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I'd like to start a thread regarding any science questions you'd like to see answered that don't seem to be able to be answered with a quick Google search.

To start off, I'm looking to find a somewhat comprehensive list of celestial objects in the sphere between 5,000 and 15,000 light-years away from Earth. I've been able to easily find individual things, but no map, list, or anything.

It'd also help to narrow the search if I could find out whether everything in this range is in the Orion Arm or not. I'm not sure if it extends into things like the Perseus or Sagittarius Arms.

I looked for something like this a couple years back but was returned only magnitudes of brightness of stars. Very annoying.

AdmiralMemo: I can answer the question of whether everything with in 5 000 to 15 000 light years (ly) of the Sun is in the Orion arm quite easily. It isn't.



As seen above, in the plane of the disc of the Milky Way, a few other spiral arms are with in 15 000 ly. You almost are getting to the Galactic Bar. Additionally, the disc of the Milky is relatively thin being only a couple thousand ly thick. If you were 5 000 ly above or below the disc, you would be way out of any of the spiral arms. This brings up an important point - not all stars live in spiral arms.

Asking for a comprehensive list of celestial objects is a bit open ended. If you are interested in planets, we do not even have a complete list with in 100 ly. I am going to assume that you mean stars. The faintest stars produce thousands of times less light compared to the Sun. At a distance of 5 000 ly these stars would be about a hundred times too faint to detected by the most sensitive all sky surveys we have such as the 2MASS survey. If we restrict ourselves to stars bright enough to be easily observed at that distance, we still do not have have a comprehensive catalogue since we do not have distances to vast majority of stars available. Currently, the best survey of stellar distances that is complete, the Hipparcos survey, only contains stars out to about 1 600 ly. Directly measuring the distance to a star is quite a difficult measurement. It should be noted that we have observed and measured distances to stars that are much further away, but we have only done so for small patches of sky. The situation will improve in a few years when the Gaia survey is complete as it will measure distances out to tens of thousand of light years.

In summary, a comprehensive catalogue of objects between 5 000 and 15 000 ly does not exist because we currently have not measured distances to stars that far away in a uniform manner. However the spherical shell between 5 000 and 15 000 ly contains a lot of space outside the Orion arm.

Elomin Sha: The magnitude system is how astronomer talk about the brightness of stars. It is a bit confusing though.

And the higher the magnitude the relative dimmer the star, because why the fuck not?

It's because some genius (Ptolemy, I think?) decided to start with a brightness of 1 and didn't have anywhere to go but up because decimal numbers hadn't been invented by that point [citation needed].

In all seriousness, the misleading part is the word "Magnitude." It should really be called "Dimness," but that didn't market as well. Actually, it's like how pH should really be called "Un-Acidity" because the whole scale is backwards from what you would intuitively expect. I maintain that both of these systems were designed specifically to screw with introductory physics/chemistry students.

well it's that damn tricksy negative log scale, isn't it. I loved getting into it with students in relation to negative pH

The pH scale made perfect sense to me once I was taught how it's calculated. Of course, now I can't remember anything from chemistry.

Tycherin wrote:It's because some genius (Ptolemy, I think?) decided to start with a brightness of 1 and didn't have anywhere to go but up because decimal numbers hadn't been invented by that point [citation needed].

In all seriousness, the misleading part is the word "Magnitude." It should really be called "Dimness," but that didn't market as well. Actually, it's like how pH should really be called "Un-Acidity" because the whole scale is backwards from what you would intuitively expect. I maintain that both of these systems were designed specifically to screw with introductory physics/chemistry students.

My teacher just said 'think of pH as 'alkalinity' for now and we'll get back to what it means at A level'. Worked alright for us as I remember.

Rational numbers were known to the Greeks but the decimal number system did not reach Europe until centuries after Ptolemy. The Greeks did not accept the concept of negative numbers (unlike contemporary or near contemporary Indian and Chinese mathematicians).

It was either Hipparchus or Ptolemy who invented the magnitude scale. They took the brightest stars in the sky to be of the first magnitude, the next brightest stars to be of the second magnitude and so on to the sixth magnitude for the faintest stars visible with the naked eye in the sky (the telescope of course was not invented until about 1600). In the 19th century is was noticed that the first magnitude stars were about a hundred times brighter than the six magnitude stars so the magnitude scale was formally defined so that a factor of hundred in brightness corresponds to five magnitudes. Thus a star one magnitude brighter than another is a tad over 2.5 times brighter in units of power. One of the brightest stars in the sky, Vega, was chosen to be the zero point of the magnitude scale.

There is such a wide range of luminosities observed in astronomy that using a logarithmic scale is necessary. However, using a negative log scale with a somewhat arbitrary scale factor does make things more confusing than it should be. Astronomy has a bad habit of using nomenclature or units long after after the theory underpinning it has been disproven.

Maybe a better question would be a list of all the stars in that distance that we've surveyed.

It wouldn't be even close to comprehensive, but it'd be the best you could ask for...

...and for that the Sloan Digital Sky Survey is probably a good start.

Thanks everyone. First, I was saying "celestial objects" to include not just stars, but stuff like nebulae, too. Second, it satisfies me for the time being that the answer is "We don't know yet."

So... Any other science questions bugging anyone that don't have quickly google-able answers?

If you want exoplanets, wikipedia has a handy list that you can sort by distance.

As far as nebulae go, at least for some types wikipedia has good lists:
Planetary
Protoplanetary
Supernova remnants

The Wikipedia lists are only a starting point as they are rather incomplete. For example the list of planetary nebula contains a few dozen objects while about 3000 planetary nebula are known in our galaxy.

The most complete catalogue of stars I could find is the NOMAD catalogue. Unfortunately it does not contain distances. Be warned that it contains 1.1 billion stars.

Astronomy is kind of weird in what is easy to measure and what is difficult to measure. You can measure positions on the sky quite easily but measuring distances is extremely difficult. Conversely velocities are quite easy to measure perpendicular to the sky but incredibly difficult to measure in the plane of the sky.

Also AdmiralMemo, can I ask why you are interested in knowing what is between five to fifteen thousand light years away?

Lord Chrusher wrote: Astronomy has a bad habit of using nomenclature or units long after after the theory underpinning it has been disproven.

Such as the word "astronomy", that should really have been left for astrology. . .

Agreed; ANOTHER task I'm gonna undertake if I ever get unlimited access to a time machine that permits 'round trips' into the past: gradually influence the culture of our ancestors that the scientific study of the heavens gets the "Astrology" appellation, while the burdening of same with mysticism gets "Astronomy"

(then I'll also go back and prevent the two solitaire games "Canfield" and "Klondike" swapping names at some point )

While there was historically a distinction between astronomy and astrophysics - with astronomy being concerned with measuring the positions and properties of celestial bodies and astrophysics being the application of physics to astronomy - astrophysics is pretty much synonymous with astronomy these days. Where something using astronomy or astrophysics in its name is somewhat historical. My undergrad was formally in Physics and Astronomy while my PhD is in Astrophysics despite my undergraduate astronomy classes being more about physics than my PhD is.

Astrophysics is likely the better term as no one confuses it with astrology and it makes you sound quite smart.

Lord Chrusher wrote:Also AdmiralMemo, can I ask why you are interested in knowing what is between five to fifteen thousand light years away?

Since it's also five to fifteen thousand years ago that the light started its journey, I was simply curious to know what was going on in the sky at roughly the start of human civilization.

empath wrote:Agreed; ANOTHER task I'm gonna undertake if I ever get unlimited access to a time machine that permits 'round trips' into the past

While you're at it, can you also get the signs for negative and positive electromagnetic charges flipped, too?

Ooh, and fix the hydrogen/oxygen mix-up too.

(The words for hydrogen and oxygen in German translate to "sour stuff" and "water stuff" respectively which as you might hear with your ears is the wrong way around. The story goes the alchemists back in the day figured out hydrogen is a vital component of acids and oxygen features prominently in water, and then there was a feud between one German alchemist and someone trying to translate his work to another language, and you can guess how that worked out.)

Here's one:

If you jump, that exerts a force on the Earth, then when you land, that exerts another force, again pushing the Earth in the same direction.

Since this is a closed system, does that mean that the gravitational pull of your body being away from the Earth pulls the Earth back up that imperceptible amount that you just pushed it away?

Yes.

What family of Animals did the Mammals evolve from?
I've heard Reptiles or Amphibians and they both seem plausible. I just want to know what the general scientific educated consensus is.

According to the evolution of mammals Wikipedia page, mammals split off from reptiles after mammals and reptiles had spilt off from amphibians. This really is not my field but I think it is a bit wrong to talk of mammals evolving from reptiles. It is correct to say that reptiles and mammals shared a common ancestor and that that ancestor was more recent than the common ancestor of amphibians and mammals.

It makes sense to me, as you have Spiny mammals like hedgehogs or Porcupines who have scaley aspects.
And the Platypus eggs look more reptilian than amphibian.

Ok I need this thread. I got a story that while using some purely unscientific science fiction ideas; still needs a lot of science explanations on a lot of other stuff, because I try not to use "techno-babble" to hand-wave away things if I can find a scientific explanation that might actually work... kind of.

But that is for a later time