Star date: 12:06:98
Open clusters are collections of perhaps a few hundred to a few thousand stars orbiting large galaxies such as the Milky Way. Since the galaxies have such a tremendous distance between them, only the closest of these open clusters are visible to amateur astronomers.
These are, of course, the open clusters which surrond the Milky Way galaxy. They orbit around the center of our galaxy in many different directions. Although each open cluster is confined to a certain orbit due to orbital mechanics, the orbits of each of these clusters bear no relationship to each other. An edge on picture of a galaxy bearing open clusters taken over the course of billions of years would produce an image of a spherical cloud around the center of the galaxy. These open clusters also appear to be made from the original building blocks of the galaxy, and at the same time as the galaxy itself. These structures have little mechanisim for new star formation, and so the stars inside of them must be as old as the galaxy itself. Therefore, if we can measure the age of the stars, we can measure the age of the galaxy. It turns out there is a way of measuring the age of stars.
Perhaps the most important chart in all of astronomy is called the Hertzsprung/Russell diagram. This is known as the H/R diagram for short. It measures the luminosity of a star versus it's spectral class. When you plot many stars on such a diagram, you will find that most stars fall along a single straight line running diagonally across the chart. This is known as the main sequence. It is only when a star is dying that it "travels" off the main sequence. Since physics tells us that certain spectral classes of stars live a certain amount of time, if we can measure how far it is off the main sequence, then we know how much "life" is left in the star. Thus, we can figure the age of the galaxy.
It turns out that the open clusters orbiting the Milky Way are about seven billion years old. Thus, that is also the age of our galaxy. Interestingly enough, there is a possibilty of some star formation within open clusters. If there are enough supermassive stars in a small enough of a cluster, then the supernovas produced by these short lived supergiant stars can trigger new star formation. If each of these supernovas, on average, causes the formation of at least one more short lived supermassive stars, a chain reaction can occur, and new star formation will continue.
This is very similar to what happens in the chain reaction of a fission bomb. Most open clusters, however, do not fit these criteria. There are numerous bright open clusters in the eastern sky this week. They are each about the same brightness, and seem to form a curve you can easily trace through the sky.
Start by going outside about 10 pm, and looking ten degrees north of east, and 76 1/2 degrees above the horizon. This is very high above the horizon. Remember, straight above your head (the zenith) is 90 degrees above the horizon. This object is NGC1582. It is over 1/2 of a degree across, and magnitude 7. 1/2 degree north and 3 1/2 degrees closer to the horizon brings us to NGC1664. This object is about half the size of NGC 1582, and not quite as bright. About half the size of NGC 1664, NGC1778 can be found three degrees north and five degrees closer to the horizon from NGC1664. Next travel to NGC1857. This can be found nearly due east, and 2/3 of the way above the horizon. It is less than 3 1/2 degrees from our last stop. Our next hop will take us down three degrees closer to the horizon, and 2 1/2 degrees further north. This is a pair of open clusters, NGC1912, and NGC1907. Three more of these clusters can be found in a six degree drop to the horizon. These are M36, and M37. M37 appears nearly as wide across as a full moon.
Clear skies, and good viewing.
WHAT WE'RE READING Enter keywords... Computer Games Jim Maynard is the head of the astronomy department at Earth Treasures. In addition, he has been an amateur astronomer for nearly 25 years, and is a physics major at Keene State college. His articles may also be accessed on the world wide web at http://www.geocities.com/Athens/Styx/2842/index.html
Enter keywords... Computer Games
Jim Maynard is the head of the astronomy department at Earth Treasures. In addition, he has been an amateur astronomer for nearly 25 years, and is a physics major at Keene State college. His articles may also be accessed on the world wide web at http://www.geocities.com/Athens/Styx/2842/index.html
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