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Elliptical Orbit

  1. Aug 29, 2015 #1

    I'm new to this forums (and please forgive my grammer and sentence structure as my first language is not English and I'm trying my best :) I'm just a teenager who has great interest in these types of stuff, never did any proper education relating to this, just some research out of curiosity and interest)

    Question: I've been wondering, do our sun orbits in a circular or elliptical manner around milky way. If it is elliptical, at a point where it is the closest to Sagittarius A* will our sun loses its velocity and gets "sucked" into the supermassive blackhole? (Senario 1) or will we just spun out like the rest of the stars and continue orbiting (Scenario 2). From what I understand from wiki as well as other sources, it would take around 200 million years to complete 1 orbit around milky way. Are we halfway there? or are we still far from it.

    Scenario 2: If we are very close to the BH, will that outwardly huge amount of gravity affects us? Gravitational Time Dilation? and due to the immerse speed where we are being thrown out the BH, by any chance will those planets such as Neptune, Uranus that is far away from our sun's gravity break free from our solar system and become a rogue planet?

    Just a note..I might get some of my understandings wrong..please correct me, I really want learn more. Thanks!!!
    Last edited: Aug 29, 2015
  2. jcsd
  3. Aug 29, 2015 #2


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    Gold Member

    Hi iLeo, welcome to PF!

    The Sun doesn't get appreciably closer to the centre of the Galaxy as it orbits. The orbit is not really circular, and it's got some serious wobble in the direction normal to the galactic plane, but it can be approximated to a circle with a radius equal to about 1/3rd of the galactic radius for all purposes not in need of great accuracy.
    This is true of most matter in any galaxy - on average, it tends to stay roughly at the same distance from the centre once the galaxy has formed, and barring collisions with other galaxies.

    Even if the Sun did get closer, the mere fact that it would do so would not result in it being sucked in by the black hole. Without some means to shed the velocity, any object in orbit around any other object (including black holes) will just swing by and return to where it started from - distance-wise. This is similar to how comets behave in the Solar System - they come closer, swing by the Sun and get back to the far reaches of the system. (unless they evaporate along the way, or manage to hit the Sun or a planet, which are very rare events).
    Black holes are especially easy to swing-by, since they're very compact, so it's hard to 'hit' them. It would take a lot of energy to slow down the Sun enough to make it get anywhere near the centre.

    In space, the only feasible way of shedding velocity is to interact with some third body. E.g., a close encounter with another star could send one closer in and eject the other one further out. But since galaxies are mostly empty space, and distances between stars are large, such encounters happen extremely rarely, so stars keep orbiting at roughly constant distances.

    One would also need to get really close to the black hole before any noticeable time dilation effects come into play. The central black hole, despite being estimated at (iirc) some 1 million solar masses, has exactly the same gravitational effect on objects 1000 light-years away* from it as the Sun has on objects 1 light-year away.
    It's not really the black hole that the Sun orbits - it's the combined mass of all the stars contained in-between our orbit and the centre, and that there is a black hole in there at all matters little as far as the orbit of our system is concerned.

    *We're currently orbiting at about 30 000 light-years from the centre.
  4. Aug 29, 2015 #3
    Thanks for the reply! Was really fasinated about the event horizon and blackholes . Some say that blackholes are not a vacuum cleaner and does not literally "suck" anything including light, it is just that the huge amount of gravitational force beyond the escape zone making the escape velocity faster than the speed of light (am i getting this right? Or it is impossible for anything to go beyond the speed of light) hence making it red shifted instead of being sucked in. Objects would seemed frozen moments before it crosses the horizon.
    So is it true that BH is an event rather than an object? Apologies if this goes off topic or i could do more research and start from the basics (are there any reliable sources which includes all the methods and calculations used by astrophysicist? All the laws and rules) . Thanks for your reply! Really appreciate it!
  5. Aug 29, 2015 #4
    I'm interested in this topic, too, and perhaps have watched too many documentaries on the topic. They make it sound like super massive black holes vacuum up everything in a hurry.
  6. Aug 29, 2015 #5


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    Staff: Mentor

    Yep. That they do! It makes black holes sound exciting, which makes people want to watch the show.
  7. Aug 30, 2015 #6
    Thanks for the replies guys! I've always been fasinated by these wonderful creation ever since I was a child..I almost lost hope as I could not get into a degree course relating to this. Almost all of the universities in Singapore where I lived in have little or no courses related to this other than aerospace which I already had a certificate for it. Until recently i found out that a school is offering astrophysic course, so I wanted to learn as much in case there is an interview..Thanks alot guys!
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