At first, glance stars pretty much all look alike. Twinkling dots scattered across the sky. When we look closely we see the difference.
Some look brighter and some look dull, sometimes that due to them being at different distances but it’s also true that stars emit a different amount of light too.
If we look through Binoculars or take pictures of them, we'll see that they're all different colors, too. Some looks white and some looks red, orange.
A spectrum is a result when we divide the incoming light from an object into individual colors or wavelengths. This reveals the vast amount of physical data about the object. But in the late 1800s we were only starting to figure out, Interpreting stellar spectra was a tough problem. The spectra we measure from the star are 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 of all wavelengths.
However, stars also have an atmosphere, thinner layers of gas above the dense 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 stars that has gaps in it, darker bonds where different elements absorb different colors.
At first, stars were classified by the strength of their hydrogen lines. The strongest were called ‘A’ stars, the next strongest B and then C and so on.
But in 1901, a new system was introduced by a Spectroscopist Annie Jump Cannon, who dropped or merged a few of the old classification and arranged them into one classified star by the strengths and appearances of many different absorption lines in their spectra.
A few years later, Max Planck solved a thorny problem in physics, showing how objects like stars give off their light of different colors based on their temperature.
According to Hr diagram, Hotter stars put on 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 tougher problem; how atoms give off light at different Temperature.
Two decades later, The Astronomer ‘Cecelia Payne Gaposehkin’,
put all these pieces together. She showed that the spectra of stars depending on the temperature and elements in the atmosphere.
This helps astronomers to understand their composition along with many other physical traits.
At the time, it was thought that stars had roughly the same composition as the Earth, but she showed that stars were overwhelming composed of hydrogen, with helium as the second most abundant element.
The classification scheme proposed by Cannon and decoded by Payne is still used today, and arranges stars by their temperature, assigning each a letter.
The hottest stars are O type stars, slightly cooler are B, followed by A, F, G, K and M
We also discovered even cooler stars in the past few decades and these are assigned the Letter L,T, and Y.
The sun has a surface temperature of about 5500 degree Celsius and is a 42 Star.
A slightly hotter star would be a G1, a slightly cooler star 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 are red, In between there orange and yellow stars but there are no green stars.
A Star can put out lots of green light, but it also emits red, blue and yellow but our eye mixed them together to form other colors.
We can measure how bright the stars appears to be in our telescope and by using the distance we can calculate how much energy it’s actually giving offer what astronomer call its Luminosity.
An intrinsically faint light looks bright if it’s nearby but so does a very luminous light far away. A lot of star physical characteristics are related; it’s luminosity depends on its temp and size.
If 2 stars are on the same size, but one is hotter and the hotter one is more luminous.
If 2 stars are on the same temperature, but one is bigger, the bigger one will be more luminous. Knowing the temp. and distances means knowing the stars themselves.
A centuries ago, spectra were taken of hundreds of thousands of stars. The best way to understand a large group of objects is to look for trends.
A century ago, Astronomers Ejnar Hertzprung and Henry Norris Russel made a graph, in which they plotted a star’s luminosity versus it’s temperature. When they did, they get very strong trend. This is called an Hr diagram, after Hertzprung and Russell.
It's not an exaggeration to call it the single most important graph in all of the 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 are on the right. There’s a thick line running diagonally down the middle, the clump to the upper right to the lower left,
This took a long time to fully understand but now we know this diagram is showing us how stars live there lives.
Most stars fall into that thick line, and that’s why astronomer call it the Main sequence. The term is a little misleading, it’s not really a sequence.
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 core.
A star that fuses into 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 the star can squeeze their core harder, so they fuse faster and get hotter than low mass stars.
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 merely 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 to the right and so on, the sun is there too more or less in the middle.
Stars on the lower left are hot, blue, and white but very faint. That means they must be small and we call them White Dwarf. They’re the result of a star like a sun eventually running out Hydrogen fuel. The stars on the upper right are luminous but cool.
Therefore, they must be huge. These are red giants, also part of the dying process of stars like the sun. Above them is red supergiant, massive stars beginning their death stage.
You can see some stars that are also that Luminous but at the upper left, those are blue supergiant. They’re rare but they too are the end stage for some stars.
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 glance just what a star is doing with itself.
The 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.
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