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Stars and Planetary Systems

NCEA Level 2 Science External Standard

Kev Knowles

on 23 September 2013

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Transcript of Stars and Planetary Systems

HR Diagram
In the Hertzsprung-Russell (HR) Diagram, each star is represented by a dot. There are lots of stars out there, so there are lots of dots. The position of each dot on the diagram tells us two things about each star: its luminosity (or absolute magnitude) and its temperature.
The vertical axis represents the star’s
or absolute magnitude. Luminosity is technically the amount of energy a star radiates in one second, but you can think of it as how bright or how dim the star appears. Luminosity is a common term, as is

Absolute magnitude is the intrinsic brightness of a star. In either case, the scale is a "ratio scale" in which stars are compared to each other based upon a reference (our sun).
The horizontal axis represents the star’s surface temperature (not the star’s core temperature – we cannot see into the core of a star, only its surface)! Usually this is labelled using the Kelvin temperature scale. This is also a "ratio scale."
Star Formation
Stars & Planetary Systems
Link 1
Link 2
Stars form in nebulae [Giant Molecular Clouds - GMCs]
A nebula is a cloud of dust and gas, composed primarily of hydrogen (97%) and helium (3%). Within a nebula, there are varying regions when gravity causes this dust and gas to “clump” together.
As these “clumps” gather more atoms (mass), their gravitational attraction to other atoms increases, pulling more atoms into the “clump.”
It has to be very cold for this to happen otherwise the molecules fly away from each other.
Eagle Nebula
Adding atoms to the center of a
is a process astronomers call accretion. Because numerous reactions occur within the mass of forming star material, a protostar is not very stable.
In order to achieve life as a star, the protostar will need to achieve and maintain.
What is equilibrium?
It is a balance, in this case a balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center. Achieving and keeping this balance is tough to do. When a star can no longer maintain equilibrium, it dies.
Equilibrium: How it Works!
Equilibrium is a battle between gravity and gas pressure. It works like this:

1. Gravity pulls gas and dust inward toward the core.
2. Inside the core, temperature increases as gas atom collisions increase.
3. Density of the core increases as more atoms try to share the same space.
4. Gas pressure increases as atomic collisions and density (atoms/space) increase.
5. The protostar’s gas pressure RESISTS the collapse of the nebula.
6. When gas pressure = gravity, the protostar has reached equilibrium and accretion stops
There are two options for a protostar at this point:

Option 1:
If a critical temperature in the core of a protostar is not reached, it ends up a brown dwarf. This mass never makes “star status.”
Option 2:
If a critical temperature in the core of a protostar is reached, then nuclear fusion begins. We identify the birth of a star as the moment that it begins fusing hydrogen in the core into helium.
Link - Brown Dwarf
Link - Protostar
Black Body Radiation
Astrophysics Science Project
Main Sequence
Helium Burning
How Stars Die
Star Gazing...
Look up in the Night Sky
What do you see...?
What can you measure...?
What does this tell us...?
Full transcript