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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

The incredible variety of stars – and how much we can learn from simply noticing their color!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Even a candle flame has a lot to say when you dissect it's light. Think of what the stars can tell us!

Look at a star. Can you tell it’s color? Don’t be surprised if you can’t. At first all stars just seem white to many people.  Yet amateur astronomers joyfully describe them in exciting hues of red, orange, yellow, and blue.  The truth is, it would probably be more accurate to talk of these “colors” as “taints.”  A red star doesn’t look like a red Christmas tree light. But once you’ve taken a good close look – once you’ve made comparisons between key bright stars, such as Spica and Antares in the summer sky, or Rigel and Betelgeuse in the winter sky – well, you definitely should see that Antares and Betelgeuse are tainted red and Spica and Rigel are tainted an icy blue.

This drawing - details and copyright can be found here - does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system.

This drawing does a better job than anything I've seen of depicting star colors as we see them with our naked eye. Keep this in mind as you learn more about the OBAFGKM classification system. For more details and copyright information click image.

And from that single piece of information – the color –  you can make reasonable guesses about a star’s size, temperature, intrinsic brightness, life expectancy – even how rare it is and the way it is going to die. How? By knowing the secret of a wonderful little classification system that uses the sequence of letters OBAFGKM – a system built on more than 150 years of breaking star light into its constituent colors, studying the result, and comparing those results with what we can learn in the laboratory about light and its relationships to various elements.  Oh – and if you don’t mind risking a little political incorrectness, you can remember that sequence of letters by this wonderful little  mnemonic device – Oh Be A Fine Girl (Guy) Kiss Me.

And what all of this tells us about stargazing in general is that what we see when we look at the stars on a typical night are not the ordinary stars, but the extraordinary ones.  That is,  the stars we see with our naked eyes are a sampling of the unusually bright, unusually close – and in some cases – unusually large . With our naked eye we cannot see a single example of the most common type of star – not one.

From a BB shot to a mountain

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

Color and brightness diffeences can be obvious, but the real differences are only revealed through careful study with sophisticated instruments.

And while we certainly notice the differences in brightness of the stars, and we may have now tuned ourselves to detect the subtle tints of color that separate red stars from yellow and orange from blue, there’s little we can discern with our naked eye that prepares us for the incredible variety of stars. Take one measure alone – spatial dimensions. All stars appear to us as a point source of light – they show no disk, except as an artifact of our telescopes. In short – they all look the same size – very, very small.

Yet this sameness is a far cry from reality. In reality stars have an incredible range of sizes. Let’s scale things down to the graspable. We’ll reduce our home planet to the size of a small bead, 2mm in diameter – less than one tenth of an inch. We’ll let that small bead represent the size of a white dwarf star. On that same scale, our star – the Sun – is about 9 inches in diameter. And on that scale the largest stars we know would be almost 2,000 feet in diameter.  Think of it! Stars range from something smaller than a BB shot to something larger than the tallest man-made structure. Then consider this – that BB shot represents something the size of the Earth, a typical size for a white dwarf star – but there’s actually a type of star much smaller than a white dwarf.  It’s called a “neutron star,” and while it may be as massive as our Sun, it’s about as big as a large city! I couldn’t figure out how to include that in our little model without using a powerful microscope.

(Magic interlude: to really get a quick handle on the size of planets and stars, go here (link opens in a new window) – then return 😉

This incredible size range alone should tell you that stars are quite different, one from the other. We first began to notice how different about 150 years ago when scientists combined a prism with a telescope and broke down the light from the stars into its various wavelengths, displaying a spectrum. In such spectra we see not only colors but thousands of dark lines at locations where specific wavelengths of light are absent, and these dark lines hold the key. From them, scientists can deduce chemical composition, temperature, and much more.

The Harvard connection

In the late 19th Century astronomers at Harvard University started to classify the spectrum of stars. There were obvious vast differences, and they applied an alphabetical system to rank stars by the strength of the lines that represented hydrogen.  At first it all made sense and the letters were in alphabetical order, but as their knowledge grew, the letters got scrambled until we finally ended up with a system that runs in this order:


So Astronomy students, in an age when we were less tuned in to sexism, learned the simple mnemonic Oh Be A Fine Girl Kiss Me.  More recently they have messed things up a bit with additional letters, L and T, and within each class there are nine different sub-classes, which are represented by numbers.  But for now let’s stick with the basic letters, as they cover the vast majority of stars.  Learn it anyway you like, but learn it. The order of the spectral letters is one of the few things I find worth memorizing, for it turns out to be an almost ideal classification system covering in a single order several major characteristics of the stars.


The mains pectral classes of stars with the class designation on the left, a represenative spectrum for that class of star, and then the temperature for that spectral class on the right in degrees Kelvin.Clcik on image for information about its source, copyright, etc.

First, these letters represent a temperature sequence. The hottest stars are at the top. and classified as “O” stars. “M” stars are the coolest common stars.

And OBAFGKM represents a mass sequence, with the stars at the top being the most massive.  (Don’t get mass and spatial dimensions confused, however.  A star can have the same mass, yet be as tiny as the Earth, or far, far larger than our Sun. )

But these same letters also represent a color sequence going from blue-white to red, with yellow stars in the middle. Our Sun, for example, is a “G” star – basic yellow.

The sequence also indicates something about life span. The hotter a star is, the shorter its life. “O” stars will live a few million years, furiously exhausting their nuclear fuel. “G” stars such as our Sun, are destined to live billions of years – about 10 billion for the Sun. And “M” stars may go on for trillions – much longer than the universe has existed to date.

And the sequence tells you something about how the stars will die – those near the top, the “O” stars, will go out with a bang, those near the bottom, a whimper.  So temperature, mass, color, life span, and the end game are all related.

They also represent a sort of frequency distribution. “O” stars are one in a million – very rare.  And “M” stars are most common. In fact, about 80 percent of the stellar population is believed to consist of “M” stars, yet we don’t see a single normal “M” star with our naked eye.  (Catch the hedging there?  We do see some abnormal “M” stars. More on that later.)

Know the spectral type, know the star

But the OBAFGKM sequence really hits the jackpot when it comes to dropping stars into convenient little boxes.  If you look up at Vega and note that it is blue/white, then you know it’s nearer the upper end of the list – probably a “B” or “A.” This means it is hotter than the Sun, more massive, and will live a much shorter time. That’s a lot to know just from a crude estimate of its color, but it’s pretty close to what astronomers have learned with more sophisticated techniques. Vega is classified as an “A” star which  has a temperature of 9600K compared to the 5777K of our Sun.  It has half again as much mass as the Sun and is expected to live less than a billion years, compared to the 10 billion for our Sun. In short, know the color – or better yet, the letter assigned to a star and you immediately know something about its color, temperature, mass, life expectancy,  and death,  as well as how common it is or isn’t.

But this really intersting thing come when you make a very simple graph that ranks stars by just two qualities – their actual luminosity  and their temperature/color. “Luminosity” means the actual brightness of a star independent of its distance from us. Color and temperature are indicators of the same thing. The result is what’s known as a Hertzprung-Russell diagram. Here’s an example with each dot representing a star.


What you should notice is the bulk of stars fall into three areas of the charts – the lower left quadrant where oyu find the white dqaefs, the upper right region where you find various giants, and a thick, swirling band that cuts across from one corner to the other. This band hold the most stars by far and it is known as the “main sequence.”

One thing I don’t like about the word “sequence,” however, is people assume that to be “on the main sequence” means a star is sort of sliding down it – that the sequence is a developmental or evolutionary sequence.  In fact, that was what scientists believed at first – that stars started out hot (top left), then got cooler as they aged, eventually ending up in th ebottom right.  Sounds logical. But the main sequence is not a developmental sequence.  Stars don’t start out life at one end and end up at the other. Instead they stay on the main sequence until their final stages of life, at which time they may move off the main sequence into another realm entirely.  Such moves signal dramatic changes in the star’s life, size, behavior, appearance, and life expectancy.

But start witht he idea that when a star starts life it does so at some particular point ont he mains equence and it stays at that point for most of its life. Where stars go as they leave the main sequence and exactly what happens to themt hen is a matter of mass. In fact, mass is probably the single most important characteristic of a star. You would think it would be chemical make up, but notice we haven’t talked much about star chemistry. That’s because for most stars its basically the same – 92 percent hydrogen, 8 percent helium, and everything else crammed into a fraction of one percent, if it’s there at all.  In a few cases, however, chemistry is significantly different and those stars get their own special classifications. Again, we’ll leave the exceptions to another time.

To appreciate the importance of mass you need to understand what a star is. Reaching for a definition that will cover the whole range of stars, James B. Kaler calls stars “self-luminous condensates of the fragmented dusty gases that fill interstellar space.” OK. As you get near the low mass end, stars do get a bit freaky and it’s difficult to fit them into the general picture. But let’s stick with the general picture where it’s easiest to think of a star as a huge ball of gas, expanding under the pressure of the nuclear furnace at its core, and held in together by the opposing inward pressure of its own gravity.

And that’s the key – gravity. There’s another word we could explore forever without getting an ultimate answer.  Gravity is the force that causes stuff to be attracted to other stuff resulting in more stuff. Honest. The interesting thing about gravity is it works on “the more the more.” That is, the more stuff you have, the more attractive the force of gravity, and so the more likely you are to have even more stuff.

In the ISM – the interstellar medium where stars are born – there’s a lot of stuff – mostly gas and dust – that is spread quite thinly. But various disturbances pass through the ISM, sort of priming the gravity pump, encouraging stuff to get together in clumps which then become bigger clumps until you finally have so much stuff in one spot that the inward pressure becomes crushing. At this point a star is born, for what happens is the hydrogen atoms start bumping into one another, and when they do this, they fuse together and form helium atoms.  At the same time, a small fraction of the matter in the hydrogen atoms gets consumed by the process – matter is turned directly into energy by that most wonderful formula E=MC square – and, of course, the amount of energy is tremendous as we found out when we built atomic bombs and later atomic power plants. A little matter goes a very long way, for we’re multiplying the converted mass by the speed of light (186,200) squared, a very big number. (Mind you, I am not a physicists. I try to recount these things as I understand them from my study. But having read a great book that traces the history and development of that equation it still leaves me completely bewildered why the energy produced should have anything to do with the speed of light. )

What’s so elegant about a star undergoing nuclear fusion is the wonderful way it counterbalances the force of gravity. Left on its own, gravity would have just kept squeezing the stuff tighter and tighter. But the very force of gravity causes the nuclear ignition and that creates the outward force to work against the inward force of gravity.

When we see a star we’re not really staring into a nuclear furnace.  The nuclear fusion in the core is a just the starting point for an exchange of energy that takes something in the order of a million years to reach the surface of the star and send out radiation in the form of the light that we see.  But oh my, what a mighty engine!  A star such as our sun “burns”  4 million tons of hydrogen a second, and according to Leon Golub and Jay Pasachoff in “Nearest Star,” the result is that in just one-thousandth of a second the Sun emits enough energy “to provide all of the world’s current energy needs for 5,000 years.”

And we look up there and it’s hard to not hear the echoes from our nursery –  “Twinkle, twinkle, little star.” Twinkle indeed!

OK – back to OBAFGKM.  How hot a star gets – and many other characteristics – depends entirely on how much stuff is gathered together in this ball of gas. The usual measure of this is to compare the amount with how much is in our Sun. Count the Sun as “one” and a typical “O” star is made of somewhere between 10 and 100 times as much stuff, while an “M” star may have just one tenth what our Sun has.

Other boxes for the stars

Without going into tremendous detail – it’s enough to try to swallow the main sequence in a single gulp – there are four other ways that stars are classified.  I want to mention them here because you’re bound to hear these terms, and some of them sound downright crazy – like calling our Sun, which is larger and brighter than most stars, a “dwarf.”

Stars fall into two broad evolutionary categories, with a third one that exists only in theory. Stars we see belong to either Population I or Population II.  Our Sun and most other stars we see are Population I. The oldest stars we see – such as the ones that are found in globular clusters – are Population II stars.  Population III stars were the first stars formed after the Big Bang. No one has yet seen a Population III star – they are a theoretical concept, but generally well accepted as such. A Population III star would be almost entirely hydrogen and helium. Population II stars were formed next – after the Population III stars had exploded and added “metals” to the universe – metals being elements other than hydrogen and helium.  So Population II stars have a more complex chemistry. Population I stars tend to be richer in metals.  They are the stars being formed today.

(Confused? Do you get the feeling astronomers count backwards?  Well stay tuned. )

Where dwarfs aren’t dwarfed by anything but giants

Another classification system looks at the size – the spatial dimension, not mass – of stars, and it would be confusing, if it weren’t so laughable. I’m not quite sure how astronomy ended up in this quandary, but I assume it’s another instance where new discoveries played havoc with the established naming process. Sort of like trying to straighten up a room as the kids are playing in it. Thus we have a size classification system that goes from smallest to largest stars and reads like this:

VII – white dwarfs
VI –  subdwarfs
V –  dwarfs – main sequence
IV –  subgiants
III –  giants
II –  bright giants
I –  supergiants, and yes
0 –  hypergiants

Oh boy! Wonder what happened to “normal?” Well it’s there. Normal is “dwarf.” In fact the main sequence holds about 95 percent of the stars, and you can consider “dwarf” and “main sequence” synonymous.  And that, of course, means our Sun is a “dwarf.”

OK – maybe I’m being too hard onthe astronomers. As mentioned earlier, the difference in the size of stars is mind-boggling. Here’s a good graphic thatc aptures only the top half od these differences – the difference between our “dwarf” Sun and the giants.

Star sizes 2

Remember, stars do not evolve along the main sequence, but they can fall  – or jump – off it. That’s where these classifications come in. Essentially, as a star nears its life’s end, it goes careening off the main sequence in what results in graceful curves when you start plotting temperature and mass on a graph.  The path to the end is complex and varies according to the initial mass of the star. In the later years of their lives, stars can swell up to incredible sizes and become red giants. An example is Betelgeuse, the star that marks the right (eastern) shoulder of Orion. Such a star doesn’t grow in mass, but it expands like a balloon and while its surface is relatively cool – thus the red color – it’s huge and so the star radiates a lot of light despite being relatively cool..

Stars actually can make the climb off the main sequence more than once and the second time they do this, they may turn into slowly pulsating giants, varying their output significantly over long periods. (Long, that is, from our viewpoint.) Such a star is Mira, “the wonderful.” It goes through an 11-month cycle where it reaches a peak making it easily visible to the naked eye, then it falls back to a point where it is so dim that’s it difficult to detect with binoculars. Chi Cygni is another such star, and there are many more.

In these end games, stars may go through a nearly explosive stage where they blow off a huge amount of their substance and create an expanding cloud that we see as a beautiful planetary nebula. The Ring Nebula in Lyra and the Dumbbell Nebula in Vulpecula are examples.

Stars can also undergo an incredible collapse where their core shrinks to the size of the Earth. Such stars still retain a significant amount of their mass, and they are known as “white dwarfs.” In this case the word “dwarf” makes sense, but the word “white” is another name that has been overtaken by new discoveries. White dwarfs can be red, or any other color normally associated with stars.

And, of course, stars can go out with a bang if they start life with enough mass. The result is a supernova that momentarily shines with incredible brightness, then leaves behind a ragged cloud such as we see with M1, the Crab Nebula.

This doesn’t mean, however, that the star was destroyed in the massive explosion.  In fact, although everyone seems to write about the life and “death” of stars – and I’ve fallen into that pattern as well – I’m not at all sure it’s appropriate. Stars don’t die. They go through incredible changes that may make them difficult or impossible for us to see – but some of the stuff of the star is still there and in most cases it still continues to radiate light. In the case of the Crab Nebula, the star that exploded left a significant core in the form of an incredibly compact neutron star – a star where all the atoms have been stripped of their electrons and protons and are crunched together so tightly that they are about the size of a large city.  Such a neutron star spins as it collapses, much as a figure skater does when she pulls in her arms. It also beams radio energy in one direction and this beam sweeps the heavens like a powerful lighthouse. When we’re aligned with such a neutron star, our radio telescopes pick up a regular pulse of energy, many times a second, from the rapidly spinning neutron star. We call this a pulsar, and again, many examples have been discovered.

In the case of our Sun the current betting is that it will “burp” a time or two creating a complex planetary nebula, then retreat to the white dwarf stage – and white dwarfs go out with a whimper, not a bang. But while theys top radiating, there is still star stuff there so I’m not sure if this really represents the detah of a star.

Whatever the evntual fate, however, it all depends on the initial mass. A star like our Sun ends up as a white dwarf. A star 2-to-3 times larger than our Sun ends up as a neutron star. And a star that starts out even bigger, ends up as a black hole.

The bigger they are, the farther they fall!


I know we’ve covered a lot of ground. There’s actually much, much more that could be said about stars.  But the basic message is simple. Most stars fit into a single classification system that tells us at a glance several major things about the star. Learning that sequence of letters – Oh Be A Fine Girl Kiss Me – goes a long way in helping you make some sense  out of those distant,  twinkling,  dots of light.

Here’s a summary of this system in table form


Click table for a larger version.

  • Ninety-five percent of all stars are on the main sequence.  Most of the stars that are not on the main sequence are white dwarfs. Roughly one percent of the stars fall into one of the giant categories.
  • Stars near the top of the main sequence are rare, as are giants of any spectral class, yet when we look at the constellations we are seeing mostly A and B main sequence stars, and a variety of giants, since these are the brighter stars.
  • Notice that stars in the last two categories,  L and T, are either barely visible to us as red stars, or not visible to our eyes at all because they shine only in the infrared. We don’t know how many of these stars there are, but they could be as common as the main sequence “M” stars.
  • The lower limit for the mass of a star is 1/80th the mass of our Sun – or about 13 times the mass of Jupiter.
  • Temperatures are for a star’s surface. The interior is much hotter. The Kelvin temperature scale is the same as the Celsius one, except it starts at absolute zero. This adds 273 degrees to the Celsius scale, a minor consideration when you look at the typical temperature of a star.
  • Age – we can date star clusters by seeing what class of star remains. “O” stars die first, then “B,” etc. No dwarf “K” or “M” star has died yet – the universe isn’t old enough.
  • Understand in all of this we’re dealing with a continuum, so the numbers are just guides.

The stars are not a WYSIWYG world.  What you see is NOT what you get. But what you get is far more exciting, interesting, and elegant than what you see. That said, don’t forget that the goal here is to see, really see – to go out in the night and let the light from these unimaginably distance fires ping your brain, and when it does, mix that experiential knowledge with your abstract knowledge in the hopes of a greater awareness. Good luck!

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