All About Galaxies

Galaxies are large systems of stars and interstellar matter, containing millions to trillions of stars, ranging from the smallest "dwarfs" to the largest "giants."; with masses reaching a million to a trillion times that of our Sun, extending diameters of a few thousand to several 100,000 light-years, separated by distances of millions of light-years. They are made up of a combination of varying numbers of both star clusters and nebulae. These galaxies emit light of every spectrum, from the long radio wavelength and microwave end-over the IR, visual, and UV light-to the short wavelength, high-energy X- and gamma-rays. However, since interstellar matter's temperature is cool, the best visible waves are in radio and IR, while supernova remnants are most visible in the high-energy part of the electromagnetic spectrum (Frommert and Kronberg.)

To begin with, there are many theories on how these remarkable galaxies were created. The first theory: through extensive research, astronomers have learned that they must have formed during a short period within the first billion years after the universe started to expand from an initial hot state. So basically, they're almost as old as the universe itself, which is currently thought to be about 10-15 billion years old (Frommert and Krongberg.) According to computer simulations, it's thought that galaxy formation started when clouds of gaseous matter-hydrogen and helium-the proto-galaxies, were singled out and started to collapse by any of the following: their own gravity due to their amount of initial angular momentum, different parameters of the proto-galaxies, as well as interacting with other neighboring galaxies.

The second theory: according to an article interviewing Dr. Peter Meszaros of Pennsylvania State University, on "Gamma Rays and the Birth of Galaxies", Gamma-Ray bursts not only light up the universe, but may also illuminate the history of the universe by showing dim ancient galaxies. Here's a little summary of Gamma-Ray bursts and neutron stars: Gamma-Ray bursts are a relativistic fireball from the collisions of two neutron stars, which tends to happen only once in each galaxy every million to 100 million years; neutron stars are the collapsed remains of super massive stars, which expend the last of their energies in a supernova explosion, yielding gamma-rays (Dooling.) If the star is massive enough, the explosion all but squeezes the core out of existence, leaving a black hole, which is only detectable by its gravity and the energy released when it swallows more matter.

Neutron stars and black holes are commonly found orbiting with visible companions that fade into dwarfs or neutron stars. Eventually, tidal forces wear these binary systems down, and the two bodies collide. The collision releases incredible amounts of energy in a rapid spiral, causing the neutron star to loop outward, then crash into the black hole, occurring in the matter of a few milliseconds. At this point, the two stars produce a massive cloud of neutrinos and anti-neutrinos, which are nearly massless particles that carry away energy from nuclear reactions (Dooling.) Because this cloud is so dense, the neutrinos collide with each other to make electrons and positrons, or anti-electrons, which annihilate each other to form gamma-rays. The energy then becomes so intense, that it accelerates the material in the fireball to almost the speed of light-which is where time and space are changed by the effects of relativity-hence the nickname "relativistic fireball." It then expands, causing the fireball to thin and more photons to escape. This then gives a sudden pulse of radiation, followed by a second pulse as energy emerges from within the fireball. The energy then crashes through slow interstellar gas, producing synchrotron radiation and inverse Compton radiation. This then produces the gamma-ray spectra-reflecting light that can even be seen through telescopes on Earth (Dooling.)

The third theory: speaking of telescopes, by peering through a giant cosmic lens, scientists have found some of the first-born stars in our Universe. If you think about it, a telescope is kind of like a time machine, believe it or not, astronomers can "travel" billions of years into the past. By peering through a telescope, you can see stars and galaxies not as they are now, but as they were when the starlight first began its journey. According to an article, interviewing astronomer Richard Ellis of the California Institute of Technology, he explained:

"At some point a billion or so years after the Big Bang, gravitational attraction caused the gas that filled the Universe to collapse and form the first stars."

Searching for the First Light stars is a very challenging task in modern astronomy, because these stars lie more than 10 billion light-years from Earth (Barry and Phillips.) Not only are they faraway, but they're also very faint; even powerful instruments like the Hubble have trouble detecting them. So Ellis and his colleagues used a magnifying glass, or gravitational lens-a gigantic cosmic magnifier formed from a group of galaxies, located 2 to 3 billion light-years away-along with the Hubble Space Telescope and the Keck Telescope in Hawaii. They observed a faint cloud of stars lying 13.4 billion light-years from Earth, a very small galaxy, containing about a million stars; believed to be a building block of a full-sized galaxy that now populates our Universe. If the universe is 14 billion years old, then the star cloud existed less than one billion years after the Big Bang (Barry and Phillips.) Though the answers are unclear now, the NASA's Next Generation Space Telescope (NGST), which is scheduled to launch in 2009, will reveal the answers to the astronomer's questions, because this telescope will be able to detect objects without any help from gravitational lenses. There are, however, many other theories, but these are just a few.

Next, we'll discuss the different types of galaxies: Spiral, Lenticular, Elliptical, and Irregular. The first are the Spiral Galaxies which are made up of both stellar population I and II stars. Stellar Population I stars can either be flat, large disks which often contain a lot of interstellar matter, appearing as a reddish diffuse emission nebulae and dark clouds. Or they can be young, or open, star clusters and associations which have emerged from them, appearing as a bluish light since they're hot, they are short-living, and the most massive stars. Stellar Population II stars are an ellipsoidally formed bulge, consisting of an old stellar population without interstellar matter, they're often associated with globular clusters. These galaxies are arranged in prominent and striking spiral patterns or bar structures. Disk pattern structures are mostly transient phenomenon only, which is caused by interacting with neighboring galaxies. Their luminosity and mass relation vary in a wide range, and they're made up of several 100 billion stars, such as our Sun and Milky Way (Frommert and Kronberg.)

The second galaxy type are the Lenticular Galaxies, which are similar to the spiral galaxies, but without a spiral structure. Instead they're smooth disk galaxies, whose stellar formation had stopped long age, because their interstellar matter was used up. They only consist of old population II stars, and due to their appearance and stellar contents, they can hardly be distinguished from ellipticals observationally (Frommert and Kronberg.)

The third type of galaxy is Elliptical which are of ellipsoidal shape, and from observation, they've been discovered as triaxil, kind of like cosmic footballs. They look like luminous bulges of spirals, without a disk component, but small disk components have been discovered in some ellipticals. They have little or no global angular momentum, so they don't rotate as a whole; however, the stars still orbit the centers of these galaxies, but the orbits are statistically oriented so that only little net orbital angular momentum sums up. They contain very little or no interstellar matter, and only consist of old population II stars (Frommert and Kronberg.)

The fourth and final galaxy type is the Irregular Galaxies, which exhibit peculiar shapes, and subclasses of distorted disks frequently occur (Frommert and Kronberg.)

Continuing with Galaxy Clusters-defined as isolate "island universes", which float independently through an otherwise empty region of the universe. Since space is so densely crowded with them, they form groups of dozens of galaxies, or even large clusters of thousands of galaxies (Frommert and Kronberg.) They also have a mutual gravitational interaction which may significantly influence their appearance.

Finally, as previously mentioned, galaxies are made up of both star clusters and nebulae. Globular Star Clusters are large, but quite compact agglomerations of some 100,000 to several million stars. They have about the same mass as the smallest galaxies, and are also among the oldest objects in the galaxies, they also vary in a wide range of richness between individual galaxies (Frommert and Kronberg.) They often form conspicuous systems, and occur in galaxies of every type and size.While the older stars tend to form an ellipsoidal bulge, the interstellar gas and dust tends to accumulate in clouds near an equatorial disk. Interstellar clouds are the places of star formation-huge diffuse nebulae are places where open clusters and associations of stars are formed.

On the other hand, Active Galactic Nuclei (AGNs), are intense sources of light made up of all wavelengths. They're caused by gaseous matter falling into, and interacting with, the super massive central objects. Sometimes the spectra of these nuclei indicate enormous gaseous masses in rapid motion (Frommert and Kronberg.) Another type of nuclei, are Quasars, or Quasi-Stellar Objects, which are extremely compact and bright, outshining their whole parent galaxy. They're not only incredibly active, but so rare and the nearest quasar is so remote, that the brightest of them is about two billion light-years away with a magnitude of only 13.7, which is very low compared to Earth (Frommert and Kronberg.)

Whether galaxies were among the first to light up the heavens or not, astronomers and scientists alike are searching but have yet to find out. Either way, these magnificent, not to mention, enormous systems of stars and interstellar matter are definitely appealing to the eye; with their variety of shapes, sizes, and rainbow waves of luminous colors. They not only light up our universe, but leave us in never-ending awe of their mysterious beauty and existence.

Works Cited

Berry, Patrick L. and Phillips, Dr. Tony. "The First Starlight: By Peering Through a Giant Cosmic Lens, Scientists Have Found Some of the First-Born Stars in Our Universe." February 8, 2002. Science @ NASA. Access Date: April 17, 2006. http://science.nasa.gov/headlines/y2002/08feb_gravlens.htm

Dooling, Dave. "Gamma Rays and the Birth of Galaxies: or 'Chinese Lanterns in Space'". September 22, 1997. Access Date: April 17, 2006. http://science.nasa.gov/newhome/headlines/ast_23sep97_1.htm

Frommert, Hartmut and Kronberg, Christine. "Galaxies". January 25, 1998. Jones and Bartlett Publishers. Access Date: April 17, 2006. http://www.jbpub.com/starlinks/explorations_visit.cfm?ID=47&URL=http://seds.lpl.arizona.edu/messier/galaxy.html