If you have been reading my first
6 5 posts, by now you know one thing that “MS” stands for. For those of you who have looked at my blog’s tagline and wondered “What the heck is a main sequence”, this post’s for you. My goal is to avoid all things medical, today.
I can tell you the exact moment that I decided to become an astronomer. I was in the 8th grade, and I had just finished reading a book about stellar evolution. This book explained how, in about 5 billion years from now, the Sun will start to transform itself: it will start to expand. At its largest girth, the Sun will likely engulf the Earth and will shine 3000 times brighter that its current brightness. This bloated type of star is called a “red giant”. Needless to say, the effect that the Sun’s future red giant phase will have on Earth will make the current global warming pale by comparison. But hey, no worries, we’ve got about 5 billion years to figure out what to do!
What fascinated me most about this book was not so much my morbid fascination of a toasted Earth, but the physics that was able to explained it all. Physics gives we mere mortals the power to look beyond our puny existences and predict, with startling accuracy, what will happen billions of years into the future. Obviously, none of us are going to be around long enough to confirm that these educated prophesies come true, but there are other ways to validate these claims. All we have to do is to look for other Sun-like stars that are at least 5 billion years older than the Sun and see what is happening to them. Fortunately (for more than one reason), our Sun is a bit of a late-comer to the Universe, with the Universe having been around for about 8-9 billions of years before the Sun even formed. Therefore, many generations of stars formed long before the Sun, so there are plenty of other Sun-like examples to look at that can give us a glimpse of the future of our own Sun. Another fortunate aspect of the Sun having formed well after the Universe got started, is that all of the many previous generations of stars helped “cook up” the heavier elements that we enjoy here on Earth. The calcium in your bones? That iron skillet you used to make breakfast? Thank a star! That platinum ring, gold necklace and grandma’s silver teapot? Thank a supernova explosion (a really massive star that explodes).
Dr. Sagan was right: we are literally made of star stuff. Yesterday, I was listening to the painfully poignant Newtown, CT memorial/vigil service when a rabbi spoke. He had a profound closing statement that resonated deep in my heart. He said: “There is no death, only transformation”. What beautiful, elegant, truthful words that speak to anyone, no matter where one happens to fall on the religious spectrum, even to this agnostic! The atoms that make up our bodies originated from stars long ago, supernova explosions, or perhaps from the very Big Bang explosion itself. After perhaps several cycles of ending up in star, then getting blown out into the Universe by strong stellar winds or explosions, they ended up in a large rotating cloud of gas and dust that started collapsing by its own weight. In the center of this collapsing cloud, the heat and density are so high that hydrogen atoms hit each other with enough force to fuse together and make another element (helium). A star is born (our Sun). Some of these atoms — the very ones that become part of our body eventually — end up in the left-overs after the Sun has been created, the stuff that ends up forming the planets. The atoms become part of the rock that we call home, are spewed out into the atmosphere by volcanoes, are inhaled by the first appearance of amoeba and farted back out into the atmosphere where … eventually … perhaps some of them become part of the front tooth of a Tyrannosaurus Rex. The mighty lizard dies, those atoms are absorbed the by soil, become the fur on a mammal, flow through the veins of one of our bi-ped ancestors, and so on and so forth … until … they are, one day, absorbed by a tomato plant, which is consumed by your mother while pregnant with you. Those atoms then provide the building blocks that become you! Clearly, the atoms of our bodies are on loan. Someday we free them back to the Universe, where they become part of another living thing or, perhaps many billions of years from now, they seed some distant, not-yet-formed planet and help provide life-sustaining conditions there. The scientist in me (and the avid recyclist!) derives much comfort from wondering about the journey these atoms have already taken, and the fascinating voyage that lay ahead of them, in perpetuity.
OK, but what does any of this have to do with the “main sequence”, you are undoubtedly asking? The main sequence is the category of stars that are in their “normal” phase of life — like our own Sun right now. They are in the prime of their life, happily converting Hydrogen to Helium in their cores. That conversion (a nuclear fusion reaction!) produces the energy that we see and feel as sunshine and warmth from the Sun. When the Sun starts to expand 5 billion years from now, it will be “leaving” the main sequence at that time. This event is triggered by the Sun running out of Hydrogen in its core.
One way to understand the main sequence is to look at an every day examples — let’s look at human hair! Suppose we were to walk around and poll a young group of people, say aged 10-30, and determine their natural hair color and count how many hairs are on their head. We would see a trend … turns out that blondes have more strands of hair than redheads. I can’t vouch for the scientific accuracy of this statement, but here’s my reference (http://milady.cengage.com/imgs/Page238_C11_stdCos2012.pdf). Let’s just say, for the sake of argument, that this trend is indeed accurate. OK, on the X-axis we plot hair color, and on the Y-axis we plot the corresponding number-of-strands-of-hair. We’d end up with something like the below graph, which indeed shows that blonds have more strands, and redheads have fewer strands. This relationship between color and number-of-hairs is a kind of “main sequence” for humans, in this graph. All healthy, youthful humans would fit somewhere along this line, which we will henceforth declare to be the totally made-up name of “human main sequence”.
Now let’s expand our poll to include everyone else — all the way up to the age of 90! Two new things are going to happen: we are going to start seeing white hair (and many – 50?! — shades of grey), and that white hair is going to either be thick or nearly non-existent (if, for example, you happen to be one of my many hair-challenged male relatives). None of those folks were born with that hair … some started with blond hair, others with brown hair, etc and as their hair changed to gray, they left the “human main sequence” and moved to the right in this graph. The points that are not on the main sequence represent the older folk. The graph below illustrates both the human main sequence (which still exists because we included the same young people in our poll) and the older, off-the-main-sequence people:
OK, now let’s get back to stars! The stellar main sequence is similar to the “human main sequence”, in that it is a line in a graph along which all stars that are in the prime of their life (e.g., haven’t yet run out of hydrogen) lay. But this graph is one of color (x-axis) versus brightness (y-axis). If one only includes these “prime life” stars, one would get a graph similar to the one below, which only shows the main sequence:
But now let’s consider ALL stars, what would the graph look like? I bet that you can already guess … the older stars (the ones that have run out of core hydrogen) have gone off the main sequence into other regions of this graph, sort of like this:
The vertical line of stars above “red” are the Red Giants (which the Sun will some day become), and the Red Supergiants (which stars much more massive than the Sun become). Do you see the little clump of points(two points in the chart) below the Main Sequence? Those stars are called White Dwarfs. They are fascinating objects, and represent the end-game for the Sun. After the Sun becomes a red giant, it will gradually lose its bloated gaseous envelop, until all that is left is its carbon core. That carbon core is the white dwarf star, and because no more nuclear reactions happen, it just sits out there in space, gradually cooling off, like an ember. Now here’s the truly fascinating aspect of white dwarfs: their densities are so high that one teaspoon of white dwarf would be about as much weight as a dump truck could handle!
In a future post, I’d like to delve into more juicy details. But here is a good place to stop, as I think I have at least accomplished my goal: to explain my blog’s tagline.