A Bright Gravitational
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.
this object earned a hallowed place in history as the first
confirmed example of a gravitational lens.
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
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.
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
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.
image from the DSS shows a 1/2o field. North is down and
east is to the right.
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
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.
of Einstein's Gravitational Lensing
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.
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
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.
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.
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.
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.
information see the Double
Quasar page at the excellent Adventures
in Deep Space site.