Admin and Founder of ‘The Secrets Of The Universe’ and former intern at Indian Institute of Astrophysics, Bangalore, I am a science student pursuing a Master’s in Physics from India. I love to study and write about Stellar Astrophysics, Relativity & Quantum Mechanics.
There are approximately 1 trillion trillion stars in our Universe, a number approximated by European Space Agency (ESA). Stars in the Universe come in all sizes. Their radius can be as small as 20 km and as big as 1 billion km. One such extreme star is UY Scuti, a red supergiant star and a beast inside of which 5 billion Suns can fit.
How was UY Scuti discovered?
UY Scuti is a red supergiant star in the constellation of Scutum and was earlier known as the largest star in the universe. It lies approximately 9,500 light years away, where 1 light-year is roughly 6 trillion miles. In the summer of 2012, astronomers using the Very Large Telescope in the Atacama Desert in Chile measured the parameters of three red supergiants near the Galactic Center region: UY Scuti, AH Scorpii, and KW Sagittarii. They determined that all three stars are over 1,000 times bigger and over 100,000 times more luminous than the Sun. The stars’ sizes were calculated using the Rosseland Radius, where the optical depth is 2/3, with distances adopted from earlier publications.
How Big Is UY Scuti?
The star was found to be the largest and the most luminous of the three stars measured, at 1,708 ± 192 R0, which is larger than the orbit of Jupiter. Its size can be realized from the fact that even if we build a hypothetical object that travels at the speed of light, it will take seven hours to travel around it, whereas for the Sun, it will just take 14.5 seconds. Thus, as many as 5 billion Suns can fit inside UY Scuti.
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UY Scuti is 340,000 times more luminous than the Sun. This means that the total energy output of this star is 340,000 times greater than our star. However, its surface temperature is about 3,300 K, roughly half the value for Sun. The surface temperature can be estimated by using Wein’s law in physics which states that the wavelength of light emitted from a black body is inversely proportional to the 4th power of its absolute temperature. Hence, red-colored stars are cooler.
Although the star is very luminous, it isn’t visible to the naked eye as it is a ninth magnitude star. We can only see stars up to the sixth magnitude with the naked eye.
A star spends 90% of its life on the main sequence of the Hertzsprung Russell diagram. On the main sequence, it converts hydrogen into helium in its core. The Sun has been on the main sequence for the past 5 billion years and will remain there for another 5 billion years or so. On the main sequence, the inward gravitational collapse of the star is balanced by the outward gas and radiation pressure from the core nuclear reaction – a situation termed as hydrostatic equilibrium.
Stellar evolution models tell that UY Scuti has already left the main sequence and has started fusing helium in its core. This means it is left with just 10% of its life. Stars like these have a short lifespan of a few million years compared to trillions of years for red dwarfs. After helium, heavier elements like carbon, neon, oxygen, magnesium, sulfur, silicon, iron, and nickel will be created. Nickel-56 would be the last major fusion product in its core.
The reaction sequence ends at Nickel-56. This is because it peaks the binding energy per nucleon curve. In other words, the next nuclear reaction, Ni-56 to Zn-60, will consume energy rather than releasing. Hence it is not thermodynamically favorable. So at Ni-56, the core becomes inert. Without the core nuclear reaction, gravity will gain the upper hand and start crushing the star. No subsequent fusion reaction will be possible. This will give gravitational collapse an upper hand, and a catastrophic collapse will ensue, forming a stellar-mass black hole.
The largest stars found to date
After its discovery, UY Scuti topped the chart of the biggest stars found to date for a long time. But as of August 2021, Stephenson 2-18 is the largest star known to us. The star’s radius is about 2150 times that of the Sun, which means if it is placed at the center of the Solar System, its photosphere could potentially engulf the orbit of Saturn.
Learn Astrophysics at Home
Did you always want to learn how the universe works? Then read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!