Huge, Massive, Mega, Gargantuan. These are the words that barely begin to describe the most massive stars in the universe. Let’s start with the familiar and take it from there. We all know and love the Sun, it gives us light, energy and life. Without it, we would not exist and life as we know it could not occur. Yet it isn’t the biggest of stars, in fact it is only a mid-sized main sequence star. The Sun is - on the cosmic scale - a rather regular and boring star, which is good for us as it gives stability and great temperatures where liquid water and thus life can occur.
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Let’s look at what is normal and build upwards from there. Below is a diagram known as the Hertzsprung-Russell diagram. It is the main tool used to map most stars given their size, luminosity and mass. We can even see the position of the Sun which is slightly below and to the right of the center of the diagram.
Image source: https://www.eso.org/public/images/eso0728c/ (Credit: ESO)
Generally as we go up and to the left, the pattern of main sequence stars (that slash through the middle of the diagram) shows larger blue stars which have a large luminosity (the amount of light they put out) and higher surface temperature. They also tend to be bigger. More light, generally means a larger amount of matter in the star.
An example of a large main sequence star is Mu Columbae. This star is a star in the constellation Columba. It is a naked eye blue star which means that if you are lucky enough to look the right way, then you might be able to see it in the night sky. It has a mass of approximately 16 solar masses which means that it is approximately 45,000 solar luminosities. So it is 45,000 brighter than the Sun. While this is not necessarily the biggest blue star in our galaxy, it is a good example of what possible values come from the largest of main sequence (MS) stars.
Blue Main Sequence (MS) Stars:
These blue MS stars are able to produce the heavy elements in our universe which are beyond Carbon, beyond Oxygen and on to heavier elements like metals all the way up to Iron. They do this using the immense pressure in their core and the process of stellar nucleosynthesis which couples with quantum tunneling and leads to incredible and complicated fusion processes. The rest of the elements which are heavier and more dense than Iron are made during the supernovae process caused by the dying of these large stars. Any element beyond Iron which is element 26 is made by the dying of these large main sequence stars. Elements such as Copper, Gold, Lead, Iodine etc. all come from those stellar sized explosions.
This is what makes the blue MS stars the elemental engines that they are. Without that immense size and pressure from all the matter above, the core would not be able to perform its job of producing enough energy and heavier products - through fusion - to sustain the core. The strong gravity from all the matter in the massive star is balanced by all the energy output pushing outwards from the core of the star.
Elemental Engines:
Image Source: https://hubblesite.org/contents/media/images/4501-Image.html (Credit: NASA, ESA, Leah Hustak (STScI))
While the largest blue MS stars are large indeed and they give us many of the heavier elements that we know, rely upon and use everyday. These are not the largest stars, they are actually far from it.
Beyond the Main Sequence (MS):
If, as this sub-title suggests, we move beyond the main sequence, there are supergiants which you can see at the top of Hertzsprung-Russell diagram. These can be larger in size than main sequence stars. Rigel for example is a blue supergiant star and has a mass of about 18 Suns and has a luminosity which is 47,000 times that of the Sun. You might notice that there is an overlap between the largest main sequence stars and blue supergiants and there is indeed comparison in size. Yet, the truly largest and biggest stars belong in a category all of their own.
Hypergiants, the largest stars in the galaxy (and likely all galaxies) have incredible size and scale. These beasts range in size massively, but reach to the absolutely largest of stars. Some comparisons here help show the scale of them below. The stars shown on the right are VY Canis Major (most right), then VY Cephi, WOH G64. They are all red hyper-giants. In this image, a single pixel represents the size of the entire Sun.
Image Credit: https://www.flickr.com/photos/feb28/4820647230
The largest star that we currently know of and is universally accepted is UV Scuti. This unbelievably massive star has rather insane and incredible characteristics that are difficult to understand how it has puffed up so much. UV Scuti has a diameter that is 1,700 times larger than that of the Sun. The diameter is approximately 2.4 billion kilometers. If UY Scuti were placed where the Sun currently sits, its outer layers would extend all the way to the orbit of Jupiter. Now that is an unfathomable size!
Image credit: Philip Park/CC by SA 3.0 (link: https://www.livescience.com/largest-smallest-known-stars-universe)
Interestingly since UY Scuti has puffy properties and the layering in the star is extended, this star only has an estimated mass of 7 to 10 Solar masses. While it is the largest star we know of, it is not actually the most massive. That honor belongs to R136c, the mass values are estimated to be between 170 to 230 solar masses. Regardless of the definite number, it is truly a colossal star with epically massive proportions. This star is so incredibly massive that it is difficult for it to keep sustained fusion because when it starts fusion it throws off its outer layers before they are brought back by gravity and the star starts fusion again.
Understanding what dynamics the largest stars have inside of them is an intriguing puzzle and can give us new insight into fusion and the limits of what is possible for stars in our universe. I hope you have enjoyed the above and perhaps learning something new.
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Thanks for reading.
Cheers and stay curious
Oliver - The Teaching Astrophysicist
(Note: This blog post was NOT generated by AI and is conceived, typed and uploaded by a real person.)
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