There are four fundamental forces that have been identified in nature. The strong and weak nuclear forces dictate how the nucleus of an atom holds together (and falls apart). The electromagnetic force holds electrons to the nucleus to form atoms and all matter. You will study these three forces next semester. But this semester, you are studying the weakest of all the fundamental forces, gravity. Although the weakest, it is gravity that holds the Universe together. In fact, gravity is responsible for ... well, just about everything that's interesting.
Let's Make Some Stars
The most convincing theory of how our Universe came into being is the Big Bang theory. In this model, the Universe began from a singularilty (a point!) and quickly expanded. The early Universe was extremely hot and was dominated by radiation and subatomic particles. As the Universe expanded and cooled, these subatomic particles condensed into electons, protons and neutrons from which hydrogen and helium formed. That was essentially all there was for a long time. (There were trace amounts of Li and a few higher elements.) If you are interested in more of the bizarre details concerning the origin and demise of the universe, check out the Big Bang FAQ site about the origins of the Universe by Wright at UCLA.
Without gravity, the Universe today would at most be a rarefied gaseous cloud of hydrogen and helium atoms floating about. But gravity provided a way to create much more. The atoms in the slightly denser regions of space started falling together under the influence of gravity. These atoms gained speed as they fell towards the region's center of mass and began to heat up from the collisions with one another. If the mass was sufficient (about two tenths the mass of our sun or more), the temperature would increased enough to start a sustained fusion reaction. A star was born! (The fusion process fuses lighter nucleii such as hydrogen and helium into larger nucleii such as Li, Be, etc. and releases tremendous amounts of energy.) Stars are still being created today by the same process. The Hubble Space Telescopes photo of M16- the Eagle Nebula, shown to the right, is a prime star factory. There are many stars being born within the massive gaseous clouds seen. If you would like to see a larger version of this beautiful and amazingly detailed photo, just click on the image.
An incredibly tiny portion of the Universe is something other than stars. There are small, insignificant, nearly invisible, but potentially interesting specks that contain a significantly higher proportion of atoms other than hydrogen and helium... such as the earth, for example. Where did these other atoms come from?
Star Dust
All heavier elements in the Universe were manufactured in the nuclear fusion furnace of stars. Essentially, gravity is supplying the energy for the fusion process. During the normal life of a star, a balance between the inward pull of gravity and the outward fusion radiation pressure is achieved. A star may exist for billions of years in this state of equilibrium. But when the hydrogen is used up, the death of the star begins. This is the time (the death may last millions of years) when all the elements heavier than helium are created. If conditions are right, some of these stars will explode in a supernova and spread these heavier elements out into the surrounding space. There is a reason that Joni Mitchell sang, "We are stardust ..."!
Structure in the Universe
Gravity is also responsible for all other structure we see in our Universe. As far as our telescopes can see, gravity has clumped mass together at every conceivable level. First, most stars are found to be part of double star systems in which both stars orbit about their common center of mass. On a much larger scale, stars are gravitationally bound together into galaxies. Our solar system is part of the Milky Way galaxy and is located about two thirds the way from the center. You can see the Milky way at night as a white band that could easily be mistaken for a whispy cloud. But a pair of ordinary binoculars will reveal the band to be densely pack stars ... about 200 billion of them!
And the Milky Way is part of a cluster of over 30 galaxies called the Local Group. The picture to the left is the Andromeda galaxy which belongs to our local cluster. Andromeda is a spiral galaxy like the Milky Way. You can't see the spiral structure of the Milky Way very well from earth without special instruments. But any Andromedans looking our way would see a picture similar to that on the left.
And finally, the clusters of galaxies are generally bound together into super clusters.
The picture to the right is of the Coma Cluster, a modest cluster which contains thousands of galaxies. Nearly every object seen in this photo is a galaxy! Click on the picture if you would like more information. Rough estimates indicate that there are about 100 billion galaxies within the visible Universe. Since it is not yet certain just how big the Universe is, this may be only a fraction of the total number. The numbers are absolutely astronomical.
And the most bizarre structure ...
There is another structure that gravity "might" create that is perhaps the most bizarre and perplexing object in the Universe. It is the black hole. A black hole is an object that is so massive that nothing, not even light, can escape once it has come too close. The idea has been around since shortly after Newton published his Universal Law of Gravitation. Both John Mitchell and Pierre Laplace discussed the idea in the late 1700's. But they based their ideas on Newton's theory and simply imagined an "ordinary" object so massive that even particles of light couldn't get away. It simply did not occur to them that if an object was massive enough, something truly catastrophic might happen.
But the picture today is quite different. Einstein's theory of gravitation attributes gravity to a bending of the very fabric of space. Although his theory is far more complicated than Newton's and not as easy to understand, it is considered to be correct by essentially everyone in the scientific community. It makes many predictions (such as perturbations in Mercury's orbit and the bending of light by the sun) that have been proven to be absolutely correct. Newton's theory, good as it is, fails at these predictions.
So what does Einstein's theory say about black holes? When an object becomes too massive, it warps the fabric of space to the breaking point. The object collapses in on itself and becomes a singularity ... a point in space of infinite density and no volume! Far away, the gravity of the black hole seems the same as for any ordinary object. But the black hole is a point, which means you can get as close as you want! Every black hole has a certain distance, called the Schwartzchild radius or event horizon, that is the point of no return. Any object which passes beyond that point will fall inescapedly into the singularity. This radius is proportional the mass of the black hole. A black hole with the mass of our sun would have a Schwartzchild radius of about 2 miles. There is nothing special that you would notice as you passed through the event horizon, except that you would die soon afterwards. If you'd like more info, check out this very extensive black hole FAQ page.
Only recently have astronomers been able to produce convincing evidence of a black hole. Cygnus X-1 is an extragalactic object found in the early seventies that emits x-rays. It has been long suspected that it was a black hole, but recent data from the Hubble Space Telescope has shown the x-ray spectrum to be precisely that emitted by a black hole as predicted by astrophysicists. There have been numerous other black hole objects similarly identified. Most scientists in the field believe the controversy over the the existence of black holes to be over.
Be sure to enter your full name. Enter your email address ONLY if you want
the number of points awarded e-mailed to you. Responses should be brief
but complete. You must click the SUBMIT button to submit your entry.
Entries are due by noon on Monday of next week.
Questions:
Who was the astronomer whose data showed that the Universe was expanding?
Were the heavier elements, such at those found here on earth, created in stars like our sun? If not, what kind of stars?
Where does the Milky Way rank in size within our local cluster?
Could our sun collapse into a black hole? Why or why not?
How big would the radius be if the earth became a black hole?
Without gravity, the Universe today would at most be a rarefied gaseous cloud of hydrogen and helium atoms floating about. But gravity provided a way to create much more. The atoms in the slightly denser regions of space started falling together under the influence of gravity. These atoms gained speed as they fell towards the region's center of mass and began to heat up from the collisions with one another. If the mass was sufficient (about two tenths the mass of our sun or more), the temperature would increased enough to start a sustained fusion reaction. A star was born! (The fusion process fuses lighter nucleii such as hydrogen and helium into larger nucleii such as Li, Be, etc. and releases tremendous amounts of energy.) Stars are still being created today by the same process. The Hubble Space Telescopes photo of M16- the Eagle Nebula, shown to the right, is a prime star factory. There are many stars being born within the massive gaseous clouds seen. If you would like to see a larger version of this beautiful and amazingly detailed photo, just click on the image. 
Gravity is also responsible for all other structure we see in our Universe. As far as our telescopes can see, gravity has clumped mass together at every conceivable level. First, most stars are found to be part of double star systems in which both stars orbit about their common center of mass. On a much larger scale, stars are gravitationally bound together into galaxies. Our solar system is part of the 
But the picture today is quite different. Einstein's theory of gravitation attributes gravity to a bending of the very fabric of space. Although his theory is far more complicated than Newton's and not as easy to understand, it is considered to be correct by essentially everyone in the scientific community. It makes many predictions (such as perturbations in Mercury's orbit and the bending of light by the sun) that have been proven to be absolutely correct. Newton's theory, good as it is, fails at these predictions.