The Largest Stars - Supergiants

Welcome to the world of the extremely large stars ... the so-called Supergiants and Hypergiants. We have looked at cool and faint stars, cool and bright stars, hot and dim stars, hot and bright stars, and a lot of average stars. Now we move back up and to the extreme right of the HR Diagram, into the realm of star so huge, their diameters are measured in Astronomical Units instead of mere kilometers. These supergiant stars are cool at their surfaces, with temperatures below 3800 K. Unlike the bright supergiants of the previous page, these supergiants are much cooler. In order for them to have similar luminosities, the Red Supergiants must be bigger than their Blue Supergiant cousins. Anyone who looks at the constellation Orion cannot help but be struck by the beauty of Blue Supergiant Rigel at the lower left foot, and Red Supergiant Betelgeuse at the upper right shoulder. While Rigel is a huge star, it is dwarfed by the dimensions of Betelgeuse. If Betelgeuse were placed at the position of our Sun, its outer envelope would extend beyond the Asteroid Belt and near the orbit of Jupiter. In the summer seasons, you can look southward and see beautiful Red Supergiant Antares, the heart of the Scorpion. This star is even bigger than Betelgeuse, with a radius out to the Jupiter's orbital position.

A list of the naked eye Red Supergiant stars is given below. It is important to note the lack of 1st and 2nd magnitude stars in the class. Indeed, the Red Supergiant is a very rare star, even more so than the hot O class stars. There is perhaps only one Red Supergiant for every million stars in our Galaxy, and maybe even far fewer. In fact, only 200 have been catalogued and studied.

Star

Apparent visual magnitude (mv)

Spectral class

Association

Distance (ly)

Absolute visual magnitude (Mv)

Radius (AU)

Variable class

Betelgeuse

0.50

M2 Iab

Ori OB1

430

-5.1

3.6

5.8 yr + irregular

Antares

0.96

M1.5 Ib + B2.5 Ve

Sco OB2

600

-5.4

4.2

Lc?

Alpha Herculis

3.48

M5 Ib-II

...

400

-1.9

2.0

SRc

Eta Persei

3.76

M3 Ib-IIa

...

1300

-4

...

...

Mu Cephei

4.08

M2 Iae

Cep OB2

2000

-7.3

5.7

SRc-Lc

119=CE Tauri

4.38

M2 Iab

...

2000

-6

2.9

SRc

Psi-1 Aurigae

4.91

M0 Iab

...

...

-6

...

Lc

VV Cephei

4.91

M2 Iaep + O8 Ve

Cep OB2

2000

-8

8.8

Lc (small); eclipsing, period 20.2 yr

KQ Puppis

4.97

M2 Iabpe + B2V

...

3000?

-6

...

Lc?

HR 8164 (Cephei)

5.66

M1 Ibep + B2pe + B3V

Cep OB2

2700

-5

...

...

6=BU Geminorum

6.39

M2 Iab

Gem OB1

4900

-6

...

Lc; eclipsing, period 32 yr?

e and p respectively stand for "with emission" and "peculiar"
SR: semi-regular; L: irregular; c is an old designation for a "supergiant"
HR means "Harvard Revised," the designation of the Bright Star Catalogue

Spectral Characteristics of Red Supergiants

Like the Red Giants and Red Dwarfs, the Red Supergiants all belong to the same spectral class M, meaning their surface temperatures are less than 3800 K. There is some overlap in the luminosities of the AGB stars and the dimmer Red Supergiants, with the two types being found at luminosities of -2 to -4. But the similarity stops there. Most stars like our Sun evolve into Red Giants, then briefly into K subgiants, before becoming Red Asymptotic Giant Branch stars, and then turning into White Dwarfs surrounded by a planetary nebulae. As you learned from the chapter on hottest stars, the White Dwarfs evolve from stars whose initial mass is 8 Suns. The Red Supergiants start out with 10 solar masses or more, and their internal structure is very different from the Carbon/Oxygen core, Helium shell, and thin Hydrogen shell of the AGB variety. The Red Supergiant stars have spectral patterns that indicate a warm surface in the range of M0 to M5 at the coolest, while the truly "cold" AGB stars are of spectral class M5 to M9.

Furthermore, the spectra of the Red Supergiants is distinct from Red Giants, AGB stars, and Red Dwarfs in terms of the weaker neutral Calcium lines, weaker Titanium Oxide bands, and stronger Hydrogen lines. With this spectral identification, it was possible to place the location of the Red Supergiants within specific regions of the Milky Way. The Red Supergiants are typically found among OB associations. This finding indicates that Red Supergiants are evolved descendants of OB stars, and if the stars of an OB association form simultaneously, as predicted, then the presence of Red Supergiants means that these stars must have had higher initial masses and moved through their lifecycles more rapidly than the surrounding OB stars. Adding to the growing knowledge of these supergiant stars was the discovery of companion stars orbiting alongside. The chart above shows some of the naked-eye supergiants and their companions. Notice that the companions are B class main sequence dwarfs. The Red Supergiant has evolved to its super-sized state and left the smaller companion behind. In order for the supergiant star to be evolved before a B class star, it must have had a higher mass. The initial mass of the Red Supergiants must be really large, and in the realm of the very bright O class stars.

Size

The diameters of the supergiants is immense compared to our Sun. Even though some are hundreds of light years distant, it is possible to measure their diameters by various kinds of interferometry. Betelgeuse has been found to boast a diameter of 700 times that of the Sun ... placing the orbits or Mars and the asteroids within its envelope were it to be placed in our Sun's location. It is so large that its disk (left) has been visualized by the Hubble Space Telescope. Because this star is so close and so large, we can measure its angular diameter with a high resolving power telescope, but even the largest telescopic mirrors are unable to resolve anything more than points for stars more distant than 600 light years. Fortunately, some stars have their own method for helping us determine their size.

If two stars orbit each other very closely, there is a pretty good chance that their orbital paths will be oriented to our line of sight in such a manner that one star eclipses the other, and then vice versa. The most famous of these eclipsing star systems is Algol (Beta Persei), which dims by a full magnitude every 2.9 days as a small and bright B star partially hides behind a fainter K giant. When the B star moves in front of the K giant, there is a much smaller dimming.

There are several eclipsing binaries found among the supergiants, among which the most famous is VV Cehpei. The primary eclipse occurs when the brilliant O dwarf companion passes behind the M2 supergiant (which owing to its large size dominates the sky). While the Algol eclipse of the dwarf star in front of the K giant last for only a few hours (this is because it is a partial eclipse, but if it were total it would last for only a day at the most), the O8 star of the VV Cephei system disappears for almost 1.2 years. From the light curve measurements we can determine the tilt of the orbit, and from measuring the Doppler-shifts in the spectrum lines we can determine the orbital speed. Therefore, the duration of the eclipse can give us an accurate measurement of the size of the supergiant. The M2 supergiant has a radius 1900 times that of the Sun, or 8.8 AU, which is just smaller than the orbit of Saturn! This is really one very BIG star. Below is a schematic that demonstrates how an eclipsing binary system works, with either a partial eclipse or total eclipse of the system. Both can be useful in determining the actual diameter of the supergiant.

The sheer size of these stars stupefies most anyone who dares to think about them. While it took the Voyager 2 spacecraft almost 8 years to travel from the Sun our to Uranus, it would take the same spacecraft, traveling at 45,000 km/hr almost 12 years just to cross from one side of VV Cephei to the other. In volume, VV Cephei would hold seven billion Suns. A representation of the 4 largest supergiant stars is shown in the schematic below, with the orbital distances of Jupiter, Saturn, and Uranus drawn in for comparison.

The extreme sizes of these Red Supergiants makes them unstable, causing them to pulsate erratically. This is why they are listed in the table above as "variable stars." The study of such variable stars is a passion for my Uncle Bill, as well as thousands of other amateur and professional astronomers worldwide. They are members of a group called the "American Association of Variable Star Observers," and these people regularly study the changing brightness of a long list variable stars. When an observation is made, and the time noted, the stellar magnitude is then submitted to the AAVSO headquarters and a light curve of that star is then generated from the observations of many other astronomers. These light curves over periods of time are then used by professionals to understand the evolution of stars. For this reason, the study of variable stars such as Betelgeuse and VV Cephei is of great importance, for it is only a relatively short matter of time before one or the other explodes in a supernova ... a subject to be visited next.

While many stars on the AAVSO list vary consistently over short time periods of days or weeks, the Red Supergiants show a longer and more irregular period. Mu Cephei varies by over 1.5 magnitudes over a two or three year period. Alpha Herculis exhibits a variation of one magnitude over a 6-year period. VV Cephei shows an interesting variability. In the blue spectrum, VV Cephei is a regular eclipsing variable, but in the infrared, where the M Supergiant is the dominant component, the star behaves unpredictably.

Betelgeuse is a special star whose variation has caused it to be listed as the Alpha star of Orion by Johannes Bayer. Today, Rigel is the brightest star in Orion ... at 0.4 magnitudes more bright than Betelgeuse. Since the Greek letters are used to designate the relative brightness scale of a constellation's star family, Rigel should be designated the Alpha star. However, Betelgeuse is a variable, but on a scale of days, weeks, months, and even a slow 6-year period. Quite possibly, it was at a maximum brightness when Bayer assigned the Greek letter system to the Orion stars. What causes this variability?

As we learned in our study of planetary nebula formation, extremely large stars lose mass due to powerful stellar winds. The large size results in a very low gravitational attraction between the outermost envelope and the interior of a supergiant. The gravity of a typical Red Supergiant is 1/10,000th that of the surface of our Sun, and but a tiny fraction of Earth's surface gravity. Beside the low surface gravity, these Red Supergiants are extremely luminous (over 1 million times solar) and the outward pressure forces the outer envelope out into space.

A large cloud of escaped stellar material surrounds Antares in Scorpius. The cloud is so great that it engulfs the B companion star of Antares. However, because the B star is so hot, it ionizes the gas and dust around Antares into a bright nebula. The location of Antares in Scorpius is seen in the image below and left, while the right image is a look more closely at the ionized nebula surrounding it and its B companion.

Betelgeuse is better understood than Antares because it is physically closer to us ... at a distance of only 430 light years. It has been actually imaged by the Hubble Space Telescope, and this image is seen to your left. What is striking from the visible image is the bulge and irregularity of the "surface." This irregularity is even more pronounced in the image of the star taken by a radio telescope (seen below)

 

 

 

As you might be able to notice, there is an extensive region of dust and gas surrounding the actual Red Supergiant. These dust grains are being observed out to a distance of 12,000 AU, which is 3,000 times the radius of the actual star. The radio image shows some of the unusual perturbations of the physical star that may be the result of pulses of stellar winds, blowing material out into deep space. Even more perplexing is the belief that Betelgeuse may have 2 companion stars ... one at a distance of 20 stellar radii, and another at only 2 radii. The innermost of these companions may orbit the supergiant in only two years, and may actually be INSIDE the chromosphere of Betelgeuse. The orbit of the inner companion may be responsible for the observed physical bulges in the supergiant. Clearly, the Betelgeuse system is undergoing great upheaval and may be transitioning ever closer to the fateful day when it implodes and then explodes in a supernova.

Stellar Evolution

We can now make connections between the various stars on the HR Diagram. The brightest O and B class stars (blue supergiants) evolve into the largest stars (cool red supergiants). Less massive main sequence dwarfs evolve into Red Giants, and most follow on to become Asymptotic Giant Branch M stars before ending their lives as White Dwarfs within blown out gas and dust clouds of planetary nebulae. The upper mass limit for the formation of a white dwarf/planetary nebular appears to be 8 solar masses. Above 8-10 solar masses, the evolutionary tracks of the HR Diagram lead into the domains of the blue and red supergiants. While Red Giants and Red Supergiants may appear to be similar, their interior structures are greatly different. The Red Giant interior is typically a Carbon/Oxygen core blowing material out into space and then collapsing into a White Dwarf. The Red Supergiant interior is full of more layers of nuclear burning into elements heavier than the Carbon/Oxygen mixture, and these stars collapse too, but then suddenly explode.

Here is the theoretical lifecycle of a 30 stellar mass star:

1) It begins as a spectral class O4 star, burning up its Hydrogen core in only 7 million years.

2) As it ages it brightens some, expands and cools a bit, transforming itself into a type B1 star at the right hand edge of the main sequence.

3) The star may ionize its surrounding gas, causing a diffuse nebula to appear in the night sky, similar to the great Orion Nebula. When it reaches the end of the main sequence life stage, it can no longer produce ionizing ultraviolet radiation, and the diffuse nebula turns into a "reflection nebula" in which the interstellar dust and gas merely reflect the star's blue radiation .

4) Finally, the core runs out of Hydrogen fuel and it rapidly contracts. The inward gravitational pressure heats up the Hydrogen layer around the collapsing core to thermonuclear ignition, and the new outward pressure causes the star to swell in size and cool at the surface layer. It moves off the main sequence, toward the M class, but at a constant luminosity. It grows into a Blue Supergiant, and expands further into a Red Supergiant, at remarkable speeds (this is why A,F,G, and K supergiants are so rare). At some point, the core reaches a critical temperature threshold of 100 million K, and fuses Helium ash into Carbon and Oxygen. The evolution stalls during this phase, and the life of a Red Supergiant slows. What is a mystery here is the amount of stellar mass lost during the change from O main sequence to M Supergiant. Perhaps as much as half of the initial mass is lost to stellar winds. The first two stages of Hydrogen burning, and then Helium burning are shown below left and center respectively. The final stage is seen to the far right below.

 

5) The Helium in the core is used up in 1/10th the time of Hydrogen burning. Following a continued core collapse when the Helium ash is fused into the Carbon/Oxygen mixture, gravitational pressure may increase the core temperature to 600 million K, at which time the Carbon/Oxygen mixture fuses into Neon. In stars whose initial mass is near the lower end of supergiant status, the final result may be the rare Neon/Oxygen White Dwarf. Since Betelgeuse and Rigel are both at this lower end, this may be their eventual fate.

6) In truly massive supergiants, the Carbon ash core may fire up and fuse into Oxygen, Neon, and Magnesium. The core will collapse when the Carbon supply is gone, raising the core temperatures over 1 billion K, and the Oxygen/Neon/Magnesium ash will ignite and fuse into a mixture of Silicon and Sulfur. At the same time, nuclear fusion continues to move outward in concentric shells of different stages of element fusion. A the final stage, the Silicon/Sulfur ash fuses into Iron. During these evolutionary steps, the star moves back and forth across the HR Diagram (seen above), becoming Blue Supergiants again, then Red Supergiants. This "onion" star is seen in the image below. What needs to be remembered here is that a huge envelope of non-burning Hydrogen and other blown out dust and gases are surrounding the outermost layer of Hydrogen burning.

7) Iron is the endpoint of nuclear fusion. Energy can be produced by fission, such as the breaking of Uranium in an atomic bomb, or energy can be released from fusion, such as the manufacture of Helium from Hydrogen in a hydrogen bomb. Elements heavier than Iron have stored energy that can be released upon fission, while elements lighter than Iron have stored energy that can be released upon fusion. Of all the naturally occurring 92 elements of the Periodic Table, ONLY Iron is unable to release or store energy from either fusion or fission. You might think that the endpoint of this star life would be some exotic Iron White Dwarf. Indeed, when the very massive Red Supergiant finally dies, it does collapse into an Iron White Dwarf, but only for the briefest moments of time ... literally a fraction of a second. Gravity that has been held at bay for so long by outward pressures of fusion and/or electron degeneracy, suddenly wins the battle, and the entire core implodes at fantastic speed, only to be almost instantly followed by a spectacular explosive detonation called a Supernova. These events are the subject of the next chapter, and the cause of the picture of the Crab Nebula below.

What about those Hypergiants?

The most visibly luminous of the stars are the Red Hypergiants. These gigantic stars are so incredibly huge that they have a real struggle just to keep themselves together. The outward pressure from internal fusion pushes the outer envelope out to a point where the surface may experience zero gravity. The star effectively begins to tear itself apart. This theoretical point where outward pressure is greater than the inward pressure of gravity is called the "Eddington Limit." How does this happen?

Let's begin this star lifecycle with a mass anywhere between 40 and 120 solar masses. Throughout their Hydrogen-burning phase on the main sequence, they are losing mass due to strong stellar winds. Finally, with their core supply of Hydrogen depleted (in less than 10 million years of time), the core contracts, a shell of Hydrogen begins to burn, and the core of Helium will ignite, pushing the outer envelope outward. The star moves from the O class through the B class, becoming a supergiant or even hypergiants. In the process, the mass-loss rate increases so quickly, that they cannot evolve into M class supergiants. Their evolution on the HR Diagram stalls in the B class stage, while half of the initial mass may now have been flung out into space. These stars become those wonderfully interesting Luminous Blue Variables (LBVs), whose winds blow out a solar mass every 100,000 years. Like the evolution of a Planetary Nebula, these stars may have disks of material at their equators, formed by slow winds, while fast wind ejection of material at either pole creates massive lobes of gas and dust ... like Eta Carinae (seen below).

At some yet unknown point in the lifetime of these massive stars, the LBV will shift to a more extreme wind flow rate, increasing by a factor of 100 at the lower mass end, and up to 1000 times faster in stars like Eta Carinae. The external shell of expanding gas hides the highly luminous star within. The expanding gas gets cooler as the diameter of the could increases, while the interior star continues to shine brightly, and with excess ultraviolet radiation. The star brightens visibly, but effectively hides the highly luminous internal star. Once enough mass has been lost to space, some modicum of stability returns, and the star is seen in a smaller nebular form. An excellent example of this evolutionary step is P Cygni, a star in Cygnus that is waiting for another explosive fast wind episode to begin. Indeed, the AAVSO has a group of variable stars that follow the pattern of the prototype P Cygni star, and this class is called P Cygni stars. Perhaps the superbright Cygnus OB2 #12 from the brightest star chapter may be an LBV waiting to happen.

No matter what the initial mass of the star is, between 40 to 120 solar masses, the star will blow out its entire Hydrogen envelope and leave an exceedingly hot core that is rich in the products of various levels of nuclear fusion. The next-to-the-last stage is the Wolf-Rayet star with either its Carbon enriched core, or Nitrogen-enriched core. Those stars that still retain a hefty amount of mass are now destined to the most extreme ends ... titanic supernovae and ultra-small core remnants.

Please proceed first to the Supernovae page, and then move ahead to Neutron Stars and Black Holes ... the smallest of the stars, or go to the Syllabus .


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