Originally posted by Daminc
As with everything I started out with researching and I found lots of data with stella coordinates in the Polar format.
Okay, let's begin here. What stars are listed? I'm going to make the (educated) assumption that they are the coordinates, probably in the form (right ascension, declination, distance), of stars
in the Milky Way galaxy. No one, to my knowledge, has made a catalog of stars in other galaxies. Why not?
Well, you first must understand something about measuring distance. Measuring distance is one of the most difficult things astronomers have to do. Think about why: a nearby dim star and a distant bright star
look the same to us. Since stars come in all kinds of inherent brightnesses (called
luminosities), you can't use brightness as an indicator of distance.
Within some fairly small distance from the Earth, we can use an effect called
parallax to measure distances. When you view the same object from two or more different positions, the object appears in a different position relative to distant background objects. You have two eyes -- each eye sees a slightly different picture of the things in front of you. Your brain uses those slightly different pictures to measure distances to things. Astronomers do the same thing. They precisely measure a nearby star's position relative to much more distant stars. Then they wait six months (so the Earth is on the opposite side of the Sun) and they do it again. The small change in the star's position relative to the background allows them to calculate distance.
All of the naked-eye stars, and quite many others, have had their distances measured this way. The most recent experiment conducting precision parallax studies was the Hipparcos satellite, which I believe measured about 10,000 distances. Chances are, you have a catalog of these nearby stars.
Now, things get substantially harder when you start dealing with stars in other galaxies. Parallax no longer works -- the angles are far, far too small to be measured accurately (with existing technology anyway). Astronomers generally measure the distances to the galaxies using tools like Cepheid variable stars, which are a special class of stars which vary in brightness in such a way that we can determine their luminosities from the period of their variation.
Galaxies are so far away (millions or billions of light-years) that astronomers generally just give one distance estimate for the entire galaxy, and do not generally ever bother with the distances to individual stars.
I suspect that you have a catalog of nearby galaxies (was that you asking about the Tully catalog a while ago?), which gives the 3D positions of some galaxies.
What you're failing to realize is that your datasets
do not overlap. Your star catalog lists stars near the Earth (some hundreds of light-years away), while your galaxy catalog lists entire galaxies (some millions of light-years away). There are no stars in your star catalog that are not in the Milky Way.
I do find this a bit confusing..
Good of you to mention this. Let's talk about constellations a bit.
The sky appears two-dimensional to us -- we can imagine a giant sphere surrounding the earth, with white dots painted on its inside surface. We can ascribe two coordinates to any point on the inside surface of this imaginary sphere. In a very similar way, we can ascribe two coordinates (latitude and longitude) to any point on the surface of the earth.
Now for the sake of naming, astronomers have chosen to break the sky up into small interlocking regions -- like a jigsaw puzzle. In a similar fashion, we've also chosen to break up the surface of the Earth into interlocking regions. We call the regions on the Earth countries, states, counties, and so on. These so-called 'political boundaries' are not physical -- you don't trip over a big black line when you cross from one state to another. The regions on the sky are called 'constellations,' and the boundaries are no more physical than the boundaries between states.
The states have cities within them; you would then say that San Francisco is within the state of California. Constellations have stars and galaxies and so on with them, too -- so you might say that the Andromeda Galaxy is within the constellation of Andromeda.
The only difference is
distance. All of the cities on the Earth are, quite obviously, on the surface, at more or less the same altitude -- the same distance, say, from the Earth's center.
The stars and galaxies and other objects inside a constellation, however, can be a
vastly different distances. The stars in a constellation are pretty close to us. The galaxies are very, very much further -- and the quasars further still. The things in a constellation are not all physically related.
So it sounds to me like you have a mixture of concepts and datasets which isn't going to accomplish much. I believe you have:
1) a catalog of nearby stars' 3D positions (all of which are in the Milky Way).
2) a catalog of nearby galaxies' 3D positions (none of which contain any specific star information).
3) a list of the boundaries between the constellations on the sky.
(Keep in mind that 'nearby' means very different things for stars and galaxies! A nearby star is a few hundred light-years or less distant, and a nearby galaxy is a few tens of millions of light-years or less.)
With this information, you're not going to be able to accomplish what you want -- what you're looking for is data on the positions of individual stars in other galaxies (like Andromeda), and this data simply doesn't exist. It isn't very important to us astronomers!
- Warren
but then I've always had difficulty with that thing.
