Humanity has long known that there are many stars in the night sky, but it was not until the invention of the telescope in the 17th century that humans were able to see that the milky white swath across the night sky was, in fact, many thousands upon thousands of faint stars. As telescopes improved, more new discoveries were made within this band on the sky, and above and below it as well. William Herschel identified different types of ‘nebulae’ in the 1780s, which was the name he gave to fuzzy objects in the sky that didn’t look like a star. But before we talk about those things that we can see in the night sky that are not obviously stars, we should first answer the question: What is a star?

Milky Way Galaxy: The Milky Way Galaxy can be seen rising above the horizon in this image.
Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

On Earth, we often see that hot objects glow, such as the wick of a candle, a log on a fire, or gas in a lamp. In the same way the heated gas in gas lights shine, stars shine because the gas that makes them has been heated. The hotter the star, the more light we see from the star. The heat that makes a star glow comes from the nuclear reactions deep within the star. The hottest stars shine with a blue-white light, while cooler stars have a rich red glow. Don’t be fooled, though. These “cooler” stars are still hotter than molten steel! The Sun, which is the closest star to Earth, has a yellow color, which means it is a medium temperature for stars.

It was realized in the second half of the 19th century that spectra of some nebulae were similar in many ways to the spectra of stars. They showed a range of colors just as stars do: bluer stars are hotter than redder stars, and bluer galaxies include more hot stars than redder galaxies. Some nebulae had very strong emission, but some of them had absorption lines just as stars do; I will tell you more about that later. It was thus eventually realized that many of the nebulae consist of billions of unresolved stars. With this realization, it became clear that the primary direct source of optical light in the Universe is stars. Hence, most of what we see in the night sky is optical light that originates from stars.