The largest stars in the known universe
There are stars hundreds, and even thousands, of times as massive as our star, the sun
Despite being the largest object in our solar system, and its importance to the viability of all life on Earth, our sun is a pretty average-sized star compared to other stars in our galaxy, and in the universe as a whole.
There are stars hundreds, and even thousands, of times as massive as our star, the sun. Stars can range in size from that of Jupiter (often considered a "failed star") to those larger than the orbit of Jupiter around our sun.
A star's size is dependent on two factors - its mass ( i.e., the amount of gas and dust it accumulates during its formation) and its volume (i.e., how much space it occupies). It would appear to hold that the more mass a star has, the larger it should be, but in reality, this is not always the case; most massive stars (i.e., possessing large amounts of matter) are not all that remarkable size-wise, while the most voluminous stars (i.e., physically large) can actually contain a lot less matter, considering their size.
MEASURING A STAR'S SIZE
Astronomers use a variety of techniques to measure a star's size. Direct imaging takes photos of a star through a telescope, from which astronomers measure the angular size of the star, and, multiplying it by the star's estimated distance, arrive at the star's diameter.
Lunar occultations, where a star is covered or occulted by the moon as it crosses the night sky, is another technique used, which measures the light from the star, as a function of time, to calculate the star's size from its light curve (i.e., the angular size of the star divided by the angular speed of the moon).
The measurement of the light from eclipsing binary stars (two stars orbiting one another) is sometimes used to determine a star's size,
whereby the amount of light coming from such a system is measured in order to detect the decrease in the total intensity as a portion of one of the stars is covered.
Finally, the technique of interferometry, the use of the interference of superimposed electromagnetic waves to extract information in a host of scientific fields, including astronomy, is another technique used. All these techniques suffer from challenges due to diffraction and other technical shortcomings, making the end results sometimes inaccurate.
A STAR'S ORIGINS
Stars are born from the vast nebulous cloud of dust and molecular gas, primarily neutral hydrogen and helium, that was created when the universe first formed. As the dust and gas swirl around each other, the suspended atoms of gas within the cloud are drawn into clusters, causing portions of the cloud to collapse and fragment into denser pockets.
These pockets, in turn, draw in more and more matter inward, becoming larger and denser, until the accumulated dust and gas undergoes its own gravitational collapse, and the atoms becoming so heated (reaching temperatures between 10-15 million Kelvin) and compressed that the resulting pressure and energy strips the atoms of their electrons, creating a plasma of ionized gas.
The exposed hydrogen nuclei, moving faster and faster, become so compacted together that nuclear fusion ignites within the star's core.
As the nuclei collide, they fuse together, releasing enormous amounts of energy, which forces the plasma outwards, increasing the star's size. However, the star's mass creates a gravitational influence which pulls the expanding plasma back towards the star's center; the outward and inward forces counteract one another, until what is referred to as "hydrostatic equilibrium"
achinevedd, is whereby the gravitational pull and the electromagnetic radiation pressures reach a point of equal balance, marked by the star's outer surface, known as the "photosphere" (the solar layer from which light is emitted). The photosphere's edge is used by astronomers to determine the star's absolute radius, and, thus, its size.
'SOLAR RADIUS'
The radius of our sun (approximately 695, 700 km), referred to as a "solar radius" or SR, is used as the benchmark in referring to a star's radius. Until a few years ago, the star UY Scuti, a red supergiant star in the constellation of Scutum - the Shield, initially estimated as having a radius of 1,7008SR, was believed to be the largest star by physical size ever observed. However, an error in its estimated distance from Earth (now calculated at 5,900 ly) led to a revision of its size, reducing its estimated radius to about 909 SR, knocking it from top spot. The star Stephenson 2 -18 DFK-1, also in the constellation of Scutum, a possible red extreme hypergiant star estimated to be 18,900 ly from Earth, was initially estimated to have a radius of 2,150 SR; however, like UY Scuti, its distance from Earth is in dispute, having been possibly overestimated by as much as 50 per cent, thereby calling its estimated size into question.
SUPERGIANT STARS
Other large red supergiant stars observed include: Antares A in the constellation of Scorpius - the Scorpion, with an estimated mass of 11-14 solar masses and a radius of 685 SR; Betelgeuse in the constellation of Orion - the Hunter, with an estimated mass of 14-19 solar masses and a radius of 764-1,021 SR; UY Cephei, in the constellation of Cepheus - the King (of Aethiopia in Greek mythology), with an estimated mass of 18.2 solar masses and a radius of 1,050 SR; Mu Cephei, also in Cepheus, with an estimated mass of 19.2 solar masses and a radius of 972 SR (plus/minus 228 SR); and UY Canis Majoris, a red hypergiant star 3,900 ly from Earth in the constellation of Canis Major - the Big Dog, having an estimated mass of 17 solar masses, and a radius of 1,429SR, approaching the theoretical limit.
ESTIMATING A STAR'S SIZE
The huge estimated sizes of these giant-sized stars forced astronomers to consider if there wasn't a theoretical limit to just how large a star could become. Although there is no fixed equation for determining the upper limit of a star's size (that size is not only based on stellar mass, but also the star's composition, evolutionary stage, and the strength of the stellar winds), astronomers theorize that the limit should be in the area of about 1,500SR. While the outer atmosphere of truly massive stars could, theoretically, continue to expand outward without a limit, at some point the diffuse outer layer would merge with the interstellar medium, and would no longer be considered the stellar surface.
BAT 99-98
The current largest star by mass is thought to be a star labelled Bat 99-98, a red hypergiant star in the Tarantula Nebula in the R136 cluster of the Large Magellanic Cloud, a satellite galaxy to our Milky Way galaxy. At a distance of 165,000 ly from Earth, it has an estimated mass of 226 solar masses (one solar mass equals the mass of our sun), and an estimated radius of 37.5SR, it is considered one of the largest stars discovered to date, and also to be a fairly young star, with an age of only 7.5 myr, compared to our sun's current age of 4.6 byrs.
It is theorized that Bat 99-98 may have formed from the merger of two stars, and that it will most likely end its life in dramatic fashion, exploding as a supernova.
WOH G64
Astronomers currently estimate the star WOH G64 as the largest star by volume in the known universe. This star is a five myr old, red supergiant located 160,000 ly from Earth located in the Large Magellanic Cloud satellite galaxy. With an estimated mass of 25 solar masses, and an estimated radius of 1,540SR (plus/minus 77SR), it is consistent with the theoretical limit of how large stars can grow, and with the measurement of other red supergiant stars discovered elsewhere. Despite its young age, WOH G^4 is believed to be in the later stages of its stellar evolution, with the majority of its nuclear fuel expended, Aand its outer layers swollen to a truly cosmic size.
THIS WEEK'S SKY
Mercury (mag. +0.5, in Pisces - the Fish), although approaching its greatest elongation from the sun, is not observable this week, as it sits just on the southeast horizon at dawn.
Venus (mag. -3.9, in Aries - the Ram), soon to pass behind the Sun (superior solar conjunction), is only 6 degrees separation from the sun, and is not observable. Mars (mag.+1.1, in Pisces) is not observable, sitting only 6 degrees above the southeast horizon by dawn.
Jupiter, also approaching superior solar conjunction, is too close to the Sun to be observed.
Saturn (mag. +1.2, in Aquarius - the Water-bearer) is the only planet readily visible in the night sky this week, rising in the southeast around 3:25 a.m., and reaching 13 degrees above the horizon before fading from view as dawn breaks around 4:50 a.m.
Uranus (mag. +5.9, in Aries) is not observable, as it is very close to the Sun.
Neptune (mag. +7.9, in Pisces) is also not observable this week, as it reaches a height of no higher than 3 degrees above the southeast horizon by dawn.
Remember to keep watching the constellation of Corona Borealis on any clear night for a sign of the CRB T nova.
Until next week, clear skies.