The purpose of this page is to provide some basic background for understanding some of the objects in our universe; and that’s a good place to start. Just what is the universe?
[Universe] I am sure that a scientific definition for “universe” exists, but I tend to think of it as a container. The universe is everything we see when we look out into space, especially with a telescope like the Hubble. The universe contains all the collections of stars called galaxies, and that includes our own galaxy, the Milky Way. It contains all the matter and energy, no matter what form they may be in. Believe it or not, but not everything contained in the universe has been discovered or fully defined yet. It is also possible that our universe is part of a multiverse. However, that topic is beyond the scope of this page. We are going to confine our discussions to the visible universe and the things (objects) that we can see with our eyes. So think of our universe as the big bag that holds everything else.
[Galaxy] Next, let’s talk about galaxies. As stated above, it can be helpful to think of a galaxy as a collection of stars and a few other objects described later. Galaxies are big! Distances in the universe are most often described in terms of light-years. A light-year is the distance that light travels in a year. Light travels at a constant speed of 299,792,458 meters per second – this is usually rounded up to 300 million meters per second. In English units, about 186,000 miles per second. In round figures, a light year is nearly 10 trillion kilometers or nearly 6 trillion miles. Don’t worry, almost everyone struggles when trying to grasp numbers that are in millions, billions, and trillions or more. So, getting back to galaxies – Our Milky Way is about 100,000 light years in diameter. Other galaxies can measure over a million light years across! To put the size of the Milky Way galaxy into perspective, if someone on the other side of our galaxy turned on a light right now, it would take 100,000 years before someone on our side would see it. In essence, when we look out into space, we are looking back in time. Even light from our own star, the Sun, takes 8 minutes to reach the surface of the Earth. So, how many stars are in a galaxy? Ours contains about 100 billion stars. The number of stars in galaxies varies with the size of the galaxy. It is safe to say that galaxies in general contain a few hundred billion stars! So, how many galaxies are there in the visible universe? The best guess from astronomers is at least 100 billion galaxies!
[Stars] What’s a star? All visible stars are balls of gas, typically hydrogen and helium, with other gases and elements. However, and this is a big “however,” a star shines because it is a nuclear fusion reactor. In other words, atoms of hydrogen are being smashed together with such force that they fuse into bigger atoms of helium. The fusion causes other parts of the atom to escape; the most noticeable are photons of light. It is important to know that stars have a life. They are “born” and they “die.” While the processes for creating stars may vary across the universe, perhaps the most common model is that of stellar material condensing into a star. Since stars are made of atoms and atoms have mass, they have a gravity. Atoms attract one another with this gravity, and they start combining. So a star may start from an area of gases and other stellar material that is combining. As material become more dense, it creates more gravity, the more gravity it has, the more it attracts material. This process continues over time until the density and gravity become great enough (critical mass) to force a nuclear reaction. The nuclear reaction causes an outward pressure, but the gravity holds the pressure in check. In other words, a balance is maintained. How long the balance is maintained is a function of how much fuel (hydrogen) is available. In essence, a star dies when in runs out of hydrogen to convert into helium. What the star becomes at the end of its life is dependent on that same factor. Our Sun will start swelling when it nears the end of its fuel. It will expand outward to about the orbit of Jupiter. This is the red giant phase. Then, it will start collapsing until it becomes a white dwarf star. It is glowing white from the heat of the collapse. If a star has enough mass to start with, it may continue the collapse until bits and pieces of atoms are compressed into a very tiny space, at which point the gravity of the compressed material is so great that even light cannot escape. These are called black holes.
[Constellations] Constellations are visible patterns of stars in our galaxy. Ancients made up names for the various star patterns they could see in the night sky. Currently, there are 88 constellations. Perhaps the best known and most easily recognized are Orion, the Big Dipper, and Cassiopeia. Constellations have a great deal of mythology surrounding them, because they most often represent mythical characters. It is important to note that the grouping we see from Earth is an apparent grouping or pattern. If we could see the constellations three-dimensionally, we would see that are not on the same plane; they only appear that way from Earth. One of the most handiest attributes of a constellation is using it as a sign post to various objects. Most objects are referenced by what constellation they appear to be in or near. Perhaps the most noted is the Big Dipper. The two end stars of the dipper part point to the North star, which is last star in the handle of the Little Dipper. The North or Pole star is one of the most important stars to be able to locate in the night sky if your are going to use an equatorial mounted telescope. The North star appears to be located almost directly over the Earth’s North Pole. Unlike the rest of the visible stars that appear to “move” through the night sky, the North Star stays in the same apparent location. More about this topic in using the North Star for orienting telescopes to the night sky.
[Star Clusters] These are groupings of stars. Two major types are open clusters and globular clusters. The globular clusters are tightly bound (gravitationally) stars in a spherical shape. Not only are they spectacular to view through a telescope, but they are some of the more mysterious objects in the Universe. Scientists do not know how or why they are formed, or even what keeps them together. They tend to be grouped near other galaxies. Our own Milky Way has some, and at least two are visible near the Andromeda galaxy. Perhaps one of the most beautiful is M13 in the constellation of Hercules.
[Nebulae] Some of the most beautiful objects to observe are nebulae (singular: nebula). Simply put, nebulae are gaseous clouds. They are areas where various gases, dust, and other particles have or are collecting. Some nebulae are star nurseries where new stars are being formed. Other nebulae are what is left after a star has gone super nova (exploded). Perhaps two of the best known nebulae are the one in Orion, and the other is the nebulosity in the constellation of the Pleiades. Both of these are visible to the naked eye. Since nebulae are relatively faint objects, the human eye is not going to be able to pick up the colors very well, however, long term photo exposures will bring out the colors. Do a search on the Internet, and you will find many images of nebulae.
[Solar System] A solar system is a group of objects, like planets, that are in orbit around a star. Our Sun is such a star and the planets, like Earth, Mars, Venus, and Jupiter are in orbit around the Sun. There are eight major planets and many minor planets.
[Planets] With the advent of space telescopes and newer technology in Earth-based telescopes, we know that a great many stars in our galaxy and in other galaxies have planets orbiting around them. For our purposes here, we will discuss the planets in our own solar system. The Sun is at the center of our solar system, and the first planet out from the Sun is Mercury. The next planet out is Venus. Then comes Earth. The next one is Mars. These planets all form what is known as the inner planets or rocky planets. Then comes a curious formation of bit and pieces forming the Asteroid Belt. The current thinking is the Asteroid Belt is bits and pieces of a planet that did not form. Next to come is Jupiter, the biggest planet in our solar system. Jupiter is a gas giant, as are the remaining major planets. It means that these planets do not have a solid surface; it is a gas ball. Next out from Jupiter is Saturn. Saturn is best known for its spectacular ring system. Saturn is also the last planet that is visible to the naked eye. Out from Saturn is Uranus followed by Neptune. Out from Neptune come the minor planets, Pluto being the best known. For years, Pluto was considered as one of the major planets, but as scientists refined their classification system to take into account all of the objects in orbit around the Sun, Pluto no longer met the parameters for being a major planet.
Perhaps the best way to wrap up this topic is to mention that for centuries men have been looking at the night sky, and over time, have noticed a great many objects. Several catalogs have been created that contain listings for all these objects. Only the best known objects have names. Most of the objects are referred to by some catalog number or reference. For example, M13, mentioned earlier, is a catalog number from the Messier list, hence the designation M in the number. This is where having a good star atlas is important. It will give the catalog number and location of the various objects. In the case of the better known objects, their locations will also be shown on the star charts in the atlas. Please consult the reference section of this site for recommendations on atlases.