 More than half of all stars are members of star systems ( groups of two or more stars). Our sun is not.  Star systems with two stars are called double stars or binary stars.  Star systems with three stars are called triple stars.

 Often astronomers can detect the presence of a star in a binary system without seeing it, they can tell it is there by observing the effect of its gravity on the second star  Sometimes with binary stars, one star blocks the light from the other star and the system is called an eclipsing binary.

 Scientists have discovered planets around stars by observing how a star “wobbles” very slightly back and forth  Over 300 “extrasolar” planets have been found according to Space.com  Most of the extrasolar planets found so far are massive gas giants with large influence on their star’s gravity.

“First ever photo of an extrasolar planet, a Jupiter-sized gas giant.”

 The so-called "habitable zone" around a star is a belt in which liquid water could exist on the surface in lakes, rivers or oceans. Too close to its stellar parent and a planet would be too hot, while an orbit too far out would yield only a frozen world, NASA scientists have said. Quote from

 Galaxies are giant structures that contain hundreds of billions of stars, Oh, by the way…There are billions of galaxies in the universe  Galaxies contain single stars, double stars, star systems and lots of gas and dust between the stars.  Astronomers classify most galaxies into three main categories: › spiral galaxies, › elliptical galaxies, › irregular galaxies

 Spiral galaxies have arms that spiral outward, like pinwheels tutorial/example_face_on_spiral.jpg HubbleBeauty/NGC1512BarredSpiralGalaxy.jpg

 Our solar system exists in the Milky Way galaxy, and is about 25,000 light-years away from the center of the Milky Way  Our solar system is about two-thirds of the way out on one of the spiral arms of the Milky Way  We can’t see the center of the Milky Way due to the massive cloud of gas and dust between the sun and the center

 Elliptical galaxies look like flattened balls  Have little gas and dust between the stars so new stars can not form › Ellliptical galaxies only contain old stars jpegMod/PIA08696_modest.jpg

 Some galaxies don’t have a regular shape, they are called irregular galaxies  The Large Magellanic Cloud is an irregular galaxy main_image_feature_666_ys_4.jpg