CHAPTER 10: The Sun – Our Favorite (and Ordinary) Star
WHAT DO YOU THINK?
How does the mass of the Sun compare with that of the rest of the Solar System?
Are there stars nearer the Earth than the Sun is?
Does the Sun have a solid and liquid interior like the Earth?
What is the surface of the Sun like?
Does the Sun rotate?
What makes the Sun shine?
INTERIOR of the Sun – 3 layers
ATMOSPHERE of the Sun – 3 layers
Layers of the Sun
Sun’s interior
Core – where hydrogen fusion happens.
Radiative zone – energy carried toward surface by radiation (as light).
Convective zone – energy carried toward surface by convection (as heat).
Sun’s atmosphere
Photosphere – lowest layer – emits visible light – what we see.
Chromosphere – middle layer – transparent.
Corona – upper layer – transparent.
The bright visible surface of the Sun is called the photosphere.
When looking at the Sun, the edges appear orange and darker than the central yellow region. This is known as limb darkening.
Upon closer inspection, the Sun has a marbled pattern called granulation, caused by the convection of gases just beneath the photosphere.
During an eclipse, sometimes you can see the layers of the Sun’s atmosphere just above the photosphere, which emits only certain wavelengths of light, resulting in a reddish appearance. We call this the sphere of color, or chromosphere.
The solar chromosphere is characterized by jets of gas extending upward called spicules.
THE SOLAR CORONA – source of the Solar Wind
Seen in visible light during an eclipse.
This x-ray image shows the million-degree gases.
The Sun undergoes differential rotation.
The rotation period of the Sun’s gases varies from 25 days in the equatorial region to 35 days near the solar poles.
Therefore, the magnetic field lines of the Sun become intertwined after several rotations, creating regions of intense magnetic fields and thus producing sunspots and other spectacular features.
The Sun’s Magnetic Field Creates Different Features
Sunspots – areas of concentrated magnetic field lines.
Prominences – magnetic loops above sunspots, can carry plasma (hot ionized gas).
Flares – twisted magnetic field lines relax and release huge amounts of X-rays.
Coronal Mass Ejections (CMEs) – twisted magnetic field lines relax and release huge amounts of plasma (up to 4 million mph).
Coronal Mass Ejections (CMEs) – twisted magnetic field lines relax and release huge amounts of plasma (up to 4 million mph).
Sunspots have two regions: the inner, darker umbra and the outer penumbra.
Overlapping sunspots
Sunspots
Sunspots are regions of intense magnetic fields
The number of sunspots on the photosphere varies over an eleven-year cycle.
Sunspot Maximum
Sunspot Minimum
Sunspots can be used to determine the rate of the sun’s rotation.
Ionized gases trapped by magnetic fields form prominences that arc far above the solar surface.
Sometimes these gases are ejected into space.
Violent eruptions called solar flares release huge amounts of X-rays. Solar flares are often associated with coronal mass ejections.
On the sun, coronal mass ejections occur when solar magnetic field lines snake around each other, forming the letter "S". Usually, they go past each other. But if they connect, it's like a short circuit. The mid-section breaks loose and drives out a coronal mass ejection.
Coronal Mass Ejections (CMEs) typically expel 2 trillion tons of plasma at up to 4 million mph.
An x-ray view of a coronal mass ejection
It reaches Earth two to four days later, and is fortunately deflected by our magnetic field.
By following the trails of gases released during a coronal mass ejection, we can map the Sun’s magnetic field.
The Sun is powered by thermonuclear fusion, which converts hydrogen into helium. Matter gets turned into energy in the process.
E = mc2
Fusion of Hydrogen into Helium
E = mc2
The Sun’s interior is held stable by a balance between radiation pressure forces and gravity, in a condition called hydrostatic equilibrium.
GRAVITY – pulls in
RADIATION PRESSURE FROM
HYDROGEN FUSION –
pushes out
THE SOLAR INTERIOR
WHAT DID YOU THINK?
How does the mass of the Sun compare with that of the rest of the Solar System?
The Sun contains almost all (99.85%) of the Solar System’s mass.
Are there stars nearer the Earth than the Sun is?
No, the Sun is our closest star.
Does the Sun have a solid and liquid interior like the Earth?
No, the Sun is composed of hot gases.
WHAT DID YOU THINK?
What is the surface of the Sun like?
The Sun has no solid surface, and no solid or liquids anywhere. The surface we see is composed of hot, churning gases.
Does the Sun rotate?
The Sun’s surface rotates differentially; once every 35 days near its poles, and once every 25 days near its equator.
What makes the Sun shine?
Thermonuclear fusion in the Sun’s core.
Chapter 11:
Characterizing
Stars
How near is the closest star other than the Sun?
Is the Sun brighter than other stars, or just closer?
What colors are stars?
Are brighter stars hotter?
What sizes are stars?
Are most stars isolated from other stars, as the Sun is?
WHAT DO YOU THINK?
Apparent Magnitude Scale – brightness of a star as seen from Earth
Several stars in and around the constellation Orion labeled with their names and apparent magnitudes
Astronomers give the
brightness of objects in the sky by apparent magnitudes. Stars visible to the naked eye have magnitudes between m = –1.44 and about
m = +6.
The Inverse-Square Law
The farther a star is from Earth, the dimmer it looks to us. Doubling the distance makes the star look one-fourth as bright. Tripling the distance decreases the star’s brightness by a factor of 9.
Absolute Magnitude – the actual brightness of a star
Absolute magnitude tells how bright a star really is, no matter how far from Earth it is.
Are the car lights actually dimmer as the car moves away?
No. Their actual brightness (absolute magnitude) is the same no matter the distance.
But they look dimmer (apparent magnitude) to us when the car is farther away.
Temperature and Color (review)
Hottest = blue color
Medium = orange/yellow color
Coolest = red color
“Oh, Be A Fine Guy/Girl, Kiss Me!”
Spectral Classes (Color and Temperature)
Hertzsprung-Russell (HR) Diagram
(Textbook
page 305)
Hertzsprung-Russell (HR) Diagram
Star brightness is plotted against star spectral types (color / temperature).
Brightness and spectral type are related.
Main-sequence stars (fusing hydrogen to helium) fall along the red curve.
Giants are to the upper right and super-giants are on the top.
White dwarfs are below the main sequence.
HR Diagram Basics
Thanks to Dan Enriquez
Star Size Is Also Important
Hotter stars are brighter than cooler stars (of the same size).
Bigger stars are brighter than smaller stars (of the same temperature).
So the brightest stars are the biggest, hottest ones.
L = R2T4 (L = brightness, R = radius, T = temperature)
Each dot = a main-sequence star.
The dot’s number is the mass of that star in solar masses (Sun = 1).
Mass, brightness, and temperature of main-sequence stars increase from lower right to upper left.
Mass-Temperature-Brightness
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FIGURE 11-2 Apparent Magnitude Scale (a) Several stars in
and around the constellation Orion are labeled with their names
and apparent magnitudes. For a discussion of star names, see
Guided Discovery: Star Names. (b) Astronomers denote the
brightnesses of objects in the sky by their apparent magnitudes.
Stars visible to the naked eye have magnitudes between
m = –1.44 (Sirius) and about m = +6. CCD (charge-coupled
device) photography through the Hubble Space Telescope or a
large Earth-based telescope can reveal stars and other objects
nearly as faint as magnitude m = +30. (a: Okiro Fujii, L’Astronomie)
WHAT DID YOU THINK?
How near is the closest star other than the Sun?
Proxima Centauri is about 40 trillion kilometers (25 trillion miles) away. It takes light about 4 years to reach the Earth from there.
How luminous is the Sun compared with other stars?
The most luminous stars are about a million times brighter and the least luminous stars are about a hundred thousand times dimmer than the Sun.
What colors are stars?
Stars are found in a wide range of colors, from red through violet, as well as white.
Are brighter stars hotter than dimmer stars?
Not necessarily. Many brighter stars, such as red giants, are cooler but larger than hotter, dimmer stars, such as white dwarfs.
What sizes are stars?
Stars range from more than 1000 times the Sun’s diameter to less than 1/100 the Sun’s diameter.
Are most stars isolated from other stars, as the Sun is?
No. In the vicinity of the Sun, two-thirds of the stars are found in pairs or larger groups.
WHAT DID YOU THINK?
Chapters 12 and 13: The Lives and Deaths of Stars - Stellar Evolution
NGC 2264
WHAT DO YOU THINK?
How do stars form?
Are stars still forming today?
Do more massive stars shine longer?
Will the Sun someday stop shining? If so, how?
Where do heavy elements on the Earth like carbon, silicon, oxygen, iron, and uranium come from?
What is a pulsar?
You will discover…
The remarkable transformations of older stars into giants and supergiants.
That some dying stars eject material that creates new generations of stars, while others act as beacons that enable astronomers to pinpoint distant galaxies.
You will discover…
What happens when stars “run out of fuel.”
How heavy elements are created.
What happens at the end of stellar evolution.
Why some stars go out relatively gently, while others go with a bang.
The incredible densities of neutron stars and how they are observed.
Star Formation
Stars form from the mutual gravitational attraction between gas and dust inside giant molecular clouds.
HR Diagram (Main Sequence = the red line).
Main Sequence Star definition:
1. Star is fusing Hydrogen to Helium in its core.
2. Star is not expanding or contracting.
If a star is above or below the Main Sequence, then something else is going on. We need to find out what that is.
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FIGURE 13-9 The Structure of an Old High-
Mass Star Near the end of its life, a high-mass
star becomes a supergiant with a diameter almost
as wide as the orbit of Jupiter. The star’s energy
comes from six concentric fusing shells, all
contained within a volume roughly the same size
as the Earth.
What if our Sun were 1.5 times as big as it is?
What if the Sun were 3 times bigger?
106 years = 1 million years
Summary of Stellar Evolution
The evolution of stars depends on their masses.
We will look at three sizes of stars:
Stars like our Sun.
Big stars (8-25 times the Sun’s mass).
Huge stars (more than 25 times the Sun’s mass).
Stars like our Sun (MO = 1) will turn into Planetary Nebulae and White Dwarf Stars, then end up as Black Dwarfs that give off no light.
Big stars (8-25 MO) will end up as Neutron Stars.
Huge stars (>25 MO) will end up as Black Holes.
Summary of Stellar Evolution
The evolution of stars depends on their masses.
Stars Like Our Sun (MO = 1)
Main Sequence
Red Giant
Red Supergiant
Planetary Nebula and White Dwarf
Black Dwarf
The Sun “burns” hydrogen now. In about 5 billion years, it will almost run out of hydrogen, and turn into a Red Giant.
The Sun Today and
as a Red Giant
Red Giant Stars in a Star Cluster
Life History of Stars Like Our Sun
Main Sequence
Red Giant
Red Supergiant
Planetary Nebula and White Dwarf
Black Dwarf
Our Sun in Old Age –
a Red Supergiant
Near the end of its life, the Sun will become a Supergiant.
From Supergiant to White Dwarf
Our Sun will “puff off” its outer layers to form a Planetary Nebula, and the Sun’s remaining core material will become a White Dwarf star.
It might look like this (Helix Nebula)
More Planetary Nebulae
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FIGURE 13-6 Sirius and Its White
Dwarf Companion (a) Sirius, the
brightest-appearing star in the night sky,
is actually a double star. The smaller star,
Sirius B, is a white dwarf, seen here at the
five o’clock position in the glare of Sirius.
The spikes and rays around the bright star,
Sirius A, are created by optical effects
within the telescope. (b) Since Sirius A
(11,000 K) and Sirius B (30,000 K) are hot
blackbodies, they are strong emitters of X
rays. (a: R. B. Minton; b: NASA/SAO/CXC)
Sirius’s White Dwarf Friend
Sirius B, a white dwarf, at the five o’clock position
Both emit X-rays
Big Stars and Huge Stars
(8-25 MO) (>25 MO)
Main Sequence
Bright Supergiant
Supernova Explosion
Big Star → Neutron Star
Huge Star → Black Hole
Structure of Big and Huge Stars in Old Age –
a Bright Supergiant
The old star’s core is now made of Iron. Oops!
Disaster is not far away.
The Supergiant’s core is made of Iron, which cannot be “burned” to make any heavier elements.
So the star’s core collapses because of gravity, then rebounds, and the star explodes.
We have a Supernova Explosion.
HOW FAST DOES ALL THIS HAPPEN?
Supernovae Make a Mess
Computer simulations showing how chaotic the supernova is deep inside the star as it begins to explode.
A Supernova Remnant
X-ray image of the Cygnus Loop
HST image of the Cygnus Loop
Gum Nebula
The Gum Nebula, created by a supernova 11,000 years ago, is the largest known supernova remnant. It now has a diameter of about 2,300 ly.
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FIGURE 11-7 A Hertzsprung-Russell Diagram On an H-R
diagram, the luminosities of stars are plotted against their
spectral types. Each dot on this graph represents a star whose
luminosity and spectral type have been determined. Some well known
stars are identified. The data points are grouped in just a
few regions of the diagram, revealing that luminosity and
spectral type are correlated: Main-sequence stars fall along the
red curve, giants are to the right, supergiants are on the top,
and white dwarfs are below the main sequence. The absolute
magnitudes and surface temperatures are listed at the right and
top of the graph, respectively. These are sometimes used on H-R
diagrams instead of luminosities and spectral types. (
Supernova 1987A (LMC)
Big and Huge Stars –
the Final Stage
Neutron Stars result from Supernova Explosions of Big Stars.
Black Holes result from Supernova Explosions of Huge Stars.
Neutron Star’s Interior
The neutron star has a superconducting, superfluid core 9.7 km in radius, surrounded by a 0.6-km-thick mantle of superfluid neutrons. The neutron star’s crust is only 0.3 km thick.
82
FIGURE 13-11 The Gum Nebula The Gum Nebula is the
largest known supernova remnant. It spans 60º across the sky
and is centered roughly on the southern constellation of Vela.
The nearest portions of this expanding nebula are only 300 ly
from the Earth. The supernova explosion occurred about
11,000 years ago, and its remnant now has a diameter of about
2300 ly. Only the central regions of the nebula are shown here.
(Royal Observatory, Edinburgh)
Neutron Stars
How Big? How Dense?
A typical Neutron Star would fit between Loyola Academy and Chicago’s Loop.
One teaspoonful of Neutron Star material would weigh one billion tons on Earth.
A Pulsar is a Rotating, Magnetized Neutron Star
Charged particles are accelerated near a neutron star’s magnetic poles and produce two beams of radiation. These beams act like the light from a lighthouse when seen from Earth.
84
FIGURE 13-14 Supernova
1987A A supernova was
discovered in a nearby galaxy
called the Large Magellanic Cloud (LMC)
in 1987. This photograph shows a portion
of the LMC that includes the supernova
and a huge H II region called the
Tarantula Nebula. At its maximum
brightness, observers at southern
latitudes saw the supernova without a
telescope. Insets: The star before and
after it exploded. (European Southern
Observatory; insets: Anglo-Australian
Observatory/David Malin Images)
Radio Signals or X-rays
from a Pulsar
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FIGURE 13-10 Supernovae Proceed Irregularly
Images (a) and (b) are computer simulations showing
how chaotic the supernova is deep inside the star as it
begins to explode. This helps account for the globs of iron and
other heavy elements emitted from deep inside, as well as the
lopsided distribution of all elements in the supernova remnant,
as shown in (c), (d), and (e). The latter three are X-ray images
of a supernova remnant taken by Chandra at different
wavelengths. (a and b: Courtesy of Adam Burrows, University of
Arizona and Bruce Fryxell, NASA/GSFC; c, d, and e: U. Hwang et al.,
NASA/GSFC)
Crab Nebula and Pulsar
The Crab’s visible flashes and X-ray pulses have identical periods of 0.033 seconds.
87
Colliding Neutron Stars
Collisions of Neutron Stars may cause creation of elements heavier than Iron, such as Gold, Silver, Platinum, Uranium.
You can thank the stars for your jewelry, as well as for the elements you are made of (Carbon, Oxygen, Phosphorus, Nitrogen, Iron, etc.). They made it all from Hydrogen.
Black Hole
Summary of Stellar Evolution
The evolution of stars depends on their masses.
90
FIGURE 13-2 The Structure of an Old
Low-Mass Star Near the end of its life, a
low-mass star like the Sun travels up the
AGB and becomes a supergiant. (The Sun
will be about as large as the diameter of
the Earth’s orbit.) The star’s core, the
hydrogen-fusing shell, and the heliumfusing
shell are contained within a volume
roughly the size of the Earth.
Summary of Stellar Evolution
The evolution of stars depends on their masses.
Stars like our Sun (MO = 1) will turn into Planetary Nebulae and White Dwarf Stars, then end up as Black Dwarfs that give off no light.
Big stars (8-25 MO) will end up as Neutron Stars, after a Supernova Explosion.
Huge stars (>25 MO) will end up as Black Holes, after a Supernova Explosion.
Material from old stars (Planetary Nebulae, Supernova Remnants, etc.) gets recycled to form new stars.
Stars – The Ultimate Recyclers
Material from old stars (Planetary Nebulae, Supernova Remnants, etc.) gets recycled to form new stars.
WHAT DID YOU THINK?
How do stars form?
Stars form from the mutual gravitational attraction between gas and dust inside giant molecular clouds.
Are stars forming today?
Yes. Astronomers have seen stars that have just arrived on the main sequence, as well as infrared images of gas and dust clouds in the process of forming stars.
Do stars with greater mass shine longer?
No. Lower-mass stars last longer because the lower gravitational force inside them causes fusion to take place at slower rates compared to the fusion inside higher-mass stars.
WHAT DID YOU THINK?
Will the Sun someday cease to exist? If so, how?
The Sun will shed matter as a planetary nebula in about 6 billion years and then cease nuclear fusion. Its remnant white dwarf will dim over the succeeding billions of years.
What are the origins of the carbon, silicon, oxygen, iron, uranium, and other heavy elements on Earth?
These elements are created during stellar evolution, by supernovae, and by colliding neutron stars.
What is a pulsar?
A pulsar is a rotating neutron star in which the magnetic field’s axis does not coincide with the rotation axis. The beam of radiation it emits sweeps across our region of space, like the light from a lighthouse.
94
FIGURE 12-20 Evolution of Stars from the Main Sequence
(a) Hydrogen fusion occurs in the cores of main-sequence stars.
(b) When the core is converted into helium, fusion ceases there
and then begins in a shell surrounding the core. The star
expands into the giant phase. This newly formed helium sinks
into the core, which heats up. (c) Eventually, the core reaches
108 K, whereupon core helium fusion begins. This causes the core
to expand, slowing the hydrogen shell fusion and thereby forcing
the outer layers of the star to contract.
STELLAR EVOLUTION (star types are underlined)
1. OUR SUN
Object (and transition) What's Happening Other Stuff
OUR SUN Hydrogen core fusion lifetime = 10 Billion years
↓ (makes Helium core)
RED GIANT Hydrogen shell fusion size = out to Venus, toasts Earth
↓ (Helium core flash)
shrinks Helium core fusion
↓ (makes Carbon & Oxygen core)
RED SUPERGIANT Helium shell fusion size = out to Earth's orbit
↓ (Helium shell flash)
PLANETARY made of dust & gas star's outer layers "puff off”
NEBULA
and
WHITE DWARF made of Carbon & Oxygen size = the Earth
↓ (cools off)
BLACK DWARF gives off no light
2. BIG STARS (8-25 times Sun's mass) and HUGE STARS (more than 25 times Sun's mass)
Object What's Happening and Other Stuff
BIG STARS lifetime = 15 Million years
and
HUGE STARS lifetime = 3 Million years
↓ (fusion of Hydrogen, Helium, Carbon, and Oxygen - makes Iron core)
BRIGHT SUPERGIANTS size = out to Jupiter's orbit or bigger
↓
SUPERNOVA EXPLOSION star collapses, rebounds, explodes
↓
BIG STARS
end up as
NEUTRON STARS made of Neutrons & other stuff, size = Chicago
HUGE STARS
end up as
BLACK HOLES gravity is so strong that light cannot escape