Cosmos 9: The Lives of the Stars

by Shane L. Larson

A distant memory is anchored in my mind of a warm fall afternoon shortly after I started college. I was sitting in a musty old university classroom, in a battered wood and steel desk chair from around World War II. The gentle breezes that wafted in the window carried the promise of frisbees and afternoon picnics in the shade, but not until my art appreciation professor was finished droning on.

At one point in the lecture, my professor’s voice pierced my reverie and longing for the end of class.  “Art is a way for people to know themselves. If you let it, art is a window into your feelings, your past, your future, your aspirations, your dreams.”  This is, more or less, the classic explanation of why we appreciate art and need art as part of our society.  By contrast, science is often portrayed as the opposing activity — while science is a way to know something, it is not yourself, but rather the world around you.  Science is a window into the inner workings of the Cosmos.

But I don’t think there is a difference between art and science, not really. Both are a way for humans, the stuff of the Cosmos made animate, to try and discover what everything is really about.  To explore that idea, let’s consider the stars.  Most of us are aware of the stars, though maybe we haven’t looked at them much.  When asked, we might describe them as small, cold white points of light in the sky.  But to someone who has really looked at the stars, they have depth and character that most of us have scarcely imagined!

Several paintings featuring the stars, by Vincent Van Gogh. (L) The Starry Night, (C) Country Road in Provence by Night, (R) Starry Night over the Rhone.

Perhaps the most famous representation of stars ever created is Vincent Van Gogh’s De sterrennacht (“The Starry Night”) painted in the summer of 1889 while he was committed to the sanitarium in Saint-Rémy-de-Provence (he had painted many other starscapes, including another the previous fall, known as “Starry Night Over the Rhone”). One may consider the stellar presentations in Van Gogh’s paintings to be fanciful and abstract in the extreme, but it is clear that Van Gogh had discovered the wonder and diversity of the stars.  In a letter to his sister, Willemina, in September of 1888, he wrote:

“The night is even more richly coloured than the day… If only one pays attention to it, one sees that certain stars are citron yellow, while others have a pink glow or a green, blue and forget-me-not brilliance. And without my expiating on this theme, it should be clear that putting little white dots on a blue-black surface is not enough.”

Van Gogh’s observation is not that far from the truth. Even to the naked eye, many stars show distinct colors — Sirius blazes bluish-white, while Antares glows a baleful red, and Mira fades to be invisible then reappears as its cool yellow-orangish self over the course of 11 months. Through the telescope, even more stars show color — the twins stars in Albireo are blue and gold, and Herschel’s garnet star is definitely red.

A gallery of stars. Left to right: (1) Sirius, the brightest star in the night sky. (2) Antares, embedded in the cloud known as the Rho Ophiuchi complex, near the globular cluster M4. (3) Mira, a dramatic variable in the constellation Cetus. (4) Albireo, a blue and yellow double in Cygnus. (5) Herschel’s Garnet Star, mu Cephei.

Annie Jump Cannon.

What do the colors of the stars mean? The understanding of stellar color is a story intimately tied to how we came to understand the lives of the stars. There have been many players over the years, but the story really begins with an astronomer named Annie Jump Cannon. In 1896, Cannon went to work at Harvard College Observatory to work on the completion of the Henry Draper Catalogue, an ambitious project to identify and map every star in the sky down to a brightness about 10 times fainter than the eye can see (photographic magnitude ~9); all told, the catalogue had 225,300 stars. Working on the Draper Catalogue, Cannon simplified two disparate systems for classifying stars based on their spectra.

The spectrum of the star Vega, clearly showing strong hydrogen absorption lines.

What was Cannon looking at? If you dump the light of a star through a prism, the light is split into a familiar rainbow, known to astronomers as a spectrum. If you look closely at the spectrum, you will see that some colors are missing, like they have been deleted, appearing as a black line. These lines are called “spectral lines” or “absorption lines”.  They come in sets that when looked at together form a unique fingerprint that identifies each kind of atom that the star is made of. Cannon was looking at the strength of lines that identified hydrogen, and developed the system that is still used today known as the “Harvard Classification System.”  What she discovered was that there was a natural way to order all the stars based on their color.  She took the old classification systems that had been established before she started working on the project, simplified them, and rearranged the order into a sequence that is familiar to astronomers: “OBAFGKM.”  Every star has a letter that describes its color, known as the “spectral class.”

Cecilia Payne Gaposchkin

The meaning of the classification system was not well understood until 1925, when theoretical astronomer Cecilia Payne-Gaposchkin demonstrated in her PhD thesis that the stars were, primarily, composed of hydrogen and helium, and that the spectral class and behaviour that Cannon observed were related to temperature — the Harvard Classification System was a sequencing of stars from hottest to coolest. Otto Struve, soon to become the director of the Yerkes Observatory, said it was “undoubtedly the most brilliant Ph.D. thesis ever written in astronomy.”

(L) Typical spectra for stars of different spectral type. Note how the central color shifts with type. (R) The Harvard Classification System for spectral type.

The Harvard Classification System has the letters out of order because it was derived from older systems that didn’t understand that temperatures were affecting the spectra. Once that connection was understood, it became clear that the reason “OBAFGKM” was the correct order was because it is the correct temperature sequence.  But you have to remember that silly order of letters!  To help, mnemonics have been developed over the years to help you get the order right, including a few by me and my students… 🙂

Some helpful mnemonics for remembering the correct order of spectral types! The first on the left is the classic many of us learn when we first take astronomy. The others are suggested alternatives.

Understanding the color and temperature of the stars leads directly to an understanding of how stars are born, live, and ultimately perish. The final fate of any star, like the fate of any human, is governed by how they live their lives.  Stars that burn hot and fast in their youth, die young. Stars that take it slow and easy, live to old age.  For stars in the prime of their lives (what astronomers call “on the main sequence”) temperature is a measure of how fast they are burning their bodies up, consuming their fuel in nuclear fires that burn down in their core.

At this point, you may be scratching your head.  How do we know how stars live their lives? Humans live to around 100 years of age, but even the youngest stars live to be tens of millions of years old; the oldest stars live to be billions. No human ever has, nor ever will, live to watch a star live through its entire life cycle.  But we don’t have to, because there are millions of stars to watch, all at different stages of their evolution.  Consider the pictures of people below.  Do you think you can sort them into age order?  Most of you probably can.  How?  You can’t even see the faces of some of them!  But you have encountered people in your life at all stages of a human lifetime, and have learned what to look for to identify age: youngsters play with toys and are smaller than old people; older folks have grey hair, and or wrinkles.  And so on.  Using very quick, visual cues, you can get the age pretty quick. 

Can your sort these people (none of whom you know) by age?

For stars, the cues that tell us something about their status in life are the brightness, and the color.  In the prime of their lives, the brightness and color are directly related, a fact that results from the physical laws that govern the nuclear fusion that burns in the stellar hearts.

But what happens when stars get older? Like many of us, the onset of their elder days causes stars to swell up in size.  A star like the Sun sits well within the orbit of Mercury today, but when it reaches old age it will swell up and expand to roughly the size of Earth’s orbit, consuming all the worlds of the inner solar system. This swelling spreads the energy of the nuclear fires more thinly across the surface of the star, so the temperature goes down.  We know from our spectral classification scheme that cooler stars appear redder — the Sun will expand into a “red giant star.”

Antares is in its red giant phase, and is larger than the orbit of Mars! [Image from Wikimedia Commons]

Stars will live their short elder years as red giants stars.  What is their ultimate fate? Once again, it depends on the genetics of their youth: their fate is strictly dependent on how massive they are.  Midsize stars like the Sun, shrug off their outer atmosphere to form one of the great spectacles of the Cosmos, a planetary nebula. After this, what is left of the star slowly shrinks down to a dying ember, about the size of the Earth, called a white dwarf.  Big stars try to keep burning hard, but eventually their nuclear fuel completely runs out; when this happens, the outer layers of the star spontaneously collapse in on the star, crushing the core in a titanic explosion known as a supernova. The explosion blasts almost all the material that was the star out into space, synthesizing all the “stuff” that you and I are made of.  All that is left behind is a skeleton of its former self: a dark, compact remnant smaller than a city.  Slightly large stars leave a skeleton known as a neutron star, and the biggest stars leave a skeleton known as a black hole.

(L) The Helix Nebula, a planetary nebula. (R) The Cygnus Loop, an 8000 year old supernova remnant.

Cradle to grave, the story of the lives of the stars is as complex and compelling as the story of any life on Earth.  To be sure, I have cast this story in language that is a reflection of our understading of biological life, but it is still a wondrous tale. It astounds me still that not a single person now or in the long history of our species has ever visited a star (not even the Sun), but we have still managed to figure the story out.  It has taken the tale more than a century to unfold, through the diligent work and pioneering efforts of astronomers like Cannon and Payne-Gaposchkin.  But in the end, we begin to understand that the stars are more than just little white points of light in the sky.  Every time I stand out in the backyard, staring up at the stars, I’m always reminded of van Gogh.

A little reminder from Van Gogh, at the eyepiece of my telescope.

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This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage.  The introductory post of the series, with links to all other posts may be found here:  http://wp.me/p19G0g-dE