Google Answers Logo
View Question
Q: optics and astronomy ( Answered,   3 Comments )
Subject: optics and astronomy
Category: Science > Astronomy
Asked by: avramala-ga
List Price: $2.00
Posted: 07 May 2004 04:44 PDT
Expires: 06 Jun 2004 04:44 PDT
Question ID: 342623
Why is it that a magnified star's light still appears as small as it
did unmagnified? i.e. if I look at a hugely magnified star photo, the
stars still appear as dots of light.  If it has size shouldn't that
size be doubled if we look at it through a 2X telescope?  If the
source of the star light is not magnifiable then how do we see it's
light in the first place?
Subject: Re: optics and astronomy
Answered By: willie-ga on 07 May 2004 05:01 PDT
Hi, and thanks for the question

Here's a summary from "How do telescopes work?" at

"Telescopes are used for two different kind of objects; pinpoint
objects (stars) and extended objects.

Pinpoint objects: For these objects magnification is used to 'thin'
out the background brightness, while it leaves the brightness of the
pinpoint object concentrated in that pinpoint. The result is a greater
contrast between the object and the background, resulting in a better

The background light can be considered as a certain amount of light
per area. If by magnification the area appears bigger, the light gets
more spread out so the surface brightness will be lower. A pinpoint
object will stay a pinpoint object, even when magnified. Thus the
contrast between the object and the background gets better with
increased magnification.

So why do bigger scopes show fainter stars? Well, because the usable
magnification range depends on the aperture of the scope. The bigger
the scope, the higher the minimum and maximum magnification. BTW, you
can easily check this effect. Put a high power eyepiece in your scope
and take a look. Remember the place of the faintest star you see. No
put in a low power eyepiece and check if you can still see that star.
(Remember this is still the same scope, the aperture did not change,
only the magnification. So the visibility of that faint star depended
on magnification and NOT aperture)

Unfortunately the magnification can not be increased forever. When it
reaches 50 times the aperture in inches (or 2 times the aperture in
mm) the pinpoint nature of a pinpoint object disappears. It becomes a
small disk with so-called 'diffraction' rings. This disk is now
increased with still higher magnifications, so now its surface
brightness decreases.

You'll find a full explanation at the site above.

Google Search used
star magnification size
"how do telescopes work"
Subject: Re: optics and astronomy
From: iang-ga on 08 May 2004 15:11 PDT
The simple answer is that the stars are too far away to be resolved as
disks. The larger the scope the smaller (more distant) the object that
can be resolved, but it's practicaly impossible to build a telescope
capable of showing a star as a disk.

Ian G.
Subject: Re: optics and astronomy
From: rnt20-ga on 28 May 2004 12:37 PDT
The answer from the Google (subcontracted) researcher is terrible!!!
The comment above is mostly correct but note that large telescopes
*can* make out the disks of the largest stars. You need to have a
telescope at least 3 metres (10 feet) in diameter, though.

Examples of images of the disks of other stars are shown at this website:
The left hand image shows three bright star spots. It was generated
using the 4.2m William Herschel Telescope (see ). The star in the image is

If you can view Postscript files (.ps) then you can also look at:
These show images of the disks of other stars from the Keck Telescopes.

A free viewer for postscript files can be downloaded from

Even the largest and nearest stars are so small that with smaller
telescopes they just appear as single points of light (the resolution
of the telescopes is not good enough to see the disk of the star).

Most instruments on most telescopes do not have the resolution to
see the disks of any stars.
Subject: Re: optics and astronomy
From: rnt20-ga on 18 Jun 2004 01:21 PDT
Some nice new images of star-spots on supergiant stars have recently
been put on the web here:

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy