Where Is the Sun Located in the Milky Way? | Hacker News Reader

Where Is the Sun Located in the Milky Way?

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222 pointsSantosh83100 comments

Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue green planet whose ape-descended life forms are so amazingly primitive that they still think digital watches are a pretty neat idea.

Wouldn’t it be wondrous if the galaxy had plentiful intragalactic public transit and we don’t have access because we’re the planetary equivalent of the suburbs?

We just need to build a station in the neighborhood: https://www.technologyreview.com/s/613127/a-halo-drive-could...

This is kind of a plot element in Mass Effect, but the humans are lucky enough to have a stop in our system.

What do you mean the humans are lucky enough to have a stop in _our_ system?!

I think the Reapers may have arrived 167 years too early.

Come on giant space Cthulhus, we haven't even discovered the Prothean Ruins in Mars yet!

Did New Horizons categorically rule out that there's something hidded inside Charon?

my pet theory is that galaxies are more like fireworks wheels, and so we aren't strictly orbiting the galactic center, we're also slowly getting farther and farther from it (that also takes care of the dark matter conundrum in my view), and thus couple billions of years ago we were in the much better and more populated part of the town than the current 'burbs.

You are not very far from what could be the reality. According to some research the Solar System has 10% less hydrogen than other galaxies, so Bussard ramjets will have an harder time

What if dark matter turns out the key to a decent space drive? There appear to be galaxies without it (a good reason for believing it exists where there is evidence) and so any aliens there would be SOL.

Astonishingly enough, you might not be far off. That is the operating principle behind the Alcubierre Drive. https://en.wikipedia.org/wiki/Alcubierre_drive

Edit: Apologies, it seems it does not in fact use dark matter.

The Alcubierre drive relies on negative matter, not dark matter.

Unfortunately, Alcubierre doesn't use dark matter. It uses negative mass (or even imaginary mass) matter. Dark Matter is believed to have regular positive mass (if it were negative mass, we'd be wondering why galaxies seemed to have less matter than they should).

The existence of negative mass exotic matter remains an open question.

No doubt stolen by the untrustworthy Andromedites!

All the planning charts and demolition orders for it have been on display in the local planning department in Alpha Centauri for fifty Earth years

...in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door...

What do you mean you’ve never been to Alpha Centauri? Oh for heaven sake mankind it’s only four light years away! I’m sorry but if you can’t be bothered to take an interest in local affairs that’s your own regard.

It’s just going to be a bunch of NIMLICs at the meeting anyway (Not In My Local Interstellar Cloud).

'beware of the leopard'.

Or the wilderness

It's unlikely. Closer to the core there is too much radiation for life to be possible.

And farther away there are not enough metals.

As best as we can tell we are at the distance where all other life would be.

Suburbs intentionally avoid mass transit.

You are thinking of... a ghetto.

This is probably a really silly question, but how do astronomers and the like define "west" in this case? I assume it must be useful to have some sort of reference orientation for the galaxy, but how is it defined?

The directions are defined the same way as directions on the Earth or in the solar system using the right-hand rule to define north and east based on counter-clockwise rotation.

Ah, I forgot that the galaxy rotates as well. That definitely makes things a bit simpler.

It's a quote from The Hitchhiker's Guide to the Galaxy. Only hit me at the very end of it... "digital watches are a pretty neat idea".

West can be defined in terms of spin direction, but western ( as in hemisphere) requires a meridian to define the boundary. I'd have assumed that we'd have put Earth on the galactic meridian, much like the Brits put London on the terrestrial one.

I expect it goes through Greenwich, as always!

Okay, so once you have north, which way is east in a circle?

East on Earth is the direction in which we rotate. Should be the same for a galaxy.

That's because earth can be viewed as a plane. The galaxy can't, you need a median.

1. Milky Way is much more like a plane than Earth

2. it doesn't matter anyway any object which rotates can have direction named basing on that rotation

You are correct, it is determined by spin. However, for some reason that has never been explained to me, we define galaxies to spin in the opposite direction from planets.

I never thought about this. Will most solar systems form roughly on the same plane as the galactic disk, and will they generally have orbits in the same or the opposite direction as the galactic rotation of their star?

It's basically random; the rotation of a solar system depends on the rotation of the gas clump that collapses to form the star and its system, and that depends on local details about the relative orientation of local gas clumps, the possible influence of winds from nearby massive stars and supernova shock waves, and general turbulent motions within molecular clouds.

It's not very random. The galaxy also formed from a clump of rotating gas, and this heavily influences the direction all of the sub-bodies will rotate and orbit. Systems influenced by other forces would be the exception to the norm.

No, it is pretty random, just as the orbits of binary stars are randomly oriented. The scales of star and solar-system formation are much, much smaller than the scale of the galaxy, so the memory of the latter has been scrambled by all the intermediate and local processes.

From this Cornell "Ask an Astronomer" page (http://curious.astro.cornell.edu/about-us/159-our-solar-syst...)

"The size of a solar system is so much smaller than the size of the Galaxy, that the Galaxy's structure has no impact on the orientation of a solar system. What determines their orientations is the direction of the angular momentum that the system had when it formed, and that's pretty much random."

Or take a look at these ALMA images of protoplanetary disks (solar systems in the making) around pre-main-sequence stars. All are at very similar distances, and most are at very similar positions relative to the Galactic plane, so they should all show very similar projected shapes and orientations if they followed the galaxy's orientation -- but they are all over the place.

https://www.eso.org/public/images/potw1904a/

We don't define galaxies in general to spin in the opposite direction.

Just the Milky Way.

(The reason for this seems to be that Galactic "North" was defined long before we had the data to determine which way the Milky Way rotated -- or even that it rotated. By the time the rotation was definitively determined, it was too late to override convention. This may go back to William Herschel deciding that the "north pole" of the Galaxy should be in the same hemisphere of the sky that the Earth's north pole pointed toward. In retrospect, he had a 50% chance of being right...)

You can try Polaris

They don't, AFAIK. You can only get away with it if you're writing exceptionally goofy science fiction.

It's more like spinwards and widdershins, really.

Spinwise/turnwise, and rimwards :)

They use giant supercomputers to do simulations which results in these counter intuitive answers. The trick is to ask the right question.

digital watches are a pretty neat idea. they're vastly more accurate than mechanical watches while using incredibly little energy.

Just bought a Casio digital watch this morning because it has a countdown timer you can adjust up and down. I'm a happy ape right now.

Also the Galaxy song is funny and mostly accurate

http://www.montypython.net/scripts/galaxy.php

It makes me sad how many of the replies here seem to have completely missed the reference. The fact that it's upvoted to the top gives me some hope, at least.

Slightly offtopic, but what extraordinarily well done alt text for the major images on this page. An example:

> "The most current map of the Milky Way is shown in an artist’s representation. The Sun is directly below the galactic center, near the Orion Spur. The Scutum-Centaurus arms sweeps out to the right and above, going behind the center to the far side."

They aren't quite following best practices with attribute usage, a screenreader may read both the alt and the title text for example and they're the same here, but to have the level of detail is really commendable.

One of the local programming forums I frequented was also frequented by a blind guy who had an audio web device. His most frustrating experiences were sites that put the same thing in the title and the alt text. Unfortunately his device always had to read both, because so few websites actually followed the correct practices.

It's also a really good example of how captions (this is a caption really, not exactly alt text) are context-dependent.

The caption you mention continues:

> The maser observed is almost directly opposite the Sun from the center in the S-C arm, 65,000 light years away.

...which makes no sense (there's no maser visible in the image or mentioned in the article) until you realise that this image was uploaded for another article and is being reused here.

Evidently the website's content management system asks for caption text when uploading the image, rather than when selecting it for use in the article. The latter is the right way to do it.

Its also a great time to replay the Contact Opening Scene https://www.youtube.com/watch?v=EWwhQB3TKXA which has to be one of the greatest scenes in history.

Related Abstruse Goose cartoon.

https://abstrusegoose.com/163

http://www.lightyear.fm was inspired by the opening scene of Contact

Off topic, but after my daughter was born I realized how sad and worried I was that she wouldn't be familiar with looking up at the sky on a clear night and seeing the Milky Way. Stuff like what's in this article is much more intuitive if it's part of your schema of the night sky; it's more difficult to appreciate when it's invisible.

She's too young to appreciate it now when I point it out to her when we have the opportunity, and I worry that when she's old enough to appreciate it she'll be someplace she can't see it. For me it was a common thing up until I was a bit older, and now it's uncommon to be in the right place at the right time to see it. I wonder if the unlit night sky will become something like old-growth forests or other ecological sights that are long gone.

Go to the Southern Hemisphere, the stars are still epic there.

Story Time: I'm from Australia, but didn't got for almost 10 years. I lived in the Yukon, roamed all of Alaska, etc. etc.

When I got to Australia after 10 years I walked outside in the middle of a city of 50,000 people and almost fell over the stars were so bright and colourful and (seemingly) close. They were so good, in fact, I took photos right there in the middle of that city that are better star photos than anything I have ever taken in Yukon/Alaska.

If you're still in doubt, go to Australia!

Can confirm that the Southern sky is great from my time in South Africa. We had a nicer view of the milky way during many braais in a small town backyard than I have seen in the Northern hemisphere (including back country camping in Yellowstone National Park). The combination of high elevation, dry air and little light pollution is great.

So yes, when I lived in the Europe one of the joys of coming back to Oz for holidays was that you can actually see stars. Even inside a big city, the nights tend to be clearer than in Europe and the light "pollution" more limited.

That said, I've never seen the Milky Way.

I probably could if I went out somewhere far from the city, but I can't remember ever actually seeing it, even when doing star-gazing at school camp.

> That said, I've never seen the Milky Way.

I'm really shocked to hear that. I feel like it's a certainty any night in Oz when it's not cloudy.

I'm 30 and until a few years back I thought the pictures with the milky way showing were fake.

My most breathtaking glimpse of the world: Stopping one night while hitching through the Nullarbor, and walking to the coast very close by. Pitch black, but then not quite, for all over the sky were insanely more and brighter stars than I have ever seen before or since. And then the crashing waves at the foot of the cliffs, and wind roaring in from the south, with the just the right amount of arctic chill to make it awesome and remind you that across that ocean lies Antarctica. That's more than thirty years ago, but words about 'the end of the world' still take me back.

The most incredible night sky I've seen is on the top of Mt John (there's a small observatory) on the southern island of New Zealand. Mauna Kea wasn't close.

You could see the band of the milky away across the sky. It was like something from Hubble.

There's nothing like looking up in the sky and getting some perspective.

I could very clearly see the bands of the Milky Way from the summit of Haleakala. My wife initially thought it was a cloud.

Interesting. I think I've heard that before somewhere else. I'll have to ask one of my colleagues about it.

Where my family is from, in the northern US, the night sky is very clear and visible (barring clouds etc which aren't uncommon for various reasons). But where we live you can barely see much other than the moon and planets. I wish more attention were paid to light pollution.

I never really saw it, until I was about 18 and spent a weekend in the Swiss Alps. That was in the mid-'80s. I'd seen it as a dim smear before, but not in its full glory. It looked like the pictures. I wished I could jump on a spaceship and just head out there.

I live in Switzerland and while I have seen it plenty, I, and I mean me, hadn't properly seen it in till the last few years.

By seeing it, realising that I am looking out from our planet across that galaxy plane. It was a very special evening.

As they say, you can be told things but you have to experience to know. I hope more people get to realise it.

Skies in the alps (at least french and swiss) have too much light pollution to have properly clear picture most of the time. Every valley has some village, and even if you get to 3000+ metres (or even 4000+), glows from valleys are never too far. Depends on cloud/haze situation too.

I camp up there a lot, all seasons, even have full frame camera with tripod with me for night shots but you simply can't escape the glow (at least I didn't manage so far).

But to be honest, even in himalayas 2 years ago (3 passes hike in Everest region), or on Aconcagua (6000m camp) last winter the skies were mostly just OK. Of course with 30-second exposure things come to light, but I don't like those overly-photoshopped pictures of milky way so popular these days. Very little reality, too much painting with brushes with regard to color representation.

I'm from the inner city. Growing up, I always recalled just seeing a twinkle here and there in the night sky.

However, my mother's side of the family is from the deep south. I remember visiting relatives, and one time staring at the night sky. There were so many "things up there" I was both in awe and a little bit frightened.

Indeed! It is that sight, the infinity of stars and space, that has probably been the greatest creative inspiration for humans ever since we gained sentience. The reminder and assurance that there is more to reality than the artifice of society and its petty concerns we've cocooned ourselves in.

With the stars gone from the skies in cities and their surroundings it is truly a dystopia, where humans will live their lives out thinking that our cities are all there is.

I can see it most clear nights, walking out my back door. But I live in rural Iowa.

When I was a kid, in the back of a sci-fi magazine I saw a t-shirt advertised, with a picture of the galaxy. It had an arrow pointing towards the edge saying "You are here." And another arrow pointing to the center, saying "All the action is here."

(Edited to correct/clarify)

Out of curiosity, what is the galactic centripetal acceleration for the Milky Way that the Solar System is experiencing?

Can you clarify the question? Big G is a constant everywhere. I can't think of an equivalent for little g (9 81). If you chose to define something it would depend where you are.

I meant the galactic centripetal acceleration we're experiencing (which I saw as the analog of little g, but I guess it isn't really); sorry if that was confusing.

As experienced on earth, as in during the day vs during the night? (I don’t know which side is facing galactic center but I’m sure it’s almost half the day one way and almost half the day the other way.)

I'd want it to be invariant to the time of day, so I guess as experienced by Earth's center? Or the sun's center. Just wanted a vague ballpark number, which someone already mentioned.

It's not easy to define a gravitational acceleration for the milky way since for planets it's defined on the surface and the milky way has no clear boundary per se.

That said, the escape velocity of our galaxy is 537 km/s; almost 50 times that of the earth. Most of it is due to dark matter.

https://www.newscientist.com/article/dn24249-stars-escape-ve...

Thanks! I worded the question poorly but this is also a cool number to know.

You can calculate it from the orbital speed and distance to the center of Galaxy.

It comes to something like 2*10^-11 g

Yeah I didn't have a chance when I posted the question, thanks! But wow that's small!

I thought there was some recent paper that claimed our galaxy was not in fact a spiral galaxy but more spherical than we thought?

this is very surprising, considering that you can actually see it with your own eyes at night and it is not spherical at all. There are also countless panoramic photos of the whole thing, which always look more or less the same, and there is no way it can be spherical. See for example https://www.eso.org/public/images/eso0932a/

OP might be thinking of the recent finding that the galaxy is not flat.

https://techcrunch.com/2019/08/01/the-galaxy-is-not-flat-res...

Or maybe the older discovery that the Milky Way is a barred spiral galaxy. And may not have a pronounced central bulge.

It may well be, with the vast majority of the solar mass in the visible disk, and a handful of stars too dim to easily perceive forming a sphere. The Andromeda galaxy actually takes up a sizable portion if the earth’s sky in its full extent, we just don’t see it.

Is there any working directory of stars of the Milky Way with [x,y,z, t] coordinates ? a galaxy class coordinate system ?

I've seen databases of nearby stars, with just the 3D coords. Biggest I think had 100,000 stars.

I don't think we have detailed data for the 100 million stars of the entire Milky Way.

There’s at least 100 billion (not million). Maybe as many as 800 billion. We can’t see most of them.

Oops, of course you're right.

If you're looking for a dataset, the GAIA project public data releases[1] are probably the best you'll find.

[1] https://www.cosmos.esa.int/web/gaia/dr2

I'm assuming something like the Gaia dataset was used to create this representation of stars near us?

https://stars.chromeexperiments.com/

> But where is it vertically in the cake? Right in the center of that frosting layer, or off by a bit up or down?

Did the author answer these questions? How far above the center of the cake is 55 light years?

He mentions that the height is 2,000 light years, so the deviation from the center is 55 ÷ 1000 = 5.5% (+- 0.16%)

I had no clue that the Sun's orbit looked like that. Bobbing up and down relative to the galactic plane. More or less in a sine wave.

so the Sun has both a vertical orbit as well as a horizontal one, with the vertical orbit in 1/4 phase with the horizontal orbit. interesting.

The period of the vertical orbit is closer to 1/3 that of the planar orbit: roughly 70 million years, versus about 225 million years for the azimuthal ("horizontal") period. Since the periods aren't in an integer ratio (and you wouldn't generally expect them to be for galactic orbits), it's also not neatly closed like the schematic suggests.

Is the galactic center moving and if so how?

Relative to other galaxies? Yes, in fact I believe there’s a galaxy or two on a collision course.

AFAIK we (Milky way) are on a collision course with Andromeda.

Sol (our sun) is located between the Sagittarius and Perseus arms of the Milky Way spiral galaxy. Specifically, our home is located on the Orion Arm.

The article is about how much above or below the plane, specifically the central plane of the galaxy we are. Spoiler: We're currently above (north) of the central galactic plane by tens of light years, and still traveling upwards, but it's cyclical and we'll be below the plane again in a few million years.

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