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Stars are like people. They come in all sizes and colors, and they are born, live and die. Our sun is a star and it is directly related to all of the points of light we see in the night time sky (except for planets). Stars are incredible by-products of the universe and are critical to it as there would be no light, energy or life, without them.

This Week's Astrophoto: Stellar "eggs" emerge from a molecular cloud (Star-Birth Clouds in M16) (Photo courtesy of NASA Hubble Space Telescope)
This Week's Astrophoto: Stellar "eggs" emerge from a molecular cloud (Star-Birth Clouds in M16) (Photo courtesy of NASA Hubble Space Telescope)

Stars are very plentiful – it is estimated that there are more than 200 billion of them in our Milky Way Galaxy alone. The stars we see at night are part of the Milky Way Galaxy and may be very close to Earth or far away. On a clear night from a dark sky site you can see thousands of stars as opposed to perhaps a dozen or more from the city.

To the eye, stars have two qualities – color and brightness. A star’s color gives us a valuable clue to one physical property of a star – its surface temperature. Just like the colors of a flame, stars follow the same convention of increasing temperature with color – red, orange, yellow, and blue.

Stars that are orange or red in color have surface temperatures that are cooler than those that are yellow or blue. Our sun, an average star – in terms of size and mass – has a surface temperature of 6,000 degrees. Stars are assigned to a classification according to their temperature – O,B,A,F,G,K,M – O being the hottest at 40,000 degrees and M the coolest at 3,000. The sun is a G class star.

The brightness of a star is a far trickier quality to interpret. Is a bright star one that is “big” and close to Earth? Is a dim star small and far away? The answer is you really can’t make a judgment as to a star’s size and distance just by looking at how bright it is in the night sky.

Think of two light bulbs – one a 25 watt, the other a 100 watt. If you place them at the same distance of course the 100 watt will be brighter. But if you move the 100 watt back far enough it will eventually be the same brightness as the 25 watt even though it is more luminous. Move the 25 watt close enough and it will eventually outshine the 100 watt. To overcome this dilemma astronomers have developed methods of determining a star’s true distance and luminosity.

For the stars we see at night they are classified according to their apparent magnitude – how bright they are in relationship to one another. Stars with higher numbers are dimmer – the dimmest that can be seen in a dark sky is about 6th magnitude. From a suburban setting 3rd magnitude stars are probably the limit. There are stars of 1st and 0 magnitude with the brightest star in the night sky being Sirius with a -1.4 apparent magnitude. The planet Venus can reach -4.4 magnitude and Mars will top out at -2.5 this fall.

Read more about magnitudes.

The single most important property in a star’s life is its mass or how much “fuel” does it have? Stars are composed of gas, mainly hydrogen and helium plus a gaseous mix of the other elements found in the Periodic Table of Elements.

Stars that are massive – anywhere from several to hundreds of times the mass of the sun - live quick and furious lives measured in millions of years and die interesting deaths.

Stars less massive than the sun -- which is used as the standard to determine “solar mass” – live long and boring lives lasting hundreds of billions of years and pass into the night quietly.

Stars like the sun – one solar mass -- last about 10 billion years and go out with a last gasp. Don’t worry, the sun has another five billion years or so to go.

Today we know a lot about stars, but one of the biggest mysteries to astronomers throughout the ages was what made the sun (and stars) shine. We can see it in the sky and feel its heat, but what caused these processes to happen was a big scientific question until scientists in the 1930’s came up with the solution -- nuclear fusion.

Nuclear fusion is the process by which two atoms are fused together as opposed to nuclear fission where they are broken apart. Both result in a tremendous release of energy as defined by Albert Einstein’s famous equation E=mc2 , where energy is equal to mass times the speed of light squared. Bottom line – a very small amount of mass packs a lot of energy. But to get to the point where stars can use nuclear fusion, they have to be born and mature.

Read more about nuclear fusion.

Stars are formed in cocoons of huge molecular clouds where the raw materials of gas and dust abound. A precipitating event such as an exploding supernova’s shock wave (more on this later) or a gravitational “bump” can cause these materials to start binding gravitationally into a growing accumulation. As the mass of this structure increases over millions of years, it eventually begins to collapse under its own gravity into an ever-shrinking spherical volume.

Read more about molecular clouds.

Read more about protostars

As this occurs, the temperature and pressure within the core of the structure begins to increase. Eventually, a spherical form is developed and the temperature and pressure continue to rise as the sphere continues its collapse. At a specific point, the temperature and pressure of this “protostar” reach a point where the atomic nuclei at its core are subjected to nuclear fusion – they slam into each other to form a new atomic nucleus and release energy in the process – and a new star is born.

Stellar eggs emerge from a molecular cloud This Week's Astrophoto: Stellar "eggs" emerge from a molecular cloud (Star-Birth Clouds in M16) (Photo courtesy of NASA Hubble Space Telescope) Because hydrogen is the most abundant (and simplest) element in the universe, stars use mostly hydrogen as their nuclear core fuel. When two hydrogen nuclei fuse, they form a single helium nucleus and release energy. Take another hydrogen nucleus and fuse it to this newly created helium nucleus and you now have a lithium nucleus.

Take two helium nuclei and fuse them and you have a Beryllium nucleus. Fuse two Beryllium nuclei together and you have an oxygen nucleus – two oxygen nuclei fused together make a sulphur nucleus.

Get the idea? Stars, by “burning” their hydrogen fuel make the other elements of the universe as a byproduct. This process is called nucleosynthesis.

Read more about nucleosynthesis.

The very first stars, formed some time after the Big Bang, only had hydrogen, helium and perhaps some lithium to fuel their nuclear cores. Over the course of billions of years this first stellar generation manufactured the elements of the Universe and as we shall see in part two next week, seeded the ocean of space with the natural resources necessary to make planets and us.

Do you have a topic that interests you? I literally have a whole universe of topics to select from for my column. But I'm interested in hearing from WTOP readers about what interests them. Feel free to contact me at with your suggestions and comments.

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