Cool Red Giants

Greetings, my Astronomy students, and welcome to an additional page on the coolest and faintest stars. We have been looking at the bottom of the main sequence of the HR Diagram at the M and L class dwarf stars, such as the tiny, but very numerous Red Dwarfs, the star wannabe Brown Dwarfs, and the less massive planets. Certainly these objects are both faint and cool. But this webpage will deal with a group of stars that are just as cool, but far, FAR more bright. When Hertzsprung and Russell constructed their famous diagram, they discovered that 90% of the stars in their survey followed a relationship between spectral class and luminosity or absolute magnitude. However, there were some groups of stars that did not follow their peers. Toward the right of the main sequence, and above that line, is a group of stars that went against the norms. Now, I am not trying to portray these stars as strays, deserters, or outsiders. These stars are in the process of dying, and have spent their core's supply of Hydrogen fuel. We use the term "evolved" stars to describe those that have left the main sequence line. They have initiated a new form of nuclear fusion of heavier elements, pushed their outer envelope to immense diameters, and evolved into the Red Giants. These M Giant stars in some instances have surface temperatures that rival the L dwarfs. To be sure, there are stars far more faint that M class Red Dwarfs. White Dwarf stars are tiny objects, and for that single reason appear very dim in the sky. Yet their surface temperatures are so high that we place them among the hottest stars. Neutron stars are fainter still, and to date only two have been optically found. Their smaller size than White Dwarfs and emission of high frequency radiation makes them exceedingly difficult to visually find. And, at the very bottom of the faintness scale are supermassive collapsed stars whose own gravity will not even allow light to escape, therefore they are present by invisible. We will devote our attention on this page to the M Giant stars, and learn about the other extreme versions later.

Like the dwarfs of the main sequence, the giant stars come in a variety of spectral classes G through M, and in colors from yellowish to red. There are O,B, and A giants too, but they not quite as big, and there are some Red Supergiants that are even larger than the Red Giant stars of this chapter, and those monsters will be looked at later. These Red Giants are much brighter than their Red Dwarf cousins of the same M spectral class. It would take 100 million M8 dwarfs to equal the light output of one M8 giant. They are much bigger with diameters out to the distance of our inner planets here. And, they follow a different luminosity-spectral class direction. While the Red Dwarfs get smaller and more dim as we descend the main sequence spectral classes, the Red Giants get larger and more bright through the same spectral descent. At their largest size and coolest temperatures, their radii extend beyond the orbital distance of Mars.

While we cannot visually see a single Red Dwarf in the night sky, Red Giants dominate it. Half of the 40 brightest stars are giants that are not only easy to find, but easy to discern their color. The only difficulty for astronomers is discerning whether the reddish star is a Giant or Supergiant. However, the supergiants are very rare ... Betelgeuse in Orion and Anteres in Scorpius are the only first magnitude stars in the northern sky. Some of the common Red Giants are: Arcturtus in Bootes, Aldeberan in Taurus, Dubhe in Ursa Major, Kochab in Ursa Minor, Pollux in Gemini, some of the stars in the Hyades Cluster of Taurus, and the classic M class Red Giant star, Mira in Cetus.

To give you a sense of the sheer size of these stars compared to the Sun, I have included a schematic the two types of giants. Aldeberan on the left, and Arcturus on the right are Giant class stars, while Betelgeuse in the middle is a Supergiant.

Some representative Red Giant stars are shown in the chart below, arranged according to descending spectral class (average value for variables), or decreasing temperature (which changes with variation). The "period" refers to the length of time to complete a cycle from brightening to dimming and back again. A study of such stars whose brightness changes is the purpose of the American Association of Variable Star Observers (AAVSO) of which my Uncle Bill Albrecht is a prominent observing contributor and I am merely a paying member at this time. The absolute magnitudes in the chart correspond to the visual peak and a warmer spectral class.

Star

Spectral Class

Temp K

Apparent Magnitude

Period (days)

Variable Class

Distance (ly)

Absolute Magnitude

Beta Andromeda (Mirach)

M0 III

3600

2.06

 

 

200

-1.87

Mu Ursa Majoris

M0 III

3600

3.05

 

 

250

-1.36

Delta Virginis

M3 III

3200

3.38

 

 

200

-0.58

Mu Geminorium

M3 III

3200

2.88

 

 

230

-1.38

R Lyrae

M5 III

2800

3.9-5.0

46

SRb

350

-0.7

19 (TX) Piscium

C5 II

3500?

5.5-6.0

 

Lb

760

-1.1

30 Hercules

M6 III

2700

4.7-6.0

70

SRb

360

0.1

Chi Cygni

S6e III

 

3.3-14.2

407

Mira

350

-1.8

R Leporis

C7e III

3000?

5.5-10.5

433

Mira

820

-1.5

R Aquarii

M7 IIIe

2500

5.8-11.5

386

Mira

640

-0.7

Omicron Ceti (Mira)

M7 IIIe

2300

3*-10

331

Mira

420

-2.5

R Cassiopeiae

M7 IIIe

2000?

5.5-13.0

350

Mira

350

0.4

R Leonis

M8 IIIe

2000?

4.4-11.3

372

Mira

325

-0.6

* Mira has reached a peak of 1st magnitude in its observed history.

These Red Giant stars begin their lives with up to 10 times as much mass as our own Sun, and thus burn faster due to the effects of the additional mass on gravitational compression of their cores. Thus they burn more brightly on the main sequence, and unlike the Red Dwarf stars, of which none have begun their formal death sequence, Red Giants are stars already in their final stages of their lives.

As you recall from earlier reading, a star will spend 90% of its lifetime on the main sequence burning Hydrogen into Helium. At the point when the core's supply of Hydrogen is depleted, there is no longer any outward thermonuclear pressure, and gravity collapses the core. During this process of internal collapse, the Hydrogen nuclei surrounding the core suddenly fuse into Helium. The combination of core heating and shell fusion generates tremendous outward pressure and, in the case of our Sun, the star will brighten by a factor of 1000, its absolute magnitude climbing from +4.6 to -3. Long-lived observers on Earth would witness the expansion of the Sun as it engulfs Mercury and nearly reaches Venus. The Luminosity of the star is increasing due to the intense inner reactions, but the expanding outer envelope is increasing even faster, and the surface temperature drops from 6000 K down to about 3200 K. The star drops in spectral sequence from G to K, and finally settles in the M realm as a Red Giant, perhaps M2 or M3.

Eventually, the collapsing core increases the internal pressure to densities of a metric ton per cubic centimeter, 100,000 times that of Lead. The increased pressure also raises the kinetic energy of the core nuclei to 100 million K. At this incredibly hot temperature, the helium ash from previous fusion reactions becomes fuel for a new series of reactions. Two normal Helium atoms fuse to make a very unstable isotope of Beryllium (8Be), which immediately falls apart - typically within 10e-16 seconds - back into two Helium nuclei. Since this accomplishes nothing as far as the star is concerned, the incredible pressures of this evolved star force three Helium nuclei together at once to make Carbon (12C). Sometimes, an attacking Helium nucleus can turn a Carbon nucleus into Oxygen (16O).

The star suddenly jumps back to "life" with a new source of fuel for thermonuclear reactions, and the collapse of the core is halted. Not only is it halted, but the core actually expands a bit. The star stabilizes in hydrostatic equilibrium, with a slight shrinking of the outer envelope and a movement up in spectral class to the orange K giants, where it will survive, in the case of our own Sun, for another billion years as a dimmer version of Alderberan or Acturus, and with an absolute magnitude near +1. The Red Giant star lifecycle is shown below left, and compared to the Red Dwarf that is below and right.

When the Helium fuel runs out in the core, what remains is a tiny, hot, dense ball of Carbon and Oxygen. Now, this star is set to really experience dramatic change. With the Helium gone, the dying star loses its internal outward support, and the core resumes its contraction under gravity's pressure. The core shrinks down to the size of Earth. The result is an increased pressure and temperature in the Helium surrounding the core. That Helium ignites into Carbon and Oxygen, while another layer outward from it ignites Hydrogen into Helium. This evolving giant star now has two shells of nuclear burning. They work in a sequence where the outer shell extinguishes its fuel while simultaneously igniting the inner shell fuel in a violent explosion and pushes the outer envelope outward with great speed and force. The star expands and brightens, climbing up and out from the K Giant stage and back to the realm of the M Red Giants. The second time this star climbs the giant branch of the HR Diagram, it has a track pattern asymptotic to the first (I have never liked mathematics a whole lot, so when I saw this word in my reference book, I had to look it up. "Asymptotic" means a line that is curving outward and to the right or left, limited from going higher by some factor). These stars have received the name "Asymptotic Giant" stars and are grouped on the HR Diagram in a place called the "Asymptotic Giant Branch." So we call them AGB stars.

As these stars evolve toward their deaths, they move rapidly up the HR Diagram toward the upper right, increasing in diameter and brightness. As the size increases, the surface temperature cools and the spectral class drops to near M8. The most famous of these AGB stars is Mira (Omicron Ceti), a star that is 420 light years from us. With a size over 500 times that of the Sun, Mira's diameter would reach halfway to Mars. Its total output of luminous energy exceeds 15,000 Suns. If Earth were to be in a safe orbit around this star, it would be twice as far away as Pluto is from our Sun, and still Mira would be over 5 times bigger in the sky that our Sun is here! What is amazing is that there are Mira-like stars that are 100,000 times more luminous than our Sun.

These Red Giant stars have absorption spectra different from our Sun, with complicated bands of elements and various molecules. The chief molecule in these spectra is Titaniun Oxide (TiO). This molecule exists in hotter stars, but the increased temperature of those surfaces causes the other elemental gases to obliterate the TiO bands from view. The star Mira was photographed recently by the Hubble Space Telescope, and its image appears below and to the right. The location of Mira is shown in the constellation map of Cetus to your lower left. Notice the bulge to the left side of the star. Mira is a binary system where one is a tiny White Dwarf, and the other is a large Red Giant. The White Dwarf is accreting matter from the Red Giant onto itself. This image shows what may be the gravitational influence of the White Dwarf companion on the larger star. On a "regular" basis, the material from the larger star will build up into a large "pile" and suddenly rush onto the surface of the White Dwarf. This sudden addition of star material causes the entire system to suddenly increase in overall brightness, increasing by a factor of over 7 magnitudes. Mira is therefore classified as a variable star, at times being invisible and at other times shining rather brightly. It is the closest such binary system to our Sun, and therefore was quite a mystery for the ancients who saw it come and go over periods 332 days. If you did not know what was going on, it would be hard to explain a star that appears and disappears.

 

Okay, you might be saying about now. What is Spock doing in this section of my course, to say nothing about what is Spock doing with a woman. Here is the ONLY time when Spock truly let go of his feelings. In one classic episode, spores on a particular planet caused the inhabitants to feel unusual joy. Spock is obviously under the influence of those spores. The stardate is 2263. Apparently this woman fell for Spock in 2261, and could not hide her feelings when Spock happened to return to the colony with the Enterprise crew. The name of the planet ... Omicron Ceti. To see a few more pictures from this episode, and one rare photo of Spock kissing the woman, click on the image. I could not pass up this moment :)

 

 

Variable Stars

As I mentioned above, if you watch Cetus over a long period of time, you will see Mira come in and then out of view. Most of the time Mira is invisible, lying at an apparent magnitude of 10. When it brightens to its peak of 2, it is quite apparent in the night sky. The brightening and dimming of Mira leads to a short introduction into a class of stars whose brightness changes in a periodic manner. There is an entire class of " Mira variables" that are named after the prototype version in Cetus, and thousands of them are known. The light curve for Mira is shown to your left, and by clicking on the image, you can connect to the entire archive of observed variability for this star from the AAVSO website. Astronomers study the patterns of brightening and dimming and discover clues about the evolution of stars. A star like Mira, with its long history, has a well-understood pattern of variability. A group of Astronomers is dedicated to studying variable stars, The American Association of Variable Star Observers, of which my Uncle Bill has been a longtime member. To visit their website and learn what variable star observing is all about, click on AAVSO. You will be asked to go there later for an assignment on variable stars.

 

The M Dwarf stars that are variable change because their magnetic fields suddenly release impressive amounts of luminous material. These objects are called Flare Stars. The Mira variables change in brightness because their diameters are changing, or their temperatures, or spectral class. The most important class of variable stars are the Cepheids. Cepheids are F and G supergiants that occupy a spot at the very top of the HR Diagram. These stars pulsate because of internal effects that cause them to expand and contract with remarkable regularity. They typically vary by a magnitude or two over a period of a few tens of days down to just a few days. Their period of brightening and dimming is tightly correlated with their luminosity, and such allow astronomers a means to determine the distance to faraway objects like whole galaxies. Here is an example. Suppose we see a distant galaxy, but cannot determine its distance from us. By using a powerful telescope, we can find Cepheid variables in that galaxy, like those in the galaxy M100, and measure the period of brightening and dimming. Once we know the period, we can also know the absolute magnitude because the two are so directly related to each other. Knowing the absolute magnitude, we can then also measure the apparent magnitude, and using the Magnitude-Distance Formula, get a close measure of the distance to the star itself, and thus to the galaxy in which the star resides. With these particular stars, we can measure the distance to galaxies that are many millions of light years away!

A great many other forms of variable stars exist that astronomers study, but these are beyond the scope of this course, if the entire course is not already there. I think it is time to move on.

Death of Mira-like Red Giants and Birth of New Stars

As the Mira star continues to evolve, with is carbon-oxygen core burning furiously, the star begins to evaporate under the repeated force of shock waves generated inside. The star acts like a giant piston, some gravitational collapse of the core with neighboring shell ignition of Helium to Carbon and Oxygen, and a sudden outward pressure that flings stellar material of the envelope out into space. Some of the matter that is blown away condenses into tiny grains of dust that are practically invisible. This dust is pushed outward, enveloping the star in a large dirty cloud. Mira itself is blowing mass away from itself at a rate of 10e-7 solar masses per year. While this may seem like a small number, remember that the mass of the Sun is 1.989x10e30 kg. Mira is blowing 1.989x10e23 kg away per year, or a number that looks like this: 198,900,000,000,000,000,000,000 kg ... this is 198 septillion kg of mass! PER YEAR! And the star can exist in this state for tens of thousands of years. The outflow of matter is only enhanced when shells of helium gas ignite into carbon burning. In the case of our Sun, a full 50% of its mass will be lost in its lifetime due to events like this, as the carbon-oxygen White Dwarf interior is gradually exposed. Stars more massive than our Sun may blow 80% of their matter away in their lifetimes. The total lifecycle of an average main sequence dwarf is seen to your left. Protostar - Sun - Red Giant (lower right pink star in the giant branch) - K (lower left pink star in the giant branch) - Red Giant (upper right pink star in the giant branch) - White Dwarf. Remember, it is the change in spectral pattern that tells astronomers what stage of Red Giant the star is in ... first climb up and off of the main sequence, or second climb and ready to die!

What is really cool is that these Mira variable stars that pulsate are spewing vast amounts of dust into space carbon (stuff the same as graphite) from carbon stars, silicates from oxygen-rich Mira-like stars, etc. These tiny grains accumulate ices and metal atoms, and help form the giant clouds of the Milky Way, giving it the wondrous structure that we see with the naked eye from a really dark site.

This fantastic image to your left is taken during a warm summer evening, looking south toward Sagittarius. The dust in the clouds generated from dying Mira-like stars shields background starlight resulting in the dark regions of the Milky Way, while some foreground stars will reflect light behind themselves against dust clouds resulting in bright cloudy patches of our galaxy.

This same dust also shields the interior of the clouds from the heating effect of neighboring starlight, and the cloud interiors drop in temperature to near absolute zero. The extreme cold temperatures allow the denser portions of the clouds to collapse under their own gravity. This contraction heats the interiors of the collapsing blobs, and when they grow hot enough, they ignite Hydrogen into Helium and new stars are born. This sequence is shown in the three images below. The top image is the center of the Trifid Nebula. The interior has dark clouds of very cool gas. The middle image is of the top of one dust pillar in M16 ... the Eagle Nebula, and you can see globules of dust collapsing into new stars. At the bottom of the set of images is a group of protoplanets forming in the interior of the Great Orion Nebula. If the collapsing cloud has any rotation to it, material will be flung out from the collapsing protostar, and some might begin to coalesce into comets, small rocks and asteroids, and then into actual planets.

Stars are here seen to be vast recycling machines. Stars like our Sun will make Helium from Hydrogen, and then Carbon and Oxygen from that Helium. As these middle-weight stars begin their death cycles, they release much of what they have built back out into space, so new stars can form from the old material. When we look at cool Red Giants, we are looking at stars in the final stages of their lives. Stars that began and lived similar to our own Sun. And stars that will replenish a supply of heavy elements into the galaxy for future stars and planets to be built of.

Crosby, Stills, Nash, and Young sang a song at the 1969 Woodstock festival entitled, "Stardust." In this song, they remind us that the very atoms that make up our physical bodies were formed in some long-since dead star. Since our Sun has a spectral signature demonstrating the presence of over 70 different elements, we can conclude that it probably formed from the collapse of a previous large star which died a long time ago. As that collapsing cloud of dust and gas rotated, most fell into the Sun, but enough material coalesced into this planet that we occupy, and some of those atoms are in the very bodies your are using to look at my beautiful pictures in this course. You may never by America's next idol, but you are all stars!

You can now go forward to the Hottest Stars, or back to Main Sequence Dwarfs, or the Introduction to Stars, or the Syllabus.


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