The stars we observe in the night sky appear as mere pinpricks of light, yet some celestial bodies defy comprehension of scale. Among the most colossal objects in the Milky Way galaxy is UY Scuti, a star whose physical size pushes the boundaries of what a star can be. This stellar giant is so immense that comparing it to our own Sun offers only a starting point for understanding its true magnitude.
What Is UY Scuti?
UY Scuti is classified as a Red Supergiant or Red Hypergiant, stars that have exhausted the hydrogen fuel in their cores and expanded dramatically. It is located in the constellation Scutum, near the plane of the Milky Way galaxy. Distance estimates generally place it around 5,900 to 9,500 light-years from Earth.
Its spectral type is M4Ia–Iab, indicating a cool surface temperature of approximately 3,365 Kelvin and giving it a characteristic reddish hue. This low temperature and extraordinary luminosity signal its enormous surface area and late stage of life. UY Scuti is also a semiregular variable star, meaning its brightness and size fluctuate over a period of about 740 days as its outer layers pulse.
The Volumetric Comparison: Fitting Earths Inside
The question of how many Earths can fit inside UY Scuti requires a comparison of their respective volumes. UY Scuti’s radius is estimated to be around 1,700 times that of the Sun. Since the Sun’s radius is approximately 109 times greater than Earth’s, UY Scuti’s radius is roughly 185,300 times larger than Earth’s radius.
Volume is calculated by cubing the radius. Cubing the radius ratio of 185,300 shows that the theoretical volume of UY Scuti is large enough to contain approximately 6.3 quadrillion Earths. This figure represents the direct volume ratio.
A more practical calculation accounts for the reality of packing spherical Earths inside a sphere, which reduces the number due to unavoidable empty space. Even with this sphere-packing consideration, the star could still physically hold about 4.8 quadrillion complete Earths within its boundary.
Understanding the Immense Scale
To truly grasp the size of UY Scuti, it helps to place it in the context of our own solar system. If this star were to replace our Sun, its vast photosphere, or visible surface, would extend far beyond the orbits of the inner planets. Mercury, Venus, Earth, and Mars would all be instantly engulfed by the star’s atmosphere.
The star’s outer edge would stretch past the orbit of Jupiter and could even approach the orbit of Saturn. The familiar structure of our solar system would be entirely contained within the boundary of this single star. Even light, which travels at an incredible speed, would take approximately seven hours to circumnavigate UY Scuti’s circumference.
The star’s volume is nearly 5 billion times greater than the volume of our Sun. This enormous size makes it a striking example of the upper limits of stellar expansion near the end of a massive star’s life cycle.
How Astronomers Measure Stellar Giants
Measuring a star as distant and large as UY Scuti presents significant technical challenges for astronomers. The primary method involves combining two separate measurements: the star’s angular diameter and its distance from Earth.
The angular diameter is the tiny, apparent width of the star as seen in the sky, which can be measured directly using advanced instruments like astronomical interferometers. Interferometers link multiple telescopes together to achieve the resolution of a much larger single dish, allowing for the detection of the star’s disc.
The distance to the star is then calculated using the parallax method, which measures the apparent shift in the star’s position as the Earth orbits the Sun. With the angular size and distance, simple trigonometry allows scientists to calculate the star’s true physical radius.
Estimates for UY Scuti are complicated because it is a semiregular variable star, meaning its size fluctuates. Its immense distance and location within the dusty plane of the Milky Way introduce errors in parallax measurements, leading to a wide range of radius estimates. Newer data, such as that from the Gaia mission, suggests a smaller radius for UY Scuti, closer to 909 solar radii, but it remains one of the most physically large stars known.