Sloan Digital Sky Survey: Mapping the Universe

Sloan Sweeps the Sky

One sacrifice for Sloan’s wide field of view is its resolution—its images aren’t nearly as crisp as Hubble’s (click to compare). But its sensitivity is such that many very dim objects, both inside and outside our galaxy, are being registered for the first time. 

2D vs. 3D

Some of the panoply of luminous points in the night sky are bright because they are nearby stars. Others, such as quasars, are intrinsically bright, yet are billions of light-years from Earth. Sloan’s digital picture-taking registers the precise brightness of objects shiny and dim, and measures their positions relative to one another in two dimensions. Determining an object’s distance from Earth—the added dimension of depth—requires an extra step  called spectroscopy.

Spectrographs act as prisms by measuring a space object’s spectrum, or its light broken down into its constituent colors. “With spectra, you can measure very important physical parameters about the object, such as its distance, its temperature, chemical composition, and magnetic fields,” says Gillespie. “You do real physics with the data that from a spectrograph, rather than just take a picture of something.” Before Sloan, acquiring a single object’s spectra could take about an hour. Sloan’s spectrographs measure 640 spectra at once.

To save time, only a selected number of the 100 million space objects registered by Sloan get the extra spectrographic step: about 700,000 at current count. The distance data on the objects, acquired from the spectrum analysis, make up the points on the three-dimensional Universe map. A sample size of about a million objects is the minimum needed to get a sense of the overall distribution of the Universe. “Theorists want to understand the large-scale structure of the Universe, because our ideas about how the Universe began predict how galaxies are distributed in it today,” explains Turner. Turner and his colleagues at Fermilab are just one of the teams eager to test ideas about the early Universe against Sloan’s real-life, current-day data.

Then vs. Now

The Sloan survey team publicly releases its data in batches about 18 months after it acquires them. Using the data, astronomers have so far discovered the Universe’s most distant quasars, its dimmest stars and galaxies, a new class of dwarf stars, and even a previously unnoticed galaxy stuck right in our own Milky Way. “That aspect of the unknown—of seeing things that no human being has ever seen before, of going to distances that no one has even really conceived of before—is probably the most exciting aspect of this work,” says Barentine. The potential for going this distance has the scientists at Apache Point looking way beyond the clouds.

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