1. Chapter Twenty-Seven: Stars
27.1 The Sun
Stars
The Life Cycle of Stars

2. NGC 3532 in the constellation Carina ESO/G.Beccari

3. 27.2 How are stars classified?
Astronomers classify stars according to:

4. 27.2 How are stars classified?
Astronomers classify stars according to:
size/mass,

5. 27.2 How are stars classified?
Astronomers classify stars according to:
size/mass,
temperature,

6. 27.2 How are stars classified?
Astronomers classify stars according to:
size/mass,
temperature,
color, and

7. 27.2 How are stars classified?
Astronomers classify stars according to:
size/mass,
temperature,
color, and
brightness.

8. 27.2 What is the size of stars?
Stars come in a range of sizes and masses.

9. 27.2 What is the size of stars?
Stars come in a range of sizes and masses.
Our Sun is a medium- sized star.

10. 27.2 What is the size of stars?
Stars come in a range of sizes and masses.
Our Sun is a medium- sized star.
The largest stars, giant stars have a mass of about 60 times the mass of the Sun.

11. The sun is considered a __________ star.
Giant
Medium-sized
Dwarf

12. 27.2 The size of stars
There are two types of giant stars.

13. 27.2 The size of stars What are two types of giant stars?
Red giants are cooler than white stars.

14. 27.2 The size of stars What are two types of giant stars?
Red giants are cooler than white stars.
Blue giant stars are hot and much more massive than our sun.

15. Which star is cooler, a blue giant or a red giant?

16. 27.2 The size of stars
Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars.

17. 27.2 The size of stars
Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars.
Sirius, the Dog Star, is the largest known white dwarf.

18. 27.2 How is temperature and color related with stars?
If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars.

19. 27.2 How is temperature and color related with stars?
If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars.
This is because their surface temperatures are different.

20. 27.2 Temperature and color
The color of light is related to its energy.

21. 27.2 Temperature and color
The color of light is related to its energy.
White light is a mixture of all colors at equal brightness.

23. 27.2 Brightness and luminosity
Brightness, also called intensity, describes the amount of light energy per second falling on a surface.

24. 27.2 Brightness and luminosity
For a distant source of light like a star, the brightness decreases as the inverse square of the distance.

25. What does the inverse square law mean?

27. 27.2 Brightness and luminosity
Luminosity is the total amount of light given off by a star in all directions. .

28. 27.2 Brightness and luminosity
Luminosity is the total amount of light given off by a star in all directions.
Luminosity is a fundamental property of a star whereas brightness depends on both luminosity and distance.

29. 27.2 Temperature and luminosity
In the early 1900s, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell developed an important tool for studying stars.

30. 27.2 Temperature and luminosity
In the early 1900s, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell developed an important tool for studying stars.
Their graph showed luminosity on the y axis…

31. 27.2 Temperature and luminosity
In the early 1900s, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell developed an important tool for studying stars.
Their graph showed luminosity on the y axis…
…and surface temperature on the x axis

33. 27.2 Temperature and luminosity
H-R diagrams are useful because they help astronomers categorize stars into groups: ?

34. 27.2 Temperature and luminosity
H-R diagrams are useful because they help astronomers categorize stars into groups:
Main sequence stars, like the Sun, are in a very stable part of their life cycle.

35. 27.2 Temperature and luminosity
H-R diagrams are useful because they help astronomers categorize stars into groups:
Main sequence stars, like the Sun, are in a very stable part of their life cycle.
White dwarfs are hot and dim and cannot be seen without a telescope.

36. 27.2 Temperature and luminosity
H-R diagrams are useful because they help astronomers categorize stars into groups:
Main sequence stars, like the Sun, are in a very stable part of their life cycle.
White dwarfs are hot and dim and cannot be seen without a telescope.
Red giants are cool and bright and some can be seen without a telescope.

37. 27.2 Temperature and luminosity
H-R diagrams are useful because they help astronomers categorize stars into groups:
Main sequence stars, like the Sun, are in a very stable part of their life cycle.
White dwarfs are hot and dim and cannot be seen without a telescope.
Red giants are cool and bright and some can be seen without a telescope.
Can you locate blue giants on the H-R diagram?

41. Chapter Twenty-Seven: Stars
27.1 The Sun
Stars
The Life Cycle of Stars

42. 27.3 The life cycle of stars
A star, regardless of its size, begins its life inside a huge cloud of gas (mostly hydrogen) and dust called a nebula.

43. 27.3 The life cycle of stars
A star, regardless of its size, begins its life inside a huge cloud of gas (mostly hydrogen) and dust called a nebula.
The Eagle Nebula is the birthplace of many stars.

44. 27.3 The life cycle of stars
A protostar is the earliest stage in the life cycle of a star.

45. 27.3 The life cycle of stars
A protostar is the earliest stage in the life cycle of a star.
The Orion Nebula was the birthplace of these protostars.

46. 27.3 The life cycle of stars
A star is born when temperature and pressure at its center become great enough to start nuclear fusion.

47. 27.3 The life cycle of stars
A star is born when temperature and pressure at its center become great enough to start nuclear fusion.
Once nuclear fusion begins, a star is in the main sequence stage of its life cycle.

48. 27.3 The life cycle of stars
The time a star stays on the main sequence depends on the star’s mass.

49. 27.3 The life cycle of stars
The time a star stays on the main sequence depends on the star’s mass.
High-mass stars burn brighter, and hotter, using up their hydrogen faster than low-mass stars.

50. 27.2 The old age of Sun-like stars
With no more energy flowing outward, nothing prevents gravity from crushing the matter in the core together.

51. 27.2 The old age of Sun-like stars
With no more energy flowing outward, nothing prevents gravity from crushing the matter in the core together.
When hydrogen fusion stops, the core glows brightly and is called a white dwarf.

52. 27.2 The old age of Sun-like stars
A planetary nebula forms when a star blows off its outer layers leaving its bare core exposed as white dwarf.

53. 27.2 The old age of Sun-like stars
A planetary nebula forms when a star blows off its outer layers leaving its bare core exposed as white dwarf.
Planetary nebulae are one of nature’s ways of recycling the matter in old stars and distributing new elements.

54. 27.3 Supernovae and synthesis of the elements
Scientists believe the early universe was mostly hydrogen, helium and a trace of lithium.

55. 27.3 Supernovae and synthesis of the elements
Scientists believe the early universe was mostly hydrogen, helium and a trace of lithium.
Heavier elements are created by nuclear fusion inside the cores of stars.

56. 27.3 Supernovae and synthesis of the elements
Scientists believe the early universe was mostly hydrogen, helium and a trace of lithium.
Heavier elements are created by nuclear fusion inside the cores of stars.
Nuclear fusion reactions are exothermic, releasing energy only up to iron.

57. 27.3 Supernovae and synthesis of the elements
Scientists believe the early universe was mostly hydrogen, helium and a trace of lithium.
Heavier elements are created by nuclear fusion inside the cores of stars.
Nuclear fusion reactions are exothermic, releasing energy only up to iron.
When the core of the star contains mostly iron, nuclear fusion stops.

58. 27.3 Supernovae and synthesis of the elements
If a star’s iron core reaches 1.4 times the mass of the Sun, gravity becomes strong enough to combine electrons and protons into neutrons.

59. 27.3 Supernovae and synthesis of the elements
If a star’s iron core reaches 1.4 times the mass of the Sun, gravity becomes strong enough to combine electrons and protons into neutrons.
During this brief period, heavier elements such as gold and uranium are created, as atomic nuclei are smashed together.

60. 27.3 Supernovae and synthesis of the elements
If a star’s iron core reaches 1.4 times the mass of the Sun, gravity becomes strong enough to combine electrons and protons into neutrons.
During this brief period, heavier elements such as gold and uranium are created, as atomic nuclei are smashed together.
The core of the star collapses and the result is a spectacular explosion called a supernova.

61. 27.3 Supernovae and synthesis of the elements
The Crab nebula is the remains of a supernova.

62. 27.3 Supernovae and synthesis of the elements
The Crab nebula is the remains of a supernova.
Chinese astronomers recorded it’s demise 1054 AD.

63. 27.3 Examining light from stars
Spectroscopy is a tool of astronomy in which the light produced by a star or other object (called its spectrum) is analyzed.

64. 27.3 Analyzing light from stars
A spectrometer splits light into a spectrum of colors and displays lines of different colors along a scale.

65. 27.3 Analyzing light from stars
A spectrometer splits light into a spectrum of colors and displays lines of different colors along a scale.
Each element has its own unique pattern of spectral lines.

66. 27.2 Analyzing light from stars
In 1861, Sir William Huggins used spectroscopy to determine that the Sun and the stars are made mostly of hydrogen.

67. 27.2 Analyzing light from stars
A few years later, Sir Joseph Norman Lockyer observed a line at the exact wavelength of nm.
He concluded that this must be an undiscovered element and named it helium, after the Greek name for the Sun, Helios.

68. 27.2 Analyzing light from stars
A few years later, Sir Joseph Norman Lockyer observed a line at the exact wavelength of nm.

69. 27.2 Analyzing light from stars
A few years later, Sir Joseph Norman Lockyer observed a line at the exact wavelength of nm.
He concluded that this must be an undiscovered element and named it helium, after the Greek name for the Sun, Helios.