Saturday, 22 April 2017

S is for Star

S is for Star, and really, you cannot talk about astronomy without mentioning stars. Impossible.

Anyone who's ever expressed the slightest interest in astronomy does so because of stars.  You look up in the sky, see the stars, and go, "Cool!" Stars are what get us interested in space.

Here's an earworm to annoy you for the rest of the day, "Twinkle Twinkle Little Star" (the astronomically correct version):

Stars are the most obvious part of the universe because they shine so brightly along all the electromagnetic spectrum. I consider them a key component of the cosmos. They are the movers and the shakers. They are responsible for all the "metals" (ie elements heavier than H and He) out there. E V E R Y T H I N G out there that's not Hydrogen/Helium, is because of stars.

As you know, hydrogen makes up a good three-quarters of the universe. When that hydrogen (usually in a nebula) gets enough gravity to collapse together, fusion starts and you get a star.

Vital Stats of Stars


Stars range in weight (measured in solar masses or M) from about approximately 0.2 M  to humongous beasts pushing 150 M. Now, that might not seem like a very big range, seeing that one solar mass (M) equals the weight of our Sun at 1.98855 × 1030 kg. But if you look at the radii of these stars, the smallest is about 20% wider than Jupiter, but the biggest is a whopping 1500 times as wide as our Sun. Frickin' huge.

Here's a size comparison:

Also, the bigger a star, the faster it burns through its hydrogen. Really, really big hypergiants have been known to live for the brief moments of a few million years. Living fast, dying young, they leave a really impressive supernova/hypernova before letting its corpse collapse into a neutron star or even a black hole.

Smaller stars last a lot longer. Our own Sun will live about 10 billion years, whereas some red dwarfs could possibly live for trillions of years.

OGLE-TR-122b is the smallest main sequence star we've discovered so far that's still fusing hydrogen. If it got any smaller, it wouldn't have enough gravity to ignite fusion. Unless you're fusing, you ain't a star.


Stars range in colour, which also correlates with temperature.

Stars are classified according to colour(temperature), with blue stars being the hottest and red stars being the coolest.

The spectra of star types. See how the spectra peak in certain colour ranges? That's why stars appear coloured.

Blue O-type stars tend to be 30-40,000 Kelvin.
Blue-white B-types are 20,000 K,
White A and F stars are about 8-10,000 K
G-type stars, like our yellow Sun, are about 6000 K
Red K and M-type stars can be as cool as 3000 K  (For reference, you are around 310 K).

Look up in the night sky and see if you can tell what colour a star is? While most of them appear "white", compare nearby stars to see if you can detect a faint bluish cast or reddish cast.  Betelgeuse in Orion is distinctively red, as is Antares in Scorpio. Rigel in Orion is rather blue.

Why aren't there any green stars?

Actually, there are. Any "white" star is actually radiating in the green part of the visible (optical) spectrum. Green happens to be right in the middle of the spectrum, so when a star is emitting green, it also emits red and blue. Combine all these together (additive colours), and they look white. Our Sun, a G2-type star is generally classified as Yellow-White. But if white is really green, that makes our Sun a yellow-green star. (Consider how much green light gets reflected by plants on Earth. That light's gotta come from somewhere.)

Birth and Death of a Star

So, a star is born from the gas of a nebula. After it gets over its initial teething phase, it settles into the Main Sequence, happily fusing its hydrogen into helium. It'll spend about 90% of its life like this.

Once it runs out of hydrogen, they move off the main sequence, do a few interesting things (like helium flashes, variable pulsing, puffing up like balloons), then die.

When it comes to the death of stars, the manner of its demise depends on its mass.  For smaller mass stars, like our Sun, it'll inflate into a red giant, then with a gentle poof, shed its outer layers, leaving the cooling cinder of a white dwarf.

But if it's a massive star, especially of the live-fast-die-young category, it starts fusing everything into onion layers of elements until it reaches iron. Once that happens, fusion stop, the pressure keeping the star puffy ceases, the star collapses in on itself, rebounds, and dies most violently B A N G!! in a  spectacular supernova.

Don't let the gentle spread of this light echo fool you. This time-lapse covers a period of four years. The light echo is about six light years across. We're talking some serious velocities here.

The energy released in this explosion is enough to start a new wave of nuclear reactions that fuses iron into the higher elements like gold and uranium and blowing them out into the universe.

Whatever is left over gets fused into neutrons and collapses down into either a neutron star or a black hole in the middle of a brand new nebula.

Meanwhile, the shockwave of a supernova can extend for several parsecs, rolling through any neighbourhood nebular clouds of hydrogen, possibly triggering some of it to collapse into new stars.

And thus, the cycle begins again.

Some people just like looking up in the skies. And that's okay. Do you have any favourite stars?  What make them your favourites?  I'm fond of Betelgeuse and Canopus.

No hardcore stuff today, unless you want to investigate more about V838 Mon which occurred in 2002.

Her Grace is sometimes content just to stare up at the stars with MK-1 eyeballs.


Kalpanaa M said...

I really enjoyed your ear worm and your information packed post about stars. Such a pleasure.

writingiscommunication said...

My oldest boy is really into stars and planets right now. He is only 6, but he has been bringing home these intense library books about planets. He seems fascinated. Loved your post.

Stopping by from A to Z: S for Strange Games

Karnika Kapoor said...

Wonderful post! I have always been very enticed by the stars.
That detail on green stars is fascinating.
I think Betelgeuse is my favorite. I have this crazy fantasy of looking at it one day casually and watching it explode, highly unlikely ... Still, every time I look at it I secretly hope for the impending supernova.
Thanks for sharing!
Best Wishes!

Nick Wilford said...

So much I didn't know here, like the different colours of stars. I would have thought blue to be the coolest. And that a supernova can create new stars. Excellent stuff!

Sue Bursztynski said...

Ooh, pretty! I'm afraid I failed Year 10 maths, so couldn't do physics and therefore no astronomy. But stars are pretty from here. I like constellations. The Southern Cross, of course - it's where I live. And Orion. I never really saw it as it should be seen till I went out into the country, where lights were not getting in the way. Then I was overwhelmed by its beauty.

Nilanjana Bose said...

Comprehensive! and vastly enjoyable. Stars are delightful - both poetically and scientifically, whichever way you want to look at them. Loved your take on S!

Best wishes,

Deborah Weber said...

What a fabulously informative post. I'll look at the stars differently tonight.

Sandra Williamson said...

What a great post, and what a wonderful idea to teach children a song that help them remember the fact!
Sandra dropping by from the A to Z Challenge

Sandra, Aspiring family historian, fellow participant in the #AtoZchallenge

Sandra's Ancestral Research Journal