Life and times of the blue supergiant that occupies the central position in one of the most famous and recognizable asterisms of the sky

In the northern hemisphere during winter, the Orion constellation presents a stunning view that captivates even casual stargazers. A key feature of Orion is its Belt, which has been recognized since ancient times and consists of three prominent stars arranged in a straight line. Descriptions of these stars have been passed down through various cultural traditions.

In Western cultures, the names of Orion's Belt stars, like many other bright stars in the night sky, have Arabic origins. The easternmost star, Alnitak, is a blue supergiant part of a triple star system. To the west lies Mintaka, which appears as a single star to the naked eye but is actually a system of at least six stars. The central star of the Belt is Alnilam, another blue supergiant whose name translates from Arabic to "row of pearls," highlighting its role in the asterism rather than just as an individual star.

The Pearls of the Belt

These three luminous stars are often cited in scientific literature under their designations from Johann Bayer's 1603 star atlas, Uranometria: Alnitak as Zeta Orionis (? Ori), Mintaka as Delta Orionis (? Ori), and Alnilam as Epsilon Orionis (? Ori). Their apparent visual magnitudes are approximately 1.79, 2.41, and 1.69, making Alnilam the brightest star in Orion's Belt and the 29th brightest star overall.

These stars, along with many others in their vicinity, form a rich star-forming region characterized by massive young stars, molecular clouds, emission and reflection nebulae, and numerous smaller stars that have recently emerged.

This stellar grouping is part of the Orion Molecular Cloud Complex and consists of a relatively young and cohesive population known as the Orion OB1 Association. The designation "OB" refers to the presence of many bright and hot blue stars within the association classified as spectral types O or B.

In 1964, astronomer Adriaan Blaauw divided the Orion OB1 Association into four subgroups based on stellar age. He proposed that this complex has experienced several episodes of star formation over the last 8-12 million years due to feedback processes. The oldest subgroup is OB1a, while the youngest, containing the famous M42 (Orion Nebula), is OB1d. The Belt's stars, along with the Sigma Orionis system, belong to OB1b, which comprises stars estimated to be 4-6 million years old.

These classifications are inherently approximate, as the distance of over 1,000 light-years from Earth complicates the assessment of the three-dimensional structure of these associations and the precise ages of the stars. A study in 2017 suggested that the eastern section of OB1b, which includes Alnitak, the Flame Nebula, and the Horsehead Nebula, contains a younger stellar population than the rest of the association.

Regardless of their exact ages, the brightest stars in this subgroup are only a few million years old, making them significantly younger than the Sun, which formed over 4.5 billion years ago. However, they are not particularly young when considering their expected life spans, as massive stars like these consume their nuclear fuel much more rapidly than the Sun.

Stellar Parameters of Alnilam

Alnilam exemplifies this pattern. Research published in 1974 by Dutch astrophysicist Henny Lamers estimated that this central star of Orion's Belt is between 3 and 4 million years old and has reached an advanced stage in its evolution, having exhausted its hydrogen fuel and now burning hydrogen in a shell surrounding its core.

Classified as a B0 Ia blue supergiant, Alnilam was designated this classification in 1953 during a revision of the Morgan-Keenan stellar classification system. Its spectrum became the standard reference for B0 Ia stars.

The main physical parameters identified in Lamers' study include: - Effective temperature: 28,800 K ± 2,000 K - Radius: 31.3 solar radii (±8.2 and ±2.3 solar radii) - Mass: 35 solar masses (±25 and ±8 solar masses) - Surface gravity: 10 m/s² - Bolometric magnitude: ?9.6 - Total brightness: 600,000 times solar luminosity

This immense brightness indicates the extraordinary energy emitted by Alnilam. If it replaced the Sun, its output would deliver energy equivalent to 817 megawatts per square meter, a lethal dose of radiation for Earth.

How Are These Measurements Obtained?

Lamers derived the effective temperature from visible light spectrum analysis, corrected for ultraviolet energy distribution. Surface gravity was also inferred from the visible spectrum. The radius was estimated using angular diameter measurements, recorded as 0.69 thousandths of an arc second at a wavelength of 4,430 Å.

The distance from Earth was another vital measurement for calculating Alnilam's radius, but direct measurements were lacking. Prior to Lamers, various authors had estimated the average distance of the Orion OB1 Association through photometric studies, considering color excess due to extinction along our line of sight to Orion. By comparing apparent and absolute magnitudes, they derived a distance modulus to estimate distance.

Lamers utilized a distance modulus of d = 8.1 magnitudes (±0.5 and ±0.1 magnitudes), resulting in an estimated distance of 420 parsecs (or 1,370 light-years) for Alnilam, with an uncertainty of ±110 parsecs. This considerable uncertainty extends to all derived measurements such as radius, mass, and brightness.

To refine Alnilam's parameters, more accurate distance measurements were essential, but these were challenging to obtain. The most reliable method for nearer stars is parallax angle calculation, but for a bright, distant supergiant like Alnilam, this was difficult with Earth-based telescopes.

The parallax was ultimately measured from space by the Hipparcos satellite during its mission from 1989 to 1993, with official data released in June 1997. Alnilam's parallax was recorded as 2.43 thousandths of an arc second, translating to a distance of just under 412 parsecs, closely aligning with Lamers' earlier estimation. However, the margin of error was substantial — 0.91 thousandths of an arc second, suggesting Alnilam's actual distance could range from 300 to 658 parsecs, thus complicating the calculations of its radius, mass, and brightness.

In 2007, astronomer Floor van Leeuwen published an updated Hipparcos catalog, purportedly improving the accuracy of parallax measurements for bright stars. Alnilam's parallax in this new version was 1.65 ± 0.45 thousandths of an arc second, which indicates a distance of 606 parsecs (or 1,977 light-years) with an uncertainty of ±227 parsecs.

Consequently, this new distance suggests Alnilam could be larger, more massive, and brighter than previously recorded.

Extraordinary Brightness

When factoring in this greater distance, Alnilam's parameters become truly remarkable. The radius expands to 42 solar radii (approximately 29.22 million km), equating to about 4,581 Earth radii. The mass escalates to 64.5 solar masses, a staggering figure that translates to 21.5 million times Earth's mass.

With this updated distance, Alnilam's brightness reaches an astounding 832,000 solar luminosities, equating to an energy output of 3.2 × 10³² watts. Considering the uncertainty in the distance, the upper estimate could exceed 1,738,000 solar luminosities, potentially ranking Alnilam among the intrinsically brightest stars in the Milky Way.

However, given the uncertainties surrounding these measurements, the true distance of Alnilam is likely between 410 and 600 parsecs, resulting in an absolute brightness ranging from 600,000 to 800,000 solar luminosities, confirming its position as one of the brightest stars in our galaxy.

Despite this, Alnilam ranks only 29th in brightness among observable stars from Earth. Why does Deneb, which is both more distant and less intrinsically luminous than Alnilam, appear slightly brighter to the naked eye?

Understanding this requires considering various factors influencing perceived brightness. While a star's distance and intrinsic brightness are crucial, extinction caused by interstellar dust and the spectral distribution of emitted energy also play significant roles. Alnilam, with a photospheric temperature around 28,000 K, emits much of its radiation in the ultraviolet spectrum, which is less visible to the human eye. Conversely, Deneb has lower ultraviolet emissions but higher output in the visible range, making it seem brighter.

Mass Loss, Rotation, Variability, Disintegration

To provide a complete picture of Alnilam, it's essential to address its mass loss and rotation speed.

Hot, massive stars like Alnilam experience significant mass loss due to powerful stellar winds. The star's radiation pressure drives this process, as energetic photons are absorbed by metallic ions in the atmosphere, resulting in an outward acceleration.

Numerous studies have estimated Alnilam's mass loss rate. A 1986 study reported 3.1 × 10?? M?/yr, which, while seemingly modest, is substantial compared to the Sun's rate, which is eight orders of magnitude lower. Subsequent research, including a 1993 study by Lamers, corroborated this estimate, and other studies have yielded similar values, with some indicating rates between 1.7 and 2.5 millionths of a solar mass per year. However, a 2015 study suggested a higher loss rate of 5.6 × 10?? M?/yr.

There have also been attempts to estimate Alnilam's rotation speed, but without knowing the inclination of its rotation axis relative to Earth, estimates only provide a minimum value. Studies since the 1970s suggest a minimum rotation speed between 42 and 85 km/s, corresponding to rotation periods of about 20 to 50 Earth days, indicating that Alnilam is not a rapid rotator.

Spectroscopic observations have also revealed that Alnilam is an Alpha Cygni variable, displaying a period of 1.9 days likely linked to pulsations related to its advanced evolutionary stage.

Future Fate of Alnilam

What lies ahead for this magnificent star? Researchers have inferred from the carbon-to-nitrogen ratio in Alnilam's atmosphere that it is undergoing the evolutionary phase that will transition it into a red supergiant. This will lead to further expansions, contractions, temperature fluctuations, and additional mass losses, culminating in a powerful supernova explosion that will likely result in a stellar-mass black hole.

When Alnilam eventually detonates, observers will witness a spectacular event. Its relative proximity will allow future astronomers to study the explosion and resultant supernova remnant with unprecedented detail, far surpassing what we gleaned from SN 1987 A, the closest and most studied supernova since the advent of the telescope.

Notes

[1] Stellar magnitudes are inversely proportional; lower values represent higher brightness.

[2] The classification of brightness into classes I and Ia was introduced during this period.

[3] No binary companion of Alnilam has been confirmed, which would have allowed for mass calculation through orbital parameters.

[4] Surface gravity is expressed as the base-10 logarithm of gravitational acceleration in CGS units.

[5] This magnitude represents the luminosity Alnilam would have if observed from a distance of 10 parsecs.

[6] The exact value of luminosity reported includes uncertainty factors.

[7] Angular diameter measurements have been obtained through interferometric observations.

[8] Alnilam's mass corresponds to approximately 1.283 × 10³² kg.

[9] Deneb's apparent visual magnitude is 1.25, while Alnilam's is 1.69.

[10] The Sun's mass loss is significantly lower than that of Alnilam.

[11] Alnilam emits across the electromagnetic spectrum, from X-rays to radio waves.