The Big Bang

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 radiation.

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.

Galaxies

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 elliptical shape.

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.

Virga A 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.

Clusters

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 heavier elements.

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*.

Nebulae

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.

Planetary 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.

Star Formation

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.

Density Waves

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 formation.

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 stars.

Hertzsprung Russel Diagram

 
 

Stellar Luminosity

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).

Stellar Lifetimes

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.
 
 

Stellar Classifications

New Stars

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 a star.

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.

Stellar Instability

Variable stars

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.

Stellar Deaths

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 stage yet.

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" intact.

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 light.

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 it here].

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.

Definitions

Asteroids

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

Planets are full sized spherical worlds which orbit a star. See rogues and asteroids.

Planetesimals

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 gravity.

Rogues

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. See Planets.

Satellites

Satellites (often called moons) are either worlds or planetesimals which orbits a planet.

Worlds

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 large moons.