UY Scuti vs VY Canis Majoris – Which is the Largest Star in the Universe?

There are thought to be between 100 billion and 400 billion stars in our galaxy, the Milky Way, and way more beyond that – so which is the largest star in the universe according to current calculations in 2019?

In this article, we are going to examine the biggest known stars, dig into how they are discovered and what the challenges are in measuring their size, and compare the giant stars of UY Scuti and VY Canis Majoris to Earth and our star, the Sun.

How big are Earth and the Sun?

To start with let’s just make a point of comparison in noting how big the Earth and the Sun are.

Earth sits in the universe as part of our Solar System of eight planets. These revolve around one star – our Sun.

The Earth has a circumference of just over 40,000 km and a surface area of 510 million square kilometers (source).

The Sun has a circumference roughly 109 times bigger than the Earth, at over 4 million km. Whilst the surface area is over 6 trillion square kilometers – which is 12,000 times that of Earth (source).

Therefore, in terms of our Solar System, the Sun is huge compared to our Earth and even the biggest planet in our Solar System – Jupiter – is tiny compared to the Sun.

How is the size of the largest stars measured?

The largest stars are measured in a unit known as solar radius. That is how many times bigger is the star’s radius than that of our Sun (which is rounded to 695,700 km for one solar radius).

This measurement is used to simplify estimates, but it is complicated by the fact that super-giant stars change in size over time; thus, changing the size of the quoted radius significantly.

Notably, a star’s size correlates with its temperature – the bigger the star, the colder it tends to be. However, these stars are “cold” in the sense that we would understand. They are hot, just not as hot as their smaller counterparts.

So which are the biggest stars in our universe?

VY Canis Majoris – previously the largest star

Up until 2012, VY Canis Majoris was considered to be the largest star in the universe. At that time, the star was estimated to be at least 2,800 – 4,000 solar radii in size.

However, more research revealed that the figures could be much lower than what had been stated before, and the current consensus is that VY Canis Majoris has a solar radii of around 1,420.

VY Canis Majoris is a massive red supergiant star. It is within our galaxy, the Milky Way, and can be located in the constellation of Canis Major.

The star is a pulsating one, which means that its size reduces and increases irregularly from time to time. This aspect also contributed significantly to the decrease in the size of this star.

Check out this video by the European Southern Observatory (ESO) which zooms in on VY Canis Majoris from a high-definition image of the Milky Way:

UY Scuti – currently the largest known star in the universe

Currently, then, it is believed that UY Scuti is the biggest star in the known universe.

It is also a massive red supergiant and a variable star.

Again, it resides in the Milky Way and is located in the constellation Scutum.

Current measurements, put its solar radius is 1708. To give you a perspective of how big this star is, we would require 489 trillion Earths to fit into the volume of this star.

UY Scuti
A zoomed-in picture of UY Scuti processed through the Rutherford Observatory’s telescope (Credit: Haktarfone)

Other potential biggest stars

Feeling small already? Even with these stars topping the charts at the moment, there are others that could be bigger than them. However, due to dust debris of stars exploding within the Milky Way, it is difficult, at times even impossible to catch a glimpse of these stars.

There are stars like WOH G64 which have been identified as possibly being bigger but their positioning and motion are not yet clearly defined.

It should also be noted that the size of stars can also be calculated by mass and the most massive stars (measured in terms of solar masses) are different from the largest stars (measured in solar radii).

Limits on our calculations of the biggest stars

It is important to remember that these stars, UY Scuti and VY Canis Majoris, both reside in the Milky Way, and the extent of mapping and measuring all the stars in this galaxy is currently extremely limited.

To illustrate we don’t even know how many stars there are in our galaxy with any certainty – the current best estimate is between 100 billion and 400 billion – let alone have been able to accurately measure their size.

Further to this, the current best estimate is that our galaxy (remember, containing up to 400 billion stars), is only one of 2 trillion galaxies in the observable universe.

If you want your mind blown further, then read our ‘How many planets are there?‘ article to dig into what we know about the size of the universe.

    Anthony Robinson founded Skies & Scopes in 2017 and has been practicing and writing obsessively about astrophotography and astronomy ever since. He has written for Amateur Astrophotography and Dark Sky Travels magazines. Skies & Scopes has also been featured in Forbes, Digital Camera World, Peta Pixel, and many other publications.


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