Chapter 8 - The Sun and Other Stars
Is it on FIRE? ... NO!
(Chemical energy content / Luminosity) ~10,000 years
Is it CONTRACTING? ... NO!
(Gravitational potential energy / Luminosity) ~ 25 million years
Remembering Spectral Types
(Hottest) O B A F G K M (Coolest) • Oh, Be A Fine Girl/Guy, Kiss Me • Only Bad Astronomers Forget Generally Known Mnemonics • Old Bob Always Favors Green Ketchup More
It can be powered by NUCLEAR ENERGY! (E = mc^2)
(Nuclear potential energy (core) / Luminosity) ~ 10 billion years
A solar model is used to calculate the expected temperature and density at all depths within the Sun. These results are then used to calculate the expected fusion rate within the Sun. We have confidence that the model is correct because it agrees with the observed characteristics of the Sun. Which of the following observations can be used to check that we really do know the Sun's internal fusion rate?
- Observations of neutrinos coming from the Sun - Measurements of the Sun's total energy output into space
Which of the following must be true if we are to infer (calculate) a star's luminosity directly from the inverse square law for light?
- We have measured the star's distance. - We have measured the star's apparent brightness - No interstellar gas or dust absorbs or scatters light between us and the star.
You should now see that the reason the mass of the star in Part A must be inferred is that the star has no known orbiting objects, which means we cannot apply Newton's version of Kepler's third law. Which of the following must be true if the star's inferred mass is to be accurate?
- We have measured the star's spectral type. - We have determined that the star is a main-sequence star.
What is net result of the Proton-Proton chain?
1 Helium atom.
About what is Rigel's surface temperature?
10,000 K
Rigel's luminosity is about _____ times the Sun's luminosity.
100,000
According to modern science, approximately how old is the Sun?
4 1/2 billion years
If I measure the parallax of a star to be 0.125 arc seconds, what is its distance in parsecs?
8
Rigel's radius is about _____ times the Sun's radius.
80
Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s).
All five stars appear at the same place along the horizontal axis showing spectral type. Because spectral type is related to surface temperature, all five stars must have the same surface temperature. Now proceed to Part C to determine how these stars vary in luminosity.
Five stars are shown on the following H-R diagrams. Rank the stars based on their luminosity from highest to lowest; notice that these are the same five stars shown in Part D. If two (or more) stars have the same luminosity, drag one star on top of the other(s)..
All five stars have the same luminosity because they are all at the same height along the vertical (luminosity) axis. Continue to Parts F and G for more practice in reading surface temperature and luminosity on the HR diagram.
Suppose you look at a detailed spectrum of visible light from some object. How can you decide whether it is an emission line spectrum or an absorption line spectrum?
An emission line spectrum consists of bright lines on a dark background, whereas an absorption line spectrum consists of dark lines on a rainbow background.
Pioneers of Stellar Classification
Annie Jump Cannon and the "calculators" at Harvard laid the foundation of modern stellar classification.
Five stars are shown on the following H-R diagrams; notice that these are the same five stars shown in Part F. Rank the stars based on their luminosity from highest to lowest. If two (or more) stars have the same luminosity, drag one star on top of the other(s).
As always, the H-R diagram shows surface temperature along the horizontal axis and luminosity along the vertical axis.
Which of the following is the best answer to the question, "Why does the Sun shine?"
As the Sun was forming, gravitational contraction increased the Sun's temperature until the core became hot enough for nuclear fusion, which ever since has generated the heat that makes the Sun shine.
Alpha Centauri and the Sun have about the same luminosity. Which one appears brighter? A. Alpha Centauri B. The Sun
B. The Sun
What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up slightly. B. The core would expand and cool. C. The Sun would blow up like a hydrogen bomb.
B. The core would expand and cool. The solar thermostat keeps burning rate steady.
Most continuous spectra are examples of what we also call thermal radiation spectra. Why do we call them "thermal" spectra?
Because the peak wavelength of the spectrum depends on the temperature of the object producing the spectrum.
Most interstellar clouds are made mostly of hydrogen (because hydrogen is the most common element in the universe). Why are these clouds usually dominated by the color red?
Because the strongest visible emission lines from hydrogen are red.
Why are sunspots dark?
Because they are cooler than the surrounding area.
The following figure shows how four identical stars appear in the night sky seen from Earth. The shading is used to indicate how bright (white) or dim (dark gray) the star would appear in the sky from Earth. Rank the stars based on their distance from Earth, from farthest to closest.
Brightest is closest, dimmest (or darkest) is most distant
Listed following are several fictitious stars with their luminosities given in terms of the Sun's luminosity (LSun) and their distances from Earth given in light-years (ly). Rank the stars based on how bright each would appear in the sky as seen from Earth, from brightest to dimmest. If two (or more) stars have the same brightness in the sky, show this equality by dragging one star on top of the other(s).
Brightest: Nismo, then (overlapping because they're the same) Shelby and Ferdinand, then Enzo, then Lotus (dimmest)
Listed following is the same set of fictitious stars given in Part A. Rank the stars based on how bright each would appear in the sky as seen from Jupiter, from brightest to dimmest.
Brightest: Nismo, then (overlapping because they're the same) Shelby and Ferdinand, then Enzo, then Lotus (dimmest)
The relationship between apparent brightness and luminosity depends on distance:
Brightness = Luminosity/4pi(distance)^2
Which kind of star is hottest? A. M star B. F star C. A star D. K star
C. A star
How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright B. It would be only 1/6 as bright. C. It would be only 1/9 as bright. D. It would be three times brighter.
C. It would be only 1/9 as bright.
Sort each item into the appropriate bin based on which type of spectrum it represents.
CONTINUOUS SPECTRUM - a graph of this spectrum shows a smooth curve - arises from relatively dense objects like light bulb filaments, rocks, and people - the only one of the spectra below that does not give us information about chemical composition EMISSION LINE SPECTRUM - a graph of this spectrum has upward spikes - produced by thin or low-density clouds of gas ABSORPTION LINE SPECTRUM - a graph of this spectrum is a curve with sharp, downward dips - produced when starlight passes through a thin or low-density cloud of gas
Suppose we obtain a single, detailed (high-resolution) spectrum of a star located many light-years away. What can we learn about the star? Sort each of the following characteristics of a star into the correct bin based on whether we can or cannot learn it from the single spectrum.
Can learn from single spectrum: - surface temperature - speed toward or away from us - chemical composition (surface) Cannot learn from single spectrum: - speed across our line of sight - interior temperature - size (diameter) - distance - mass
What are the layers of the Sun (not including the Solar Wind) from the inside out?
Core, radiative zone, convection zone, photosphere, chromosphere, corona
Following are the different layers of the Sun's atmosphere. Rank them based on the order in which a probe would encounter them when traveling from Earth to the Sun's surface, from first encountered to last.
Corona, chromosphere, photosphere
Comte was proven wrong in his claim that we could never learn the composition of stars. What do we know today that Comte did not know when making his claim, and that makes it possible for us to learn the chemical compositions of stars?
Every chemical element produces a unique spectral fingerprint.
Listed following are the different layers of the Sun. Rank these layers based on their distance from the Sun's center, from greatest to least.
Greatest distance corona, chromosphere, photosphere, convection zone, radiation zone, core. least distance.
Earth is about 150 million kilometers from the Sun, and the apparent brightness of the Sun in our sky is about 1300 watts/m2. Using these two facts and the inverse square law for light, determine the apparent brightness that we would measure for the Sun if we were located at the following positions.
Half Earth's distance from the Sun.: 5200 watts/m^2 Twice Earth's distance from the Sun.: 325 watts/m^2 10 times Earth's distance from the Sun.: 13 watts/m^2 [1300* 1/10^2 = 13]
Main-Sequence Star Summary
High-Mass Star: • High luminosity • Short-lived • Larger radius • Blue • Hot Low-Mass Star: • Low luminosity • Long-lived • Small radius • Red • Cool
Consider the four stars shown following. Rank the stars based on their surface temperature from highest to lowest.
Highest to lowest: - a blue white dwarf star - Sun - an orange main-sequence star - a red supergiant star
Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are __________.
Hotter and smaller in radius
What is the connection between hot main sequence stars and their luminosity?
Hotter main sequence stars are more luminous
How would the interior temperature of the Sun be different if the strong force that binds nuclei together were 10 times stronger?
If the strong force were ten times stronger, the force would be strong enough to overcome repulsion, and the protons would end up sticking together, making helium. Nuclear fusion wouldn't take place, so the Sun's core would be cooler than it is now, the Sun itself may not have existed at all or for a much shorter period of time without hydrogen.
Describe what the Sun would look like from Earth if the entire photosphere were the same temperature as a sunspot.
If the temperatures were the same, we wouldn't get the contrast of light stemming from the difference in heat of the photosphere and sunspots. The Sun would end up being entirely dark in this case since the photosphere wouldn't be as hot and therefore less visible to the human eye. We are currently able to view sunspots since their cool regions are surrounded by the photosphere.
According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
Its apparent brightness will decrease by a factor of 9.
Which of the following best describes why the Sun emits most of its energy in the form of visible light?
Like all objects, the Sun emits thermal radiation with a spectrum that depends on its temperature, and the Sun's surface temperature is just right for emitting mostly visible light.
We can determine a star's luminosity if we can measure its distance and apparent brightness
Luminosity = 4pi(distance)^2 * Brightness
Five stars are shown on the following H-R diagrams; notice that these are the same five stars shown in Part B. Rank the stars based on their luminosity from highest to lowest. If two (or more) stars have the same luminosity, drag one star on top of the other(s).
Luminosity is shown along the vertical axis, with stars higher up more luminous than those lower down. Note that each tickmark along the luminosity axis represents a change by a factor of 10 from the prior tickmark, so the range of luminosities is quite large. Continue to Parts D and E to investigate surface temperature and luminosity for a different set of five stars.
How do we measure stellar luminosities?
Luminosity: • Amount of power a star radiates (energy per second = watts) Apparent brightness: • Amount of starlight that reaches Earth (energy per second per square meter) - The brightness of a star depends on both distance and luminosity • The amount of luminosity passing through each sphere is the same. Area of sphere: 4pi(radius)^2 • Divide luminosity by area to get brightness.
How is the lifetime of a star related to its mass?
More massive stars live much shorter lives than less massive stars.
Based on its location on the HR diagram, what can we say about Rigel's mass and lifetime?
Nothing, because it is not on the main sequence.
From hottest to coolest, the order of the spectral types of stars is __________.
OBAFGKM
To understand the interplay of observations and models you must first be able to distinguish between things that we observe and things that we infer from models. Consider the following statements about the Sun. Classify each statement as an observation or as an inference based on the current, accepted model for the Sun.
Observations: - The photosphere emits mostly visible light. - The photosphere is made mostly of hydrogen and helium. - The Sun emits neutrinos. - The corona is hotter than the photosphere. Inferences from a model: - The core temperature is 10 million K. - The composition of the photosphere is the same as that of the gas cloud that gave birth to our solar system. - The convection zone is cooler than the radiation zone. - The Sun generates energy by fusing hydrogen into helium in its core.
Consider a relatively nearby, single star, that is, a star that is not a member of a binary system and has no known orbiting planets. Listed below are a few properties of this star. Classify each property as either something that we can observe or measure directly (with the aid of a telescope and instruments such as cameras or spectrographs) or something that we must infer indirectly (and hence is correct only if all of our assumptions are correct).
Observe - color, parallax angle, spectral type, apparent brightness Infer - luminosity, surface temperature, mass, radius
What phase of matter is the Sun?
Plasma
Solar facts
Radius: 6.9 * 10^8 m (109 Earths) Mass: 2 * 10^30 kg (300,000 Earths) Luminosity: 3.8 * 10^ 26 watts Spectral class: G (main sequence,surface temp 5200-6000K) Composition: 70% hydrogen, 28% helium, 2% heavier elements
There are more _________ stars than other.
Red
Listed following is a set of statements describing individual stars or characteristics of stars. Match these to the appropriate object category.
Red Giant or Supergiant Stars: - Very cool but very luminous - Found in the uppper right of the H-R diagram Main-sequence stars: - The majority of stars in our galaxy - The Sun, for example - the hottest andmost luminous stars White dwarfs: - not much larger in radius than Earth - Very hot but very dim
How do we measure stellar temperatures?
Reminder: 1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average energy. • Every object emits thermal radiation with a spectrum that depends on its temperature.
Suppose you could travel to Jupiter and observe changes in positions of nearby stars during one orbit of Jupiter around the Sun. Describe how those changes would be different from what we measure from Earth. How would your ability to measure the distances to stars be different from the vantage point of Jupiter?
Since Jupiter's radius is about five times larger than Earth's, we would see that the parallax of stars is five times larger compared to being viewed from Earth. With the larger parallax, we would be able to measure distances five times further away than we would be able to from Earth.
Listed following are events or phenomena that occur during either the part of the sunspot cycle known as solar minimum or the part known as solar maximum. Match these items to the correct part of the sunspot cycle.
Solar Maximum: - Occurs about 11 years after a solar maximum (on average) - Solar flares are most common - Auroras are most likely in Earth's skies - Sunspots are most numerous on the Sun - Orbiting satellites are most at risk Solar Minimum: - Occurs about 5 to 6 years after a solar maximum (on average)
Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s)...
Spectral type is related to surface temperature, with stars of spectral type O having the highest surface temperature and stars of spectral type M having the lowest surface temperature. In other words, spectral type increases to the left on the H-R diagram.
Other than Hydrogen and Helium, where are most other elements made?
Stars
H-R diagram depicts:
Temperature Color Spectral type Luminosity Radius Mass Lifetime/Age
Which of the following correctly compares the Sun's energy generation process to the energy generation process in human-built nuclear power plants?
The Sun generates energy by fusing small nuclei into larger ones, while our power plants generate energy by the fission (splitting) of large nuclei.
How does the Sun's mass compare to Earth's mass?
The Sun's mass is about 300,000 times the mass of the Earth.
Observations of the Sun
The Sun, as seen from low Earth orbit overlooking the International Space Station. This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum
The "extraordinary" part of Comte's claim was his statement that we could never learn the composition of stars. Which of the following best summarizes the key lesson we should learn from the fact that his claim was ultimately proven wrong?
The advance of science and technology may someday provide ways to answer questions that seem unanswerable today.
How can we best observe the Sun's chromosphere and corona?
The chromosphere is best observed with ultraviolet telescopes and the corona is best observed with X-ray telescopes.
One statement about the Sun from Part A is "The corona is hotter than the photosphere." Which of the following statements provides observational evidence for this claim?
The corona primarily emits X rays while the photosphere primarily emits visible light.
Astronomers can measure a star's mass only in certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
The star is a member of a binary star system.
Why do sunspots appear dark in pictures of the Sun?
They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere.
Why do sunspots appear darker than their surroundings?
They are cooler than their surroundings.
Why do they know white dwarfs are so small in radius?
They are hotter and less luminous
Why do they know Giants are so large in radius?
They cooler and more luminous
What is the common trait of all main-sequence stars?
They generate energy through hydrogen fusion in their core.
The Sun's surface seethes and churns with a bubbling pattern. Why?
We are seeing hot gas rising and cool gas falling due to the convection that occurs beneath the surface.
Now consider the statements in Part A that are inferred from models. A solar model is used to calculate interior conditions based on certain "known" characteristics of the Sun, such the Sun's total mass. How do we know the Sun's mass?
We can calculate it by applying Newton's version of Kepler's third law with Earth's orbital period (1 year) and Earth's average distance from the Sun (1 AU).
Visual Binary
We can directly observe the orbital motions of these stars.
Eclipsing Binary
We can measure periodic eclipses
Which of the following choices is not a way by which we can study the inside of the Sun?
We can send a space probe into the Sun's photosphere.
Spectroscopic Binary
We determine the orbit by measuring Doppler shifts.
How we know what is happening inside the Sun?
We learn about the inside of the Sun by ... • making mathematical models • observing solar vibrations • observing solar neutrinos • Patterns of vibration on the surface tell us about what the Sun is like inside. • Here, vibrations revealed by Doppler shifts are shown. • Data on solar vibrations agree very well with mathematical models of solar interior. • Neutrinos created during fusion fly directly through the Sun. • Observations of these solar neutrinos can tell us what's happening in core.
Which of these stars has the coolest surface temperature?
a K star
What is the sunspot cycle?
a cycle in which the number of sunspots on the Sun at any one time gradually rises and falls
The light radiated from the Sun's surface reaches Earth in about 8 minutes, but the energy of that light was released by fusion in the solar core about __________.
a few hundred thousand years ago
Which of these stars is the most massive?
a main-sequence A star
For an object producing a thermal spectrum, a higher temperature causes the spectrum to have ___________.
a peak intensity located at shorter wavelength
Study the graph of the intensity of light versus wavelength for continuous spectra, observing how it changes with the temperature of the light bulb. Recall that one of the laws of thermal radiation states that a higher-temperature object emits photons with higher average energy (Wien's law). This law is illustrated by the fact that for a higher temperature object, the graph peaks at __________.
a shorter wavelength
What is the solar wind?
a stream of charged particles flowing outward from the surface of the Sun
What type of visible light spectrum does the Sun produce?
an absorption line spectrum
What do we need to measure in order to determine a star's luminosity?
apparent brightness and distance
The absorption line spectrum shows what we see when we look at a hot light source (such as a star or light bulb) directly behind a cooler cloud of gas. Suppose instead that we are looking at the gas cloud but the light source is off to the side instead of directly behind it. In that case, the spectrum would __________.
be an emission line spectrum
How can we measure the strength of magnetic fields on the Sun?
by looking for the splitting of spectral lines in the Sun's spectrum
Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity
On an H-R diagram, stellar masses __________.
can be determined for main-sequence stars but not for other types of stars
Five stars are shown on the following H-R diagrams. Rank the stars based on their surface temperature from highest to lowest. If two (or more) stars have the same surface temperature, drag one star on top of the other(s)..
circle moves from top left to top right when going from highest to lowest temp
Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are __________.
cooler and larger in radius
In which of the following layer(s) of the Sun does nuclear fusion occur?
core
From the center outward, which of the following lists the "layers" of the Sun in the correct order?
core, radiation zone, convection zone, photosphere, chromosphere, corona
Rank the layers of the Sun based on their density, from highest to lowest.
core, radiation zone, convection zone,photosphere, chromosphere, corona.
Rank the following layers of the Sun based on the pressure within them, from highest to lowest.
core, radiation zone, convective zone, photosphere.
Rank the following layers of the Sun based on their temperature, from highest to lowest.
core, radiation zone, convective zone, photosphere.
Rank the layers of the Sun's atmosphere based on their temperature, from highest to lowest.
corona, chromosphere, photosphere
Rank the layers of the atmosphere based on the energy of the photons that are typically emitted there, from highest to lowest.
corona, chromosphere, photosphere
The intricate patterns visible in an X-ray image of the Sun generally show ________.
extremely hot plasma flowing along magnetic field lines
Which of these groups of particles has the greatest mass?
four individual protons
At the center of the Sun, fusion converts hydrogen into
helium and energy.
On an H-R diagram, stellar radii __________.
increase diagonally from the lower left to the upper right
We found that mass must be inferred for the star described in Part A. However, we can measure a star's mass directly if __________.
it is a member of an eclipsing binary system
Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is __________.
less luminous, cooler, smaller, and less massive
The total amount of power (in watts, for example) that a star radiates into space is called its __________.
luminosity
The axes on a Hertzsprung-Russell (H-R) diagram represent __________.
luminosity and surface temperature
Scientists estimate the central temperature of the Sun using
mathematical models of the Sun.
Click "show" for the emission line spectrum, then click "choose gases" and study the emission line spectrum for neon. The neon "OPEN" sign appears reddish-orange because __________.
neon atoms emit many more yellow and red photons than blue and violet photons
The source of energy that keeps the Sun shining today is __________.
nuclear fusion
The fundamental nuclear reaction occurring in the core of the Sun is __________.
nuclear fusion of hydrogen into helium
Solar energy leaves the core of the Sun in the form of
photons
The Sun's visible surface (that is, the surface we can see with our eyes) is called the _____.
photosphere
Which of these layers of the Sun is coolest?
photosphere
Rank the layers of the Sun's atmosphere based on their density, from highest to lowest.
photosphere, chromosphere, corona
Which of the following layers of the Sun can be seen with some type of telescope? Consider all forms of light, but do not consider neutrinos or other particles.
photosphere, chromosphere, corona
Coronal mass ejections
send bursts of energetic charged particles out through the solar system
From Part A, you know that surface temperature is a stellar property that we infer indirectly. What must we measure directly so that we can infer a star's surface temperature?
spectral type
Suppose you want to know the chemical composition of a distant star. Which piece of information is most useful to you?
the wavelengths of spectral lines in the star's spectrum
Solar wind:
• A flow of charged particles from the surface of the Sun
Stellar Luminosity Classes
• A star's full classification includes spectral type (line identities) and luminosity class (line shapes, related to the size of the star): I - supergiant II - bright giant III - giant IV - subgiant V - main sequence Examples: Sun - G2 V Sirius - A1 V Proxima Centauri - M5.5 V Betelgeuse - M2 I
Ionization
• Absorption lines in star's spectrum tell us its ionization level • Level of ionization also reveals a star's temperature.
Hertzsprung-Russell (HR) diagram
• An H-R diagram plots the luminosity and temperature of stars. • Most stars fall somewhere on the main sequence of the H-R diagram. • Stars with lower T and higher L than main-sequence stars must have larger radii. These stars are called giants and super giants • Stars with higher T and lower L than main-sequence stars must have smaller radii. These stars are called white dwarfs.
Sunspots
• Are cooler than other parts of the Sun's surface (4000 K) • Are regions with strong magnetic fields • Charged particles spiral along magnetic field lines. • Loops of bright gas often connect sunspot pairs. • Magnetic activity causes solar flares that send bursts of X rays and charged particles into space. • Magnetic activity also causes solar prominences that erupt high above the Sun's surface. • The corona appears bright in X-ray photos in places where magnetic fields trap hot gas.
Fission
• Big nucleus splits into smaller pieces. • (Example: nuclear power plants)
How does solar activity affect humans?
• Charged particles streaming from the Sun can disrupt electrical power grids and can disable communications satellites.
Solar Thermostat
• Decline in core temperature causes fusion rate to drop, so core contracts and heats up. • Rise in core temperature causes fusion rate to rise, so core expands and cools down.
We measure mass using gravity.
• Direct mass measurements are possible only for stars in binary star systems. p^2 = [4pi^2 / G(M1 + M2)] * a^3 p = period a = average separation
Solar neutrino problem:
• Early searches for solar neutrinos failed to find the predicted number. • More recent observations find the right number of neutrinos, but some have changed form.
Core:
• Energy generated by nuclear fusion ~ 15 million K
How does the energy from fusion get out of the Sun?
• Energy gradually leaks out of radiation zone in form of randomly bouncing photons. • Convection (rising hot gas) takes energy to surface. • Bright blobs on photosphere show where hot gas is reaching the surface.
Gravitational equilibrium:
• Energy supplied by fusion maintains the pressure that balances the inward crush of gravity
Radiation Zone:
• Energy transported upward by photons
Convection Zone:
• Energy transported upward by rising hot gas
Stellar Temperatures
• Hottest stars: 50,000 K • Coolest stars: 3000 K • (Sun's surface is 5800 K.) • Lines in a star's spectrum correspond to a spectral type that reveals its temperature. O > 33,000 K B 33,000K-10,000K A 10,000K-7500K F 7500K-6000K G 6000K-5200K K 5200K-3700K M< 3700K *Other spectral types: L, T, Y -usually brown dwarfs, cooler than stars of type M (not true stars because they cannot sustain nuclear fusion in their cores) *All stars above 6000K look mostly white to the human eye because they emit radiation at all possible wavelengths
Stellar Properties Review
• Luminosity: from brightness and distance - (0.08M_Sun) 10^-4 L_Sun - 10^6 L_Sun (100M_Sun) • Temperature: from color and spectral type - (0.08M_Sun) 3000 K - 50,000 K (100M_Sun) • Mass: from period (p) and average separation (a) of binary star orbit 0.08M_Sun - 100M_Sun
The significance of the main sequence
• Main-sequence stars are fusing hydrogen into helium in their cores like the Sun. • Luminous main-sequence stars are hot (blue). • Less luminous ones are cooler (yellow or red) • Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones. • The mass of a normal, hydrogen-burning star determines its luminosity and spectral type. • Core pressure and temperature of a higher-mass star need to be larger in order to balance gravity. • Higher core temperature boosts fusion rate, leading to larger luminosity.
Chromosphere:
• Middle layer of solar atmosphere ~ 10^4-10^5K
Stellar luminosities
• Most luminous stars: 10^6 L Sun • Least luminous stars: 10^-4 L Sun (L_Sun is luminosity of Sun)
Stellar Masses
• Most massive stars: 100M_Sun • Least massive stars: 0.08M_Sun • (M_Sun is the mass of the Sun.)
Location and Motion
• Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being the red dwarf Proxima Centauri at approximately 4.2 light-years), the Sun ranks fourth in mass • The Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. The Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due increased impact events • It takes the Solar System about 225-250 million years to complete one orbit through the Milky Way (a galactic year) • The orbital speed of the Solar System about the center of the Milky Way is approximately 251 km/s (156 mi/s). At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU - The Sun is close to the inner rim of the Milky Way's Orion Arm, in the Local Interstellar Cloudat a distance of about 25,000-28,000 light-years from the Galactic Center.
Corona:
• Outermost layer of solar atmosphere ~1 million K
Gravitational contraction:
• Provided the energy that heated the core as Sun was forming • Contraction stopped when fusion began
Fusion
• Small nuclei stick together to make a bigger one. • (Example: the Sun, stars) • High temperatures enable nuclear fusion to happen in the core.
Off the Main Sequence
• Stellar properties depend on both mass and age: Those that have finished fusing H to He in their cores are no longer on the main sequence. • All stars become larger and redder after exhausting their core hydrogen: giants and supergiants. • Most stars end up small and white after fusion has ceased: white dwarfs.
Future of the Sun
• Sun is fusing hydrogen into helium (will take about 10 billion years) • Gradually becoming hotter (shrinking because He occupies less space) • Higher pressure causes increase in rate at which fusion occurs • Brightness increases by about 1% every 100 million years • As brightness increases, outer layers expand • Once hydrogen is exhausted in the core, the sun will start burning helium in the core while still burning hydrogen in the outer layers • More expansion happens until the sun becomes a red giant, increasing its radius about 200 times
Solar activity is like "weather".
• Sunspots • Solar flares • Solar prominences • All these phenomena are related to magnetic fields.
sun energy
• The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus. • The proton-proton chain is how hydrogen fuses into helium in Sun. - IN: 4 protons - OUT: ^4He nucleus, 2 gamma rays, 2 positrons, 2 neutrinos - Total mass is 0.7% lower.
How does solar activity vary with time?
• The number of sunspots rises and falls in an 11-year cycle. • The sunspot cycle has something to do with winding and twisting of the Sun's magnetic field. • Sunspot cycle seems to affect climate on earth - Little Ice Age (1645 to 1715) • Unusually low temperature were detected in Europe • Solar activity was exceptionally low
Binary star systems
• The orbit of a binary star system depends on strength of gravity.
Energy Balance:
• The rate at which energy radiates from the surface of the Sun must be the same as the rate at which it is released by fusion in the core
Types of Binary Star Systems
• Visual binary • Spectroscopic binary • Eclipsing binary - About half of all stars are in binary systems.
Mass and Lifetime
•Sun's life expectancy:10 billion years [Until core hydrogen (10% of total) is used up] • Life expectancy of 10M_Sun star: - 10 times as much fuel, uses it 10^4 times as fast - 10 million years~ 10 billion years * 10/104 • Life expectancy of 0.1M_Sun star: - 0.1 times as much fuel, uses it 0.01 times as fast - 100 billion years~ 10 billion years * 0.1/0.01
Photosphere:
•Visible surface of Sun ~ 6000 K