STARS, GALAXIES
AND THE UNIVERSE
The beginning…
What are Stars?
Stars are large balls of hot gas.
They look small because they are a long way away, but in
fact many are bigger and brighter than the Sun.
The heat of the star is made in the centre by nuclear fusion
reactions.
There are lots of different colours and sizes of star.
A STAR IS BORN…
A STAR IS BORN…
1st Step:
Stars form from nebulas Regions of concentrated
dust and gas.
Gas and dust begin to collide,
contract and heat up,
All due to gravity
2nd Step:
As a nebula contracts, a small star is
formed
Called a protostar.
Eventually, the protostar will begin
nuclear fusion.
Hydrogen protons attract to each
other and fuse together.
Nuclear fusion = Hydrogen into
Helium
This necessary for stars to survive.
3rd Step:
Star joins the main
sequence.
90% of stars spend their
life here.
The mass of star
determines its location on
the main sequence.
Beginning of the End:
Stars begin to die when
they run out of hydrogen.
Gravity begins to take
over.
Star begins to shrink; outer
core of hydrogen begins to
fuse.
The outer part of the Star
gets bigger.
Beginning of the End:
When a star gets bigger, it
cools down and becomes a
Red giant.
Eventually, the star can
fuse helium into other
elements such as
Carbon, oxygen, and
other heavier elements.
Beginning of the End:
Once a star runs out of
“fuel”, the star shrinks
under its own gravity.
It can Turn into a white
dwarf, neutron star, or
black hole.
Death of Stars:
What stars end up as depend on
their mass.
Low and Medium mass stars
Planetary nebula --------white dwarf.
High mass stars
Supernova --------- neutron
star or black hole.
Death of Stars: Low and Medium Mass
Main Sequence Star
Red Giant
Planetary Nebula
White Dwarf
Death of Stars: High Mass
Main Sequence Star
Red Super Giant
Supernova
Neutron Star
Black Hole
High Mass Stars:
Mass greater than 8x our
sun
Create high mass
elements such as iron
Neutron Star
Formed if remaining
star < 3x sun’s mass
Black Holes
Formed if remaining
star > 3x sun’s mass
Distances To The Stars
Stars are separated by vast distances.
Astronomers use units called light
years to measure the distance of stars
A light-year is the distance that light
travels in a vacuum in a year
Proxima Centauri, is the closest star to
the sun.
9,461,000,000,000 trillion km or
5,878,000,000,000 trillon miles.
A star is made up of different elements in the form of gases.
The inner layers are very dense and hot. But the outer layers
are made up of cool gases. Elements in a star’s atmosphere
absorb some of the light that radiates from the star. Because
different elements absorb different wavelengths of light,
astronomers can tell what elements a star is made of from the
light they observe from the star. This is called the spectrum.
The spectrum consists of millions of colors, including
red, orange, yellow, green, blue, indigo and violet. A hot
solid object gives off a continuous spectrum—a spectrum
that shows all colors. However the spectrum of a star is
different. Astronomers use an instrument called a
spectrograph to break a star’s light into a spectrum. This
information tells astronomers information about the
composition and temperature of a star.
Brightness of a Star.
Absolute magnitude – how bright a star really is.
Apparent magnitude – how bright a star looks from
Earth.
Motion of stars.
Different types of stars.
• We have learned that stars are classified by their size, mass,
brightness, color, temperature, spectrum and age. Some types of
stars include main sequence stars, red giants, supergiants, and
white dwarf stars. A star can be classified as one type of star
early in its life cycle and hen be classified as another star when
it gets older. The life cycle for an average star, such as our sun
includes different stages. The first stage is star formation. The
second and longest stage is the main sequence. The third stage a
star can become a red giant or super red giant. The last stage is
a white dwarf.
Main sequence stars are stars that are fusing hydrogen
atoms to form helium atoms in their cores. Most of the
stars in the universe — about 90 percent of them — are
main sequence stars. The sun is a main sequence star.
These stars can range from about a tenth of the mass
of the sun to up to 200 times as massive.
Red Giants and Super Red Giants
A red giant star is a
dying star in the last
stages of stellar evolution.
In only a few billion
years, our own sun will
turn into a red giant star,
expand and engulf the
inner planets, possibly
even Earth.
Stars spend approximately a few
thousand to 1 billion years as a red
giant. Eventually, the helium in the
core runs out and fusion stops. The star
shrinks again until a new helium shell
reaches the core. When the helium
ignites, the outer layers of the star are
blown off in huge clouds of gas and
dust known as planetary nebulae.
The core continues to collapse in on
itself. Smaller stars such as the sun
end their lives as compact white
dwarfs.
Super Red Giants
Red supergiant stars are
the largest stars in
the Universe by volume
(meaning they also have
the greatest diameter),
however, they are not
necessarily - and almost
never are - the largest stars
by mass.
The star will remain a red giant
until the core reaches a high
enough temperature to begin
fusion helium into carbon and
oxygen. At this time the star
shrinks down slightly into a
yellow giant.
White Dwarf
White dwarfs are the burned-out cores of collapsed stars that, like dying
embers, slowly cool and fade away. They are the remnants of low mass stars,
among the dimmest objects observable in the Universe. They are low to
medium (less than ten solar mass) Main Sequence stars which have burned
through their reservoirs of both hydrogen and helium, passed through the
giant phase, were not hot enough to ignite their carbon, puffed off their outer
layers to form colorful planetary nebula, and then collapsed and cooled into
small glowing coals. This beautiful Hubble Space Telescope image shows a
nearby white dwarf, and the outer layers of the former star's atmosphere
which have been blown away. The resultant planetary nebula will shine for
the next 20,000 to 50,000 years, expanding outwards and fading slowly
with time.
This beautiful Hubble Space Telescope image shows a nearby white dwarf, and the
outer layers of the former star's atmosphere which have been blown away. The
resultant planetary nebula will shine for the next 20,000 to 50,000 years, expanding
outwards and fading slowly with time.
When massive stars die…
A massive star is a star with a
mass eight times greater than
that of the Sun. It is difficult for
stars to get this large, as a
number of factors influence
stellar development and these
factors often limit size, but
astronomers have been able to
observe massive stars up to 150
times larger than the Sun,
illustrating that it is possible
under the right conditions.
Massive stars use their
hydrogen much faster than
stars like the sun. Massive stars
generate more energy and are
very hot! Massive stars do not
have long lives. At the end of its
life the massive star may
explode in a bright flash of light
called a supernova.
Supernova explosion
A blindingly bright star bursts into view in a corner of the night sky —
it wasn't there just a few hours ago, but now it burns like a beacon.
That bright star isn't actually a star, at least not anymore. The brilliant
point of light is the explosion of a star that has reached the end of its life,
otherwise known as a supernova.
Supernovas can briefly outshine entire galaxies and radiate more energy
than our sun will in its entire lifetime. They're also the primary source of
heavy elements in the universe. According to NASA, supernovae are “the
largest explosion that takes place in space.”
What happens after a supernova explosion…
A supernova is a cataclysmic event that occurs as a result of the
final uncontrolled nuclear reactions in a very high mass star
at the end of its life. The giant star explodes violently due to the
collapse of its core, hurtling all or most of its material outward
at extremely high velocity. In some cases, a supernova will
produce more light, for several weeks following the explosion,
than the entire galaxy in which it resides. The remains of this
titanic explosion consist of an expanding debris cloud and
possibly an imploded remnant of the core, such as a neutron
star, pulsar, or black hole.
Neutron Star
When stars four to eight times as
massive as the sun explode in a
violent supernova, their outer
layers can blow off in an oftenspectacular display, leaving
behind a small, dense core that
continues to collapse. Gravity
presses the material in on itself so
tightly that protons and electrons
combine to make neutrons,
yielding the name "neutron star."
Pulsar
A pulsar is a neutron star that
emits beams of radiation that
sweep through Earth's line of
sight. Like a black hole, it is an
endpoint to stellar evolution. The
"pulses" of high-energy radiation
we see from a pulsar are due to a
misalignment of the
neutron star's rotation axis and
its magnetic axis
Black Holes
A black hole is a place in space where gravity pulls so much
that even light can not get out. The gravity is so strong
because matter has been squeezed into a tiny space. This
can happen when a star is dying.
Because no light can get out, people can't see black holes.
They are invisible. Space telescopes with special tools can
help find black holes. The special tools can see how stars
that are very close to black holes act differently than other
stars.
Scientists can't directly observe black holes with telescopes that detect x-rays,
light, or other forms of electromagnetic radiation. We can, however, infer the
presence of black holes and study them by detecting their effect on other
matter nearby. If a black hole passes through a cloud of interstellar matter,
for example, it will draw matter inward in a process known as accretion. A
similar process can occur if a normal star passes close to a black hole. In this
case, the black hole can tear the star apart as it pulls it toward itself. As the
attracted matter accelerates and heats up, it emits x-rays that radiate into
space. Recent discoveries offer some tantalizing evidence that black holes
have a dramatic influence on the neighborhoods around them - emitting
powerful gamma ray bursts, devouring nearby stars, and spurring the
growth of new stars in some areas while stalling it in others.
Galaxies
• There are three types of galaxies in our
universe:
• Spiral
• Ellipical
• Irregular
Spiral Galaxy
Spiral galaxies are complex objects and have several
components: a disk, a bulge, and a halo. The disk contains
gas, dust, and young stars in its spiral arms. The dense bulge
in the center of the disk contains mostly old stars and no gas
or dust.
The four distinguishing characteristics of the spirals are: (a) they have
more orderly, rotational motion than random motion (the rotation refers
to the disk as a whole and means that the star orbits are closely confined
to a narrow range of angles and are fairly circular); (b) they have some
or a lot of gas and dust between the stars; (c) this means they can have
new star formation occurring in the disk, particularly in the spiral
arms; and (d) they have a spiral structure.
We live in a spiral galaxy…The Milky Way.
How do we know the Milky Way is a spiral galaxy?
How do the astronomers tell the shape of our galaxy (the Milky Way), even
though it is not possible to take a photograph of it because to do that we would
have to go away from it?
The clues we have to the shape of the Milky Way are:
1) When you look toward the galactic center with your eye, you see a long,
thin strip. This suggests a disk seen edge-on, rather than a ellipsoid or
another shape. We can also detect the bulge at the center. Since we see spiral
galaxies which are disks with central bulges, this is a bit of a tipoff.
2) When we measure velocities of stars and gas in our galaxy, we see an
overall rotational motion greater than random motions. This is another
characteristic of a spiral.
3) The gas fraction, color, and dust content of our galaxy are spiral-like.
Elliptical Galaxies
Elliptical galaxies are the most abundant type of galaxies found in the
universe. However, because of their age and dim qualities, they are
frequently outshone by younger, brighter collection of stars.
Elliptical galaxies lack the swirling arms of their more well-known
siblings, spiral galaxies. Instead, they bear the rounded shape of an ellipse, a
stretched-out circle. Some stellar collections are more stretched than others.
Because elliptical galaxies contain older stars and less gas, scientists think that
they are nearing the end of the evolution line for galaxies. The universe is a
violent place, and collisions between galaxies are frequent — indeed, the Milky
Way is due to crash into the Andromeda Galaxy in a few billion years. When
two spirals collide, they lose their familiar shape, morphing into the lessstructured elliptical galaxies.
A supermassive black hole is thought to lie at the center of these ancient
galaxies. These gluttonous giants consume gas and dust, and may play a role
in the slower growth of elliptical galaxies.
Born from collision, elliptical galaxies are more commonly found around
clusters and groups of galaxies. They are less frequently spotted in the early
universe, which supports the idea that they evolved from the collisions that
came later in the life of a galaxy.
Irregular Galaxy
A galaxy that does not have the clearly defined shape
and structure of typical elliptical, lenticular, or spiral
galaxies. Irregular galaxies typically contain large
amounts of gas and dust, and their stars are often
young. They account for only a small percentage of
known galaxies. Some irregular galaxies are the result
of gravitational interactions or collisions between
formerly regular galaxies. Many irregular galaxies orbit
larger regular ones; the Magellanic Cloud galaxies
orbiting the Milky Way are examples.
Irregular galaxies have no particular
shape. They are among the smallest
galaxies and are full of gas and dust.
Having a lot of gas and dust means that
these galaxies have a lot of star formation
going on within them. This can make
them very bright.
Contents of Galaxies
1.Nebulas
2.Globular clusters
3.Open Clusters
Nebulas
A nebula is a cloud of gas and dust in space. Some nebulae
(more than one nebula) are regions where new stars are being
formed, while others are the remains of dead or dying stars.
Nebulae come in many different shapes and sizes. There are four
main types of nebulae: planetary nebulae, reflection nebulae,
emission nebulae, and absorption nebulae. The word nebula
comes from the Latin word for cloud.
. Nebulae are the basic building blocks of the universe. They contain
the elements from which stars and solar systems are built. They are
also among the most beautiful objects in the universe, glowing with
rich colors and swirls of light. Stars inside these clouds of gas cause
them to glow with beautiful reds, blues, and greens. These colors are
the result of different elements within the nebula. Most nebulae are
composed of about 90% hydrogen, 10% helium, and 0.1% heavy
elements such as carbon, nitrogen, magnesium, potassium, calcium,
iron. These clouds of matter are also quite large. In fact, they are
among the largest objects in the galaxy. Many of them are dozens or
even hundreds of light-years across.
Globular Clusters
Globular clusters are
groups of older stars that
looks like a ball. There
may be up to one million
stars in a globular
cluster. They are located
in the spherical halo that
surrounds spiral galaxies
and are also found near
giant elliptical galaxies
Open Clusters
Open Clusters are groups of
closely grouped stars that
usually located along the
spiral disk of a galaxy.
Newly formed open clusters
have many bright blue
stars. There may be a few
hundred to a few thousand
stars in an open cluster.
Quasars…what we know
very little about.
Quasars are extremely distant objects in our known universe. They
are the furthest objects away from our galaxy that can be seen.
Quasars are extremely bright masses of energy and light. The name
quasar is actually short for quasi-stellar radio source or quasistellar object.
Quasars are the brightest objects in our universe, although to see one
through a telescope they do not look that bright at all. This is because
quasars are so far away. They emit radio waves, x-rays and light
waves. Quasars appear as faint red stars to us here on Earth.
A quasar is believed to be a supermassive black hole surrounded by an
accretion disk. An accretion disk is a flat, disk-like structure of gas that
rapidly spirals around a larger object, like a black hole, a new star, a
white dwarf, etc. A quasar gradually attracts this gas and sometimes
other stars or or even small galaxies with their superstrong gravity. These
objects get sucked into the black hole. When a galaxy, star or gas is
absorbed into a quasar in such a way, the result is a massive collision of
matter that causes a gigantic explosive output of radiation energy and
light. This great burst of energy results in a flare, which is a distinct
characteristic of quasars.
The light, radiation and radio waves from these galaxies and stars being
absorbed into a black hole travel billions of light years through space.
When we look at quasars which are 10-15 billion light years away, we are
looking 10-15 billion years into the past. Pretty amazing, right?
According to the best estimates of astronomers there are at least one
hundred billion galaxies in the observable universe. They've counted
the galaxies in a particular region, and multiplied this up to
estimate the number for the whole universe.