1. Astronomy: Horizons
10th edition
Michael Seeds

2. In their general properties, stars are very simple.
They are great balls of hot gas held together by their own gravity.
Their gravity would make them collapse into small, dense bodies—were they not so hot.
The tremendously hot gas inside stars has such a high pressure that the stars would surely explode—were it not for their own confining gravity.

3. Another reason to study the sun is that life on Earth depends critically on the sun.
Very small changes in the sun’s luminosity can alter Earth’s climate.
A slightly larger change might make Earth uninhabitable.
Nearly all our energy comes from the sun—oil and coal are merely stored sunlight.
Furthermore, the sun’s atmosphere of very thin gas reaches out past Earth’s orbit—and any change in the sun, such as an eruption or a magnetic storm, can have a direct effect on Earth.

4. The sun is 109 times Earth’s diameter and 333,000 times Earth’s mass.
The Solar Atmosphere
The sun is 109 times Earth’s diameter and 333,000 times Earth’s mass.

5. The sun is only a little bit denser than water.
The Solar Atmosphere
The sun is only a little bit denser than water.
So, although it is very large and very massive, it must be a gas from its surface to its center.
When you look at it, you see only the outer layers of this vast sphere of gas— the solar atmosphere.

6. When you look at the sun, you see a hot, glowing surface.
Heat Flow in the Sun
Heat Flow in the Sun
When you look at the sun, you see a hot, glowing surface.
Simple logic tells you that energy in the form of heat is flowing outward from the sun’s interior.
The solar spectrum reveals that the sun is a G2 star with a temperature of about 5,800 K.
At that temperature, every square millimeter of the sun’s surface must be radiating more energy than a 60-watt lightbulb.

7. Heat Flow in the Sun
With all that energy radiating into space, the sun’s surface would cool rapidly if energy did not flow up from the interior to keep the surface hot.
Not until the 1930s did astronomers understand how the sun makes its energy.
Nuclear reactions occur in the core of the sun and generate energy—which flows upward as heat and keeps the surface hot.

8. The Photosphere
The Photosphere
The visible surface of the sun looks like a smooth layer of gas marked by a few dark sunspots.
Although the photosphere seems to be a distinct surface, it is not solid.
In fact, the sun is gaseous from its outer atmosphere right down to its center.

9. The Photosphere
The photosphere is the thin layer of gas from which Earth receives most of the sun’s light.
It is less than 500 km deep and has an average temperature of about 5,800 K.
If the sun magically shrank to the size of a bowling ball, the photosphere would be no thicker than a layer of tissue paper wrapped around the ball.

10. The Photosphere
For comparison, the chromosphere lies above the photosphere and is only a few times thicker in extent.

11. The Photosphere
In contrast, the corona, beginning above the chromosphere, extends far above the visible surface.

12. The Photosphere
Below the photosphere, the gas is denser and hotter and therefore radiates plenty of light.
That light, though, cannot escape from the sun because of the outer layers of gas.
So, you cannot detect light from these layers.
Above the photosphere, the gas is less dense and so is unable to radiate much light.

13. The Photosphere
Although the photosphere appears to be substantial, it is really a very-low-density gas.
Even in the deepest and densest layers visible, the photosphere is 3,400 times less dense than the air you breathe.

14. The Photosphere
To find gases as dense as the air at Earth’s surface, you would have to descend about 70,000 km below the photosphere—about 10 percent of the way to the sun’s center.
With fantastically efficient insulation, you could fly a spaceship right through the photosphere.

15. The spectrum of the sun is an absorption spectrum.
The Photosphere
The spectrum of the sun is an absorption spectrum.
That can tell you a great deal about the photosphere.

16. The Photosphere
In good photos, the photosphere has a mottled appearance because it is made up of dark-edged regions.
The regions are called granules.
The visual pattern they produce is called granulation.

17. Each granule is about the size of Texas.
The Photosphere
Each granule is about the size of Texas.
It lasts for only 10 to 20 minutes before fading away.
Faded granules are continuously replaced by new granules.

18. The Photosphere
In the sun, rising currents of hot gas heat small regions of the photosphere, which, being slightly hotter, and look brighter.
The cool sinking gas of the edges emits less light and thus looks darker.
The presence of granulation is clear evidence that energy is flowing upward through the photosphere.

19. Above the photosphere, lies the chromosphere.
Solar astronomers define the lower edge of the chromosphere as lying just above the visible surface of the sun, its upper regions blending gradually with the corona.
You can think of the chromosphere as being an irregular layer with an average depth of less than Earth’s diameter.

20. The Chromosphere
As the chromosphere is roughly 1,000 times fainter than the photosphere, you can see it with your unaided eyes only during a total solar eclipse—when the moon covers the brilliant photosphere.
Then, the chromosphere flashes into view as a thin line of pink just above the photosphere.

21. The Chromosphere
The word chromosphere comes from the Greek word chroma, meaning ‘color.’
The pink color is produced by the combined light of three bright emission lines—the red, blue, and violet Balmer emission lines of hydrogen.

22. The Chromosphere
Astronomers know a great deal about the chromosphere from its spectrum.
Since the chromosphere produces an emission spectrum; the chromosphere must be an excited, low-density gas.
The density is about 108 times less dense than the air you breathe.

23. The Chromosphere
Just above the photosphere, the temperature falls to a minimum of about 4,500 K and then rises rapidly to the extremely high temperatures of the corona.

24. The Chromosphere
Spicules are flamelike jets of gas rising upward into the chromosphere that last 5 to 15 minutes.

25. The Chromosphere
Seen at the limb (edge) of the sun’s disk, spicules blend together to look like flames covering a burning prairie.
However, they are not flames at all.
Spectra show that spicules are cooler gas from the lower chromosphere extending upward into hotter regions.

26. The Solar Corona
The outermost part of the sun’s atmosphere is called the corona, after the Greek word for crown.
The corona is so dim that it is not visible in Earth’s daytime sky because of glare of scattered light from the sun’s brilliant photosphere.

27. The Solar Corona
During a total solar eclipse, however, when the moon covers the photosphere, you can see the innermost parts of the corona.

28. The Solar Corona
Observations made with specialized telescopes called coronagraphs on Earth or in space can block the light of the photosphere and record the corona out beyond 20 solar radii—almost 10 percent of the way to Earth.

29. Just above the chromosphere, the temperature is about 500,000 K.
The Solar Corona
Just above the chromosphere, the temperature is about 500,000 K.
In the outer corona, it can be as high as 2 million K or more.

30. Its density is very low, only 106 atoms/cm3 in its lower regions.
The Solar Corona
The spectrum of the corona reveals that it is exceedingly hot gas, but it is not very bright.
Its density is very low, only 106 atoms/cm3 in its lower regions.
That is about a trillion times less dense than the air you breathe.
In its outer layers, the corona contains only 1 to 10 atoms/cm3—better than the best vacuum on Earth.
Due to this low density, the hot gas does not emit much radiation.

31. The Solar Corona
Observations made by the Solar and Heliospheric Observatory (SOHO) satellite have mapped a magnetic carpet of looped magnetic fields extending up through the photosphere.
As the gas of the chromosphere and corona has a very low density, it can’t resist movement in the magnetic fields.

32. The Solar Corona
Turbulence below the photosphere seems to flick the magnetic loops back and forth and whip the gas about.
That heats the gas.
In this instance, energy appears to flow outward as the agitation of the magnetic fields.

33. These flares extend far out into the corona.

34. The white spot is the Earth as seen by SOHO.

35. The Solar Corona
Not all of the sun’s magnetic field loops back—some leads outward into space.
Gas from the solar atmosphere follows along the magnetic fields that point outward and flows away from the sun in a breeze called the solar wind.
Like an extension of the corona, the low-density gases of the solar wind blow past Earth at 300 to 800 km/s—with gusts as high as 1,000 km/s.
Earth is bathed in the corona’s hot breath.

37. Due to the solar wind, the sun is slowly losing mass.
The Solar Corona
Due to the solar wind, the sun is slowly losing mass.
However, it is a minor loss for an object as massive as the sun.
The sun loses about 107 tons per second.
That is only about of a solar mass per year.
Later in life, the sun, like many other stars, will lose mass rapidly.

38. Random motions in the sun constantly produce vibrations.
Below the Photosphere
Random motions in the sun constantly produce vibrations.
However, these rumbles are much too low to hear with human ears--even if your ears could survive a visit to the sun’s atmosphere.
Some of these vibrations resonate in the sun like sound waves in organ pipes.

39. Below the Photosphere
This covers the surface of the sun with a pattern of rising and falling regions that can be mapped using the Doppler effect.

40. Below the Photosphere
Just as geologists can study Earth’s interior by analyzing vibrations from earthquakes, solar astronomers can use helioseismology to explore the sun’s interior.

41. Below the Photosphere
Helioseismology has allowed astronomers to map the temperature, density, and rate of rotation inside the sun.
They have been able to detect great currents of gas flowing below the photosphere and the emergence of sunspots before they appear in the photosphere.
Helioseismology can even locate sunspots on the back side of the sun—sunspots that are not yet visible from Earth.

42. Nuclear Fusion in the Sun
Thus, the gas is an atomic soup of rapidly moving particles colliding with each other at high velocity.
When you discuss nuclear reactions inside the sun and stars, you should be careful to refer to atomic nuclei and not to atoms.

43. Nuclear Binding Energy
There are only four forces in nature—the force of gravity, the electromagnetic force, the weak force, and the strong force.
The weak force is involved in the radioactive decay of certain kinds of nuclear particles.
The strong force binds together atomic nuclei.
Thus, nuclear energy comes from the strong force.

44. Hydrogen Fusion
The sun needs 1038 reactions per second, transforming 5 million tons of mass into energy every second.
It might sound as if the sun is losing mass at a furious rate.
However, during its entire 10-billion-year lifetime, the sun will convert less than 0.07 percent of its mass into energy.

45. Atomic nuclei must collide violently.
Hydrogen Fusion
Atomic nuclei must collide violently.
Violent collisions are rare unless the gas is very hot.
In that case, the nuclei move at high speeds and collide violently.

46. You can symbolize the hydrogen fusion reaction with a simple equation.
4 1H → 4He + energy
1H represents a proton, the nucleus of the hydrogen atom.
4He represents the nucleus of a helium atom.
The superscripts indicate the approximate weight of the nuclei—the number of protons plus the number of neutrons.

47. Hydrogen Fusion
The proton-proton chain is a series of three nuclear reactions that builds a helium nucleus by adding together protons.

48. This process is efficient at temperatures above 10,000,000 K.
Hydrogen Fusion
This process is efficient at temperatures above 10,000,000 K.
The sun manufactures over 90 percent of its energy in this way.

49. Hydrogen Fusion
The three steps in the proton–proton chain entail the following reactions.
1H + 1H → 2H + e+ + v
2H + 1H → 3He + γ
3He + 3He → 4He + 1H + 1H

50. Hydrogen Fusion
The energy appears in the form of gamma rays, positrons, the energy of motion of the particles, and neutrinos.

51. Energy Transport in the Sun
Now, you are ready to follow the energy from the core of the sun to the surface.
The surface is cool, only about 5,800 K.
The center, though, is over 10 million K.
So, energy must flow outward from the core.

52. As the core is so hot, the photons being emitted are gamma rays.
Energy Transport in the Sun
As the core is so hot, the photons being emitted are gamma rays.
Each time a gamma ray encounters an electron, it is deflected or scattered in a random direction.
As the ray bounces around, it slowly drifts outward toward the surface.
That carries energy outward in the form of radiation.

53. Astronomers refer to the inner parts of the sun as the radiative zone.
Energy Transport in the Sun
Astronomers refer to the inner parts of the sun as the radiative zone.

54. Energy Transport in the Sun
The energy backs up like water behind a dam, causing the gas to churn in convection.
Hot blobs of gas rise and cool blobs sink.
In this region, known as the convective zone, the energy is carried outward as circulating gas.

55. Energy Transport in the Sun
The granulation visible on the photosphere is clear evidence that the sun has a convective zone just below the photosphere carrying energy up to the surface.

56. Sunlight is nuclear energy produced in the core of the sun.
Energy Transport in the Sun
Sunlight is nuclear energy produced in the core of the sun.
The energy of a single gamma ray can take a million years to work its way outward—first as radiation and then as convection—on its journey to the photosphere.

57. Inside a star, the gas is so hot it is ionized.
Building Scientific Arguments
Inside a star, the gas is so hot it is ionized.
This means that the electrons have been stripped off the atoms and the nuclei are bare and have a positive charge.

58. For hydrogen fusion, the nuclei are single protons.
Building Scientific Arguments
For hydrogen fusion, the nuclei are single protons.
These atomic nuclei repel each other because of their positive charges.
So, they must collide with each other at high velocity to overcome that repulsion and get close enough together to fuse.

59. Observing the Sun
You should never look at the sun with any optical instrument—unless you are certain it is safe.
The figure illustrates a safe way to observe the sun with a small telescope.

60. Observing the Sun
In the early 17th century, Galileo observed the sun and saw spots on its surface.
Day by day, he saw the spots moving across the sun’s disk.
These are sunspots.
He rightly concluded that the sun was rotating.

61. There are various points to note about sunspots.
They are cool spots on the sun’s surface caused by strong magnetic fields.

62. Sunspots
They follow an 11-year cycle not only in the number of spots visible but in their location on the sun.

63. Sunspots
The cycle can vary over centuries and appears to affect Earth’s climate.

64. The sunspot groups are merely the visible traces of active regions.
Sunspots
The sunspot groups are merely the visible traces of active regions.
What causes this magnetic activity?
The answer appears to be linked to the waxing and waning of the sun’s overall magnetic field.

65. Sunspots are magnetic phenomena.
The Sun’s Magnetic Cycle
Sunspots are magnetic phenomena.
So, the 11-year cycle of sunspots must be caused by cyclical changes in the sun’s magnetic field.
To explore this idea, you need to begin with the sun’s rotation.

66. The sun does not rotate as a rigid body.
The Sun’s Magnetic Cycle
The sun does not rotate as a rigid body.
It is a gas from its outermost layers down to its center.
So, some parts of the sun rotate faster than other parts.

67. The Sun’s Magnetic Cycle
The equatorial region of the photosphere rotates faster than do regions at higher latitudes.
It rotates once every 25 days.
At a latitude of 45°, one rotation takes 27.8 days.

68. Helioseismology can map the rotation throughout the interior.
The Sun’s Magnetic Cycle
Helioseismology can map the rotation throughout the interior.
This phenomenon is called differential rotation.
It is clearly linked with the magnetic cycle.

69. The Sun’s Magnetic Cycle
The magnetic behavior of sunspots provides an insight into how the magnetic cycle works.
Sunspots tend to occur in groups or pairs.
The magnetic field around such a pair resembles that around a bar magnet—with one end magnetic north and the other end magnetic south.

70. The Sun’s Magnetic Cycle
At any one time, sunspot pairs south of the sun’s equator have reversed polarity compared with those north of the sun’s equator.
At the end of an 11-year cycle, the new spots appear with reversed magnetic polarity.

72. The Sun’s Magnetic Cycle
The differential rotation wraps the field around the sun like a long string caught on a hubcap.
Rising and sinking gas currents twist the field into ropelike tubes, which tend to float upward.

73. The Sun’s Magnetic Cycle
Where these magnetic tubes burst through the sun’s surface, sunspot pairs occur.

74. The Sun’s Magnetic Cycle
After about 11 years of tangling, the field eventually becomes so complex that adjacent regions of the sun begin changing their magnetic fields to agree with neighboring regions.
Quickly, the entire field rearranges itself into a simpler pattern.
Differential rotation begins winding it up to start a new cycle.

75. Chromospheric and Coronal Activity
The solar magnetic fields extend high into the chromosphere and corona—where they produce beautiful and powerful phenomena.

76. There are various important points to note about these phenomena.
Chromospheric and Coronal Activity
There are various important points to note about these phenomena.

77. Second, tremendous energy can be stored in arches of magnetic field.
Chromospheric and Coronal Activity
Second, tremendous energy can be stored in arches of magnetic field.
These are visible near the limb of the sun as prominences and, seen from above, as filaments.

78. Chromospheric and Coronal Activity
When that stored energy is released, it can trigger powerful eruptions.
Although these eruptions occur far from Earth, they can affect us in dramatic ways, including auroral displays.

79. Chromospheric and Coronal Activity
Finally, in some regions of the solar surface, the magnetic field does not loop back.
High-energy gas from these regions flows outward and produces much of the solar wind.

80. Chromospheric and Coronal Activity
You may have heard the common misconception that an auroral display is caused by sunlight reflecting off of the ice and snow at Earth’s North Pole.

81. Chromospheric and Coronal Activity
It is fun to think of polar bears standing on sunlit slabs of ice—but that doesn’t cause auroras.
Auroras are produced by gases in Earth’s upper atmosphere excited to glowing by energy from the solar wind.