Module 2: The Universe
Stars makeup most of the matter in the universe. In this module you will learn how stars are formed from the gravitational condensation of a nebula, and once they become massive enough they will turn on through a flare-up event.
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CC2.1 Analyze the science of stars and galaxies
★ LO2.1.1 Determine the origin of stars and their life cycle
★ LO2.1.2 Perform computations involving a star’s brightness, color, and temperature
LO2.1.3 Identify galaxy properties
LO2.1.4 Identify properties and predictions of the Big Bang theory
Read Chapter 14 (14.1 through 14.2) Physical Science, 13th edition by Bill Tillery McGraw Hill Education
Getting ready for the learning objectives that involve calculations, this video follows the development of concepts and calculations through time and the scientists that were involved.
Time: 11:15
This video introduces the formation of a star, and where it is placed on the HR diagram, and how its life-cycle evolves.
Time: 4:27
ASL version of Mini Lecture - Formation of Stars - Structure of StarsTime: 14:14
This video will introduce you to black holes, and explain how they are so massive that not even light can escape them.
Time: 8:20
Try the following custom interactive activities to explore how star temperature and color are related.
Star Color and Temperature - For the best experience, go directly to the interactive but it is also embedded below for your convenience. Click into the panel below. You can scroll within the panel if needed.
hr-diagram - For the best experience, go directly to the interactive but it is also embedded below for your convenience. Click into the panel below. You can scroll within the panel if needed.
Test your knowledge on the lesson concepts by answering the following questions.
PROMPT When does the “stable” life of a star begin?
Answer
The outward pressure from the fusion reaction balances the inward force of gravity.
PROMPT The apparent magnitude of a star is defined as how bright a star appears as viewed from Earth. What does this depends upon?
Answer
It depends on the distance of the star from Earth
PROMPT Based upon the apparent magnitude scale established by Hipparchus, a Level 1 star is ____ times brighter than a Level 3 star.
Answer
6.3
PROMPT What is the method used by astronomers to calculate the brightness of a star which compensates for the different distances from Earth?
Answer
absolute magnitude
PROMPT What is the name of the graph that relates the absolute magnitude of a star to its temperature- luminosity.
Answer
Hertzsprung-Russell diagram
PROMPT What is the property of a main sequence star that determines its brightness, temperature and location on the H-R diagram?
Answer
mass
PROMPT What is the class of stars that regularly changes brightness and is very useful for determining distances in space?
Answer
Cepheid variable
PROMPT What type of star has the same temperature as a main sequence star but is much brighter?
Answer
red giant
PROMPT What processes in a massive star lead up to a supernova?
Answer
1. It contracts, builds up heat, goes through many fusion stages to the formation of iron.
2. After iron is produced, energy is no longer released; the star collapses and rebounds into a catastrophic explosion.
PROMPT If the remaining core of a supernova has a mass of at least three solar masses. Theoretically, what might it collapse into being?
Answer
A black hole
Test your knowledge on the lesson's computational content.
Difference in Brightness:
Discussion: The part in front of the equal sign is more like a label than something you do math with. The right side is where all the action is. When you get an answer, you are finding how many times brighter one star is compared to the other. So what is absolute magnitude? It is the total amount of energy radiated into space each second. Often, comparisons are made to our own sun, a fairly average star, to find x or how many times brighter or dimmer the star being compared is. Interestingly, stars with small absolute values are the more luminous stars compared to the larger numbered absolute values.
PROMPT Since our star is pretty average, let's compare it to the brightest star we see from Earth, Sirius A. Our sun has an absolute magnitude of 4.85. Sirius A is 1.43.
Answer
Answer: Sirius A is approximately 21.72 times more luminous than the Sun
Not what you got? Study the walkthrough to see where you may have gone wrong.
Given:
Msun = 4.85
MSirius = 1.43
Formula:
Step 1: Find the difference in absolute magnitude
∆M = Msun - MSirius
∆M = 4.85 - 1.43
∆M = 3.42
Step 2: Calculate the brightness ratio
101.3368 = 21.72
This result shows that Sirius A is approximately 21.72 times more luminous than the Sun.
Discussion: So, is Sirius the brightest star? No, it appears bright due to proximity to us, 8.6 light years away. righters stars are in our night sky but they do not appear so bright due to how far away they are. You might find the YouTube short Sun vs Brightest Known Star to be interesting. It compares the size of Sirius to our solar system's largest plat and then compares Sirius to the most luminous known star, the Godzilla Star.
Wien's Displacement Law or Working with Star Color (wavelength) and temperature (in Kelvin):
T = b / λpeak
Where ...
T is temperature in Kelvins
'b' is Wien's constant. You might see this online as approximately 2.897 x 10⁻³ meter-kelvin (m⋅K) or you might see it in the Angstrom unit version of 2.897 x 107 K⋅Angstroms. It is the same number, but technically the match to the units you are given for λ is there so that you can cancel the units.
λ is wavelength which you can use with a chart to find what the color is. Some equations will use λpeak while others will word it λmax. Don't let that throw you if you see it online or in another text. It means the same thing.