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This webpage covers the life cycle of stars, HR diagrams, and galaxies. Stars start as stellar nebulas and two different paths in their life cycle depending on their starting mass. An HR diagrams graphs stars based on their temperature and luminosity. Galaxies are made of stars and the type of galaxies depend on the stars inside.
This concept is important because it helps us understand what path our Sun will take and how stars are formed to begin with.
Life Cycle of Stars
Every star is born in a stellar nebula. Over time, the dust and gases in the nebula get push together by gravity into a star. The amount of dust and gases will determine the star's mass. The route a star takes throughout its lifetime depends on the mass of the star.
An average star, like the Sun, will lose heat over time and expand into a red giant. When it no longer has the energy to stay together, it will shed its outer layer into a planetary nebula. The core of the star that's left behind become a white dwarf.
A massive star will also lose heat over time and expand into a red supergiant. When it no longer has the energy to stay together, it will explode into a supernova. From here, the supernova will either become a black hole, or a neutron star.
Hertzsprung–Russell Diagram
Image by gstarub from Shutterstock
A Hertzsprung–Russell (HR) Diagram is a graph that shows the temperature and luminosity of stars in reference to the Sun. The x-axis (bottom) shows the temperature of the star. The y-axis (left) shows how bright the star is.
There are 4 different groups of stars on an HR diagram.
The main sequence is where most stars (including our Sun) fall under. These stars are still in the average or massive star stage of their life cycle.
The giants are the average stars that have progressed to the red giant stage of their life cycle.
The supergiants are the massive stars that have progressed to the red supergiant stage of their life cycle.
In the Main Sequence, the stars get hotter and brighter as you move towards the left. The temperature on the x-axis is measured in Kelvin (K) rather than Fahrenheit (F) or Celsius (C). 1 K to equal to about -272.15 C or -457.87 F.
The 4 Corners of the HR Diagram :
Top Left - these stars are bright and hot (supergiants)
Top Right - these stars are bright but cool (giants)
Bottom Left - these stars are dim and hot (white dwarfs)
Bottom Right - these stars are dim and cool (main sequence)
Galaxies
A galaxy is a collection of stars, gas, and dust held together by gravity.
Galaxies are classified by the age of the stars inside of it.
Types of galaxies :
have no star formation or any visible features
most have thousands of globular clusters
contain mostly old stars
have brighter centers and very little dust and gas
looks like a sphere
mostly have grainy, highly irregular groups of bright areas
they have no noticeable symmetry nor obvious center core
made of younger stars
they don't fit any other class
found near spiral galaxies
have a bulge in the middle and spiral arms
the center of a spiral galaxy is made of old stars
arms made with young stars that formed recently out of gas and dust
shape caused by the rotation of the galaxy
we live in the Milky Way, a spiral galaxy
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