What is a Star?
Before we take a deeper look at the Alnilam Star, let’s take a quick look at what it is exactly that defines a star.
A star is an astronomical object, like a massive ball, that consists of hydrogen and helium, with enough mass to enable it to sustain nuclear fusion at its core.
The best-known star is the Sun, which is the only star in our solar system. The luminosity of the Sun is measured as 1 on the standard luminosity scale and anything larger than 1 signifies that Star emits more energy than the Sun.
Basically the more luminous a star is, the more energy it produces.
Characteristics of any Star
There are 5 main characteristics that define a star:
- Brightness – luminosity and magnitude
- Mass – using the Sun as the benchmark
- Size – using the Sun as a benchmark
- Color – depends on the temperature of the star
- Surface temperature – measured on the Kelvin scale
The temperature of a star is measured on a scale known as the ‘Kelvin scale’. On the Kelvin scale, zero degrees is theoretically absolute and equivalent to -273.15 degrees Celsius.
The approximate temperatures of stars by color range from:
- Hot Stars (Blue) – 50,000 K
- Alnilam, Epsilon Orionis (Blue) – 25,000K – 27,000K estimated
- The Sun – 5,500 K
- Cooler Stars (Red) 2,500K
So let’s take a look at one of the more notable Stars in our night sky.
- The Star Alnilam (Epsilon Orionis, e Ori)
The Alnilam Star– Facts in brief:
What is it?
Alnilam is a large variable blue supergiant star, located in the Constellation of Orion, which is located on the celestial equator and visible from across the world in the night sky. Alnilam appears as a single star to the naked eye.
The luminous star Alnilam (epsilon Orionis, e Ori or 46 Orionis its Flamsteed designation) from the Arabic language is a main star forming the outline of the well-known Constellation of Orion.
It is also prominent as the central star, between two other bright stars, that makes up an easily recognized asterism known as Orion’s Belt, or the Belt of Orion.
The three stars often referred to as Orion’s Belt stars are:
- Alnilam (epsilon Orionis), located 2,000 light years distant
- Alnitak (zeta Orionis), located 1,260 light years distant
- Mintaka (delta Orionis), located 1.200 light years distant
Alnilam in Orion
Alnilam is a hot double supergiant star that is blue in color, located in the Constellation of Orion, in the northern hemisphere, (and associated with the image from Greek Mythology, the Hunter Orion with a sword and shield in his hand).
It is the fourth brightest star in Orion, ranked behind three stars that each have with a very bright apparent magnitude
- Rigel, beta Orionis (7th brightest star overall in night sky)
- Betelgeuse, alpha Orionis, a red supergiant, (10th brightest star in night sky)
- Bellatrix, gamma Orionis (25th brightest star in the night sky)
Alnilam has a variable apparent magnitude of between 1.64 and 1.74, making it a luminous first magnitude bright Star, and varies between 275,000 to 832,000 times more luminous than the Sun.
It ranks as the 29th brightest star overall in the night sky. At this brightness it is easily located in the night sky and is one of 58 bright stars commonly used for celestial navigation and marking our position.
Alnilam is at its highest in the sky in the middle of December. It is estimated to be a young star, younger the Sun, at around 5.7 million years in age but it is not known how long its lifespan will be. It is thought that it will end up as a less massive star, possible a red supergiant and end life in a supernova explosion.
The names associated with Alnilam
The proper star name, approved by the International Astronomical Union (IAU), of this variable star is Alnilam, from the Arabic ‘al-nizam’, which means a ‘string of pearls’.
From Arabic culture it is also known as the middle star of the three belt stars.
These three stars were collectively referred to as the belt (al Nijad), the golden grains or nuts (al Alkat) or the accurate scale beam (Al mizan al hakk).
All names in some way refer to the position of these three bright stars, sitting in a row, like pearls, grains or even a weighing beam pattern.
This theme is mirrored in other cultures, such as its meaning in:
Chinese mythology – ‘the weighing Beam’ or the second star of the three stars, or the middle star of the three belt stars.
English names for this asterism include: the three Kings (the Magi), ‘Peter’s Staff’, ‘the yard stick’, ‘the golden yard arm’, or ‘the three Marys’.
In several Hispanic countries this asterism is called ‘the Three Marys’ or ‘the Three Marias/ Tres Marias’, or ‘the Three Kings’, Los Tres Reyes Magos.
In ancient Egypt – Orion and the brightest Star Sirius were associated with the Egyptian gods – Osiris and Isis. It was believed that their gods came from Orion’s Belt and the human race originated from Sirius.
Characteristics of this supergiant star
Alnilam is of the spectral type BO la categorizing it as a large blue supergiant star. It is also a variable type star, showing alpha Cygni variables behavior.
It is also regarded as a pulsating star.
The brightness of Alnilam has been used as the standard benchmark for luminosity by which other stars are categorized.
It has been categorized in the BO spectral class with a luminosity figure of I, which classifies it as a hypergiant star, with an absolute magnitude of -6.37 and a temperature of 27,000K, at the latest estimate. It is visible by naked eye.
Alnilam is regarded as one of the Navigation stars used for celestial navigation because of its luminosity and the fact that it is easy to identify as a part of a familiar asterism in the sky
The latest Hipparcos distance estimate in 2007 for this bright blue supergiant star locates it around 1975.8 light years from Earth (around 606 parsecs). Its exact distance is not certain.
FACT: 1 light year equals 0.3066 parsecs
- Alnilam is now located about 1,341.56 light years (411.52 parsecs) from the Sun.
However we now know that everything moves around in space and Alnilam is moving away from the Sun at speeds of around 26 kilometers per second.
Its Radial Velocity is the speed at which Alnilam is moving towards or away from the Sun, measured in milliarcseconds. If the Radial Velocity measure is negative it means the star is moving closer to the Sun. If the number is positive it means the star is moving away from the Sun.
- Alnilam is moving 1.44 milliarcseconds east (away from the Sun) and -0.78 milliarcseconds north (towards the Sun)
According to the Hipparcos 2007 apparent magnitude list, it has an apparent variable magnitude of between 1.64 and 1.74; making it one of the bright stars in the Milky Way (although this has been disputed by certain researchers)
Alnilam (epsilon Orionis) 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 Orion 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 Orion parent constellation’s Latin name – ‘Orionis’, sometimes abbreviated to ‘Ori’.
The main sequence stars of Orion are listed by their apparent magnitude (luminosity) from Earth, listed from the brightest to faintest star in decreasing order:
1. Rigel (beta Orionis) is the most luminous star in Orion, with a highly luminous apparent magnitude of 0.18
2. Betelgeuse, (name from Arabic), alpha Orionis, – the second brightest star in Orion with a visual/apparent magnitude of 0.42
3. Bellatrix, (name from Arabic) gamma Orionis – third brightest Star with a visual/apparent magnitude of 1.64
4. Alnilam, epsilon Orionis– the fourth brightest star with a variable magnitude of 1.69 (an alpha Cygni variable star)
5. Alnitak A, zeta Orionis – the fifth brightest component in Orion and is a triple star system. The primary star Alnitak Aa is a blue supergiant star with a visual magnitude of 1.88.
It is the brightest class O star in the sky. The 3 components of this triple star system are members of the Orion OB1 Association of other hot giant stars from the spectral types O and B.
Where did the name come from?
This Star name Alnilam derives from Arabic, ‘al-nizam’, meaning ‘string of pearls’ is also referred to by other names in other ancient cultures.
In ancient Chinese the name Alnilam is ‘Shen Su er’, which means the second star of the three stars, referring to Alnilam’s position in the Belt of Orion.
In Chinese mythology the 3 belt stars were also referred to as the weighing beam.
It is known in other cultures as versions of the middle star.
Where is it in the shape of Orion?
The outline of the Constellation of Orion is in an hourglass shape, representing the body of the hunter from Greek Mythology.
Different neighboring stars of Alnilam form part of the body shape:
- Bellatrix and Betelgeuse – represent the shoulders of the Hunter
- Rigel and Saiph – represent the feet of the Hunter
- Mintaka, Alnilam and Alnitak – represent the belt near the waist of the Hunter
- Meissa – represents the Head, and is surrounded by a small star cluster known as Collinder 69.
The traditional name “Alnilam’, of the star epsilon Orionis, has been officially recognized by the International Astronomical Union (IAU).
The Constellation of Orion has a total of 10 formally named stars, which are approved by the IAU, including:
- Alnilam, Alnitak, Bellatrix, Betelgeuse, Hatysa, Meissa, Mintaka, Rigel, Saiph and Tabit
Properties of the Alnilam Star
Alnilam is a young massive star, classified as a bright blue supergiant star. and the fourth brightest star in the Constellation Orion. It islocated approximately 2000 light years distant from Earth.
Alnilam is located in Orion –
- South of the Constellations Gemini and Taurus
- North of the Constellations Monoceros, Lepus and Eridanus
Alnilam, the fourth brightest star in Orion is referred to as epsilon Orionis or sometimes NGC 1990, which is its surrounding molecular cloud.
William Herschel, discovered NGC 1990 in 1786, using a telescope.
Alnilam is considered to be the biggest and most massive star of the three Stars in the Belt of Orion.
There is disparity between the stellar evolution and the mass of this star, reaching different results over the years. Generally the rate at which stars evolve depends on their mass. The greater the mass the quicker the star evolves from the main sequence.
The mass of the sun, referred to as its stellar mass, and enumerated as the Sun’s mass as a proportion of solar mass. The mass of the Alnilam star components is around 40 solar masses, meaning it is 40 times the mass of the Sun.
It has strong stellar winds which means it can lose mass quickly.
The surface gravity of Alnilam is 3.0 cgs.
The Alnilam Star has a solar radius of 24 times that of the Sun, referred to as 24 solar radii. (Although some estimates suggest it has solar radii of 20)
How bright is this Star
Alnilam, the blue supergiant star, has a variable brightness that ranks it as the 29th most luminousstar of all the known stars viewed from Earth. Its luminosity is the amount of energy emission from this giant star relative to the Sun. Its luminosity is estimated to be an around 537,000 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 Alnilam are:
- An apparent magnitude (also referred to as its visual magnitude) of +1.69 variable
- An absolute magnitude of -6.38
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 color of the Alnilam Star
The Alnilam Star is classified as a single Star, which according to the spectral type of b0 ia is a Blue 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 hotter 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 Alnilam 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 Alnilam Star is approximately 125,007,977.41A.U. from 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 Alnilam is located approximately25,303.75 light years (7,758.00 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. This is because of the molecular clouds of interstellar dust along its line of sight.
Where can it be seen?
The Alnilam Star is one of the main Stars in the outline of the Constellation Orion and a bright star you can see by the naked eye. It is visible as part of the belt stars of Orion.
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 Alnilam Star lies at a 05 hours 36 minute right ascension, and a declination of -01 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
Alnilam is located close to the celestial equator and is therefore partly visible from both the northern hemisphere and the southern hemisphere at certain times of year.
The age of the Alnilam Star
It is not certain what age the Star Alnilam is or how long it is likely to continue to survive.
One estimate of the age of this Star Alnilam is that it is still young and possibly only 5.7 million years old.
How can you identify the Alnilam 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.
Orion is probably one of the easiest to spot as it is formed from some very bright stars in a distinctive pattern. The Constellation of Orion is located on the celestial equator. That means it is visible in most locations of the world.
The best time of year to spot the bright star Alnilam is in the evening sky is in December.
Like all stars, Alnilam is constantly on the move.
Fun Facts about Stars – Did you know that?
- Alnilam is sometimes referred to as Sao 132346
- It is listed as HIP 26311 in the Hipparcos Star Catalogue
- 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.
- Alnilam star image – https://upload.wikimedia.org/wikipedia/commons/2/28/Alnilam_star.png
- Johann Bayer – https://laexuberanciadehades.files.wordpress.com/2012/10/johann-bayer.jpg
- Orion stars – https://telescopeobserver.com/wp-content/uploads/2019/06/orion-stars.jpg