The universe formed some
12.5 billion years ago. Much is speculation, but somehow from a tiny speck
everything including space, time, matter and energy unfolded into something
that became recognizable as an early version of the universe we see about
us today. Initially, the temperature was too intense to allow matter to
condense from energy. All of the energy was in the form of fierce gamma
After expanding for many
thousands of years, the temperature of the Universe had cooled to the
point where gamma radiation could form neutrons, protons and electrons.
Almost all of this matter was in the form of hydrogen (91+%) and helium
(8%) and less than a percent lithium, an isotope of hydrogen (deuterium)
and an isotope of helium (helium 3). Almost no other elements were created
at this time. It is a matter of debate whether primordial
black holes were also created. The forces were enough so that dense
knots of matter could create black holes. These black holes may be the
"seed" around which galaxies formed.
The Universe Today
Today large clouds of
gas exist throughout the universe. Most of it is simple atoms, but some
of these clouds contain simple molecules and dust. Most of it has collected
in and around galaxies. While we see star formation throughout the universe,
and we see the absorption of smaller galaxies when the encounter larger
galaxies, we no longer see the formation of new galaxies.
The nature of the interstellar
gas is very different today from the original gas. While hydrogen and
helium still abound, other elements can be found in densities as high
as 7%. This has profound consequences for the type of stellar systems
that can form. Most importantly, heavy elements allow rocky planets such
as the Earth to form. These new elements came from the transmutation of
elements in the hearts of first generation stars. Elements up to iron
in weight can be formed in normal stars and ejected into space as solar
winds and exploding shells when the stars reach the red giant stage. Elements
heavier than iron are created and distributed by a much more dramatic
process - supernovas.
Edwin Hubbell assigned a naming convention to galaxies which remains in
use today. Galaxies come in three main forms, irregular galaxies with
shapes that are amorphous, elliptical galaxies with a large core and almost
no disk, and spiral galaxies which come in two forms, those with a large
central cylinder of stars (barred) and those where the spirals go all
the way to the core.
The shapes of galaxies
appears to start as spirals of one sort or the other. Over time galaxies
pas near or actually through each other. Currently, a small galaxy [the
Sagittarius Galaxy] is colliding with our Milky Way. When they do this
the smaller galaxy loses many of it stars to the larger galaxy. If the
smaller galaxy comes into too close a contact it may be simply swallowed
by the greater galaxy. If it is somewhat farther away it may escape badly
tattered as an irregular galaxy. This is what seems to have happened to
the Large and Small Magellenic Clouds. After swallowing enough smaller
galaxies, the spiral shape disappears and the galaxy assumes an ever more
The Great Nebula in Andromeda is a classic spiral nebula. At the leading
edges of reaching rotating arm, a wave of gas compression occurs triggering
areas of star formation. Andromeda is nearly edge on in this image but
it would look like M74 if we saw it face on.
also known as M87 is many hundreds the times the mass of our own galaxy.
Trillions of stars are believed to populate this great object. Virgo A
is the largest of the galaxies in the so called Realm of the Galaxies
which spans parts of Virgi, Coma Berenices and Leo. This area has more
galaxies to seen than it has stars visible to your eye. While it looks
rather like a globular cluster, it is billions of times larger.
NGC1300 is a clear example
of a barred spiral. Unlike a traditional spiral, the swirling arms star
at the ends of a cylinder of stars which extends for many tens of thousands
of light years from the center of the barred spiral.
Open clusters of stars are formed in a common stellar
nursery. In time birthing grounds like M42 and the Omega Nebula in the
southern hemisphere will drive out gas which has not been included in
the newly formed stars. In some open clusters like the Plieades traces
of the gas still can be seen in photographs sensitive to blue and ultraviolet
light. [This gauzy gas is not visible to the human eye]. When the stars
were formed they were densely packed. However they each had their own
motions which over time causes them to disperse.
Globular clusters are effectively satellites of the
galaxy in which they reside. They travels as units into the central areas
of the galaxy on orbits somewhat like comets around the Sun. Stars in
globular clusters are quite unlike stars in the main disk of the galaxy.
Main disk stars like the Sun carry a great deal of heavier elements (metals
to astronomers no matter what the chemists call them). Sun like stars
are called Population I stars. Stars found in globular clusters lack more
than a tiny percentage of heavy elements and form the Population II stars.
The thin halo of stars outside the plane of the galaxy are also Population
II stars. Population II stars formed before there had been many supernovae
to create heavier elements. They are first generation stars for the most
part. Population I stars form in the wake of supernovae and contain the
As the globular clusters
orbit about the central core, they tend to be densest near the core and
more sparse farther out. In our galaxy, the great concentration of globular
clusters in Sagittarius confirms that this constellation harbors the center
of the Milky Way. Further tests have shown that the exact center is a
small volume in Sagittarius with at least a mass of 2 billion Suns. This
dense area is believed to be a black hole and is called Sagittarius A*.
An emission nebula is gas excited by ultraviolet
radiation from fierce new blue violet stars. This is the same conditions
which occur in fluorescent and neon lamps. This type of nebula is typified
by M42, the Great Nebula in Orion. The Trapezium as well as many unseen
embedded stars provide the' sources of ultraviolet radiation.
An absorption nebula is a mixture of gas and particularly
dust dense enough to absorb, redden and even blot out light. These cause
dark nebulae like the Horsehead and the Coal Sack. They cause dark areas
like Sagittarius and the Sombrero. However they do not include "empty
lanes" in the Milky Way.
Sometimes a nebula can
have regions which emit while other regions absorb. This is the case in
the Horsehead Nebula shown here. The dark regions are areas where light
has been absorbed so heavily that the area looks like a dark cloud. The
bright regions are where hydrogen gas is fluorescing emit a reddish frequency
called the hydrogen beta line. Near the edges of the dark regions areas
absorb much but not all of the light and it is possible to use a spectrograph
to determine the clouds chemical make up.
When light from a foreground stars shines on background clouds, a reflection
nebula is formed. In some cases, the foreground star can be hidden
by an absorbing nebula. Reflection nebula can sometimes be precisely mapped
and measured by timing the pulses of light from a variable star or a supernova.
nebulae arise when an aging stars sheds shells of gas as the fusing
of hydrogen leaves the core and moves towards the star's surface. Large
explosions (but not as large as supernovae), progressively strip the star
of material. If the star manages to shed enough material, the it will
end up as a white dwarf with a ring about it which grows year by year.
Eventually these rings become so large and thin that they are no longer
illuminated by the hot central white dwarf.
Stars are continually
being formed from the huge reservoirs of hydrogen gas the fill the galaxies.
It was once thought that gravity played the role of "gas compressor",
but we now know that there hasn't been time since the formation of the
universe to have had many clouds compress naturally into stars. A triggering
event is required. The two principle events are density waves and supernovae.
The center of every
galaxy appears to contain a black hole. This is by no means certain, but
something large and dense exists there. Lines of magnetic force stream
outwards and are bent along the leading edges of the galactic arms. This
creates a density wave which sweeps up and compresses hydrogen and helium
along with any other elements which may be in the region. Although we
cannot look down on the Milky Way to see such area, we can see similar
areas in thousands of other galaxies. Along the leading edge of their
arms, young fierce glowing blue white stars abound, a sure sign of star
Role of Supernovae
"We are such things as dreams are made on" said Shakespeare. I wonder
what he would have said if he realized that it is also quite literally
true that once our very elements were forged in the hearts of the largest
stars. Look at the Crab Nebula as the explosion which tore it apart sends
material through space. However, the material which pours out of a supernova
is not just the hydrogen and helium which formed the star but nitrogen,
oxygen, carbon, silicon, sulfur, magnesium, neon, iron and in fact to
some degree or other every element in the natural world.
One role of a supernova
is to create the elements from which Population I (metal rich) stars are
formed. These are the stars that can have rocky, watery worlds where life
can form. The other crucial role that supernova play is as another source
of gas compression and the triggering of new stars. Like density waves,
the bow wave of a supernova explosion pushes everything before it and
compresses gas until its own gravity can take over forming a new set of
Hertzsprung Russel Diagram
The luminosity (the total emitted energy) of a star is directly proportional
to the fourth power of it mass. To maintain this power output, the star
must consume its fuel proportional to its fourth power as well. If one
main sequence star is 3 times as massive as another star, it will shine
81 times as brightly. It also fuses its fuel 81 times as rapidly. As stars
leave the main sequence this relationship is disrupted.
The term luminosity is
the preferred to describe the brightness of a star. For historical reason,
the portion of a star's spectrum that lies in the visual range is measured
by a magnitude scale. Stars of the first magnitude seem to be twice as
bright as those of the second magnitude which in turn seem to be twice
as bright as those of the third magnitude. In fact, a closer relation
ship is that every five magnitudes in brightness represent a 100 fold
change in luminosity. Luminosity is measured directly. Magnitude is measured
on an inverse logarithmic scale. Larger magnitudes mean dimmer stars which
is counterintuitive. Larger luminosities mean brighter stars exactly as
you would think.
Do not confuse apparent
luminosities (or magnitudes) with absolute luminosities (or magnitudes).
Apparent brightness depends on how a star looks to us on Earth. Absolute
brightness depends on how bright a star would be at the standard distance
of ten parsecs (33.26 light-years).
The time that a star spends on the main sequence is INVERSELY proportional
to the cube of its mass. This is a direct result of the luminosity relationship
we just discussed. Since a stars luminosity (and hence its rate of fuel
consumption) is proportional to the fourth power of the mass but its mass
is only the first power, stars have a lifetime which is proportional to
M/M4 or simply M-3.
Large stars have very
short lifetimes. A maximal sized star of about 100 solar masses will live
1 MILLIONTH as long as the Sun. A minimal sized star of 0.08 solar masses
will live 1950 times as long as the Sun. Since the Sun will live about
10 billion years, the largest stars burn out in just about 10 THOUSAND
years but smallest stars will live 19.5 TRILLION years.
When stars coalesce
from interstellar gas clouds, their temperature and pressure rise from
frictional heating and gravity. Once nuclear processes begin gas already
falling in from the spinning disk collides with gas expanding from nuclear
fusion. One way that Herbig-Haro stars relieve this problem is to eject
mass at the poles of the new star.
Young stars have yet
to achieve hydrostatic balance between the rate of energy production and
the size of the star. As much as ten times the material that will eventually
form the finished star exists in the new stellar system. This material
must be driven back into the interstellar medium. Stars in this stage
of development are called T-Tauri stars.
Brown dwarfs weighing
between 0.01 and 0.08 stellar masses are neither true stars nor planets
but intermediate objects. They radiate in the infrared. Most of their
heat comes from gravitational contraction. However, sometimes their central
cores are hot enough to fuse deuterium, lithium or beryllium. These elements
fuse at a temperature several million degrees cooler than the minimum
required for hydrogen fusion. However, there are so few of these atoms,
that they are unlike to encounter each other in a core that is largely
hydrogen and helium. When these elements do fuse, they expand the core
cooling it enough to shut down the reactions. far between to really ignite
Once a body of hydrogen
reaches 0.08 solar masses, it has enough material so that gravitational
contraction will raise the central core to 15 million degrees. Hydrogen
begins to fuse. A true star is born.
When the new star has
a mass between 0.08 and 0.4 solar masses, it forms a small dim red dwarf
star. Of the 100 nearest stars 92 are red dwarfs. They form in great numbers
but their total luminosity is so low that galaxies seem blue white. Indeed,
Proxima Centauri, the nearest star to the solar system is 13th magnitude
- no brighter than dim little Pluto.
Most normal sized stars
are the so called main sequence dwarfs. They are in the spectral classes
K, G, F and A with masses between 0.4 and 3.3 solar masses. The term "dwarf"
is unfortunate because it seem to imply a star of small dimensions. In
fact they are much larger and brighter than an average star. For example
the Sun is a yellow G2 dwarf, yet of the 100 nearest stars only 3 are
a bit larger and another is just a bit smaller. 95 stars have diameter
which are less than 60% of the Sun and masses which are less than 40%
of the Sun. No nearby star is really large, although Sirius is almost
twice the mass of the Sun.
Some orange, yellow,
white (green) stars fall into a category of sub-giants. Sub-giants are
large stars which are in the process of leaving the main sequence. These
stars swell as the hydrogen fusion shell approaches the surface. Most
of these stars are variables.
The largest main sequence
stars are the blue giants. They are between 3.3 and 100 solar masses.
While they are called blue giants, they can be blue, violet or even ultraviolet
in color. These stars are extremely bright and short lived. Of the roughly
6000 stars that can be seen by the human eye, all but 50 are either red
or blue giants. Blue giants of necessity are all very young stars. Some
of these blue giants become unstable - like Dschubba and Gamma Cassiopeia
- throwing off huge shells of gas and briefly becoming very bright. A
few actually become supernovae without first becoming red giants.
Red giants posed a paradox
to early astronomers. They were very red (hence they were cool) and they
were very bright (which seemed impossible - because the black body laws
[which we shall learn about in the Physics Section] say the red objects
emit light dimly). Finally, astronomers realized that a star with a very
low brightness per square meter could actually put out a huge luminosity
if its surface area was enormous.
Red giants have HUGE
volumes although they have low density. A typical red giant like Antares
or Betelgeuse will have a volume as large as the orbit of Mars. The largest
known red giant VV Cassiopeia is calculated to have a diameter as large
as the orbit of Saturn.
Red giants are aging
stars which have converted a large portion of their hydrogen to helium
(typically 40-50%). As the core fills up with helium "ashes" the fusion
zone approaches the surface. However at some point the gas above the star
has too little remaining mass and the star stops being stable and begins
to swell. The swollen star emits more light that before cooling it at
a new less healthy stage. Red giants with lower mass (such as the Sun
will become) will eventually simply become white dwarfs. High mass red
giants are rapidly on their way to becoming supernova.
Eclipsing binaries are
binary stars have the plane of their orbit edge on to the solar system.
As the stars revolve around their barycenter they will regularly pass
in front of one another. Since at least some of the total surface area
is masked, the luminosity will drop. If one star is much brighter than
its companion, there will be alternating large and small dips in the luminosity.
By timing the dips precisely and determining the stars mass and velocity
by applying Newton's laws of gravitation, it is possible to determine
the diameters of the stars very accurately.
Flare stars appear to
change more profoundly than they really do. All main sequence stars appear
to emit flares. Against a bright star such as the Sun, Sirius or Rigil,
a flare is lost in the overall brightness of the star. Against a dim red
dwarf however, the flare can actually be brighter than the rest of the
star's surface. All stars have flares where a pocket of overheated gas
erupts at the surface. Momentarily, the star emits radiation of shorter
wavelengths (blue, violet, ultra violet and x-rays). On a moderate star
like the Sun, a flare tends to fade into surface brightness. Flares are
unnoticeable on large blue stars. However, on a small red dwarf, a flare
can actually be brighter than the star itself. For periods of a few minutes
to a few hours the star may brighten several magnitude. Some amateurs
watch a collection of red dwarfs looking for these flares.
Certain yellow orange
sub-giants (called Cepheid variables) pulsate in a very regular manner.
It is possible to determine exactly how far these stars are from the solar
system by timing the pulse rate. What makes these Cephied variables unusually
useful is that they are bright enough to be seen in distant galaxies.
Hydrostatic balance is
the balance between the expanding forces from the heat produced by fusion
and a compressive forces from gravity. Imbalances between the expansion
and compression can cause pulsations. These stars expand when they are
hottest, emit radiation more rapidly when they are inflated, cool and
contract in a cycle. Cepheid variables are examples of pulsating stars.
Supernovae are the deaths
of very large stars. Stars which start out at least 10 times the mass
of the Sun cannot shed enough mass by ejecting shells by the time their
core reaches 1.4 solar masses (Chandreskar's limit) [details to follow
in Physics]. This results in an enormous explosions where all the elements
of the periodic table beyond the first groups are produced. Supernovae
can outshine their galaxy (billions and even trillions of star power)
for a few weeks. Even this most titanic of nuclear explosions does not
totally destroy the star. A core of compressed material remains. If the
core is less than 1.4 solar masses it creates a white dwarf. If it is
between 1.4 and 3 solar masses it forms a neutron star. More than 3 solar
masses results in a black hole.
White dwarfs can result from supernovae, but they also are the end product
of stars which go through the red giant stage without going supernova.
The sun will someday become a white dwarf after it swells into a red giant
stage. You can see a white dwarf at the center of the Cat's Eye nebula.
White dwarfs no longer
fuse hydrogen into helium. The core is composed of helium or some heavier
element (usually, carbon, oxygen, neon, silicon, magnesium or sulfur).
Since there is no steady source of fusion energy, white dwarfs slowly
cool down eventually become cold inert [hundreds of trillions of years]
black dwarfs. No white dwarfs is believed to have entered black dwarf
Astronomers used to think
that nova and supernova were differing degrees of the same thing - stellar
explosions. However, they are really quite dissimilar. Supernovae are
titanic explosions which rip stars apart scattering elements into the
universe. Novae are recurring small explosions which leave their "star"
Novae are white dwarfs
or neutron stars in close orbit around a main sequence star. The fierce
gravity of the burnt out star strips the outer layers of hydrogen from
the main sequence star. When enough accumulates on the burnt out star,
a hydrogen bomb type explosion takes place.
We have already seen
that neutron stars are supernovae remnants where the core is greater than
3 solar masses. These objects are very odd things indeed. In "normal"
white dwarfs, the elements left after the supernova explosion are left
as a plasma (sort of a gas where the electrons have been stripped away).
The white dwarf does not have fusion energy to hold the star up from collapse,
but the "electron pressure" (like charges repel) keeps the white dwarf
steady at about the size of the planet Earth in diameter.
All this changes in neutron
stars. Once the mass reaches 1.4 Sol, the gravity becomes so intense that
the electrons are dragged kicking and screaming into the core. They get
squished into the protons (positively charged nuclear particles) neutralizing
them and becoming neutrons (uncharged nuclear particles). The star loses
the pressure of the "degenerate electrons" and it collapses into a ball
about 10 miles in diameter spinning at hundreds and thousands of times
per second. The surface of a neutron star spins very near the speed of
Effectively, this neutron
star is a single giant (fiercely radioactive) atom. It is very nearly
the densest object in the universe. A sugar cube chunk of this stuff would
weigh more that Mount Everest.
Pulsars (a type of neutron
star) spin extremely rapidly. Near their poles, they emit charged particles
at very near the speed of light. Think of them as swizzle sticks spinning
around blindingly fast. The swizzle sticks of charged particles sweep
up and stir around the gas in the system they reside in causing a form
of electromagnetic radiation. Some of this is in the radio frequencies
and the rest in higher frequencies up to visible flashes. If the beam
of charged particles is lined up in the direction of the Solar system,
the electromagnetic radiation will flash on us. When these very regular
flashes were first detected many astronomers suspected they were artificially
produced by alien species.
Supernova remnants greater
than 3 solar cannot remain stable at the neutron star stage. They become
the most exotic of all stellar objects - black holes. Gravity again begins
its relentless pull. The gravity reaches a point where no particles, not
even light can escape because they would have to travel above the speed
of light (the universal maximum) to leave the ex-star. [There is an odd
form of radiation (Hawking radiation) which can leave the event horizon
of a black hole through quantum mechanical processes but we will not discuss
For our purposes, the
event horizon marks the point where anything that enters the black hole
cannot leave. There is a false belief that black holes are all powerful
vacuums which slurp anything and everything into their maw. This is not
so. For example if you squeezed the Earth into a black hole (an event
horizon about the size of a marble), and stood at a distance of 6,400
kilometers from it (our current distance from the center of the Earth)
the gravity would be exactly 1 G. The field would only become great as
we came very close to the black hole.
Quasars are extremely
bright objects which can be seen across the universe. The only known source
of such power would be a huge black hole swallowing the gas from stars
unfortunate enough to get too near the black hole. The light is not emitted
by the black hole itself, but a disk of material spiraling into the black
hole. Quasars normally have long jets of material shooting out at nearly
the speed of light. This jet can be luminous for light-years.
Active galactic nuclei
are suspected of containing black holes in their centers. In fact some
theories say that all galaxies arose around a central black hole formed
at the big bang [a pure guess so far]. Those which are so suspected have
something very energetic at the core.
Seyfert galaxies have
centers which seem very much like quasars without their jets. They appear
to be ex-quasars or quasars which no longer have enough nearby gas to
power the quasar. The heart of our galaxy is a black hole located at the
object we call Sagittarius A*. It looks as if the Milky Way is (or at
least was) a Seyfert type galaxy. The central black hole solar masses
some 2-3 million solar masses. We can see stars orbiting Sagittarius A*
in as little as decades.
- Asteroids (sometimes
called planetoid) are planetesimals which orbit a star. Ideally, all
asteroids would be planetesimals, however some larger asteroids are
actually worlds. The dividing line is an arbitrary 1000 km.
- Dwarf Stars
- Dwarfs are regular
stars like the Sun which have modest masses and modest volumes. Stars
which are not some sort of "giant" are called dwarfs no matter what
their size. super dense star is called a white or a black dwarfs.
- Giant Stars
- Giant Stars have
volumes many thousands of times that of the Sun. Some "sub-giants" and
"blue giants" have masses much greater than the Sun, but volumes which
are not radically larger than the Sun.
- Main Sequence Stars
- Main Sequence Stars
are huge bodies which derives the vast majority of their energy primarily
from fusing hydrogen to helium. Main sequence stars are in hydrostatic
balance between the forces of gravity and nuclear fusion. Stars
too young to have achieved this balance throw off huge amounts of material
via jets and fierce solar winds. Stars that have used up their hydrogen
fuel supply swell enormously.
- Planets are full
sized spherical worlds which orbit a star. See rogues and asteroids.
- Planetesimals are
bodies which is too small to attain spherical shape simply through their
own gravity. A planetesimal melted by passing too close to a star and
becoming spherical due to surface tension (a result of electromagnetic
force) does not count, because the forming was not done primarily by
- Rogues are suspected
(but unproven) worlds like planets that do not orbit stars. These are
believed to be ejected from star systems as the systems grow older.
- Satellites (often
called moons) are either worlds or planetesimals which orbits a planet.
- Worlds are bodies
large enough to be pulled into roughly spherical shape by their own
gravity. All stars fall within this definition as do major planets and