Chandra EducationChandra Education
Chandra Education Home PageInstallation PageLearning ds9 PageImages & Activities PageEvaluation Page
[Contact Help] [Chandra Public Page]


Section I: Introduction
Section II: 6 Activities
Section III: Summary

Education Activities To Accompany Chandra Data Analysis Software

How Big Is It?

So how big is 3C273? Note that the x-ray image of the object is much smaller than that of Cas-A, but of course, that could be because of the stupendous distance to the quasar. In fact, the size of the object is consistent with a point source of light, such as you might see when you look at an ordinary star that resides in our galaxy. However, we can certainly see the jet emanating from the side of the quasar, and measure its length.

Activity 4: How big is 3C273?
  1. Zoom in and see how many pixels represent the length of the jet. Be careful! The jet is at almost a 45 degree angle, and just counting pixels will result in an error. Remember each pixel along a side spans half an arc-second, so along a diagonal, the angle spanned is 1.4 times that. (It might be helpful to rotate the image so that the jet is horizontal. You can do that by going to edit: rotate in the menu for ds9, and then clicking on the image. Note that the cursor has changed to indicate rotation. Now rotate the image! This is fun! When the image is horizontal you can do a horizontal cut by using the analysis menu, and thereby get a good estimate for the extent of the jet. [Along the diagonal, the jet is about 15 pixels long, so the extent in the sky is 15 x 0.5 x 1.4 arc-seconds or about 10 arc-sec. Therefore, L = 10 x 800 Mpc / 206,265 or about 40,000 pc. This is bigger than the size of our Milky Way.] Indeed, recent observations using the Chandra satellite have shown a faint connection between the jet and the central part of the quasar, so the jet is probably about twice this size.

    By now you're probably wondering: the main part of the quasar looks like a ball about 20 pixels across. Doesn't that represent the size of 3C273? The answer is no. The reason is very similar to what happens when you take a photograph of a very bright light. The picture "spills over" into adjacent regions on the film. Since the jet is much fainter, it really does represent a true size. In astronomical lingo, we say that the jet is "resolved", because we can see features over its entire image, but the main quasar is "unresolved", since it is a featureless blob consistent with a point-like object that is bright. It is exactly the same way with stars in optical telescopes. All the stars, other than the Sun, are so far away as to be point-like in a telescope, although the brighter ones will appear to be bigger blobs on a picture because of the spill-over effect. So we need another way to measure the size of the quasar. The answer comes from an unexpected place: the quasar's time variations.

    To see how this helps us, imagine a football field, with you a standing a few blocks away on the outside. When a team scores a touchdown, a roar goes up from the crowd. All at once, everyone is screaming. But you don't hear the loudness immediately. The sound from the part of the stadium nearest you arrives first, followed by the sounds from the more distant parts. It takes time for the sound to build up. In fact, if the velocity of sound is given as "c", the amount of time it takes is the length of the stadium divided by the velocity, t = r/c. So measuring how long it takes for the light coming from the quasars to change in intensity gives us an idea of how small the central engine is that is responsible for emitting the radiation. Typical optical variations are shown below:

    Optical Variations
    Optical Variations
    larger image

    These variations in the quasar 3C279 were discovered from a study of the Harvard Survey Plates, which are optical photographs, by Eachus and Liller, and established time variability on the order of months. Since then, some quasars have exhibited variations within minutes.

    Thus, the size of the central engine of these objects must be incredibly small, considering their stupendous output. If r = ct, (distance = velocity of light x time) with now "c" representing the velocity of light, we have objects that must be no larger than our solar system in size. And yet their output is hundreds of times that of an entire galaxy.

    3C273 tends to be fairly constant in x-ray output, so we have no direct evidence for its size. But in terms of investigating this class of objects, it is clear we have our work cut out for us; we need to explain how they produce prodigious amounts of energy in a very small space.
What can possibly allow us to do this? [next] [back]

Last updated: 11/08/02


Chandra Ed. Home Page | Installation | Learning ds9 | Activities & Images | Evaluation
Activities: Cas-A | 3C273 | M31 & Coma | Cen X-3 | Seeing Through Different Eyes
Resources: ds9 | Chanda Public Information & Education

Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Cambridge, MA 02138 USA
Phone: 617.496.7941  Fax: 617.495.7356
Comments & Questions?

This site was developed with funding from NASA under Contract NAS8-39073.