First Light Guides #14: Algol


A detailed guide to finding / observing the variable star

First Light Guides 14 graphicFirst Light Guides #14 is a detailed written and video reference to finding and observing the variable star Algol in the constellation of Perseus. This star's proper name of Algol means "the demon" or "ghoul" in Arabic; it is also known as Beta Persei. This guide is ideal for beginners, but many advanced amateur astronomers may find it useful. It is designed to help you locate this variable star with almost any telescope. There is a single video to help anyone with any telescope locate the star.

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 Algol in the sky

Though not known for certain if ancient astronomers knew of it's variable nature, Algol is nonetheless notable in that those astronomers saw fit to place the "winking" star as the eye of Medusa - the decapitated head of the Gorgon that Perseus holds in his hand. Today of course we know the star is not evil or "ghoulish" in any way, but it does "wink" at us quite regularly due to the eclipsing nature of the dual-star system. As a dimmer secondary star passes in front of the brighter primary, the star dims from magnitude 2.1 down to magnitude 3.4 every 2.867 days (2 days, 20 hours, 49 minutes). The whole process takes 10 hours to complete from bright, down to dim, back to full brightness again.

Here is how anyone with a small telescope can locate and observe Algol:

Observing tips:

From nearly any location, Algol can be seen naked eye, except for the worst light polluted city areas. While we cannot split the two stars into separate points of light from here on Earth, we get to see the dimming effect that the smaller, less-bright secondary has on the overall light when it eclipses the brighter primary star. 

While a telescope is not actually required to see this occur, it is worthwhile to aim a telescope this way and watch the star over time change from bright, to dim, to bright again when it occurs at time of night you can see part or all of this change occur. While no online tables exist to link to easily, keep watching the current Eyes on the Sky videos on YouTube for updates on good times to see Algol "wink" at you.

The magnification used to see the star is not terribly relevant given that the two stars cannot be split by amateur instruments. 

Useful filter(s): None needed or required.

What should I see?

A single star, easily seen naked eye at maximum, and usually seen easily at minimum from most areas. 

Details of Algol

Type: Eclipsing variable star

Distance: 93 light years, ± 2.0 

Apparent magnitude: 2.1 at maximum, 3.4 at minimum

Period: 2.867 days

Right ascension: 03h 08m 10.1s

Declination: +40 57' 20"

Algol minima for early 2015

Upcoming time for deepest minima of Algol that are well-timed for American / European observers:

Mar 12, 2015 @ 03:51 UT

Mar 15, 2015 @ 00:40 UT

Mar 17, 2015 @ 21:29 UT

Mar 20, 2015 @ 18;18 UT

Mar 23, 2015 @ 15:07 UT

Mar 26, 2015 @ 11:56 UT

Mar 29, 2015 @ 08:45 UT

Apr 01, 2015 @ 05:34 UT

Apr 04, 2015 @ 02:23 UT

Apr 06, 2015 @ 23:12 UT

Apr 09, 2015 @ 20:01 UT

Apr 12, 2015 @ 16:50 UT

Apr 15, 2015 @ 13:39 UT

Apr 18, 2015 @ 10:28 UT

Apr 21, 2015 @ 07:17 UT

Apr 24, 2015 @ 04:06 UT

Apr 27, 2015 @ 00:55 UT

Apr 29, 2015 @ 21:44 UT

This calculator converts Universal Time (UT) to your time zone. 

The constellation: Perseus

General information about Perseus the Hero, where Algol is located, can be found below.  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: Perseus

Abbreviation: Per

Genetive form: Persei

Common names: The Hero

Associated asterisms: No major ones

Original 48 of Ptolemy: Yes

Area by size: 615 square degrees

Relative size: 24 out of 88 (Serpens is next larger, Cassiopeia is next smaller)

First Light Guide objects: Messier 34, Double Cluster, Algol

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). Graphic shows constellation at zenith as if looking up with body facing north.

  • α (alpha) Persei (Mirfak) magnitude 1.79, is the 37th brightest star in the sky. It is 510 light years away from us. The traditional name is derived from the Arabic Mirfaq al-Thurayya, which means, "the elbow."

  • β (beta) Persei (Algol) is an eclipsing variable star of 2.1 that dips to 3.4 every 2.86 days. At maximum, it is the 59th brightest star in the sky. Traditional name is from the Arabic رأس الغول ra's al-ghūl : head (ra's) of the ogre (al-ghūl) - or "ghoul."

  • ζ (zeta) Persei is magnitude 2.84, and about 750 light years from Earth.

  • ε (epsilon) Persei is magnitude 2.88, and approximately 640 light years distant.

  • γ (gamma) Persei is an eclipsing binary system, with an combined magnitude of 2.93, and around 243 light years distant.

  • δ (delta) Persei is magnitude 3.01 and is 520 light years away.

  • ρ (rho) Persei is magnitude 3.39 and is 308 light years distant.