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Watsisname
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24 May 2018 20:40

"Quasar" or "quasi-stellar object" is kind of an older term referring to the first examples of these objects being discovered.  They looked like very small (star-like) points on the sky, yet having a distinctly non star-like spectrum.

We now know of course that a quasar is the central region of a galaxy, powered by an especially active accretion disk around the supermassive black hole.  But this activity is not unique to the most distant galaxies, or to quasars.  Blazars, quasars, radio galaxies and Seyferts are all powered in the exact same way.  In general we refer to these as "active galaxies", with an "active galactic nucleus" or AGN.  It wasn't realized until fairly recently that all these different things are actually the same kind of object.  The difference in what we see and classify them as just depends on our viewing angle:

Image

In other words, different types of active galaxies are the same things as quasars.  Or, quasars are AGN seen at a particular viewing angle.


So we can ask, "Was the Milky Way ever an active galaxy?"  Definitely!  We do see the evidence left behind from previous activity like you mention with the fast-moving bubbles and shocks.  I think it is unlikely that this event was so luminous that it would dominate over all the starlight of the galaxy, but astronomers in nearby galaxies would be able to tell.

Another near certainty is that it will become active again in the future, especially with the merger with M31!


Also, are these "bubbles" of pressure created by the gas responsible for the creation of arms in a spiral galaxy? Kind of like tree-rings, multiple arms might mean multiple Quasar stages or periods of time?


No, the effect of these ejections is mostly to remove gas from the galactic center, and especially to blow it out in polar outflows.  This mechanism is very important for regulating the black hole's growth, and also plays a part in a dynamical relationship between the black hole's mass and the properties of the galactic bulge, like the M-Sigma relation.  But the gas in these ejections is too thin and lacks the oomph to be able to set up the spiral structure throughout the disk.

The spiral arms are instead generated by density waves, which move slower than the material of the disk itself.  As dust and gas pass through a density wave they get compressed, triggering star formation and the arm structure we observe.  (This is a very good model for "grand design" spirals).  In computer simulations we also sometimes see the star formation process itself become self-propagating and lead to spiral structure.  Gravitational perturbations from mergers or neighboring galaxies can also play a role.

So in short there are a lot of mechanisms behind spiral formation, and nature probably uses a combination of them.  But the central black hole probably has little to do with it, even though it does have significant and surprising influences on other large features of the galaxy. :)
 
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Julian
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31 May 2018 18:22

Watsisname wrote:
The difference in what we see and classify them as just depends on our viewing angle:

Why is one hemisphere of the black hole radio-loud and the other radio-quiet?
In an unrelated question, do all Earthlike planets orbiting G-class stars have to have green plants?
 
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Watsisname
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31 May 2018 22:10

Julian wrote:
Source of the post Why is one hemisphere of the black hole radio-loud and the other radio-quiet?

Ah, that was a failure in my explanation earlier.  One hemisphere of a particular AGN doesn't look much different than the other (presumably).  Rather, the graphic is showing simultaneously the effect of viewing angle on AGN which are either radio loud or radio quiet.

Radio loud AGN have emission dominated by powerful relativistic jets, and appear to be more common in galaxies involved in mergers.  Radio quiet AGN have more quiescent jets and appear to be more common in isolated galaxies.  There is possibly a lot more to in than that in terms of the detailed physics happening around the black hole and this is actually a very big research question.

Julian wrote:
Source of the post In an unrelated question, do all Earthlike planets orbiting G-class stars have to have green plants?

This is a fascinating subject and you might enjoy checking out this thread from the old forum. :)

I think it is far from certain that all plants on planets orbiting G stars must be based on green chlorophyl, but there is good physics for why it is the most efficient choice, and how the colors may be different around other temperature stars.
 
 
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01 Jun 2018 05:41

Watsisname wrote:
Julian wrote:
Source of the post In an unrelated question, do all Earthlike planets orbiting G-class stars have to have green plants?

This is a fascinating subject and you might enjoy checking out this thread from the old forum. :)

I think it is far from certain that all plants on planets orbiting G stars must be based on green chlorophyl, but there is good physics for why it is the most efficient choice, and how the colors may be different around other temperature stars.
 

Earth may have been purple: linkwiki, but the wiki page is severely behind in my opinion.
 
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Julian
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03 Jun 2018 15:54

The period of a pulsar's signal is its precession period, not its rotation period, right? Do we have any way of determining what the real rotation period of a neutron star is?
 
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Watsisname
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03 Jun 2018 18:13

The period of a pulsar signal is its rotation period.  What's happening is that, like with Earth, the magnetic axis of a neutron star is not necessarily aligned with the spin axis, so as the neutron star rotates the magnetic axis may sweep past us once per rotation if the alignment is right.

Image

If a pulsar precesses, then what we'll see are periodic changes in the pulse timing.  There is evidence that this happens with some pulsars.  It can be caused by either the shape of the star (misalignment of the symmetry axis with spin axis), or by interaction with a neighboring star.  However, the effect is slow -- in most cases the precession has a timescale of months to years.

There is an excellent thesis about pulsar precession (especially the free precession of isolated pulsars) here if you're interested in reading more. :)
 
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04 Jun 2018 10:18

Do astrophysical jets follow the rotation axis or the magnetic axis? Can there be instances where the jets aren't perpendicular to the accretion disc? 
 
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FastFourierTransform
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04 Jun 2018 13:50

Julian wrote:
Source of the post Do astrophysical jets follow the rotation axis or the magnetic axis?

The jets follow the magnetic axis. Since the magnetic axis can be missaligned with the rotation axis the jets are constantly sweeping around. The pulses you see in a pulsar are the beams passing in the direction of Earth, like a lighthouse. If the jets were to follow the rotational axis then this wouldn't happen, the jets would be pointing constantly in a certain direction and only precession would account for the pulses you would see from Earth (pulses in timescales of months or years and not in millisenconds as was explained by Whatsisname). By the way, Whatsisname's diagram explicitly answers your question, look which axis the jets follow.

Julian wrote:
Source of the post Can there be instances where the jets aren't perpendicular to the accretion disc?

Yes they can. For high inclinations is a completely different regime of accretion (a much more violent one) but they can. Have a look at page 15 of this article were they show a nearly 45º inclination between a neutron star's magnetic field and the accretion disk.
 
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Julian
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04 Jun 2018 21:34

FastFourierTransform wrote:
By the way, Whatsisname's diagram explicitly answers your question, look which axis the jets follow.

I wanted to make sure that the areas of radio emission always coincide with the jets, but thanks for the explanation. I can understand why Vladimir hasn't been able to implement astrophysical jets in SE yet, because there are certainly a lot of issues to work out, like how to model the distortion in the accretion disc when the jets are off-axis, and ensuring that the magnetic axes of known pulsars are oriented so that their jets can point toward Earth.
 
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Propulsion Disk
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05 Jun 2018 14:39

Has the planet Tyche been verified? Has it been unverified? Or are we still not sure.
I'm good when it comes to Physics, Algebra, Relativity, Space, and SpaceEngine. But I could still use a LOT of help on the things I still don't know. So I hope I get a lot of help on how all that works, here!
 
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FastFourierTransform
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05 Jun 2018 15:06

Propulsion Disk wrote:
Source of the post Has the planet Tyche been verified? Has it been unverified? Or are we still not sure.

The hypothesis of the existence of Tyche was never a very rigorous one in scientific terms. No data provided so far points to its existence at all. And currently, since 2014, it's a rejected statement inside the scientific community as WISE data should have shown direct evidence of it.

There is an interesting and wonderfull series of mini-documentaries in youtube that shows the entire history of this. Do not confuse Planet X (rejected), Tyche (rejected by science and still part of new-age pseudoscientific speculation), Hercolubus (never proposed in science but still part of pseudoscientific discussion), Nibiru (never proposed in science but invented to make astrology fit some esoteric design) and Planet 9 (compeling evidence is been gathered since the last year and it is expected to be directly imaged in the next decade) hypothesis.



The entire series is 4 hours long but is very rigorous, educational and entertaining at the same time. I recommend all of you to take a look. I got addicted to this guy since then. Understanding the history of astrophysics is understanding why certain decisions were made and that in the end allows a more pure understanding of the topics.
 
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08 Jun 2018 10:26

Does anyone know a good website to research space engineering and other science?
I'm good when it comes to Physics, Algebra, Relativity, Space, and SpaceEngine. But I could still use a LOT of help on the things I still don't know. So I hope I get a lot of help on how all that works, here!
 
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Watsisname
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10 Jun 2018 06:22

I have not used it myself, but I have been told that brilliant.org is very good.
 
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Propulsion Disk
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10 Jun 2018 06:30

Watsisname wrote:
Source of the post I have not used it myself, but I have been told that brilliant.org is very good.

But my computer can't handle it so yeah, I go in, says it wants to reopen the page, and then it fails. :(
I'm good when it comes to Physics, Algebra, Relativity, Space, and SpaceEngine. But I could still use a LOT of help on the things I still don't know. So I hope I get a lot of help on how all that works, here!
 
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15 Jun 2018 10:33

Would a spacecraft that is aerodynamic move faster? Because if my studies are correct, space mainly has hydrogen gas in it, which means that if a spacecraft was aerodynamic it would move faster sense the hydrogen gas would flow over it, and under it, would it not?
I'm good when it comes to Physics, Algebra, Relativity, Space, and SpaceEngine. But I could still use a LOT of help on the things I still don't know. So I hope I get a lot of help on how all that works, here!

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