Northern Star

[Mistermouse]

My son had a question from school- Why in a photo of the Northern Star do all the surrounding stars appear to have rings of light around them.

[jake-the-peg]

The Nortern star is in an almost unique position. It is nearly exactly in line with the North Pole. As the earth spins on it's axis it therefore appears to remain in the same position while the other stars seen to move over time.

The result is that a picture of the North star over a few hours looks like this:

http://www.nrc-cnrc.gc.ca/multimedia/picture/i mages/html/nrc-hia_gemini_polaris-lr.jpg

[!ightoftruth]

that link is dead

[mibn2cweus]

Click on image for large view of star trails.

[gen2]

this is jake-the-peg's link that works

http://www.nrc-cnrc.gc.ca/multimedia/picture/images/html/nrc-hia_gemini_polaris-lr.jpg

It would appear to be approximately a 5-hour exposure.

[Geo1000]

Do you mean that each surrounding star has a small circle neatly around it, or that each star forms a circle centred around the pole star?

If the latter then it is indeed due to 'star trailing'.

But if you mean the former then it could be due to diffraction effects caused by the telescope. No telescope yet invented can really give a perfect star image of a minute 'dot'. Dim stars can appear like dots, but bright stars tend to be surrounded by haloes, rings, or even have spikes because of diifraction efects caused by intenal bits and pieces.

[Mistermouse]

Thanks gang I think these postings have solved my problem.