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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

what happened if moon replace Jupiter
Naresh Reply
we well never exist because Jupiter's mass and gravity is toooo high so it will protect us from asteroids comments etc... but moon can not do that.
Yabtsega
that's accurate
Joel
What is the definition of spacial relativity
Yabtsega Reply
what is a black hole
Jesse Reply
why do people use eclipses
Jesse
black hole is very massive object which is made up of explosion of huge dead stare
Yabtsega
a supernova
Joel
it's actually an implosion
Joel
it's actually an implosion
Joel
it's actually an implosion
Joel
it's actually an implosion.
Joel
yep It is the explosion
Yabtsega
thanks for correction
Yabtsega
my pleasure
Joel
but It It depend on the supernova It either explode or implode .
Yabtsega
Exactly if it implodes is formed a black hole due to the massive gravity but if the supernova explodes is formed a neutron star.
Joel
if a star at its white dwarf stage has mass equal to 1.4 times the mass of our sun then it has a highly chance of a becoming a black hole
Mahesh
It is known as chandrasekhar limit
Mahesh
Right
Joel
when two Neutron Stars merge , they can also make black hole
Gurjinder
Has the start of another big bang started to happen yet?
LAURA Reply
are there any home learning courses linked to jobs and carriers in space?
LAURA
i dont know for sure but as yourself i am also very interested on it
Paco
does tidally locked planet have magnetic field?
SCIENCE Reply
no its bc it doesn't rotate at all so it doesn't create friction in its interior and that in turn doesn't create no magnetic field
NARUTO
There is no way for us to find it out because their cores might revolve although their crust is not rotating
Tanmay
what is mean by dwarf planet
Mohamed Reply
Nearly round shaped planet, bil smaller compared with other planets
Murrino
thank you murrino I was thinking another type of planet but now i remember
NARUTO
These dwarf planets are nearly like Mercury but these planets have not cleared every matter present in their orbital path. Like most of the dwarf planets are present at asteroid belt as their neighborhood is not cleared of matter
Tanmay
But since Mercury was capable to clear its path so it is called a planet rather than a dwarf planet
Tanmay
Of course they should directly orbit our Sun rather than any other planet
Tanmay
this on our solar system
Youssef
not necessarily
Tanmay
The mass of a planet is twice the mass of the Earth and the radius is half of the Earth. If an object is placed in the equatorial region of the planet, it cannot be held by the gravitational force; Any other area can hold. How long is a day on the planet? (Earth's redius is 6370 km)
Muhtasim Reply
what is energy made up of?
Suman Reply
It is a fundamental property that exists, it is not made of anything
Tanishk
what is the correct arrangement of the horoscope
edwin Reply
what causes voids in space like bootes void?
NARUTO Reply
I think that it has somehow managed to get a lot of dark energy which makes star formation hard
Tanmay
Or this area might have a lot of gas clouds which obstruct our view of its star. that means it has normal amount of galaxies but we can't see them
Tanmay
but why scientists are saying that it bounces galaxues away while containing some inside it 🤔🤔🤔
NARUTO
I never heard anyone say that
Tanmay
well a YouTube channel called the seeker just said that scientist doing observations on this phenomenon is causing galaxies to bounce of
NARUTO
hi
Bas_
can you send the link to that video I didn't got any video saying galaxies to bounce
Tanmay
***youtu.be/I4Yc8Zw_NJo
NARUTO
there
NARUTO
ok I will see
Tanmay
I might get the wrong idea but that's what I understood
NARUTO
ok
Tanmay
who knows how to get dark materia
Jordan
its literally next to impossible, even the best scientist are only able to get 5 nanograms of it
NARUTO
I think we can get it from dark hole
Jordan
yeah but getting a black is nearly impossible at this moment
NARUTO
yeah I know but they can just think
Jordan
what are you talking about
Tanmay
What black?
Tanmay
black hole*
NARUTO
So who managed to make 5 nanograms of black hole?
Tanmay
What does that even mean 5nm of black hole
Tanmay
It's Swartzchild radius might be below plank length
Tanmay
I meant 5 nanometers of dark energy
NARUTO
oh ok I got it
Tanmay
we live in a place which only has 3 dimensions and our imaginations are only limited till there neither is the space infinity nor are our imaginations!
Samuel Reply
hi guys can we all discuss about a thought which I have?
Samuel
yes why not tell us
Tanmay
I do not know
Clint
it's up to u to do your homework
Clint
we live in many dimensions, x,y,z, time, gravity, light, ...... an n num of Dem,
Clint
Is this the question? we live in a place which only has 3 dimensions and our imaginations are only limited till there neither is the space infinity nor are our imaginations!
Tanmay
ppl thought we lived on a flat earth, 2 Dem, them 3, then 4 Einstein, ...... the list goes on
Clint
ur turn to work
Clint
work
Clint
young buzzards chew at the various of the old, find new food
Clint
work, there is no free ride in learning
Clint
read everything, process it later, keep reading
Clint
one day, you will make a contribution to science, but u have to read, u have to take ur bumps
Clint
I think we should read and understand what it wants to tell us what you learnt where can it be used and than go forward
Tanmay
Infinity is beyond our imaginations
Samuel Reply
maybe, maybe not.... it depends on the limit of ur thinking
Clint
have u stop thinking?
Clint
no?
Clint
then u do not know you own limits of ur imagination....
Clint
I think our imagination is beyond of infinity...
Suman
u may choose to stop imagining, but it's u that chooses
Clint
the depth of the mind has not been determined
Clint
if someone knows his imagination is limited then how does his imagination will exist, the imagination that is limited then those are just thoughts but unlimited imagination is creation of something new...
Suman
A person trapped in a well for a lifetime can never imagine the vastness of the sea so our imagination very well depends upon your physical limits
Tanmay
how would one describe the 3 dimensions, i mean what defines their boundaries . . . ?
Chalo
3 dimensions mean 3 mutually perpendicular coordinate in space
Gurjinder
how mass came to our galaxy when it was jast a point.
Nashita Reply
From energy
suvas
mass is energy, engery is mass, a very small amount of mass equals a huge amount of energy.... books tell that God is light, and if light is there, there is mass. Where it came from, why are we here, those are meta-physicial questions or religious ones
Clint
light bends toward planets, it's equilance can be computed to equal mass and vic-versi
Clint
a point? define a point?
Clint
a point has infinite points between it and any object
Clint
ur question isn't wrong, but mass, like light are fundamentals.... that means they are just there, we question them yes, dissect them yes, but why are they there? Answer that question and ur the smartest person on this rock
Clint
show me where we can create light without mass or mass without light
Clint
ur 8th grade right?..... keep thinking,... u have a future and will go far
Clint
infinite means where we humans are unable to count further, I think energy is expanding itself...
Suman
who are you Sir?
Suman
I can't argue with u there.... as long as light exists, matter will increase
Clint
good point
Clint
I think there is something opposite of light too... if there would be ...then anti- matter will also exist
Suman
actually anti photon and photon are the same thing. photons are there own anti particles
Tanmay
We theorised that all matter in our universe came from a single point which is called big bang but we don't know what exactly happened at t=0 sec
Tanmay
a point? big bang? hyper condensed matter, but even if true it was not a point, it had light, space, dimension....
Clint
that's just a theory.
Suman
what caused that infinitesimal point to get so concentrated and expand outwards , what made that happen 🤔 😳
NARUTO
ah, but u speak of God.... with God all things are possible
Clint
the matter we see in our universe came from a giant dust cloud which made most of the stars and protostar of current Sagittarius A* black hole
Tanmay
so ur point is what? .... u believe in God?
Clint
I don't believe in god
Tanmay
I would like to have a proof that God exists
Tanmay
theory of a pulsating universe is a theory, theory of big bang is a theory.,... it's up to each and every individual student to start/believe somewhere/something
Clint
Naruto we exactly don't know what happened at t=0
Tanmay
we need to believe in these theories and challenge then test them to their limits and prove enhance or disprove it
Tanmay
but a student must believe in his imagination first then others'. And try to prove his imagination. This is what we call progress.
Suman
all we can do is theorize we will never know
NARUTO
this is what happened to Newtonian mechanics we believed it tested it it's limits and then enhanced it
Tanmay
physics uses existing phenom to at best guess what in the heck it is, and what it can do... we name things with a limited vocabulary and put numbers where they fit in equations we determine to match as precisely as possible
Clint
in 10th century we never new we can ever see and work at atomic level but now we have hopes the will happen with these theories
Tanmay
those are called physical constants.... and until someone describes a vocab, and rules that's in Greek (forgive me Greece) that we try to work within..... but I'm all open to another language if u can at least match the one that we have
Clint
You should imagine a theory where other theory fails other wise it is of no good use your theory would not get recognition and would not help science to progress
Tanmay
Einstein did not develope a new language, he added a dialect
Clint
the roots of science is always asking questions and trying to solve it
NARUTO
then he showed that quantum can be reduced to classical to prove he was indeed talking the same language.... physics
Clint
This is turning to a philosophy slightly going out of topic
Murrino
points are hard to work with they are infinitesimal sphere or 1 dimensional dot? They can't have any rotation that's why string theory was born to work at that small point. These strings are easy to study and do research
Tanmay
Yes I was also thinking about it so let's converge all of the discussion
Tanmay
if string theory is true then that proves warm holes true and time traveling possible right?
NARUTO
that's right Naruto, we question first,.... but we don't always solve it... I bet if u had access to a crate, and toys u could kick
Clint
worm*
NARUTO
As per big bang theory all the matter came from a point at t=0 . Everything including time and space emerged from it. But we can't prove this theory with current technology and we also don't know what exactly happened at t=0
Tanmay
As per big bounce theory tells that universe was always there and time and again the universe goes into big bounce and the previous universe converges into a point and new universe emerge from it.
Tanmay
Can someone tell me what multi verse theory states?
Tanmay
doesn't it states that there's other universes exist and parallel universes to ours
NARUTO
that's right tam, but first work with what's there, and before u go off half cock, look at terminology, search 5x more.... when u know what they know, then u kick there butt and tell them what they are missing..... but be humble
Clint
other life forms.... maybe
Clint
we are not the only back water plant
Clint
say what do yall think cause those huge dark voids in the universe?
NARUTO
but I will tell each of u something, what grows here can grow there, what is here maybe there, but that single point (who or what ever it was) is a part of that
Clint
what do you mean by part of "that"?
Tanmay
Naruto do you mean the great void which lack a lot of stars than the other areas?
Tanmay
the sun is cooling down alot & could do with warming up again.
LAURA
something's like that maybe, the ways to know them
Dennis Reply
oh ok you can actually reply to your own comment by tapping it once
Tanmay
you should do this
Tanmay
what are laws of the universe
Dennis Reply
The law of attraction
Lambent
Are you talking about fundamental forces Dennis?
Tanmay
you can go to google and type 4 fundamental forces of the universe
Tanmay
let me give you gist of it there are 4 fundamental forces namely gravitational, electromagnetic, strong nuclear and weak nuclear
Tanmay
the came into being just after the bid bang
Tanmay
I mean big bang
Tanmay
you should watch this video for better understanding ***youtu.be/X9otDixAtFw
Tanmay
Practice Key Terms 6

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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