Thursday, January 21, 2010

Astro101: Solar Systems, Star Clusters, and Galaxies

Today I'm going to start an occasional series that I'll call Astro101.  In this series, I intend to try and talk about some basic astronomy concepts that come up a lot in astronomy news but that many people don't really understand.  My goal here is to give short and clear explanations, not exhaustive discussions covering every possible angle on a topic. 

All of these posts will be labeled with "Astro101", so if you come back and search the blog on that term, these posts should come up, and you can read them as often as you wish. 

Today's topic involves some definitions for three types of objects that are often confused: a solar system, a star cluster, and a galaxy.  I've even known some very intelligent and knowledgeable amateur astronomers to get these mixed up, so there's no shame in not knowing (at least until you've finished reading this post).

Solar systems: A solar system is a star and any planets, comets, asteroids, or other small objects that orbit the star.  Our own Solar System consists of the Sun, the eight planets, Pluto, the asteroids, all the comets that we know, Kuiper Belt objects (like Sedna, Eris, and Makemake -- small icy things further from the sun than Neptune), the Oort Cloud, and everything else that is orbiting the sun.  Whenever I refer to our Solar System, I use capital letters in "Solar System".  Other solar systems (with lower case  "s") would be similar, with individual stars, or sometimes groups of two or three or even a few more stars (as long as they all orbit each other), and any planets orbiting those stars.  Our Solar System has only one star -- the sun. The total size of our Solar System is less than one light year; the Sun, all of the planets and Pluto are within one thousandth of a light year of each other.  The next-closest solar system to our own, the alpha Centauri system, is four light years away.

Star clusters: Star clusters are groups of stars, from a few dozen to a few million, that are loosely held together by gravity.  All of the stars in a star cluster were born at the same time out of the same cloud of gas.  Each of the member stars of a star cluster is its own solar system, so a star cluster can contain hundreds or even millions of solar systems.  A famous star cluster you can see without a telescope is the Pleiades, or the Seven Sisters, currently visible in the evening sky. Our Solar System is not part of a star cluster.  Star clusters can be a few light years across.

Galaxies: Galaxies are huge collections of solar systems and star clusters, often containing billions of stars.  Galaxies are held together by gravity.  Our Solar System and every single star you see in the sky are part of the much larger Milky Way galaxy.  The Milky Way is huge, about 100,000 light years across.

An analogy: If you have trouble remembering the difference between these three types of objects, you can think of them in human terms.  A solar system is like a single family, including the people, their pets, and their belongings.  A star cluster is like an apartment building: lots of families living in one big building, each family being its own solar system.  Just like not all families live in apartment buildings, not all solar systems are in star clusters.  A galaxy is like a city -- a large collection of families and apartment buildings.

Some pictures and artwork to help clarify:

  1. Solar systems:

  2. Star clusters:

  3. Galaxies:

    • The Milky Way, as seen from the Earth (remember, we are part of the Milky Way)
    • The Milky Way, an artists's concept of the view from outside the Milky Way
    • The Pinwheel Galaxy, a neighbor galaxy to the Milky Way.  If you click on the picture, you can see the brightest individual stars and star clusters that make up the bigger galaxy

1 comment:

  1. in need of help :02:46 PM

    can you get more facts on in less than 2 days i have a huge project due plaese help