A Detailed Explanation of Stars' Spectra // HSC Physics

Summary

This video explores the electromagnetic radiation spectrum of stars, focusing on the absorption spectrum that reveals a star's chemical composition, surface temperature, and velocities through the Doppler effect. It explains how absorption lines are formed and how they can be used to determine a star's properties, including its density, which affects the width of these lines due to increased pressure and molecular collisions.

Takeaways

• 🌟 Electromagnetic radiation (EMR) from a star's core travels through its outer layers before propagating into space, where elements in gas form absorb certain wavelengths, creating an absorption spectrum.
• 🔬 The script discusses three types of spectra: continuous spectrum, emission spectrum, and absorption spectrum, with a focus on the latter, also known as a stellar spectrum.
• 🌈 If no gases are present to absorb energy, the radiation from stars would form a continuous spectrum, but the presence of gaseous elements in a star's outer layers produces an absorption spectrum.
• 📊 Absorption spectra can provide information on a star's surface temperature, chemical composition, translational and rotational velocity, and density.
• 🌡️ William's Displacement Law relates the peak wavelength (λ max) of a star's spectrum to its surface temperature, with the formula T = b / λ max, where b is Wien's constant (2.898 x 10^-3 m K).
• 🔭 By observing the shift in absorption lines (redshift or blueshift) due to Doppler's effect, we can determine a star's translational velocity towards or away from us.
• 🌌 The broadening of absorption lines in a star's spectrum indicates the star's rotational velocity, as different parts of the star's surface move towards or away from the observer.
• 🧪 The absorption spectrum reveals a star's chemical composition by comparing the star's absorption lines with the characteristic emission spectra of known elements.
• 🌠 The density of a star can be inferred from the breadth of its absorption lines, as higher density leads to increased pressure and more frequent collisions between gas molecules, affecting electron energy levels.
• 🔍 The script emphasizes that the absorption lines in a star's spectrum are crucial for understanding various star features, including temperature, composition, velocity, and density.

Q & A

What is the electromagnetic radiation (EMR) produced by stars known as?

The electromagnetic radiation produced by stars is known as the star's spectrum, which includes different types such as continuous, emission, and absorption spectra.

What causes the absorption spectrum seen in stars?

The absorption spectrum in stars is caused by elements in the form of gases in the outer layers of the star absorbing certain wavelengths or frequencies of EMR as it travels through these layers.

What are the three types of spectra mentioned in the video?

The three types of spectra mentioned are the continuous spectrum, the emission spectrum, and the absorption spectrum, with the latter being the focus of the video.

How can the absorption lines in a star's spectrum be related to the elements present in the star?

The position of the absorption lines in a star's spectrum corresponds to the emission lines of the elements responsible for the absorption, allowing us to identify the elements present in the star.

What is the significance of the peak wavelength (lambda max) in determining a star's surface temperature?

The peak wavelength is used in Wien's displacement law to calculate the star's surface temperature, with the relationship being inversely proportional to the surface temperature.

How is the surface temperature of a star calculated using Wien's displacement law?

The surface temperature (T) is calculated using Wien's constant (2.898 x 10^-3 m*K) divided by the peak wavelength (lambda max) in meters, resulting in the temperature in Kelvin.

What information can be derived from a star's absorption spectrum regarding its chemical composition?

By comparing the absorption spectrum of a star with the emission spectra of various elements, we can identify the elements responsible for the absorption lines, thus determining the star's chemical composition.

How does the Doppler effect influence the position of the absorption lines in a star's spectrum?

The Doppler effect causes a shift in the position of the absorption lines due to the relative velocity between the star and the observer, resulting in either a redshift (moving away) or a blueshift (moving towards).

How can the rotational velocity of a star be inferred from its absorption spectrum?

The rotational velocity affects the absorption lines through the Doppler effect, causing each line to experience both blueshift and redshift, which results in broader absorption lines for stars with faster rotational velocities.

What role does the density of a star play in the appearance of its absorption spectrum?

Higher density stars have greater pressure, leading to more frequent collisions between gas molecules and a wider range of energy absorption by electrons, which results in broader absorption lines in the spectrum.

How does the script summarize the key takeaways about the information that can be obtained from a star's absorption spectrum?

The script summarizes that a star's absorption spectrum provides information on the star's chemical composition, surface temperature (using Wien's displacement law), translational and rotational velocities, and density.

Outlines

00:00

🌟 Understanding Star Spectra and Electromagnetic Radiation

This paragraph introduces the concept of electromagnetic radiation (EMR) emitted by stars and how it interacts with the star's outer layers. It explains the production of an absorption spectrum when gases in the star's atmosphere absorb certain wavelengths of EMR. The video will focus on absorption spectra, also known as stellar spectra, and contrasts it with continuous and emission spectra. The position of absorption lines corresponds to the emission lines of elements, providing a visual representation of the relationship between different types of spectra. The paragraph also discusses how absorption spectra can reveal a star's surface temperature, chemical composition, and velocities through the study of absorption lines.

05:02

🚀 Doppler Effects and Determining Star Characteristics

The second paragraph delves into the Doppler effect's influence on a star's absorption spectrum, which affects the wavelength and frequency of light based on the star's motion relative to Earth. It explains how redshift indicates a star moving away and blueshift indicates a star approaching. The paragraph also covers how the rotational velocity of a star can broaden absorption lines due to the Doppler effect. Furthermore, it discusses how the density of a star, related to its pressure and molecular collisions, can affect the width of absorption lines. The video concludes by summarizing that the absorption spectrum provides insights into a star's chemical composition, surface temperature (via Wien's displacement law), translational and rotational velocities, and density.

Keywords

💡Electromagnetic Radiation (EMR)

Electromagnetic Radiation, or EMR, is a form of energy that travels through space as both electric and magnetic waves. In the context of the video, it is the energy produced in the core of a star that travels through its outer layers before propagating through the universe. The script explains how the absorption of certain wavelengths of EMR by elements in the star's outer layers results in an absorption spectrum, which is a key concept in understanding the star's properties.

💡Absorption Spectrum

An absorption spectrum is a visual representation of the wavelengths of light absorbed by an object, in this case, the gases in a star's outer layers. The video script describes how the absorption spectrum, also known as a stellar spectrum, is characterized by 'small pits' where specific wavelengths have been absorbed, providing crucial information about the star's chemical composition and other properties.

💡Continuous Spectrum

A continuous spectrum is a range of wavelengths emitted by a light source that appears unbroken. The script mentions that if starlight does not pass through any gases that can absorb energy, it would appear as a continuous spectrum, devoid of the 'pits' that characterize an absorption spectrum.

💡Emission Spectrum

An emission spectrum is the range of wavelengths emitted by an excited atom or molecule. The video script explains that the elements in the form of gases in a star's outer layers can produce a specific emission spectrum if the electrons are excited, which is different from the absorption spectrum.

💡Willian's Displacement Law

Willian's Displacement Law is a principle that relates the peak wavelength of the light emitted by a black body to its temperature. The script uses this law to explain how the maximum intensity wavelength, or lambda max, can be used to calculate a star's surface temperature, showing an inverse relationship between the two.

💡Peak Wavelength (Lambda Max)

The peak wavelength, also known as lambda max, is the wavelength at which the maximum intensity of light is observed. The script illustrates how this value is used in conjunction with Willian's Displacement Law to determine a star's surface temperature, with longer peak wavelengths corresponding to cooler stars and vice versa.

💡Chemical Composition

Chemical composition refers to the elements that make up a substance. In the script, the absorption spectrum of a star is used to determine its chemical composition by comparing the star's absorption lines with the emission spectra of known elements, revealing which elements are present in the star's gaseous layers.

💡Doppler Effect

The Doppler Effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. The script describes how the Doppler Effect affects the position of the absorption lines in a star's spectrum, causing redshift when the star is moving away and blueshift when it is moving towards the observer.

💡Redshift and Blueshift

Redshift and blueshift are terms used to describe the shift in the wavelength of light due to the Doppler Effect. The script explains that redshift occurs when a star is moving away from the observer, causing the absorption lines to shift towards the red part of the spectrum, while blueshift occurs when the star is moving towards the observer, shifting the lines towards the blue.

💡Rotational Velocity

Rotational velocity is the rate at which an object spins around its axis. The script explains that a star's rotational velocity affects the Doppler shift of its absorption lines, causing them to broaden due to the combined effects of blueshift and redshift on opposite sides of the star's rotation.

💡Density

Density is defined as mass per unit volume. The script relates density to the pressure within a star, explaining that denser stars have greater pressure, which increases the collision rate between gas molecules. This, in turn, affects the energy levels of electronic orbits and the wavelengths of light absorbed, resulting in broader absorption lines in the star's spectrum.

Highlights

Electromagnetic radiation (EMR) from a star's core travels through its outer layers, producing an absorption spectrum when gases absorb certain wavelengths.

There are three types of spectra: continuous, emission, and absorption (stellar spectrum), with the latter being the focus of the video.

A continuous spectrum is produced by stars without gaseous interference; an absorption spectrum is characterized by 'pits' where specific wavelengths are absorbed.

Absorption spectra can reveal a star's surface temperature, chemical composition, translational and rotational velocity, and density.

Willian's displacement law relates the peak wavelength of a star's EMR to its surface temperature, with a hyperbolic relationship.

The surface temperature of a star can be calculated using its peak wavelength and Willian's constant (2.898 x 10^-3 m K).

A star's absorption spectrum shows the presence of elements by comparing with known emission spectra, determining its chemical composition.

Doppler's effect influences the position of absorption lines based on the star's relative velocity, indicating redshift or blueshift.

Redshift suggests a star is moving away, while blueshift indicates it is moving towards us, providing information on translational velocity.

A star's rotational velocity can be inferred from the broadening of absorption lines due to the Doppler effect.

Denser stars have greater pressure, leading to increased collision rates and broader absorption lines, indicating higher density.

The absorption lines' broadening in a star's spectrum is directly related to its rotational velocity and density.

Stars of different densities show variations in their absorption spectra, with denser stars having broader lines.

The video concludes by summarizing how the absorption spectrum provides insights into a star's various features.

Transcripts

hello everybody this video is on

spectrum of stars electromagnetic

radiation emr for short from the core of

the star where it's been produced will

travel through the outer layers before

it propagates through the universe or

across space

and before it reaches earth elements in

the form of gases in these layers or

absorb certain wavelengths or

frequencies of this emr producing what

we see as an absorption spectrum so i

will review in the spectroscopy video i

talk about three different types of

spectra

a continuous spectrum emission spectrum

and what we're going to be focusing on

this video in the absorption spectrum

this is also known as a stellar spectrum

spectrum from stars so normally the

radiation produces from stars if it

doesn't pass through any gases it will

be continuous because there are no

elements absorbing any amount of energy

from it

the elements in the form gases in the

outer layers of the star will produce

specific emission spectrum if the

electrons were excited and i think this

diagram here is really great because it

allows you to see the relationship

between the three types of spectra the

position of the absorption lines in the

absorption spectrum are identical to the

emission lines of these elements

absorption spectra obtained from stars

can be used to obtain information on the

surface temperature chemical composition

translational and rotational velocity of

the star as well as the density this is

what a star's absorption spectrum looks

like the absorption lines look like

small pits throughout the spectrum

because these wavelengths have been

absorbed by the electrons in the

elements that's present in the stars

the intensity of electromagnetic

radiation emitted by a star

varies with its wavelength the

wavelength at which the maximum

intensity is observed is used in

willian's displacement law to calculate

the star's surface temperature the

wavelength with the maximum intensity is

also known as lambda max

or peak wavelength this peak wavelength

is inversely proportional to the surface

temperature

that is t is equal to a constant v which

is willian's constant that is 2.898

times 10 to the power -3 divided by the

peak wavelength if we plot the peak

wavelength with the temperature on the

graph you can see they are invisibly

proportional they exhibit what we call a

hyperbolic relationship

if the peak wavelength is longer this

will correspond to a cooler surface

temperature of the star and vice versa

if the peak wavelength is shorter it

will suggest that the surface

temperature of the star is hotter the

absorption spectrum of the star reveals

the maximum wavelength lambda max that

is a peak wavelength of at 550

nanometers

determine the surface temperature of the

star so whenever the surface temperature

equals to winds constant divided by the

peak wavelength lambda max wind's

constant is 2.898 times ten to the minus

three

divided by

the peak wavelength which is 550

but this is nanometers so we times this

by ten to minus nine to convert into

meters

and this gives me five thousand 5269

kelvins the temperature you calculate

from using wind's law will always be

kelvins if you want to calculate the

peak wavelength using the temperature

make sure you're using the temperature

with the units and kelvins not degrees

celsius

a star's absorption spectrum also

provides information on its chemical

composition specifically what elements

are present in the gaseous layers of the

star in the spectroscopy video i

explained that different elements

produce unique and characteristic

emission spectra due to their unique

atomic structure when we compare the

absorption spectrum of a star that is a

stellar spectrum with the emission

spectra of various elements we can

identify what elements were responsible

for the absorption lines in the stars

spectrum by doing this we can solve

the star spectrum to identify all the

elements that are found in the star

therefore this is how we can determine

its chemical composition a star's

absorption spectrum also provides

information on translational velocity by

way of review doppler's effect refers to

the phenomenon where the wavelength and

frequency

of electromagnetic radiation are

affected by the relative velocity of the

source that produces the radiation and

the observer that measures the radiation

in this case the source is the relative

velocity of the star and the observer is

the relative velocity of earth

since doppler's effect affects the

wavelength and frequency of light it

will affect the position of the

absorption lines that we see in the

stars spectrum

when a star has a relative velocity that

is moving away or receding from us the

wavelengths absorption lines become

longer and the position on the spectrum

are shifted towards the red part of the

visibility spectrum this effect is known

as red shift

when a star has a relative velocity that

is moving towards us the wavelengths of

absorption lines become shorter and the

positions on the spectrum will move

towards the blue part or the violet part

of the visibility spectrum

this phenomenon is called blue shift

red shift and blue shift can only be

identified when the star spectrum is

compared with an element absorption

spectrum that's obtained on earth where

there's no relative velocity otherwise

you'll have no way of knowing what the

original position of these lines are

the original spectrum of various

elements that's obtained on earth is

also known as the rest frame because the

source of radiation is at rest and not

moving compared to the observer

in summary red-shifted absorption lines

suggest that the star is moving away

from us or receding from us

and blue shifter absorption lines

suggest that the star is moving towards

us and this is how the spectrum provides

information on the star's translational

velocity using doppler's effects

equation we can actually derive a more

simplified and new equation to calculate

the star's relative velocity and that is

the velocity of the star is equal to the

change or the shift in wavelength

divided by the actual wavelength of the

absorption line multiplied by the speed

of light in this equation we can see

that faster the translational velocity

of the star greater the effect of either

blue shift or red shift that is the

delta lambda

stars spectra also provide information

on the rotational velocity a star's

rotational velocity gives rise to

relative velocity between the source of

the radiation and the observer which

means the absorption lines are again

affected by doppler's effect when a star

rotates the side that is rotating

towards earth will cause light to be

blue shifted and the side that's

rotating away from earth will cause the

absorption lines and the light to be

redshifted therefore a star's rotational

motion will result in each absorption

line to experience both blue shift and

redshift and this is why each absorption

line will become broader faster the

rotational velocity or the motion of the

star the broader each absorption line

will become

density is the last feature that can be

determined from a star's absorption

spectrum density is a star's mass

divided by its volume stars have

different densities particularly if they

are in different stages of their life

cycle density is related to the pressure

of the star denser stars have a greater

pressure which increases the collision

rate between the gas molecules in the

outer layers of the star

when the molecules and atoms collide the

energy level of the electronic orbits

will change due to the electrostatic

interaction between the electrons

when the orbits increase in energy level

the electrons need to absorb more energy

to be excited and reach the new excited

state

vice versa when orbit's decreasing

energy level the electrons can now

absorb a smaller amount of energy to

reach the excited states radiation of

shorter wavelength has greater amount of

energy so when orbits increase in energy

level the electrons will absorb a

shorter wavelength of radiation

conversely when orbits decrease in

energy level then the electron will

absorb a longer wavelength of emr

therefore in stars with higher density

the increased collision between atoms

will cause the electrons to absorb

radiation of a wider range of energy so

both shorter and longer wavelengths as

the orbits can either increase or

decrease in energy level consequently

this will produce broader absorption

lines as you can see in the diagram here

the diagram here shows that the two

absorption spectra are produced from two

stars made of the same elements but the

bottom spectrum is of a star with a

higher density

and you can see here due to the higher

pressure and density of the star each

absorption line appears to be broader or

wider compared to the thinner ones in

the top spectrum

i'll summarize the key takeaways to

conclude the video elements and outer

layers of the stars will absorb

electromagnetic radiation emr which

produces absorption lines in the

spectrum of the star the absorption

spectra will provide information on

various features of the star including

its chemical composition that is the

elements found in the star the surface

temperature using wind's displacement

law

translational and rotational velocity of

the star and the density of the star

this concludes the video on spectrum of

stars