1. Stars

3. NGC 3603

4. What is a Star? Spherical “ball” of glowing gases
Produces/produced its own energy through nuclear FUSION reactions
Typical composition
~75% Hydrogen
~25% Helium
<1% other elements

5. Stable Lifetime Stars exist because of gravity
They remain stable only by maintaining great pressures in their interiors.
Gravity acts to make it contract, pressure pushes back to balance the gravity
Stars are battlefields between gravity and pressure, and gravity ALWAYS wins in the end
© Eric Chaisson

6. Star Birth
Where astronomers find the youngest stars, they also find molecular clouds
Molecular clouds are typically:
Composed of H, He, simple molecules, and “dust”
huge - may have enough matter to form 100,000 stars the size of the Sun
Cold, about 10 K (10 degrees above absolute zero)

7. From Nebula to Star
A passing shock wave triggers a part of a cloud to begin to contract
Shock waves come from four processes:
Nearby supernovae
Ignition of a nearby star
Collision between molecular clouds
Rotation of a galaxy
As the cloud contracts atoms gather speed as they fall in a stage called free-fall contraction
Gravitational energy is converted into thermal energy and the gas grows hotter
if temperatures reach about 7 million K, nuclear fusion starts
A star is born

8. Star Formation Video

9. Distances to Stars
The closest star to the Sun is Proxima Centauri, 4.3 lightyears away
one way to determine distances to stars is to use parallax
measure the angular shift in the star's position 6 months apart
Using a little geometry you can calculate the distance
angles get too small to measure for stars more than about 150 lightyears away

10. What Process Could You use to Calculate Distance Using Parallax?
Establish where you can find a right triangle when using parallax.
What is the length of the base (short side) of the triangle?
Which trigonometric function would you use?
Construct a formula for getting the distance including both of the components above.

11. Some “Nearby” Stars Name Dist (ly) mag Mag
Proxima Centauri 4.2 ly
Alpha Centauri A
Alpha Centauri B
Barnard’s Star
Wolf BD
Luyten 726-8A
Luyten 726-8B
Sirius A
Sirius B

12. Star Brightnesses - the Magnitude Scale
Introduced by Hipparchos ( BC)
brightest stars were 1st magnitude
faintest stars were 6th magnitude (unaided eye)
larger the magnitude, the fainter the star!
Today it is similar, but more mathematical - logarithmic
1st magnitude stars are 100 x brighter than 6th magnitude stars
1 magnitude difference = x difference in brightness

13. Star Brightnesses - the Magnitude Scale
Magnitude scale has been extended to all astronomical objects
formalizing the mathematics has produced stars with negative magnitudes (Sirius = -1.42)

14. More About Magnitudes
The brightness of stars is affected by two factors:
Intrinsic brightness - how bright the star really is
How far away it is
To deal with this, astronomers use two types of magnitudes:
Apparent magnitude – how bright a star appears
Ex: Sun: mag -26.5, Sirius: mag -1.44
Absolute magnitude – how bright a star really is:
Ex: Sun: Mag 4.8, Sirius: Mag 1.45

15. Star Names
The "common" names derive from ancient Arabic, and often have meanings
in 1603, the first "modern" naming system was developed by Johann Bayer, using:
the Greek alphabet to indicate the apparent brightness of the star
the Latin name of the constellation
EX: the brightest star in Orion (Betelguese) is also called a Orionis, the 2nd brightest, is called b Orionis
one problem with this system is that some stars vary in brightness – the alpha star may not always be the brightest
There are over 10 different systems that are/have been used for naming stars
NO COMMERCIAL, FOR-PROFIT, STAR-NAMING SERVICE HAS ANY OFFICIAL STAR-NAMING RIGHTS

16. Sizes of Stars
Stars come in all different sizes, depending on their mass and their age
Masses range from 1/25 the mass of the Sun to maybe 200x mass of the Sun
largest known star is VY Canis Majoris, which would extend nearly to Saturn

17. Energy Production Stars produce energy through nuclear fusion.
Convert Hydrogen to Helium
Sun and smaller stars
Proton-proton chain
4 H  He
Requires temps of 10 million K & high densities
The Sun converts about 600 million tons of H to 596 million tons of He every second.
The “missing” million tons of mass is converted into energy:
E = mc2

18. Energy Production Stars larger than the Sun (1.1 x larger)
Use CNO cycle
Convert Hydrogen to Helium, using Carbon, Nitrogen,and Oxygen
End result is the same, 4 H+  He
Requires temps = 16 million K and higher

19. Life Expectancy of Stars
The biggest stars have the shortest lives – maybe “only” 20 million years.
While they have a large amount of fuel, they use it very quickly.
Average stars like our Sun, shine for about 10 billion years.
The smallest stars live the longest lives, maybe a trillion years!

20. The Beginning of the End
When the Hydrogen in the core is “used up”, the forces of _____ and _____ become unbalanced, and the star becomes unstable.
Energy production slows, internal pressure decreases.
Gravity causes star to shrink, increasing the temperatures.
New nuclear reactions start in the core, Helium is converted to Carbon.
Star expands, its surface cools, becoming a red giant.