Editor's Note: This page provides a brief overview of our Universe. For a comprehensive look at the Universe, visit NASA's Astrophysics Division.
When we leave the solar system, we find our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of stars, so big that even at the speed of light, it would take 100,000 years to travel across it. All the stars in the night sky, including our sun, are just some of the residents of this galaxy, along with millions of other stars too faint to be seen.
The further away a star is, the fainter it looks. Astronomers use this as a clue to figure out the distance to stars that are very far away. But how do you know if the star really is far away, or just not very bright to begin with? This problem was solved in 1908 when Henrietta Leavitt discovered a way to tell the wattage of certain stars that changed their pulse rate linked to their wattage. This allowed their distances to be measured all the way across the Milky Way.
Beyond our own galaxy lies a vast expanse of galaxies. The deeper we see into space, the more galaxies we discover. There are billions of galaxies, the most distant of which are so far away that the light arriving from them on Earth today set out from the galaxies billions of years ago. So we see them not as they are today, but as they looked long before there was any life on Earth.
Finding the distance to these very distant galaxies is challenging, but astronomers can do so by watching for incredibly bright exploding stars called supernovae. Some types of exploding stars have a known brightness - wattage - so we can figure out how far they are by measuring how bright they appear to us, and therefore how far away it is to their home galaxy.
The galaxies and clusters of galaxies that make up the visible Universe are concentrated in a complex scaffold that surrounds a network of enormous cosmic voids. However, in addition to the "normal" matter that makes up the visible parts of the Universe, scientists have discovered that there are vast amounts of unseen matter. This so-called, dark matter makes up roughly 23 percent of the matter-energy content of the Universe, while the visible pieces account for only about 5 percent of the total.
A star is a brilliantly glowing sphere of hot gas whose energy is produced by an internal nuclear fusion process. Stars are contained in galaxies.
A galaxy contains not only stars, but clouds of gas and dust. These clouds are called nebulae, and it is in a nebula where stars are born. In the nebula is hydrogen gas which is pulled together by gravity and starts to spin faster. Over millions of years, more hydrogen gas is pulled into the spinning cloud. The collisions which occur between the hydrogen atoms starts to heat the gas in the cloud.
Once the temperature reaches 15,000,000 degrees Celsius, nuclear fusion takes place in the center, or core, of the cloud. The tremendous heat given off by the nuclear fusion process causes the gas to glow creating a protostar. This is the first step in the evolution of a star. The glowing protostar continues to accumulate mass.
The amount of mass it can accumulate is determined by the amount of matter available in the nebula. Once its mass is stabilized, the star is known as a main sequence star. The new star will continue to glow for millions or even billions of years. As it glows, hydrogen is converted into helium in the core by nuclear fusion. The core starts to become unstable and it starts to contract. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and starts to glow red. The star has now reached the red giant phase. It is red because it is cooler than the protostar phase and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths the star will take.
A solar system is made up of a star and everything that travels around it. Our solar system includes eight planets and their natural satellites such as Earth's moon; dwarf planets such as Pluto and Ceres; asteroids; comets and meteoroids. There are most likely billions of solar systems in our galaxy.
A galaxy is a cluster of stars, dust, and gas which is held together by gravity. Galaxies are scattered throughout the universe and they vary greatly in size. A galaxy may be alone or it may be in a large group of galaxies called a "supercluster". Galaxies are classified by scientists according to their shape and appearance. An irregular galaxy has an undefined shape and is full of young stars, dust, and gas. A spiral galaxy is shaped like a disk. The disk tends to resemble a pinwheel with arms which spiral outward as it rotates. Spiral galaxies tend to contain more middle-aged stars along with clouds of gas and dust. The next galaxy classification is an elliptical galaxy. The elliptical galaxies contain older stars and very little gas and dust. Elliptical galaxies vary in their shape from round to flattened, elongated spheres.
Our Galaxy: The Milky Way Galaxy
Our Sun is a star in the Milky Way Galaxy. If you were looking down on the Milky Way, it would look like a large pinwheel rotating in space. Our Galaxy is a spiral galaxy that formed approximately 14 billion years ago. Contained in the Milky Way are stars, clouds of dust and gas called nebulae, planets, and asteroids. Stars, dust, and gas fan out from the center of the Galaxy in long spiraling arms. The Milky Way is approximately 100,000 light-years in diameter. Our solar system is 26,000 light-years from the center of the Galaxy. All objects in the Galaxy revolve around the Galaxy's center. It takes 250 million years for our Sun to pull us through one revolution around the center of the Milky Way. The stars we see over our head every night are also members of the Milky Way family.
The universe is a vast expanse of space which contains all of the matter and energy in existence. The universe contains all of the galaxies, stars, and planets. The exact size of the universe is unknown. Scientists believe the universe is still expanding outward. They believe this outward expansion is the result of a violent, powerful explosion that occurred about 13.7 billion years ago. This explosion is known as the Big Bang. By looking at an object's electromagnetic spectrum, scientists can determine if an object is moving away from Earth or towards Earth. When distant objects, such as quasars, are viewed from Earth, their spectrum is shifted towards red. Whenever there is a shift in a spectrum, it is called a Doppler Shift. If the shift is toward red, the light given off by the object is in longer wavelengths. When an object moves away from Earth, the light that it is giving off is seen in longer wavelengths. When an object moves toward Earth, the light that it is giving off is seen in shorter wavelengths. This causes a shift in the object's spectrum towards violet. The amount of shift in an object's spectrum is determined by how fast the object is moving. All of the distant galaxies have tremendous red shifts. Based on these data, scientists believe the universe is still expanding outward.
Black holes are extremely compact space objects that were once massive stars which collapsed inward due to the force of their own gravity. Consequently, black holes are very dense. If it were not for the effect that black holes have on the objects around them, we would be unable to detect them. A black hole has a powerful gravitational field which traps everything that goes near it. Scientists now theorize that some galaxies have huge black holes in their centers which release tremendous amounts of energy that powers the spectacular energetic events that go on within the galaxy. The fuel for the black hole, scientists believe, may be the trapped gas, stars, and dust that are pulled into the hole. Gas that is pulled into a black hole swirls down into the hole much like a whirlpool. By using a spectroscope, the Hubble Space Telescope has the ability to clock the speed of this gas as it swirls around the entrance to the hole. The speed with which the gas swirls is considered the black hole's signature. By knowing the speed of the gas, the mass of the black hole can be calculated. A black hole in the center of the M87 galaxy in the constellation Virgo, which is 50 million light-years away, has been calculated to have a mass equal to that of 3 billion Suns! An even more effective way of studying black holes is through the use of X-ray observations. X-rays have the capacity to penetrate through gas and dust much better than optical light. With the data delivered to us by X-ray observations and the Hubble Space Telescope, scientists now believe that the presence of black holes explains a lot of the powerful cosmological events which occur in the universe.
There is no current problem of greater importance to cosmology than that of dark matter. Dark matter is composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly. We know that dark matter exists because of the effect it has on objects that we can observe directly.
Scientists study dark matter by looking at the effects it has on visible objects. Scientists believe that dark matter may account for the unexplained motions of stars within galaxies. Computers play an important role in the search for dark matter data. They allow scientists to create models which predict galaxy behavior. Satellites are also being used to gather dark matter data. In 1997, a Hubble Space Telescope image (seen on the right) revealed light from a distant galaxy cluster being bent by another cluster in the foreground of the image. Based on the way the light was bent, scientists estimated the mass of the foreground cluster to be 250 times greater than the visible matter in the cluster. Scientists believe that dark matter in the cluster accounts for the unexplained mass.
Scientists have produced many theories about what exactly dark matter may be. Some believe that it may be normal objects such as cold gasses, dark galaxies, or massive compact halo objects (called MACHOs, they would include black holes and brown dwarfs). Other scientists believe that dark matter may be composed of strange particles which were created in the very early universe. Such particles may include axions, weakly interacting massive particles (called WIMPs), or neutrinos.
Understanding dark matter is important to understanding the size, shape and future of the universe. The amount of dark matter in the universe will determine if the universe is open (continues to expand), closed (expands to a point and then collapses) or flat (expands and then stops when it reaches equilibrium). Understanding dark matter will also aid in definitively explaining the formation and evolution of galaxies and clusters. As a galaxy spins it should be torn apart. This does not happen, so something is holding the galaxy together. The something is gravity; the amount of gravity required to do this, however, is enormous and could not be generated by the visible matter in the galaxy.
In 1998, by observing distant supernova explosions, scientists came to the conclusion that the universe is expanding at a faster and faster rate. This was very surprising! Before that, it was believed that the universe would be either expanding at a constant rate or expanding at a slower and slower rate (due to gravity).
So what causes the change in the expansion rate? The answer seems to be what scientists have called dark energy. At present, dark energy is a complete mystery. In fact, that is how it got its name - dark energy refers to the fact that some kind of something must fill the vast reaches of the universe in order to be able to make space accelerate in its expansion. But what exactly this something is is completely unknown to us.
Several ideas have been pursued to explain or account for dark energy. One idea is related to the work of Albert Einstein who thought that space itself might contain energy. Another idea defines a new, fifth state of matter called quintessence (to go along with the usual four states of solid, liquid, gas, and plasma). Neither of these ideas has proven to work very well to explain what astronomers see happening in the universe. So it looks like it is up to future physicists (maybe you) to solve this important problem.