Blue Giant Stars – 12 Interesting Facts

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Written By: Richard J Bartlett
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Category: Learn

You might not know it from looking at it, but our local star, the Sun, is really nothing special. In fact, it’s just a regular yellow dwarf star. It dominates our solar system, and yet there are countless others that put ours to shame.

Our own Milky Way galaxy contains billions of stars of many different sizes and colors. From cool red dwarves to hot blue giants, there are thought to be about 250 billion stars within our galaxy alone. On any given clear night, you might be able to see about 2,000 of them.

In addition to being visually stunning, blue giant and supergiant stars act as colossal metal factories, playing a pivotal role in the synthesis and distribution of heavy elements in the universe. Their ability to produce these elements and their subsequent dispersal through supernovae explosions shape the composition of galaxies and influence the formation of diverse celestial objects. In understanding the workings of blue giants, we uncover not only the secrets of these magnificent stars but also the very processes that have led to the complexity and richness of the universe. Their study provides a profound insight into the cosmic cycle of creation, destruction, and rebirth, reminding us of our deep-rooted connection with the stars.

What colors can you see? Which stars stand out? Which ones are the hottest? Or the largest? Which ones live the longest? Today we’ll consider the blue giant stars of the cosmos—the kings whose reigns are short and lonely.

Zeta Puppis, a blue star
Credit: Kryptid

1. Blue Giants Are Very Hot

Earth’s average temperature is about 58 degrees Fahrenheit (or nearly 15 degrees Celsius). However, astronomers measure temperatures in Kelvin (K), and 58F is roughly 288K. In comparison, the surface of the Sun is nearly 10,000 F (5,500 C), or roughly 5,800 K.

That’s about 172 times hotter than the Earth, but still quite cool compared to blue giants. These stars are some of the hottest and can have surface temperatures in excess of 40,000K—about four times hotter than the Sun.

So why are blue stars so hot? Or, alternatively, why are hot stars blue? It really comes down to the star’s mass. As all stars produce energy through nuclear fusion, the more massive a star is, the more nuclear fusion occurs and the more energy it gives out.

Most energy is emitted at the blue end of the spectrum, and as light itself is energy, the more energy produced, the more blue light is emitted. Hence, hot, blue stars.

2. They Are Very Luminous

A size comparison of a red dwarf (lower left), the Sun (center), a blue dwarf (right) and R136a1 (background)
A size comparison of a red dwarf (lower left), the Sun (center), a blue dwarf (right) and R136a1 (background). Credit: ESO/M. Kornmesser

There is a star, some 28,000 light years away, that’s about two million times brighter than the Sun. When LBV 1806–20 was discovered back in January 2004, it was thought to be the biggest and most luminous star known, but we now know that’s not the case.

In fact, there are a number of stars even more luminous, with the current record holder being R136a1. This blue hypergiant shines with a light that’s nearly nine million times brighter than the Sun. Not surprisingly, it’s also the most massive star, with an estimated mass of over 250 Suns and a volume large enough to contain 27,000 Suns within it.

Again, the reason blue stars are so luminous comes down to energy. The more energy produced, the more light is emitted, and energy is emitted at the blue end of the spectrum. The reason R136a1 is so luminous is that it generates more energy in four seconds than the Sun does in a year. So you have a very luminous, hot, blue giant star!

3. Blue Giant Stars Are Unstable

There’s a limit to a star’s luminosity, and this limit actually has a name, the Eddington Limit (or Eddington Luminosity). Named for the man who discovered it, Sir Arthur Eddington, it describes the balance between the force of radiation (energy as light) being pushed outward and the force of gravity pulling inward.

Super and hypergiant stars live close to the edge of this limit. Sometimes, when the limit is exceeded and the outward push of radiation is greater than the inward pull of gravity, the star ejects matter and suddenly brightens. Eventually, balance returns, the star slips below the Eddington Limit again, less radiation (light) is emitted, and the star consequently dims.

Another characteristic that can lead to blue giants’ stability is the unusually rapid rotation of some stars, which can cause them to become out-of-round, with a wide equator and squashed polar regions. 

To all intents and purposes, it’s as though the star has become a temporary supernova.

A classic example of this is the star Eta Carinae, in the southern hemisphere. This multiple star system has a combined luminosity of over five million Suns and a combined mass of about 200 Suns. Over the past 160 years, it has undergone a number of sporadic outbursts, causing it to shift back and forth from naked-eye brilliance to invisibility.

Most famously, in 1843, it became the second-brightest star in the sky as a result of an event now known as the Great Eruption. The aftermath of this outburst can be seen in photographs taken with the Hubble Space Telescope. It’s currently a relatively dim magnitude 4.3 star, but it’s been consistently brightening over the past few years.

Hubble image of Eta Carinae clearly shows the huge bubbles of material that were blown off during the Great Eruption.
Hubble image of Eta Carinae showing huge bubbles of material that were blown off during the Great Eruption. Credit – NASA

4. These Stars Don’t Live Long

“The flame that burns twice as bright burns half as long,” said Lao Tzu, the renowned philosopher of ancient China. He was, of course, talking about our mortal lives, but he could have been talking about the lives of giant blue stars.

Put simply, stars survive by fusing hydrogen into helium. The more massive the star, the quicker it burns through its supply of hydrogen and, therefore, the shorter its lifespan.

Our Sun is middle-aged; it’s been around for about four and a half billion years and probably has another five billion to go. That’s not bad. It’s certainly plenty of time for the human race to leave the Earth and find a new home.

In comparison, a red dwarf star typically has about a tenth the mass of the Sun and could theoretically survive for trillions of years. The universe itself is thought to only be about 13 billion years old, so there’s no way to test the theory yet!

(No one knows the fate of the universe or when, if at all, it will ever come to an end. Assuming the universe does come to an end, red dwarves could potentially last until the end of time!)

Now let’s consider blue giant stars. It’s not unusual for these stars to have masses of ten or twenty times greater than the Sun. Rigel, for example, has a mass of about 21 Suns. It’s burning through its hydrogen pretty rapidly and is losing mass ten million times faster than the Sun.

Despite only being about eight million years old, it’s already burned through its hydrogen and could be entering the final stages of its life. In fact, at a distance of about 800 light years, it could have already become a supernova, and we simply don’t know it yet. (Realistically, though, it’s probably still got millions of years to go.)

5. Some Blue Giants Are Quite Rare

The three stars of Orion's belt. From left to right, Alnitak, Alnilam and Mintaka
The three stars of Orion’s belt. From left to right, Alnitak, Alnilam, and Mintaka. Credit: Digitized Sky Survey

Stars are classified by their characteristics and typically fall into one of seven groups, with each group being assigned a letter of the alphabet. The seven groups (or types) are O, B, A, F, G, K, and M. The hottest and most luminous stars are known as Type O stars, while the coolest and least luminous are Type M.

Type O stars are the rarest, and you may have already guessed the reason why. They’re typically very hot, very luminous, and very massive, so they appear blue and tend to burn through their fuel very quickly and then explode.

If we could watch a time-lapse video of the night sky over the past billion years, the chances are we’d see a lot of blue stars quickly appear and then suddenly vanish, like fireworks on New Year’s Eve.

They’re rare because they simply don’t live as long as the other stars, so they’re not as numerous. If you want to spot a couple of easy examples, turn towards Orion. Of the three stars that form his belt, two are type O blue giant stars: Mintaka, the westernmost, and Alnitak, the easternmost.

Both were probably born from the same star cloud, the Orion Molecular Cloud Complex. Alnitak is actually the brightest example of a type O star in the entire night sky. It’s a multiple star system, some 1,250 light years away, with the largest member being a blue giant some 33 times the mass of the Sun and with a luminosity of over 200,000 Suns.

6. Other Blue Giants Are More Common

The other type of blue star, type B, is actually fairly common. As type B stars tend to be less massive, they take longer to burn through their fuel, hence their longer lifespans, and there are more of them around.

Type B blue giants are relatively common in the night sky because of their luminosity; despite the distance, they’re still easily seen because they’re bright.

If you want to see some examples, you only have to look again at Orion. Rigel, the star that marks the hunter’s knee, is the seventh brightest star in the sky and the brightest example of a Type B blue giant.

At a distance of over 850 light years, it must be very luminous to be so easily seen. However, estimates of its luminosity wildly vary. The star could be anywhere between about 60,000 and 360,000 times more luminous than the Sun.

Another example can, again, be found in Orion. Alnilam is the middle and brightest star in Orion’s belt, with neighboring Mintaka and Alnitak on either side. Alnilam is, arguably, more impressive than any of the other blue giants to be found in the region.

Not only is it further away (about 2,000 light years), but with some estimates placing its luminosity at over 500,000 Suns, it’s the most luminous of the seven stars that form the brightest part of the constellation.

7. Where Can Blue Super Giants be Found?

The Jewel Box star cluster, in the southern constellation of Crux
The Jewel Box star cluster, in the southern constellation of Crux. Credit: ESO

Besides Orion, a winter constellation, there’s an easy way to find blue giants at any time of year: open star clusters.

Open star clusters (such as the Pleiades) are typically formed of young, hot, blue-white stars that have yet to drift apart from one another. The Pleiades is the most famous example, but there are, of course, many others to be found across the night sky.

Northern hemisphere observers can admire the giant blue stars of the Pleiades, but those in the southern hemisphere have their own prize.

NGC 4755, the famed Jewel Box cluster in the constellation of Crux, is a stunning open cluster and contains a number of blue giant stars. Messier 47, in Puppis, another winter sight, has a multitude of giant stars, while Messier 18, in the summer constellation of Sagittarius, has nearly thirty type B stars. In the autumn, NGC 663, in Cassiopeia, has over twenty.

Blue giant stars predominantly occur in OB associations. These are loose clusters of young, massive stars classified under spectral types O and B. Blue giants in OB associations play a vital role in the formation and evolution of galaxies.

8. Blue giant stars can switch colors

Blue giant stars have fascinated astronomers with their complex behaviors and stages of evolution. One captivating feature of blue giants is their ability to change colors, a phenomenon that correlates with changes in spectral classification. This color-switching can be understood through the different phases of their life cycle, along with changes in temperature and composition.

One intriguing aspect of blue giant evolution occurs as they approach the end of their main sequence phase and enter a transition where they may change color. When the hydrogen in their cores becomes depleted and helium burning becomes prominent, the outer layers can expand and cool. Consequently, a blue giant may shift towards the yellow spectral classification, altering its appearance.

Some stars, after evolving from the main sequence, may enter the Asymptotic Giant Branch (AGB) phase. This late stage is characterized by significant changes in size and brightness, with the core contracting and the outer layers expanding and cooling. During the AGB phase, thermal pulses may occur, causing fluctuations in appearance and color, with the star potentially appearing red or yellow.

Following the AGB phase, a star without sufficient mass to burn carbon after it begins to run low on helium may enter the post-AGB phase, characterized by substantial mass loss and the ejection of outer layers into space. The remaining core heats up and may appear blue or even shift into the ultraviolet spectrum. During this phase, the star is on its way to becoming a planetary nebula, with the process of shedding its outer layers leading to further color shifts, depending on the temperature and composition of the remaining core. The shedding of outer layers may cause the star to transform into a Wolf-Rayet star as well. A supernova may or may not happen, depending on how much of the star’s mass is left after this process. If no supernova occurs, the star continues to billow away its outer layers and may leave behind a white dwarf, in a similar manner to stars like our Sun.

More massive blue giant stars don’t enter the post-AGB phase. They may fuse carbon, then neon, then perhaps a couple heavier elements for a very short period of time before they finally go supernova, leaving behind a black hole or neutron star.

It’s important to note that not all blue giants will follow this exact evolutionary path. The transitions between different spectral classifications and the physical changes that occur are highly dependent on factors such as the star’s initial mass, metallicity, and other unique characteristics. While the deaths of highly massive stars rarely follow a single regular path, generally a nebula forms (either a planetary nebula as the star continues to shed its outer layers, or a supernova remnant), and some kind of stellar remnant is left over from the star’s crushed core in the form of a white dwarf, neutron star, or black hole.

9. Many rotate very rapidly

One subgroup of blue giants that stands out even further is the rapidly rotating blue giant. These stars provide a stunning showcase of extreme physics and unique behaviors that broaden our understanding of stellar dynamics.

Rapidly rotating blue giant stars can spin at velocities of hundreds of kilometers per second at their equators. This extreme rotation sets them apart from other stellar objects and results in distinct observable characteristics. The rotational speed is so immense that it can significantly alter the star’s shape, often leading to an oblate spheroid rather than a perfect sphere. 

The rapid rotation of these blue giants can be traced back to various factors, including their formation processes, the accretion of mass from binary companions, or the conservation of angular momentum as the star evolves and contracts.

This rapid spinning can have substantial effects on the star’s life. It can lead to a redistribution of internal temperatures, alter the star’s magnetic field, and influence the way a star sheds its outer layers as it evolves. Furthermore, the rotation can have a profound effect on the process of nucleosynthesis within the star, affecting the production and distribution of elements.

Among these spinning giants, one star stands out: the blue giant VFTS 102. Located in the Large Magellanic Cloud, it rotates at more than 1 million miles per hour (over 400 kilometers per second) at its equator. This makes it one of the fastest-known rotating stars. The cause of such extreme rotation is still a subject of research and may involve previous interactions with a binary companion.

10. They usually go supernova

Massive blue giant stars greater than 8–10 times the mass of our own Sun can go supernova when they die, provided that events prior to the end of their helium-burning phase don’t cause too much mass loss (e.g., Wolf-Rayet stars, which can blow away so much material that their cores can’t collapse). 

Supernova SN1987A is one of the most famous and well-studied stellar explosions in astronomy, and it holds a unique place in the research of blue giant stars. Located in the Large Magellanic Cloud, a dwarf galaxy that is a satellite of the Milky Way, SN1987A marked the cataclysmic end of a blue giant star named Sanduleak -69° 202.

11. Some “blue giants” are actually the result of a collision or merger between stars

Blue stragglers, essentially a type of blue giant star, were first discovered in the 1950s within globular star clusters. These immense, densely packed star clusters are known to contain some of the oldest stars in the universe, often more than 10 billion years old. The presence of relatively young, blue stars within these clusters raised immediate questions and sparked extensive research. Blue straggler stars stand out because of their blue color, which indicates higher temperatures compared to the other, more evolved stars in the same cluster. They lie off the main sequence in the Hertzsprung-Russell diagram of stellar evolution and calssification, in a region where no stars of their apparent age should theoretically be found.

For some time, the existence of blue stragglers stumped astronomers, hence the name; they shouldn’t exist on the H-R diagram at all. However, it appears they are the byproduct of mergers and collisions between stars, which the dense environment of a globular cluster is ripe for. Stars in globular clusters either directly collide to form blue stragglers, or a blue straggler can form in a binary or multiple star system when one star’s outer layers are torn off and absorbed by the gravity of a larger and more massive star.

Many blue stragglers are also rapidly rotating stars. This is supporting evidence that they are produced by mergers or mass transfers between stars, as such an event would add large amounts of angular velocity and thus induce rapid rotation speed in the resulting body.

Additionally, the energy of the mergers and collisions that produce blue stragglers often has the result of ejecting the blue straggler from the globular cluster or even the galaxy in which it resides at extraordinary speed. These are known as field blue stragglers.

The study of blue stragglers has far-reaching implications for our understanding of stellar dynamics, cluster evolution, and even galactic history. By challenging conventional theories of stellar aging and evolution, blue stragglers prompt astronomers to consider more complex interactions within clusters.

12. Blue Giants: Colossal Metal Factories of the Universe

Elements like carbon and oxygen, fundamental to life on Earth, owe their cosmic abundance to the processes occurring within massive stars. The iron in our blood and the calcium in our bones were once part of a blue giant star’s core, highlighting the intricate connection between these celestial giants and life.

Blue giants are among the most massive and luminous stars in the universe. Their incredible size and heat not only make them visually stunning but also endow them with the ability to forge heavy elements, a process known as nucleosynthesis. This unique attribute has made them a focal point in understanding the creation and distribution of elements throughout the cosmos.

As with any star, the life of a blue giant or supergiant is governed by a constant battle between gravity trying to collapse the star and the pressure from nuclear fusion pushing outward. Inside the extreme environment of a blue giant’s core, temperatures can reach upwards of hundreds of millions of degrees. At these temperatures, nuclear fusion occurs at an accelerated rate, and the star can produce elements heavier than hydrogen and helium.

Through successive stages of fusion, blue giants synthesize elements such as carbon, oxygen, silicon, and iron. Each stage involves fusing lighter elements to create heavier ones, releasing energy in the process. When helium- or carbon-burning is reached, in some instances, less massive blue giant stars may become so volatile that they shed their outer layers into a planetary nebula and then eventually turn into a white dwarf, despite having started out with enough mass to go supernova.

The more powerful nuclear reactions taking place inside a massive blue giant star keep the core from collapsing even as elements heavier than helium are fused. In contrast, a lower-mass star like our Sun will essentially die when it stops burning helium.

As massive blue giants enter their final years, they will burn numerous elements such as carbon into neon, then neon into oxygen, then oxygen into silicon, and finally silicon into iron. And that’s not an exaggeration. The phase of carbon-burning, which takes place after helium, lasts a whopping few centuries in massive dying stars before the subsequent decades of neon fusion and even shorter subsequent periods for the heavier elements.

When a star’s core begins to fuse silicon into iron, the reaction is no longer capable of producing more energy than it uses to sustain itself, much like all of our fusion reactors on Earth, themselves a crude attempt to replicate a star’s core, have so far. Obviously, the star can no longer keep its outer layers from collapsing in on themselves, and the star then goes supernova.

When a blue giant exhausts its nuclear fuel and the core collapses, this will result in a colossal Type II supernova explosion. This explosion is not only a visually spectacular event but also a cosmic forge where most of the elements heavier than iron are created. The shock waves from the supernova disperse these newly created elements into space, seeding the interstellar medium throughout galaxies with the building blocks for new stars, planets, and even living things.

3 thoughts on “Blue Giant Stars – 12 Interesting Facts”

  1. 1. The term “giant” or “supergiant” is restricted to evolved stars. Some blue stars such as HD93129B (luminosity roughly 1000000 solar) or Theta-1 Orionis C1 (200000 solar) are, technically, “dwarf” stars. Stars are considered “giants” or “supergiants” only when their surface gravity is low and products of fusion reactions start to show up in the star’s spectrum.
    2. One surprising thing about massive blue stars is how long they live. Massive stars have convective interiors which allow them to burn far more of their available hydrogen than the 10% or so which sunlike stars can burn.
    3. The classical Eddington limit does not apply to blue stars. A similar limit, called the Humphries – Davidson limit, applies to them. That limit is temperature-sensitive and lies above the Eddington limit. Hot stars can be more massive than cooler ones.

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  2. Fantastic information. Thank you for putting this all together and making it available to small-time star lovers like me!

    Reply

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