First Light Guide #6: NGC637

NGC 637

A detailed guide to finding / observing the open cluster

First Light Guide 6 graphicFirst Light Guides #6 is a detailed written and video reference to finding and observing the open cluster NGC 637 in the constellation of Cassiopeia. This guide is ideal for beginners, but many advanced amateur astronomers may find it useful. Note that NGC 637 is quite small - only 4 arc minutes across - and so it will require more magnification to see well. This guide will help you locate this open cluster with almost any telescope. There is a video to using a magnified finderscope, red dot finder, or setting circles along written tips on observing NGC 637.

Visitors who are new to this site may wish to get started here, while more advanced observers can search the First Light Guides by constellation, or by right ascension


Find NGC 637 in the sky

NGC 637 is an open cluster within our own Milky Way galaxy. It is over 7,000 light years from Earth, so the photons from it reaching your eye in a telescope left that cluster about the time rice was being domesticated in China, and farming reached the Atlantic coast of Europe. The estimated age is about 10 million years, making it one of the younger star clusters in our galaxy.

If you have a magnified finderscope on your telescope, start here to find NGC 637:

If you have a red dot finder on your telescope, start here to find and observe NGC 637:

If you have a telescope that has an equatorial mount with fairly precise setting circles, you may find this video useful when trying to find NGC 637:

Video not available yet, but coming soon!

Observing tips:

This might be the first "challenge" object in the First Light Guides list. However, NGC 637 is not necessarily hard to find, but compared to other open clusters on this list, it is somewhat small so it could be easily overlooked. That makes it good for star hopping and object-locating experience. 

Remember, small: Just 4 arc minutes across! This will help you know how small that is for other objects. But, it is 8.2 magnitude, so it should stand out fairly well from most places. There is a lovely little double star right nearby, even in the same field of view as the cluster at high magnification. In fact, try using higher magnification once this cluster is centered; note that a larger scope will show 13-th magnitude stars that smaller telescopes may not. 

Useful filter(s): None necessary

What should I see?

Below is an approximate view of NGC 637 with a 70mm telescope at 60x magnification, and a 0.90 degree telescopic field of view.  

Demonstrates how view would look with objects on the meridian using a refractor telescope and a star diagonal.  Other telescopes or object sky positions may incur a differing view.  Various magnifications, eyepieces, telescope focal lengths and other variables may alter the view compared to this one.  This is a representation only intended to help the observer get some idea what they may see at the eyepiece.  Extreme local light pollution may block the view entirely.

Approximate view of NGC637 as seen through a small telescope

Details of NGC 637

Type: Open star cluster

Distance: 7,045 light years 

Apparent dimensions: 4 arc minutes

Apparent magnitude: 8.2 (integrated)

Right ascension: 01h 42m 54.0s

Declination: +64 00' 00"











Because telescopes and observers are all different, here are some alternate sketched/drawn views of NGC 637

Eric C. Graff, using 6" reflector

Jay L Eads, using 10" reflector

Rober D. Vickers Jr., using 12.5" reflector


The constellation: Cassiopeia

General information about Cassiopeia, the Queen, where NGC637 is located. This will help you know where to find the constellation in the sky and be able to locate and identify its brightest stars.

Name of constellation: Cassiopeia

Abbreviation: Cas

Genetive form: Cassiopeiae

Common names: The Queen, Ethiopian Queen

Associated asterisms: Five brightest stars form a "W" or "M" shape depending on the time of night/year

Original 48 of Ptolemy: Yes

Area by size: 598 square degrees

Relative size: 25 out of 88 (Perseus is next larger, Orion is next smaller)

First Light Guides objects: Eta Cassiopeiae, NGC225, NGC457, NGC663, NGC637

Brightest stars, in order of magnitude 

First lists the Bayer designation, then the "traditional" star name (often Arabic, but not always - see each star's notes for details).

  • α (Alpha) Cassiopeiae (Schedar, Shedir) magnitude 2.25, the 71st brightest star in the sky. The traditional name is derived from the Arabic word صدر şadr, meaning "breast," appropriate for a queen constellation.

  • β (Beta) Cassiopeiae (Caph) magnitude 2.27, is the 73rd brightest star in the sky. Its name comes from a pre-Islamic Arabic term al-Kaff al-Khadib which means "the stained hand" (referring to stain from henna). .

  • γ (Gamma) Cassiopeiae (Tsih, Navi) magnitude 2.39, is 82nd brightest star in the sky. However, it is also somewhat variable, dropping as low as 3.0, and as bright as 1.6. The star has no traditional Arabic name. "Tsih" comes from Chinese, derived from the word "whip." A nickname was applied to the star by American astronaut Gus Grissom; his middle name was "Ivan" and spelled backwards, it is "Navi."

  • δ (Delta) Cassiopeiae (Ksora, Ruchbah) magnitude 2.68. It is an Algol-type eclipsing variable star, with a period of 2 years/1 month, and varies by 0.1 magnitude. The traditional names are from the Arabic word ركبة rukbah meaning "knee."

  • ε (Epsilon) Cassiopeiae, magnitude 3.35. It has no traditional Arabic name assigned to it. 

  • η (Eta) Cassiopeiae, magnitude 3.45. It has no traditional Arabic name assigned to it. It is a very Sun-like star in size, mass and luminosity. This is a double star highlighted in the First Light Guides (link here).