1. Stellar Classification and Evolution: The origin, life, and death of stars

2. What is a star? A cloud of gas and plasma, mainly hydrogen and helium
The core is so hot and dense that nuclear fusion can occur.
The fusion converts light elements into heavier ones

3. Every star is different
Luminosity:
Tells us how much energy is being produced in the core
Can be calculated using apparent magnitude and distance
Color:
Tells us the surface temperature of the star
Determined by analyzing the spectrum of starlight
Mass:
Determines the life cycle of a star and how long it will last
Given relative to our sun’s mass (ex: 0.8 solar masses)

4. Units of luminosity
We measure the luminosity of every day objects in Watts.
How bright is a light bulb?
By comparison, the Sun outputs:
380,000,000,000,000,000,000,000,000 Watts
This is easier to write as 3.8 x 1026 Watts
To make things easier we measure the brightness of stars relative to the Sun.

5. Units of temperature Temperature is measured in Kelvin
The Kelvin temperature scale is the same as the Celsius scale, but starts from -273o.
This temperature is known as “absolute zero”
-273 oC
-173 oC
0 oC
100 oC
1000 oC
0 K
100 K
273 K
373 K
1273 K
Kelvin = Celsius + 273

6. Measuring the temperature
The temperature of a star is indicated by its color
Blue stars are hot, and red stars are cooler
Red star
3,000 K
Yellow star
5,000 K
Blue star
10,000 K

7. Colors of Stars
Stars appear different colors depending on the peak wavelength of light they emit.
The sun, whose data is depicted in this graph, appears yellow-orange to our eyes.

8. Classification of Stars

9. Spectral Class (Oh Boy, A Failing Grade Kills Me)
Determined by analyzing a star’s spectra
O stars are the hottest and most massive
M stars are the coolest and least massive
Our Sun is a G star

10. Spectral Classes

11. The Hertzsprung Russell Diagram
We can also compare stars by showing a graph of their temperature and luminosity

14. Hertzprung-Russell Diagram
What information is plotted on the H-R Diagram?
Temperature and luminosity
What are the main stages of stars?
Main sequence, giant, supergiant, dwarf,
Do stars always stay in the same stage?
No, they change throughout their “lifetimes”

15. The Life of Stars

16. “Birth” of Stars…
Stars (and their solar systems) are created in giant molecular clouds of cosmic dust and gas
When gravity causes intense heat and pressure in the core of the proto-star, it triggers fusion and a star is “born”
The planets and other solar system objects are formed from the left-over materials in the proto-planetary disk surrounding this new star

17. The Carina Nebula (HST photo)

18. Artist rendering

19. Mass and Stellar Evolution
The life cycle of a star is determined by its mass
More massive stars have greater gravity, and this speeds up the rate of fusion
O and B stars can consume all of their core hydrogen in a few million years, while very low mass stars can take hundreds of billions of years.

20. Brown Dwarf– a “Failed Star”
If a proto-star does not have enough mass, gravity will not be strong enough to compress and heat its core to the temperatures that trigger fusion
If the mass is less than 0.08 x solar mass, it will form a Brown Dwarf
Brown Dwarfs are not true stars, but they do give off small amounts of light as they cool

21. The Main Sequence Longest life stage of a star
Energy radiating away from star balances gravitational pull inward (hydrostatic equilibrium)
Main-sequence stars fuse hydrogen into helium at a constant rate
Star maintains a stable size as long as there is ample supply of hydrogen atoms
The Sun will spend a total of ~10 billion years on the main sequence

22. OUR SUN A
Main
Sequence
Star
SDO image

23. When hydrogen in the core starts to run low…
In stars with masses more than 0.4 x solar mass, fusion slows down
Outer layers of the star begin to swell and surface temperatures fall
The shell surrounding the core begins to fuse hydrogen
Stars move out of the Main Sequence

24. Giants and Supergiants
“Old” stars
Helium produced through shell fusion becomes part of the core
Star’s core temperature increases as the more massive core contracts
The increased core temperature causes the helium left to fuse into carbon atoms (triple-alpha process)

25. Size: 100 X bigger than our sun Giants: 10X bigger than our sun
Supergiants
100 X
bigger than
our sun

26. A red supergiant nearing the end of it's life
Betelgeuse
A red supergiant nearing the end of it's life

27. The Death of Low-Mass Stars

28. “Death” of Stars Depends on MASS
“Low mass stars” are less than 8 solar masses
“High mass stars” are greater than 8 solar masses

29. Low-Mass Giants/Supergiants
In low mass stars (0.4 – 8.0 x solar mass) strong solar winds and energy bursts from helium fusion shed much of their mass
The ejected material expands and cools, becoming a planetary nebula (which actually has nothing to do with planets, but we didn’t know that in the 18th century when Herschel coined the term)
The core collapses to form a White Dwarf

30. Helix Nebula (HST photo)

31. White Dwarf Stars
The burned-out core of a star less than 8 x solar mass becomes a white dwarf
The carbon-oxygen core that remains is about the size of earth, but much more dense
Theoretically, after all of the stored energy radiates out into space, these stars will become giant crystals of carbon and oxygen (Black Dwarfs)

32. White Dwarf Stars
main sequence star Sirius A
Astronomers overexposed the image of Sirius A so that the dim Sirius B could be seen.
HST photo
White dwarf Sirius B

33. White Dwarfs in binary systems can explode
Occasionally we observe a White Dwarf star that suddenly becomes dramatically brighter and then fades to its original luminosity over a period of months or years.
It may repeat this process, if the companion star is stable
This is a Nova
Artist’s rendering

34. Nova (artist’s rendering)

35. The Death of High-Mass Stars

36. Massive stars continue fusion
Massive stars (> 8 x solar mass) have more gravity than low-mass stars
When helium fusion ends, gravitational compression collapses the core and the temperature rises beyond 600 million K
Fusion of the atoms from heavier elements begins, and the star becomes a luminous supergiant
These stars produce neon, magnesium, oxygen, sulfur, silicon, phosphorous, and iron

37. Supernova explosions
The iron-rich core signals the impending violent death of the massive star
The core collapses in seconds, and the resulting temp. exceeds 5 billion K
Intense heat breaks apart the atomic nuclei in the core, causing a shock wave
After a few hours, the shockwave reaches the star’s surface, blasting away the outer layers in a supernova

38. Artist’s rendering of a Supernova

39. Crab Nebula (HST image)
Remnants of a Supernova recorded in 1064
11 ly across
Supernova remnants are strong sources of X-rays and radio waves

40. Supernova 1987A
This HST picture shows three rings of glowing gas encircling the site of supernova in February 1987.
The supernova is 169,000 ly away in the dwarf galaxy called the Large Magellanic Cloud

41. Neutron Stars
The cores left over after Supernovae can become Neutron Stars-- very small, dense balls of NEUTRONS
1 teaspoon of this would be approximately 1 billion tons on Earth
Due to the great density it rotates very rapidly, and some become PULSARS

42. Pulsars
Rapidly-spinning neutron stars with very strong magnetic fields.
Jets of charged particles are ejected from the magnetic poles of the star.
This material is accelerated, producing beams of light in all wavelengths from the magnetic poles.
We can see this “lighthouse effect” many times per second
Computer model

43. Pulsar
Chandra X-Ray Observatory image shows a pulsar at the center of the Crab Nebula

44. Black Holes
Supermassive stars (>25 x solar mass) collapse into neutron stars too massive to be stable
They collapse in on themselves, forming a region of infinite density and zero volume– a SINGULARITY at the center of a Black Hole
Space “curves inward” and traps all matter and electromagnetic radiation