The Universe  

Our Solar System occupies a very small volume within our home galaxy, the Milky Way. Very few stars visible to the naked eye are more distant than about two thousand light years. Think of a galaxy as a vast city of stars - with a population of hundreds of billions of stars for a typical spiral galaxy. When you step

The Milky Way, as most spiral galaxies, is shaped somewhat like a pizza with a large meatball in the center. The plane of the pizza represents the plane of our galaxy where most of the stars reside, and the meatball is like the central bulge, or galactic nucleus. The nucleus contains mostly older stars while the spiral arms that lie along the disk contain younger stars.

To continue with the analogy of the pizza, consider the Milky Way to be a pizza six feet in diameter. On this scale, the Solar System would be about two feet out from the center but the Sun and the Earth would be utterly invisible. What we see in our night sky, on the same scale, would be contained within a volume equal to an ordinary meatball! 

We can see a greater distance with telescopes, and the diameter of our galaxy has been measured at about 100,000 light years. This means that a pulse of light would take 100,000 years to traverse just our galaxy alone!

Things We See On The Way Out

There's a lot more to a galaxy than just the stars within it - just as there's more to pepperoni pizza than pepperoni. A galaxy is like a pond when you consider all the various contents and processes going on just under the surface. Galaxies were thought to have formed very early in the history of the universe, but since the stars that populate the galaxy go through life cycles of various lengths and result in the formation of many different structures; there's plenty of variety. Star Colors and Temperatures

When you go outside at night and notice the subtle color differences among the stars, what that indicates about the star itself is its temperature. Stars that appear more red are cooler than the hotter, bluer stars. What controls how hot a star will be is how much material it has. The hotter stars are also the more massive ones. A star must have a certain minimum mass to even become a star in the first place, and in fact most stars are small, dim red dwarfs. The hot blue stars are the minority, and these stars also live very short lives compared to their lighter cousins.

Passing through the space between the stars, we notice that there is some material out here, primarily dust. Space is sometimes considered a vacuum, but there is actually quite a zoo of different atoms and molecules out here. We even find some of the basic building blocks of life - carbon compounds and water. The hottest stars are about 80,000-degrees F, while to coolest are little more than 6,000-degrees F. The Sun, a yellow star, is in between at 10,000-degrees

On our journey across the solar system,  we pass the orange star Nash at about 120 light years and then Eta Sagittarii, distinctly reddish, at a little over 200 light years. These are all stars you can see from the Earth, but as we look back over our shoulder we notice that the Sun is definitely now one of thousands of stars fading into the Suddenly the scene changes; whereas earlier we saw thousands of stars ahead, we now find ourselves in a fog. This is not a fog of water vapor like we find on Earth, but rather hydrogen gas and lots of dust. We're in a nebula.

Star Formation

Stars form out of nebulae (plural for nebula), condensing slowly over millions of years by the force of their own gravity. Anything that has mass or substance has gravity - you have gravity, a proton has gravity, and so do stars and planets. This nebular collapse continues, heating like air in a bicycle pump, until the temperature and pressure in the center are high enough to begin nuclear fusion. The process in its simplest form produces Helium from Hydrogen and releases enormous amounts of energy. It is this energy (heat and light) that you feel from our star, the Sun, on a sunny day.

 Occasionally  we are bound to notice  a very large and bright red star, hundreds of times larger than the Sun and sometimes surrounded by a bubble of gas rushing out into space. These are the so-called red giants and supergiants, stars near then end of their lives after exhausting the hydrogen in their core. 

Stellar Evolution

We  know that the colors and temperatures of stars are controlled by the amount of material they possess...so Why then do  we see  red stars that are incredibly large and bright?

Stable stars are balanced by two forces working in opposite directions: gravity pulling the star inward and radiation pressure pushing outward. As long as these forces are balanced, the star shines steadily. All stars consume Hydrogen in their core, which builds up a sort of "Helium Ash". This Hydrogen fuel source is exhausted faster in the stars that have to burn it faster to support their own weight...large blue stars particularly.

When the Hydrogen fuel in a star is exhausted and the core begins to collapse, temperatures rise high enough to fuse Helium. The star responds by bloating out and becoming a red giant or supergiant. The outer atmosphere of the star can then be lost to form a gaseous shell surrounding the exposed core. This stage is very brief - the ultimate fate of most stars is a naked stellar core whose properties are determined by the initial mass of the star.

Some of the more beautiful inhabitants of the galaxy are star clusters - groups of stars having anywhere from a few dozen to a few hundred thousand stars. The so called open clusters are the result of recent star formation in the disk of the galaxy; most stars form in groups, or clusters, and remain together for a period of time before dispersing off in different directions. Globular clusters are very ancient groupings that reside primarily in a spherical halo centered on the galactic center but extending far above and below the plane of the galaxy. They do not disperse over time due to the large number of stars they contain - sometimes one million - and their mutual gravitational force holds them together for billions of years.

We are now almost at the galactic center, having traveled over 30 thousand light years from the Earth. From this distance, we cannot even detect the exact region of the galaxy from which we came - much less the Sun or the Earth.

The Electromagnetic Spectrum

What we call visible light is but one part of a much larger spectrum of light that we don't see. When you see a rainbow, you are seeing the entire range of what is visible to the human eye.

Although you can't see other types of light, you can experience them. Walk outside on a sunny day and you can feel the heat from the Sun which is scientifically t called infrared   and which we feel as heat. Stand in the Sun too long and you'll get a sunburn due to another type of light from the sun, ultraviolet.

Other parts of the EM spectrum are used by us every day. If you watch television while eating microwave popcorn, then you are using three forms of non-visible light: radio light waves on which the TV signal is carried, infrared light from your remote control to change the channel, and microwave light to cook your popcorn.

Two additional types of EM radiation are X-rays (which doctors use for looking inside the human body) and gamma rays which are emitted by atomic nuclei. Human eyes evolved to see in visible light since our atmosphere is most transparent to that type of electromagnetic radiation.

Astronomers, with the help of the Hubble Space Telescope, have discovered what appear to be black holes at the centers of many galaxies. A black hole is what remains of the most massive stars after they exhaust their nuclear fuel. A black hole's gravity is so strong that not even light can escape; this means that astronomers who study black holes must rely on less direct evidence of their existence - mainly the motion they cause in neighboring objects.

To avoid being pulled into a supermassive black hole we stop short of the galactic center and choose a new course out of the plane of the Milky Way. As we soar high above its disk and look down the spiral structure becomes evident. Our galaxy contains about 100 billion stars, and we now see our stellar city as a single unit as we speed off into intergalactic space.

After a minor course correction we head off in the direction of the Andromeda galaxy. This galaxy, also called M31, is the nearest spiral galaxy to the Milky Way and along with about 20 other galaxies forms what we call the Local Group.

Andromeda is a spiral galaxy like the Milky Way, only a little larger and containing more stars. Astronomers' studies of M31 have revealed  several  facts about the nature of galaxies, processes that go on inside them,  and the distances to the galaxies. In the 1920's astronomers observed individual stars inside M31 which enabled them to pinpoint its distance at over 2 million light years. Before large telescopes were built it was difficult to tell at what distance the galaxies were - or whether they were merely gaseous blobs in our Milky Way galaxy.

Cosmos...