UY Scuti – Meet a hypergiant star

Do you ever wonder about the size of the universe? Looking up at the sky, at all those stars, gives a pretty good idea, but can our brain even process how vast it really is?

Take the Sun for example. It is massive compared to our home planet, to the point that “it would take 1.3 million Earths to fill the Sun’s volume”, according to NASA. And yet in terms of size and volume the Sun is just a regular star in the cosmos.

Read more about the Sun here: The Sun, our host star.

Especially if we consider that there are countless stars in the universe (with the current technology we can only take a good guess), and around 100 billion in the Milky Way alone, according to NASA’s latest estimations.

The Sun with a large prominence and the Earth in scale – Credit: A. Anfuso ®

Among these is UY Scuti, a bright red hypergiant star located near the centre of our galaxy, approx. 9,500 light years away from us, in the small but rich constellation Scutum.

Despite being very luminous, it has a relatively low magnitude oscillating between 8.3 and 10.5 as seen from Earth, mainly due to its distance and its position in the Zone of Avoidance (ZOA), an area of the sky obscured by the Milky Way.

The fluctuation in UY Scuti’s magnitude indicates that UY Scuti is as a semiregular variable star. First catalogued in 1860 by astronomers at the Bonn Observatory, in Germany, the following observations in fact showed an approximate pulsation period of 740 days.

Rather than its magnitude though, it’s the size of UY Scuti that caught the eye of astronomers. It has nearly five billion times the volume of the Sun (yes, it means that around 5 billion Suns would fit within it!) and latest assessments suggest the star has a radius around 1,700 times larger than that our star, which makes UY Scuti the second largest star by radius, behind Stephenson 2-18 (2,150 solar radii).

Credit: Wikipedia

At the time of writing, astronomers have not discovered any companion stars or planets orbiting UY Scuti although it is widely believed that the radiations emitted by the enormous star would make life as we know it impossible, even if there were indeed some tenacious planets in its orbit.

Besides, UY Scuti is believed to be nearing the end of its cycle. Stars typically spend 90% of their life on main sequence, converting hydrogen into helium in their core, and according to current stellar evolution models, UY Scuti has already completed its main sequence phase and has started fusing helium in its core. This means the star is somewhere within the last 10% of its life cycle.

Once it runs out of helium, it will start fusing heavy elements. This will disrupt the balance the star needs to sustain itself against gravity, causing a gravitational collapse and ending into a spectacular, powerful supernova.

According to some models, the core left behind by the explosion would then become a black hole or a neutron star.

UY Scuti – Credit: NASA

Observing UY Scuti is quite easy. Despite being located in the Zone of Avoidance, in fact, the star is so large and bright that can be observed from Earth, even with amateur telescopes.

It is located just northeast of the Eagle Nebula and can be seen even with a pair of binoculars or a small telescope.

While UY Scuti’s significant size is perhaps not enough for understanding how big the universe is, it is definitely helpful to put things into perspective and to realise how small we are in the scale of the universe.

After all, “we are just a slightly advanced breed of monkeys on a minor planet orbiting an average star” as Stephen Hawking once said.

As bold as it sounds, it’s an undisputable truth.