How do astronomers know so much about distant stars, galaxies, and nebulae? By studying light. To determine what stars are made of, astronomers use telescopes that break starlight into its component colors on the electromagnetic spectrum. These colors correspond to specific chemical elements and, as Academy astronomer Bing Quock points out, are as distinctive as fingerprints.

To determine the age of stars, scientists again examine their color, but not to determine their chemical makeup but, rather, to determine their temperature. Blue-colored stars indicate hotter temperatures, suggesting that they are relatively young. Yellow, orange, and red-colored stars indicate cooler temperatures, suggesting that they are relatively older.

Scientists use the speed of light to calculate the mind-boggling distances between planets, solar systems, stars, and galaxies.  If light travels at a speed of 186,282 miles per second then a light year equals approximately six trillion miles. It’s estimated that it takes 100,000 light years to travel from one end of our own galaxy, the Milky Way, to the other!

In the last two decades, increasingly powerful observational tools have led astronomers to discover thousands of large orbiting bodies in the region of space beyond Neptune called the Kuiper Belt. One body—discovered in 2003 and named Eris in 2006—is larger than Pluto. Eris’ discovery led scientists to debate the definition of a planet and whether a tenth planet should be added to the nine previously listed in our Solar System.

In response, the field’s governing body—the International Astronomical Union (IAU)—established new, selective criteria to define planets. According to these new criteria, neither Pluto nor Eris qualifies as a planet. After all, neither planet maintains a stable, elliptical orbit around the Sun, and neither planet is of sufficient size and mass to sweep away comets, meteors, and other space debris from its orbiting path. So what do we call them now? According to the IAU, both Pluto and Eris should now be referred to as dwarf planets.