I mentioned Simbad just once in my book, Remote Astrophotography Using Slooh.com (which you can download for free here). But what is Simbad and how can you use it?
What Is Simbad?
Simbad is a large database of astronomical information. It contains over 9 million objects under over 24 million names. You can search the database for information about an object and get a lot of details about it, including an image, coordinates, distance, and the different identifiers used for the object.
Simbad is useful for figuring out an object’s designation because it gives you a listing of object cross references, making it easier to figure out just what you are looking at. Sometimes a research paper will refer to an object using its LEDA designation and through Simbad, I can find that the LEDA designation has an equivalent NGC designation.
The interface to Simbad isn’t all that user-friendly, making it look rather intimidating to use at first, but it’s actually straightforward to use once you know what to do. This tutorial walks you through quickly researching an object.
We’re going to research NGC 936 in this tutorial.
Basic Usage of Simbad
Start by opening the Simbad page here: http://simbad.u-strasbg.fr/simbad/
You’re presented with a page that’s full of tables and other information. You can certainly go over the information on the page but for now, click the “basic search” link in the “Queries” table on the left.
You’re presented with a “basic query” page with a textbox on it. Enter NGC 936 and click “SIMBAD search” to bring up the result.
You’re presented a page with quite a bit of information on it – let’s look at what some of it means.
Understanding the Data Simbad Presents
You can see an image on the right side of the page. The image is usually from the SDSS (Sloan Digital Sky Survey), but you can try other sources by clicking the radio buttons just below the image (they’re marked 2MASS and DSS).
The numbers at the top of the image are the Right Ascension and Declination of where the cross hairs are on the image. The format of the numbers is RA followed by Dec, in sexagesimal format so in this case the RA is 02:27:37.462 and the Dec is -01:09:22.61.
If you mouse over the image the coordinates will change to indicate where your mouse is pointing. You can scroll your mouse wheel to zoom in an out and you can click and drag to pan the image to look around.
The number at the bottom right of the image is the FOV, or Field of View, so you can get an understanding of how large the object is.
Over on the left side of the page is the heading which tells you that NGC 936 is a galaxy in a pair of galaxies. In this case you can’t see the other member of the pair in the image, but you can do further research using Wikipedia for example to find out that the other member is NGC 941.
Below the heading are the celestial coordinates in RA and Dec:
ICRS coord. (ep=J2000) : 02 27 37.462 -01 09 22.61 (Infrared)
The coordinates tell you that the epoch is J2000 to confirm that you are indeed looking at the current coordinates (there are other epochs and the next one is going to be J2050). The coordinates are expressed in RA and Dec so in this case the RA is 02:27:37.462 and the Dec is -01:09:22.61. The coordinates are useful when you’re researching an object that might not be in the Slooh 1000 and you want to observe it using its coordinates instead.
The next item of interest is labeled “Radial velocity / Redshift / cz” as follows:
Radial velocity / Redshift / cz : V(km/s) 1225.5 [~] / z(emission) 0.004096 [~] / cz 1228.02 [~]
(Rad) C 2004MNRAS.350.1195M
The line contains three pieces of information, but we’re interested in the middle one: the redshift. The redshift is used as a measure of the distance of the object and in this case, the z value is 0.004096. You can convert z into an approximate distance in case you cannot find, for example, a Wikipedia page about the object you’re researching. I explain how to convert z into a distance in this tutorial: Converting Redshift z to Distance. The other values in this section are interesting too if you want to understand the velocity of the object.
Under that is the object’s morphological type, in this case it is SB0, as shown:Morphological type: SB0
There’s a great Wikipedia article on what the various types mean: https://en.wikipedia.org/wiki/Galaxy_morphological_classification
Scroll down the page to the Identifiers section, and here you will find a cross-reference of the object’s designations. This is useful if you’re researching an object and want to see whether it happens to be in a catalog supported by Slooh so you don’t have to use a coordinate mission to observe the object. In this case, you can see that NGC 936 has 18 other designations that you might find in research papers.
Conclusion
This concludes the tutorial about Simbad, a very valuable resource. Simbad is also integrated with Aladin, an interactive sky atlas that brings together data from a lot of sources. I explain how to use Aladin in another tutorial where I show you how to investigate your own images using Aladin.
