The Coolest and Faintest

We have spent a great deal of time in this course looking at our Sun as the standard. Form now on, we will look at all of the rest of the star types in the Universe and determine what makes them different and also similar to our Sun. Keep in mind my opening comments about the Sun ... it is an average, middle-aged star. As you look at the HR Diagram, the Sun sits almost squarely in the middle of the chart, and in the middle of the main sequence. Ascending the main sequence to the left will bring you to brighter and hotter stars, a subject for another chapter. Now, we will be descending the main sequence to the right, out of the realm of the G class, through the K, and into the M class. Stars down there in the chart are so cool in temperature and small in size that NONE of the M class dwarfs on the main sequence are visible to the naked eye. The farther down and to the right you go on the main sequence, the more difficult it becomes to find these stars at all as the absolute magnitudes of some of these dwarfs is +20, and you even move into that new class of stars L, which contains the transition between red dwarfs and brown dwarfs (bodies too cool in temperature to even be real stars).

In the previous page, we looked up the HR Diagram main sequence from G to O class stars. Among the 40 brightest stars in the night sky, 21 are giants or supergiants. Of the remaining 19 main sequence dwarfs and subgiants, eight are spectral class B and eight others are of class A. This means that 37 are substantially hotter, brighter, and more massive than our Sun. There is only one F star, first magnitude Procyon in Canis Minor, and it is a subgiant. The final two stars are the G and K combination that makes Alpha Centauri (seen above). These stars appear bright because they are so close to us, not because they are luminous. Below Alpha Centauri are almost no visible stars. There is fourth magnitude 40 Eridani, and then the main sequence stops at the K5 and K7 components of 61 Cygni. These stars appear to be bright because they are only 11.4 light years away. We can not see any main sequence Red Dwarf M stars with our naked eye.

WE SEE WHAT IS THERE

These first two paragraphs, as well as information from the previous page only demonstrates the bias of many astronomers. They study the stars that they can see. And they see lots of B stars because they are bright and shine for great distances. The O stars are hotter and brighter, but also far more rare. On the other hand, the M class stars are BY FAR, the most common star class in the Universe, but because they are so dim, few people studied them. This is not much different that the challenge posed to me in graduate school in Zoology. Did I want to study a really cool animal but find so few that my statistical analysis of small numbers would weaken the strength of my conclusions, or did I want to study something mundane, but so common in number that my data might reveal something very significant. I chose the ubiquitous land snail over the rare polar bear! It is no different with the stars. You can either study the bright, obvious stars, but find few, or study the dim, hidden stars, and find lots of them. For example, for every O class star, there are 17,000 A stars. For every A star, there are five F dwarfs, 13 G dwarfs, 15 of class K, and an amazing 100 M stars (1.7 million for every O star). There are more of the M (and L) dwarfs in the Galaxy than any other kind of star, but all are hidden by their low luminosities. Furthermore, while they may be small in size, they make their presence known in numbers, constituting one half of the mass of the Galaxy's stellar material. It is sort of like lots of ants for every polar bear.

Red Dwarf M Stars

Here is a close-up image of a typical Red Dwarf M star, Gleise 623b. The "b" refers to the type of stellar object in a binary system, so the "b" star is the smaller of the two in this HST image. The photo was taken on December 20, 1994.

 

 

 

 

 

 

I chose this particular HR Diagram image because it is prettier to the eye, but also demonstrates the difference in numbers of M dwarfs compared to other stars. From the HR Diagram we can learn several things about the Red Dwarf stars. First, their spectral class is M, which means low surface temperature. Recalling Wein's Law which relates surface temperature to spectral wavelength, these stars burn more coolly at the surface and therefore are red in color.

Second, these stars are also low magnitude. The values of +10 to +20 make them extremely faint objects. Remembering that for each increase in level of magnitude, the star is 2.513 times more faint, a Red Dwarf star is 1000 times (M=+10) to 22 million times (M=+20) less bright than our Sun. These things are very difficult to see, even with the most powerful telescopes.

Finally, the HR Diagram I chose to display here shows that there are a lot of these Red Dwarfs in our Galaxy. As stated earlier, perhaps 50% of the visible mass of the Milky Way is made up of Red Dwarfs. They are common, dim, and red in color.

Below is a list of some of the stars in this category, "Faintest and Smallest." It is given here to show you the range of stars in M dwarf group, as well as a few in the L dwarf region of the HR Diagram. Please pay special attention to the low mass values. These stars are all significantly less massive than our Sun, and with this information at hand, let's look at the Red Dwarf family more closely.

Star

Apparent

Magnitude

Absolute

Magnitude

Distance

(ly)

Spectral

Class

Temp (K)

Mass

(Suns)

Comment

Eta Cassiopeiae B

7.51

8.7

19

M0

3800

0.19

 

40 Eridani C

11.17

12.7

16

M4e

3300

0.16

companion to white dwarf; flare star

Kruger 60A

9.85

11.9

13

M3

3500

0.27

flare star

Kruger 60B

11.3

13.3

...

M4e

3300

0.16

flare star, DO Cephei

Barnard's Star

9.54

13.3

6

M5

3100

...

highest proper motion

Proxima Centauri

11.05

15.5

4

M5e

3100

...

closest star; flare star

L726-8A

12.45

15.3

9

M5e

3100

0.10

flare star

L726-8B

12.95

15.8

...

M6e

2800

0.10

flare star, UV Ceti

Wolf 630A

9.76

10.8

21

M4e

2600

0.4

flare star

Wolf 630B

9.8

10.8

...

M5e

3100

...

flare star

Wolf 630C

16.66

17.7

...

M7

2600

...

VB 8

BD+4^4048A

9.12

10.3

19

M4

3300

...

 

BD+4^4048B

17.38

18.7

...

M8

2200

...

VB 10

RG 0050

21.5

20.0

65

M8

2200

...

 

LHS 2924

19.7

19.4

36

M9

2100

...

 

Kelu-1

22.1

22.1

33

L2

1900

...

brown dwarf

GD 165B

24

21.5

1093

L4

1850

...

brown dwarf

Gleise 229A

8.14

9.3

19

M1

3700

...

 

Gleise 229B

...

...

...

T

1000

...

brown dwarf

M Class Red Dwarf Stars have a Solar Mass value Between .08 and .5 Suns. This mass value is the all-important factor in determining the fate of the star. Red Dwarfs begin their lives as every other star does ... a cloud of dust and gas that begins gravitational collapse when triggered by some external event. The lower mass clouds collapse over several hundred million years until the inward pressure of gravity raises the kinetic energy of the atomic nuclei in the core to 7 million K. At this "temperature" Hydrogen nuclei will be so energetic that when they collide with each other, they fuse and release gamma radiation. A star is thus born.

The difference in the Red Dwarf star is that the mass of the cloud was low, and so the mass of the star is low. With less mass, there is less gravity to compress the core nuclear particles, so the fusion process happens less frequently. This is also stated as slower nuclear burning. The slower rate of thermonuclear fusion results in less outward pressure. When these stars reach "hydrostatic equilibrium" they are smaller and cooler objects. They are red in color and dwarflike when compared to our Sun. Hence the cute little name "Red Dwarf."

With a slower rate of nuclear fusion, these stars will burn much longer. Yes, there is less nuclear fuel to work with, but the rate is so slow that these little stars will drastically outlive their giant cousins farther up the main sequence. While our Sun has an expected lifetime of 10-12 billion years, the Red Dwarf stars may live for 20 billion up to several trillion years! This has very important ramifications to the visible Universe. No Red Dwarf star in the Universe has died yet! When we find Red Dwarf stars in globular clusters, they may be among the oldest objects in the Universe. Long overlooked because they were deemed dim and boring, Red Dwarf stars are now the subject of intense study, for they may hold clues to the earliest material from which the Universe is made.

Theoretically, as the Red Dwarf ages, it converts its core supply of Hydrogen into Helium. When the core fuel supply of Hydrogen is depleted, gravity will collapse the core, but never achieve sufficient pressure to give the Helium nuclei sufficient kinetic energy to collide and form Carbon. All that remains will be a Helium core surrounded by a shell of non-burning Hydrogen. The dead Red Dwarf slowly cools to become a Black Dwarf. I say "theoretically" because no Red Dwarfs in their death throes or Black Dwarf state have been found.

On an HR Diagram, the entire lifecycle of a Red Dwarf looks like the right image below, compared to the Sun lifecycle to the left below.

 

Brown Dwarfs & Planets

Fainter and Smaller than the Red Dwarf stars are the so-called Brown Dwarfs. As the name implies, a brown color is not part of the spectral classification system, so these objects do not even make their own light. Click on the title to learn more about this kind of star.

You can now go forward to the Cool Red Giants, or back to Main Sequence Dwarfs, or the Introduction to Stars, or even to the Syllabus.


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