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Learning objectives

By the end of this section, you will be able to:

  • Explain how the gravitational interactions of many bodies can causes perturbations in their motions
  • Explain how the planet Neptune was discovered

Until now, we have considered the Sun and a planet (or a planet and one of its moons) as nothing more than a pair of bodies revolving around each other. In fact, all the planets exert gravitational forces upon one another as well. These interplanetary attractions cause slight variations from the orbits than would be expected if the gravitational forces between planets were neglected. The motion of a body that is under the gravitational influence of two or more other bodies is very complicated and can be calculated properly only with large computers. Fortunately, astronomers have such computers at their disposal in universities and government research institutes.

The interactions of many bodies

As an example, suppose you have a cluster of a thousand stars all orbiting a common center (such clusters are quite common, as we shall see in Star Clusters ). If we know the exact position of each star at any given instant, we can calculate the combined gravitational force of the entire group on any one member of the cluster. Knowing the force on the star in question, we can therefore find how it will accelerate. If we know how it was moving to begin with, we can then calculate how it will move in the next instant of time, thus tracking its motion.

However, the problem is complicated by the fact that the other stars are also moving and thus changing the effect they will have on our star. Therefore, we must simultaneously calculate the acceleration of each star produced by the combination of the gravitational attractions of all the others in order to track the motions of all of them, and hence of any one. Such complex calculations have been carried out with modern computers to track the evolution of hypothetical clusters of stars with up to a million members ( [link] ).

Modern computing power.

Photograph of the rows of supercomputers at NASA’s Ames Research Center.
These supercomputers at NASA’s Ames Research Center are capable of tracking the motions of more than a million objects under their mutual gravitation. (credit: NASA Ames Research Center/Tom Trower)

Within the solar system, the problem of computing the orbits of planets and spacecraft is somewhat simpler. We have seen that Kepler’s laws, which do not take into account the gravitational effects of the other planets on an orbit, really work quite well. This is because these additional influences are very small in comparison with the dominant gravitational attraction of the Sun. Under such circumstances, it is possible to treat the effects of other bodies as small perturbations (or disturbances). During the eighteenth and nineteenth centuries, mathematicians developed many elegant techniques for calculating perturbations, permitting them to predict very precisely the positions of the planets. Such calculations eventually led to the prediction and discovery of a new planet in 1846.

The discovery of neptune

The discovery of the eighth planet, Neptune , was one of the high points in the development of gravitational theory. In 1781, William Herschel , a musician and amateur astronomer, accidentally discovered the seventh planet, Uranus . It happens that Uranus had been observed a century before, but in none of those earlier sightings was it recognized as a planet; rather, it was simply recorded as a star. Herschel’s discovery showed that there could be planets in the solar system too dim to be visible to the unaided eye, but ready to be discovered with a telescope if we just knew where to look.

Questions & Answers

what is the cause of blackhole
DazedHotdog Reply
a collapsed massive star
donot
what is the difference between An Astronomer and a Cosmologist?
Mr Reply
cosmologist only learn cosmology right. an astronomer is learn all about astronomy, including cosmology
donot
Cosmology Is The Study Of The Universe As A Whole, Origin And Development Of The Universe.....
Adam
astronomy include cosmology, astrophysics, astrodynamic and astrometry
donot
what is astronomy
Joecy Reply
A Branch Of Science That Deals With Celestial Objects And Space.....
Adam
Is the space you had mention is about the outer space?
Joecy
Yes Outer Space.....
Adam
astronomy is the study of the universe beyond the borders of the planet.
rizza
what object can protect my pyramid being affected through doorways and interaction with others?
Christian Reply
what is big bang
Nitesh Reply
This is the blast of extremely huge sphere contains some unique particles from which our lovely universe begins
Aditya
Can we say that there is constant gravitational force in outer space instance of 0 gravity
Aditya Reply
i think garvity is not constat in every place in space,depends on the cosmic that u r near for...
Israel
I think I am right if we imagine a universe in a completely circular sphere or as an atom of some unknown element ... ....
Aditya
if you think this then think about them who use material made up of that
Ashish
element
Ashish
This is not so interesting
Aditya
I also think our universe as complex as our brain and as a computer program
Aditya
yes bro we have not that powers or that is a different dimension which a human brain does not think yet but may be possible after some time
Ashish
a group of seven stars is called
raju Reply
saptarishi
Suresh
The Pleiades.....
Adam
Why our earth or other celestial bodies is approx or completely spherically bounded why this is not square ,rectangle..... .I want to know that universal force which bound them
Aditya Reply
The most stable 3 dimensional shape which keeps the spinning material of that celestial body together in an equilibrium is a sphere. So the shapes are spheres and not squares or something different.
Ujwala
gravity. this force favors spheres and spheroids because spheres allow gravity to act equally in all directions whereas a rectangle or square wouldn't.
Mark
what do you mean by flint glass?
Suresh
which glass is called flint glass?
Suresh
who said Flint glass?
Mark
Stars, planets and moons can be made of gas, ice or rock. Get enough mass in one area, and it’s going to pull all that stuff into a roughly spherical shape. Less massive objects, such as asteroids, comets, and smaller moons have less gravity, so they may not pull into perfect spheres.
Kevin
the mass needed goes by a funny name: the potato radius
Mark
where you heared
Ashish Reply
what?
Grisha
i mean when u enter to a black hole it will take u a billions of light-years away...
Israel
yes because time slows down in more gravity
Ashish
and they said they are not always black
Israel
i don't understand this
Ashish
what do wanna say
Ashish
are black holes always black? what if there is a black hole with a blue color...
Israel
I guess black hole is called black hole because it was sucking matters that around them, including dark matter.
donot
what do u mean by "dark matter"
Israel
How man.. We don't even know what's inside the Black hole.. Just know that even light can't overcome against the gravity of BH.. So BH don't reflect any type of light..
Pavan
in universe black is not colour like other colours you can't change colour of universe because it is its nature given colour if light reflects and then come to our eyes then we saw the colour but a black hole absorbs light and never reflects it back
Ashish
are you never heard of it? well, when you look at space it never illuminated by star's lights right. space itself always look black or you can say dark although there more than zillion stars around
donot
ok then what happens to the materials which are trapped in BH when it dies
Israel
that is why scientist named them by dark matter
donot
Just quit knows about the event horizon,,,.. We don't know that what's beyond the Singularity of BH.. If any others BH singularity joins other singularity then possible well.. Don't know
Pavan
oky thanks guys...
Israel
what begin the black hole? or what is the original form of black hole? its a exploded star and black hole is just making a new star by collecting matters around them If they die, well they just became a material
donot
in our universe real and natural colour is black and light is a form of energy which emit photons colour is of that objects which are in 3 dimensions and when light reflects from these objects it comes back to our eyes space is not 3d so it is not our dimension
Ashish
Its just my thought, okay. if you wanted to know more ask some professor or search it or read a book about BH
donot
i cant know more about it becaus i had just passed my 12th class
Ashish
Well even scientists are still researching about universe we cannot sure yet, if we don't prove it
donot
its my own thinking i can't prove this now but may be in future
Ashish
really, you want to prove it too
donot
yes i wamt but at present i am a student and i have no contact to reserchers if you are then plz help me
Ashish
am also a student in 11th grade...do u some advice for me...
Israel
from which country you are
Ashish
How much u got in 12?
Pavan
from Ethiopia
Israel
i got 80 percent
Ashish
i am from india so i cant know about your country
Ashish
astronomy is world wide..ok which subjects should i pay more attention?
Israel
physics >maths >chemistry
Ashish
ok then thanks again.. stay safe..
Israel
which acid is also called chamber acid?
Suresh
sulfuric because a lead chamber was used to manufacture and process it.
Mark
thank u very much.
Suresh
i heard that black holes are not always black and they can take u to another coordinate...is this true?
Israel Reply
They have no color, and no
Cyclone
black holes have no hair.
hair?... what do you mean by that..
Israel
***en.wikipedia.org/wiki/No-hair_theorem
Black holes size gets increase by Eating massive stars and black hole are made from high massive stars who were exploded in supernova.when a black hole ears a star A quasar is formed thus a black hole is not always black
Mr
what is hisboson particle
Aditya Reply
Higgs Boson Is An Elementary Particle In The Standard Model Of Particle Physics, Permeates The Higgs Field Which Gives All Particles Thier Mass.....
Adam
Simply.. A god particle..
Pavan
The God Particle Is A Little Miss Leading.....
Adam
what you mean by a god particle?
vasu
God Particle Was A Nick Name They Gave It When They Had No Clue What It Was Or Did.....
Adam
so they know what it is now?
James
A field call higgs field is one of the field which are present in vacuum, just like electric, quark and gravitational fields.. So basically befor interaction of photos to this field, all are mass less.. But after interaction, electrons get some mass.. Just like a quack.. But there is very less
Pavan
Yes, It Has Been Confirmed By All The Physicists And Engineers At Cern, Large Haldron Collider.....
Adam
Interaction between field and hisboson.. So they remain mass less..
Pavan
Simply call.. God Particles.. 😊
Pavan
why space is black?
Akash Reply
Hubble's Law, the Doppler Effect, and the lack of objects to reflect light
Cody
read on Olber's paradox
Kevin
because it night there man
Grisha
Olber's Paradox.....
Adam
Dark matter!
Pavan
what is the best book in order to study astronomy.
jawad
personally i don't use a book, but any book is good
daniel
I know it sounds funny but I picked up a few books including Astronomy For Dummies. I also got a couple from Neil Tyson, he has some simple into books.
josh
tell me any lectures that are available on YouTube for astronomy as a beginner.
jawad
Crash course- Astrophysics. that's the beginner course on YouTube.
Kevin
And anything from the brilliant Director of the Hayden Planetarium, Neil deGrasse Tyson
Bill
there is alit on you tube luke bill said NGT has alot of into stuff. Brian Green is good at makin tv thing simple and when your ready Lawernece Krauss is good too.
josh
Amy Mainzer is especially good.
Because Space is Not a Object light can't reflected but it.light simply passes thoughtout the space and about 90%of the space is Dark because their is No star or quasar to Emit light
Mr
could get a suggestion to become a good astronomer
Monisha
monisha sure
Mr
Well You should be curious about space and Astronomy and then you would must become a good Astronomer
Mr
oh thank you
Monisha
you are very welcome
Mr
can i know which course i'll choose to study in higher studies
Monisha
well it requires pHd in the field of Astronomy which would take 5-6 years to complete it
Mr
hello I am Piyush from india, I am a student of class 8th and I want to become an astronomer in future. I am confused that what course I do in college to become an astronomer. please answer me
Piyush Reply
There are many courses, and they all depend on where you are, and what college or university you go to. I hope this helps!
Cody
Thank you sir
Piyush
Math, Geometry,Trigonometry Calculus, Algebra.....
Adam
don't worry bro you would become an Astronomer in NASA
Mr
what is cosmos
SÍDDHÁRTH Reply
a space active evaluation to other sorts of things
Olivia
Cosmos is the whole space of everything happening or destroying creating cosmology which is the study of the whole Cosmos deals with the happening inside the universe but not of astrophysical bodies but of the beginning and the end of the universe
Amritansh
what do people do when sleeping
Olivia Reply
people when sleep they actually shutdown there conscious brain but the subconscious brain is it still on so that is the reason why you see dreams and suddenly wake up when you feel thirsty
Amritansh
really?
rizza
how to control your emotions/feelings over special someone?
rizza
there is no such trick honey. you just gotta distract yourself with better things.
Suraksha
yup crct
Amritansh
and always remember that you are enough. you don't need somebody else to make you feel important and loved
Suraksha
are there really twenty four hours in a day
Olivia
I heard that there are 36 hrs in a day we just think there are 24
Olivia
hey I GUESS our home clocks show 24😂😂
Amritansh
hey suraksha I didn't get what you said about imporatnce
Amritansh
cool anyway what happens when a black hole dies
Olivia
it becomes a white hole
Subhash
really I thought it sucked up it's self
Olivia
How does a black whole die?
basim
black holes just evaporate overtime through a process called Hawking radiation
Subhash
when there is nothing left for it to suck up it eats it self then there is nothing in space anymore
Olivia
Amritansh what I meant was, that you are important even though others might not show it to be so. as an individual you are valued and important
Suraksha
why do peoke love
Olivia
people
Olivia
it's just a natural feeling of bondage as well as possession. we humans call it "love" because that makes us feel more connected rather than just saying it's because we can't function without one another
Suraksha
yes I am completely agreed with what Subhas said actually what happens is that a black hole has a limited lifetime and it dies due to the evaporation and emission of Hawking's radiation but as a normal astrophysical black hole the lifetime is much longer than the current age of our universe
Amritansh
thank you Suraksha for your explanation
Amritansh
can anyone answer the question what are quasars
Amritansh

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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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