Astro 101 Final

Sun's Core

Sun's Core

- The inner most layer
- where energy is made via nuclear fusion reactions
- highest temperature
- smallest radius
- central region where hydrogen atoms fuse to form helium, releasing energy in the process
- 10-15 million Kelvin
- energy from core takes 100,000 years to reach the surface

Sun's Composition

Hydrogen (90%), Helium (9%), Metals (1%)

Helioseismology

pressure (sound) waves move through the Sun, making surface and interior waves
- Doppler shifts give the speed of wave motion
- Speeds depend on Sun's composition and depth of the convection zone

Sun's distance from Earth

1 AU (8 light minutes)

Radiative Zone

- the zone of the sun's interior that is between the core and the convective zone and in which energy moves by radiation (radiative transfer)
- Energy is absorbed, emitted, and deflected by matter in unpredictable directions
- moves outwards through this zone through radiation
- energy transported outward by photons (random)
- Hotter photons move out from the core

Convective Zone

- a region of the sun where gases circulate in convection currents, bringing the sun's energy to the surface
- Moving matter physically transports energy outward
- Energy is transported to this zone through movement of hot plasma, creating convection currents
- lifetime of convective cells = 5-10 mins
- hot gas rises, cool gas sinks

Granulation

The observable consequence of convection
- bright blobs = hot gas comes to surface
- Dark = cooler = sinking

Photosphere

the visible surface of the sun
- where most photons escape
- depth = ~ 500km
- Temp = ~ 5800K
- Highly opaque (H- ions)
- absorbs and reemits radiation produced in the solar interior

Chromosphere

The middle layer of the sun's atmosphere
- right above photosphere
- notable for its emission of H-alpha spectral lines
- indicates presence of hydrogen
- about 10^4 - 10^5 K
- temp increases with height gradually ( ~4500K - ~10,000K)
- visible during a total solar eclipse, the pink color comes from hydrogen (H-alpha line)

Transition Zone

Marks rapid increase in temp in a short distance

Corona

- The outer layer of the sun's atmosphere.
- low density
- hottest layer of Sun's atmosphere (~1 million Kelvin!)
- highly active and is influenced by the Sun's magnetic fields
- visible during a total solar eclipse (as well as the chromosphere)

Solar Wind

- A flow of charged particles (ions) from the surface of the Sun
- emanates from the corona, streams outward into space, carrying with it the Sun's magnetic field
- Velocity = 300 to 800 km/s = 186 to 497 miles/s

Amount of mass sun loses per year

10^7 tons/year
(~10^-14 of its mass per year)

Sun Spots

- Darker regions with strong magnetic fields where convection is slightly inhibited near photosphere
- cooler than other parts of the Sun's surface
-Magnetic field in sun spots is about 1000x stronger than average
- come in pairs

Magnetic Field

Originate in the Sun's convective layer
- plays crucial role in governing solar activity

Coronal Holes

Believed to be where solar winds originate
- areas of lower density and temperature in the Sun's corona

Solar Flares

sudden and intense bursts of energy, x-rays, charged particles and radiation
- highly energetic and violent
- caused by the release of magnetic energy stored in the Sun's atmosphere.

Limb Darkening

This phenomenon is observed at the edges (limbs) of the Sun. - The intensity of light appears dimmer compared to the center.
- "Limb darkening" is considered evidence of the Sun's atmosphere.
- This phenomenon showcases its layered structure and varying optical properties.

As energy moves out from the Sun's core toward its surface, it first travels by _______, then by _______, and then by _______.

radiation; convection; radiation

Solar Spicules

These are fast-changing and rising columns of gas or plasma observed in the Sun's chromosphere.
- Represent the dynamic activity in the Sun's atmosphere.
- filaments of cooler gas from the photosphere, rising up into the chromosphere
- Visible in H-alpha emission
- gives a reddish emission-line spectrum
- each one lasts about 5-15 minutes

Prominences

These are outflows of gas or plasma from the Sun's surface that follow the Sun's magnetic field lines.
- Often appear as bright loops/arcs extending from the Sun's surface into the corona.

Hydrostatic Equilibrium

Refers to the balance between the inward gravitational force and the outward pressure force within the Sun.
- Ensures that the Sun maintains its shape and structure over time.
- Pressure = gravity

Energy Generation

The primary mechanism through which the Sun produces energy is nuclear fusion.
- Process involves the fusion of hydrogen nuclei (protons) into helium nuclei, releasing immense amounts of energy in the form of gamma rays.

Energy Transport

Energy generated in the Sun's core needs to be transported to its surface and beyond. This transport occurs through two main mechanisms: Radiative and Convective

Radiative Transport

In this process, energy is carried by photons (electromagnetic radiation) as it travels outward from the core through the radiative zone.

Convective Transport

In regions where the temperature gradient is steep enough, such as the convective zone, energy is transported through the physical movement of hot plasma (convection currents).

The temperature and density of the Sun change abruptly at the interface between

the chromosphere and the corona

Sunspots, flares, prominences, and coronal mass ejections are all caused by

magnetic activity on the Sun

Layers of the Sun from highest to lowest temperature

1. Core
2. Radiative Zone
3. Convective Zone

Layers of the Sun from lowest to highest density

1. Convective Zone
2. Radiative Zone
3. Core

Regions of the Sun in order of smallest radius to largest radius

1. Core
2. Radiative Zone
3. Convective Zone
4. Photosphere
5. Chromosphere
6. Corona

How far is the nearest star other than the Sun?

4.3 LY (300,000 times farther than the Sun: Proxima Centauri

Order these in shortest to longest travel time: Core photon, Neutrino, Coronal Mass Ejection, Photosphere photon

Shortest to Longest:
1. Photosphere Photon
2. Neutrino
3. Coronal Mass Ejection
4. Core Photon

Neutron Star

- the small, dense remains of a high-mass star after a supernova
- mass is high enough to break through electron degeneracy, collapse, and then stop collapsing because of the stronger neutron degeneracy pressure
- Neutron degeneracy pressure, in turn, is broken if the total mass of the neutron star is over 3 times the mass of the Sun.

Star Mass Ranges

Low Mass Stars: less than 2 Msun

- long evolution times because they never become hot enough to fuse carbon nuclei, and end as white dwarfs

Intermediate Stars: between 2 & 8 Msun

- can make elements heavier than carbon but end as white dwarfs

High Mass Stars: greater than 8 Msun

- faster nuclear fusion

- burn fuel more quickly an have shorter life-spans

- hot enough to make iron, and end in supernova explosions

High Mass Star Evolution

Nebula - M-S - Red Supergiant - Supernova - Neutron Star OR Black Hole

- they start out the same until the hydrogen is gone
- Hydrogen core fusion (main sequence)
- they use CNO cycle NOT proton proton cycle
- Red Giant Phase: hydrogen shell burning (supergiant) no helium flash
- as they expand and cool, they can pass through the instability strip ( the star acts like a piston)

Variable Stars

A star that varies significantly in brightness with time
- the star tries to achieve proper balance between power welling up from the core and power radiated from the surface (gravity-pressure tug of war)
- alternately expands and contracts, varying in brightness as it tries to find a balance
- period of variability of pulsating star is correlated with its luminosity (measuring the period can determine its absolute magnitude, which tells us the distance

Our knowledge of stellar evolution comes from comparing ___________ ______ of stars with ____________

mathematical models; observations

A pulsar occurs when a neutron star...

has a magnetic axis that is not aligned with the rotation axis

Pulsar

A rapidly spinning neutron star that produces radio waves
- each has a unique rate of change of spin, spin rate, and location

Elements heavier than iron originated from the...

explosions of high-mass stars

In a high-mass star, hydrogen fusion occurs via the ___ cycle

CNO

White Dwarf

- Corpses of low-mass stars
- kept from collapsing under their own gravity by the outward pressure of electron degeneracy
- white dwarfs with the same mass as the Sun are about the same size as Earth
- Higher mass white dwarfs are smaller (the more massive, the smaller the star is)
- More mass >> more contraction >> higher density >> more degeneracy pressure

The White Dwarf Limit

the maximum possible mass for a white dwarf, which is about 1.4Msun
- also known as the Chandrasekhar limit

Brown Dwarf

- Stars that lack sufficient mass and so do not achieve nuclear fusion
- first theorized in early 1960s

Black Hole

The result of an object whose gravity is so powerful that not even light can escape
- gravity crushes all the matter into a single point known as singularity
- stars mass must be > 3 Msun
- black hole's mass strongly warps space and time in vicinity of event horizon

Parts of a black hole

- singularity
- event horizon
- normal space

Event Horizon

- the location (spherical circle) around a black hole where the escape velocity equals the speed of light; the boundary of a black hole
- the hole's size is related the location of the event horizon
- the radius of the event horizon is known as the Schwarzschild radius

Stellar Parallax

The apparent shift in position of a nearby star against the background of more distant stars as the Earth orbits the Sun
- astronomers can triangulate its distance by observing a star's position relative to background stars at different times of the year
- observations taken six months apart when Earth is on opposite sides of its orbit
- The amount of shift observed is directly related to the distance to the star
- the closer the star, the larger the apparent shift

Apparent Magnitude

- A measure of how bright a celestial object appears from Earth
- Influenced by luminosity (intrinsic brightness) and distance
- Uses logarithmic scale: smaller numbers = brighter objects

Absolute Magnitude

-Measure of the luminosity of a celestial object
- Represent brightness at a standard distance of 10 parsecs (~32.6 LY) from Earth
- Allows for comparison of true brightness of different objects regardless of distance

The steps of the evolution of a low-mass star

1. A clump forms in a giant molecular cloud then a protostar forms
2. Hydrogen fusion begins
3. the star moves onto the red giant branch then a helium flash occurs.
4. The star moves onto the horizontal branch
5. The star moves onto the asymptotic giant branch then the star sheds mass producing a nebula
6. the white dwarf cools

Use in Distance Estimation

-Enables estimation of distance to celestial objects if apparent and absolute magnitudes are known.
- Apparent and absolute magnitudes provide key data for this estimation.

Wien's Law

Blue stars = hotter
Red stars = cooler

Stefan-Boltzmann Law

A square meter of a hot star's surface radiates more energy than a square meter of a cool star's surface... so a hot star is more luminous than a cool star of the
same size.

How to find luminosity of a star

brightness and distance
Luminosity = energy/time

How to find temperature of a star

the color! (red = cooler, blue = hotter)

Types of stars in order of hottest to coldest (hint: silly acronym!)

O, B, A, F, G, K, M
"Oh Be A Fine Girl, Kiss Me!"
O = hotter than 25,000K
B = 11,000 to 25,000K
A = 7500 to 11,000K
F = 6000 to 7500K
G = 5000 to 6000K
K = 3500 to 5000K
M = cooler than 3500K

The Harvard "Computers"

Group of women hired by Harvard College Observatory in the late 19th and 20th century to process astronomical data
- Performed complex calculations and analysis, including the classification of stellar spectra
- Notable: Annie Jump Cannon, Williamina Fleming, Antonia Maury, and Henrietta Swan Leavitt
- crucial for developing classification system for stars based on spectral characteristics, which laid the foundation for our understanding of stellar evolution

Lower Mass Limit

Objects need about 0.08 times the Sun's mass (~ 80 times Jupiter's mass) to start fusion and become stars. Below this they are considered brown dwarfs

Upper Mass Limit

Stars can have masses between 60 and 100 times that of the Sun at most, beyond this they become unstable

Mass Limits help astronomers understand the range of:

- stellar masses observed in the universe
- The processes governing their formation evolution and ultimate fate

H-R Diagram

a plot of stars' luminosity (brightness) versus their surface temperature (color), or alternatively, their spectral type
- a fundamental tool in astronomy for understanding stellar properties and evolution
- can tell you how old a star cluster is

H-R Diagram General Trends

Main Sequence:The majority of stars fall along a diagonal band called the main sequence. This is where stars like our Sun spend most of their lives, fusing hydrogen into helium in their cores
Luminosity: Stars' luminosities increase as you move up the diagram, from lower luminosity (dimmer) stars at the bottom to higher luminosity (brighter) stars at the top.
Giants and Supergiants: Stars that have exhausted their core hydrogen fuel expand into giants and supergiants, appearing above and to the right of the main sequence
White Dwarfs: Once a star exhausts its nuclear fuel, it sheds its outer layers and leaves behind a hot, dense core called a white dwarf. These are found at the lower-left portion of the HR diagram.
Temperature: Surface temperature decreases as you move from left to right along the main sequence. Hotter stars are found on the left side of the diagram, while cooler stars are on the right

Star Clusters

- gravitationally bound groups of stars
- useful because they contain stars of different mass that formed about the same time, located at around the same distance, and evolve from the same nebula thus having the same composition
- creates a time-dependent story
- a cluster's H-R diagram will change as it evolves over time

Star moves off the main sequence when there is no more ________ in the core and the core is ______

hydrogen; helium

Electron Degeneracy

a state where electrons are very tightly packed together

Light

electromagnetic radiation made up of photons (particles) that travel in waves
- used as the primary tool by astronomers to explore and understand the universe by giving insights into its vastness, complexity and evolution

Atomic Structure

Electron on outer shell, neutron and proton in center, nuclei
- Nuclei: protons and neutrons
- Neutrons: no electrical charge
- Protons: positive electrical charge
- Electrical forces push protons apart
- The strong nuclear force binds protons together

Fusion requires ramming _______ together at ____ speed (i.e. at high temperature)

protons; high

Helium Fusion

- requires higher temps than hydrogen fusion, larger charge leads to greater repulsion
- occurs in the inner shell around the core
- fuses to carbon via the triple-alpha process
- must combine three He nuclei to make carbon, two doesn't work
- Keeps going until the core is all Carbon and then the core becomes electron-degenerate again

Helium Flash

- only occurs with stars less than about 2 Msun
- none for those greater than 2 Msun because their core is not degenerate
- happens when the temps get high enough for helium to start fusing, the onset is called the helium flash

After Helium Flash...

- Helium fuses to Carbon in the core
- Hydrogen is fusing to Helium in a shell around the core
- The star is smaller and hotter and luminosity increases
- now radiation in the core is balancing gravity and the envelope of star is contracting

Planetary Nebulae

- the star expels its outer layers and leaves behind the exposed inert carbon (electron degenerate core)
- leftover core of star remains as a white dwarf
- the white dwarf will be primarily carbon and oxygen because the low mass star never grows hot enough to produce (fuse) heavier elements and then cool off since no nuclear fusion is occurring
-the star will ionize the gas in the expanding outer layers
- will last for about 50,000 years before the gas expands too far and disperses

Neutrinos

- light atomic particles, no charge
- very weak interactions with matter
- should escape the core freely
- emitted by hydrogen fusion
- can penetrate huge amounts of material without being absorbed
- theory predicts about once a day a solar neutrino will convert a chlorine atom in the tank into radioactive argon (this is how they are detected)

Fusion vs Fission

Fusion - small nuclei stick together to make a bigger one.
Fission - big nucleus splits into smaller pieces.
High temps enable nuclear fusion to happen in the core by overpowering the repulsion between atoms.

Overall reaction (input and output)

Start with: 4 protons
Finish with:
- He4 nucleus
- 2 gamma rays
- 2 positrons
- 2 neutrinos
- Lots of energy!

Short summary of atomic physics

- Slow moving protons approach each other but are pushed apart by electric repulsion
- but the faster they are going, the closer together they can get
- at high enough temperatures, protons may move rapidly enough to overcome the electric repulsion and fuse...
- ... releasing energy and leaving behind a deuterium nucleus, a positron, and a neutrino

Light Spectra (3 Types)

Continuous, emission, and absorption

Continuous Spectra

A spectrum of light that contains all wavelengths without interruption or gaps, typically produced by incandescent solids, liquids, or dense gasses

Emission Spectra

These spectra arise when electrons in atoms transition from higher energy levels to lower energy levels, emitting photons of specific wavelengths characteristic of the

Absorption Spectra

When light passes through a cold gas, it can be absorbed by the electrons, causing them to transition to higher energy levels. This results in dark lines (absorption lines) in the spectrum, corresponding to the wavelengths of light that were absorbed.

Sun's Spectrum

- Cooler outer layer absorb some of the light from hotter, deeper layers
- this produces a complex absorption spectrum with more than 70 elements

The Roche Limit

the critical distance inside which a large moon will be pulled apart by tidal forces
- for giant planets in our Solar System, the Roche Limit explains how RINGS form