The Milky Way Galaxy
Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A galaxy is a large group of stars, gas, and dust bound together by gravity. They come in a variety of shapes and sizes.
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- It is very difficult to count the number of stars in the Milky Way from our position inside the galaxy.
- Our best estimates tell us that the Milky Way is made up of approximately 100 billion stars. These stars form a large disk whose diameter is about 100,000 light years.
- Our Solar System is about 25,000 light years away from the center of our galaxy – we live in the suburbs of our galaxy.
- Just as the Earth goes around the Sun, the Sun goes around the center of the Milky Way. It takes 250 million years for our Sun and the solar system to go all the way around the center of the Milky Way.
GLOBULAR CLUSTER
The largest and most massive star clusters are the globular clusters, so called because of their roughly spherical appearance. The Galaxy contains more than 150 globular clusters
Globular clusters are extremely luminous objects. Their mean luminosity is the equivalent of approximately 25,000 Suns. Most globular clusters are highly concentrated at their centres.
OPEN CLUSTERS
Clusters smaller and less massive than the globular clusters are found in the plane of the Galaxy intermixed with the majority of the system’s stars
The peak absolute luminosity appears to be about 50,000 times the luminosity of the Sun, but the largest percentage of known open clusters has a brightness equivalent to 500 solar luminosities.
all the stars in a cluster have very nearly the same age and chemical composition, the differences between the member stars are entirely the result of their different masses.
NEAREST GALAXY
The Andromeda Galaxy is a larger galaxy that can be seen from the northern hemisphere (with good eyesight and a very dark sky).
It is about 2.5 million light years away from us, but its getting closer, and researchers predict that in about 4 billion years it will collide with the Milky Way.
It takes light 2.5 million years to reach us from one of our "nearby" galaxies.
The other galaxies are even further away from us and can only be seen through telescopes.
