Stars: Crash Course Astronomy #26

Stars: Crash Course Astronomy #26

Twinkle, twinkle little star. Oh. I know who
you are. At first glance, stars pretty much all look
alike. Twinkly dots, scattered across the sky. But as I talked about in episode 2, when you
look more closely you see differences. The most obvious is that some look bright and
some faint. As I said then, sometimes that’s due to them being at different distances, but it’s also
true that stars emit different amounts of light, too. If you look through binoculars or take pictures of
them, you’ll see that they’re all different colors, too. Some appear white, some red, orange, blue, and
for a long time, the reason for this was a mystery. In the waning years of the 19th century,
astrophotography was becoming an important scientific tool. Being able to hook a camera
up to a telescope and take long exposures meant being able to see fainter objects,
revealing previously hidden details. It also meant that spectroscopy became a force,
say it with me now, for science. A spectrum is the result when you divide the
incoming light from an object into individual colors, or wavelengths. This reveals a vast
amount of physical data about the object. But in the late 1800s, we were only just starting
to figure that out. Interpreting stellar spectra was a tough problem.
The spectrum we measure from a star is a combination of two different kinds of spectra. Stars are
hot, dense balls of gas, so they give off a continuous spectrum; that is, they emit
light at all wavelengths. However, stars also have atmospheres, thinner
layers of gas above the denser inner layers. These gases absorb light at specific wavelengths
from the light below depending on the elements in them. The result is that the continuous
spectrum of a star has gaps in it, darker bands where different elements absorb different
colors. At first, stars were classified by the strengths
of their hydrogen lines. The strongest were called A stars, the next strongest B, then
C, and so on. But in 1901, a new system was introduced by spectroscopist Annie Jump Cannon,
who dropped or merged a few of the old classifications, and then rearranged them into one that classified
stars by the strengths and appearances of many different absorption lines in their spectra. A few years later, physicist Max Planck solved
a thorny problem in physics, showing how objects like stars give off light of different colors based
on their temperature. Hotter stars put out more light at the blue end of the spectrum, while cooler
ones peaked in the red. Around the same time, Bengali physicist Meghnad Saha solved another tough problem:
how atoms give off light at different temperatures. Two decades later, the brilliant astronomer
Cecelia Payne-Gaposchkin put all these pieces together. She showed that the spectra of stars
depended on the temperature and elements in their atmospheres. This unlocked the secrets
of the stars, allowing astronomers to understand not just their composition but also many other
physical traits. For example, at the time, it was thought that stars
had roughly the same composition as the Earth, but Payne-Gaposchkin showed that stars were
overwhelmingly composed of hydrogen, with helium as the second most abundant element. The classification scheme proposed by Cannon
and decoded by Payne-Gaposchkin is still used today, and arranges stars by their temperature,
assigning each a letter. Because they were rearranged from an older system the letters
aren’t alphabetical: So the hottest are O-type stars, slightly cooler are B, followed
by A, F, G, K, and M. It’s a little weird, but many people use the mnemonic, “Oh Be
A Fine Guy, Kiss Me,” or “Oh Be A Fine Girl, Kiss Me,” to remember it — which
was dreamed up by Annie Jump Cannon herself! Each letter grouping is divided into 10 subgroups,
again according to temperature. We’ve also discovered even cooler stars in the past few decades,
and these are assigned the letters L, T, and Y. The Sun has a surface temperature of about
5500° Celsius, and is a G2 star. A slightly hotter star would be a G1, and a slightly cooler
star a G3. Sirius, the brightest star in the night sky, is much hotter than the Sun, and is classified as an
A0. Betelgeuse, which is red and cool, is an M2. Stars come in almost every color of the rainbow.
Hot stars are blue, cool stars red. In between there are orange and even some yellow stars. But there are no green stars. Look as much as you want, and you won’t
find any. It’s because of the way our eyes see color. A star can put out lots of green
light, but if it does it’ll also emit red, blue, and orange. And our eyes mix those together
to form other colors. A star can actually emit more green light than any other color,
but we’ll wind up seeing it as white! How do I know? Because if you look at the
sun’s spectrum, it actually peaks in the green! Isn’t that weird? The Sun puts out more
green light than any other color, but our eyes see all the mixed colors together as
white. Wait, what? White? You may be thinking the
Sun is actually yellow. Not really. The light from the Sun is white,
but some of the shorter wavelengths like purple and blue and some green get scattered away
by molecules of nitrogen in our air. Those appear to be coming from every direction but
the Sun, which is why the sky looks blue. The Sun doesn’t emit much purple, so the sky doesn’t
look purple, and the green doesn’t scatter as well as blue. That gives the Sun a yellowish tint to our
eyes, and looking at the Sun is painful anyway, so it’s hard to accurately gauge what color
it appears. That’s also why sunsets look orange or red: You’re looking through more
air on the horizon to see the Sun, so all the bluer light is scattered away. So we can use spectra to determine a lot about
a star. But if you combine that with knowing a star’s distance, things get amazing. You can measure how bright the star appears
to be in your telescope, and by using the distance you can calculate how much energy it’s actually
giving off — what astronomers call its luminosity. An intrinsically faint light looks bright
if it’s nearby, but so does a very luminous light far away. By knowing the distance, you can
correct for that, and figure out how luminous the objects actually are. This was, no exaggeration,
the key to understanding stars. A lot of a star’s physical characteristics are related:
Its luminosity depends on its size and temperature. If two stars are the same size, but one is
hotter, the hotter one will be more luminous. If two stars are the same temperature, but one
is bigger, the bigger one will be more luminous. Knowing the temperature and distance means
knowing the stars themselves. Still, it’s a lot of data. A century ago,
spectra were taken of hundreds of thousands of stars! How do you even start looking at
all that? The best way to understand a large group of
objects is to look for trends. Is there a relationship between color and distance? How
about temperature and size? You compare and contrast them in as many ways possible and
see what pops up. I’ll spare you the work. A century ago astronomers
Ejnar Hertzsprung and Henry Norris Russell made a graph, in which they plotted a star’s
luminosity versus its temperature. When they did, they got a surprise: a VERY strong trend. This is called an HR Diagram, after Hertzsprung
and Russell. It’s not an exaggeration to call it the single most important graph in
all of astronomy! In this graph, really bright stars are near
the top, fainter ones near the bottom. Hot, blue stars are on the left, and cool, red
stars on the right. The groups are pretty obvious! There’s that thick line running diagonally
down the middle, the clump to the upper right, and the smaller clump to the lower left.
This took a long time to fully understand, but now we know this diagram is showing
us how stars live their lives. Most stars fall into that thick line, and
that’s why astronomers call it the Main Sequence. The term is a little misleading;
it’s not really a sequence per se, but as usual in astronomy it’s an old term and
we got stuck with it. The reason the main sequence is a broad, long
line has to do with how stars make energy. Like the Sun, stars generate energy by fusing
hydrogen into helium in their cores. A star that fuses hydrogen faster will be
hotter, because it’s making more energy. The rate of fusion depends on the pressure
in a star’s core. More massive stars can squeeze their cores harder, so they fuse
faster and get hotter than low mass stars. It’s pretty much that simple. And that explains the main sequence! Stars spend most of their lives fusing hydrogen
into helium, which is why the main sequence has most of the stars on it; those are the ones merrily
going about their starry business of making energy. Massive stars are hotter and more luminous,
so they fall on the upper left of the main sequence. Stars with lower mass are cooler
and redder, so they fall a little lower to the right, and so on. The Sun is there, too,
more or less in the middle. What about the other groups? Well, the stars
on the lower left are hot, blue-white, but very faint. That means they must be small
and we call them white dwarfs. They’re the result of a star like the Sun eventually
running out of hydrogen fuel. We’ll get back to them in a future episode. The stars on the upper right are luminous
but cool. They must therefore be huge. These are red giants, also part of the dying process
of stars like the Sun. Above them are red SUPERgiants, massive stars beginning their
death stage. You can see some stars that are also that luminous but at the upper left;
those are blue supergiants. They’re more rare, but they too are the end stage for some
stars, and again we’ll get to them soon enough in a future episode. But I’ll just say here that it, um, doesn’t
end well for them. But, on a brighter note, we literally owe
our existence to them. And this implies something very nifty about
the HR diagram: Stars can change position on it. Not only that, but massive stars versus
low mass stars age differently, and go to different parts of the HR diagram as they
die. In many ways, the diagram allows us to tell at a glance just what a star is doing
with itself. This difference between the way low mass stars
like the Sun and higher mass stars age is actually critical to understanding a lot more
about what we see in the sky… so much so that they’ll be handled separately in later
episodes. I’m sorry to tease so much about what’s to come, but this aspect of stars
— finally understanding them physically — was a MAJOR step in astronomy, leading
to understanding so much more. And don’t you worry: we’ll get to all
that. Today you learned that stars can be categorized
using their spectra. Together with their distance, this provides a wealth of information about
them including their luminosity, size, and temperature. The HR diagram plots stars’
luminosity versus temperature, and most stars fall along the main sequence, where they live
most of their lives. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel and catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.


  • Sebastien Guenette says:

    Williamina Fleming, Annie Jump Cannon, Henrietta Swan Leavitt et Antonia Maury Were pioneers

  • d. c. says:

    I’d like to jump annies cannon if you know I mean

    ( ͡° ͜ʖ ͡°)

  • Noakai Aronesty says:

    "Once, Beatrice, A Fat Glacier, Kicked Mutton" is much easier to remember, in my opinion

  • Noakai Aronesty says:

    Are you a doctor, or is this a bad joke?


    0:03 either he was in a boy choir, or they used auto tune. He nailed every note!

  • James French says:

    I wish they made more movies like Contact that show what another star system most likely looks like.

  • KitCat22 says:

    Intro had me quaking in my converse shoes.

  • Random Guy says:

    That intro was savage.

  • Steampunk Ass Monster says:

    thanks for that intro, I hated it

  • courtney barrett says:

    Umm creep

  • Jonathan Shamo says:

    How is beetleguse an m2 when it’s absolutely huge and incomprehensibly hot. That’s at least what I’ve heard.
    Edit: I’m exaggerating when I say incomprehensibly

  • TED LINN says:

    OBAFGKM….Oh Boy An F Grade Kills Me….seems to be a better way for students to remember it….But…..w/e

  • Sutton Johnson says:

    This is exactly what my wet dreams are made of.

  • Sidney Rose says:

    I can finally explain to that one five year old why the sky is blue!

  • maxis2k says:

    I learned the stars classifications with the acronym KGB MOAT. I guess the person who taught me didn't care about L or N. But it worked well enough to learn the main sequence. I also found it funny, because I imagine a bunch of soviet secret agents defending a stereotypical British castle.

  • Aydin Mutchler says:

    Watched this in Science class got over 1 page of notes in 3 mins wow!

  • Johnny Sanford says:

    "Dr. phil 😂😂😂

  • Lina Langemark says:

    Imagine being Phil, going through the comment section and seeing that the vast majority of the comments are about people loving him. Must be nice.

  • Arturo Hurtado Rodriguez says:


  • pk good says:

    Thank you! You are a ROCK STAR! 🙂

  • Kahvi Patel says:

    intro was very creepy but rest of video was great

  • EricTuravaniVlogs says:

    0:03 so creepy

  • Ethan Bishop says:

    Really good!

  • Dmitry R says:

    Wow, this was an incredibly informative video, probably the most informative 10-minute video I have ever seen! It helped me understand a whole chapter for my science test, I couldn't thank you enough for making this video.

  • FnafGamer2709 says:

    Anyone watching in 2019?

  • DeathbyPixels says:

    Finally! Someone answered “why are there no green stars?”

  • Saul Galloway says:

    Phil is the Steve Shives of the scientific community. Like Shives he's a moron when it comes to politics but a fackin' genius when discussing Star Trek/Star Trekkin'.

  • SATYAM PATEL says:

    How small a star is possible

  • Legoracers says:

    The orbit of sirius star 49.9 years is mentioned in the Quran 1400 year ago. 😊

  • Paulo Pombal says:

    The good thing about this "CrashCourse" is that the subject of each vídeo is clearly explained in therms of script but… Is he on drugs??… In fact, It feels the whole production of this is on drugs! Why is he talking like he's on the run?? And why the whole frantic graphic information rollercoaster all the time?? Is that just because is the visual and behavioural trend, nowadays?… It wouldn´t surprise me that, most of the times, viewers were replaying it over and over ( and replaying at 75% speed, for example ) and, maybe, that's why it has so many viewings and so little interaction. Maybe, that's on purpose in therms of viewing, advertising and sponsorship… Multitasking has been proved not to be what our brain does better, so why insisting on this kind of formats on a channel which is suppose to teach well everyone??

  • wubba lubba lub lub says:

    I would like to see Phil and Neil DT wondering about the universe lol

  • Dos Peterson says:

    You would need 10000 planets to have a chance to have this many collisions

  • Afra The Cat Girl says:

    Thank u 4 answering the question "Why is the sky blue?"
    That was great!!!

  • JOREL FARIA says:

    Three words… KERBAL SPACE PROGRAM.

  • Chaos A.D. says:

    So you're sure all of that information is "Cannon?" 😁

  • Christopher Trujillo says:

    That intro was the smallest bet stalkerish

  • DECEPTION says:


  • Melina Currá says:

    The sun is green

  • kombrig 2 says:

    On 3:55 you've made a big mistake. Sirius A is just twice of mass of the Sun and 3 times bigger in radius. So, it classified as F8 type. It is the brightest, because it's only 8.6 yrlight away. A0 class stars has at list 70 mil km in size.

  • Random Obsessions says:

    first decent-to-awessome science assignment ever!

  • JustJosPEH :D says:

    You are the best. I love learning from you because you have the best teaching style for me so when I grow up I have more experience of how astronomy works because of your amazing teaching.

  • Nouaman Moukassi says:

    Shouldn't the sun look white to people in space then? like people in the iss?

  • Paul Cooper says:

    Could more information be packed into this video?
    I don't think so.
    Great presentation!!!!
    Don't know how I've missed seeing this series of videos

  • John Montgomery says:

    you can see a lot just by looking

  • Joshua Garner says:

    The sun is a white star, goes on to use a yellow star to picture the sun

  • Barton Paul Levenson says:

    Some A-type stars are greenish-white.

  • Arceus The God Pokemon says:

    Phil Plait (not Swift) here, with Hydrogen fusion! to show you the power of fusion… i sawed this star in half!

  • Fluffy Scruff says:

    I wish i had hands

  • Nicole Elizardo says:

    that intro scared the F*** out of me…

  • Henry_Ballins says:

    thank you Phil

  • Alexander Martinez says:

    to stars fly ?bc chief keef cant

  • ShadowCat 1442 says:

    Twinkle Twinkle Little Star,
    I want to hit you with a car.
    Like a diamond in the sky,
    I'm going to sell you and get rich.

  • Xiaohan Yi says:

    Our astronomy teacher on OBAFGKMr: Oh Boy An F Grade Kills Me. lol she is the best

  • Schulze says:

    Amazing opening. Probably one of my favorites. Seriously

  • Dave K says:

    Poor Max Planck, you guys gave him green skin and blue hair!
    Is that better than being an old white guy?

  • Cartoon Fanatic says:

    I got into astronomy at a young age. In senior year, I used to watch astronomy videos everyday. AWESOME CONTENT IN THIS SUBJECT!!! What's even better, crash course makes astronomy even more interesting. Thanks crash course

  • WhittyWhitts says:

    The sun is a planet

  • HeavenFoundWithin says:

    Twice in my life I had a star appear to me in front of my eyes, wide awake. I could not move, I could not blink my eyes shut. All sound went away as if i was in a perfect quiet vacuum. I felt no fear nor did I remember what fear was, only love, peace and calm. The first time the star opened and I saw silhouettes of people but they were angels. Three of them walked out. 2 stood at either side of the doorway/portal that had opened when the star expanded to reveal the fog / silhouettes. Then one of them came and put an arm around my shoulder and said "God is love." We smiled at each other then the angel said "God IS love" stressing the word "IS" — and said "And everyone has some love in them when they are born, everyone has some of Gods Holy Spirit in them when they are born. God IS LOVE." Then as the angels said that our eyes went to the doorway again and I saw something like water pour out into my entire room and we were engulfed in it and we were flooded with a love so powerful that I could say it was eternal perpetual bliss. This was the Spirit of God and I was given a chance to see.

    The 2nd time a few weeks later the star appeared again, this time I was outdoors about to sit down after a big lunch to read my bible….. the star came, could not move, blind, hear anything but felt only love, peace and calm. All negativity left me. This time the star kind of exploded open and 'ate away this realm' like a negative melting, revealing a new world as it expanded… the entire world melted away before my eyes!! My eyes saw the sky above but it was not blue anymore…. the entire sky was lightening. All of it, one giant lightening field! In the middle was something or someone that looked like human shape. He came down slowly… he was so bright and glowing it was amazing. It was Jesus Christ. And he floated right down to me stood me up, stretched my arms out as if I were being crucified and kept my eyes locked to his eyes. I could spend eternity looking at his face and eyes and never get bored! It was amazing like looking into the eyes of the creator. I felt a powerful flood of energy and love pour out of him into me as He had me standing up with outstretched arms and he himself outstretched upon me, hand on hand, head on head, feet on feet. Mirroring me. I felt his power enter my body. My eyes were healed from glasses… all pain left me. 20 years now my eyes are perfect still and I still feel that power! I have met Him in countless dreams where Jesus gives me his name and tells me to tell people to ask for the Mind of Christ and Spirit of Christ to be given to them and to ask to see him face to face and warned me to tell them that it does not happen right away that they must ask with all their heart for a time with shameless persistence.


  • Jose BERGER says:

    Maybe green is posible because one day when i was watching the sunset i noticed a tiny bareor between blue and yellow the barior was green but only a tiny tiny tiny tiny little little bit

  • Don't, Jim Angius says:

    STARS! 😍🤩😍😵

  • Persca99 says:

    That was awesome 👏

  • Evadne Johnson says:


  • Ant_Dies _Alone says:

    6:42 luminosity and temp
    (Its the reason why im here so i thought it might help u guys)

  • Dani says:

    3:58 Are you sure about that?

  • Blessing Forreal says:

    I am so shooketh this is amazing!

  • scdriver007 says:

    I like micelle thallar

  • The Non-Believer says:

    I just came here to understand Superman's powers better

  • KingofSchubi says:

    But how can you calculate the mass and distance of an object thats so far away? Its crazy

  • Manish Singh says:

    What a thumbnails you pick for you videos, amazing… Wonderful thumbnails

  • Dorothy Isidro says:

    The intro was pretty accurate and funny.
    🤣 I see what u did there

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