First Light Guide #9: Lambda Ari

Lambda Arietis

A detailed guide to finding / observing the double star

First Light Guide 9 graphicFirst Light Guides #9 is a detailed written and video reference to finding and observing the double star Lambda Arietis in the constellation of Aries. This guide is ideal for beginners, but many advanced amateur astronomers may find it useful. It is designed to help you locate this double star with almost any telescope. There is a video to using a magnified finderscope, red dot finder, or setting circles along written tips on observing Lambda Arietis.

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 Lambda Ari in the sky

Lambda Arietis is a double star within our own Milky Way galaxy. It is an F-class, white star at magnitude 5, with a secondary that is somewhat cooler, and dimmer at magnitude 7. The resulting difference in temperature and visual brightness sometimes shows an actual color difference to some observers. The separation of the two components has remained the same since William Herschel observed these stars in 1777. The orbital period of the two stars is likely on the order of 33,000 years!

If you have a magnified finderscope on your telescope, start here to find Lambda Arietis:

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

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 Lambda Arietis:

Video not available yet, but coming soon!

Observing tips:

Lambda Arietis is an easily split double star, at 37.4 arc seconds of separation between the primary and secondary components. As a zodiac constellation, Aries is visible from most anywhere on Earth, and is ideally placed for best viewing when it is at or near the meridian. In this way, it will be at it's highest point (culmination), providing the least amount of atmosphere to look through in order to find and observer the star. Use a long to medium focal length eyepiece.

Though the two stars are close in spectral type (see below), the nearly 3 magnitudes of difference between them causes an interesting effect at the eyepiece with respect to the colors seen. An F class star should be white, and G class slightly yellow in comparison, but these two stars have an almost yellow/blue appearance due to their properties as seen from our perspective. What color(s) do you see?

Useful filter(s): None needed.

What should I see?

Below is an approximate view of Lambda Arietis and the surrounding stars as seen with a 70mm telescope at 60x magnification, and a 0.90 degree telescopic field of view.  

Approximate view of Lambda Arietis at 60x

Details of Lambda Arietis

Type: Double star

True binary: -

Orbital period: -

Distance: 129 ± 2 light years

Apparent separation: 37.4 arc seconds

Apparent magnitudes: 4.95, 7.75

Right ascension: 01h 57m 55.7s

Declination: 23° 35′ 45









Because telescopes and observers are all different, here are some alternate sketched/drawn views of Lambda Arietis:

Eric C. Graff, using a 6" reflector

Jeremy Perez, using a 6" reflector


The constellation: Aries

General information can be found here about Aries the Ram, where Lamda Arietis 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.

General information about Aries

Name of constellation: Aries

Abbreviation: Ari

Genetive form: Arietis

Common names: Ram

Associated asterisms: No major ones

Original 48 of Ptolemy: Yes

Area by size: 441 square degrees

Relative size: 39 out of 88 (Canes Venatici is next larger, Capricornus is next smaller)

First Light Guides objects: Lambda Arietis, 30 Arietis

Brightest stars in Aries, by magnitude 

First lists the Bayer designation, then the "traditional" star name (often Arabic, but not always - see each star's notes for details). Graphic shows constellation at culmination on the meridian, facing south.

  • α (Alpha) Arietis (Hamal) magnitude 2.01 it is the 49th brightest star in the sky, and resides at 66 light years distance. The traditional name comes from the Arabic phrase for "head of the ram."

  • β (Beta) Arietis (Sheratan) magnitude 2.64, and is 60 light years distant. Traditional name means the Arabic الشراطان aš-šarāţān "the two signs", a reference to the star having marked the northern vernal equinox together with Gamma Arietis several thousand years ago.