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What is the diameter of the sun?
109
How did the sun begin?
As a cloud of gas undergoing gravitational collapse (gravity squeezing towards the center)
This caused the core of the sun to get hot and dense enough to start nuclear fusion reactions
The fusion reactions generated energy which provided an outward pressure
perfectly balances the inward force of gravity
What is gravitational equilibrium?
The pressure of balance, of inward force, of gravity and outward force of fusion (means equal)
Photosphere
the sphere of light
visible surface of sun
most visible light comes from here
5,800 K
composition of the sun
We know it by identifying the absorption lines in the sun’s spectrum
3/4 hydrogen
1/4 helium
what is absorption lines
They are observed as dark lines or gaps in an otherwise continuous spectrum of electromagnetic radiation.
certain colors are characteristics of different elements
Light is the cosmic messenger
Gives us information about what is the body temperature or objects far away and what is the object’s composition
Matter in the universe interacting with light leaves its fingerprints in the light
The study of the way in which atoms absorb and emit electromagnetic radiation
what is spectroscopy?
The study of the way in which atoms absorb and emit electromagnetic radiation
the process of dispersing light into its spectrum (different wavelengths)
allows astronomers to determine the chemical composition of stars
Dispersion of light
splitting of light into component colors
Electromagnetic spectrum
radio (lowest energy)
infrared
visible
ultraviolet
x-ray
gamma ray (highest energy)
The E M spectrum
The visible part of the spectrum runs from
blue (violet) light
red light
radiation with a wavelength longer than that of red light is infrared and that with wavelength shorter than blue (violet) light is known as ultraviolet
types of spectra
Continuous: a smooth, uninterrupted distribution of electromagnetic radiation across a range of wavelengths or frequencies. It spans a broad range of colors or energies, without any distinct dark or bright lines.
Emission line: also known as a bright-line spectrum, consists of bright, discrete lines against a dark background. It occurs when atoms or molecules emit light at specific wavelengths as they transition from higher energy levels to lower ones. Each line corresponds to a specific energy transition in the atom or molecule. Emission line spectra are characteristic of hot, low-density gases or excited substances.
Absorption line: also called a dark-line spectrum, appears as a continuous spectrum with dark lines or gaps at specific wavelengths. It occurs when a continuous spectrum passes through a cooler, lower-density medium, such as a gas cloud or a cool atmosphere. Atoms or molecules in the medium absorb certain wavelengths of light, resulting in dark lines at those wavelengths. The positions of the absorption lines correspond to the wavelengths absorbed by specific elements or compounds. Absorption line spectra are commonly observed in the spectra of stars, where the star's atmosphere absorbs certain wavelengths of light emitted from its interior, producing dark lines in the spectrum.
Continuous spectrum
spans all visible wavelengths without interruption
continuous spectra are observed from hot, dense objects due to the motion and collisions of particles
Absorption line spectrum
atoms are absorbing certain wavelengths of light, that’s why you don’t see them in the rainbow (black lines)
Thermal radiation
nearly all large or dense objects emits it
the radiation emitted by an opaque object is called blackbody radiation
a blackbody’s continuous thermal radiation spectrum depends on only one property
it’s temperature
Properties of thermal radiation
Stefan-Boltzman Law: hotter objects emit higher energy than cooler objects
Wein’s law: hotter objects emit photons with a higher average energy. The wavelength of peak intensity decreases (shifts towards blue) as the temperature increases
the thermal spectrum can tell us the temperature of a star
Which is hotter?
Blue star, red star, or a planet that emit only infrared light
Blue star
Emission line spectrum
A thin or low-density cloud of gas when HEATED emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines
absorption line spectrum
a cloud of gas between us and a light source can absorb light of a specific wavelengths, leaving dark absorption lines in the continuous spectrum
chemical fingerprints in light
each type of atom has a unique spectral fingerprint of absorption or emission lines
observing the fingerprints in a spectrum tells us which kinds of atoms are present
atom
building block of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons
ground state
the lowest energy state that an electron can have within an atom
excited state
state of an atom when one of its electrons is in a higher energy orbital than the ground state. Atoms can become excited by absorbing a photon of a specific energy, or by colliding with a nearby atom
ionized
state of an atom that has had at least one of its electrons removed
The Bohr model of the hydrogen atom
an electron circles the nucleus (proton) only in allowed orbits
n= 1, 2, 3
where n= 1 is the smallest orbit
when the electron is in the n-1 orbit (closest to the nucleus) the atom is said to be in the ground state which represents the “normal” state
this is the lowest energy state
Number of protons determines what?
which element it is
EX: helium has 2 protons, carbon has 6 proton
Neutral atom
same number of protons and electrons
Protons and Neutrons roughly have the same mass, while electrons mass is
2000 times less
Planets are kept in their respective orbits by what?
gravitational force
electrons keep orbiting ____ due to the attractive electric force between the protons and the electrons
the nucleus
By gaining or losing the correct amount of energy, an electron can do what?
Can jump from one orbit to another (the Bohr model of the atom)
to jump from an inner to an outer orbit, the electron must absorb a specific amount of energy
electron must emit (lose) a specific amount of energy to jump from an outer to an inner orbit
The number of protons in the nucleus is unique to each element (T/F)
True
each kind of element has its own pattern of permitted orbits
Electron shells
Definition: regions of space surrounding an atomic nucleus where electrons are found. They represent the energy levels at which electrons can exist within an atom. Electrons occupy these shells based on their energy and distance from the nucleus.
when the electron is in a higher orbit (n=2, 3…) the atom is said to be in an excited state
when an atom absorbs energy beyond a certain maximum value, the electrons(s) is(are) no longer bound to the nucleus, and can roam free
the atom has one (or more) less electrons(s)
atom is positively charged
such an atom is called an ion
The excitation of atoms
Definition: the process by which electrons in an atom gain energy and move to higher energy levels or orbitals. This excitation can occur through various mechanisms, such as absorption of photons, collisions with other particles, or exposure to external energy sources.
the shorter wavelength (higher-energy, bluer) photons can excite the electron to higher levels
a photon with too much or too little energy cannot be absorbed
as the hydrogen atom has many more energy levels than shown, it can absorb photons of many different wavelengths
the wavelengths (colors) emitted and absorbed by leaping electrons are determined not by starting or ending energy level of the jump, buy by the difference between the levels
Absorption of light by atoms
The atom can absorb the energy from the photons if the energy of the light matches the difference in energy between the atom's energy levels
When a photon with the right amount of energy comes along, an electron in the atom can jump up from a lower step to a higher one
Emission of light by atoms
When atoms get excited, they absorb energy from their surroundings. This can happen when they collide with other particles or when they absorb light.
As the excited atom tries to calm down, it releases the extra energy it gained. This release of energy is emitted as light (like a little burst of energy in the form of light)
The emitted light has a specific color, and the color depends on the type of atom and the amount of energy released. Different atoms release light of different colors.
In hydrogen atom, the electron jumps from where?
from orbit 3 to 2 by emitting a photon
the energy of the photon emitted exactly equals the difference in energy between the 2 orbits
Each type of atom, ion and molecule has a unique ladder of energy levels that electrons can occupy (T/F)
True
the only allowed changes in energy are those corresponding to a transition of an electron between energy levels
Each transition of an electron between energy levels corresponds to what?
A unique photon energy, frequency, and wavelength
downward transitions produce a unique pattern of emission lines for each atom/ion/molecule
because those atoms/ions/molecules can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelength
chemical fingerprints in light
every atom produces a specific set of lines that we can recognize and tell exactly which element it is
how does light tell us what things are made of?
electrons in atoms have distinct energy levels
each chemical element, ion, molecule, has a unique set of energy levels
Stefan-Boltzmann Law: Temperature-Energy relation
from everyday experience we know that the higher the temperature of an object, the higher the total amount of energy radiated
this express by the following relation:
total energy radiated per sec E = σ x T4
Why is the sky blue during daytime?
Air molecules scatter blue light more effectively than other wavelengths
small particles scatter blue light more effectively
air molecules are smaller than visible light they scatter blue light
Why is the sky red during sunset?
sunlight has to travel through a larger portion of the Earth's atmosphere. The longer red and orange wavelengths are less scattered by the particles in the atmosphere and are able to reach our eyes more directly. This gives the sky a reddish or orange hue during those times.
blue wavelengths are more stretched out, leaving more room for other wavelength colors to make their appearance
From spectra, astronomers learn
temperature of a hot body (continuous spectrum)
elemental and molecular composition of stars/hot clouds or absorbing gas/dust clouds (emission and absorption spectrum)
relative velocity of approach or recession (doppler shift)
A neon light produces which type of spectrum?
emission
quantum leap
electron jumping from one energy level to another
how is intensity of light look like from a glowing body?
intensity peaks at a certain wavelength and tapers off on both sides
What is the Doppler effect?
the change in the frequency of a wave caused by relative motion between the source of the wave and the observer
Doppler shift tells us only what?
about the part of an object’s motion toward or away from us
I measure a line at the lab at 500.7 nm
the same line in a star has a wavelength 502.8 nm
what can I sat about this star?
It is moving away from me
Doppler effect summary
blueshift (shorter wavelength) → motion toward you
redshift (longer wavelength) → motion away from you
greater shift → greater speed
Inner parts of the sun
core
radiative zone
convective zone
outer parts of the sun
photosphere
chromosphere
corona
Sun’s energy source
the sun is the main source of light and heat in the solar system
without the light (energy) from the sun, there would be no life on earth
emits radiation of all wavelength, with peak emission in the visible region of the EM spectrum
The sun closely approximates a blackbody with a surface temperature of what?
5,800 K
How do we know the sun’s surface temperature?
Wein’s law (wavelength of frequency)
what is luminosity?
one of the basic properties used to characterize stars, defined as the total energy radiated by a star each second, at all wavelengths
solar winds
a flow of charged particles from the surface of the sun
corona
outermost layer of solar atmosphere
chromosphere
middle layer of solar atmosphere
photosphere
visible surface of sun
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
Sun’s energy source
The sun is powered by nuclear fusion
Albert Einstein’s special theory of relativity predicted that matter can be converted to energy according to the equation
E= m c 2
what type of process will convert mass into energy?
thermonuclear fusion: fusing together of two light nuclei to form a heavier nuclei
nucleus 1+ nucleus 2 → nucleus 3 + energy
when particles come together under strong nuclear force and untie to form nuclues
energy is released
strong nuclear force
positive protons in the nucleus would repel to electrical force, but there is a stronger force that acts at this level (short range) and attract “nucleons”
The number of protons determine what
the element
neutral atom
same amount of protons and electrons
isotopes
same element- same number of protons (and electrons) BUT different numbers of neutrons
EX: the 3 isotopes of hydrogen have the same number of protons but different number of neutrons
what do isotopes differ by, what is the same in 3 isotopes of the element
protons, the number of protons is the same
what is different in isotopes?
the number of neutrons
thermonuclear fusion
can take place only at extremely high temperature and pressure:
under these conditions atoms are completely ionized
in the core of the sun there are
the proton-proton chain OR hydrogen burning: series of nuclear reactions that convert hydrogen nuclei (protons) into helium nuclei, releasing energy in the process
no atoms, like a soup
Thermonuclear reactions in the core of the sun produce its energy
4H → He + energy + v
(hydrogen fusion)
antimatter
has particles with the same mass as protons, neutrons, and electrons but with opposite charges.
antimatter of electron is called what?
positron (same mass, opposite charge)
photons come from where?
positron and electron joining
what is positron?
anit electron
The sun has enough hydrogen to keep burning for how long?
5 billion years
how long has the sun existed?
4.5 billion years
temperature needed of fusion for hydrogen to fuze into helium
has to be greater than 10 to the power of 7 kelvin (roughly 10 to 15 million degrees)
The temperature of the sun’lls visible surface
5,800 K
why is the surface of the sun colder than the core?
prediction is that surface is so cold that fusion has to occur in the very core
how is energy produced at the core of the sun come out?
astronomers use laws of physics to construct theoretical models
sun doesn’t go any dramatic changes
it’s not expanding or collapsing
it’s not significantly cooling or heating up
hydrostatic equilibrium
balance between the forces of gravity and pressure within the sun or other gaseous planets
perfect amount of pressure and gravity that it doesn’t collaspe in on itself
what would happen inside the sun if a slight rise in core temperature led to a rapid rise in fusion energy?
the core would expand and cool
thermal equilibirum
a state where two or more objects or systems are at the same temperature and there is no net transfer of heat between them.
the temperature is constant, it doesn’t change with time
2 mechanisms where energy is transported in the sun
convection: circulation of gases (fluids) between hot and cold regions
hot gases rise to the surface and the cooler gases sink to the interior
radiative diffusion: photons created in the core diffuse outwards
in and near the core, the atoms are stripped off their electrons because of extremely high temperature
they can’t capture photons
the results is a slow migration of the photons towards the surface
we learn about the sun by
making mathematical models
running computer simulations and testing the results against actual observations
observing solar vibrations
inner parts of the sun
core: where energy is produced (thermonuclear fusion)
15 million kelvin
convective zone: the temperature is low enough for nuclei to join with electrons and form hydrogen atoms, and these absorb light very efficiently
gases are opaque to light, thus convection is the transportation mechanism
radiative zone: transparent to EM radiation
energy is carried away from core as electromagnetic radiation (photons) by the radiative diffuison mechanism
helioseismology
measuring vibrations of the sun as a whole
methods of probing the interior of the sun
solar neutrinos: the only direct evidence of the thermonuclear reaction at the core
only the neutrino survives the journey through the solar interior
has energy but no charge and almost no mass
travels almost at the speed of light and interacts with nothing: goes right through earth
photosphere
surface of sun that we see. radiates energy as continous spectrum (5,800 k)
lowest of all 3 layers
all visible light that we see is emitted by this layer
emits radiation like a nearly perfect blackbody at a temperature of 5,800 k
is heated from below by the energy streaing out from the solar interior
low density gas, primarily hydrogen, and helium
know this by identifying the absorption lines in the sun’s spectrum
chromosphere
low density gases form “atmosphere”- red color comes from hydrogen emission line
corona
outer part of atmosphere- extremely hot
solar winds
consists of mainly electrons, hydrogen ions, and helium ions, they interact with earth’s magnetic field causing aurora’s (aka northern lights!)
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