Celestial Treasure Hunt

Astronomy and Space G and T Celestial Treasure Hunt

The Sloan Digital Sky Survey

A Sloan Digital Sky Survey image of an area of sky about 13 arcminutes across. For comparison, the Moon is about 30 arcminutes in diameter (0.5 degrees).


If you look up at the night sky from a dark location you will see a few thousand stars, probably a planet or two, possibly the Moon, and occasionally a comet or shooting star (meteorite). With a pair of binoculars the number of stars you can see rises to about 10,000, and with a 15 cm telescope more than two million can be seen. The bigger the telescope, the more light it gathers, so the fainter the stars and other objects that it can see.

Most of the bright stars that we see in the sky with the naked eye lie quite close to us within the disc part of our own Milky Way Galaxy, typically no more than a thousand light years away. This is of course an enormous distance, but it is small compared with the size of our galaxy which is about 160,000 light years across and about 1000 light years thick near to our position in it, which is about a third of the way out from the centre of the disc. In astronomical terms, therefore, all the stars we see on a clear night at a dark location away from light pollution are actually quite close to us.

With the aid of telescopes, however, we are able to see a lot more than just fainter stars. We can detect the remains of stars that have run out of hydrogen fuel and died – so called 'planetary nebulae' and 'supernova remnants'. We can also see 'globular clusters', collections of 100000 or so stars orbiting around the outside of our galaxy, each of which is rather like a giant collection of bees buzzing around inside a swarm. We are also able to see some of the smaller bodies in the solar system such as asteroids and moons circling other planets.

All these objects lie in or around our own galaxy, the Milky Way, but with a telescope we can see way beyond this, out into the further reaches of space, where we find millions upon millions of galaxies, stretching away in every direction, as far as our telescopes will allow us to see. We realise that the Milky Way is just one galaxy among many thousands of millions extending throughout the universe.

The further away that we look, the fainter these galaxies appear to us, and the larger the telescopes, and the longer the exposure times needed to see them. Fortunately, modern telescopes no longer need to rely on the human eye looking through an eyepiece. Instead they use electronic detectors ('CCDs') which are very similar to the chips found in ordinary digital cameras (except that instead of recording the basic colours of red, green and blue simultaneously, they record one colour at a time by using coloured filters). CCDs are much more sensitive than either the human eye, or photographic plates (which astronomers used to use), and they can be exposed for minutes, or even hours, at a time in order to detect the faintest objects.

(Left) Apache Point Observatory in New Mexico, with the 2.5 metre Sloan Digital Sky Survey telescope on the left; (right) a close-up of the SDSS telescope, showing the unusual wind baffles. (Image credits: Fermilab Visual Media Services)

The Sloan Digital Sky Survey (SDSS) telescope in New Mexico, for example, has a main mirror 2.5 metres in diameter, and using its standard exposure time of 50 seconds, it can image objects as faint as 'magnitude' 22 in the visible part of the spectrum. [For those who are unfamiliar with the idea of measuring the brightness of astronomical objects in 'magnitude's, a decrease in brightness of one magnitude is equivalent to about 2.5 times as much light reaching the telescope, while a decrease of five magnitudes represents exactly 100 times as much light.] A magnitude 22 galaxy is therefore 100x100x100times fainter than a magnitude seven star, and this is 2.5 times fainter than a magnitude six star, the faintest visible to the naked eye. In other words, a 50 second exposure with the Sloan telescope can detect objects which are 2.5 million times fainter than those you can see with your unaided eye!

The SDSS telescope has now finished imaging a full quarter of the whole sky (about 10,000 square degrees). It has five cameras which simultaneously image in five wavebands, from ultraviolet to infrared. High quality colour images are produced using the three middle wavebands and these can be easily accessed from the SDSS website using a range of easy to use 'visual tools'.

In your Celestial Treasure Hunt, you will be using SDSS images to survey small patches of sky in order to discover what sort of objects are to be found 'out there' in space. By combining your results with those of others, you will help build up a picture of what the universe looks like on the grand scale, i.e. on scales larger than the Milky Way galaxy.

Here is a typical image from the SDSS telescope.

A typical image from the Sloan Digital Sky Survey (slightly enhanced so as to show more clearly when printed out). SDSS detects objects millions of times fainter than the human eye can see (magnitude 22)

Because the speed of light is not infinite, but has a finite value of 300,000 kilometres per second, the further away we look in distance, the farther back we look in time as well. The Sloan survey has imaged galaxies out to a distance of about 6000 million light years, and this means that we see the farthest galaxies appearing in its images as they were 6000 million years ago (ignoring a relativistic correction for the farthest galaxies).

The area of sky imaged by SDSS is that directly 'above' the plane of the Milky Way. In other words, the survey avoided looking through the disc of our galaxy, which would have resulted in images of extra-galactic objects being affected by absorption, emission and scattering from dust and gas in our own Milky Way.

SDSS mainly imaged the area around the North Galactic Pole (RA = 193 deg, Dec = 27 deg). This is a point directly 'above' our position in the Milky Way Galaxy.

Because of this the survey includes hardly any supernova remnants or planetary nebulae. This is because these objects are remnants of dead stars and therefore mainly to be found in the disc of the Milky Way. The survey also imaged very few globular clusters as these are spread out around the whole galaxy so that few of them happen to be in a direction directly above the Sun's position in the disc of the galaxy.

From the point of view of doing a survey of the whole sky it is a pity that a representative sample of supernova remnants, planetary nebulae and globular clusters is not included. On the other hand, SDSS does provide an excellent sample of extragalactic objects, i.e. other galaxies, galaxy clusters and quasars, and it is these objects that you will be discovering in your sky samples.

However, if you are able to obtain time on a large robotic telescope (such as one of the 2 metre Faulkes Telescopes or the Liverpool Telescope), you may want to extend the SDSS survey activities described here to areas of sky nearer to the plane of the Milky Way, and include supernova remnants, globular clusters and planetary nebulae in your treasure hunt.

Richard Beare, Institute of Education, University of Warwick. 14th August, 2007


Richard Beare, 14th August, 2007 Version 1.2 Page 1 of 1