This video is sponsored by Magellan TV. Stars are the primary building blocks of
the universe. They bring light to the darkness, are home to planetary systems,
come together to form clusters, and sculpt entire galaxies. Along the way,
they create the heavy elements needed to build rocky planets and even the life
that inhabits them. But stars are far away; even the closest star to Earth, our
Sun, is a hundred and fifty million kilometres away.
That means everything we know about stars - how they are born, what they're
made, of how they work, how long they'll live, and how they'll even die - we learn
by decoding the information imprinted in starlight. Welcome back to Launch Pad, I'm
Christian Ready, your friendly neighborhood astronomer. The universe is
home to 10 to the 21st stars. That's 1 sextillion stars! In other words, a
billion-trillion stars! In fact, there are more stars in the known universe than
there are grains of sand on every beach on planet Earth. If you're lucky enough
to get away from the city lights on a dark clear night you'll see a few of
them. But even a cursory look at the stars reveals that they're not all quite
the same. Some of them are brighter than others while others are barely visible
at all. That could mean that the faint ones are farther away, or it could
mean that there are some really bright stars that are far away that are just
emitting more light. In fact if you look closely you'll notice that stars come in
different colors. Most appear white but some of them take on a orange or reddish
tint while others take on like a sapphire blue color. For a long time this
was a mystery because observations were made by eye and recorded by hand, and
that made it hard for astronomers to compare their observations. Towards the
end of the 19th century astrophotography became a thing and that meant that long
exposures could record the brightness and positions of stars precisely on
photographic plates. Even better, spectroscopy had become a thing!
Starlight could be passed to a prism and their resulting spectra could be
recorded on photographic plates. Spectroscopy was a game-changer. It
allowed astronomers to dissect a star's components remotely and reveal their
chemical makeup. A star's spectrum is complicated. Its surface is hot and dense so
it emits light at all wavelengths, producing a continuous spectrum. But
they're also surrounded by atmospheres of gas. The gas is made of atoms and the
electrons in those atoms absorb some of the star's light at specific wavelengths.
For example, hydrogen atoms absorb light at one set of wavelengths, helium at
another set, and so on. This makes for a jumbled mess of overlapping absorption
lines all overlaid on top of a continuous spectrum. However, different
stars show different patterns in their absorption spectra, and that allowed
astronomers a way to compare and classify them for the first time. In the
late 19th century, Harvard astronomer Edward Charles Pickering set about
cataloguing every star in the sky according to their spectra.
His students were all men at the time because in those days women weren't
supposed to be doing things like astronomy. The story goes that Pickering's
students didn't want to sit around classifying spectra all day so they
basically blew him off. Pickering got frustrated and he eventually proclaimed
that his Scottish maid could do a better job than these guys. His maid was a
Scottish immigrant named Williamina Fleming who set about the task of
classifying thousands of spectra. To help her with the work, she recruited
secretaries at Harvard to create an all-women cadre of human computers.
Initially, Fleming classified stars according to the strengths of their
hydrogen absorption lines. The stars with the strongest lines were Type A, followed
by Type B with the next strongest set, and so on all the way down to Type O.
When Annie Jump Cannon took over the team, she quickly realized the
classification system could be simplified. She merged or dropped half of
the classifications and rearranged the rest according to the strengths of
multiple spectral features. Two decades later astronomer Cecilia Payne-Gaposchkin
realized that the appearance of the stars spectra were due to their
temperatures. So the hottest stars are Type O, followed by Types B, A, F, G, K, and M.
It's not alphabetical, but they didn't want to have to change all of their star
catalogs so Annie Jump Cannon came up with a clever mnemonic to help us
remember their order: "Oh be a fine girl (or guy), kiss me." A reality of stars is
that they come in a broad range of temperatures and therefore don't fit
neatly into one spectral type or another. Annie Jump Cannon realized this and
expanded her classification system to reflect this reality. For
example, our Sun is a G2 type star. That means it's a little bit cooler than a G1
star but a little bit warmer than a G3 star. Vega by comparison is a lot hotter
than our Sun so it's classified as a type A0, while Proxima Centauri is a
cool red star of type M 5.5 Cecilia Payne-Gaposchkin also showed that stars
are overwhelmingly made of hydrogen, followed by some helium and then trace
amounts of other elements. This meant that all stars had something in common
and that could serve as the basis for understanding how they work. We're
gonna find out how stars work in a moment but first I'd like to thank
MagellanTV for sponsoring this video.
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MagellanTV is a great service and I highly recommend checking it out. In fact,
the first 100 visitors to go to Magellantv.com/LaunchPadAstronomy will get a
one-month free trial offer. I'll have a link in the description below so make
sure you check it out. Ultimately a star is a sphere of hot gas
radiating energy. A star's energy originates in the core - a lot of energy
in fact. So much radiation pours out of the core it exerts a pressure on the
surrounding layers of the star this pressure is strong enough to hold itself
up against the weight of the surrounding layers, preventing the star from
collapsing in on itself. this balance between pressure and
gravity is called "hydrostatic equilibrium", and it's essential to understanding how
the star makes energy. And when it comes to understanding how stars produce
energy, there is no better example than the one right here at the center of our
cosmic neighborhood, our Sun. The Sun is large enough to fit 1 million Earths
inside of it. It weighs in at 2 times 10 to the 30 kilograms. that's 2 million
trillion-trillion kilograms! that means the Sun is capable of accommodating a
lot of fuel. But how is this fuel actually burned? 19th century astronomers
were trying to figure out the same thing, and they came up with some 19th century
proposals. The breakthrough came in the early 20th century when Albert Einstein
showed there was a relationship between matter and energy - namely, energy is equal
to the mass of matter times the speed of light squared. The speed of light squared
is a really big number so even if you multiply that by a relatively small
amount of mass you still get a lot of energy. But how does matter in a star get
converted into energy? To understand that let's think about how our Sun is
constructed. The outer layers of the Sun bear down hard on the core, driving
pressures up to 250 million times the Earth's atmosphere.
Temperatures are incredibly high, reaching 15 million Kelvin. At those
temperatures, hydrogen atoms are stripped of their electrons and become ionized.
Hydrogen nuclei are just protons and protons carry positive charge. Normally
particles with the same charge push each other away via an effect
called electric repulsion. But under such intense pressure, hydrogen nuclei are
moving so fast that occasionally some of them will collide, producing helium
nuclei in a sequence of steps. In the process, some of that mass is converted
into gamma ray photons. In other words, mass is converted into pure energy!
This is nuclear fusion in a nutshell, but protons have extremely teeny tiny
amounts of mass so not a lot of energy is really produced in each individual
reaction. But the Sun converts about 700 million tons of hydrogen into
696 million tons of helium nuclei every second! That means about 4 million tons
of hydrogen is ultimately converted into pure energy every second of every day in
the Sun's core. That's a lot of energy - enough to power a star. About 34% of the
Sun's mass is concentrated in its core and most of that mass is in the form of
hydrogen fuel. That's enough to keep the Sun going for about 10 billion years.
This isn't just how the Sun works, this is how all 10 to the 21st stars in the
universe work! Stars have different colors and temperatures and the two
properties are closely related. Cool stars radiate mostly at infrared
wavelengths so they appear reddish in color to our eyes. Meanwhile hot stars
radiate most of their energy in the ultraviolet, so they appear blue. Our Sun
is somewhere in between these two extremes and radiates mostly in the
middle of the visible part of the spectrum. We can tell a star's
temperature by examining its spectrum but if you happen to know the distance
to the star then you can calculate how much energy the star is really giving
off. It's a property that we call "luminosity". Think of it this way:
a spot light is intrinsically more luminous than a flashlight, but if you
move the spotlight far enough away, it'll appear no brighter than the flashlight.
That means a star's apparent brightness depends on its distance as well as it's
true luminosity. We can measure brightness of stars fairly easily so if
we happen to know the star's distance we can easily calculate its luminosity. A
star's luminosity depends on its size and its temperature. If two stars are the
same size the hotter star will be more luminous. If both stars are the same
temperature but one is bigger, the larger star will be more luminous. Stars are
spheres, so it has a surface area defined by the square of its radius
multiplied by some constants. The amount of energy radiating through a single
square meter of its surface every second is proportional to the fourth power of
its temperature multiplied by another constant - a quantity we call flux. If we
multiply the star's flux over its entire surface area, we get its luminosity
it's such an elegant relationship between luminosity, temperature, and
radius that I like to call this the 'star equation'. It's remarkable how much we can
tell about a start just by analyzing its star light. But there are thousands and
thousands and thousands of stars out there. How do we even begin to make sense
of all of that data? Well the easiest way to do that is to make some comparisons
between different parameters and see if there's any interesting relationships
between them. So if we look at a cluster of stars we can examine each star
spectrum to work out their temperatures. We'll arrange them from hottest to
coolest so the hottest stars go on the left while the coolest stars go on the
right. If we know the distance to these stars we can work out their luminosities
as well. We'll put the most luminous stars toward the top and the least
luminous stars at the bottom. If we repeat these measurements for a few thousand
more stars a very clear pattern emerges. This diagram was first plotted around
1910 by Danish astronomer Enjar Hertzsprung and American astronomer Henry
Norris Russell. It's called the Hertzsprung-Russell diagram in their
honor and it is easily the most important diagram in all of astronomy.
The graph shows three clearly defined groups. Most of the stars in this graph
are plotted along a diagonal line called the Main Sequence. It's really not a
sequence at all, it's just a holdover from when astronomers thought that stars
started their lives very hot and then gradually cooled down. That's not the
case at all but the name stuck. Our Sun is here,
roughly in the middle of the Main Sequence. the Main Sequence's shape
reveals how different stars burn their fuel. The more massive a star is the
harder it squeezes on its core and therefore the faster it burns its fuel.
As a result it's a higher temperature and that's part of the reason why they
are more luminous. The opposite is true of stars on the other end of the main
sequence - there are less massive so they exert less pressure on their cores and
burn their fuel slower. As a result they're less luminous and cooler than
other stars. In the upper right of the diagram are a group of stars that are
cooler, but also very luminous. That means they have to be very large in order to
have such high luminosities. We call these stars red giants. In the lower
left are a group of very hot yet very low luminosity stars.
These are called white dwarfs and they're about the size of
Earth. Since the star's luminosity is related to its size we can map out
regions of common radii on the diagram as well. That means a star of a different
temperature or spectral type can have a range of possible radii and luminosities.
For example an A0 type star will have a surface temperature of about 10,000
Kelvin. But it could have a radius anywhere from five to a hundred times
the Sun and a corresponding luminosity from ten to as much as a hundred
thousand times the Sun. To help keep track of a star's size within a given
spectral type, astronomers assigned stars to one of six luminosity classes
represented by Roman numerals. The luminosity class gets appended to the
star's spectral type, producing an overall spectral classification. For example, a K0
star on the main sequence has a spectral class of K0-roman numeral five (K0V). It's
about 75% the radius and 60% the luminosity of the Sun. a K0III red
giant is twenty times larger and a hundred times more luminous than the Sun.
and a K0Ib red supergiant is two hundred solar radii and forty thousand
times more luminous than the Sun! But what do all three of these stars have in
common? Yep, they all have the same spectral type
of K0. They're all the same temperature! Our Sun is a G2V
Main Sequence star. Most of the stars on this diagram spend most of
their lives on the Main Sequence fusing hydrogen into helium. But stars don't
live forever. Over time they'll exhaust that supply of hydrogen and helium and
then eventually move around on this diagram becoming red giants and
eventually white dwarfs. Or at least most of the stars will become white dwarfs -
some of these more massive stars turn into some interesting things and
we're going to learn about those in future videos. The Hertzsprung-Russell
diagram is much more than a simple graph - it's the rosetta stone for understanding
how stars form, live, and die. And the one property that predetermines all of those
factors is the star's mass. The more massive the star is the
larger it tends to be and the more hydrogen fuel it has in its core. But all
that extra mass squeezes that core much harder, causing it to burn hotter and
burn through its fuel faster. That means that massive stars don't live
nearly as long as low mass stars. Mass determines everything about a star, but
it does raise the question: is there a limit to a star's mass? The short answer
is yes...probably. What makes a star a star is its ability to fuse hydrogen into
helium in its core and that requires a minimum amount of mass. The cutoff seems
to be around 7% the Sun's mass. Objects below this limit can't fuse hydrogen and
helium but they can fuse an isotope of hydrogen called deuterium. These objects
occupy a middle-ground between the lowest mass star and the most massive
planet. We call these objects "brown dwarfs" and we're gonna be talking about
those in an upcoming video as well. The lowest-mass star we know of is SCR 185-6357
located 12 light-years away in the constellation Pavo. At 0.07 solar
masses, it's barely a star at all with a spectral type of M8.5. The star
happens to have a brown dwarf companion but both objects are so cool they can
only be imaged in the infrared so these colors represent where in the infrared
part of the spectrum they radiate the most. As far as the upper end goes, stars
are thought to top out at around 150 solar masses. That's because stars
form in huge clouds of molecular hydrogen. As they form they are very, very
luminous and a massive enough star's radiation pressure would prevent it
from accreting any more gas from the collapsing cloud. At least that's the
prevailing theory. But in 2010 astronomers discovered a star in the Large
Magellanic Cloud called R136a1 weighing in at a colossal 315 solar
masses! The star is only about a million years old which means it's already
halfway through its lifetime. Its surface temperature is a mind-melting
53,000 Kelvin and shines with the luminosity of seven million Suns. It's so
luminous, radiation pressure rips off its outer layers, blasting material out into
space in a fast stellar wind. That means it probably was 350 solar masses at
birth - a very big baby. This stellar behemoth and the O, B, and A-type stars
make our Sun look like a tiny insignificant dwarf star by comparison.
Tut the reality is that our Sun is really not all that dwarf-y. Massive
stars are pretty rare. It turns out the most common type of star that are out
there are K and M-type red dwarf stars. But red dwarf stars have the super power
of long life, as in trillions of years! I talked about red dwarfs in a previous
video and I'll have a link to that in the description below. I'd like to thank
my patrons for helping to make this video and channel possible, as well as a
thanks to Magellan TV for sponsoring today's video. And if you would like to
learn how our Sun will eventually meet its demise, well please make sure you
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new videos. Until next time, stay curious my friends.