Stars and Galaxies
Stars and Their Characteristics
VOCABULARY
constellation
apparent magnitude
astronomical unit
Hydrogen and helium are the two most
abundant elements in stars.
light-year
parsec
Stars can be grouped into constellations.
luminosity
HERCULES
LYRA
CYGNUS
absolute magnitude
DRACO
BOÖTES
CANES
VENATICI
URSA
MINOR
CEPHUS
LACERTA
COMA
BERENICES
CASSIOPEIA
PEGASUS
URSA
MAJOR
CAMELOPARDALIS
LEO
LEO MINOR
LYNX
PERSEUS
PISCES
Stars and Their Characteristics
Stars and Galaxies
Stars and Their Characteristics
Stars differ in mass, size, and surface temperature.
Surface temperature affects the color of stars.
Apparent magnitude, luminosity, and absolute
magnitude are used to describe the brightness of
stars.
Betelgeuse
Rigel
There are stars of different
brightness in the constellation
Orion, including two of the
brightest stars as viewed from
Earth—Betelgeuse and Rigel.
Stars and Their Characteristics
Stars that show variation in brightness are known
as variable stars.
Distances in space are measured in astronomical
units, light-years, and parsecs.
Light-Year The distance that light travels in
one year, about 9.5 trillion kilometers.
Parsec A unit of measurement used to
describe distances between celestial objects,
equal to 3.258 light-years.
Spectral Types
Apparent Magnitude
Some stars appear
very bright but are
actually fainter stars
that lie closer to us.
Similarly, we can see
stars that appear to
be faint, but are
intrinsically very
bright ones lying far
away from Earth.
Apparent Magnitude
Apparent Magnitude
The measure of
how bright a star
appears to be to
an observer on
Earth.
Stars and Their Characteristics
Luminosity: The brightness of a star or the power radiated
by the star.
The luminosity is a quantity
that depends on the star
itself, not on how far away it
is. For this reason a star's
luminosity tells you about
the internal physics of the
star and is a more
important quantity than the
apparent brightness.
What does the luminosity of
a star depend on?
Temperature (proportional to T4)
Size (proportional to R2)
Full blown formula? L=4pR2sT4
A star can be luminous because it is hot or it
is large (or both!).
The luminosity of
an object = the
amount of energy
every square
meter produces
multiplied by its
surface area.
Caution!
 Do not confuse the size of an object with
the mass of an object.
 Just because an object is large in
dimension does not necessarily mean it is
also large in mass.
 For example, you can have a forty foot tall
by three foot across marshmallow that
looks “large,” but that does not mass as
much as that of a “small” football sized
hunk of lead.
Absolute Magnitude
Absolute
Magnitude: The
measure of
how bright a
star would be if
it were located
10 parsecs
from Earth.
On the left-hand map of Canis Major, dot sizes indicate stars'
apparent magnitudes; the dots match the brightnesses of the
stars as we see them.
The right-hand version indicates the same stars' absolute
magnitudes — how bright they would appear if they were all
placed at the same distance (32.6 light-years) from Earth.
Absolute magnitude is a measure of true stellar luminosity.
Inverse Square Law
As the light from a star goes into space it fills a
larger and larger spheres.
The area of a sphere is given by its radius: A =
4 p d2
d is the radius of the sphere
The amount of light we
receive from a star
decreases with the
square of our distance
from the star:
Amount of light = L0 / d2
Flux=“amount of light”
Measuring the Distance to Stars
Measuring
distances is
difficult.
The best method for measuring distances of
nearby stars is called parallax.
It relies on observing a star from two different
places.
Measuring the Distance to Stars
Measuring the Parallax Angle:
The parallax angle p is illustrated in the
following figure.
Measuring the Distance to Stars
Parallax, or more accurately motion
parallax (Greek: παραλλαγή (parallagé) =
alteration) is the change of angular
position of two stationary points relative to
each other as seen by an observer,
caused by the motion of an observer.
Simply put, it is the apparent shift of an
object against a background caused by a
change in observer position.
The Distance to the Stars
 We obtain a different perspective
on a star by observing it at
different times of the year.
 In 6 months the Earth has moved
2 AU away.
 (2AU = 300 million km)
 The parallax method lets us
measure the distance to stars
about 1000 light years away.
Measuring Distances: Parallax
 The larger the star’s distance, d, the smaller its
parallax p.
 So distance and parallax are inversely related.
d= 1/p
Measuring Distances: Parallax
 Most stars have a parallax angle, p, which is very small.
 The angle of parallax, p, is usually measured in arc seconds
 60 arc seconds = 1 arc minute
 60 arc minutes = 1 degree.
 Distances to stars are measured in either: light years, or
parsecs.
 1 parsec = 3.2 light years
(parsec = PARallax of one arcSEC)
 If a star’s parallax is 1 arc second, then its distance is 1 parsec.
Parallax Examples
If a star’s parallax is 1 arc second its distance
is 1 parsec
Question: If a star has a parallax of 0.1 arc
seconds what is its distance in parsecs?
Answer: d = 1 / p
d = 1/ (0.1) = 10 parsecs = 3.2 light years
Constellations:
A group of stars that
appear to form a pattern in the sky.
Constellations
 Constellations are easily recognizable
patterns that help people orient themselves
using the night sky. There are 88 “official”
constellations.
 Hundreds of the brightest stars, those visible
with the unaided eye, were given names in
ancient times.
 Today stars are named by their coordinates
on the celestial sphere. This is an imaginary
sphere surrounding Earth.
Constellations
All stars and objects in
space, can be mapped
relative to the poles
and equator of the
celestial sphere. Their
position north or south
of the celestial equator
— essentially their
latitude — is called
“declination.” Their
position east or west
essentially is their
longitude, or right
ascension, measured
in hours, minutes, and
seconds.
Constellations
The stars are distant objects. Their
distances vary, but they are all very
far away. Excluding our Sun, the
nearest star, Proxima Centauri, is
more than 4 light years away. As
Earth spins, the stars appear to
move across our night sky from
east to west, for the same reason
that our Sun appears to “rise” in the
east and “set” in the west.
Constellations
If observed through the year, the
constellations shift gradually to the
west. This is caused by Earth’s orbit
around our Sun. In the summer,
viewers are looking in a different
direction in space at night than they are
during the winter.
Constellations
Stars close to the celestial poles, the
imaginary points where Earth’s north
and south axes point in space, have a
very small circle of spin. Polaris,
Earth’s north “pole star,” will appear to
move very little in the night sky. The
farther from Polaris, the wider the circle
the stars trace.
Constellations
 Stars that make a full circle around a
celestial pole, like those in the Big and Little
Dippers in the northern hemisphere, are
called “circumpolar stars.” They stay in the
night sky and do not set. At the equator,
there are no circumpolar stars because the
celestial poles are located at the horizon. All
stars observed at the equator rise in the east
and set in the west.
Constellations
HERCULES
LYRA
CYGNUS
DRACO
BOÖTES
CANES VENATICI
URSA MINOR
CEPHUS
LACERTA
COMA BERENICES
CASSIOPEIA
PEGASUS
URSA MAJOR
CAMELOPARDALIS
LEO
PERSEUS
LEO MINOR
LYNX
PISCES
Stars and Galaxies
VOCABULARY
Life Cycles of Stars
main sequence
giant star
supergiants
white dwarf
nebula
planetary nebula
supernova
neutron star
pulsar
black hole
The Hertzsprung-Russell diagram plots a star’s
luminosity against its surface temperature. The
diagram’s groups
Red Supergiants
Blue Supergiants
of stars
represent lifecycle stages of
stars. Most stars
are mainsequence stars.
Red Giants
White
Dwarfs
HertzsprungRussell
diagram
Hottest
Temperature
Red Dwarfs
Coolest
Hertzsprung-Russell Diagram:
– Ejmar Hertzsprung (1873-1967) – Copenhagen –
Began his career as a Chemical Engineer. While
working and independently at the same time…
– Henry Norris Russell (1877-1957) – Princeton –
Student then professor.
– A graph that separates the effects of temperature
and surface area on stellar luminosities.
– The HR Diagram is much like the same thing as
producing a graph of people’s height vs. weight.
Stars and Galaxies
Life Cycles of Stars
A star’s fate depends on its mass.
A star with a mass similar to the sun’s will become
a white dwarf.
Stars and Galaxies
Life Cycles of Stars
A star with a mass eight or more times greater
than the sun’s will either become a black hole
or a neutron star.
Properties of stars
Color and temperature
• Hot star
• Temperature above 30,000 K
• Emits short-wavelength light
• Appears blue
• Cool star
• Temperature less than 3000 K
• Emits longer-wavelength light
• Appears red
Properties of stars
 Color and temperature
• Between 5000 and 6000 K
• Stars appear yellow
• e.g., Sun
 Binary stars and stellar mass
• Binary stars
• Two stars orbiting one another
• Stars are held together by mutual gravitation
• Both orbit around a common center of mass
Hertzsprung-Russell diagram
Shows the relation between stellar
• Brightness (absolute magnitude) and
• Temperature
Diagram is made by plotting (graphing)
each star's
• Luminosity (brightness) and
• Temperature
Hertzsprung-Russell diagram
Parts of an H-R diagram
• Main-sequence stars
• 90% of all stars
• Band through the center of the H-R diagram
• Sun is in the main-sequence
• Giants (or red giants)
• Very luminous
• Large
• Upper-right on the H-R diagram
Hertzsprung-Russell diagram
Parts of an H-R diagram
• Giants (or red giants)
• Very large giants are called supergiants
• Only a few percent of all stars
• White dwarfs
•
•
•
•
•
Fainter than main-sequence stars
Small (approximate the size of Earth)
Lower-central area on the H-R diagram
Not all are white in color
Perhaps 10% of all stars
Hertzsprung-Russell diagram
Birth of a Star: Nebula
 Stars are born in a glowing cloud of
interstellar gas and dust (mostly hydrogen),
called a nebula.
 Gravity causes every atom and every bit of
dust to pull on every other one and all move
to the center, causing the protostar to
collapse.
 Because the atoms move faster and faster
as they fall toward the center, friction is
created as they rub together and the
temperature rises.
Birth of a Star: Nebula
 Heat causes the protostar to glow in with its own
light, giving off even more light than our Sun even
though it is not nearly as hot.
 When a temperature of about 27,000,000°F is
reached, nuclear fusion begins. This is the nuclear
reaction in which hydrogen atoms are converted to
helium atoms plus energy. This energy (radiation)
production prevents further contraction of the star.
 The protostar is now a stable main sequence star
which will remain in this state for about 10 billion
years. After that, the hydrogen fuel is depleted and
the star begins to die.
Birth of a Star: Nebula
Black
Widow
Nebula
Birth of a Star: Nebula
Crab
Nebula
Birth of a Star: Nebula
Main Sequence Stars
Main Sequence: A star that is at the point in its life
cycle in which it is actively fusing hydrogen nuclei into
helium nuclei;
Our sun is a
main sequence
star.
Giant Stars
A Giant Star: is large star with great luminosity and a
diameter 10 to 100 times greater than that of the
sun.
A giant star is one of two
kinds very large stars
the other being a Red
giant or Supergiant
Red giants are stars of 1000 times the
volume of the Sun which have exhausted
the supply of hydrogen in their cores and
switched to fusing hydrogen in a shell
outside the core.
Supergiants
Supergiants: are the most luminous, most massive
stars, with diameters greater than 100 times the
diameter of the sun.
The best known example
is Rigel, the brightest star
in the constellation of
Orion. It has a mass of
around 20 times that of
the Sun and gives out
more light than 60,000
suns added together.
White Dwarf :
The remnant of a giant star that has
lost its outer atmosphere; the glowing stellar core.
 A white dwarf is what stars like our Sun become
after they have exhausted their nuclear fuel. Near
the end of its nuclear burning stage, such a star
expels most of its outer material, creating a
planetary nebula.
White Dwarf:
Sirius–B, this white
dwarf is very hot due
to high density and
rapid spin.
Neutron star: The superdense remains of a
massive star that collapsed with enough force to push all of its
electrons into the nuclei they orbit, resulting in a mass of neutrons.
 A neutron star is formed from the collapsed remnant a Type II,
Type Ib, or Type Ic supernova.
•Pulsar – general
term for neutron
stars that emit
directed pulses of
radiation towards
us at regular
intervals due to
their strong
magnetic fields.
Supernova: The brilliant burst of light that
follows the collapse of the iron core of a massive star.
 Supernovae are the main source of all the elements
heavier than oxygen, and they are the only source
of many important elements.
X-ray image of the
remnant of Kepler's
Supernova
Supernova Remanent
Black hole: The final life stage of an extremely
massive star, with a gravitational field so intense that
not even light can escape.
Black holes are areas in
space where there is a huge
amount of mass in a very small
space. The gravity of this
mass is so great that
everything in the area is pulled
toward the mass. Even light,
with its tiny mass, is pulled into
the center of the hole. No
object can escape the
gravitational pull of a black
hole.
Black hole: The final life stage of an extremely
massive star, with a gravitational field so intense that
not even light can escape.
 We can't see a black hole because no light
escapes the event. Astronomers use other
ways to look for black holes. Since they have
large masses and gravities, they affect the
surrounding stars and systems. They have
found evidence of black holes in the dark
centers of galaxies and systems that emit
large amounts of x-rays.
How does a black hole form?
 A black hole forms when any object reaches a
certain critical density, and its gravity causes it to
collapse to an almost infinitely small pinpoint.
Stellar-mass black holes form when a massive
star can no longer produce energy in its core.
With the radiation from its nuclear reactions to
keep the star "puffed up," gravity causes the core
to collapse. The star's outer layers may blast
away into space, or they may fall into the black
hole to make it heavier.
Black hole: The final life stage of an extremely
massive star, with a gravitational field so intense that
not even light can escape.
 Supermassive black holes containing millions to
billions of times the mass of the sun are believed
to exist in the center of most galaxies, including
our own Milky Way.
 Intermediate-mass black holes, whose size is
measured in thousands of solar masses, may
exist. Intermediate-mass black holes have been
proposed as a possible power source for ultraluminous X ray sources.
Galaxies and the Universe
VOCABULARY
galaxy
quasar
Galaxies contain
millions or billions
of stars. There are
three major types
of galaxies: spiral,
elliptical, and
irregular.
Galaxies and the Universe
Normal galaxies emit as much radiation as that
given off by their stars. Active galaxies emit much
more radiation than that given off by their stars,
possibly due to supermassive black holes at their
center.
Quasar: A very distant, extremely luminous
celestial object that scientists consider to be
a type of active galactic nuclei.
Galaxies and the Universe
 Galaxies are defined as large groupings of
stars, dust, and gas held together by gravity.
They vary greatly in size and shape. Most of
the objects we know of in space are
contained within galaxies. They contain
stars, planets, moons, comets, asteroids,
nebulae, dust, neutron stars, and black
holes. Many probably even contain large
amounts of unseen dark matter. Since most
of the space between galaxies is thought to
be empty, a galaxy is essentially an oasis in
space.
Galaxies and the Universe: Types of
Galaxies
Spiral Galaxy - Spiral galaxies are
characterized by a distinct flattened
spiral disk with a bright center called
the nucleus. Our own Milky Way is a
spiral galaxy. Spiral galaxies are
represented by the letter S and are
divided into subgroups.
Galaxies and the Universe: Types of
Galaxies
Galaxies and the Universe: Types of
Galaxies
Barred Spiral Galaxy - A barred spiral
galaxy is very similar to a spiral with
one important difference. The arms
spiral out from a straight bar of stars
instead of from the center. About one
third of all spiral galaxies are barred
spiral in shape.
Galaxies and the Universe: Types of
Galaxies
NGC 1365 is one
of the most
prominent barred
galaxies in the sky.
It is a supergiant
galaxy with a
diameter of about
200 000 light
years.
Galaxies and the Universe: Types of
Galaxies
Elliptical Galaxy - Elliptical galaxies
vary in shape from completely round to
extremely elongated ovals. Unlike
spiral galaxies, they have no bright
nucleus at their center. Elliptical
galaxies are represented by the letter E
and are divided into seven subgroups
according to their shape.
Galaxies and the Universe: Types of
Galaxies
NASA's Chandra
X-ray Observatory
shows hot gas in
nine different
elliptical galaxies.
Galaxies and the Universe: Types of
Galaxies
 Irregular Galaxy - A fourth type of galaxy is
known as the irregular galaxy. These
galaxies have no discernable shape or
structure. Irregular galaxies are divided into
two classes, Im and IO. Im class galaxies
are the most common and show just a hint of
structure. Sometimes the faint remnants of
spiral arms can be seen. IO class galaxies
are completely chaotic in form.
Galaxies and the Universe: Types of
Galaxies
 Irregular galaxy, as observed by the Hubble Space
Telescope.
Galaxies and the Universe: Types of
Galaxies
Paired Galaxies
Life Cycle of a Star
Colliding Galaxies
Galaxy Cluster
Galactic Research
Edwin Hubble was the first person to
figure out how to tell the distance of a
galaxy. He used a type of pulsating star
known as a Cepheid variable as a kind
of galactic yardstick.
Galactic Research
 Hubble noticed a correlation between the
period required to complete one pulsation
brightness and the amount of energy the star
gives off. This was the first major
breakthrough in galactic research. Hubble
also discovered that there was a correlation
between the red shift of a galaxy and its
distance. This is known today as the Hubble
constant.
Galactic Research
Today astronomers are able to
measure the speed and distance of a
galaxy by measuring the amount of
redshift in its spectrum. We know that
all galaxies are moving away from
each other. The farther a galaxy is from
us, the faster it is moving.