The Brightest Stars

We have been looking at the realm of small and very hot stars ... those residing in planetary nebulae. These white dwarf objects are the remnants of a former life on the main sequence of the HR Diagram. I have encouraged "couples" at Hopkins High School to go to the International Star Registry and buy a star, have it named after their loved one, and be romantic. I tell the kids to get a white dwarf because it is like buying an earth-sized diamond. With this said, you must now realize that white dwarfs are ALL invisible to the naked eye. There are very luminous, reaching levels 10,000 times that of solar, but due to their small size and majority of light being emitted in the ultraviolet, we simply do not see them without a telescope. If it were possible to observe a hot planetary star in the visible and ultraviolet bands, they would shine 1000 times more brightly in the sky than they appear to. These are pretty bright stars, but they are not king of the brightness hill.

To find the brightest stars, we need to look at stars far more massive than those tiny white dwarf cores. Down toward the right of the main sequence and into the realm of the red dwarfs, we see small stars with cool surface temperatures. The mass of these objects is anywhere between 0.5 and 0.08 solar masses, and the result of such a relatively low mass is weaker gravitational compression in the core and slow nuclear burning. The most dim of the red dwarf stars are over 1 million times less bright than our Sun. Moving up and to the left of the main sequence, the stars behave in an opposite manner. As these stars become more massive, gravitational compression increases, and thus too the core temperatures and rate of nuclear burning. The result is a tremendous increase in intrinsic brightness. At the top of the main sequence, among the realm of the O stars, stellar masses climb from 10 solars to over 100, apparent magnitudes reach -5 to -6, well in excess of the visual luminosities of the AGB or planetary stars. These stars are so massive, and so hot inside, that their cores do not fuse Hydrogen into Helium via the proton-proton chain like our Sun, but relies instead on the carbon cycle. All main sequence dwarfs above class F0 and hotter use carbon as the nuclear catalyst. Lots of heavy elements are forged in these cores, and shells of Hydrogen and Helium gases around the core are fusing with conventional pathways.

At the upper limit (spectral class O3), a 100 solar mass star on the main sequence will have a surface temperature of nearly 50,000K. As the planetary nebulae core stars do, these hot main sequence stars radiate much of their energy in the ultraviolet. When this additional energy is accounted for in the Absolute Bolometric Magnitude, such stars will be 4 magnitudes more bright (Mva = -6 becomes Mvb = -10), making them over 1 million times brighter than the Sun . And to think, some can become brighter as they age!

Sad thing for backyard astronomers like you and I, none of these faintest stars are visible to the naked eye, nor are any of the hottest stars, simply because they are so small and emit so much of their energy in the ultraviolet. Only a few of the coolest stars can be seen, and these are the "highly evolved" giants like Mira and Chi Cygni, and even these big stars are visible because they are close to us. But ... the brightest stars pose no such difficult ... afterall, they are the brightest. But, remember from our discussion earlier about the properties of starlight, that a star that is close to us will "appear" to be bright, but might not actually be very "luminous." On the converse, a star that is very "luminous" may "appear" quite dim, if it is far away. Because we know that spectral signatures of stars tell us about their actual luminosity, or absolute magnitude, we can use the HR Diagram's light relationship and determine how far away the star really is, and then plug in the numbers into the Magnitude-Distance formula to determine how bright the star is. Even better, is the discovery that the Absolute Bolometric Magnitude follows the mathematical set of rules relative to spectral signature and the Absolute Visual Magnitude, but this web connection is highly mathematical and perhaps daunting for the weak-hearted math grunt.

For example, Sirius in the brightest star that we see in the night sky, but parallax methods have determined it to be only 9 light years away. Though it is a class A star and 25 times brighter than our Sun, it is hardly exceptional. Now, look at the constellation Cygnus on some autumn evening. The tail of the swan is a beautiful white first magnitude class A star called Deneb. It appears to be as bright as Vega (25 ly) but is actually 2600 light years away. Deneb has an absolute visual magnitude of -8.4 ... meaning it is 200,000 times brighter than what our Sun would appear to be if placed next to it. If we were somehow able to tow Deneb to a location next to Vega, it would shine at magnitude -9, fifty times brighter than Venus at its best, and would rival a crescent moon. It would be easily visible in broad daylight, and even cast shadows at night!

Deneb is the starting place for these superbright beauties ... stars that have moved off the main sequence ... stars that are so massive that they burn through their nuclear fuel at enormous speed. They become not mere giants, but supergiants. Deneb is the brightest class A supergiant known in the galaxy. Just a quick reminder about these classes. Astronomers classify stars by their brightness in the HR Diagram, but also have a means to designate stars according to their luminosity. Below are the basic luminosity classes of stars. We are looking in this page at the top two luminosity classes ... Luminous Supergiants, and Hypergiants.


Type of Star



Extreme, luminous supergiants; hypergiants

rho Cas; S Dor


Luminous supergiants

Betelgeuse, Deneb


Less luminous supergiants

Antares, Canopus


Bright giants

Polaris, theta Lyrae



Aldeberan, Arcturus, Capella





Main sequence dwarfs

Sun, Sirius, Vega




White dwarfs

Sirius B, Procyon B


The Brightest Known Stars

For each spectral class the table lists the brightest star that is known in the Galaxy, the most luminous with either Bayer names or Flamsteed numbers, and some of special interest. "LBV" means "luminous blue variable." All are supergiants except for Theta-1 Orionis-C, which is still on the main sequence. "Location" refers to the OB association or constellation of residence.


Apparent Visual magnitude


Spectral Class

Absolute Visual magnitude


Absolute Bolometric magnitude





Spectral properties

HD 93129A


O3 If





most luminous

Zeta Puppis


O4 Iaf





runaway star

Theta-1 Orionis-C


O6 V



Orion OB1


in Orion Nebula

Tau Canis Major


O9 Ib



NGC 2362



Cygnus OB2#12


B5 Ia+e



Cyg OB2


10 visual magnitude extinction

Eta Carinae


B0 0






P Cygni


B2 Ia-0



Cyg OB1


LBV. loses 4x10^-4 solar masses per year

Zeta-1 Scorpii


B1.5 Ia+



Sco OB1


loses 5x10^-5 solar masses per year



B8 Ia



Ori OB 1


S Doradus


A5 0








A2 Iae



Cyg OB7


loses 3x10^-10 solar masses per year

6 Cassiopiae


A3 Ia+e



Cas OB5


shell star



F8-G0 Ia


-9.2 to -10.2



dust-enshrouded IR star

AG Carinae








Rho Cassiopiae


F8 Ia



Cas OB5


odd variable






Cep OB1


variable, 0.4 apparent magnitude shell

RW Cephei


K0 Ia-0



Cep OB1


Mu Cephei


M2 Iae



Cep OB2



VV Cephei


M2 Iae



Cep OB2


Lc, eclipsing

Roman numeral I denotes that the star is a supergiant; Ia that it is a bright supergiant; Ib a less-bright supergiant. Class 0 (zero) denotes that the star is an even brighter hypergiant.

You may notice an unusual star designated by the title Cygnus OB2#12. This is the most visually luminous star in our galaxy, glowing at an absolute magnitude of -10. However, we cannot even see it without a telescope boasting an 8 inch mirror and a really dark sky, since its apparent magnitude is a lowly 11.5. This dimness is the result of the star being hidden by thick clouds of dust that happen to lie in our line of sight between us and it. If there were no dust to obscure this star's light, we could see it with the naked eye even if it were 60,000 light years away. If we were to tow it to a place next to Alpha Centauri, it would shine ten times brighter than the full Moon, and you could read a book at night by its light.

Without going into great detail here, these bright stars are exceedingly hot, and as such they are producing more than just light energy in the form of extra ultraviolet radiation. So, when you are looking at the chart above, you will notice that these hot stars have magnitude corrections called bolometric magnitudes that will take into account the extra radiation emitting from these hot and bright stars in the ultraviolet band lengths. Rigel would be about 40,000 to 50,000 times brighter than the Sun, but with the bolometric correction would be 66,000 times brighter. Zeta-1 Scorpii is 25 times brighter still, and with the bolometric magnitude of -11.9, Eta Carinae (to your left) becomes one of the most brilliant stars in the Galaxy, 4.5 MILLION times brighter than the Sun. The hotter O class stars have larger ultraviolet corrections. Tau Canis Majoris appears to be just a fourth magnitude star to the eye, but it is an O9 supergiant with an absolute visual magnitude of -7.0, and then bolometrically it lies at -10.1.

At the top of the brightness list is another supergiant star in the Carina constellation visible in the southern hemisphere. This particular star, HD 93129A (a name within the Henry Draper catalogue), has an absolute visual magnitude of -7, spectral class of O3, and a distance of 11,000 light years. Its bolometric correction puts it at a -12 ... a whopping 5 million times brighter than the Sun, and the champion brightest star in the Milky Way.

Now, let's try to imagine if our planet were to orbit one of these bright stars. To be properly warmed by a typical dim red dwarf like LHS 2924, we would need to be barely a million kilometers away and our year would be less than an Earth day. If we were to replace this red dwarf with a bright supergiant like Deneb, Earth would have to orbit at a distance of 450 astronomical units, 15 times farther away than Neptune. Our orbit would require 2000 years to complete one revolution, and Minnesota winter would last for 500 years! Orbiting a star such as HD 93129A would put us out even farther and winter would last much longer still.

What does the OB Designation Mean?

I think this is pretty fascinating stuff, so I am including some pretty interesting details about star formation here. The OB designation refers to the consistent association of massive O and B stars in a close proximity, congregations we call "OB associations." Dozens of them are known throughout our Galaxy as well as our close neighbor Andromeda. These stars apparently form from the same giant gas clouds and live relatively short lives. Due to their short lifespans, they do not experience a great deal of separation from each other after formation, and hence are found in somewhat close connections to each other. Within the constellations that lie in the line of sight between us and the center of the Galaxy are numerous of these OB associations. Constellations that have more than one OB group will list them as OB1, OB2, etc., and the stars in an association are numbered. Hence you derive the name Cygnus OB1, or even Cygnus OB2#12.

While the O and F stars of the Pleiades (left) are close together, the OB associations are spread farther apart relative to our line of sight. One of the best known is easily visible to your naked eyes in the middle of winter. Orion OB1 (seen below) consists of Beta (Rigel) Orionis, that is Orion's left leg, and lesser B supergiants Zeta (Alnitak) and Epsilon (Alnilam) Orionis of the belt, and Kappa (Saiph) that marks Orion's right leg. Then add the O star of the belt, Delta (Mintaka); and Lambda (Meissa) that is Orion's head; and Sigma and Theta-1 of the sword, and a few others, and you have Orion OB1.

The stars of Orion are in the picture below. The OB association of primary interest are written in yellow ink, while those not in the OB association are in violet.

What makes the OB associations interesting is that they are short-lived in terms of galactic time. Due to neighbors exploding and gravitational influences, the stars in an OB association are moving away from each other. Seldom will they last more than a few tens of millions of years. While this seems like a long time to you, it is nothing compared to galactic time that is measured in billions of years. Sometimes, the stars of an association are flung out by differing forces that they may even occupy a different constellation before they finally burn out. However, in spite of these motions, most of the B stars and all of the O stars in an association stay together throughout their lifetimes because they live and die so rapidly. Therefore, these O and B stars that we see and also the youngest stars in the galaxy.

Where do these hot and bright stars come from?

We commonly find OB associations in or nearby the giant gas clouds in which they originally formed. The Great Orion Nebula is a wonderful example of such a "diffuse" nebula in which these stars form. This cloud of ionized Hydrogen extends out to a distance of 10 light years, and it is almost entirely lit and ionized by a central star, Theta-1 Orionis C. The great nebula hold over 100 solar masses of Hydrogen alone, not to mention the additional dust particles and other elements. Scoripus and Ophiuchus have similar diffuse nebulae within their boundaries, as does Sagittarius, Serpens, and Cygnus, as well as many other. Below is a nice peak at some of these nebulae. The Great Orion Nebula of Orion is in the upper left, the Eagle of Aquila in the upper right, the Trifid and Lagoon of Sagittarius are in the middle left and right respectively, and Carina is in the lower left. The lower right is an image of galaxy M101, and if you look closely at the spiral arms, you can see the knots of gas and dust in which these bright OB associations reside. All images are courtesy of the NOAO/AURA/NSF.


While these photographs are really cool to look at, they tell a very important story. Giant OB class stars form from this material, and the gas and dust you see above is the remains of these formation processes. The Red Supergiant stars we find in these nebulae are highly evolved OB stars in the final stages of their short lives. When we look back at the HR Diagram and see the apparent relationship between OB main sequence dwarfs and supergiants, we are able to visually confirm what we see on the chart. These massive O and B stars follow the established theories of stellar evolution based on both visual observation and computer modeling. We can therefore conclude that if we are right about the relationship of main sequence stars to their supergiant descendants at the top of the chart, we might be correct about the relationship of mid-level and low-level main sequence dwarfs to the lesser giant and white dwarf descendants at the bottom of the HR Diagram.

Finally, we have discovered that these bright O and B class stars are beautiful and obvious, but also very rare. A studious count of the stars on the HR Diagram reveals that 72% of the stars in our Galaxy are of the M Class Red Dwarf variety, and the O stars comprise only 0.00004% of the galactic population. Part of the reason for their apparent dearth is their short lifetimes, but perhaps we may learn that "nature" simply doesn't like to make a lot of them. Interstellar dust and gas clouds have a great many small mass stars forming within them, and this star type must be preferred. But we study them because they are so intrinsically bright, and therefore visible from great distances. In the image set above, and at the lower left is galaxy M101. You can see OB associations in the spiral arms. Their known spectral patterns and brightness relationship to spectra makes them pretty good aids in determining the distance to galaxies.

More importantly, when these stars conclude their existence following the supergiant phase, they often explode in spectacular fashion as supernovae. It is during these incredible explosions that the heavy elements of the Periodic Table are formed. Without supernovae, the metals of gold, silver, and platinum would not exist, nor would biologically necessary elements like zinc, magnesium, and cobalt.

In extreme situations, the O stars cluster together into tight, bound groups. When we look closely at the interior of the Orion Nebula, we see a quartet of stars called the "Trapezium" (above and to the left). These four stars are all O and B stars, and the brightest, Theta-1 Orionis C is lighting up the entire works. Nearby (at a distance of 150,000 light years) in the Large Magellanic Cloud is the Tarantula Nebula. This nebula is lit by a compact group of stars known as 30 Doradus (above right). The center of this association is so densely packed that at one time astronomers though it was a single star of over 1000 solar masses. We now know that over 50 very hot O3 stars are packed into a volume less than 8 light years across. Life on an planet could not possibly exist among such powerful radiation and stellar winds, but if we could somehow go there, can you imagine the sight at night? You would see hundreds of stars brighter than Venus, and an almost perpetual daylight. Below is another look at the Tarantula Nebula in the LMC (left) and a close-up at the heart of this nebula, R136 (right). Both images are from Gary Bernstein and Megan Novicki at the University of Michigan. A bigger version of this area taken by the Hubble Space Telescope is available by clicking on Doradus.

OB Stellar Spectra and Distances

Theoretically, if we know the absolute magnitude of a star and also the apparent magnitude, we should be able to determine the distance to the star using the Magnitude-Distance Formula. However, as we are learning, the hottest stars may be very bright, but much of their radiation is emitted in the ultraviolet bands, and creates difficulty for the formula. The largest stars also appear very bright, but significant amounts of infrared radiation are also emitted and this affect the formula. To get a real handle on the stars, we need to turn to the star's spectrum, for it holds the secret to its type and brightness.

Modern techniques, developed with increasingly better resolving telescopes, and built on techniques of Henry Draper, Annie Canon, and the Hertzsprung-Russell collaboration have allowed us to look very closely at the absorption lines of a star's spectrum. Much of the spectral analysis is well beyond the scope of this Astronomy course, but here are some of the broad details. From a stellar spectrum, astronomers can determine whether the star is moving rapidly or slowly, far away or near. There are differences between O class stars that are main-sequence dwarfs and O class stars that are supergiants. There are differences between Red Giants, Red Supergiants, Red Hypergiants, and Red Dwarfs. It is a simple matter to look at the absorption spectrum of a star and thus determine its luminosity class. Once the luminosity class is know, we can therefore extrapolate its distance using the Magnitude-Distance Formula. In cases where this is too difficult, astronomers have developed a hybrid version of the HR Diagram that plots OB associations against apparent and absolute visual magnitudes and the relationship can be used to derive the distance. All of this is incredibly interesting to me, but still difficult to comprehend to an extent where I might be comfortable to explain it.

Some representative stellar spectra of O Class stars are shown below. To see more stellar spectrum, check out this spectral class site. The images are from "An Atlas of Stellar Spectra" by Morgan, Keenan, and Kittman; Astrophys. monographs, University of Chicago Press, 1943. They only serve here to show the difference between O supergiants and O main sequence dwarfs.

It was Morgan and Keenan who, in the 1940s, developed the luminosity classification system and also broke down the difference between Ia and Ib supergiants. But, after observing enough stellar spectra, Morgan saw a need for a group of stars more bright than the I supergiant designation. Since the Romans did not have a 0 in their numeric system, and because 0 can be easily confused with O, it was a while before the designation of 0 was accepted and the term "hypergiant" used widely among the Astronomy community. The massive star S Doradus, seen in an image above is classified as an A5 0 star. The star Eta Carinae, in our own Galaxy is classified at B0 0, a true hypergiant with the mass of 100 suns. Welcome to the world of the Ia-0 hypergiants.

If you look into the internet more deeply, or also at advanced Astronomy textbooks, you will find other designators after these supergiant stars. Among the 19 O class stars in the table on top of this page, seven have an "e" or "f" after their formal designations. These little letters indicate strong emission lines. In order for a star to have emission lines, the star must be surrounded by a cloud of low-density gas ... matter that is lost from strong stellar winds. Some of these supergiant stars are literally blowing themselves into space at astonishing rates, and an analysis of the emission spectra of some of these stars will tell us the rate of the mass lost to these winds. Zeta-1 Scorpii has a wind measured to blow away from 500 hundred thousandths of a solar mass per year. Wind velocities and amounts vary from star to star, but indicate just a little bit how hostile the environment can be around these stars. The windiest star is P Cygni, in the body of Cygnus. This star is visible to the naked eye and was the first star found with such an unusual spectrum, such that astronomers call lines like them in other stars "P Cygni lines." P Cygni is blowing a wind at a rate of four ten-thousandths of a solar mass per year. It blows out the mass equivalent to the Sun in less than 10,000 years! What planets are here would be either obliterated by this wind, or have no gas coverings.

Eta Carinae

Once again, let's go over the top of these huge objects and look at the truly awesome Eta Carinae (seen above near the top of this page, and again to your left). I just think this is the coolest star to look at, even though it is incredibly hot. This amazing star is a definite hypergiant, luminosity of 4.5 million Suns, and an absolute bolometric magnitude of -12. Its surface temperature is between 20,000 and 30,000 K and the spectral class between B1 and B0, with a current mass outflow of one solar mass every thousand years! What happened here, and why does it look so peculiar?

Based on past observations and historical spectral images, 150 years ago the star erupted in a wind whose flow was 100 times greater than the present rate. Over this eruption, perhaps an entire solar mass was blown out. The gas was thickest at the equator of the star and hence expanded more slowly there, resulting in the waist-like appearance. The gas at the poles escaped more rapidly giving the bi-polar lobe look. As the gas hit the cold temperatures of interstellar space, some of it condensed into dust to form the dirty cloud we now see. The giant star is now buried in its own gaseous outflow!

But there is even more here. The star's emission lines change periodically over every 5.5 years, indicating the presence of a companion star. Also, the nebula is rich in Nitrogen, but the visible star is not. The only possible explantion is a super large pair of stars, one of which is more evolved than the other. To have evolved faster than the 100 solar mass visible star, the less visible companion must have originally had even more mass to have evolved more quickly. Together, the pair easily comprised over 200 solar masses! The visible star has perhaps ejected up to 30 solar masses so far, and the dimmer (or perhaps hidden) companion must have lost more than 40. The sheer mass loss of 70 Suns easily dwarfs all of the stars within several light years of our own solar neighborhood.

Eta Carinae belongs to a strange group of stars known as "Luminous Blue Variables." These supergiants and hypergiants blow out large quantities of themselves into space in periodic episodes. Some outbursts are so sensational that P Cygni was confused for a 'nova" in 1600. No one knows why some of these massive stars suddenly brighten and blow out much of their material, but the lack of anything more massive in observable space leads to the conclusion that a star cannot form from a mass greater than 200 Suns, and most often with this large mass amount, the cloud fragments into many smaller stars. The top of the list may be the newly discovered "Pistol Star," (image to your left) whose initial mass was near 200 Suns. Whatever these things are, whether single or binary system, they are blue in spectral class, hot at their surfaces, and blowing fierce winds ... literally tearing themselves to shreds. Our Sun is stable because gravity and thermonuclear fusion pressures are equal. At the top of the theoretical "Eddington Mass Limit," the radiation pressure from the dense core becomes greater than gravity's ability to compress the star, and much of it begins to evaporate. This evaporation is not smooth, but episodic, and the mechanisms behind them are unknown.

Wolf-Rayet Stars

These very unusual stars are named after the French astronomers who first discovered them in 1867 (Charles Wolf and Georges Rayet). These stars are bright supergiants with temperatures in the same range as the O star class, but their spectral pattern is very different, displaying ONLY emission lines. Furthermore, their spectra do not show the presence of Hydrogen. Like the LBV stars in the paragraph above, these Wolf-Rayet stars are losing mass at tremendous rates, shine with luminosities between 100,000 and one million times that of the Sun, and show the dominant element to be Helium.

They come in two varieties ... nitrogen-rich (WN) and carbon-rich (WC). While these stars do contain some Hydrogen, the relative amount of it is quite different from main sequence dwarfs. The latter stars typically have 10 times as much Hydrogen as Helium, while these Wolf-Rayet stars may have anywhere from 3 to 10 times as much Helium as Hydrogen. Carbon and Oxygen may be practically absent in the WN stars , but they contain 10 times as much Nitrogen than Helium (and thus vastly more than Hydrogen). If that suspected companion of Eta Carinae is not already a WN star, it will be soon! The WC stars are even more strange. In these stars, NO Hydrogen is seen at all, and neither is there any Nitrogen, while the ratio of Carbon to Helium is 100 times normal. In the extreme, the number of Carbon nuclei is almost equal to the number of Helium nuclei.

These stars have apparently blown out their outer envelope to expose deeper layers now rich in the byproducts of different kinds of nuclear burning. The WN stars, with their excess Nitrogen, show us the results of the Carbon Cycle. What typically happens is that the Nitrogen from Carbon fusion turns back into Oxygen and then to Carbon. Since the Nitrogen is being fused from other elements before degrading back into Carbon, we see for a while, a star with more Nitrogen than there is supposed to be. A star very rich in Carbon must be making it from Helium fusion. Whether the WN stars will eventually become WC stars or the other way around is still being debated.

Since they are always blowing their material into space, they take on a similar appearance to the classic planetary nebulae, and are indeed collectively called "Ring Nebulae." This term is not to be confused with the Ring Nebula of the constellation Lyra, but merely refers to its appearance. Their difference from the planetaries is found in the spectra of the ring material. In the planetary, the ring is made of the same material as the central star, and this material is Hydrogen and Helium. In the WN and WC rings, the nebulae consist of lots of Nitrogen or Carbon ... elements commonly found only in massive stars, unlike the lower mass planetaries. These large mass clouds contain many heavier elements that will provide material for a future generation of stars and perhaps planets. The star WR124 in Sagittarius is shown to your left, as photographed by the HST.


These Wolf-Rayet stars are probably the remnants of once mighty supergiant stars. They have blown out most of their outer shells of gas and are left with the hotter inner core regions. Since they are so massive, their fate is probably to explode in a spectacular supernova and die as a neutron star or black hole. Our Sun simply lacks the mass to yearn for such a glorious demise.

The next chapter is devoted to the largest of all the stars ... the great Red Supergiants. You can go there now, or return to the Star Introduction , or go to the Syllabus .

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