Tag Archives: Stars

Feeling Small in a Big Cosmos 02: Discovery

by Shane L. Larson

Suppose we wanted to imagine some very big numbers, to somehow develop an appreciation for how BIG the Cosmos truly is. Sitting on a the beach somewhere, one might idly wonder “how many grains of sand are there on all the beaches and in all the deserts of Earth?”  Counting is certainly out of the question, so how might you figure that out?

Bear Lake, Utah.

How many grains of sand, on all the beaches and in all the deserts of Earth?

You would do it the same way we “counted” the galaxies in the sky using the Hubble Extreme Deep Field. You count all the grains in some small amount, perhaps a handful of sand picked up off the shores of Lake Michigan. Then you figure out how long and wide all the beaches and deserts are, and how deep the shifting sands run, and figure out how many handfuls of sand would cover them all. Multiplying my the number of grains in my hand, you would find there are some 10 billion billion (1019) grains of sand on the planet Earth.  That’s a BIG number; a number that is beyond ordinary human understanding, beyond our everyday experience.

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove -- it has seen almost 30 million nights like this one, but very little has changed. The constellations change over thousands of years, but the sky is still full of stars, and the Milky Way still arches over the sky, giving the impression that the Universe is unchanging. [Image: Shane L. Larson]

The night sky over the Pando Forest in central Utah. Pando is an 80,000 year old aspen grove — it has seen almost 30 million nights like this one, a sky full of stars [Image: Shane L. Larson]

But imagine for a moment comparing it to the total number of stars in all the Cosmos. The Hubble Deep Fields have convinced us there must be something like 100 billion galaxies in the Cosmos. A galaxy like the Milky Way has more than 100 billion stars in it, so multiplying those two numbers together, there are some 10,000 billion billion (1022) stars in all the Cosmos, more than all the grains of sand on Earth. An even bigger number, well beyond our everyday experience.

When there is so much we don’t understand here on our own small planet, it is easy to be overwhelmed by the immense size, the immense possibilities of what we don’t understand in a Universe far larger than our brains can easily imagine. We could very easily crawl into our shells, hide from the immensity, and turn our vision inward, with nary another glance outward into the deep vastness that doesn’t even notice we are here.

But we don’t do that. We have, for countless generations, stared into the immensity in an ongoing  (and surprisingly successful) camapign to understand and explain all we can about the Universe. But when everything is so impossibly far away, when the Cosmos is full of so many different and unknown things, how is it that we can know anything?  The answer to that question is that we ask questions.

questionMarkConsider a popular game that most of us have played since we were kids (I have a 9 year old — I get to play this A LOT).  Here is a box (with a question mark on it). You want to figure out what is under this box by asking 20 “Yes-No” questions. Go!

  • Is it alive? No.
  • Is it something made by humans? Yes.
  • Is it small enough to hold in my hand? Yes.
  • Is it edible? No.
  • Does it have batteries? No.

So there we have asked just 5 questions. The answers are nothing more than a simple yes or no. But the tremendous power of asking questions is clear. Despite the vastness of the Cosmos, despite its immense size and the mind-boggling large number of things it contains, you have eliminated almost ALL of it from consideration with only 5 simple questions. You know it is not something huge (galaxy, star, planet, white dwarf, asteroid, comet, …). You know it is not alive, so every organism on Earth — plant, animal, bacteria, fungus, protozoan — is eliminated.  Your attention is now focused on only things that humans make, and only those things that aren’t powered by batteries.

me_ndgt_legoAnd you have 15 questions left! With 20 carefully constructed questions, you will be able to figure out almost anything I wanted to hide under that question mark, with a high degree of success! If we went on and I let you ask the rest of your 15 questions, I am confident you would eventually arrive at the fact that hiding in my question mark box is a little Lego version of me and Neil deGrasse Tyson.

We could have done this with anything in the Cosmos. I could have had anything under that box — an elephant, a quasar, a piece of Pluto, the left foreleg of a carpenter ant, a circle of paper from a hole punch, a cough drop wrapper, an oyster shell, that little plastic do-hickey that holds your gas cap on your car, a Calving & Hobbes sketch, a molybdenum atom, Marie Curie’s lab notebook, a lost pawn from a Sorry game, and so on. ANYTHING!

But you can figure out what it is with only a few questions so reliably we’ve made it into a game children can play and enjoy! It’s usually called “20 questions,” but it also goes by the name science. Except when we play science, we don’t limit ourselves to just 20 questions — we ask as many as we want! You can learn a LOT with carefully constructed questions. And we have learned a lot. We have collected and gathered and recorded our knowledge of the Cosmos so effectively that much of it has passed into the communal memory of our species, integrating itself into the fabric of who and what we are so effectively that we often don’t give it a second thought. We’ve forgotten how hard it was to earn that knowledge, the struggle our forbears went through to wrest some secrets from Nature and then understand what they meant.

A 1/2 globe of the Moon, roughly 5 feet in diameter, made before spacecraft had ever flown to the far side. You can see this in the Rainbow Lobby of the Adler Planetarium in Chicago.

A 1/2 globe of the Moon, roughly 5 feet in diameter, made before spacecraft had ever flown to the far side. You can see this in the Rainbow Lobby of the Adler Planetarium in Chicago.

To understand this, consider the Moon. What do you know about the Moon? It orbits around the Earth. It is spherical, and is illuminated by the Sun. The near side always faces the Earth. It is covered with lowlands (called maria, lunar “seas”), highlands (called terrae, the brighter areas), mountains, craters, and canyons. All of this is common knowledge, which if you didn’t know it you could have found out using the electronic web that girdles our world. I’m pretty sure almost everyone reading this has not been to the Moon. In all the history of our species, only 24 humans have ever crossed the gulf between the Earth and the Moon; only 12 humans have ever walked on the Moon and seen what we know with their own eyes. The pictures of the Moon, taken by the Apollo astronauts and robotic emissaries have virtually erased from our memory what it was like to not know what the Moon was like.

Consider the globe of the Moon shown here. It is about 5 feet in diameter, and lives up to our expectations of a rugged, desolate landscape covered in mountains and craters. How far away from this globe would I have to stand, for it to look roughly the same as the Moon in the sky?  About 140 feet. The full moon in the sky, is about the size of a US dime, held at arm’s length.

When you see the Moon in the sky, it is quite small, roughly the size of a dime held at arm's length. The detail your eye can see is minimal -- mostly just dark and light shading, with no topography! [Image: Shane L. Larson]

When you see the Moon in the sky, it is quite small, roughly the size of a dime held at arm’s length. The detail your eye can see is minimal — mostly just dark and light shading, with no topography! [Image: Shane L. Larson]

When the Moon is that small, you can’t tell it has any topography at all. It is clearly shaded in some irregular pattern (which allows you to make the famous Moon shadows), but there are no craters or mountains to be seen. Go out and look, but don’t look with your brain plugged in to what you know; just look at what you can see. This is how the Moon has always look to the naked eye; it wasn’t until the  application of the telescope to astronomy that we knew anything different.

Galileo's early views of the Moon through his telescope revealed previously unknown topography.

Galileo’s early views of the Moon through his telescope revealed previously unknown topography.

In 1609, Galileo Galilei was the first person to plumb the depths of the sky with a telescope, and what he saw shook the foundations of what we thought we knew about the Cosmos. In 1610, he published one of the seminal works in astronomy: Sidereus Nuncius, “The Starry Messenger,” wherein he described all that he had seen during his first excursions in 1609.  He wrote of the Moon

“... the Moon certainly does not possess a smooth and
polished surface, but one rough and uneven, and, just
like the face of the Earth itself, is everywhere full
of vast protuberances, deep chasms, and sinuosities.”

Two things stand out to me about this passage. The first is how he initially describes the Moon: a smooth and polished surface. This is how people thought of the Moon — it is, in a very real sense, what the Moon looks like, and what you would think if you had never been taught that there were craters and mountains on its surface. The second is when he describes what he saw on the Moon: just like the face of the Earth itself. The telescope allowed us to see that the Moon had features and topography that were at once recognizable and intimately familiar, appearing just like the topography we see here on Earth. In a singular moment of discovery, the telescope deprovincialized our view of the Earth. The Moon is, in a very real sense, the first world other than the Earth that we ever discovered, and this is how it happened.

Galileo's planet sketches, while not showing the detail of his lunar observations, were no less revolutionary.

Galileo’s planet sketches, while not showing the detail of his lunar observations, were no less revolutionary.

There were many other startling revelations Galileo had looking through the telescope. In addition, he was the first person to look at the planets through a telescope. And what he found was that the planets were not stars at all, but also were other worlds. Every planet showed size, and round shape. The planet Saturn had odd protrusions; Galileo wrote “Saturn has ears.”  Turning his telescope to Venus, Galileo found that it went through phases, just like the Moon, a fact that was easily explained by the still new Copernican idea that the Sun was at the center of the solar system.  But Jupiter revealed one of the greatest secrets of all — it held in its grasp its own entourage of moons, that orbited the great world much as our own Moon orbits the Earth. Today, they are known as Io, Europa, Ganymede, and Callisto — the Galilean moons.

When I think about these momentous discoveries, my mind always wanders to the following, often overlooked fact: even though Copernicus’ De revolutionibus orbium coelestium had been published more than 60 years before Galileo’s observations, and placed the Earth in orbit around the Sun, Galileo’s observations were the first to reveal the planets were indeed other worlds. To put an even finer point on it, Galileo’s observations were the first to definitively show that the Earth was a planet, possibly not unlike the other planets that orbit the Sun. Galileo’s telescope allowed us to discover the planet Earth.

Galileo's telescopic observations of the Pleiades revealed stars that could not be seen with the naked eye. There was an unseen -- an unknown -- part of the Cosmos to discover.

Galileo’s observations of the Pleiades revealed stars that could not be seen with the naked eye. There was an unseen — an unknown — part of the Cosmos to discover.

Galileo also peered at stars. He found that when he looked at the Pleiades, the Seven Sisters, the telescope revealed stars that could not be seen with the unaided eye. When he peered at the diaphanous glow of the Milky Way, arching horizon to horizon in the dark skies of 17th Century Italy, he found it was comprised of uncountable numbers of individual stars, so far away and so dim that without the telescope their combined light looked no more than an evanescent fog in the dark.  The scale of the Universe was suddenly much larger. The structure of the Universe was suddenly more complex. Larger and more complex than humans had ever imagined. The revelation of the Cosmos had begun.

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This post is the second in a series of three that capture the discussion in a talk I had the great pleasure of giving for Illinois Humanities as part of their Elective Studies series, a program that seeks to mix artists with people far outside their normal community, to stimulate discussion and new ideas for everyone.  The first post can be found here:  http://wp.me/p19G0g-xB

The idea of describing science in the context of 20 Questions is one I was introduced to at a very young age, by Carl Sagan in “Cosmos: A Personal Voyage” (in Episode 11: Persistence of Memory).

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Cosmos 9: The Lives of the Stars

by Shane L. Larson

schoolDeskA 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!

A collection of 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.

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.

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.

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 Vego, clearly showing strong hydrogen absorption lines.

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 first person to understand what the stars are made of.

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.

(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!

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?

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]

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.

(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.

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