Stars are the most numerous objects visible in the sky. On a clear dark night, you’ll see about 3,000 stars with your unaided eye. Binoculars or a telescope reveal tens of thousands more. Every star you see belongs to our own galaxy, the Milky Way, which holds some 200 billion stars altogether.
Take a look at the stars on the next clear night. Aside from differences in brightness from star to star, you will also see differences in color. Some stars, like Rigel in the constellation Orion, are blue. Others, like Altair in Aquila, are white. Arcturus, a bright star in the northern spring sky, is yellow-orange. Yet others, like Betelguese in Orion or Antares in Scorpius are a deeper orange-red.
These color differences are real. Just as hot coals in a fire will glow red, and hotter coals will glow white, the color of a star depends on its temperature. Red stars are coolest. They have a temperature of 4,000-5,000 Kelvin at their surface. Yellow-white stars like our Sun are hotter… about 6,000-10,000 K. And blue stars are the hottest of all, with surface temperatures of 15,000 to 30,000 K.
Blue stars are massive… some 5x to 20x the mass of our Sun or more. Massive stars burn hottest and emit most of their light in the blue and ultraviolet part of the spectrum. White and yellow stars are young-to-middle-aged middleweights. Most of the red stars you can see in the night sky were once massive blue or white stars that now near the end of their lives. Most of the stars of the galaxy are dim red stars, much less massive than our Sun, and too faint to see without a large telescope.
Stars have “life cycles”. They are born out of cold dust and gas in the Milky Way, then begin shining when they become hot enough in their cores to start turning hydrogen into helium, a process called nuclear fusion in which a huge amount of energy is released. The biggest stars burn for a few tens of millions of years before running out of fuel, and mid-sized stars like our Sun burn for some 10 billion years. While the details are complex, each star changes drastically as it runs out of fuel, then eventually goes dark. Some of its material is returned to the galaxy where it eventually gets “recycled” into new stars.
Many stars—astronomers suspect most stars—are born together and gravitationally connected to one or more companion stars. Some stars with widely-spaced companions, especially if they are nearby, can be resolved directly with a small telescope. Many of these “double star” and “triple star” systems are quite lovely in a small telescope, particularly when there is a vivid color contrast between the stars. Other stars, especially as they get older, go through a period where they change brightness over days, months, or years. These are the so-called variable stars, many of which can be seen in a small telescope or binoculars.
Some stars look bright because, well, they ARE bright. Others appear bright because they are close to us. Deneb, the brightest star in Cygnus, is one of the brightest stars known… about 60,000x brighter than our Sun. Deneb is 1,500 light years away (one light year is the distance light travels in one year, about 9.5 trillion kilometers). The star Sirius, on the other hand, appears even brighter than Cygnus (in fact Sirius is the brightest star in the sky as seen from Earth). But it’s only 25x brighter than our Sun and only 0.0004x as bright as Deneb. Sirius only appears brighter than Deneb because it’s 175 times closer… just 8.6 light years away.
Stars are organized into constellations, groupings of stars first conceived by ancient astronomers. Some constellations are large and obvious, such as Orion and Pegasus and Scorpius. Some are dim and small such as Triangulum, Lynx, and Lepus. Modern astronomers now recognize 88 constellations in total, and every object in the night sky lies within a constellation.
An asterism is a recognizable group of stars within a constellation, or which may comprise stars of more than one constellation. The Big Dipper is an example. It’s made of a subset of stars of the constellation Ursa Major. The “Great Square” of Pegasus is also an asterism. So is the “Summer Triangle” (or the Northern Triangle), which is made from the bright stars Vega, Altair, and Deneb in the constellations Lyra, Aquila, and Cygnus. There are many more asterisms.