R136a1

## Observation

### **Discovered by**

* Michael W. Feast
* Andrew D. Thackeray
* Adriaan J. Wesselink

## Properties

### **Characteristics**

* Brightest known star
* Most massive known star
* Sits in the central part of a nebula
* Grouped with 2 other very massive stars

## Kurzgesagt

### **First appearance**

The Largest Star in the Universe – Size Comparison

### **Latest appearance**

The Largest Star in the Universe – Size Comparison

## General Information

R136a1 is a blue giant, Wolf–Rayet, main-sequence star. It is located at the core of the Tarantula Nebula.

At 196 M☉ and around 4.7 million L☉, R136a1 is among the most massive, brightest, and hottest stars known to science. Its temperature is also rather high, at about 46,000 K. The star at the core of R136, the Tarantula Nebula (30 Doradus) in the enormous Magellanic Cloud, is a Wolf-Rayet star. It is part of the central concentration of stars in the enormous NGC 2070 open cluster. Using a small telescope or binoculars, one may observe the cluster in the far southern celestial hemisphere at a magnitude of 7.25. R136a1 itself is only resolvable with speckle interferometry, as it is 100 times fainter than the cluster.

R136a1 is a Wolf–Rayet star (WR star), one of the rarest types of stars. R136a1 is the brightest and most massive member of the R136 star cluster, which sits at the center of Tarantula Nebula, itself located in the satellite galaxy Large Magellanic Cloud.

### History

#### Formation

High mass star formation is not well understood, partly because direct observations are hampered by their creation in dense, dusty areas far from the Sun, and partly because radiation pressure is thought to induce outflows that impede accretion (though there are counterarguments).

According to the current agreement, high mass stars originate very quickly, maybe within a couple of hundred thousand years, and may include stellar mergers in very dense systems. The total stellar mass is estimated to be 50,000 solar masses within a few parsec (10 light years) of R136a1, meaning that a total of about 100,000 stars have formed or are in the process of creating. Merely twelve stars out of them have estimated masses more than 100 solar masses.

#### Discovery

The brightness and spectra of luminous stars in the Large Magellanic Cloud were systematically measured in 1960 by a team of astronomers at the Radcliffe Observatory in Pretoria. A multiple star system is likely what the observers decided was happening to RMC 136, the center "star" of the Tarantula Nebula, among the objects cataloged. Following investigations, it was determined that R136 was situated in the midst of an enormous H II region, or area of ionized interstellar hydrogen, a hotspot for active star formation right next to the detected stars.

R136 was divided into three parts in 1979 using ESO's 3.6 m telescope: R136a, R136b, and R136c. There was a great deal of debate on the precise nature of R136a. A star 3,000 times the mass of the Sun was thought to be the most plausible answer, since estimates suggested that the brightness of the core area would need up to 100 hot O class stars within half a parsec at the cluster's center.

Weigelt and Beier presented the first evidence that R136a was a star cluster in 1985. The cluster's center, R136a, was found to consist of eight stars within 1 arcsecond, with R136a1 being the brightest, according to the results of the speckle interferometry method.

Following the Hubble Space Telescope's launch, R136a's nature was finally confirmed. R136a was resolved into at least 12 components by its Wide Field and Planetary Camera (WFPC), demonstrating that R136 contains more than 200 extremely bright stars. 46 enormous bright stars in half a parsec and almost 3,000 stars within 4.7 parsec of R136a may be studied with the more sophisticated WFPC2.

### Properties

It has been determined that R136a1 may have a binary partner, albeit there is a 25% chance that this is just a coincidence. The Chandra X-ray Observatory picked up X-ray radiation from R136. R136a could not be resolved, however R136c and R136a were both definitely recognized.18] A different investigation distinguished R136a1/2 from R136a3. Relatively mild x-rays were seen in R136a1/2, which are not believed to be signs of a colliding winds binary.

#### Spectral Class

R136a1 is located in the very top left corner of the Hertzsprung-Russell diagram as a high-luminosity WN5h star. The broad, strong emission lines in the spectrum are indicative of a Wolf–Rayet star. Ionized nitrogen, helium, carbon, oxygen, and occasionally silicon are included in this; however, hydrogen lines are often faint or nonexistent. When ionized helium emission from a star is significantly greater than neutral helium emission lines and exhibits nearly similar emission intensity from NIII, NIV, and NV, the star is categorized as WN5. Significant hydrogen emission is indicated by the "h" in the spectral type, and by mass, hydrogen accounts for 40% of the surface abundance.

#### Mass

Based on HST visual spectra, an evolutionary mass of 215 M☉ is determined. After blazing for nearly a million years, R136a1 closely resembles the anticipated parameters for an initially quickly spinning 251 M☉ star with LMC metallicity.

A previous examination with UV spectroscopy determined an initial mass of 325 M☉ and a current mass of 315 M☉. Similar investigation utilizing PoWR (Potsdam Wolf–Rayet) atmosphere models with optical and ultraviolet spectra and a mass–luminosity ratio, assuming a single star, finds a current mass of 256 M☉.

Extreme mass loss is occurring in R136a1 due to a stellar wind that is traveling at 2,600±150 km/s. The reason for this is because the extremely hot photosphere's powerful electromagnetic radiation accelerates material away from the surface faster than gravity can hold it. With low surface gravity and elevated concentrations of heavy elements in the photosphere, high-luminosity stars experience the greatest mass loss. Over a billion times higher than the Sun's annual loss, R136a1 sheds 1.6×10−4 M☉ (3.21×1018 kg/s) and is predicted to have shed roughly 35 M☉ since creation.

#### Size

R136a1 has a radius of more than forty times that of the Sun (42.7 R☉; 29,700,000 km; 1.7 au), meaning that its volume is about 80,000 times bigger than that of the Sun. Unlike the Earth and the Sun, R136a1 does not have a well defined visible surface. A thick atmosphere encircling the star's hydrostatic main body is being driven outward into the stellar wind. The surface used to determine the radius is specified as an arbitrary place inside this wind, and multiple writers may use various interpretations.