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The Twin Quasar

Lensed Quasar
aka Q0957+0561A
RA: 10h01m20.8s, Dec: +5553'54" (2000) in Ursa Major
Magnitude: 17.1 + 17.4 (somewhat variable)
Separation: 6"

Distance: 8.5 Mly

Minimum requirements to detect: 16-inch scope under dark skies


A Bright Gravitational Lens

The most famous gravitational lens is Einstein's cross, but if that has eluded you, this object is brighter and easier to see. The Twin Quasar is officially designated Q0957+0561A/B, and you will also see it referred to as the Double Quasar and the Binary Quasar.

In 1979 this object earned a hallowed place in history as the first confirmed example of a gravitational lens.

Observing the Twin Quasar

Our challenge begins at the beautiful edge-on spiral galaxy NGC 3079. Even if you think you may not be able to see the lens, this galaxy itself is worth a visit, as is the surrounding rich field. It subtends 8.1' x 1.4' and is listed as magnitude 11.5, making it within reach of an 8-inch scope. Several other fainter galaxies lie nearby.

Our prey lies off the end of the galaxy to the southeast, and nearly 200 times farther away! Look for a parallelogram of stars; Q0957+0561A/B lie just off the southeast corner. Like most quasars, this one is slightly variable in brightness, but generally speaking the two components are 17.1 and 17.4 magnitude, separated by 6". Observers with very large instruments (25-inch or greater) have reported seeing the two objects cleanly split.

Users of smaller instruments most often report seeing the two objects blended together, which is a good thing--the light of these two objects combine into a considerably brighter single object of 16.5 magnitude. Otherwise, they might not be detectable in even an 18-inch.

This also explains why they are so difficult to split and why intermediate magnifications may be best in such instruments. Many observers report seeing a faint, slightly elongated star.  

This image from the DSS shows a 1/2o field. North is down and east is to the right.

I observed the Twin Quasar in my 18-inch Dob in January 2000. I noted that the field was easy to find near the very nice edge-on spiral galaxy NGC 3079. The night was far from ideal, but I was definitely able to see something at the location of these quasars. The best view came in a simple, 12.4mm Meade Super Plossl with a 2X barlow. It just goes to show that sometimes the fancy, expensive eyepieces aren't always the best for the job at hand. It was enough for me to know that I had sighted the pair at all -- splitting them will have to wait for another, better night. I'm pretty sure that at 9 Billion light years, this is the most distant object I have ever observed visually.  

These views simulate that seen in an 18-inch f/4.5. On the left is at the field at 82x. The right is as seen in a 4.8mm Nagler (425x). The quasars are the double-object at the center.

Confirmation of Einstein's Gravitational Lensing

Einstein's General Relativity is based on the simple proposition that the weightlessness you feel as you fall to earth is identical and indistinguishable from that of being far out in deep space, away from any gravity at all.

The classic "thought experiment" which Einstein considered involved two people enclosed in identical elevators with no windows. One elevator was falling toward earth, where it experienced an acceleration due to gravity. The other elevator was far out in space somewhere. He asked the question, what if the people inside these elevators could perform no experiment to tell which one they were in?

The magic comes when you consider a beam of light directed across the elevator. The weightless person in the free falling elevator is weightless because she is falling right along with the elevator. But does light fall too? If not, the beam of light will hit the other side of the elevator a bit higher than where it was when it left. This is because the elevator will have moved downward while the light traveled across the room. This represents an experiment that could be done to tell which elevator you were in. Either that, or light must fall right along with everything else.

Up to that time it hadn't really been considered that light would be affected by gravity. After all, light is massless, and it had been supposed that gravity required two objects with mass to work. The result of considering the consequences of light being affected by gravity led to a whole new way to look at gravity. In this view, gravity is no longer a force between two objects, but the result of spacetime being curved in an unseen dimension around any object with mass. For instance, spacetime is curved about the earth, so the moon, which "thinks" it is going in a straight line, appears to us to follow a curved path around the earth.

If that's confusing, think of a person who believes the earth to be flat. As he drives his car he thinks he's going in a straight line, but in reality he's following the curvature of the earth. To our senses we see a universe with only three dimensions, but like the guy who thinks he lives on a flat earth, we too are unwittingly following curved paths every time we are affected by gravity.

It turns out that light too follows this curved path around massive objects. This has been measured in many different ways, the most famous of which involved a tiny shift in the apparent positions of stars observed near the sun during a solar eclipse. Another example is the bending of the light from a distant quasar by a galaxy or galaxies that lie between it and us. The result is that the light is bent in a way similar to that of a poor lens, producing multiple images of the quasar in the sky. It is apparent that these are the same object because the redshifts for these images are identical.

In the case of the Twin Quasar, the gravity of a cluster of galaxies that lies an astounding 3.5 billion light years away is splitting the light of the more distant Q0957+0561 into multiple images. Two of these images are much brighter than the others; these are what we observe. The above image shows the two quasars (bright blue) as well as the many, tiny, faint intervening galaxies.

For more information see the Double Quasar page at the excellent Adventures in Deep Space site.

Millennium Star Atlas Vol II Chart 578
Sky Atlas 2000 Chart 2
Uranometria 2000 Vol I Chart 45
Uranometria 2nd Ed. Chart 25
Herald-Bobroff Astroatlas B-05 C-03