutron Star
• The imploded core of a massive
star produced by a supernova
explosion
• The most dense known objects
in the universe!
– A sugar cube size of neutron
star material weighs 100
million tons!!!!!!!
BLACK HOLES
• A supermassive star that
undergoes supernova and the
core is when the star is
“swallowed” by its own gravity
• readily attracts any matter and
energy that comes near it including light…
3,700 LY wide dust-disk encircling a 300 million solar mass
blackhole in the center of an elliptical galaxy.
The disk is a remnant of an ancient galaxy collision and could
be “swallowed” up by the blackhole in a few billion years.
Galaxies & the
Universe
Big Bang Theory
• The Big Bang Theory is the dominant
scientific theory about the origin of
the universe
• According to the big bang, the
universe was created sometime
between 10 billion and 20 billion years
ago from a cosmic explosion that
hurled matter and in all directions
Galaxy Formation
• The formation of all the galaxies is
explained by the Big Bang Theory
• Simply put, it states that the
universe was a big ball of hydrogen
gas that exploded outward
• The expanding cloud had areas that
condensed into galaxies that are still
expanding out from the center (the
universe is getting larger)
– We can see this via RED SHIFT!
The Solar
System
Galaxies
• system containing millions to billions of
stars
– Ex. the Milky Way galaxy contains over 500,000
million stars
• Milky Way galaxy is a spiral shaped galaxy
with a large central cluster of stars, and
thinner “arms” radiating out from the
center
– The solar system is located on one of the arms
of the Milky Way galaxy about 2/3 away from
the center
Origin of the Milky Way
• Formed 10-12 billion years ago
• Possibly collided with smaller
galaxies
• Globular star clusters formed
• Stars and solar systems formed
roughly 5 billion years ago
Milky Way
Andromeda Galaxy – 2.9 million LY from earth
Our CLOSEST neighbor!!!
Ursa Major’s galaxy (a galaxy that we can see
within the constellation) –
50 million LY from earth!
Sombrero Galaxy – 50 million LY from earth – 100,000
LY across
Overlapping Spiral galaxies – 140 million LY from earth
– within the constellation HYDRA
Whirlpool Galaxy – 37 million LY from earth –
60,000 LY across
Hoag’s Galaxy – 120,000 LY wide – perfect ring
of blue stars surrounding older nucleus of
yellow stars
When galaxies collide…