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Let’s pause here for a moment of perspective. We are now discussing numbers by which even astronomers sometimes feel overwhelmed. The Coma cluster may have 10, 20, or 30 thousand galaxies, and each galaxy has billions and billions of stars. If you were traveling at the speed of light, it would still take you more than 10 million years (longer than the history of the human species) to cross this giant swarm of galaxies. And if you lived on a planet on the outskirts of one of these galaxies, many other members of the cluster would be close enough to be noteworthy sights in your nighttime sky.

Central region of the coma cluster.

Small Galaxies Outnumber Large Galaxies. This combined visible-light image from the Sloan Digital Sky Survey and infrared Spitzer Space Telescope view of the central region of the Coma Cluster has been color coded so that faint dwarf galaxies are seen as green. Large ellipticals and spirals are few compared to the number of dwarf galaxies.
This combined visible-light (from the Sloan Digital Sky Survey) and infrared (from the Spitzer Space Telescope) image has been color coded so that faint dwarf galaxies are seen as green. Note the number of little green smudges on the image. The cluster is roughly 320 million light-years away from us. (credit: modification of work by NASA/JPL-Caltech/L. Jenkins (GSFC))

Really rich clusters such as Coma usually have a high concentration of galaxies near the center. We can see giant elliptical galaxies in these central regions but few, if any, spiral galaxies. The spirals that do exist generally occur on the outskirts of clusters.

We might say that ellipticals are highly “social”: they are often found in groups and very much enjoy “hanging out” with other ellipticals in crowded situations. It is precisely in such crowds that collisions are most likely and, as we discussed earlier, we think that most large ellipticals are built through mergers of smaller galaxies.

Spirals, on the other hand, are more “shy”: they are more likely to be found in poor clusters or on the edges of rich clusters where collisions are less likely to disrupt the spiral arms or strip out the gas needed for continued star formation.

Gravitational lensing

As we saw in Black Holes and Curved Spacetime , spacetime is more strongly curved in regions where the gravitational field is strong. Light passing very near a concentration of matter appears to follow a curved path. In the case of starlight passing close to the Sun, we measure the position of the distant star to be slightly different from its true position.

Now let’s consider the case of light from a distant galaxy or quasar that passes near a concentration of matter such as a cluster of galaxies on its journey to our telescopes. According to general relativity, the light path may be bent in a variety of ways; as a result we can observe distorted and even multiple images ( [link] ).

Gravitational lensing.

Illustration of Gravitational Lensing. At left is a blue ball labeled “Observer on Earth”. At center a “Galaxy” is drawn as a white ellipse, and at far right a “Quasar” is drawn as a white circle. Two yellow arrows are drawn from the quasar pointing to the left representing light from the quasar. One points horizontally and one points at an angle toward the bottom center of the diagram. Where these arrows are closest to the galaxy at center, they change direction, with each arrow now pointing toward Earth. The angle between the arrows where they contact Earth is labeled with the Greek letter “theta”. To the observer on Earth looking along the lines separated by “theta”, two images of the quasar would appear: “Image A” above the galaxy, and “Image B” below the galaxy.
This drawing shows how a gravitational lens can make two images. Two light rays from a distant quasar are shown being bent while passing a foreground galaxy; they then arrive together at Earth. Although the two beams of light contain the same information, they now appear to come from two different points on the sky. This sketch is oversimplified and not to scale, but it gives a rough idea of the lensing phenomenon.

Gravitational lenses can produce not only double images, as shown in [link] , but also multiple images, arcs, or rings. The first gravitational lens discovered, in 1979, showed two images of the same distant object. Eventually, astronomers used the Hubble Space Telescope to capture remarkable images of the effects of gravitational lenses. One example is shown in [link] .

Multiple images of a gravitationally lensed supernova.

Multiple Images of a Gravitationally-lensed Supernova. The background image is of a distant galaxy cluster through which the light of an even more distant supernova has passed (white box at center). The enlargement at right shows the four images of the supernova (arrowed) around the lensing galaxy.
Light from a supernova at a distance of 9 billion light-years passed near a galaxy in a cluster at a distance of about 5 billion light-years. In the enlarged inset view of the galaxy, the arrows point to the multiple images of the exploding star. The images are arranged around the galaxy in a cross-shaped pattern called an Einstein Cross. The blue streaks wrapping around the galaxy are the stretched images of the supernova’s host spiral galaxy, which has been distorted by the warping of space. (credit: modification of work by NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI))

General relativity predicts that the light from a distant object may also be amplified by the lensing effect, thereby making otherwise invisible objects bright enough to detect. This is particularly useful for probing the earliest stages of galaxy formation, when the universe was young. [link] shows an example of a very distant faint galaxy that we can study in detail only because its light path passes through a large concentration of massive galaxies and we now see a brighter image of it.

Distorted images of a distant galaxy produced by gravitational lensing in a galaxy cluster.

Distorted Images of a Distant Galaxy Produced by Gravitational Lensing in a Galaxy Cluster. The distorted images of the galaxy are circled in white and lie outside the galaxy cluster at the center of the image. A small box near the center of the cluster marks the position of the background galaxy being lensed by the cluster. The image in the large box at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster.
The rounded outlines show the location of distinct, distorted images of the background galaxy resulting from lensing by the mass in the cluster. The image in the box at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster, based on a model of the cluster’s mass distribution, which can be derived from studying the distorted galaxy images. The reconstruction shows far more detail about the galaxy than could have been seen in the absence of lensing. As the image shows, this galaxy contains regions of star formation glowing like bright Christmas tree bulbs. These are much brighter than any star-formation regions in our Milky Way Galaxy. (credit: modification of work by NASA, ESA, and Z. Levay (STScI))

We should note that the visible mass in a galaxy is not the only possible gravitational lens. Dark matter can also reveal itself by producing this effect. Astronomers are using lensed images from all over the sky to learn more about where dark matter is located and how much of it exists.

Questions & Answers

why the dark side of moon never face us? because moon dont rotate? why tho
Rishabh Reply
im in 8th standard and my school teach us nothing about astronomy but i want to be an astronomer so i study from youtube and apps like this but sometimes i get language problems
The Moon does actually rotate. It takes 27 days for the Moon to make a full rotation. Because it takes it so long and the Earth is rotating around the sun, it seems like the Moon is pretty much standing still. That's called "synchronous rotation".
I ment to say "The Earth is orbiting around the sun"
Tidal Locked.....
thats cool
ya gives you a look at how the moon orbits around us too and a perspective of how fast we are going. also gives the moon phases for the year! enjoy 😁
yes you are absolutely correct, it just like something like the milk of heaven
the far side of the moon is not the dark side of the moon.
what phases of the moon occurs when spring tide?
Florence Reply
the moon has a 28 day cycle. you can download a moon calander or follow the pattern and make your own calendar. 1st full moon of the year 21/22nd Jan, its also a blood moon and a wolf moon. if this info helps at all. 😊
How to determine the velocity of light?
using this formula c=1/√absolute permeability×absolute permittivity of free space i.e. c=1/√8.854×10^-12×4π×10^-7~3×10^8ms^-1
what is mean by Big Bang
block Reply
What it means by Big Bang is how the world was made and came in to exist
The Big Bang Is The Best Explanation For What We See Around Us, And For The Origin Of The Universe, All Our Evidence Points To The Big Bang Theory..... And Was Proposed By A Belgian Priest.....
if the big bang theory is really true than i would like to know what exploded and where did it came from and where did the energy came from and where come from for the matter to expand into amd where did the organization come from and where did the information come from.........
Some say that on the otherside of the singularity was a black hole. They say that at the other end of all black holes is another universe. Hence the multiverse. It's all theoretical at this point but less than a 100 years ago people believed that there was only 1 galaxy in the universe. So why
should there be only 1 universe?
is astronomy a hard subject to learn because i want to be an astronomer
FNAF Reply
No it not it just takes time and effort to learn
if your already interested/observing, your already an astronomer ! The more you look and read the more you'll learn.
I want to become a scientist but my knowledge is very low. How I can improve my knowledge?
any form of science courses, there's free crash course channel on you tube. I've learned alot from this channel. covers most topics, science: biology chemistry physics, astronomy, ranges to anatomy, history and many many more. it's a never ending subject!
suggest a channel as a example
CrashCourse is the name of the channel and there is a picture of an apple. look through their play lists. They will have other related channels they follow also.
thank you so much
your very welcome
Why are they so far away that their light takes hundreds of millions of years to reach us on earth?
LadyCatTM Reply
what is comets , astroids ,
Sumit Reply
comets are broken pieces of dust frozen debris Asteroids are big soiled pieces of Ice that burst into flames when it in to earth's atmosphere
hi, I am yamini, I am in class 8 but very much interested in astronomy and go to NASA, what are the subjects in which I can master and lead to NASA.
yamini Reply
Hello Yamini, Im Ken and I'm avery intrested in joining the NASA too. Is nice to meet you.
nice to meet u ken
What I know about the subjects is that you have to be a master on science and math, also if you know about aviation is better too
I read that the Russian language is very important, is not a requirement but it's like an extra point!
yeah like physics, chemistry and maths, they are my most favorite.
than you can go free of cost
Yes, That's right!
Oh! Free of cost?
hello yamini nice to meet you
wow from NASA ur so lucky
hello yamini nice to meet you
what is time
Abdul Reply
Time is relative
pls elaborate
the clear defination.I know that.
In planet mars there the life exits or not and is there water there
Eshwarsa Reply
see till now nothing can be found as u know that the curiosity rover has struck in mars
It has been proven that there are water molecules on Mars but not enough that most lifeforms could thrive upon.
There are huge amounts of water in the ice caps and under the surface. The surface and chemistry indicate that Mars had cosiderable amounts of water on its surface in the past.
what is your opinion about the theory of Vedas about modern physics..
Manish Reply
i think in some ways vedas are also correct but not everytime
I agree
hmm even I agree
Is there any patened theory about time relativitg in growth and development?
donot Reply
some astronomer's says that there is no alien exist but why search for extra terrestrial intelligence center is established
Eshwarsa Reply
No One Knows That For Absolute Fact, The Universe Is Too Huge To Have Any Type Of Idea About What Exist In The Far Reaches Of Our Universe.....
Check Out The Drake Equation.....
their should be aliens as like ours there would be another planet
which could have existed life on it
adam i want to ask a question
can kepler 1st law be applied on all the planets of the universe
hello, anyone home?
guys listen we cannot ever find aliens because our technology is not that great.we don't know whether they exist or no because our universe is very large . Just for an example;even if we spot out aliens we cannot reach there because lot of time must have been passed and before us reaching there
what if their species or existence might get vanished our have been extinct!! getting my point
sjskskfhjkkktewqqw and try?
Lanika Reply
what is this ?
hi I am Samuel from India mumbai
nice to meet you
thats my question, what is this?
penzias and wilson's a discovery of the cosmic microwave background is a nice example of scientific serendipity-something that is found by chance but turns out to have a positive outcome
Jacqueline Reply
Practice Key Terms 6

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