The Night Sky

No introduction to astronomy would be complete without some basic introduction to the night sky—the hemisphere of stars and other objects (e.g. planets) that we Earthlings observe. As astronomers, the night sky is our observational arena. It may be the most consistent aspect of our world, and so we are able to create a map of all the things in it, above and below our observational horizon, which is known as the celestial sphere.

The celestial sphere is a two-dimensional sphere, like the surface of a ball viewed from its centre. Every day, the stars revolve once around the Earth as it rotates, but their relative positions never change (aside from a few nearby stars whose motions we’ve detected). For this reason, the stars are plotted with fixed coordinates on the celestial sphere. As the Moon revolves around the Earth and the Earth and other planets revolve around the Sun, the positions of the Sun, Moon and planets change relative to the background stars, and we can map this movement on the celestial sphere.

The remainder of the learning material for this module will provide you with an introduction to the many different aspects of the celestial sphere—things like constellations, conventions for measuring stellar brightnesses, the daily and annual evolution of the celestial sphere, and the reason why we experience seasons on Earth.

Constellations

Today, constellations are specific regions of the celestial sphere with boundaries that have been defined by the International Astronomical Union (IAU). However, the modern boundaries are based on the traditional constellations of ancient Greece. As with many ancient cultures, the Greeks defined constellations based on patterns of the brightest stars in the sky, which they used to represent mythical heroes. (Note that the word asterism is the general term for a pattern of stars, regardless of whether an asterism is a constellation). The constellations were then used in the telling of traditional stories that were handed down from generation to generation, but were also useful to astronomers as they mapped the sky.

Because the ancient Greeks could not see the southern half of the celestial sphere, they passed down only constellations in the north. In order to complete the delineation of the celestial sphere, early explorers defined new constellations based on the asterisms in the southern part of the celestial sphere.

While constellation stars appear close to one another in our night sky, it is important to be conscious that they may not be. Due to the projection of each star’s light onto the celestial sphere resulting from our observational perspective, two stars appearing very close together may in fact be much further apart than two stars on opposite sides of the celestial sphere. For example, two stars located to either side of the Earth might each be 10 light-years away from us, so they are 20 light-years apart. Two stars located next to one another in our night sky could be separated by much more than this if, say, one is 10 light-years from Earth and the other is 1000 light-years away.

Similarly, the apparent brightness of two stars does not represent intrinsic brightness. A star that is 10 times brighter than another will appear 10 times brighter only if they are the same distance from Earth. Otherwise, a bright star may even be located further away from us than one that appears relatively dim.

Star Names

As with constellations, star names are useful conventions for astronomers. If the celestial sphere is a map of the sky, constellation boundaries can be compared to province borders on a geographical map. Similarly, star names are comparable to the names of towns and cities.

All of the brightest stars in the sky have common names derived either Greek (e.g. Arcturus, Castor and Pollux, Sirius), Arabic (e.g. Aldebaran, Altair, Betelgeuse, Rigel, Vega) or Latin (e.g. Regulus, Spica) origins. The brightest stars visible by the unaided eye were assigned the first systematic names by the German astronomer Johann Bayer in 1603. In Bayer’s designation, the brightest 24 stars in a constellation were assigned lower-case Greek letters α, β, etc., in order of decreasing apparent brightness. For constellations with more than 24 bright stars, Bayer went on to assign Latin letters. Bayer designations involve both letters and the genitive (possessive) forms of constellation names. For example, Betelguese, the brightest star in the constellation Orion, is also known by its Bayer designation α Orionis (shortened as α Ori), which literally means “the alpha of Orion.”

A list of all the constellation names, both nominative and genitive, along with their pronunciations, can be found on the Sky and Telescope website.

Star Brightness

Years before Bayer introduced his designation for star names, the first attempt to systematically map the stars and classify their brightnesses was made by Hipparchus, a Greek astronomer who lived in the second century BCE. Hipparchus was one of the greatest astronomers of antiquity, and will be encountered again in Module 2.

Hipparchus invented the magnitude scale that astronomers still use today. He designated the brightest stars in the sky as first magnitude, and the faintest stars as sixth magnitude. The modern convention follows this same designation, but quantifies it: there is a 100x difference in brightness between first and sixth magnitudes, evenly divided between each intermediate magnitude. Therefore, because there is a five-magnitude difference between first and sixth magnitude, a difference of one magnitude means a star is 100^1/5 = 2.512 times brighter, a difference of two magnitudes (e.g. between a third magnitude star and a fifth magnitude star) is 100^2/5 = 6.310 times brighter, etc.

Due to digital technology astronomers are now able to precisely measure an object’s apparent brightness to multiple decimal places. The brightness difference between a 2.34-magnitude star and a 2.95-magnitude star is 100^{(2.95-2.34)/5} = 1.75, meaning that the 2.34-magnitude star is 1.75 times brighter.

This modern quantitative measure of apparent magnitude sets the scale for relative brightness. Then, in order to assign an actual value for the brightness of every star in the sky we only need to arbitrarily fix the brightness of a single one. This star is Vega (α Lyr), which is defined to have a magnitude of 0.

Note that the magnitude scale is unfortunately backwards; the brighter the star, the smaller its magnitude. In fact, objects brighter than Vega (which is the sixth brightest star in the sky, including the Sun) have negative apparent magnitude. For example, Venus’ brightness ranges from -3.8 to -4.9.