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Our Star Why does the Sun shine?

Gravitational equilibrium:  Energy supplied by fusion maintains the pressure that balances the inward crush 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.

Gravitational contraction:  Provided the energy that heated the core as Sun was forming  Contraction stopped when fusion began. What is the Sun’s structure?

Radius: 6.9  108 m (109 times Earth) Mass: 2  1030 kg (300,000 Earths) Luminosity: 3.8  1026 watts

What is the Sun’s structure?

Solar wind: A flow of charged particles from the surface of the Sun Corona: Outermost layer of solar atmosphere ~1 million K Chromosphere: Middle layer of solar atmosphere ~ 104–105 K Photosphere: Visible surface of the Sun ~ 6000 K Convection zone: Energy transported upward by rising hot gas Radiation zone: Energy transported upward by photons Core: Energy generated by nuclear fusion ~ 15 million K

 Parts of the Sun. This illustration shows the different parts of the Sun, from the hot core where the energy is generated through regions where energy is transported outward, first by radiation, then by convection, and then out through the solar atmosphere. The parts of the atmosphere are also labeled the photosphere, chromosphere, and corona. Some typical features in the atmosphere are shown, such as coronal holes and prominences. (credit: modification of work by NASA/Goddard)

 The Sun’s Atmosphere. Composite image showing the three components of the solar atmosphere: the photosphere or surface of the Sun taken in ordinary light; the chromosphere, imaged in the light of the strong red spectral line of hydrogen (H-alpha); and the corona as seen with X-rays. (credit: modification of work by NASA)

 Temperatures in the Solar Atmosphere. On this graph, temperature is shown increasing upward, and height above the photosphere is shown increasing to the right. Note the very rapid increase in temperature over a very short distance in the transition region between the chromosphere and the corona.

What have we learned?

 Why does the Sun shine? — Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years. — The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

What have we learned?

 What is the Sun’s structure? — From inside out, the layers are: ▪ Core ▪ Radiation zone ▪ Convection zone ▪ Photosphere ▪ Chromosphere ▪ Corona

How does nuclear fusion occur in the Sun?

How does nuclear fusion occur in the Sun?

High temperatures enable nuclear fusion to happen in the core.

How does nuclear fusion occur in the Sun?

The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.

How does nuclear fusion occur in the Sun?

Proton–proton chain is how hydrogen fuses into helium in the Sun.

How does nuclear fusion occur in 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. The solar thermostat keeps burning rate steady. Solar Thermostat

How does the energy from fusion get out of the Sun? How does the energy from fusion get out of the Sun?

 Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons.

a) Convection (rising hot gas) takes energy to surface. b) Bright blobs on photosphere show where hot gas is reaching the surface.

 Granulation Pattern. The surface markings of the convection cells create a granulation pattern on this dramatic image (left) taken from the Japanese Hinode spacecraft. You can see the same pattern when you heat up miso soup. The right image shows an irregularshaped sunspot and granules on the Sun’s surface, seen with the Swedish Solar Telescope on August 22, 2003. (credit left: modification of work by Hinode JAXA/NASA/PPARC; credit right: ISP/SST/Oddbjorn Engvold, Jun Elin Wiik, Luc Rouppe van der Voort)

 Coronal Hole. The dark area visible near the Sun’s south pole on this Solar Dynamics Observer spacecraft image is a coronal hole. (credit: modification of work by NASA/SDO) How do 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

Solar Vibrations

 Patterns of vibration on the surface tell us about what the Sun is like inside.

Solar Vibrations

 Data on solar vibrations agree with mathematical models of solar interior.

 Photon and Neutrino Paths in the Sun. (a) Because photons generated by fusion reactions in the solar interior travel only a short distance before being absorbed or scattered by atoms and sent off in random directions, estimates are that it takes between 100,000 and 1,000,000 years for energy to make its way from the center of the Sun to its surface. (b) In contrast, neutrinos do not interact with matter but traverse straight through the Sun at the speed of light, reaching the surface in only a little more than 2 seconds.

Solar Neutrinos

 Neutrinos created during fusion fly directly through the Sun.  Observations of these solar neutrinos can tell us what’s happening in the core.

Solar Neutrinos

 Solar neutrino problem:  Early searches for solar neutrinos failed to find the predicted number.  Solar neutrino problem:  Sudbury Neutrino Observatory (SNO)  More recent observations find the right number of neutrinos, but some have changed form.

What have we learned?

 How does nuclear fusion occur in the Sun? — The core’s extreme temperature and density are just right for the nuclear fusion of hydrogen to helium through the proton–proton chain. — Gravitational equilibrium acts as a thermostat to regulate the core temperature because the fusion rate is very sensitive to temperature.  How does the energy from fusion get out of the Sun? — Randomly bouncing photons carry it through the radiation zone. — The rising of hot plasma carries energy through the convection zone to the photosphere.  How do we know what is happening inside the Sun? — Mathematical models agree with observations of solar vibrations and solar neutrinos.

Solar activity is like “weather” on Earth.

Sunspots

 Sunspots  Solar flares  Solar prominences All these phenomena are related to magnetic fields.  Are cooler than other parts of the Sun’s surface (4000 K)  Are regions with strong magnetic fields

Zeeman Effect We can measure magnetic fields in sunspots by observing the splitting of spectral lines.

Charged particles spiral along magnetic field lines.  Loops of bright gas often connect sunspot pairs

 Sunspots. This image of sunspots, cooler and thus darker regions on the Sun, was taken in July 2012. You can see the dark, central region of each sunspot (called the umbra) surrounded by a less dark region (the penumbra). The largest spot shown here is about 11 Earths wide. Although sunspots appear dark when seen next to the hotter gases of the photosphere, an average sunspot, cut out of the solar surface and left standing in the night sky, would be about as bright as the full moon. The mottled appearance of the Sun’s surface is granulation. (credit: NASA Goddard Space Flight Center, Alan Friedman)  Sunspots Rotate Across Sun’s Surface. This sequence of photographs of the Sun’s surface tracks the movement of sunspots across the visible hemisphere of the Sun. On March 30, 2001, this group of sunspots extended across an area about 13 times the diameter of Earth. This region produced many flares and coronal mass ejections. (credit: modification of work by SOHO/NASA/ESA) Solar Flares

 Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space.  Solar Flare. The bright white area seen on the right side of the Sun in this image from the Solar Dynamics Observer spacecraft is a solar flare that was observed on June 25, 2015. (credit: NASA/SDO)

Solar Prominences

 Magnetic activity also causes solar prominences that erupt high above the Sun’s surface.

Corona

 The corona appears bright in X-ray photos in places where magnetic fields trap hot gas.  Coronagraph. This image of the Sun was taken March 2, 2016. The larger dark circle in the center is the disk the blocks the Sun’s glare, allowing us to see the corona. The smaller inner circle is where the Sun would be if it were visible in this image. (credit: modification of work by NASA/SOHO)

How does solar activity affect humans?

 Coronal mass ejections send bursts of energetic charged particles out through the solar system.

 Flare and Coronal Mass Ejection. This sequence of four images shows the evolution over time of a giant eruption on the Sun. (a) The event began at the location of a sunspot group, and (b) a flare is seen in far-ultraviolet light. (c) Fourteen hours later, a CME is seen blasting out into space. (d) Three hours later, this CME has expanded to form a giant cloud of particles escaping from the Sun and is beginning the journey out into the solar system. The white circle in (c) and (d) shows the diameter of the solar photosphere. The larger dark area shows where light from the Sun has been blocked out by a specially designed instrument to make it possible to see the faint emission from the corona. (credit a, b, c, d:

modification of work by SOHO/EIT, SOHO/LASCO, SOHO/MDI (ESA & NASA)) How does solar activity affect humans?

 Charged particles streaming from the Sun can disrupt electrical power grids and disable communications satellites.  Aurora. The colorful glow in the sky results from charged particles in a solar wind interacting with Earth’s magnetic fields. The stunning display captured here occurred over Jokulsarlon Lake in Iceland in 2013. (credit: Moyan Brenn)

How does solar activity vary with time?

 The number of sunspots rises and falls in 11-year cycles.

 Solar Cycle. This dramatic sequence of images taken from the SOHO satellite over a period of 11 years shows how active regions change during the solar cycle. The images were taken in the ultraviolet region of the spectrum and show that active regions on the Sun increase and decrease during the cycle. Sunspots are located in the cooler photosphere, beneath the hot gases shown in this image, and vary in phase with the emission from these hot gases—more sunspots and more emission from hot gases occur together. (credit: modification of work by ESA/NASA/SOHO) How does solar activity vary with time?

 The sunspot cycle has something to do with the winding and twisting of the Sun’s magnetic field.

What have we learned?

 What causes solar activity? — The stretching and twisting of magnetic field lines near the Sun’s surface causes solar activity.  How does solar activity affect humans? — Bursts of charged particles from the Sun can disrupt communications, satellites, and electrical power generation.  How does solar activity vary with time? — Activity rises and falls in 11-year cycles.

Surveying the Stars

How do we measure stellar luminosities? How do we measure stellar luminosities?

 Brightness of a star depends on both distance and luminosity.

The amount of luminosity passing through each sphere is the same. Area of sphere: 4 (radius)2 Divide luminosity by area to get brightness. How do we measure stellar luminosities?

Most luminous stars: 106LSun Least luminous stars: 10−4LSun (LSun is luminosity of the Sun) These two stars have about the same luminosity— which one appears brighter? A. Alpha Centauri B. The Sun

Parallax

 Parallax is the apparent shift in position of a nearby object against a background of more distant objects.  Apparent positions of the nearest stars shift by about an arcsecond as Earth orbits the Sun.  The parallax angle depends on distance. p = p a ra lla x a n g le

d (in p a rs e c s ) =

1 p (in a rc s e c o n d s )

d (in lig h t-y e a rs ) = 3 .2 6 

How do we measure stellar temperatures?

1 p (in a rc s e c o n d s )

 Every object emits thermal radiation with a spectrum that depends on its temperature.  An object of fixed size grows more luminous as its temperature rises. Hottest stars: 50,000 K Coolest stars:

3000 K (Sun’s surface is 5800 K) How do we measure stellar temperatures?

Pioneers of Stellar Classification

 Level of ionization also reveals a star’s temperature.  Absorption lines in a star’s spectrum tell us its ionization level.  Lines in a star’s spectrum correspond to a spectral type that reveals its temperature:  (Hottest) O B A F G K M (Coolest)

Which of the stars below is hottest? A. M star B. F star C. A star D. K star Only buy apples from Grandma Kims mart  Annie Jump Cannon and the “calculators” at Harvard laid the foundation of modern stellar classification.

How do we measure stellar masses?

Types of Binary Star Systems

 Visual binary  Eclipsing binary  Spectroscopic binary About half of all stars are in binary systems.

Visual Binary

 We can directly observe the orbital motions of these stars.

Eclipsing Binary

 We can measure periodic eclipses.

Spectroscopic Binary

 We determine the orbit by measuring Doppler shifts.

How do we measure stellar masses?

We measure mass using gravity. Direct mass measurements are possible only for stars in binary star systems. p = period a = average separation

Need two out of three observables to measure mass:

1) Orbital period (p) 2) Orbital separation (a or r = radius) 3) Orbital velocity (v) For circular orbits, v = 2r/p.

Stellar Masses

Most massive stars: 100MSun Least massive stars: 0.08MSun (MSun is the mass of the Sun.)

What have we learned?

 How do we measure stellar luminosities? — If we measure a star’s apparent brightness and distance, we can compute its luminosity with the inverse square law for light. — Parallax tells us distances to the nearest stars.  How do we measure stellar temperatures? — A star’s color and spectral type both reflect its temperature.  How do we measure stellar masses? — Newton’s version of Kepler’s third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a).

Patterns Among Stars

What is a Hertzsprung– Russell diagram?

An H-R diagram plots the luminosities and temperatures of stars. Hotter on the left

The Main Sequence

 Most stars fall somewhere on the main sequence of the H-R diagram.

Giants and Supergiants

 Stars with lower T and higher L than mainsequence stars must have larger radii. (top right)

White Dwarfs

 Stars with higher T and lower L than mainsequence stars must have smaller radii. (bottom left)

Classifying Stars

 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

Which star is the hottest? Which star is the most luminous? Which star is a main-sequence star? Which star has the largest radius? What is the significance of the main sequence?

ACDC  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.

Stellar Properties Review

Mass and Lifetime

 Luminosity: from brightness and distance 10−4LSun–106LSun  Temperature: from color and spectral type 3,000 K–50,000 K  Mass: from period (p) and average separation (a) of binary-star orbit 0.08MSun–100MSun Sun’s life expectancy: 10 billion years (Until core hydrogen (10% of total) is used up) Life expectancy of a 10MSun star: 10 times as much fuel, uses it 104 times as fast 10 million years ~ 10 billion years ´ 10/104 Life expectancy of a 0.1MSun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years ´ 0.1/0.01

Main-Sequence Star Summary

Off the Main Sequence

Off the Main Sequence

What are giants, supergiants, and white dwarfs?  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.  Giants and supergiants are far larger than main-sequence stars and white dwarfs.

Which star is most like our Sun? Which of these stars will have changed the least 10 billion years from now? Which of these stars can be no more than 10 million years old? BCA

Why do the properties of some stars vary?

Variable Stars

 Any star that varies significantly in brightness with time is called a variable star.  Some stars vary in brightness because they cannot achieve proper balance between power welling up from the core and power radiated from the surface.  Such a star alternately expands and contracts, varying in brightness as it tries to find a balance.

Pulsating Variable Stars

 The light curve of this pulsating variable star shows that its brightness alternately rises and falls over a 50-day period. Cepheid Variable Stars

 Most pulsating variable stars inhabit an instability strip on the H-R diagram.  The most luminous ones are known as Cepheid variables. What have we learned?

 What is a Hertzsprung–Russell diagram? — An H-R diagram plots the stellar luminosity of stars versus surface temperature (or color or spectral type).  What is the significance of the main sequence? — Normal stars that fuse H to He in their cores fall on the main sequence of an H-R diagram. — A star’s mass determines its position along the main sequence (high mass: luminous and blue; low mass: faint and red).  What are giants, supergiants, and white dwarfs? All stars become larger and redder after core hydrogen burning is exhausted: giants

and supergiants. — Most stars end up as tiny white dwarfs after fusion has ceased. Star Clusters What are the two types of star clusters?

How do we measure the age of a star cluster? How do we measure the age of a star cluster?

 Massive blue stars die first, followed by white, yellow, orange, and red stars.

 Pleiades now has no stars with a life expectancy less than around 100 million years.  The main-...