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complete beginner here, only got into star gazing a few days ago using my friend's telescope and I don't really understand at all what any of the measurements mean.

Anyway, talk about jumping in the deep end, but I'm poking around Nasa's Intergalactic Database, but sadly it's all greek to me: http://ned.ipac.caltech.edu/

Here's a galaxy: NGC 7590

Here's its basic data:

EquJ2000.0 RA

EquJ2000.0 DEC

Velocity km/s

Redshift / z

Magnitude / Qual Filter

Separ arc min

Now I'm afraid I don't really understand what any of these measurements and their values actually mean.

I remember from school what Red Shift is, its the wavelength increasing becoming 'stretched' making the light seem to appear more reddish the faster an object is accelerating away from us. But what is that z unit?

I'm afraid I really don't have a clue regarding anything else... spear arc min, magnitude / qual filter?

Is there a book for a brief understanding of these units? Not out to solve problems regarding celestial mechanics, I just want the NED to make a bit more sense to me so I can visualize these objects.

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3 Answers 3

If you browse further down the page, there are details for each of the top table data.

So RA and Dec give you coordinates (Right Ascension and Declination, like longitude and latitude). There is a designation J2000.0 which tells you the zero point to which the coordinates relate (remember, the Earth wobbles, so any coordinate system tied to Earth changes over time, hence a zero point), in this case the year 2000. The Equ. in front of that means equatorial, which ties is directly to the earth axis (poles & equator; the latter explains the name).

Redshift (z is the symbol for redshift) for a distance estimate.

Qual probably stands for qualification, for example the type of galaxy. It's likely not related to the filter. The type of galaxy would be the classification, which is further down the page.

Magnitude is the brightness, and Filter the filter it is measured in (most astronomy is done in a filter, to obtain a brightness in a specific wavelength range. Eg, red, blue, green, uv, infrared, H-alpha, Oxygen filters).

Separ arc min: the separation (or more likely, the diameter) in minutes of arc (so in 1/60 of a degree). You can't really measure the actual size (unless you trust the distance estimate), but you can measure the size as a angle projected on the sky. That's what it says here (e.g., the sun and the full moon are about 0.5 degree, or 30 arcminutes, "wide").

As mentioned at the start, browse down to get more in depth information, which may shed a bit more light on what the top table abbreviations actually mean and indicate.

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RA and DEC are the coordinates on sky.

  • RA (Right Ascension) is the angular position measured eastward in a full circle along the celestial equator.
  • Dec (Declination) is the angular position measured from the celestial equator in either the north (positive) or south (negative) direction. $+90\,^{\circ}$ is the north celestial pole, $-90\,^{\circ}$ is the south.

Velocity (km/s) is the radial velocity that the galaxy has away from or towards the earth.

Redshift (z) is basically a distance estimate based on observing the amount that the light of the object is redshifted due to it's movement away from us. (Since galaxies farther away will appear to move away faster according to Hubble's Law) If you understand the Doppler effect, the equation for its definition is pretty simple: (from Wikipedia's "Redshift" page)

From wikipedia on 'redshift'

The higher z is, the further away in distance (and time) the galaxy is.

Magnitude / Filter : Magnitude is the measure of brightness of the object. This depends on which filter (at which wavelength range) the observation is made in. For this measurement: smaller values = brighter. The sun is something like -27, the brightest star Sirius, is around -1.5, the faintest stars visible to the naked eye are around +6 to +8.
The galaxy page you linked only lists the magnitude - I don't know why the filter doesn't seem to be there.

Separ. arcmin : The separation in arc minutes. (Probably meaning size, or diameter of the galaxy) An arc minute is an angular unit of size on the sphere of the sky (the same units as declination). So one arc minute is 1/60th of a degree, and there are 360 degrees spanning the entire celestial sphere.

Sorry, I don't know the "Qual" measurement.

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DEC :Declination

RA :Right ascension

Redshift / z :use to calculate velocity over earth z axis (azimuth)

Magnitude/ Qual Filter : zoom into image filter quality (??)

Separ arc min : if is a group as cloud magallans

here more information

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