Download About SDSS - Astro Projects

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).
____________________________________________________________________________________
Richard Beare, 14th August, 2007
Version 1.2
Page 1 of 6
Astronomy and Space G and T
Celestial Treasure Hunt
____________________________________________________________________________________
Introduction
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.
Orion and the Milky Way from Iran (Credit:
Babak A. Tafreshi, www.dreamview.net )
Showing the position of our Sun and Solar System in the Milky
Way Galaxy. (This is actually an infra-red image obtained using
NASA's COBE satellite, but it shows nicely what our galaxy would
look like if seen edge-on from a great distance.)
____________________________________________________________________________________
Richard Beare, 14th August, 2007
Version 1.2
Page 2 of 6
Astronomy and Space G and T
Celestial Treasure Hunt
____________________________________________________________________________________
Two famous planetary nebulae taken with the Hubble Space Telescope.
Left: the Ring Nebula; right: the Butterfly Nebula
The Crab Nebula: a two colour
image of the best known supernova
remnant. (Credit: Jay Gallagher and
WIYN Telescope)
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 100 000 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.
Globular Cluster M13
(© 2007 Thomas V. Davis,
www.tvdavisastropics.com)
A selection of galaxies as imaged by the Sloan Digital Sky Survey
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.
____________________________________________________________________________________
Richard Beare, 14th August, 2007
Version 1.2
Page 3 of 6
Astronomy and Space G and T
Celestial Treasure Hunt
____________________________________________________________________________________
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 100 x 100 x 100 times 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.
____________________________________________________________________________________
Richard Beare, 14th August, 2007
Version 1.2
Page 4 of 6
Astronomy and Space G and T
Celestial Treasure Hunt
____________________________________________________________________________________
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.
____________________________________________________________________________________
Richard Beare, 14th August, 2007
Version 1.2
Page 5 of 6
Astronomy and Space G and T
Celestial Treasure Hunt
____________________________________________________________________________________
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 6 of 6