Tuesday, May 10, 2011

Astro-Lesson: The Star Life Cycle

This week's topic kinda picks up from last weeks topic, the Sun. From our short and limited view of the cosmos, we have been able to discern something amazing, the life cycles of stars. It turns out stars have a birth, then long lives of constant work converting hydrogen to helium and other elements, and then they die, either by withering away or massive violent explosion.

Art of the life of a Sun-like star.
 There is a significant reason why knowing this is important. A star can last for BILLIONS -- or even TRILLIONS!-- of years, and yet humans have only been around for about 200,000 years, but a brief moment of any star's life. But if it isn't possible for us to observe the entire life span of a star, how have we figured it out? Well, there are lots of stars out there, of all different ages that show features that relate to other stars, this allows us to work with something to make a model out of.

The best analogy I can provide is this: Imagine being in the woods, deep in a seemingly infinite forest that stretches to the horizon. You are surrounded by trees, while you are stuck to a tree yourself, the only one that you can see up close. Your tree appears to be a sturdy adult tree, analogous to our Sun. Looking out into the forest you see a variety of trees; without knowing the life cycle of the tree, they may not seem otherwise connected. You have seeds on the ground, saplings sprouting up, small trees, big trees, really big trees, and rotting logs on the ground.

Sagittarius Star Field - Our Forest.

This analogy can go further; those rotting logs provide subsistence for the next generations of trees, much like supernovae provide matter for new stars. And occasionally you might hear the crash in the distance of a tree falling, much like a supernovae or gamma ray burst going off. And what lies beyond the horizon remains a mystery since you are tied to one tree, just like massive size of the Universe.

Now the trick is, you have only a minute to gaze out into this forest to try discern what is what. And that brings the analogy home with what humanity has been able to accomplish. In our brief glimpse, we have been able to identify saplings, healthy full-grown trees, various species of trees, and dying or dead trees, of course in stellar terms. So lets now look at a few of these stages of the life of a star.

Concept art of the birth of a star in a molecular cloud. (NASA)

Protostar: This is where a star begins to form. A molecular cloud or nebula of dust and gas that starts "clumping" together. As the atoms gather, their gravitational attraction pulls in more atoms, making a larger "clump". This process is known as accretion. Lots of reactions happen inside an unstable protostar, which can have influences on a possible planetary system. Once the protostar achieves and maintains equilibrium, balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center, it becomes a star.

Not all stars are born equal! Stars are born into a variety of sizes and colors, depending on their composition, mass of formation nebula, and temperature. These stars lead vary different lives, of varying length, and die in dramatically different ways.


Small-Medium Stars: The overwhelming majority of stars, at least in the Milky Way, fall into this category, including the Sun, up to 1.5 times the mass of the Sun. These are amongst the longest living stars, the Sun is predicted to last 14 billion years, and it is roughly 4.6 Billion years old now. Smaller stars than the Sun can last billions of years longer. The smaller a star is, the longer it will live.

Hertzsprung–Russell diagram, commonly used to classify stars.

The majority of the life of a star will take place during a phase called the main sequence. This is when the star fuses hydrogen to produce helium in high-temperature and high-pressure reactions near the core.

After a small star like the Sun burns up all of its hydrogen, it leaves the main sequence. It begins to burn helium instead, converting helium atoms into carbon. The star loses its previous equilibrium, so in order to maintain it and keep cool, the star 'puffs out.' The star is now called a red giant, and is the first step in old age. The Sun may expand out to roughly the distance of Earth, 250 times its current size!

A red giant is very unstable, and may expand and contract, these stars are known as variable stars. This period if very short, only lasting a couple million years over the billions of years in a star's life. Soon the helium will burn up and and the star will again change as it switches to the last phase of fusion -- carbon burning.

The Cat's Eye Nebula is a planetary nebula. (NASA)

When a star switches to fusing carbon, the core contracts inward, down to about the size of the Earth. The outer layers of a star are blown off into space into a planetary nebula. Often beautiful to see from Earth, these are the dying throws of a star. The star becomes a white dwarf, densely packed, but not massive enough to become an neutron star or black hole. A white dwarf burns slowly and will gradually fade into a black dwarf, a cold dark mass. However, the Universe is not old enough for any black dwarf stars to exist yet.

A VERY Giant Star (the biggest known)

Huge and Giant Stars: These are stars much more massive then the Sun, with drastically shorter lifetimes. There are two classifications here based on the predicted end results of the star. Here we will call a star that is between 1.5 to 3 times the mass of the Sun a huge star; and a star greater than 3 times the mass of the Sun will be a giant star.

These stars spend a relatively brief period on the main sequence, since they are larger and burn hotter, they burn their hydrogen quickly. When they begin helium burning, they enter a special sort of phase, they become red supergiants, the largest known stars in the Universe by volume.

The onion-like layers of a massive star just before core collapse. (Not to scale.)

These stars are so large, they begin to fuse elements in an onion like structure near the end of their lives. Fusion of elements all the way up to iron on the periodic table is possible before a core collapse. The core of the star essentially is unable to support its weight against gravity. A massive explosion then occurs, a supernova, which violently blasts the upper layers of the star into space, leaving behind the core.

One of two things happens to the core: If it was a huge star, the core becomes a neutron star. If it was a giant star, it becomes a black hole. You can read more about black holes and neutron stars in one of my older articles.

The Crab Nebula, the remains of a supernova first observed around 1050 AD.

Tiny Stars: A special case in stellar physics, these are stars smaller then 0.5 times the mass of the Sun. These stars never fuse helium into higher elements, they do not have the mass to exert the pressure on the core. They are known as red dwarfs, like Proxima Centuri. These stars can exist on the main sequence for up to trillions of years and have life spans longer then when the Universe is predicted to end. They may eventually fade to white dwarfs, but there is no real way of knowing.

That accounts for an overview of star life cycles. I hope this helps you to understand some of the differences between stars and their end results. Astronomers were able to piece this together looking at the clues that the Universe has given us, from our brief glimpse. We have been able to determine the amazing course of existence for the very things that gave us life.


34 comments:

Dave said...

Im right into this stuff so Ill be back for more.

ed said...

feels like im taking astro again. stars are huge

VenomForMasses said...

Canis Majoris is one big dog!

Zombie said...

And soon enough our sun will engulf us all. lol.

BigMike said...

I still love that in the mere moments we have had to study the universe we have come to so many good conclusions... I just wish I had more time... this is a great era to be alive in...

Kicking Rocks said...

I remember learning that the sun wasn't that big and compared to other stars it was a speck!

Rob said...

That was very informative! I'm always interested in the various other heavenly bodies in space.

Jay said...

mind blowing.

what are the chances that a star forms near planets (like the earth)?

and i remember reading about two stars (or suns) orbiting each other. is that true?

Astronomy Pirate said...

A star couldn't just randomly form near the Earth, mostly because we already have a star near the Earth, the Sun. The clouds that form stars can be light years across and planets tend to form with stars, during that period of clumping. Planets are essentially large clumps that didn't get absorbed into the star by the time it reached equilibrium, and to massive to be blown away by the solar wind.

And yes, two stars orbiting each other are actually fairly common. I don't think there is a good explanation as to why, it's just that several stars tend to form from one large cloud, and if they are close to each other, they become gravitationally bound. And stars can come in triples and four star systems, a few even have 6.

Malkavian said...

Ok so a black dwarf but the universe isnt old enough that it hasnt happened yet but what about dark matter? i thought a dying star was related to the black matter when a star died leaving the black dwarf to be part of the Dark matter. Damn i was so wrong then.

Astronomy Pirate said...

Dark matter is something entirely different. A black dwarf would still be made out of 'normal' baryonic matter. The stuff that makes up us and stars and pretty much everything else we know about. Dark matter is non-baryonic and only interacts with baryonic matter gravitationally, as far as we know. The source of dark matter is much more ancient and having to deal with evolution of the Universe.

LifeHacks said...

Excellent article, but remember dark matter is a theory!

Astronomy Pirate said...

@LifeHacks, you are confusing the scientific use of the word theory and the casual use of the word theory. Dark matter is pretty well understood to exists... and really has little at all to do with this article... I did write another article on dark matter though!

Schla.mp3 said...

it's hard to believe how big things can be

Ephedrine said...

I remember finding out pluto isn't a planet. Never trusted a teacher since then.

Dejch said...

ill digg in a little later, thanks for sharing

Necroticism said...

We're doomed! doomed I tell you!

The Angry Lurker said...

This is really interesting because you don't realise the details involved, the forest analogy is good.

MassiveUps said...

So how big is the universe?

Nom de Plume said...

coooool :)

JayPower said...

Very interesting! never knew this stuff ;D

elexerdelex said...

Wow this is pretty nice... I never really got into astronomy but this blog rocks my socks!

Question: If no Black Dwarfs exist yet, how do we know they will come to be?

Because of your sunshine award, I have nominated you for an award of my own! I hope you like this!

Solsby Kid said...

I agree with necroticism... WE'RE DOOMED ;D

Banacek said...

I feel kind of insignificant now.

Dave said...

Really great pics there.

MRanthrope said...

loved that forest analogy. All this talk about stars makes me think of Superman and his red sun. poor bastard.

ScottD said...

thank god it says not to scale on the layers pic

Venus said...

the first photo looks like one of the crop circles.

WanderingWriter said...

The picture of the star field is simply mind boggling...the lights of the galactic neighborhood (sorry, you know me).

M Pax said...

Thanks for the brush up. Star lifecycles is one of the things I usually have to explain up at the observatory.

kgp318 said...

What a great post! I really loved the forest analogy...made whole concept more understandable.

Also, I remember learning about suns in elementary school...this was a much better lesson

Keep up the good work :)

MAX POWER said...

Yeah that tree analogy is a good one for explaining how scientists see how stars change. good post :) very clear!

AS said...

Welp. Way to make me feel small. Lol.

HiFi said...

Very interesting!

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