Many of us might think of the stars as unchanging. From night to night we see the same constellations, invariable in appearance and shifting predictably in the sky, thanks to the rotation and revolution of the Earth. However, we know by now that there are all sorts of changes happening, from very small to very big. So, how did we know this in the first place?
Studying and Tracking Changes in the Sky
There are lots of changes happening in the space—from the motion of tiny asteroids and the subtle variations of Cepheid stars, to the brief but brilliant flare of a supernova.
How do we study and track these kinds of changes? It seems inefficient to observe the same patch of sky again and again, waiting for something to happen. It would make more sense to search large patches of sky, but how do we know where to start, and how do we know that something we see on one night wasn’t there the night before?
To do this properly, astronomers need to explore a different method of observing, one that often involves more of the patience of cartography and record-keeping than the excitement of serendipitous discovery. We need surveys and detailed catalogs of the whole night sky. But how do we do this? What’s the best approach, and what are our limitations?
Today, it can be easy to take for granted that we have a map of the night sky available at our fingertips. We might have a star atlas on our bookshelf, or a stargazing app on our smartphone, or we might simply turn to the internet in search of a specific database. But how did these maps come about?
This article comes directly from content in the video series Great Heroes and Discoveries of Astronomy. Watch it now, on Wondrium.
Surveys of the Night Sky
There are an immense number of variables involved when an astronomer decides that they want to conduct a survey of the night sky. First, what sky is the astronomer looking at? A map of the constellations made in California will look very different from a map of the constellations made in Chile.
Second, just how deep should survey map go? Our eyes alone show us countless stars when in a dark location, but just add binoculars and an entirely new host of dimmer stars immediately appear. This effect is further pronounced when we shift to bigger and bigger telescopes.
How faint should stars be before a map of the sky is considered complete? And what about wavelength? We know about astronomical research that spans the entire electromagnetic spectrum. Do we want to survey the stars in visible light, the same light we detect with our eyes? Other astronomers might prefer a radio survey, or an X-ray survey, each carrying their own technical challenges.
There are a number of other questions that need to be addressed, but one thing is clear: no one survey will be perfect, or truly complete. Still, knowing the stars and knowing them well is a crucial first step to studying how they work and, eventually, how they change.
Passing Down Knowledge of the Sky
Astronomers have known this for millennia. Polynesian navigators had an encyclopedic knowledge of the stars, recognizing precisely where they should rise and set as a function of time and location, and naming hundreds of stars and astrophysical phenomena.
This knowledge was passed down through training and oral traditions, and the navigators’ mental charts and naked-eye measurements were so accurate that the Polynesians are widely recognized today as being among the greatest and most far-reaching explorers in history.
In Australia, Aboriginal astronomers similarly passed down their detailed knowledge of the sky through everything from storytelling to rock engravings. Aboriginal constellations included the traced-out lines connecting stars that we’re all familiar with today as well as dark constellations; many different Aboriginal groups marked a giant emu sprawled across the Milky Way, a shape traced by dust lanes obscuring the center of our galaxy that’s only visible from the Southern Hemisphere.
The Boorong people have detailed oral records of an outburst from a star named Eta Carinae. The star brightened dramatically in the 1830s and 1840s and became one of the most brilliant stars in the sky, an event noticed by Boorong astronomers and incorporated into stories. In fact, Aboriginal astronomy observations were so detailed, they even recognized that stars like Betelgeuse would slowly vary their brightness with time.
The New General Catalogue
Beginning in the 1780s, William and Caroline Herschel began to build a systematic catalog of the Northern night sky for wide publication. The brother-sister team constructed telescopes together and used them from their home in England to painstakingly improve on past stellar catalogs, sweeping their telescopes methodically across the sky and making note of both known and newly-discovered objects.
The catalog became an invaluable part of the scientific literature, listing objects’ positions, sizes, and brightnesses; it also considered how they might be classified. Their observations also turned up numerous new objects: William discovered the planet Uranus, while Caroline discovered numerous comets. Her work was recognized with a Gold Medal from the Royal Astronomical Society, an honor that wouldn’t be given to a woman again until Vera Rubin received it for her work on dark matter in 1996.
The Herschels’ final result, the New General Catalogue, is still used today, and many famous astronomical objects are identified by their NGC number.
The Herschels’ catalog set an early standard for the best sky surveys of the day: a clear and well-organized compendium of objects in the night sky, organized by position and listing key properties like brightness, shape, and what each object might be. As a reference it proved crucial—an astronomer wondering about a strange object that had appeared through their telescope could simply check the catalog to see whether that object was previously known, a new discovery, or represented some kind of surprising change.
Common Questions about How Surveys Help Studying the Night Sky
In Australia, Aboriginal astronomers passed down their detailed knowledge of the sky through everything from storytelling to rock engravings.
The Herschels’ catalog became an invaluable part of the scientific literature. It listed objects’ positions, sizes, and brightnesses; it also considered how they might be classified.
Caroline Herschel discovered numerous comets. Her work was recognized with a Gold Medal from the Royal Astronomical Society.