The Algol Star– Facts in brief:
What is it?
Algol is a variable and multiple main sequence star. It is a triple-star system, made up of the 3 bright stars – Algol A, Algol B and Algol C.
This is a hot star that is bluish-white in color, located in the Constellation of Perseus, in the northern hemisphere, (and associated with the image of the Greek Hero Perseus, carrying the severed head of Medusa, the Gorgon medusa).
Algol (Beta Persei, B Persei) is the second brightest star in the Constellation Perseus, after Mirfak (alpha Persei), and one of the best known variable stars in the night sky.
Algol variable stars have a spherical shape. It is a popular sight from Earth in the night sky for many amateur astronomers.
The Constellation of Perseus is the 24th largest Constellation in the Northern Sky and borders Constellation Cassiopeia to the north and Constellation Andromeda to the east.
Algol has an apparent magnitude of between 2.12 and 3.5, making it a second/third magnitude Star, and at its brightest is around 180 times more luminous than the Sun. It ranks as the 61st brightest star overall in the night sky.
The proper star name of this variable star is Algol (from Arabic) and it is also referred to as ‘the Demon’s Head.’
Characteristics of this Star System
Algol is not a single star, but a multiple star system with three confirmed and a further two suspected stellar components. There are another five dimmer stars in this star system that are listed as companions.
Algol is the prototype in a group of eclipsing binary stars known as the ‘Algol Variables’, which have variability in their apparent luminosity.
There is also a third star, a fainter star, in this star system forming a triple star system, within the Constellation Perseus. The triple star system comprises of:
- Beta Persei Aa1 (also known as B Persei A) – a hot blue-white main sequence star, spectral type B8V
- Beta Persei Aa2 (also known as B Persei B) – a cooler orange subgiant, stellar classification KOIV
- Beta Persei Ab, the third star (also known as B Persei C) – a cooler white star, spectral type A7M
Within Algol, Beta Persei Aa1 (Algol A) and Beta Persei Aa2 (Algol B), form a very close binary system. They are known as eclipsing binary stars meaning they regularly eclipse each other, like clockwork. This occurs because as we view them their orbital plane contains the line of site to Earth.
This eclipsing binary action lowers the combined brightness from 2.1 to around 3.4.
Beta Persei Aa1, is a bluish star and the brighter star in the binary system and is estimated to be 180 times as luminous as the Sun.
Beta Persei Aa2, the fainter star in the binary system, is a yellow star and is estimated to be around three times more luminous than the Sun.
Although these two bright binary stars form a binary system, orbit each other and at times eclipse each other they are actually separated by a distance of 4.65 million miles (which by solar system standards is quite close!)
They are distanced from each other by an estimated 0.062 astronomical units (abbreviated to au), whereas the third star in the system is around 2.69 au further away from these two stars. All three stars in Algol orbit each other like clockwork over 681 Earth days.
The brightness of Algol during the secondary eclipse is comparable with the brightness of two neighboring stars – Almach, the bright star in Constellation Andromeda and another star in the Constellation of Perseus, called Epsilon Persei.
Algol A is a bright and bluish supergiant star was one of the first non-nova variable stars to be discovered the night sky, and is visible by unaided eye.
The latest Hipparcos distance estimate in 2007 for this blue-white supergiant star locates it around 2,120 light years from Earth (28,50 parsecs), although its exact distance is not certain as this is one of the distant stars.
FACT: 1 light year equals 0.3066 parsecs
Distance From Our Sun
Algol is now located about 92.8 light years from the Sun, however around 7 million years ago it was much closer as it passed within 9.8 light years from our solar system. At this time its apparent magnitude was about -2.5, which today would make it the brightest Star, even brighter than Sirius, which is -1.46.
Algol is a main sequence star that ranks as the second brightest star within the Constellation Perseus that is located in the Northern Hemisphere and crosses the Milky Way.
According to the Hipparcos 2007 apparent magnitude list, it has an apparent magnitude of 1.25; making it one of the brightest stars in the Milky Way.
Beta Persei is a type of variable star, which means its brightness can vary or fluctuate over time. It is regarded as the main star in a group of variable stars, known as the ‘alpha Cygni variables’.
Who named the stars?
In 1603, the German Astronomer – Johann Bayer, systematically assigned names to the brightest stars in each constellation and cataloged them in his Star atlas – ‘Uranometria Omnium Asterismorum’.
The Bayer designations are stellar designations where the stars within Constellations such as the Constellation Perseus are initially identified by a name or a letter from the Greek Alphabet from Alpha through Omega (in order of brightness).
1. The names of the Stars begin with a letter of the Greek alphabet starting with– Alpha, then beta, gamma, etc.
2. Followed by the genitive form of their parent constellation’s Latin name – ‘Persei’, sometimes abbreviated to ‘Per’.
The main sequence stars of Perseus are listed by their apparent magnitude (luminosity) from Earth, listed from the brightest to faintest star in decreasing order:
1. Mirfak (Alpha Persei), is the most luminous star in Perseus, with a highly luminous apparent magnitude of 0.120
2. Algol, (name from Arabic), beta Persei – the second brightest star in Perseus with a visual/apparent magnitude of 1.25
3. Menkeb or Atik, (name from Arabic) zeta Persei– third brightest Star with a visual/apparent magnitudeof 2.84,
4. Epsilon Persei– the fourth brightest star with a variable magnitude of 2.90 (a beta Cephei variable star)
5. Gamma Persei – the fifth brightest star with an apparent magnitude of 2.91
In 1667, Geminiano Montanari an Italian astronomer first recorded the variability of Algol.
However it was not until 1783 that the British astronomer, John Goodricke, presented his research on the reasons for the variability of this star to the Royal Society in the United Kingdom. He suggested that the luminosity of this star varied because of a dark object passing in front of it from time to time.
Later in the 19th century, it was revealed that Algol was actually an eclipsing binary star, and eventually some of the confusion over measuring the reflection between the two stars was solved by the discovery of a dimmer third star, making it a star system, not a single star as originally thought.
Where did the name come from?
This Star name Algol derives from Arabic, ‘ra’s al ghul’, or ‘al ghoul’, is also referred to by other names, such as
- The Demon Star (the English name)
- Carput Larvae, from Latin, the Spectre’s Head (Specter’s Head),
- The head of the ogre
- Rosh ha Satan, the Head of Satan (in Hebrew)
- Rosch hassatan, Divil’s Head (from Hebrew into English writings)
This is possibly in reference to the location of this bright Star in the outline of the Hero Perseus from Greek mythology. He is portrayed holding the severed head of Medusa, the Gorgon Medusa in his hand as a trophy, which we now associate with the Constellation Perseus, in the Northern Hemisphere.
Different versions of the word Algol, referring to the gorgon’s head and suggesting a Satan type creature, or associations with blood and violence are also used to describe this star.
The alternative names include:
- The Demon Star, Al Ghoul, Gorgona, Gorgonea Prima, the Gorgon of Perseus, death by decapitation and the Spectre’s Head.‘
In many cultures, including ancient Chinese astronomy, the Star Algol is considered to be one of the unluckiest in the sky.
It is also listed as one of the 15 Behenian fixed stars, used in medieval times in astrology for magic and represented the occult, in Europe and across the Arab world.
The traditional name “Algol’,for the Star Beta Persei, has been officially recognized by the international Astronomical Union (IAU).
The Constellation of Perseus has 8 named stars, which are approved by the IAU.
Properties of the Algol Star
Algol is a bright multiple star and the second brightest star in the Constellation Perseus. It is located approximately 90 light years distant from Earth.
Algol is located –
- South of the Constellations Cassiopeia (in the same area of the sky as the Great square of Pegasus)
- West of the Constellation Andromeda
- East of the Constellation Auriga
- North of the Constellations Taurus, Aries and Triangulum
Algol, the second brightest star in Perseus is named Beta Persei, it is part of a star system that emits x-rays and radio-wave flares. The magnetic fields of this star system are around 10 times stronger that the Sun’s.
There is disparity between the stellar evolution and the mass of this eclipsing binary system. In fact Algol did not appear to evolve in the established pattern of stellar evolution and this became known as the Algol paradox.
Generally the rate at which stars evolve depends on their mass.
The greater the mass the quicker the star evolves from the main sequence.
Algol is an exception to this rule, as its binary components do not leave the main sequence in the normal manner. The primary component in Algol (the more massive star) remains on the main sequence, and the secondary component (the less massive star) evolves as a subgiant.
The Algol paradox is due to a mass transfer that is typical in close binary stars.
This mass transfer process occurs when the more evolved secondary component (B) was not always the less massive one, but instead it originally was the more massive star but lost much of its mass to its companion (A). This companion (A) then became the primary component of Algol, the more massive star.
The mass transfer is thought to cause the magnetic fields of the two components to interact and generate powerful x-ray flares and radio wave flares
The mass of the Algol components are:
- Algol A – around 3.17 solar masses
- Algol B – around 0.70 solar masses
- Algol C – around 1.76 solar masses
The mass of the sun, referred to as its stellar mass, and enumerated as the Sun’s mass as a proportion of solar mass.
Algol Star components have a solar radius of
- Algol A – around 2.73 solar radii
- Algol B – around 3.48 solar radii
- Algol C – around 1.73 solar radii
How bright is this Star
Algol is a triple star system and its brightness is variable reflecting this. Overall its combined brightness is estimated it to be the 62nd most luminous of all the known stars viewed from Earth.
Its luminosity is the amount of energy emission from this giant star relative to the Sun. It is estimated to 98 times brighter than the Sun.
The brightness of a star as seen from earth is measured by its magnitude of which there are three classifications: Apparent Magnitude, Visual Magnitude and Absolute Magnitude.
The measurements for the brightness of Algol are:
- An apparent combined magnitude (also referred to as its visual magnitude) of +2.12 (the range of the three stars is 2.1-3.39 variable)
- An absolute magnitude of -0.18 to -0.11
FACT: The Apparent Magnitude is how bright we see a Star from Earth, and the Absolute Magnitude is the Apparent Magnitude of that star from a 10 parsecs distance (32.6 light years), assuming there are no molecular clouds, or dust in the line of sight.
The lower the number of magnitude the brighter the Star.
The stellar luminosity (Lsun) figure is 119.2200000
The color of the Algol Star
The Algol Star is classified as a variable trinary Star system, which according to the spectral type of its three stars –
- Algol A, spectral B8-type (Blue star)
- Algol B, spectral KO IV (Orange star)
- Algol C, spectral A7-type (blue-white star)
The temperature of this blue supergiant
Based on the spectral type of this Star it is estimated to be at least 12,500 degrees Kelvin. This makes it one of the hot Stars in the Universe.
Where is it located? –
The distances vary depending on how it’s measured and there are certainly margins of error.
According the latest 2007 Hipparcos measurement the parallax data gave an uncertain result. The Algol Star is estimated to be around 92.82 light years (28.46 parsecs) from Earth.
FACT: A parallax is a difference in the apparent position of a star or any solar system object viewed from two different lines of sight. The parallax is measured by the angle between the two lines of sight.
The distance between the Earth and the Sun is known as an Astronomical Unit.
The figure of A.U. is calculated as the number of times that Star is from Earth, in relation to the Sun. It is estimated that the Algol Star is approximately 5,686,681,08 A.U. from the Earth to Sun in distance.
FACT: All Stars and Planets orbit round a central point, the planets orbit the Sun and the Stars orbit the Galactic Centre.
The Star Algol is located approximately 24,214,37 light years (7,424.00 parsecs) from the Galactic Center, the center of the galaxy we call the Milky Way.
The Galacto-Centric distance is measured as the distance from that Star to the center of the Galaxy (Sagittarius A).
FACT: the Galactic Center, or Galactic Centre, is a supermassive black hole and the rotational center of our Milky Way galaxy. It is not possible to view it at visible, ultraviolet, or even soft or low-energy X-ray wavelengths because of the molecular clouds of interstellar dust along its line of sight.
Where can it be seen?
The Algol Star is one of the 17 main Stars in the outline of the Constellation Perseus and a bright star you can see by the naked eye.
The luminous Algol Star is of great interest to amateur astronomers as it twinkles.
Co-ordinates of a right ascension, or left ascension and their declination are used to locate all of the notable objects in the sky
The Deneb Star lies at a 03hours 08 minute right ascension,and a declination of +40 degrees.
The right ascension – is the angular distance of any sky object along the celestial equator from the March (Spring) equinox.
- If it has a positive number it is east of the March equinox.
The declination – is the angle of the sky object from the celestial equator.
- If it has a positive number it indicates it is located in the Northern Hemisphere
The age of the Algol Star
It is not certain what age the Star Algol is or how long it is likely to continue to survive.
One estimate of the age of this Star Algol is around 570 million years.
How can you identify the Algol Star?
The simplest method for spotting any particular Star from Earth is to first of all locate an easy to recognize neighboring Constellation or object in the night sky.
The easy to spot W-shape of neighboring Constellation Cassiopeia, in the area of the Great square of Pegasus, would be a good starting location for finding the Algol Star, in Perseus. The best time of year to spot it is in the evening sky from autumn to spring in the Northern Hemisphere.
Like all stars, Algol is constantly on the move at the same time every day.
The celestial pole
The celestial pole defines the poles of the celestial equatorial coordinate system.
An object at the Celestial pole has a declination of 0 degrees.
- The declinations for the north celestial pole is +90 degrees
- The declinations for the south celestial pole is -90 degrees
The celestial poles are not permanently in a fixed position against the background of the stars as everything moves in Space.
The Algol Star is located in the Northern celestial sky at a +40 57’ degrees declination, and an average 03 hours, 08 min right ascension.
Fun Facts about Stars – Did you know that?
- As the two main components of Algol eclipse each other they appear to wink at observers. Known as the winking star it is a favorite with amateur astronomers
- After the Sun, Sirius is the brightest star in the sky with an apparent magnitude of -1.46
- The Star we call The Sun does not belong to any constellation
- The planet Jupiter is often cited when making size comparisons between planets or stars. The Jupiter mass is a unit of mass equal to the total mass of planet Jupiter
- The rate of formation of stars in a starburst galaxy is more than 10 times faster than the star formation in the Milky Way galaxy
- The center of a Galaxy does not contain a Giant Star it contains a Supermassive Black Hole.
- A Red Dwarf is not a Dwarf Planet it is a Star. Most common Stars are Red Dwarf (cool Stars)
Commonly Asked Questions
Q. What prevents us seeing the Stars in the night sky?
A. Light pollution, fog, city lights and artificial lights all limit our visibility of the objects in the sky at night.
Q. Will the locations of Stars change over time?
A. Stars are continually on the move.
The images we form in our imagination to make objects, shapes and patterns out of the constellations have already shifted over time.
As we view the night skies from Earth they are likely to continue to shift and possibly in time the images may look very different.
- Algol star image – https://cosmicpursuits.com/wp-content/uploads/2018/09/1600px-Artist%E2%80%99s_impression_of_eclipsing_binary.jpg
- Algol location – https://starrynight.com/starry-night-8-professional-astronomy-telescope-control-software.html
- Nebula – By Credit: NASA, Jeff Hester, and Paul Scowen (Arizona State University) – http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/34/image/a, Public Domain, https://commons.wikimedia.org/w/index.php?curid=129538
- Algol 3 star image – https://nineplanets.org/wp-content/uploads/2020/02/Algol-view.jpg
- Supernova Remnant – Cassiopeia A. Credit: NASA/CXC/SAO – https://scitechdaily.com/lonely-origin-of-cassiopeia-a-revealed-one-of-the-most-famous-supernova-remnants
- Milky way – Source: Nick Risinger – https://sedsvit.medium.com/just-us-d4ab577099a2
- Celestial Pole – By I, Dennis Nilsson, CC BY 3.0,https://commons.wikimedia.org/w/index.php?curid=3262268
- Johann Bayer – https://laexuberanciadehades.files.wordpress.com/2012/10/johann-bayer.jpg